Method for determining influence of ferrihydrite on colloidal state As content in soil solution

Abstract

The invention discloses a method for determining the influence of ferrihydrite on the content of colloidal As in a soil solution. The method comprises the following steps: S1, preparing raw materials; s2, carrying out a reduction-reoxidation microcosm experiment: S2-1, carrying out culture in a reduction period; s2-2, culturing in an oxidation period; s2-3, sampling: in each control group, carrying out destructive sampling on 4 parts of samples in the first, tenth, 20th and 35th days of the culture in the reduction period, and carrying out destructive sampling on the rest 2 parts of samples in the fifth and tenth days of the culture in the oxidation period; s3, extracting data; and S4, data analysis: obtaining the relative weight of each influence factor to the colloidal state As content in the soil solution. According to the method, the sensitivity of the soil to the ferrihydrite can be judged, the method is convenient and rapid to operate, high in practicability and high in accuracy, can be applied to various scenes, can provide theoretical guidance for the amount of the ferrihydrite needing to be added and the adding time of the ferrihydrite in the subsequent soil treatment process, and has high practicability.

Description

Technical Field

[0001] The invention relates to the technical field of soil heavy metal pollution control, and in particular to a method for determining the influence of ferrihydrite on the content of colloidal As in a soil solution. Background Art

[0002] Arsenic (As) is widely present in the earth's environment and is the most abundant element in the earth's crust. It usually coexists with iron ore, copper ore, and lead ore. Due to natural activities and human factors, As is released into groundwater, and farmland is polluted through groundwater and industrial wastewater during mining and smelting activities. At the same time, the use of some pesticides can also cause As pollution to farmland soil.

[0003] As in soil can be divided into two forms: solid phase As and soil solution As. Among them, As in soil solution can be divided into colloidal As and dissolved As according to particle size. Colloidal particles are usually defined as dispersed particles with a size range of 1 to 1000 nm. Colloids and nanoparticles in soil usually exist in the form of iron (hydroxide) oxides and iron-containing clay minerals and are closely related to organic matter. As has a strong association with iron-rich colloids. These associations can enhance the mobility of As in the soil environment by bypassing adsorption on the solid phase, thereby increasing the mobility and availability of As in the soil-rice system and promoting its long-distance transport to aquifers and surface water in neighboring areas.

[0004] Ferrihydrite has the characteristics of small particle size, large specific surface area, high reactivity, and amorphous porosity, which make it have a strong affinity for As and other substances. Therefore, it is often used for in-situ remediation of As-contaminated soil, which can effectively reduce farmland pollution and is the most commonly used soil conditioner in As-contaminated areas. However, the structure of ferrihydrite in the environment is extremely unstable and is easily affected by the interaction of various environmental factors. Over time, it dissolves and transforms into hematite, goethite, and magnetite, which significantly affects its efficiency in fixing and repairing the As element. Therefore, further research is needed on the effect of ferrihydrite on the content of colloidal As in soil solution. Summary of the invention

[0005] In view of the above problems, the present invention provides a method for determining the effect of ferrihydrite on the content of colloidal As in soil solution.

[0006] The technical solution of the present invention is:

[0007] A method for determining the effect of ferrihydrite on the content of colloidal As in soil solution comprises the following steps:

[0008] S1. Raw material preparation: remove impurities from the soil to be tested, crush, sieve, and dry it for later use, and take synthetic ferrihydrite for later use;

[0009] S2. Reduction-reoxidation microcosm experiment:

[0010] S2-1, reduction period culture: the treated soil to be tested is treated with the synthetic ferrihydrite to obtain two mixtures with a mass proportion of 0.1% and 1% of the synthetic ferrihydrite, respectively, which are recorded as the first mixture and the second mixture as experimental groups, and a blank control group is set at the same time. The blank control group is the soil to be tested after the treatment without the addition of synthetic ferrihydrite. 2 to 3 times the weight of deoxygenated ultrapure water is added to the first mixture, the second mixture and the blank control group, respectively, and mixed evenly. The centrifuge tube is placed in a nitrogen atmosphere box for 2 to 3 hours and then sealed, and then placed in a constant temperature shaking box for constant temperature shaking and light-proof culture for 35 days as a soil sample. Multiple parallel soil samples are set in each of the experimental groups or blank control groups;

[0011] S2-2, oxidation period culture: open several small holes on the top of the centrifuge tube, and continue to culture in a constant temperature oscillating box in the dark for 15 days;

[0012] S2-3, sampling: In each of the experimental groups or blank control groups, several soil samples are destructively sampled during the reduction period to obtain soil supernatant and residual solids, and the remaining several soil samples are destructively sampled during the oxidation period to obtain soil supernatant and residual solids;

[0013] S3, data extraction: characterize the water-dispersible colloids in the soil supernatant collected by destructive sampling in step S2, obtain the contents of colloidal organic matter, colloidal Fe and colloidal As, and determine the total As and total Fe contents in the soil sample;

[0014] S4. Data analysis: Based on the data in S3, the dynamic changes of various influencing factors over time during soil cultivation were obtained: pH value, Eh value, dissolved organic matter content, total As content, total Fe content, As form ratio, colloidal organic matter, colloidal As, and colloidal Fe. Multilinear models of the effects of various influencing factors on the colloidal As content in soil solution were drawn, and the relative weights of various influencing factors on the colloidal As content in soil solution were obtained to evaluate the relative importance of various influencing factors on the colloidal As content in soil solution.

[0015] Furthermore, the soil to be tested in S1 is crushed and passed through a 60-70 mesh sieve, and the drying temperature is 80-85°C.

[0016] Furthermore, the conditions for the constant temperature shaking light-proof culture in S2-1 are: temperature 25-27°C, rotation speed 180-200 rpm.

[0017] Furthermore, in S2-1, n parallels are set in each of the experimental groups or blank control groups, 6≤n≤12, and in S2-3, the interval between two samplings in the reduction period is 5 to 15 days, and the interval between two samplings in the oxidation period is 3 to 10 days.

