Research on the effects of warming and seawater intrusion on the migration and transformation of arsenic in coastal soil

Abstract

The application discloses a research method of influence of temperature increase and seawater intrusion on migration and transformation of coastal soil arsenic, and belongs to the technical field of biogeochemistry. The method simulates global weather warming and seawater intrusion by using temperature increase and addition of sulfate, explores the influence of temperature and sulfate on migration process of As in solid-liquid phase under the condition of flooding and driving mechanism, and explores the influence of biochar on migration and transformation of As in soil in the process of seawater intrusion by applying biochar, so as to finally obtain that temperature increase and SO4 2‑ play an important role in migration and transformation of As, Fe and S in solid-liquid phase of soil under the condition of flooding. The temperature increase can accelerate the reaction speed of As, Fe and S, the introduction of SO4 2‑ into the soil containing As under the condition of flooding can promote the processes of reductive dissolution and release and fixation of As and Fe in the soil, increase the potential risk of release of As in solid phase, and the experimental results can provide a reference for understanding of the process of migration and transformation of As in coastal soil under the influence of climate warming and seawater intrusion.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of biogeochemistry, and particularly relates to a method for studying the influence of warming and seawater intrusion on the migration and transformation of coastal soil arsenic. BACKGROUND

[0002] Due to the greenhouse gas emissions caused by human activities, the concentration of greenhouse gases in the atmosphere has increased significantly, and after a long period of accumulation, it has formed a greenhouse effect, causing global warming. The sea level rise caused by global warming is also an issue that cannot be ignored. Sea level rise can lead to coastal recession, submergence of coastal lowlands and delta areas; salt tide rise intensifies, and originally fertile farmland soil salinization causes serious damage to agriculture and farmland environment, and causes serious damage to the entire coastal ecosystem; and it promotes the frequency and intensification of extreme weather such as typhoons, storm surges and floods.

[0003] Climate warming has a profound impact on terrestrial ecosystems, and soil microorganisms play a key role in this process. Climate warming significantly changes the community composition and structure of soil microorganisms. Under the condition of warming, the changes in soil microbial community will accelerate the biogeochemical cycles such as the utilization of soil organic matter, nitrogen turnover, and carbon emission. Coastal areas are the most densely populated core areas of global human production and life, and the economic and social development level is the highest. The problem of heavy metal pollution of soil in these areas has become increasingly prominent.

[0004] As (Arsenic), element symbol As, is a kind of metalloid element, located in the fourth period of the periodic table of chemical elements, VA group, atomic number 33, atomic mass 74.9216. As has four common valence states, which are -3, 0, +3 and +5. Arsenic exists in various forms in the natural environment, which can be divided into inorganic arsenic and organic arsenic, among which the most common form is inorganic arsenic, mainly including arsenate, arsenite and arsenic sulfide. Eh and pH are two important factors affecting the form of arsenic. Under the condition of oxidation, when pH<2.24, As mainly exists in the form of H3AsO4 0 ; when 2.24 4- < pH < 6.9, H2AsO 2- -4 is dominant; when pH > 6.9, arsenic mainly exists in the form of HAsO4 0 ; under the condition of reduction, when pH < 9.2, arsenic exists in the form of uncharged As(OH)3 .

[0005] In recent years, the global natural environment, especially the climate environment, is experiencing a trend of global warming, and the diversity of the ecosystem and the biogeochemical process of pollutants are also changing significantly. Arsenic (As) is a kind of metalloid element widely distributed in nature, and high concentrations of As are found in farmland soils and various coastal areas, which poses potential risks to human health. Under the background of global warming and sea level rise, the release mechanism and extent of As in contaminated coastal soils are unclear. SUMMARY

[0006] In view of the above problems, the present application aims to provide a research method for the influence of warming and seawater intrusion on the migration and transformation of coastal soil arsenic, which uses warming and the addition of sulfate to simulate global weather warming and seawater intrusion, and explores the influence of temperature and sulfate on the migration process of As in the solid-liquid phase under the condition of flooding and the driving mechanism. The application also explores the influence of biochar on the migration and transformation of As in the soil during the process of seawater intrusion by applying biochar.

[0007] To achieve the above purpose, the technical solution adopted by the present application is as follows:

[0008] The research method for the influence of warming and seawater intrusion on the migration and transformation of coastal soil arsenic, characterized in that it comprises the following steps,

[0009] S1: preparing a sulfate-containing and a sulfate-free reduction bacteria culture medium, respectively;

[0010] S2: adding a contaminated soil sample and a biochar-treated contaminated soil sample to the two culture media of step S1, respectively, to finally form four groups of soil treated differently;

[0011] S3: performing biological treatment and sterilization treatment on the four groups of soil treated differently, respectively, and placing the biological treatment group and the sterilization treatment group in a constant temperature incubator for light-free constant temperature incubation, and taking samples regularly;

[0012] S4: performing bioavailable extraction on the solid phase of the soil samples obtained after different treatments and at different sampling times, to study the bioavailability of arsenic;

[0013] S5: performing solid phase continuous extraction on the soil samples obtained after different treatments and at different sampling times, to study the different extraction states of arsenic;

[0014] S6: performing content determination analysis on the liquid phase elements in the soil samples after different treatments.

[0015] Further, the preparation method of the sulfate-containing reduction bacteria culture medium in step S1 comprises the following steps,

[0016] S101A: KH2PO4, NH4CI, CaCl2·6H2O, Na2SO4, lactic acid and yeast extract are added to a three-necked round-bottom flask containing ultrapure water, the sample is shaken to dissolve thoroughly, and then the volume is made up to the mark;

[0017] S102A: The three-necked round-bottom flask containing the medicine in step S101 is heated while creating an oxygen-free environment in the flask;

[0018] S103A: After the medium in step S102 is heated to boiling, the oxygen-free atmosphere is maintained for cooling, and after the temperature drops to room temperature, it is dispensed.

[0019] Further, the preparation method of the sulfate-free reducing bacteria culture medium in step S1 specifically includes the following steps,

[0020] S101B: KH2PO4, NH4CI, CaCl2·6H2O, lactic acid and yeast extract are added to a three-necked round-bottom flask containing ultrapure water, the sample is shaken to dissolve thoroughly, and then the volume is made up to the mark;

[0021] S102B: The three-necked round-bottom flask containing the medicine in step S101 is heated while creating an oxygen-free environment in the flask;

[0022] S103B: After the medium in step S102 is heated to boiling, the oxygen-free atmosphere is maintained for cooling, and after the temperature drops to room temperature, it is dispensed.

[0023] Further, the four groups of different treated soils in step S2 include: no sulfate added and no biochar applied group, sulfate added and no biochar applied group, no sulfate added and biochar applied group, and sulfate added and biochar applied group.

[0024] Further, in step S3, the biological treatment group is set to five temperature conditions of 10℃, 16℃, 22℃, 25℃ and 28℃, and the sterilization treatment group is autoclaved at 121℃ for 25min. The serum bottles containing soil samples and culture medium are then placed in a constant temperature incubator for incubation in the dark.

[0025] Further, the specific operation of sampling in step S3 includes the following steps,

[0026] S301: Shake the sample in the serum bottle, use a sterile syringe to extract the mixed sample, and use a filter membrane to filter the extracted sample;

[0027] S302: Inject the filtered liquid into a sterilized headspace bottle, seal the headspace bottle with a blue butyl stopper, tighten the aluminum cap, and store it to obtain the liquid sample for step S6;

[0028] S303: The filtered filter membrane is placed in a sterilized centrifuge tube and immediately frozen in a -20℃ environment, and then freeze-dried using a freeze dryer, and the dried solid sample is used in steps S4 and S5.

[0029] Further, the specific operation of step S4 includes the following steps,

[0030] S401: An HCl solution of 1 mol / L is prepared as an extractant using oxygen-free water.

[0031] S402: The extractant is added to the solid-phase soil sample, and the extraction is shaken at room temperature for 2 hours.

[0032] S403: The extractant is filtered using a filter membrane.

[0033] Further, the extraction state of arsenic in step S5 includes a phosphate extraction state, a hydrochloric acid extraction state, a reducing agent extraction state, an oxidizing agent extraction state, and a residue state.

[0034] Further, the liquid-phase elements in step S6 include As elements, Fe elements, and S elements.

[0035] The beneficial effects of the present application are:

[0036] The present application discloses a research method for the influence of warming and seawater intrusion on the migration and transformation of arsenic in coastal soil, which simulates global weather warming and seawater intrusion by using warming and adding sulfate, explores the influence of temperature and sulfate on the migration process of As in the solid-liquid phase under the condition of flooding and the driving mechanism, and explores the influence of biochar on the migration and transformation of As in soil in the process of seawater intrusion by applying biochar, so that the research results can provide a theoretical basis for the research on the biogeochemical cycle process of As in coastal soil under the conditions of seawater intrusion and climate warming.

[0037] 2、The present application can draw the following conclusions through research: warming and SO4 2- play an important role in the migration and transformation of As, Fe and S in the solid-liquid phase of the soil under the condition of flooding. The increase of temperature will accelerate the reaction speed between As, Fe and S. When SO4 2- is introduced into the As-containing soil under the condition of flooding, it will promote the processes of As, Fe reduction dissolution and release and fixation in the soil, and increase the potential risk of As release in the solid phase. The experimental results can provide a reference for the understanding of the migration and transformation process of As in coastal soil under the conditions of climate warming and seawater intrusion. BRIEF DESCRIPTION OF DRAWINGS

[0038] Figure 1 The present application is a change curve of the total As concentration in the liquid phase under different temperatures.

[0039] Figure 2 The liquid-phase Fe concentration at different temperatures in the application 2+ The concentration change curve.

[0040] Figure 3 The liquid-phase S concentration at different temperatures in the application 2- The concentration change curve.

