Method for rapid removal of arsenic from groundwater by synergistic effect of biochar and ferrous salt

Abstract

The present application belongs to the field of groundwater arsenic pollution treatment, and discloses a method for rapidly removing arsenic in groundwater by using the synergy of biochar and ferrous salt, which is specifically for groundwater containing pentavalent arsenic elements with a pH value of 5.5-7.5, adding biochar and ferrous salt into the groundwater in sequence, so that the mass ratio of ferrous ions to arsenic elements is 2:1-5:1, then using oxygen-containing gas to perform aeration stirring reaction for at least 5 min; after the reaction is completed, liquid-solid separation is performed, and the obtained liquid is the groundwater after the removal of arsenic elements. The method can greatly reduce the addition amount of ferrous reagent, utilize the synergy of biochar and ferrous salt, improve the arsenic fixation efficiency, and avoid the problem of water acidification after the repair through the combined action of biochar.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of groundwater arsenic pollution treatment, and more particularly relates to a method for rapidly removing arsenic in groundwater by synergistic effect of biochar and ferrous salt. BACKGROUND

[0002] Arsenic is a toxic metalloid element, and arsenic-contaminated groundwater is a non-negligible cause of harm to human health. In recent years, arsenic pollution in groundwater has been considered a very important problem, and the World Health Organization has lowered the limit of arsenic in drinking water from 50 ug / L to 10 ug / L, further illustrating the severity of arsenic hazards. Arsenic pollution in multi-regional groundwater is original and difficult to treat from the source. The problems faced in treating arsenic-containing groundwater include large groundwater volume, and secondary pollution caused by remediation. The large volume of arsenic-containing groundwater requires short remediation period, short hydraulic retention time, and high treatment efficiency. In addition, chemical agents are usually used for precipitation removal in the treatment process, which can easily introduce new pollutants and cause secondary pollution. Therefore, there is an urgent need for a rapid, efficient and green arsenic removal technology for arsenic-containing groundwater.

[0003] The main forms of arsenic in water bodies are inorganic arsenic in the forms of trivalent arsenic and pentavalent arsenic, and the content of organic arsenic is very small. For the removal of inorganic arsenic, trivalent arsenic is usually oxidized to pentavalent arsenic. Patent 201910435744.9 has achieved in-situ oxidation of trivalent arsenic in groundwater to pentavalent arsenic by regulating biochar, but no effective method has been proposed for subsequent treatment. There are many types of reagents for removing pentavalent arsenic (hereinafter referred to as arsenic) at present, including calcium-based remediation reagents, iron-based remediation reagents, and carbon-based remediation reagents. Calcium reagent remediation is usually suitable for the removal of arsenic in strongly alkaline groundwater. Patent 201910436474.3 discloses a method for removing arsenic in calcium-rich and high-arsenic groundwater using biochar. The biochar is added to the alkaline, calcium-rich and high-arsenic groundwater, and stirred for at least 4 hours to remove the dissolved arsenic in the water body. The step of removing arsenic is simple and efficient, but it is only suitable for the removal of arsenic in strongly alkaline water. Calcium-based remediation reagents are not suitable for water bodies with pH < 8 (such as arsenic-containing groundwater with pH < 8). In the context of non-strongly alkaline arsenic-containing groundwater, iron-based remediation reagents are widely used for the removal of arsenic in groundwater bodies. The principle is to form iron-arsenic precipitates by iron oxides and arsenic, and then remove the dissolved arsenic. In the prior art, trivalent iron (Fe(III)) reagents (such as ferric chloride) are usually used to form precipitates with arsenic. However, the current remediation method has many shortcomings. First, iron-arsenic is prone to form colloids or micron-sized particles during the reaction process. Although the arsenic is removed from the dissolved phase, the mobility of arsenic does not decrease effectively because the mobility of colloids and micron-sized particles is strong. Second, in order to reduce the content of iron-arsenic colloids in the system and improve the stability of the iron-arsenic precipitate after treatment, an excess of Fe(III) is usually added to ensure the removal of arsenic (usually several g / L, and the Fe / As mass ratio is usually several hundred or even thousands), which causes the acidification of the water body and the need to add a pH alkalization unit (usually lime is used to adjust the pH), prolonging the treatment process and generating a large amount of solid waste, which increases the treatment cost. Third, the reaction process of iron-arsenic usually takes several hours to several days, which cannot meet the treatment requirements of large-volume arsenic-containing groundwater. The long hydraulic retention time and settling time result in low efficiency of arsenic-containing groundwater treatment. Fourth, the use of Fe(III) to remove arsenic in groundwater, especially in some areas with fluctuating groundwater levels, can easily cause Fe(III) to be reduced to ferrous iron under anaerobic conditions and the action of microorganisms, resulting in the dissolution of a large amount of iron and exceeding the standard of iron content in groundwater. Some adsorbed ferrous iron can catalyze the reduction of indigenous iron-arsenic minerals in the groundwater environment, further increasing the risk of arsenic release.