[0018] Furthermore, when destructive sampling is performed at each time point in S2-3, the centrifuge tube is centrifuged at 4000-5000 r / min for 10-15 min, then transferred to a glove box and opened, the pH value and Eh value of the soil supernatant are immediately measured, and then the soil supernatant and the remaining solid are poured out, the soil supernatant is filtered using a 0.45 μm filter membrane, 0.2-0.4 unit weight of the filtrate is taken and loaded into an ultrafiltration centrifuge tube for soil water-dispersible colloid separation, 0.1-0.2 unit weight of the filtrate is taken to determine the Fe(II) concentration, the remaining filtrate is transferred to a centrifuge tube, acidified with hydrochloric acid having a molar concentration of 6-8 mol / L, and placed in a refrigerator at 2-4°C for frozen storage as a soil test solution.

[0019] Preferably, S3 also includes using a TOC meter, ICP-OES, and ICP-MS to measure the content of soluble organic carbon DOC, soluble Fe, and soluble As in the soil test solution, respectively. The remaining solid in the centrifuge tube is used for soil solid phase element measurement, DGT extraction, XRD, and soil continuous extraction experiments after freeze-drying.

[0020] Preferably, before use, HCl with a molar concentration of 0.01 to 0.03 mol / L, NaOH with a molar concentration of 0.01 to 0.03 mol / L and ultrapure water are sequentially added to the ultrafiltration centrifuge tube to wash away other organic molecular impurities such as glycerol remaining in the tube to avoid interference with the determination of soluble organic carbon DOC.

[0021] Furthermore, the method for determining the content of colloidal organic matter, colloidal Fe and colloidal As in the S3 is as follows: after filtering the soil supernatant using a 0.45 μm filter membrane, the filtrate is placed in an ultrafiltration centrifuge tube, the ultrafiltration centrifuge tube cap is tightened and sealed with a sealing film, and centrifuged at 5000-6000 r / min for 15-20 min to obtain a dissolved soil solution of <10 kDa, and the concentrations of soluble organic carbon DOC, soluble Fe and soluble As in the dissolved soil solution are measured. The concentration difference between the filtrate passing through the 0.45 μm filter membrane and the dissolved soil solution is the content of colloidal organic matter, Fe and As.

[0022] The beneficial effects of the present invention are:

[0023] (1) A method for determining the effect of ferrihydrite on the colloidal As content in soil solution of the present invention characterizes soil colloids by means of ultrafiltration technology, scanning transmission electron microscopy-energy dispersion spectroscopy, etc. through reduction-reoxidation microcosm cultivation experiments, and uses wet chemical analysis methods to obtain the physical and chemical properties of soil solutions, explore the main components of colloidal particles in soil solutions, evaluate the effects of factors such as redox state, ferrihydrite addition, and physical and chemical properties of soil solutions on the distribution and content changes of soil colloidal particles As, analyze the effect of ferrihydrite addition on the pH value, Eh value, DOC value, total Fe concentration, and total As concentration of soil solution, draw a multilinear model of each influencing factor on the colloidal As content in soil solution, obtain the relative weight of each influencing factor on the colloidal As content in soil solution, and use it to evaluate the relative importance of each influencing factor on the colloidal As content in soil solution, thereby judging the sensitivity of soil to ferrihydrite. The operation is convenient and fast, the feasibility is strong, and the accuracy is high.

[0024] (2) The method of determining the effect of ferrihydrite on the colloidal As content in soil solution of the present invention can be applied to various scenarios, which can be in-situ soil to be treated or ex-situ soil to be treated. After implementing the method of the present invention, the sensitivity of the soil to be treated to ferrihydrite can be understood, and theoretical guidance can be provided for the amount of ferrihydrite to be added in the subsequent soil treatment process and the timing of adding ferrihydrite, which has strong practicality. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is the dynamic change process of pH value of four kinds of soils during the microcosm culture experiment in Experimental Example 1 of the present invention over time;

[0026] Figure 2 It is the dynamic change process of the redox potential Eh value of the four soils in Experimental Example 1 of the present invention during the microcosm cultivation experiment with time;

[0027] Figure 3 It is the dynamic change process of dissolved organic carbon DOC value over time of the four soils in Experimental Example 1 of the present invention during the microcosm cultivation experiment;

[0028] Figure 4 This is the dynamic change process of total Fe content of four soils during the microcosm culture experiment in Experimental Example 1 of the present invention over time;

[0029] Figure 5 This is the dynamic change process of total As content over time in the four soils during the microcosm culture experiment in Experimental Example 1 of the present invention;

[0030] Figure 6 This is the dynamic change process of the As form ratio of the four soils in Experimental Example 1 of the present invention during the microcosm cultivation experiment;

[0031] Figure 7 This is the dynamic change process of colloidal organic matter OM over time in the four soils during the microcosm culture experiment in Experimental Example 1 of the present invention;

[0032] Figure 8 This is the dynamic change process of colloidal Fe over time in the four soils during the microcosm culture experiment in Experimental Example 1 of the present invention;

[0033] Fig. 9 This is the dynamic change process of colloidal As over time in the four soils during the microcosm culture experiment in Experimental Example 1 of the present invention;

[0034] Fig.10 is the linear correlation between Fe and OM on the colloidal scale during the microcosm culture experiment of the four soils in Experimental Example 1 of the present invention;

[0035] Fig.11 It is the relative importance of various influencing factors on the colloidal As content of the four soils in Experimental Example 1 of the present invention during the microcosm cultivation experiment. DETAILED DESCRIPTION

[0036] Example 1

[0037] A method for determining the effect of ferrihydrite on the content of colloidal As in soil solution comprises the following steps:

[0038] S1. Raw material preparation: remove impurities from the soil to be tested, crush, sieve, and dry it for later use, and take synthetic ferrihydrite for later use. The synthetic ferrihydrite soil to be tested is crushed and passed through a 60-mesh sieve, and the drying temperature is 81°C;