[0041] Figure 4 The liquid-phase SO4 concentration at different temperatures in the application 2- The concentration change curve.

[0042] Figure 5 The As concentration of the phosphoric acid extractable state and the hydrochloric acid extractable state in the soil at 10 DEG C in the application changes with the incubation time.

[0043] Figure 6 The proportion of different forms of As in the soil at 10 DEG C in the application changes with the incubation time.

[0044] Figure 7 The total As concentration of the phosphoric acid extractable state and the hydrochloric acid extractable state in the soil at 16 DEG C in the application changes with the incubation time.

[0045] Figure 8 The different binding states of As in the soil at 16 DEG C in the application determined by the chemical continuous extraction method change with the incubation time.

[0046] Figure 9 The total As concentration of the phosphoric acid extractable state and the hydrochloric acid extractable state in the soil at 22 DEG C in the application changes with the incubation time.

[0047] Figure 10 The different binding states of As in the soil at 22 DEG C in the application determined by the chemical continuous extraction method change with the incubation time.

[0048] Figure 11 The total As concentration of the phosphoric acid extractable state and the hydrochloric acid extractable state in the soil at 25 DEG C in the application changes with the incubation time.

[0049] Figure 12 The different binding states of As in the soil at 25 DEG C in the application determined by the chemical continuous extraction method change with the incubation time.

[0050] Figure 13 The As(T) concentration of the phosphoric acid extractable state and the hydrochloric acid extractable state in the soil at 28 DEG C in the application changes with the incubation time.

[0051] Figure 14 The different binding states of As in the soil at 28 DEG C in the application determined by the chemical continuous extraction method change with the incubation time.

[0052] Figure 15Figure 1 is a curve of the total iron in the hydrochloric acid extractable state in the soil at 10°C in the present application versus the incubation time.

[0053] Figure 16 Figure 4 is a curve of the total iron in the hydrochloric acid extractable state in the soil at 16°C in the present application versus the incubation time.

[0054] Figure 17 Figure 5 is a curve of the total iron in the hydrochloric acid extractable state in the soil at 22°C, 25°C and 28°C in the present application versus the incubation time.

[0055] Figure 18 Figure 6 is a curve of the total As in the bioavailable state in the soil in the present application versus the incubation time. DETAILED DESCRIPTION

[0056] In order for those skilled in the art to better understand the technical solutions of the present application, the technical solutions of the present application are further described below in combination with the drawings and examples.

[0057] 1. Materials and methods

[0058] 1.1 Experimental materials

[0059] 1.1.1 Experimental reagents

[0060] The chemical reagents used in the present application are shown in Table 1 below.

[0061] 1.1.2 Experimental instruments

[0062] The experimental instruments used in the present application are shown in Table 2 below. All glassware was soaked in 5% nitric acid for at least 24 hours before use, then washed with deionized water for at least 3 times, and dried in an oven for standby use.

[0063] 1.2 Test samples

[0064] The soil sample used in the present application was collected from a coastal soil 0-20 cm plough layer soil in Longgang District, Huludao City, Liaoning Province. Since there are many smelting industries in this area, the surrounding soil is seriously polluted by various toxic heavy (class) metals. The biochar used in the present application was purchased from the biochar engineering technology research center of Shenyang Agricultural University, which was prepared by slow pyrolysis at 350-550°C, and the total carbon was 650.3 g·kg -1 , the total nitrogen was 11.3 g·kg -1 , the ash content was 14.53%, the specific surface area was 8.32 m 2 ·g -1 , and the average pore size was 20.52 nm. The content of the main metal elements in the soil was determined by XRF, as shown in Table 3 below.

[0065] 1.3 Experimental methods

[0066] 1.3.1 Preparation of the reducing bacteria culture medium

[0067] Specifically, the reducing bacteria culture medium with and without sulfate is prepared respectively.

[0068] The components of the reducing bacteria culture medium with sulfate are shown in Table 4 below. Each component shown in Table 4 is added to a three-necked round-bottom flask (3 L) containing 1500 ml of ultrapure water, shaken to fully dissolve the sample, and finally diluted to 2 L. The three-necked round-bottom flask containing the above-mentioned medicine is heated using a constant temperature heating jacket. A condensation reflux device is installed in the middle neck of the three-necked round-bottom flask, and water is injected through the water pipe to achieve condensation. At the same time, nitrogen is introduced into the other side of the neck to remove the air in the round-bottom flask and create an anaerobic atmosphere during heating of the culture medium. When the culture medium is heated to boiling, the round-bottom flask is placed in a barrel filled with ice chips for rapid cooling. Nitrogen is continuously introduced during the cooling process to maintain an anaerobic atmosphere. After the culture medium is cooled to room temperature, it is ready for dispensing. The reducing bacteria culture medium without sulfate is the same as above except that Na2SO4 is not added.

[0069] Table 1 List of chemical reagents used in the present application

[0070]

[0071] Table 2 List of experimental instruments used in the present application

[0072]

[0073] Table 3 Main element content in soil (dry weight, mg·kg -1 )

[0074]

[0075] Note: “< LOD” means below the detection limit

[0076] Table 4 Components of the reducing bacteria culture medium with sulfate

[0077]

[0078] Note: Na2SO4 is only added for external sulfur treatment, and the conversion molar concentration is 10 mmol / L

[0079] Sub-packing of the medium: Serum bottles with a volume of 100 ml were used as sub-packing containers. Nitrogen was introduced into the serum bottles using a gas needle to expel the air in the bottle. The cooled round-bottom flask was placed on the table, and then 60 ml of medium was transferred to the nitrogen-filled serum bottle using a 100 ml syringe. The three-necked round-bottom flask was kept under nitrogen throughout the process. Then the serum bottle was tightly sealed with a butyl stopper and tightly sealed with an aluminum cap. After that, the process was repeated to sub-pack the medium in the three-necked round-bottom flask into each serum bottle. Then the sealed serum bottles containing the medium were autoclaved at 121°C for 25 minutes. After sterilization, they were ready for use.

[0080] 1.3.2 Soil anaerobic culture experiment

[0081] Whether to add 10 mmol / L Na2SO4 to the medium was used to distinguish between sulfate-added medium and non-sulfate-added medium. According to the addition of exogenous and non-exogenous, two media were added to contaminated soil and biochar-treated (biochar was added to soil at a ratio of 1:100) contaminated soil samples in an anaerobic glove box, resulting in four different treatment groups: no sulfate addition without biochar group (DC4), sulfate addition without biochar group (+S), no sulfate addition with biochar group (+C), and sulfate addition with biochar group (+S+C). Biological treatment groups and sterilization treatment groups were set up. The biological treatment group was set at 10°C, 16°C, 22°C, 25°C, and 28°C. Each experimental treatment was set up in triplicate. The specific operation steps of each treatment in the experiment are as follows:

[0082] Preparation process: In the anaerobic glove box, 3g of soil sample was added to a serum bottle containing 60mL of MSM medium, with a mass-volume ratio (m:v) of 1:20. After the soil was added to the serum bottle, it was resealed with a butyl stopper and the aluminum cap was tightly closed. The sterilization treatment group was autoclaved at 121°C for 25 minutes, and then removed for use. The biological treatment group and the sterilization treatment group were then placed in a constant-temperature incubator at a specific temperature and incubated in the dark. Samples were taken at set times: 1 day, 3 days, 7 days, 14 days, 21 days, 30 days, and 45 days. The sampling process was carried out in an anaerobic glove box.

[0083] Sampling process: Before sampling, the serum bottle was shaken to mix the sample. 3 mL of the mixed sample was extracted with a 5 mL sterile syringe. The extracted sample was filtered with a 0.22 μm filter membrane. The filtered liquid was injected into a sterilized headspace bottle. The headspace bottle was sealed with a blue butyl rubber plug, and the aluminum cap was tightened. The obtained liquid sample was subjected to liquid phase determination and liquid phase element analysis. The filtered filter membrane was immediately frozen in a sterilized centrifuge tube and then stored in a refrigerator at -20°C. The dried solid sample was taken out for use, and subjected to solid phase determination and analysis.

[0084] 1.3.3 Bioavailability extraction method

[0085] Bioavailability extraction experiments were performed on the solid phases obtained at different sampling times. The bioavailable As in the soil mainly exists in amorphous ferric arsenate and iron-containing (hydro)oxides, and is significantly correlated with the amorphous and low-crystalline iron-aluminum hydroxide components. Therefore, this component is a key factor affecting the bioavailability of As in contaminated soil. Generally, weak acid-extracted As is considered to be easily bioavailable; reduced and oxidized As is considered to have potential for microbial utilization; and residual As is relatively stable and has no bioavailability.

[0086] The obtained sample was extracted by using 1 mol / L HCl as the extractant. During the extraction process, the mass-volume ratio (m:v) of the sample to the extractant was 1:10. The extraction was performed at room temperature at a shaking frequency of 170 r·min -1 for 2 hours, and then the extract was filtered with a 0.22 μm filter membrane. To avoid experimental errors caused by the intervention of oxygen, the 1 mol / L HCl solution was prepared with anaerobic water, and the addition of the extraction reagent was completed in an anaerobic operation glove box.

[0087] 1.3.4 Solid phase continuous extraction method

[0088] The soil samples obtained at different times were subjected to solid phase continuous extraction experiments after freeze-drying. The forms of As were generally divided into four types, i.e., phosphate-extracted state, hydrochloric acid-extracted state, reducing agent-extracted state, oxidizing agent-extracted state, and residual state. The mass-volume ratio of the sample to the extractant was 1:10 during the continuous extraction process. After each extraction step in the experiment, a washing process was performed with three volumes of distilled water. To avoid experimental errors caused by the intervention of oxygen, the extractants required for continuous extraction were prepared with anaerobic water, and the addition of the extraction reagent was completed in an anaerobic operation glove box. The specific method of continuous extraction is as follows:

[0089] (1) Phosphate extractable fraction (F1): for determination of adsorbed or exchangeable As in soil or sediment. A 1 mol / L solution of potassium dihydrogen phosphate (KH2PO4) / potassium hydroxide (KOH) is used as the extractant. The pH is adjusted to 5 before use, and the extraction is carried out at 20°C, with a solid-to-liquid ratio (w:v) of 1:10, at a shaking frequency of 170 r·min -1 for 24 hours, and the liquid sample is then filtered using a 0.22 μm aqueous filter head.