[0004] Patent 201710081804.2 discloses an arsenic removal material, its preparation method and application method in rural groundwater arsenic removal. By mixing kaolinite, ferric chloride and ferrous chloride in a certain mass ratio, mixing for 8-24h, and standing for 24-72h, the concentration of arsenic is reduced from 50μg / L to 3μg / L. The treatment time is long and is not suitable for the removal of arsenic in large water volume groundwater. Patent 201210309544.7 discloses a water treatment device and method for groundwater with excessive iron, manganese, fluorine and arsenic. By existing iron, manganese and arsenic in groundwater, iron and manganese are formed into iron-manganese oxides through aeration treatment, which can fix arsenic, and finally achieve the reduction of arsenic in groundwater. The reaction residence time is 30min, but the application scenario needs to be iron-containing and manganese-containing water body. After treatment, iron-arsenic colloid and micron-sized particles are easily generated, which need further filtration treatment, and the requirements for iron concentration and Fe / As mass ratio in water body are high (Fe=16.8mg / L, As=0.19mg / L, and Fe / As=88 calculated). It is only applicable to specific scenarios of arsenic-containing groundwater. Patent 201310262970.4 discloses a method for simultaneously removing arsenite and arsenate in water. By using inexpensive iron shavings as electrode material, ferrous ions are continuously and slowly generated in the water to be treated after electrode electrolysis, and then arsenic is oxidized and precipitated to achieve the purpose of removal. However, electrolysis method is not suitable for the treatment of arsenic-containing groundwater, with high cost and small water volume. Patent 201410027022.7 discloses an arsenic removal method for rural water wells in high-arsenic groundwater areas. Cement balls loaded with nano-sized iron-manganese oxides on the surface are used as granular filter material to adsorb arsenic in water, with low economic cost and high treatment efficiency. However, the treatment time often needs to reach 4h, which is suitable for small-scale arsenic-containing well water, but not suitable for large water volume arsenic-containing groundwater. Patent 201811160540.0 discloses a high-arsenic high-iron groundwater treatment device and water treatment method based on oxidation method for arsenic removal. Patent 202210311229.1 discloses a high-arsenic groundwater improvement material, its preparation method and application. Patent 201910293130.1 discloses a method for efficiently adsorbing arsenic in groundwater by using nano-iron oxide. The above patents have long treatment period, complex material preparation process, limited water volume, and high economic cost.

[0005] In summary, in the field of water treatment, it is known in the prior art that iron-based materials can effectively remove dissolved arsenic in water bodies as a core remediation agent. However, adding excess iron-based materials to achieve better arsenic removal results can easily cause serious acidification problems and secondary pollution in water bodies due to the hydrolysis of Fe(III) and the addition of excess Fe(III). Adjusting the pH can prolong the treatment process and generate a large amount of arsenic-containing sludge after treatment. In addition, the use of iron-containing remediation agents to treat arsenic groundwater usually also requires the use of flocculants for settling iron arsenic particles, which is relatively high in cost. Therefore, the existing technical method has a long processing cycle and obvious technical shortcomings in dealing with arsenic groundwater, and there is still a lot of room for improvement in efficiency and cost. SUMMARY

[0006] In view of the above defects or improvement needs of the prior art, the purpose of the present application is to provide a method for rapidly removing arsenic in groundwater by using biochar and ferrous salt in cooperation, wherein by improving the reaction participants and the action mechanism, the biochar obtained at a specific pyrolysis temperature of 600-800℃ is used, and only ferrous reagent with a Fe / As mass ratio of 2-5 needs to be added. By using biochar, the ferrous salt forms adsorbed Fe(II) and nanoscale Fe(III) formed by oxidation to cooperatively fix arsenic, which can quickly remove arsenic in groundwater. The aeration reaction time can be as short as 5 minutes (e.g., 5-20 minutes), and arsenic can be fixed (i.e., a solid product containing arsenic is obtained, which is separated from the liquid water body, and the water body from which arsenic has been removed is obtained). Moreover, the method of the present application can greatly reduce the amount of ferrous reagent added. By using the synergistic effect of biochar and ferrous salt, the efficiency of arsenic fixation is improved, and the problem of water body acidification after remediation is avoided by the combined effect of biochar. Compared with the prior art, the present application can solve the problems of excessive addition of iron reagent, low efficiency, long processing cycle, water body acidification after treatment, and easy secondary pollution during the remediation process of arsenic-containing groundwater. Moreover, the use of biochar to cooperatively fix arsenic on the surface of biochar by the present application method also does not require the additional use of flocculants for flocculation and sedimentation.