[0039] The preparation method of synthetic ferrihydrite is as follows: accurately weigh 20g Fe(NO3)3·9H2O crystals using an analytical balance, put them into a prepared 500mL beaker, add 250ml ultrapure water to dissolve them, use a pre-prepared 1mol / L NaOH solution to adjust the pH value of the mixed solution to 7-8, then add a magnetic rotor to the mixed solution, place the beaker on a magnetic stirrer and stir for about 30min to ensure that the solution is evenly mixed and in a suspended state, divide the suspension into several conical flasks, put them into a constant temperature shaking box and shake for 24h, the temperature is set to 25°C, the speed is set to 200rpm / min, after the shaking is completed, divide the ferrihydrite suspension into several 50mL centrifuge tubes, centrifuge them in a centrifuge at 4500rpm for 10min, pour out the supernatant in the centrifuge tube, add The same volume of ultrapure water is added to the centrifuge tube so that the ferrihydrite colloid at the bottom of the centrifuge tube is suspended again, and then the constant temperature oscillating box is used to oscillate for 30 minutes, centrifuged for 10 minutes, and the supernatant is poured out. The oscillation and centrifugation operation is repeated three times to wash and purify the ferrihydrite colloid. After washing and purification three times, the ferrihydrite colloid is introduced into a dialysis bag, and the two ends of the dialysis bag are sealed with a dialysis bag clamp, and placed in ultrapure water. The dialysis bag is driven to rotate by a magnetic rotor. The ultrapure water is replaced every day during the dialysis process until the conductivity of the replaced ultrapure water drops below 10μS / cm. The ferrihydrite colloid is taken out and frozen in a refrigerator for 12 hours. The frozen ferrihydrite is dried in a freeze dryer for about 24 hours. After grinding and sieving, the synthesized ferrihydrite is stored in a vacuum bag. The specific surface area of ​​the freeze-dried product is between 200 and 320m2 / g;

[0040] S2. Reduction-reoxidation microcosm experiment:

[0041] S2-1, reduction period culture: the treated soil to be tested was treated with synthetic ferrihydrite to obtain two mixtures with a mass proportion of 0.1% and 1% of synthetic ferrihydrite, respectively, which were recorded as the first mixture and the second mixture as the experimental group. A blank control group was set up at the same time. The blank control group was the treated soil to be tested without the addition of synthetic ferrihydrite. 2.5 times the weight of deoxygenated ultrapure water was added to the first mixture, the second mixture and the blank control group, respectively, and the mixtures were mixed evenly. The centrifuge tube was placed in a nitrogen atmosphere box for 2.5 hours and then sealed. It was then placed in a constant temperature shaking box for constant temperature shaking and dark culture for 35 days as a soil sample. The conditions for constant temperature shaking and dark culture were: temperature 26°C, rotation speed 190rpm, and n parallel soil samples were set in each experimental group or blank control group, and n was 6;

[0042] S2-2, oxidation period culture: open several small holes on the top of the centrifuge tube, and continue to culture in a constant temperature oscillating box in the dark for 15 days;

[0043] S2-3. Sampling: In each experimental group or blank control group, 4 samples were destructively sampled on the 1st, 10th, 20th and 35th days of reduction period culture to obtain soil supernatant and residual solids. The remaining 2 samples were destructively sampled on the 5th and 15th days of oxidation period culture to obtain soil supernatant and residual solids. When destructive sampling was performed at each time point, the centrifuge tube was centrifuged at 4500r / min for 12min, transferred to the glove box and opened, and the pH value and Eh value of the soil supernatant were immediately measured. Then, the soil supernatant and the residual solids were poured out, and the soil supernatant was filtered using a 0.45μm filter membrane. 0.3 unit weight of the filtrate was taken and loaded into an ultrafiltration centrifuge tube for soil water-dispersible colloid separation. 0.1 unit weight of the filtrate was taken to determine the Fe(II) concentration. The remaining filtrate was transferred to a centrifuge tube, acidified with hydrochloric acid with a molar concentration of 7mol / L, and placed in a refrigerator at 3°C ​​for freezing and storage as the soil test solution;

[0044] Before using the ultrafiltration centrifuge tube, 0.02 mol / L HCl, 0.02 mol / L NaOH and ultrapure water were added to the ultrafiltration centrifuge tube in sequence to wash away other organic molecular impurities such as glycerol remaining in the tube to avoid interference with the determination of soluble organic carbon DOC;

[0045] S3, data extraction: characterize the water-dispersible colloids in the soil supernatant collected by destructive sampling in step S2, obtain the contents of colloidal organic matter, colloidal Fe and colloidal As, and determine the total As and total Fe contents in the soil sample. Use a TOC meter, ICP-OES, and ICP-MS to measure the contents of soluble organic carbon DOC, soluble Fe, and soluble As in the soil test solution, respectively. The remaining solid in the centrifuge tube is freeze-dried and used for soil solid phase element measurement, DGT extraction, XRD, and soil continuous extraction experiments;

[0046] The method for determining the content of colloidal organic matter, colloidal Fe and colloidal As is as follows: after filtering the soil supernatant with a 0.45 μm filter membrane, the filtrate is placed in an ultrafiltration centrifuge tube, the ultrafiltration centrifuge tube cap is tightened and sealed with a sealing film, and centrifuged at 5500 r / min for 16 min to obtain a dissolved soil solution <10 kDa, and the concentrations of soluble organic carbon DOC, soluble Fe and soluble As in the dissolved soil solution are measured. The concentration difference between the filtrate passing through the 0.45 μm filter membrane and the dissolved soil solution is the content of colloidal organic matter, Fe and As.

[0047] S4. Data analysis: Based on the data in S3, the dynamic changes of various influencing factors over time during soil cultivation were obtained: pH value, Eh value, dissolved organic matter content, total As content, total Fe content, As form ratio, colloidal organic matter, colloidal As, and colloidal Fe. Multilinear models of the effects of various influencing factors on the colloidal As content in soil solution were drawn, and the relative weights of various influencing factors on the colloidal As content in soil solution were obtained to evaluate the relative importance of various influencing factors on the colloidal As content in soil solution.

[0048] Example 2

[0049] The difference between this embodiment and embodiment 1 is that:

[0050] S1. Raw material preparation: remove impurities from the soil to be tested, crush, sieve, and dry it for later use; take synthetic ferrihydrite for later use; crush the soil to be tested and pass it through a 70-mesh sieve; and dry it at a temperature of 85°C;

[0051] S2-1, in the reduction phase culture, 2.4 times the weight of deoxygenated ultrapure water was added to the first mixture, the second mixture and the blank control group respectively, and the mixture was mixed evenly. The centrifuge tube was placed in a nitrogen atmosphere box for 2 hours and then sealed. The conditions for constant temperature shaking and light-proof culture were: temperature 25°C, rotation speed 180 rpm.