[0090] (2) Hydrochloric acid extractable fraction (F2): for extraction of As co-precipitated with amorphous iron oxides, AVS (acid volatile sulphur) and carbonates. The solid sample obtained in step (1) is washed three times with deionized water, and 10 mL of 1 mol / L HCI extractant is added to the solid phase. The extraction is carried out at room temperature, at a shaking frequency of 170 r·min -1 for 2 hours, and the extract is filtered using a 0.22 μm filter.

[0091] (3) Reducing agent extractable fraction (F3): for extraction of As bound to crystalline iron oxides. The solid sample in step (2) is added to 10 mL of CBD solution (0.5 mol / L -1 sodium acetate and 2.5 mL of 1 mol / L -1 sodium bicarbonate solution), and placed in a water bath heating pot. When the water temperature reaches 85°C, 0.5 g of Na2S2O4·2H2O is added, and the mixture is kept at 85°C for 15 minutes.

[0092] (4) Oxidizing agent extractable fraction (F4): for extraction of As bound to pyrite. A 4 mol / L solution of nitric acid is used as the extractant, and the solid sample in step (3) is extracted at 65°C, at a shaking frequency of 170 r·min -1 for 1 hour.

[0093] (5) Residual fraction (F5): for extraction of As bound to pyrite and As bound in organic matter. The remaining solid is heated with concentrated nitric acid and perchloric acid until the solid is completely dissolved, and the liquid sample is taken for analysis.

[0094] 1.3.5 Liquid elemental content analysis

[0095] (1) Determination of As content

[0096] Sample pretreatment: Samples for measuring total As in the liquid phase were treated by reducing As(V) to As(III) using 5% thiourea and 5% ascorbic acid as reducing agents, followed by reduction with 5% HCl for at least 6 hours. During the measurement process, total As in the liquid phase was determined using a dual-channel atomic fluorescence spectrometer (model AFS-2202E), with 5% HCl as the sample carrier and 2% KBH4 + 0.3% NaOH as the hydride generator to determine As(T) in the liquid phase sample.

[0097] (2) Determination of Fe content

[0098] ①Fe 2+ Content determination

[0099] This experiment Fe 2+ The content was determined using a UV-Vis spectrophotometer (model UV-2500). Fe 2+ The colorimetric reaction requires the use of an ammonium acetate-glacial acetic acid buffer solution and 0.5% o-phenanthroline colorimetric reagent to develop the color of the sample.

[0100] ② Determination of Fe(T) content

[0101] This experiment used an atomic absorption spectrophotometer (model AA240, Varian) to determine Fe(T), in which the sample was diluted with 1% HNO3 before measurement.

[0102] (3) Determination of S content

[0103] ①S 2- Content determination: The methylene blue colorimetric method was used in this experiment to determine the S content of the solution. 2- The concentration of S. Add a certain amount of S to a 10mL colorimetric tube. 2- The water sample was then subjected to a process involving the addition of 5 mL of p-aminoaniline solution and 1 mL of ferric sulfate ammonium solution, followed by dilution with deionized water. The sample was then stored in the dark for 20 minutes before measurement. The absorbance of the sample was determined at 350 nm using a UV-Vis spectrophotometer (model UV-2500).

[0104] ②SO4 2- Content determination: This experiment used an ion chromatograph (ICS-1100) for detection. The sample was diluted with ultrapure water and then passed through a 0.2 μm water filter head for detection.

[0105] 1.3.6 Data Processing and Analysis

[0106] Microsoft Excel 2010 and Origin 8.5 were used for data organization and graphing; SPSS 21 was used for data analysis.

[0107] 2 Effect of temperature increase and seawater intrusion on arsenic release from coastal soil

[0108] 2.1 Changes in As(T) concentration in liquid phase at different temperatures

[0109] The results of changes in As(T) concentration in liquid phase at different temperatures are shown in Table 5 and Figure 2, respectively. In Figure 2, CK represents the sterilization treatment group. Figure 1 Figure 1 In Table 5, CK represents the sterilization treatment group.

[0110] Figure 2 Changes in As(T) concentration in liquid phase at different temperatures Figure 1 The results of anaerobic incubation experiments showed that different temperatures resulted in different trends in liquid As(T) concentration. Sterilization treatment did not significantly change the liquid As(T) concentration in each treatment within 0-45 days. The initial slow increase in liquid As(T) concentration was followed by a trend towards stabilization. Compared with the biological treatment group, high-temperature sterilization treatment affected the release and fixation of As in the solid-liquid phase. This proves that microorganisms in the soil are the key factors that promote the migration and transformation of As in the soil. Under anaerobic conditions, the process of microbial reduction of Fe(III) and As(V) is considered an important mechanism for the release of As from sediments into the liquid phase. The reductive dissolution of iron oxyminerals under the action of microorganisms destroys the adsorption sites of As on iron minerals, and the adsorbed As is released. At the same time, studies have shown that As-reducing bacteria are the main driving force for the release of adsorbed and combined As from the soil, and plants and microorganisms can also absorb As from the environment, affecting the changes in As chemical forms through microbial oxidation and reduction.

[0111] Table 5 Changes in total As concentration in liquid phase at different temperatures (45 days) (p<0.05)

[0112]

[0113] As can be seen from Table 5, the liquid As(T) concentration in the biological treatment group under different temperature conditions showed a trend of first increasing and then decreasing with time, while the total As concentration in the liquid phase without adding sulfate showed a trend of first increasing and then decreasing and then increasing. However, under different temperatures, there were significant differences in the changes in As(T) concentration in the liquid phase.

[0114] At 10°C, As was a continuous release process within 0-45 days, and the As(T) content in the liquid phase increased. The change trend of As(T) concentration in the solution with and without adding sulfate was consistent, but the As(T) concentration in the liquid phase with biochar was higher than that without biochar.

[0115] ​The concentration of liquid phase As(T) in the treatments of 16℃, 22℃, 25℃ and 28℃ within 0-45 days can be divided into two stages, showing the trend of first rising and then falling. The concentration of liquid phase As(T) in the treatments without adding sulfate can be divided into three stages, showing the trend of first rising, then falling and then rising.

[0116] Overall, in the treatments of 16℃ and 22℃, in the first stage of 0-7 days, the content of liquid phase As(T) is in the rising stage, and the release amount reaches the highest on the 7th day, and the treatment with biochar is higher than that without biochar. In the first stage, the highest concentration of liquid phase As(T) in the treatments of 16℃ (DC4, +S, +C, +S+C) reaches 2.44 μmol·L -1 , 2.04 μmol·L -1 , 3.25 μmol·L -1 and 2.71 μmol·L -1 , respectively. In the treatments of 22℃, the highest concentration of liquid phase As(T) reaches 2.57 μmol·L -1 , 2.78 μmol·L -1 , 3.06 μmol·L -1 and 2.71 μmol·L -1 , respectively. The highest concentration of liquid phase As(T) in the first stage of 25℃ and 28℃ occurs earlier than that of 16℃ and 22℃, and the concentration of liquid phase As(T) rises in the first stage of 0-3 days, and reaches the highest on the 3rd day, which is earlier than that of 16℃ and 22℃, and the concentration of liquid phase As(T) on the 3rd day is higher in the treatment with biochar than that without biochar, which is consistent with the results of other temperature conditions. In the first stage, the highest concentration of liquid phase As(T) in the treatments of 25℃ (DC4, +S, +C, +S+C) reaches 1.58 μmol·L -1 , 1.99 μmol·L -1 , 2.18 μmol·L -1 and 2.28 μmol·L -1 , respectively. In the treatments of 22℃, the highest concentration of liquid phase As(T) reaches 1.64 μmol·L -1 , 1.66 μmol·L -1 , 2.05 μmol·L -1 and 2.22 μmol·L -1 , respectively. The concentration of liquid phase As(T) in the second stage of 16℃, 22℃, 25℃ and 28℃ shows a downward trend. The concentration of liquid phase As(T) in the treatments with added sulfate decreases significantly, and the concentration of liquid phase As(T) continues to decrease from the first stage of liquid total As(T) reaching the peak to the end of the culture, and decreases to 0.1 μmol·L -1As shown in Table 5, at the end of the 45th day of cultivation, the liquid phase As(T) concentrations at different temperatures showed significant differences between the treatments with added sulfate and those without, with the decrease in liquid phase As(T) concentration becoming more pronounced as the temperature increased. The liquid phase As(T) concentration in the biochar-treated group was lower than that in the group without biochar.

[0117] In the treatment without sulfate addition, the concentration of As(T) in the liquid phase decreased in the second stage. The concentrations of As(T) in the liquid phase decreased to 1.79 μmol·L⁻¹ in the treatments at 16 °C, 22 °C, 25 °C, and 28 °C (DC₄, +C). -1 and 1.93 μmol·L -1 0.20 μmol·L -1 and 0.26 μmol·L -1 0.10 μmol·L -1 and 0.21 μmol·L -1 0.52 μmol·L -1 and 0.73 μmol·L -1 Subsequently, the As(T) concentration in the liquid phase increased in the treatments at 16℃, 22℃, 25℃, and 28℃ without sulfate addition. Overall, the concentration of As(T) released into the liquid phase increased with increasing temperature, and the As(T) concentration in the liquid phase was even higher in the treatments with biochar addition. At the end of cultivation, the total As concentration in the liquid phase reached 4.78 μmol·L⁻¹ in each of the 16℃, 22℃, 25℃, and 28℃ treatments (DC4, +C). -1 and 3.28 μmol·L -1 1.12 μmol·L -1 and 1.75 μmol·L -1 2.17 μmol·L -1 and 2.43 μmol·L -1 1.85 μmol·L -1 and 2.77 μmol·L -1 .