[0052] Example 3

[0053] The difference between this embodiment and embodiment 1 is that:

[0054] S1. Raw material preparation: remove impurities from the soil to be tested, crush, sieve, and dry it for later use; take synthetic ferrihydrite for later use; crush the soil to be tested and pass it through a 60-mesh sieve; and dry it at 80°C;

[0055] S2-1, during the reduction phase culture, 3 times the weight of deoxygenated ultrapure water was added to the first mixture, the second mixture and the blank control group respectively, and the mixture was mixed evenly. The centrifuge tube was placed in a nitrogen atmosphere box for 3 hours and then sealed. The conditions for constant temperature shaking and light-proof culture were: temperature 27°C, rotation speed 200 rpm.

[0056] Note: When dissolving, add enough deoxygenated ultrapure water to completely submerge the solid sample.

[0057] Example 4

[0058] The difference between this embodiment and embodiment 1 is that:

[0059] S2-3, during sampling, centrifuge the centrifuge tube at 4000r / min for 10min; take 0.2 unit weight of the filtrate and put it into an ultrafiltration centrifuge tube for soil water-dispersible colloid separation, take 0.1 unit weight of the filtrate to determine the Fe(II) concentration, acidify it with hydrochloric acid with a molar concentration of 6mol / L, and place it in a refrigerator at 2°C for freezing and storage as the soil test solution;

[0060] Before using the ultrafiltration centrifuge tube, 0.01 mol / L HCl, 0.01 mol / L NaOH and ultrapure water were added to the ultrafiltration centrifuge tube in sequence to wash and remove other organic molecular impurities such as glycerol remaining in the tube;

[0061] S3. During data extraction, centrifuge at 5000 r / min for 15 min.

[0062] Example 5

[0063] The difference between this embodiment and embodiment 1 is that:

[0064] S2-3, during sampling, centrifuge the centrifuge tube at 5000r / min for 15min; take 0.4 unit weight of the filtrate and put it into an ultrafiltration centrifuge tube for soil water-dispersible colloid separation, take 0.2 unit weight of the filtrate to determine the Fe(II) concentration, acidify it with hydrochloric acid with a molar concentration of 8mol / L, and place it in a 4°C refrigerator for freezing and storage as the soil test solution;

[0065] Before using the ultrafiltration centrifuge tube, 0.03 mol / L HCl, 0.03 mol / L NaOH and ultrapure water were added to the ultrafiltration centrifuge tube in sequence to wash and remove other organic molecular impurities such as glycerol remaining in the tube;

[0066] S3. During data extraction, centrifuge at 6000 r / min for 20 min.

[0067] Note: In Examples 4 and 5, the selected parameters are all conventional adjustments within a reasonable range.

[0068] Example 6

[0069] The difference between this embodiment and embodiment 1 is that:

[0070] S2-1. In the reduction period culture, n parallel soil samples were set in each experimental group or blank control group, and n was 8;

[0071] Correspondingly, in S2-3, sampling, in each experimental group or blank control group, 5 samples were destructively sampled on the 1st, 8th, 15th, 25th and 35th days of cultivation in the reduction period to obtain the soil supernatant and residual solids, and the remaining 3 samples were destructively sampled on the 4th, 9th and 15th days of cultivation in the oxidation period to obtain the soil supernatant and residual solids.

[0072] Example 7

[0073] The difference between this embodiment and embodiment 1 is that:

[0074] S2-1, in the reduction period culture, set n parallel soil samples in each experimental group or blank control group, and n is 12;

[0075] Correspondingly, in S2-3, sampling, in each experimental group or blank control group, 8 samples were destructively sampled on the 1st, 5th, 10th, 15th, 20th, 25th, 30th and 35th days of cultivation in the reduction period to obtain the soil supernatant and residual solids, and the remaining 4 samples were destructively sampled on the 3rd, 6th, 10th and 15th days of cultivation in the oxidation period to obtain the soil supernatant and residual solids.

[0076] Note: In Examples 6 and 7, the sampling frequency will have a certain impact on the final result. When the number of samples is greater, the experimental result will be relatively more accurate, but at the same time the steps will be more complicated. Using 6 samplings in Example 1 can obtain the experimental result more accurately and quickly.

[0077] Experimental example

[0078] We took four kinds of soil (Hengyang HY, Renhua RH, Huanjiang HJ and Guixi GX) to conduct experiments using the method in Example 1. The dynamic changes of pH value over time during the microcosm cultivation experiment are shown in the following figure: Figure 1 As shown. The pH value of each experimental group showed a trend of increasing first and then decreasing. The pH value gradually increased during the anaerobic period (0-35 days) and gradually decreased during the aerobic period (35-50 days). During the anaerobic cultivation of paddy soil, the microbial activities in the soil are mainly facultative and anaerobic organic matter fermentation. Microorganisms use nitrates, iron and manganese oxides, and sulfates as electron acceptors in the soil system to metabolize organic matter and continuously consume protons (H +), which causes the pH value of the rice soil to gradually increase during the anaerobic culture process. After entering the aerobic period, oxygen re-enters the system, the physical and chemical state of the soil gradually returns to the level at the beginning of the culture, and the pH value gradually decreases to the level at the beginning of the experiment. The pH values ​​of the three experimental groups in HY soil were generally in the range of 6.85-7.44, and all reached the maximum value on the 35th day of the anaerobic period, with an average increase of 0.52 relative to the initial stage of cultivation; the pH values ​​of the three experimental groups in RH soil were generally in the range of 6.20-7.50, and the pH of the CK group and the 0.1FH group reached the maximum value on the 20th day of the anaerobic period, and the pH of the 1FH group reached the maximum value on the 35th day of the anaerobic period. The average difference between the maximum and minimum values ​​of the three experimental groups was 1.16; the pH values ​​of the three experimental groups in HJ soil were generally in the range of 3.08-4.00, and all reached the maximum value on the 35th day of the anaerobic period, with an average increase of 0.76 relative to the initial stage of cultivation; the pH of the three experimental groups in GX soil were generally in the range of 5.72-6.95, and all reached the maximum value on the 35th day of the culture period, with an average increase of 0.92 relative to the initial stage of cultivation. The pH values ​​of the four soil experimental groups generally showed a distribution trend of CK group>0.1FH group>1FH group. This is because ferrihydrite contains abundant acidic hydroxyl groups (≡FeOH). Moreover, ferrihydrite has an unstable structure and will undergo abiotic and biotic reduction dissolution in the soil, releasing iron ions into the soil environment. The hydrolysis of iron ions will consume hydroxide ions (OH - ), lowering the pH value of the soil environment.

[0079] The dynamic changes of redox potential (Eh) of the four soils during the microcosm cultivation process are shown in Figure 2. Figure 2As shown. The Eh values ​​of all experimental groups decreased rapidly to a slow and steady decline during the anaerobic reduction period, and the Eh values ​​rose rapidly to a higher level during the aerobic oxidation period. During the anaerobic reduction period, the soil was flooded, causing the liquid phase to quickly occupy and replace the gas phase in the soil pores. The sealed environment of the centrifuge tube isolated the oxygen from the microcosm system. The remaining oxygen in the system was consumed within 24 hours under the action of microbial activity. In addition to oxygen, other inorganic soil components were also reduced by microorganisms in turn. Therefore, the Eh values ​​of each experimental group decreased rapidly during the anaerobic reduction period. After entering the aerobic period, oxygen re-entered the microcosm system, and the Eh values ​​of each experimental group rose rapidly to a higher level. The Eh values ​​of the three experimental groups of HY soil were distributed between -50.4 and 20.7 mV, and the decline in the 20 days before the flooding period accounted for an average of 98% of the decline in the entire flooding period; the Eh values ​​of the three experimental groups of RH soil were distributed between -28.5 and -124.7 mV, and the decline in the 20 days before the flooding period accounted for an average of 83% of the decline in the entire flooding period; the Eh values ​​of the three experimental groups of HJ soil were distributed between 5.7 and -101 mV, and the decline in the 20 days before the flooding period accounted for an average of 75% of the decline in the entire flooding period; the Eh values ​​of the three experimental groups of GX soil were distributed between -80.4 and -155.7 mV, and the decline in the 20 days before the flooding period accounted for an average of 87% of the decline in the entire flooding period. During the entire incubation experiment, the Eh value of the CK group was generally higher than that of the 0.1FH group, and the Eh value of the 0.1FH group was generally higher than that of the 1FH group. The application of ferrihydrite reduced the Eh value of the soil system. It is speculated that the increase in the content of iron oxides may have increased the activity of iron-reducing bacteria.

[0080] Figure 3The dynamic changes of dissolved organic carbon (DOC) in soil solution during the cultivation of each experimental group are shown. The DOC content in each experimental group of HY soil and RH soil showed a gradual downward trend throughout the experimental period. The DOC content of each experimental group of HY soil was in the range of 38.7-198.9 mg / L. The DOC content of CK, 0.1FH and 1FH experimental groups decreased by an average of 67.80% during the experimental period. The DOC content of each experimental group of RH soil was in the range of 26.9-229.8 mg / L. The DOC content of CK, 0.1FH and 1FH experimental groups decreased by an average of 77.87% during the experimental period. The DOC content of each experimental group of HJ soil showed a change pattern of gradually increasing in the reduction period and gradually decreasing in the oxidation period. The content ranged from 35.13 to 101.31 mg / L. The DOC content of the CK, 0.1FH and 1FH experimental groups increased by an average of 33.85% during the reduction period and decreased by an average of 35.08% during the oxidation period. The DOC content of each experimental group in the GX soil reached its maximum value around the 10th day, and then continued to decline until the end of the experiment. The maximum DOC content of the CK, 0.1FH and 1FH experimental groups was 133.7 mg / L, 94.11 mg / L and 68.62 mg / L, respectively. At the end of the experiment, the DOC content was 70.9 mg / L, 58.94 mg / L and 36.69 mg / L, respectively. The biomineralization process of soil organic matter refers to the process in which soil microorganisms convert organic matter into inorganic substances (such as carbon dioxide, water and nitrogen) and release them from the soil. It is also the process in which microorganisms use organic matter. Dissolved organic matter in soil solution is usually more easily used by microorganisms, so the DOC content of each experimental group in HY and RH soils continued to decline after the start of the cultivation experiment. However, soil organic matter also includes difficult-to-degrade organic matter such as plant, microbial and animal residues, which are first decomposed into small molecular DOC under the action of microorganisms and then biomineralized. The DOC produced by microbial degradation of macromolecular soil organic matter may be the source of the increase in DOC content in some stages at the beginning of the experiment in each experimental group of HJ and GX soils. The DOC content in the four experimental soil experimental groups showed the phenomenon of CK group>0.1FH group>1FH group during the experiment. Iron (hydroxide) oxides play an important role in the carbon cycle of paddy soil and are electron acceptors in the biomineralization of organic matter mediated by microorganisms. Therefore, the application of ferrihydrite will enhance the biomineralization of organic carbon by soil microorganisms. Studies have shown that the addition of ferrihydrite can promote the mineralization of short-chain fatty acids by microorganisms, and ferrihydrite has a strong adsorption effect on aromatic compounds and is an important carrier of DOC in the soil system.

[0081] The dynamic changes of total Fe content in soil solution during the experimental soil cultivation process are shown in Figure 4The total Fe content in the soil solution of each experimental group showed a change pattern of first increasing and then decreasing. The isolation of oxygen caused the Eh of the soil in each experimental group to continue to decrease during the reduction period ( Figure 2 ), low Eh value triggers the continuous reduction and dissolution of iron oxides in the soil, and various forms of colloidal Fe and Fe ions are released into the soil solution. The reason why the total Fe content in the soil solution of some experimental groups decreased in the late reduction period may be that the reduction and dissolution rate of iron oxides decreased, resulting in a decrease in the amount of Fe released into the soil solution, and the aggregation effect of iron (hydroxide) oxide colloids and colloids carrying Fe ions in the soil solution caused the Fe element in the soil solution to become a solid phase and leave the soil solution again. In the oxidation period, the Eh value increased due to the introduction of oxygen ( Figure 2 ) causes the Fe element in the soil solution to be reoxidized and precipitated as iron oxides. During the entire experimental period, the total Fe content of the soil solution in the HY soil experimental group was the highest at 53.04 mg / L, the total Fe content of the soil solution in the RH soil experimental group was the highest at 196.19 mg / L, the total Fe content of the soil solution in the HJ soil was the highest at 1657.63 mg / L, and the total Fe content of the soil solution in the GX soil was the highest at 43.94 mg / L. Since the Fe element in the soil solution is released from the soil solid phase, the total Fe content of the soil solution in each experimental group is positively correlated with the total iron content in each test soil sample. During the four soil experiments, the total Fe content in the soil solution of the 1FH experimental group was significantly lower than that of the CK experimental group and the 0.1FH experimental group. Previous research results also confirmed the experimental phenomenon that the application of exogenous iron oxide ferrihydrite led to a decrease in the total Fe content in the soil solution. This is because ferrihydrite adsorbs soluble Fe during the adsorption of As. In addition, the mineral phase change process of the amorphous iron oxide ferrihydrite in flooded soil is also accompanied by the consumption of Fe(II) ions in the soil solution.