[0118] Experimental results show that the As(T) concentration in the liquid phase increases in the early stages of cultivation. This is because As in the soil mainly exists in amorphous iron oxides. When these iron oxides undergo reductive dissolution under the action of microorganisms, a large amount of As is released and transferred to the liquid phase. Simultaneously, a large amount of Fe is generated in the solid phase during the reduction of iron oxides. 2+ Fe 2+The secondary iron oxides can catalyze the transformation of amorphous iron oxides to high crystallinity iron oxides. These minerals have a weaker adsorption capacity for As than amorphous iron oxides, causing the desorption and release of As. Therefore, whether the microbial-mediated reductive dissolution of iron hydroxides or the direct reduction of As(V) occurs, it will release As from the solid phase, resulting in an increase in the concentration of liquid-phase As.

[0119] In the treatment of adding sulfate, the concentration of As(T) in the liquid phase will decrease. This result is consistent with the results of Krik et al. who found in the investigation in Bangladesh that the sulfate in the groundwater can reduce the concentration of As in the water, because the action of microorganisms in the system with the addition of sulfate reduces the sulfate to sulfide (H2S, HS - , S 2- ), which promotes the dissolution of amorphous iron hydroxide in soil minerals, and S 2- reacts with Fe 2+ to form FeS black precipitate, while As is co-precipitated with FeS or wrapped into the mineral lattice, so that As is immobilized. At the same time of the formation of FeS, Fe 2+ reacts with H2S to form Mackinawite, so that As is re-adsorbed into the solid phase. Mackinawite is considered to be an important adsorbent of As in the sulfur-containing system. At the same time, under the action of microorganisms, S 2- forms a large amount of water-insoluble sulfide precipitates such as realgar (AsS), orpiment (As2S3) and As pyrite (FeAsS) with As, reducing the concentration of As in the liquid phase (see equation (1)). The increase in temperature will accelerate the change in the concentration of As(T) in the liquid phase. Temperature is an important factor in regulating the physiological activity of microorganisms, which promotes the growth and respiration of microorganisms. The increase in temperature will make the sulfate enter the reduction stage to generate S 2- in advance, and As forms precipitates with S 2- or co-precipitates with pyrite minerals, promoting the decrease in the concentration of As(T) in the liquid phase.

[0120] As 3+ +S 2- →As2S3 (1)

[0121] In the treatment without the addition of SO4 2- , the concentration of As(T) in the liquid phase decreases. This may be because the Fe 2+ generated under the action of microorganisms is adsorbed on the surface of the un-reduced iron minerals, forming mixed-valence iron oxides, so that part of the As is fixed in the As-iron minerals. However, as the culture process proceeds, some iron-reducing bacteria and As-reducing bacteria destroy the mineral existing form, so that more As is released. Therefore, the concentration of As(T) in the liquid phase in the treatment without the addition of sulfate is higher than that in the treatment with the addition of sulfate.

[0122] Adding biochar promotes the release of asphalt (As) from the solid phase. This is because applying biochar effectively increases soil organic matter, total soil carbon, and microbial carbon nitrogen content, enriching the soil's carbon source. Applying biochar also improves soil water retention, enhances soil fertility, improves soil quality, provides additional carbon sources, and enriches soil microbial diversity.

[0123] 2.2 Fe in the liquid phase at different temperatures 2+ Changes in content

[0124] During anaerobic culture, Fe in the liquid phase 2+ Concentration changes are shown in the attached figure. Figure 2 As shown in Table 6 below. Fe in the liquid phase of each treatment during sterilization treatment (0-45 days) 2+ The content of Fe was very low, and no significant iron reduction occurred in the system. Compared with other unsterilized treatments at other temperatures, the liquid phase Fe content in the sterilized treatment was significantly lower. 2+ No regular changes were found in the content of Fe, indicating that the Fe content in the system... 2+ The formation of [something] is mainly determined by microorganisms.

[0125] Table 6 Liquid phase Fe at different temperatures 2+ Changes in concentration

[0126]

[0127] At 10℃, Fe 2+ It is continuously released from the solid phase to the liquid phase within 0-45 days. The treatment involving the addition of sulfate to apply biochar (+S+C) results in Fe... 2+ The release rate was the fastest, with Fe in the liquid phase on day 45. 2+ The concentration reached 133.60 μmol·L⁻¹ -1 On day 45 at 10℃, the liquid phase Fe of the other three treatments (DC4, +S, +C) was... 2+ The concentrations are: 87.05 μmol·L -1 91.58 μmol·L -1 and 80.87 μmol·L -1 .

[0128] Under incubation conditions at 16℃ and 22℃, within 0-45 days, the early-stage Fe... 2+ It is continuously released from the solid phase into the liquid phase, but in the later stage, the concentration of ferrous iron decreases when an external sulfur source is added, while the concentration of ferrous iron gradually increases when no external sulfur source is added, and the rate of increase slows down in the later stage.

[0129] Treatment with added sulfate, at 16℃, showed that the concentration of ferrous iron in the liquid phase reached its maximum after 30 days in both the biochar (+S+C) and non-biochar (+S) treatments. 2+The concentrations reached 82.59 μmol·L⁻¹. -1 and 68.31 μmol·L -1 Subsequently, Fe in the liquid phase 2+ The concentration was reduced until the end of 45 days of incubation, and the liquid phase Fe was... 2+ The concentrations reached 10.85 μmol·L⁻¹. -1 and 30.17 μmol·L -1 The concentration of ferrous iron in the liquid phase treated with biochar (+S+C) and without biochar (+S) reached its maximum after 7 days at 22℃. 2+ The concentrations reached 168.48 μmol·L⁻¹. -1 and 142.14 μmol·L -1 Subsequently, Fe in the liquid phase 2+ The concentration decreased, reaching 39.52 μmol·L⁻¹. -1 and 67.86 μmol·L -1 .

[0130] In the treatment without sulfate addition, the treatment at 16°C with biochar (+C) and the treatment without biochar (DC4), Fe 2+ Continuous release into the liquid phase. After 45 days of cultivation with and without biochar (DC4), the Fe in the liquid phase... 2+ The concentrations reached 263.63 μmol·L⁻¹. -1 and 250.01 μmol·L -1 Treatment at 22℃ with and without biochar (DC4) resulted in Fe... 2+ It is also continuously released into the liquid phase. The liquid phase Fe was treated with biochar (+C) and without biochar (DC4). 2+ The concentrations reached 385.71 μmol·L⁻¹. -1 and 462.08 μmol·L -1 Fe treated at 22℃ without sulfate 2+ The concentration is higher than 16℃.

[0131] Under incubation conditions at 25℃ and 28℃, within 0-45 days, the early Fe... 2+ It continuously releases from the solid phase into the liquid phase, but in the later stages, the treatment with an external sulfur source... 2+ As the concentration decreases, the concentration of ferrous iron gradually increases without the addition of an external sulfur source, but the rate of increase slows down in the later stages. However, in the treatment with the addition of sulfate, the concentration of Fe in the liquid phase... 2+ The trend of concentration change can be divided into three segments.

[0132] In the first phase (0-7 days), Fe in the liquid phase was treated at 25℃ and 28℃ with and without biochar (+S+C). 2+The concentration of Fe in the liquid phase reached its maximum after 7 days. The treatment with biochar (+S+C) and without biochar (+S) resulted in different concentrations. 2+ The concentrations reached 196.79 μmol·L⁻¹. -1 and 244.11 μmol·L -1 175.48 μmol·L -1 and 217.68 μmol / L -1 Subsequently, Fe in the liquid phase 2+ The concentration decreased and tended to plateau. Liquid phase Fe was treated at 25°C with and without biochar (+S+C). 2+ The content decreased by 14.70 μmol·L⁻¹ -1 and 22.26 μmol·L -1 Applying biochar increases the activity of iron-reducing bacteria, accelerating the reduction of Fe. The treatment of Fe in the liquid phase at 28℃ with and without biochar (+S+C) was compared. 2+ The concentration decreased, reaching 28.99 μmol·L⁻¹. -1 and 28.10 μmol·L -1 .

[0133] As can be seen from Table 6, at the end of the 45th day of cultivation, the liquid phase Fe was treated with sulfate at different temperatures. 2+ The concentration was significantly lower than that of the treatment without the addition of sulfate.

[0134] In the treatment without the addition of sulfate, Fe in the liquid phase 2+ The concentration change trend can be divided into three stages, showing an initial increase, a slight decrease, and then a gradual leveling off. Treatments at 25℃ and 28℃ with and without biochar (DC4) were performed during the first stage (0-21 days). 2+ The Fe is continuously released into the liquid phase. Treatment of the liquid phase with and without biochar (DC4) at 25℃... 2 + The concentrations reached 362.05 μmol·L⁻¹. -1 and 462.77 μmol·L -1 Treatment at 28℃ with and without biochar (DC4) resulted in Fe... 2+ It is also continuously released into the liquid phase. The liquid phase Fe was treated with biochar (+C) and without biochar (DC4). 2+ The concentrations reached 377.02 μmol·L⁻¹. -1 and 393.81 μmol·L -1 .