[0082] Figure 5The dynamic changes of the total As content in the soil solution during the cultivation process of each experimental group were shown. The total As content showed a change pattern of first rising and then falling. As was gradually released into the soil solution during the reduction period and reprecipitated into the solid phase during the oxidation period. During the cultivation period, the maximum values ​​of the total As content in the soil solution of the HY soil CK group, 0.1FH group and 1FH group were 1742.30μg / L, 1421.73μg / L and 534.52μg / L, respectively. The total As content in the soil solution of the 0.1FH group was reduced by an average of 19.37% compared with the control experimental group CK group, and the total As content in the soil solution of the 1FH group was reduced by an average of 60.18% compared with the control experimental group CK group; during the cultivation period, the total As content in the soil solution of the RH soil CK group, 0.1FH group and 1FH group was reduced by an average of 1742.30μg / L, 1421.73μg / L and 534.52μg / L, respectively. The maximum values ​​of total As content in soil solution of CK group, 0.1FH group and 1FH group were 658.63μg / L, 493.17μg / L and 264.18μg / L, respectively. The total As content in soil solution of 0.1FH group was reduced by 28.07% on average compared with that of CK group, and the total As content in soil solution of 1FH group was reduced by 43.24% on average compared with that of CK group. During the culture period, the total As content in soil solution of CK group, 0.1FH group and 1FH group in HJ soil was significantly higher than that of CK group. The maximum values ​​of total As content in soil solution of CK group, 0.1FH group and 1FH group were 27.54μg / L, 20.62μg / L and 2.74μg / L, respectively. The total As content in soil solution of 0.1FH group was reduced by 24.71% on average compared with that in control group CK group, and the total As content in soil solution of 1FH group was reduced by 84.22% on average compared with that in control group CK group. During the culture period, the maximum values ​​of total As content in soil solution of CK group, 0.1FH group and 1FH group were 5284.41μg / L, 209 3.72μg / L and 655.24μg / L, the total As content in the soil solution of the 0.1FH group was reduced by an average of 48.67% compared with that of the control experimental group CK group, and the total As content in the soil solution of the 1FH group was reduced by an average of 70.15% compared with that of the control experimental group CK group. It can be seen that the application of ferrihydrite significantly reduced the content of total As in the soil solution. This is because the hydroxyl group (≡FeOH) on the surface of ferrihydrite can complex the arsenate and arsenite ions in the soil solution through ligand exchange.

[0083] The dynamic changes of As forms in soil solution over time during the cultivation of four soils are as follows: Figure 6As shown. During the flooding period, with the extension of incubation time, the proportion of As(Ⅲ) in the soil solution of each experimental group decreased, while the proportions of As(V), DMA(V) and MMA(V) increased. This is because As(V) was reduced to As(Ⅲ) under the reducing conditions of the flooding period. In addition, the flooding conditions also promoted the methylation of As mediated by microorganisms. The proportions of DMA(V) and MMA(V) increased in the 1FH group relative to the 0.1FH group, because methylated As species generally have a lower adsorption affinity to iron oxides than arsenate and arsenite.

[0084] like Figure 7 As shown in the figure, the proportion of colloidal organic matter (OM) in the soil solution of each experiment showed a change pattern of gradually increasing in the reduction period and continuously decreasing in the oxidation period. Combined with the results of DOC content in soil solution ( Figure 3 ), and found that the content of colloidal OM continued to increase under soil anoxic conditions. The study by Said-Pullicino et al. found that anoxic conditions can enhance the dispersion of colloids in soil and preferentially release small-particle colloidal components (<240nm). In addition, the reduction and dissolution of iron oxides under anoxic conditions will also release related OM into the soil solution, which contributes greatly to colloidal OM. During the experiment, the proportion of colloidal OM in each experimental group of HY soil, RH soil and GX soil fluctuated greatly, ranging from 15% to 90%, while the proportion of colloidal OM in each experimental group of HJ soil was relatively stable, ranging from 34% to 38%. According to the study of Jiang et al., 60% to 90% of the colloidal particles carrying OM in the pore water of paddy soil are between 20 and 450nm, and aluminosilicate clay minerals and iron (hydroxide) oxides are the main components of soil colloids combined with OM. Hu et al.'s study showed that the colloidal particles in the soil solution with a particle size between 0.3 and 40 kDa were mainly nano-silicate clay minerals, and the colloidal particles with a particle size between 130 kDa and 450 nm were mainly iron (hydroxide) colloids. Therefore, the colloidal particles carrying OM were mainly iron (hydroxide) colloids. The proportion of colloidal OM in the soil solution of each experimental group of HJ soil was relatively stable. It is speculated that the possible reason is that the strong acid environment with a lower pH value in the HJ soil culture group is not conducive to the formation of iron (hydroxide) colloid particles ( Figure 1 ), so that most organic matter exists in the form of solute molecules, while in the HY, RH and GX soil experimental groups, the pH environment is close to neutral ( Figure 1 ), iron (hydroxide) colloids are gradually generated with the hydrolysis of Fe ions and carry OM, causing the proportion of colloidal OM to continue to increase ( Figure 6), previous research results have confirmed that iron (hydroxide) colloids are mainly generated in an environment with a pH>4.5. Among the four experimental soil culture groups, the proportion of colloidal OM in the soil solution of the ferrihydrite application group 0.1FH group and 1FH group was higher than that of the CK group, and the more ferrihydrite was applied, the higher the proportion of colloidal OM ( Figure 6 ). This is because ferrihydrite, as an amorphous iron oxide, is a highly active short-range ordered (SRO) mineral. Similar minerals include allophane and imogolite. These minerals are more likely to form organic mineral complexes than ordered minerals in the soil, and play an important role in improving soil carbon storage. Therefore, the application of ferrihydrite adsorbs DOC in the soil solution and releases nano-ferrihydrite particles carrying OM, increasing the proportion of colloidal OM in the soil solution.