[0135] The second phase lasts 21-30 days, with liquid phase Fe... 2+The concentration began to decrease, with treatments at 25°C with and without biochar (DC4) showing the Fe concentration in the liquid phase on day 30. 2+ The concentration decreased to 183.63 μmol·L⁻¹. -1 and 183.33 μmol·L -1 Treatment at 28℃ with and without biochar (DC4) showed Fe in the liquid phase on day 30. 2+ The concentration decreased to 180.77 μmol·L⁻¹. -1 and 159.76 μmol·L -1 .

[0136] The third stage lasts 30-45 days, with liquid phase Fe. 2+ The concentration began to plateau. Treatments at 25°C with and without biochar (DC4) showed that on day 30, the liquid phase Fe... 2+ The concentration decreased to 183.63 μmol·L⁻¹. -1 and 183.33 μmol·L -1 Treatment at 28℃ with and without biochar (DC4) showed Fe in the liquid phase on day 30. 2+ The concentration decreased to 180.77 μmol·L⁻¹. -1 and 159.76 μmol·L -1 Fe 2+ It can catalyze the conversion of iron oxide into Fe, which has a stronger oxidizing ability. 3+ Subsequently, As was oxidized 3+ For As 5+ Thus, Fe 2+ The concentration decreased.

[0137] Overall, the Fe content in the treated liquid phase without added sulfate was low. 2+ The concentration will continue to rise, then level off in the later stages. This is because under anaerobic flooding conditions, Fe(III) acts as an electron acceptor and is converted into Fe(II), leading to an increase in the Fe(II) concentration in the soil solution (see Equation 2). 2+ The changes in the liquid phase are consistent with the trend of As(T), indicating a good correlation between arsenic and iron in the anaerobic environment. Furthermore, increasing temperature promotes an increase in the concentration of Fe(II) in the liquid phase. Studies by Zhai Siyuan et al. have shown that with increasing temperature, Fe concentration increases during the dissimilatory iron reduction process in activated sludge. 2+ The cumulative amount of Fe is on the rise. 2+ The accumulation rate also accelerates, which is consistent with the results of this invention. Temperature is the most important factor affecting soil biomineralization; increased temperature accelerates mineralization. Therefore, as temperature increases, it promotes the mineralization of minerals in the soil, leading to the formation of Fe. 2+ Released into the liquid phase (see Equation 3).

[0138] However, the addition of sulfate will make Fe 2+ re-adsorbed to the solid phase, and the sulfate reduction process will occur to generate S 2- and Fe 2+ to form FeS black precipitate, thus reducing the concentration of Fe 2+ in the liquid phase. The heating will accelerate the sulfidation reaction, and the generated S 2- will easily combine with metal ions such as Fe and As, thus accelerating the reduction of Fe 2+ in the liquid phase (see equation 4).

[0139] Fe2O3 / FeOOH + 6H→ 2Fe 2+ + 3H2O (2)

[0140] 2Fe(OH)3+ HS - + 5H + → Fe 2+ + S 0 + 6H2O (3)

[0141] Fe 2+ + HS - → FeS↓ + H + (4)

[0142] The treatment without the addition of sulfate showed a decrease in the concentration of Fe 2+ at the later stage, which may be because the iron oxide surface will adsorb some Fe 2+ on its surface, thus reducing the specific surface area of the contact between the microorganisms and Fe 3+ , and reducing the iron reduction rate in the system. On the other hand, under the action of iron-reducing bacteria, Fe 3+ is reduced to Fe 2+ , which will undergo biomineralization to generate magnetite under neutral conditions, and the Fe 3+ encapsulated in the magnetite will not be available as an electron acceptor under neutral conditions. The addition of biochar will promote the reduction of Fe 3+ , because the addition of biochar will enrich the carbon source in the soil. The application of biochar can provide additional electrons for microorganisms, and enhance the denitrification process of nitrate under the condition of Fe(0) by promoting electron transfer. Studies have shown that the addition of acetic acid has caused significant release of arsenite and Fe(II) in sediments.

[0143] 2.3 Changes in the content of S 2- in the liquid phase at different temperatures

[0144] The results of the changes in the content of S 2- in the liquid phase at different temperatures in the anaerobic culture experiment are shown in the attached Figure 3 . From the attached Figure 3It can be seen that in the treatment of adding sulfate, the S 2- accumulation amount changes differently with time, and the S 2- content in the liquid phase changes regularly with the culture time. The specific change process is as follows:

[0145] In the sterilization treatment, the S 2- content of each treatment does not change significantly within 0-45 days and remains at a low level. This is because after sterilization treatment, there is no sulfate-consuming microorganism in the system, so the S 2- content in the liquid phase does not increase significantly. In the 10℃ treatment, the S 2- content of each treatment does not change significantly within 0-45 days and remains at a low level. This may be because low temperature inhibits the growth of microorganisms, slowing down the process of SO4 2- generated by sulfate-reducing bacteria to S 2- .

[0146] The 16℃ treatment within 0-45 days, in the treatment group of adding sulfur source, the S 2- content in the liquid phase changes with time can be divided into two stages. The first stage: 0-30 days, the S 2- content in the liquid phase does not change significantly, indicating that the microbial sulfuration reaction has not occurred. The S 2- content in the liquid phase remains at a low level. The second stage: within 30-45 days, the S 2- content in the liquid phase begins to increase, and the S 2- content in the liquid phase reaches 165.79μmol·L -1 and 187.73μmol·L -1 at the 45th day.

[0147] The 22℃ treatment within 0-45 days, in the treatment group of adding sulfur source, the S 2- content in the liquid phase changes with time can be divided into three stages. The first stage: 0-7 days, the S 2- content in the liquid phase does not change significantly, indicating that the microbial sulfuration reaction has not occurred. The S 2- content in the liquid phase remains at a low level.

[0148] The second stage: the S 2- content in the liquid phase begins to rise rapidly. In the treatment of applying biochar, the S 2- content in the liquid phase reaches 247.92μmol·L -1 at 7-14 days. In the treatment without applying biochar, the S 2- content in the liquid phase reaches 200.40μmol·L -1 at 7-21 days.

[0149] The third stage, S in liquid phase 2- Concentration began to decline. Among them in the application of biochar treatment, 14-45 days S 2- content in liquid phase decreased to 119.43 μmol·L -1 . In the absence of biochar treatment, 7-21 days S 2- content in liquid phase decreased to 97.26 μmol·L -1 .

[0150] 25℃ and 28℃ treatment in 0-45 days, in the treatment of additional sulfur source, S 2- content in liquid phase changed with time had similar trends, which can be divided into three stages. The first stage: 0-21 days, S 2- in liquid phase began to accumulate in the application of biochar and no biochar treatment, indicating that the sulfidation reaction began to generate S 2- , among them 25℃ application of biochar treatment, 0-21 days S 2- content in liquid phase reached 163.91 μmol·L -1 . In the absence of biochar treatment, 0-21 days S 2- content in liquid phase reached 99.75 μmol·L -1 . In different treatments, the application of biochar treatment than the treatment of no biochar S 2- content in liquid phase was higher. 28℃ application of biochar treatment, 0-2 days S 2- content in liquid phase reached 138.38 μmol·L -1 . 28℃ in the absence of biochar treatment, 0-14 days S 2- content in liquid phase reached 89.98 μmol·L -1 . Consistent with the previous findings, the application of biochar treatment than the treatment of no biochar S 2- content in liquid phase was higher.

[0151] The second stage 21-30 days, S 2- concentration in liquid phase began to decline. Among them 25℃ application of biochar treatment, 21-30 days S 2- content in liquid phase reached 67.48 μmol·L -1 . In the absence of biochar treatment, 21-30 days S 2- content in liquid phase reached 53.65 μmol·L -1 . And 28℃ application of biochar treatment, 21-30 days S 2- content in liquid phase decreased to 45.80 μmol·L -1 . In the absence of biochar treatment, 14-30 days S 2-The content decreased to 45.52 μmol·L. -1 .

[0152] In the third stage, S in the liquid phase 2- The concentration increased slightly. Specifically, at 25℃ for 30-45 days, the concentration of S in the liquid phase decreased compared to that in cases without biochar treatment and those without. 2- The content reached 97.26 μmol·L⁻¹. -1 and 93.71 μmol·L -1 At 28℃ for 30-45 days, the sulfur content in the liquid phase was different in both cases with and without biochar treatment. 2- The content reached 75.22 μmol·L⁻¹. -1 and 53.16 μmol·L -1 S 2- The slight increase in concentration may be due to S 2 - The release rate is faster than the rate at which it forms precipitates with As and Fe.

[0153] In summary, increasing the temperature promotes an increase in the concentration of sulfide ions in the liquid phase, and increases the activity of sulfate-reducing bacteria, resulting in the reduction of more sulfate to sulfur. 2- (See Equation 5). The application of biochar will promote the liquid phase S... 2- The formation of this substance is likely due to the high specific surface area and porous nature of biochar, which provides more attachment sites for sulfate-reducing bacteria. Furthermore, the application of biochar significantly increases soil organic matter content, improves soil properties, and enhances soil nutrient content, thereby promoting the growth of sulfate-reducing bacteria and leading to the reduction of more sulfate to sulfur. 2- At the same time, the activity and quantity of soil microorganisms also increased significantly. Combined with the changes in the concentrations of As and Fe in the liquid phase, S... 2- It will form precipitates with As and Fe, which will cause S to form later. 2- Concentration decreases. In the initial stage, S... 2- The content did not change significantly, which may be because in the early stage of culture, sulfate-reducing bacteria need to adapt to the environment to activate and enrich themselves in order to start the sulfate reduction function.

[0154] 2CH2O (organic matter) + SO4 2- →H2S + 2HCO 3- (5)

[0155] 2.4 SO4 in the liquid phase at different temperatures 2- Changes in content

[0156] SO4 in the liquid phase at different temperatures 2- The content changes are shown in the attached figure. Figure 4 As shown, from the appendix Figure 4As can be seen from Figure 1, the initial concentration of sulfate in the liquid phase was about 11 mmol / L in the culture system with the addition of sulfate. With the increase of culture time, the concentration of sulfate gradually decreased. The curves of the concentration of sulfate at different temperatures were also different.

[0157] Under sterilization conditions, the content of SO4 2- in the liquid phase did not change significantly compared with the initial concentration in the 0-45 days of applying biochar treatment and in the liquid phase without applying biochar treatment. This is because the sterilization process killed the microorganisms that can utilize sulfate in the system, so the content of sulfate in the liquid phase did not change, which is consistent with the results of the sterilization of the liquid phase SO4 2- concentration.

[0158] Under 10°C, the content of SO4 2- in the liquid phase did not change significantly compared with the initial concentration in the 0-45 days of applying biochar treatment and in the liquid phase without applying biochar treatment. This may be because low temperature inhibits the growth of microorganisms, slows down the process of generating S 2- from SO4 2- by sulfate-reducing bacteria, which is consistent with the results that the liquid phase SO4 2- concentration did not increase at 10°C in each treatment.

[0159] Under 16°C, the content of SO4 2- in the liquid phase can be divided into two stages with the change of time. In the first stage of 0-21 days, the consumption of SO4 2- in the liquid phase with and without applying biochar treatment was slow, and the concentration change was not obvious. In the second stage of 21-45 days, the concentration of sulfate gradually decreased, and the concentration of SO4 2- in the liquid phase with and without applying biochar treatment decreased to 1.80 mmol·L -1 and 3.91 mmol·L -1 , respectively. The rate of consuming SO4 2- in the liquid phase with applying biochar treatment was faster than that without applying biochar treatment.

[0160] Under 22°C, 25°C and 28°C, the content of SO4 2- in the liquid phase with and without applying biochar treatment had similar change trends, and the concentration of sulfate in the liquid phase gradually decreased with the increase of culture time. In 0-45 days, it can be divided into two stages. In the first stage of 0-14 days, the concentration of SO4 2- in the liquid phase with and without applying biochar treatment decreased rapidly, and the concentration of SO4 2-4.84 mmol·L -1 and 5.22 mmol·L -1 , 2.05 mmol·L -1 and 4.03 mmol·L -1 , and 2.59 mmol·L -1 and 3.87 mmol·L -1 . The sulfate utilization rate of the treatment with biochar is faster than that of the treatment without biochar. Overall, the higher the temperature, the faster the consumption rate of sulfate. In the second stage of 14-45 days, the rate of decrease of sulfate in the liquid phase slows down, and gradually tends to be stable in the later stage. At 22℃, the rate of decrease of SO4 2- concentration is 1.20 mmol·L -1 and 2.45 mmol·L -1 , and the SO4 2- concentration of each treatment under the conditions of 25℃ and 28℃ is lower than 1 mmol·L-1, but the sulfate utilization rate of the treatment with biochar is still faster than that of the treatment without biochar.

[0161] As can be seen from the above, the increase in temperature and the addition of biochar can accelerate the consumption of SO4 2- in the liquid phase, because the changes of temperature and biochar can affect the activity of microorganisms, and the higher the temperature, the stronger the activity of sulfate-reducing bacteria. In addition, long-term liquid culture of iron oxides can cause obvious notches and linear fractures, which can expose more active sites, increase the adsorption of SO4 2- on iron minerals, and SO4 2- can combine with iron oxides to form schultenite, which also has a strong adsorption effect on As. Studies have shown that increasing the temperature can promote the activity of sulfate-reducing bacteria and make the sulfidation reaction advance. The addition of biochar can increase the content of organic matter in the soil, improve the soil environment, improve the microbial activity, and accelerate the degradation of sulfate.

[0162] In summary, in the present application, coastal cultivated soil with and without biochar is taken as the research object, the effects of climate warming and seawater intrusion on cultivated soil are simulated by increasing the temperature and adding sulfate, and the variation law of arsenic-iron-sulfur in the liquid phase is revealed. The main conclusions are as follows:

[0163] (1) In the anaerobic waterlogged soil culture experiment, the treatment of adding SO4 2- , the total As concentration in the liquid phase decreases with the progress of the culture experiment, and the total As concentration in the liquid phase of the treatment with biochar decreases faster. Without the introduction of SO4 2-During the treatment, the total As content in the liquid phase continuously increases, and the application of biochar further promotes the increase in As concentration in the liquid phase. Increasing the temperature will make the above results more pronounced.

[0164] (2) Adding SO4 2- Fe in the liquid phase during the treatment 2+ The concentration of Fe in the liquid phase of the treatment with biochar showed a trend of first increasing and then decreasing. 2+ The concentration decreased faster. However, without added SO4... 2- Treatment of Fe in the liquid phase 2+ The content continues to increase; the application of biochar will promote the liquid phase Fe 2+ The concentration increased. And as the experiment progressed, with the addition of SO4... 2- The treatment produces a black mineral that deepens in color over time, suggesting that this black solid is FeS. Increasing the temperature will make these results even more pronounced.

[0165] (3) Add SO4 2- The treatment of SO4 by microorganisms 2- The concentration of SO4 in the liquid phase will gradually decrease as the temperature increases. 2- The faster it is consumed; S 2- The concentration of S will gradually increase, and as the temperature rises, it will promote the growth of S. 2- The formation of SO42-. Treatment with added biochar increases the metabolic rate of sulfate-reducing bacteria, accelerating SO42- formation. 2- To S 2- The transformation.

[0166] 3. Effects of warming and seawater intrusion on arsenic speciation in coastal soils

[0167] 3.1 Arsenic speciation in coastal soils

[0168] The morphological changes of As in soil culture at 10℃ for 45 days are shown in the attached figure. Figure 5 and attached Figure 6 As shown. From the appendix Figure 5 and attached Figure 6As can be seen, with the extension of cultivation time, the content of As(T) extracted by solid-phase phosphate increased in the sulfate-added treatment and decreased in the sulfate-free treatment. The contents of hydrochloric acid-extractable and reduced As(T) decreased slightly, while the changes in oxidized and residual As(T) were not significant. On day 45 of cultivation, compared with day 1, the content of As(T) extracted by solid-phase phosphate increased by 28.0% and 15.7% in the sulfate-added treatment (+S, +S+C), respectively, while the content of As(T) extracted by phosphate did not change significantly in the sulfate-free treatment (DC4, +C). The proportions of total As extracted by hydrochloric acid and reduced As decreased slightly, but the changes were not significant. The content of oxidized As(T) did not change significantly. The DC4, +C, and +S+C treatments increased the content by 1.1%, 1.3%, and 0.8%, respectively, while the +S treatment decreased it by 0.4%. The residual state treated with sulfate (+S, +S+C) decreased the As(T) content by 13.8% and 6.3%, respectively, while the As(T) content treated without sulfate (DC4, +C) increased by 3.5% and 10.4%, respectively.

[0169] The morphological changes of As in soil culture at 16℃ for 45 days are shown in the attached figure. Figure 7 and attached Figure 8 As shown in the figure. Overall results indicate that the content of As(T) extracted by solid-phase phosphoric acid gradually increased over time, while the contents of As(T) extracted by hydrochloric acid and reduced forms decreased slightly, and the changes in oxidized and residual forms were not significant. On day 45 of cultivation, the content of As(T) extracted by solid-phase phosphoric acid increased by 6.8%-32.5% compared to day 1, with the treatment with sulfate addition and biochar application (+S+C) showing the largest increase in the proportion of phosphoric acid extractable As(T). The content of As(T) extracted by hydrochloric acid generally decreased, decreasing by 3.8%-11.3% compared to day 1. The content of As(T) extracted by solid-phase hydrochloric acid in the treatment without sulfate addition did not change significantly over 45 days, with the treatment with biochar application (+C) showing a 1.9% increase in hydrochloric acid extractable As(T), and the treatment without biochar application (DC4) showing a 3.8% decrease. The treatment with added sulfate (+S, +S+C) showed a greater reduction in the amount of hydrochloric acid extractable than the treatment without added sulfate (DC4, +C). The treatment with added sulfate and biochar (+S+C) reduced the amount of As extracted by solid-phase hydrochloric acid by 7.9%, and the treatment with added sulfate (+S) reduced the amount of As(T) extracted by solid-phase hydrochloric acid by 11.3%.

[0170] The morphological changes of As in soil culture for 45 days at 22℃ are shown in the attached figure. Figure 9 and attached Figure 10 As shown. From the appendix Figure 9 and attached Figure 10As can be seen, with the extension of cultivation time, the content of phosphoric acid-extractable As(T) increases, the content of hydrochloric acid-extractable As(T) decreases slightly, the content of reduced and oxidized As(T) shows no significant change, and the content of residual As(T) decreases. The changes in the content of phosphoric acid-extractable and hydrochloric acid-extractable As(T) in soil after 45 days of cultivation at 22℃ are shown in the attached figure. Figure 9 As shown, the content of solid-phase phosphoric acid extractable As(T) increased on day 45 compared to day 1, with the sulfate-treated group showing a higher content than the untreated group. The sulfate-treated groups (+S, +S+C) showed increases of 29.1% and 27.8% in solid-phase phosphoric acid extractable As(T) compared to day 1, respectively; while the untreated groups (DC4, +C) showed increases of 0.6% and 25.9%, respectively. The biochar-treated groups showed a greater increase in total phosphoric acid extractable As content than the untreated groups. The hydrochloric acid extractable As content generally decreased, ranging from 1.6% to 10.4% compared to day 1. The untreated groups showed little change in total hydrochloric acid extractable As content over 45 days, while the sulfate-treated groups showed a greater decrease, with the sulfate-treated groups (+S, +S+C) showing decreases of 10.4% and 4.5%, respectively.