[0085] Figure 8 The percentage of colloidal Fe in soil solution during the cultivation of four soils is shown. In the reduction period of the HY soil experimental group, the RH soil experimental group and the GX soil experimental group, more than 90% of the Fe element in the soil solution exists in colloidal form, while in the HJ soil experimental group, the percentage of colloidal Fe in the soil solution during the reduction period is between 10% and 20%. The colloidal Fe in the soil solution is mainly composed of iron (hydroxide) colloids with a particle size of >130 kDa, which are generated by the hydrolysis of Fe(II) and Fe(III) in a solution environment with a pH>4.5. The pH range of the HY soil, RH soil and GX soil experimental groups is between 5.72 and 7.64 ( Figure 1 ), the hydrolysis of iron ions resulted in a high proportion of colloidal Fe in the soil solution, and in the HJ soil experimental groups with a pH range of 2.97 to 4.02, the Fe element in the soil elements existed more as dissolved ions. After entering the oxidation period, the proportion of colloidal Fe in the HY soil experimental group, the RH soil experimental group, and the GX soil experimental group began to decrease, and the proportion of colloidal Fe in the HJ soil experimental group began to increase ( Figure 7 ). It is speculated that after oxygen enters the microcosm experimental system, the iron (hydroxide) colloid particles aggregate and precipitate, which reduces the proportion of colloidal Fe in the measurement results. In the HJ soil experimental groups with a low proportion of colloidal Fe, the dual effects of iron (hydroxide) colloids with a particle size of <450nm produced during the oxidation precipitation process and the reduction of the total Fe content in the soil solution increase the proportion of colloidal Fe.

[0086] The proportion of colloidal As in the soil solution of the four soils during the microcosm cultivation process is as follows Fig. 9As shown in the data, during the reduction period, the proportion of colloidal arsenic As in the soil solutions of the CK, 0.1FH and 1FH groups in the HY soil experimental group remained relatively stable, with average values ​​of 4.41%, 4.92% and 8.12%, respectively; the proportion of colloidal arsenic As in the soil solutions of the CK, 0.1FH and 1FH groups in the RH soil experimental group remained relatively stable, with average values ​​of 5.86%, 6.70% and 9.58%, respectively; the proportion of colloidal arsenic As in the soil solutions of the CK, 0.1FH and 1FH groups in the HJ soil experimental group fluctuated greatly over time, with average values ​​of 32.86%, 33.14% and 37.41%, respectively; the proportion of colloidal arsenic As in the soil solutions of the CK, 0.1FH and 1FH groups in the GX soil experimental group remained relatively stable, with average values ​​of 3.19%, 3.14% and 6.68%, respectively. The experimental results showed that the application of ferrihydrite increased the proportion of colloidal As in the soil solution. The percentage of colloidal As in the soil solution during the reduction period in HY, RH, and GX soils remained around 10 ± 5%, which is similar to the results of a study on As colloids in flooded rice fields in Bangladesh. The total As content in the soil solution of the HJ soil experimental group remained at a low level ( Figure 5 ), the extremely low pH value and ultra-high concentration of soil solid Fe content in HJ soil were caused by the dam collapse of the tailings pond in the upstream area of ​​the soil. The flood caused the tailings dam of the Beishan lead-zinc mine in Huanjiang to collapse, and the tailings were washed into the nearby farmland. After that, the reduced sulfur in the tailings (mainly pyrite) was oxidized, releasing H+ and increasing the soluble and amorphous Fe. It is speculated that the low pH value and high Fe oxide content make the solid phase of HJ soil have a strong adsorption capacity for soluble oxygen-containing anions of As, resulting in a decrease in the dissolved As content in the water phase of HJ soil and a relative increase in the colloidal As content.

[0087] The results show that the application of exogenous ferrihydrite increases the proportion of colloidal OM, Fe and As in the soil solution. The possible reason is that the content of colloidal particles in the soil solution increases after the application of ferrihydrite. Exogenously applied ferrihydrite itself is a spherical nanoparticle. Although ferrihydrite nanoparticles will undergo aggregation effects under the action of environmental factors (such as pH and solute ions) to form larger aggregates, Wang Meiling's research selected 12 typical soils with different physical and chemical properties as environmental media for stability experiments, proving the suspension stability of ferrihydrite nanoparticles in brown soil, granite brick red soil, black soil and irrigated desert soil solutions. In addition, mobile colloids of various sizes will continue to be released during the biological reduction of ferrihydrite during the soil flooding stage.

[0088] The linear correlation between Fe and OM at the colloidal scale during reduction and reoxidation of soil solutions of the four soils is shown in Fig.10, a significant positive correlation between colloidal Fe and OM was found in all soils. We speculate that the colloidal particles associated with As in soil solution are mainly Fe-As-OM complex colloidal particles. A large number of studies have confirmed the existence of ternary complexes of OM, iron (hydr)oxides and As. As combines with OM through iron metal bridges to form ternary complexes. The presence of OM is conducive to the formation of iron (hydr)oxide colloids, which can provide electrostatic and steric stability for the colloids. The OM coating can reduce the aggregation of Fe colloidal particles. Therefore, the correlation between colloidal As and OM in soil solution depends on their common dependence on Fe colloidal particles and the strong correlation between Fe and OM in soil solution colloidal particles. During soil cultivation, the interaction between OM and iron (hydr)oxides forms iron (hydr)oxide-OM complex colloids. It is the interaction of these complex colloidal particles with dissolved As that leads to the formation of ternary Fe-As-OM complex colloids. Moreover, the input of organic materials during long-term rice cultivation in paddy soil promotes the combination of soil organic matter and iron minerals. In summary, we speculate that in paddy soil solution, colloidal As mainly exists in the form of Fe-As-OM complex colloidal particles.