[0171] The morphological changes of As in soil culture for 45 days at 25℃ are shown in the attached figure. Figure 11 and attached Figure 12 As shown in the figure. Overall, it can be observed that with prolonged culture time, the total As content extracted by phosphate increases, the total As content extracted by hydrochloric acid decreases, the changes in reduced and oxidized forms are not significant, and the total As content in the residue decreases. The changes in As speciation after 45 days of soil culture at 25℃ are shown in the attached figure. Figure 5 As shown, the total As content in the solid-phase phosphate extract increased with prolonged culture time. In the sulfate-added treatments (+S, +S+C), the As content in the solid-phase phosphate extract increased by 35.8% and 30.9% respectively compared to day one; in the un-sulfate-added treatments (DC4, +C), the As content in the solid-phase phosphate extract increased by 26.6% and 31.1% respectively compared to day one. The total As content in the hydrochloric acid extract decreased. Specifically, in the sulfate-added treatments (+S, +S+C), the As content in the solid-phase hydrochloric acid extract decreased by 14.4% and 14.1% respectively compared to day one; in the un-sulfate-added treatments (DC4, +C), the As content in the solid-phase hydrochloric acid extract decreased by 19.9% ​​and 10.2% respectively compared to day one. Changes in the reduced and oxidized states were not significant. The content of residual As decreased by 17.5%, 21.6%, and 2.3% respectively under the +S, +C, and +S+C treatments compared to the first day, while the content of residual As increased by 5.4% under the DC4 treatment compared to the first day.

[0172] The changes of As speciation in soil incubated at 28℃ for 45 days are shown in Fig. 1. It can be found that the content of As(T) in phosphoric acid extractable fraction increased, the content of As(T) in hydrochloric acid extractable fraction and reduced As(T) decreased, and the content of oxidized As(T) did not change significantly, and the content of total As(T) in residual fraction decreased. Figure 13 The changes of As speciation in soil incubated at 28℃ for 45 days are shown in Fig. 1. It can be found that the content of As(T) in phosphoric acid extractable fraction increased, the content of As(T) in hydrochloric acid extractable fraction and reduced As(T) decreased, and the content of oxidized As(T) did not change significantly, and the content of total As(T) in residual fraction decreased. Figure 14 The changes of As speciation in soil incubated at 28℃ for 45 days are shown in Fig. 1. It can be found that the content of As(T) in phosphoric acid extractable fraction increased, the content of As(T) in hydrochloric acid extractable fraction and reduced As(T) decreased, and the content of oxidized As(T) did not change significantly, and the content of total As(T) in residual fraction decreased. Figure 5 The changes of As speciation in soil incubated at 28℃ for 45 days are shown in Fig. 1. It can be found that the content of As(T) in phosphoric acid extractable fraction increased, the content of As(T) in hydrochloric acid extractable fraction and reduced As(T) decreased, and the content of oxidized As(T) did not change significantly, and the content of total As(T) in residual fraction decreased.

[0173] Overall, the content of As(T) in phosphoric acid extractable fraction increased, and the content of As(T) in hydrochloric acid extractable fraction decreased with the extension of incubation time. In the system with added sulfate, this may be because the reduction and dissolution of iron minerals in soil released As fixed by iron oxides and generated Fe 2+ . Fe 2+ reacted with S 2- to form Fe-S minerals, which adsorbed As on their surface. Excess S 2-This leads to the dissolution of amorphous As₂S₃ to form thioarsenate (see Equation 6), which is then adsorbed by soil minerals. Therefore, the content of phosphoric acid-extractable As(T) increases, while the content of hydrochloric acid-extractable As(T) decreases. The addition of sulfate to biochar treatment makes it easier for As in the solid phase to be converted to phosphoric acid-extractable As. This is because the porous structure of biochar reduces competition among microorganisms, provides nutrients to promote the growth and reproduction of soil microorganisms, promotes the growth of iron-reducing and sulfur-reducing bacteria, and accelerates the reaction. In contrast, without sulfur treatment, iron oxides dissolve, and the generated Fe... 2+ with Fe 3+ Iron minerals that form mixed valence states have a certain adsorption capacity for As. 2+ Iron is oxidized on biofilms to form iron minerals, and As is adsorbed onto iron oxides, thus increasing the content of exchangeable As(T). The content of phosphate-extractable As(T) decreases with increasing temperature after sulfate treatment. This may be because lower temperatures are more conducive to the growth of sulfate-utilizing microorganisms. Sulfate-reducing bacteria can directly contact the soil or release enzymes, organic acids, and chelating agents to promote the dissolution of solid, insoluble iron in the soil, releasing As from the iron minerals into exchangeable As. As temperature increases, this advantage weakens, the activity of iron-reducing bacteria in the soil increases, and the generated Fe... 2+ It combines with sulfide ions to form pyrite minerals, such as Maginotite. Maginotite has a strong adsorption capacity for As. As is fixed on the mineral surface, thus reducing the As concentration. In the treatment without the addition of sulfate, the As(T) content in the liquid phase continuously increases, which is consistent with the corresponding increase in the content of phosphoric acid-extractable As(T) and the decrease in the content of hydrochloric acid-extractable and reduced As(T). This indicates that under the action of microorganisms, some crystalline iron oxides and amorphous iron oxides undergo reducing dissolution, leading to the redistribution of As between the solid and liquid phases.

[0174] 3As2S3+3HS - +H + →2H2AsS 6- (6)

[0175] 3.2 Changes in hydrochloric acid-extractable iron in coastal soils

[0176] At 10℃, no significant change was observed in the total iron content extracted by solid-phase hydrochloric acid within 0-45 days for each treatment. (See attached image) Figure 15 As shown. This may be because the low temperature slowed down the activity of microorganisms, resulting in no significant changes in the iron minerals extracted by hydrochloric acid.

[0177] The total iron content of the solid-phase hydrochloric acid extract during the 0-45 days of each treatment at 16℃ is shown in the attached figure. Figure 16As shown in the figure, the total Fe content of solid-phase HCl extractable state did not change significantly within 0-45 days in the treatment without added sulfate, while the total Fe content of solid-phase HCl extractable state did not change significantly within 0-30 days in the treatment with added sulfate, but increased slightly within 30-45 days, higher than the treatment without added sulfate. This corresponds to the phenomenon that the concentration of divalent Fe ions in liquid phase decreased within 30-45 days. The Fe content of the treatment with added sulfate and biochar was higher than the treatment without added biochar. This may be because the application of biochar promotes the growth of microorganisms, thus enhancing the corresponding microbial metabolic activity.

[0178] The change of total Fe content of HCl extractable state under the conditions of 22℃, 25℃ and 28℃ is shown in the figure Figure 17 As shown in the figure, the total Fe content of solid-phase HCl extractable state did not change significantly within 0-45 days in the treatment without added sulfate, while the total Fe content of solid-phase HCl extractable state did not change significantly within 0-30 days in the treatment with added sulfate, but increased slightly within 30-45 days, higher than the treatment without added sulfate. This corresponds to the phenomenon that the concentration of divalent Fe ions in liquid phase decreased within 30-45 days. The Fe content of the treatment with added sulfate and biochar was higher than the treatment without added biochar. This may be because the application of biochar promotes the growth of microorganisms, thus enhancing the corresponding microbial metabolic activity.

[0179] The results of the study show that the addition of sulfate will promote the increase of total Fe content of solid-phase HCl extractable state, and the higher the temperature, the more obvious the result. This is because the addition of SO4 2- and the increase of temperature will promote the growth of sulfur-reducing bacteria, thus activating the insoluble iron minerals in the soil to generate Fe 2+ . The S 2- generated by the reduction of sulfate will react with Fe 2+ to generate FeS and FeS2 precipitates, thus increasing the content of HCl extractable state Fe(T). The concentration of HCl extractable state Fe in the treatment without added sulfate decreased. This is because the iron oxides in the soil will undergo reductive dissolution, thus reducing the concentration of Fe in the HCl extractable state. The application of biochar will promote the above reactions, and the study shows that the application of carbon-rich silicon fertilizer will significantly increase the number of iron-reducing bacteria in the soil and promote the reduction of Fe.

[0180] 3.3 Change of bioavailable arsenic in coastal soil

[0181] The results of the change of total As in bioavailable state in soil under different temperature conditions are shown in the figure Figure 18The concentration of bioavailable As(T) in the treatment of adding sulfate increased by 5.3% to 17.5% within 0-45 days at 10℃. The concentration of bioavailable As(T) in the treatment of adding sulfate alone (+S) increased from 181.11 μmol / kg to 212.75 μmol / kg, and the concentration of bioavailable As(T) in the treatment of adding sulfate and biochar (+S+C) increased from 237.66 μmol / kg to 250.36 μmol / kg. In the treatment without adding sulfate, the concentration of bioavailable As(T) decreased by 27.8% to 31.5%. The concentration of bioavailable As(T) in the treatment without adding sulfate and biochar decreased from 263.54 μmol / kg to 180.49 μmol / kg, and the concentration of bioavailable As(T) in the treatment of adding sulfate and biochar decreased from 253.93 μmol / kg to 183.23 μmol / kg.

[0182] Within 0-45 days at 16℃, the concentration of bioavailable As(T) in the treatment of adding sulfate alone decreased from 297.15 μmol / kg to 225.14 μmol / kg, with a decrease of 24.23%; the concentration of bioavailable As(T) in the treatment of adding sulfate and biochar increased from 219.21 μmol / kg to 295.05 μmol / kg, with an increase of 34.60%; the concentration of bioavailable As(T) in the treatment of adding sulfate alone increased from 197.18 μmol / kg to 252.28 μmol / kg, with an increase of 27.94%; the concentration of bioavailable As(T) in the treatment of adding biochar decreased from 253.06 μmol / kg to 195.54 μmol / kg, with a decrease of 22.73%.