[0089] The relative importance of soil solution pH, Eh, total As, total Fe, total DOC, colloidal Fe and colloidal OM to colloidal As content during the reduction and reoxidation microcosm incubation experiments is shown in Fig.11 The relative weights of each influencing factor on colloidal As were obtained by relative weight analysis (RWA) based on a multilinear model. The results showed that the total As concentration in soil solution and the colloidal OM content were the main influencing factors determining the colloidal As content, with relative weights of 60.03% and 22.26%, respectively. The total Fe concentration in soil solution had the least influence on the colloidal As content, with a relative weight of 0.86%. Figure 4 and Figure 5 It can be seen that the total Fe concentration in the soil solution is much greater than the total As concentration. Although the As element in the soil solution is released by the reduction and dissolution process of iron oxides, the content of iron oxides in the soil is relatively large, and the iron oxides that release As are only a part of the iron oxides in the soil. Therefore, we can assume that the formation of colloidal As is mainly controlled by the total As concentration in the soil solution, followed by the interaction between Fe and As. OM can increase the space or electrostatic interaction between colloidal particles by forming an organic coating on the surface of the colloid, thereby enhancing the stability of the colloid. Therefore, the presence of colloidal OM maintains the content of colloidal As by reducing the coagulation of the colloid.

Claims

1. A method for determining the effect of ferrihydrite on the content of colloidal As in soil solution, characterized in that: The following steps are involved: S1. Raw material preparation: remove impurities from the soil to be tested, crush, sieve, and dry it for later use, and take synthetic ferrihydrite for later use; S2. Reduction-reoxidation microcosm experiment: S2-1, reduction period culture: the treated soil to be tested is treated with the synthetic ferrihydrite to obtain two mixtures with a mass proportion of 0.1% and 1% of the synthetic ferrihydrite, respectively, which are recorded as the first mixture and the second mixture as experimental groups, and a blank control group is set at the same time. The blank control group is the soil to be tested after the treatment without the addition of synthetic ferrihydrite. 2 to 3 times the weight of deoxygenated ultrapure water is added to the first mixture, the second mixture and the blank control group, respectively, and mixed evenly. The centrifuge tube is placed in a nitrogen atmosphere box for 2 to 3 hours and then sealed, and then placed in a constant temperature shaking box for constant temperature shaking and light-proof culture for 35 days as a soil sample. Multiple parallel soil samples are set in each of the experimental groups or blank control groups; S2-2, oxidation period culture: open several small holes on the top of the centrifuge tube, and continue to culture in a constant temperature oscillating box in the dark for 15 days; S2-3, sampling: In each of the experimental groups or blank control groups, several soil samples are destructively sampled during the reduction period to obtain soil supernatant and residual solids, and the remaining several soil samples are destructively sampled during the oxidation period to obtain soil supernatant and residual solids; S3, data extraction: characterize the water-dispersible colloids in the soil supernatant collected by destructive sampling in step S2, obtain the contents of colloidal organic matter, colloidal Fe and colloidal As, and determine the total As and total Fe contents in the soil sample; S4. Data analysis: Based on the data in S3, we analyzed the influencing factors in the soil cultivation process: pH value, Eh value, dissolved organic matter content, total As content, total Fe content, As form ratio, colloidal organic matter, colloidal As, and colloidal Fe. The dynamic changes over time of each influencing factor on the colloidal As content in the soil solution were plotted, and the relative weights of each influencing factor on the colloidal As content in the soil solution were obtained.

2. A method for determining the effect of ferrihydrite on the content of colloidal As in soil solution according to claim 1, characterized in that: The soil to be tested in S1 is crushed and passed through a 60-70 mesh sieve, and the drying temperature is 80-85°C.

3. A method for determining the effect of ferrihydrite on the content of colloidal As in soil solution according to claim 1, characterized in that: The conditions for the constant temperature shaking and light-proof culture in S2-1 are: temperature 25-27°C, rotation speed 180-200 rpm.

4. A method for determining the effect of ferrihydrite on the content of colloidal As in soil solution according to claim 1, characterized in that: In the S2-1, n parallels are set in each of the experimental groups or blank control groups, 6≤n≤12. In the S2-3, the interval between two samplings in the reduction period is 5 to 15 days, and the interval between two samplings in the oxidation period is 3 to 10 days.

5. A method for determining the effect of ferrihydrite on the content of colloidal As in soil solution according to claim 4, characterized in that: When destructive sampling is performed at each time point in S2-3, the centrifuge tube is centrifuged at 4000-5000 r / min for 10-15 minutes, then transferred to a glove box and opened, the pH value and Eh value of the soil supernatant are immediately measured, and then the soil supernatant and the remaining solid are poured out, the soil supernatant is filtered using a 0.45 μm filter membrane, 0.2-0.4 unit weight of the filtrate is taken and loaded into an ultrafiltration centrifuge tube for soil water-dispersible colloid separation, 0.1-0.2 unit weight of the filtrate is taken to determine the Fe(II) concentration, and the remaining filtrate is transferred to a centrifuge tube, acidified with hydrochloric acid having a molar concentration of 6-8 mol / L, and placed in a refrigerator at 2-4°C for frozen storage as a soil test solution.

6. A method for determining the effect of ferrihydrite on the content of colloidal As in soil solution according to claim 5, characterized in that: The S3 also includes using a TOC meter, ICP-OES, and ICP-MS to measure the content of soluble organic carbon DOC, soluble Fe, and soluble As in the soil test solution, respectively. The remaining solid in the centrifuge tube is used for soil solid phase element measurement, DGT extraction, XRD, and soil continuous extraction experiments after freeze drying.

7. A method for determining the effect of ferrihydrite on the content of colloidal As in soil solution according to claim 6, characterized in that: Before use, HCl with a molar concentration of 0.01 to 0.03 mol / L, NaOH with a molar concentration of 0.01 to 0.03 mol / L and ultrapure water are sequentially added into the ultrafiltration centrifuge tube to wash and remove other organic molecular impurities such as glycerol remaining in the tube.

8. A method for determining the effect of ferrihydrite on the content of colloidal As in soil solution according to claim 5, characterized in that: The method for determining the content of colloidal organic matter, colloidal Fe and colloidal As in the S3 is as follows: after filtering the soil supernatant with a 0.45 μm filter membrane, the filtrate is placed in an ultrafiltration centrifuge tube, the ultrafiltration centrifuge tube cap is tightened and sealed with a sealing film, and centrifuged at 5000-6000 r / min for 15-20 min to obtain a dissolved soil solution of <10 kDa, and the concentrations of soluble organic carbon DOC, soluble Fe and soluble As in the dissolved soil solution are measured. The concentration difference between the filtrate passing through the 0.45 μm filter membrane and the dissolved soil solution is the content of colloidal organic matter, Fe and As.

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