[0183] At 22℃, the concentration of solid-phase bioavailable As(T) in each treatment increased within 0-45 days. In the treatments with added sulfate, the concentration of solid-phase bioavailable As(T) increased from 158.02 μmol / kg to 317.26 μmol / kg with added biochar and from 224.86 μmol / kg to 266.70 μmol / kg without added biochar, representing an increase of 100.8% and 18.6%, respectively. In the treatments without added sulfate, the concentration of solid-phase bioavailable As(T) increased from 252.90 μmol / kg to 280.59 μmol / kg with added biochar and from 204.05 μmol / kg to 236.27 μmol / kg without added biochar, representing an increase of 10.9% and 15.6%, respectively. The addition of sulfate promoted the transformation of As in the solid phase to bioavailable As, and in each treatment, the addition of biochar significantly promoted the transformation of As in the solid phase to bioavailable As. In the treatments without added sulfate, the concentration of bioavailable As(T) increased, but the increase was not as significant as in the treatments with added sulfate.

[0184] At 25℃, the concentration of solid-phase bioavailable As(T) in each treatment increased within 0-45 days. In the treatments with added sulfate, the concentration of solid-phase bioavailable As(T) increased from 301.12 μmol / kg to 313.295 μmol / kg with added biochar and from 227.21 μmol / kg to 281.50 μmol / kg without added biochar, representing an increase of 4% and 23.9%, respectively. In the treatments without added sulfate, the concentration of solid-phase bioavailable As(T) increased from 276.27 μmol / kg to 304.86 μmol / kg with added biochar and from 225.20 μmol / kg to 264.07 μmol / kg without added biochar, representing an increase of 10.3% and 17.3%, respectively.

[0185] Overall, the concentration of solid phase bioavailable As(T) increased in all treatments within 0-45 days at 28℃. In the treatment of adding sulfate, the concentration of solid phase bioavailable As(T) decreased from 266.09 μmol / kg to 247.47 μmol / kg, with a decrease of 7.0%; in the treatment of not adding biochar, the concentration of solid phase bioavailable As(T) increased from 204.07 μmol / kg to 252.77 μmol / kg, with an increase of 23.9%; in the treatment of not adding sulfate and adding biochar and not adding biochar, the concentration of solid phase bioavailable As(T) increased from 262.62 μmol / kg to 274.90 μmol / kg and from 214.37 μmol / kg to 257.40 μmol / kg, with an increase of 4.7% and 20.1%, respectively.

[0186] In the present application, the transformation and distribution of As between solid and liquid phases are driven by microorganisms, so that the As in the solid phase under different treatment conditions also changes in morphology over time. The change results correspond to the change of As(T) content in the liquid phase. Overall, the bioavailability of pollutants is related to environmental conditions, soil properties, and pollutant concentrations. Through the verification of pot experiments and field experiments, the research shows that the bioavailable state of As is mainly determined by soil As(T) content, soil texture, and soil organic matter content. Under anaerobic conditions, many factors affect the activity of As, such as pH, soil texture, redox potential, soil organic matter content, cation exchange capacity, iron oxide content, and microbial activity. In the present application, the addition of sulfate can promote the increase of bioavailable As, which indicates that the addition of sulfate can mediate the transformation of other more stable forms of As to bioavailable As. This may be because the addition of sulfate promotes the reductive dissolution of minerals, releases more As, and promotes the growth of sulfate-reducing bacteria, producing S 2- , and lead sulfide (PbS) precipitates such as realgar (As4S4) / realgar (As2S3). The application of biochar compared to the treatment without biochar will promote the transformation of As in the solid phase to bioavailable As. Because the surface of biochar has a rich functional group, for example: the application of biochar can provide additional carbon source for soil, improve the activity of soil microorganisms, and accelerate the mineralization rate of soil, so that the As in the solid phase is transformed to bioavailable As.

[0187] In summary, in the present application, the influence of the mutual transformation between different forms of As in soil under the conditions of temperature increase and introduction of sulfate is explored by continuous extraction of the obtained solid phase samples, and the main conclusions are as follows:

[0188] (1) In the anaerobic flooding culture experiment of soil, the content of phosphoric acid extractable As(T) increased and the content of hydrochloric acid extractable As(T) decreased with the increase of culture time. The change of other forms was not obvious. The content of phosphoric acid extractable As(T) showed a decreasing trend with the increase of temperature. Under the action of microorganisms, the addition of sulfate and the application of biochar can promote the transformation of As in the solid phase into phosphoric acid extractable As.

[0189] (2) Under the treatment of adding sulfate, the content of hydrochloric acid extractable Fe(T) increased with the increase of temperature, and under the treatment of not adding sulfur, the content of hydrochloric acid extractable Fe(T) decreased with the increase of temperature. The application of biochar can promote the reaction, and the result is more obvious.

[0190] (3) Overall, under the condition of anaerobic flooding, the content of bioavailable As in the solid phase increased with the increase of culture time. Under the treatment of adding sulfur, the content of bioavailable As in the solid phase decreased with the increase of temperature. The addition of biochar can promote the transformation of As in the solid phase to bioavailable As.

[0191] (4) Low temperature (10℃) treatment weakens the activity of microorganisms, and the transformation of different extractable As / Fe is not obvious.

[0192] The above shows and describes the basic principles, main features and advantages of the present application. Those skilled in the art should understand that the present application is not limited to the above examples, and the above examples and descriptions in the specification are only to illustrate the principles of the present application. Without departing from the spirit and scope of the present application, various changes and improvements can be made to the present application, and these changes and improvements all fall within the scope of the present application. The scope of protection of the present application is defined by the appended claims and their equivalents.

Claims

1. A method for studying the influence of warming and seawater intrusion on the migration and transformation of arsenic in coastal soil, characterized in that, The method comprises the following steps, S1: preparing a sulfate-containing and a sulfate-free reducing bacteria culture medium, respectively; S2: adding a contaminated soil sample and a contaminated soil sample treated by biochar into the two culture media of step S1, respectively, to finally form four groups of soil samples with different treatments; S3: performing biological treatment and sterilization treatment on the four groups of soil samples with different treatments, respectively, the biological treatment group is set to five temperature conditions of 10℃, 16℃, 22℃, 25℃ and 28℃, and the sterilization treatment group is subjected to high-pressure sterilization at 121℃ for 25min; then the biological treatment group and the sterilization treatment group are placed in a constant-temperature incubator for dark constant-temperature incubation, and samples are taken regularly; S4: performing bioavailability extraction on the solid phase of the soil samples obtained after different treatments and at different sampling times, and studying the bioavailability of arsenic; S5: performing solid-phase continuous extraction on the soil samples obtained after different treatments and at different sampling times, studying different extraction states of arsenic, and studying the mutual transformation between different forms of arsenic in the soil under the conditions of temperature increase and introduction of sulfate; wherein the extraction states of arsenic include phosphate extraction state, hydrochloric acid extraction state, reducing agent extraction state, oxidizing agent extraction state and residual state; S6: determining and analyzing the content of liquid-phase elements in the soil samples after different treatments, the liquid-phase elements including As element, Fe element and S element.

2. The method according to claim 1, wherein, The preparation method of the sulfate-containing reducing bacteria culture medium in step S1 specifically comprises the following steps, S101A: adding KH2PO4, NH4Cl, CaCl2·6H2O, Na2SO4, lactic acid and yeast extract into a three-necked round-bottom flask containing ultrapure water, shaking to fully dissolve the samples, and then constant volume; S102A: heating the three-necked round-bottom flask containing the drugs in step S101, and creating an oxygen-free environment in the flask at the same time; S103A: after the culture medium in step S102 is heated to boiling, the oxygen-free atmosphere is maintained for cooling, and then the flask is divided into portions after being cooled to room temperature.

3. The method according to claim 2, wherein the method is characterized in that, The preparation method of the sulfate-free reducing bacteria culture medium in step S1 specifically comprises the following steps, S101B: adding KH2PO4, NH4Cl, CaCl2·6H2O, lactic acid and yeast extract into a three-necked round-bottom flask containing ultrapure water, shaking to fully dissolve the samples, and then constant volume; S102B: heating the three-necked round-bottom flask containing the drugs in step S101, and creating an oxygen-free environment in the flask at the same time; S103B: after the culture medium in step S102 is heated to boiling, the oxygen-free atmosphere is maintained for cooling, and then the flask is divided into portions after being cooled to room temperature.

4. The method for studying the effect of warming and saltwater intrusion on the migration and transformation of arsenic in coastal soil according to claim 1, characterized in that, The four groups of soil samples with different treatments in step S2 include: a group without adding sulfate and without applying biochar, a group with adding sulfate and without applying biochar, a group without adding sulfate and with applying biochar, and a group with adding sulfate and with applying biochar.

5. The method for studying the effect of warming and saltwater intrusion on the migration and transformation of arsenic in coastal soil according to claim 1, characterized in that, The specific operation of sampling in step S3 comprises the following steps, S301: shaking the sample in the serum bottle, using a sterile syringe to extract the mixed sample, and filtering the extracted sample using a filter membrane; S302: Inject the filtered liquid into a sterilized headspace bottle, seal the headspace bottle with a blue butyl plug, press the aluminum cap, and store it to obtain a liquid sample for step S6; S303: Place the filtered filter membrane in a sterilized centrifuge tube, immediately place it in a -20°C environment for freezing, and then use a freeze dryer to freeze dry. The dried solid sample is used in steps S4 and S5.

6. The method for studying the effect of warming and saltwater intrusion on the migration and transformation of arsenic in coastal soil according to claim 1, characterized in that, The specific operation of step S4 includes the following steps, S401: Prepare a 1 mol / L HCl solution as an extractant using oxygen-free water; S402: Add the extractant to the solid soil sample and shake it at room temperature for 2 hours; S403: Filter the extractant using a filter membrane.

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