[0007] To achieve the above-mentioned purpose, according to the present application, a method for rapidly removing arsenic in groundwater by using biochar and ferrous salt in cooperation is provided, characterized in that for groundwater containing pentavalent arsenic elements with a pH value of 5.5-7.5, biochar and ferrous salt are sequentially added to the groundwater, the mass ratio of ferrous ions to arsenic elements is 2:1-5:1, and then an oxygen-containing gas is used for aeration and stirring reaction for at least 5 minutes; after the reaction is completed, liquid-solid separation is performed, and the obtained liquid is the groundwater from which arsenic elements have been removed.

[0008] In the method, the content of silicon elements in the biochar is not more than 3wt%; and the biochar is obtained by pyrolysis of non-straw agricultural and forestry waste as raw material at 600-800℃ in the absence of oxygen.

[0009] As a further preferred embodiment of the present application, the raw material of the biochar is forestry waste, preferably at least one of bamboo, oak, camphor tree, and eucalyptus;

[0010] More preferably, the pyrolysis time is 60-150 min.

[0011] As a further preferred embodiment of the present application, the concentration of arsenic element in the groundwater containing arsenic element is 0.1-20 mg / L;

[0012] The dosage of the biochar is 0.2 g-2 g per 1 L of the groundwater containing arsenic element.

[0013] As a further preferred embodiment of the present application, the time of the aeration and stirring reaction is 5 min-20 min.

[0014] As a further preferred embodiment of the present application, the method further comprises adjusting the pH value of the groundwater containing arsenic element to be treated in advance, so that the pH value satisfies 5.5-7.5.

[0015] As a further preferred embodiment of the present application, the oxygen-containing gas is air; preferably, the aeration rate of the groundwater is 5-20 m 3 / h. 3 / h.

[0016] As a further preferred embodiment of the present application, the particle size of the biochar is >8 mm.

[0017] Compared with the prior art, by simultaneously adding biochar and ferrous salt into the groundwater containing arsenic to be treated, and combining with the oxygen exposure condition, the soluble arsenate (i.e., removing pentavalent arsenic) in the water body can be quickly and effectively removed.

[0018] (1) The present application utilizes the synergistic effect of biochar and ferrous iron. After adding ferrous salt into the groundwater containing arsenic, the ferrous ions (Fe 2+ ) can be quickly adsorbed on the biochar to form adsorbed divalent iron (Fe(II)) (about 70%), and part of the ferrous iron is quickly oxidized to form nano-iron oxide (Fe(III)) (about 30%) under the catalytic oxidation of the biochar (as shown in the following Figure 4 ), and the biochar and Fe(II) and Fe(III) thereon can quickly fix the arsenic in the water body to achieve the purpose of removing arsenic in the groundwater containing arsenic. In the process of treating the groundwater containing arsenic, Fe(II) and Fe(III) are completely adsorbed on the biochar and will not remain in the water body (as shown in the following Figure 4(a) in the presence of biochar, after 20 min of reaction, no dissolved Fe exists, and biochar-Fe-As macroparticles are formed, avoiding the formation of Fe-As colloids and nano / microparticles, and the migration of As cannot be effectively reduced.

[0019] That is, the present application utilizes the synergistic effect of biochar and ferrous iron, and the biochar can partially oxidize the adsorbed ferrous iron (Fe(II)) to ferric iron (Fe(III)) (about 30% of Fe(II) is oxidized to Fe(III)), and maintain the remaining adsorbed Fe(II) on the biochar from being oxidized by oxygen (about 70% of adsorbed Fe(II) is not oxidized), forming a mixture of Fe(III) and Fe(II) (as shown in the following Figure 4 ), which significantly enhances the removal of arsenic, while the fixation efficiency of arsenic by Fe(II) or Fe(III) alone cannot meet the requirements (as shown in the following Figure 8 ). The essence of the mechanism of the present application is that oxygen can quickly catalyze the oxidation of part of Fe(II) to Fe(III) (about 30% of Fe(II) is oxidized to Fe(III)) through biochar, but the organic functional groups and micropores on the biochar can maintain part of Fe(II) from being oxidized (about 70% of adsorbed Fe(II) is not oxidized), so that a mixture of Fe(II) and Fe(III) is formed on the surface of the biochar, and a multi-component complex of biochar-Fe(II) / Fe(III)-As is finally formed, which significantly enhances the removal of arsenic in arsenic-containing groundwater.

[0020] In the method of the present application, biochar can effectively disperse Fe (including Fe(II) and Fe(III)), which promotes the fixation of Fe on arsenic. Specifically, the present application forms Fe-As nanoparticles (<1 nm, as shown in (a) in the following Figure 2 ) on the surface of biochar, and the better dispersion of Fe on biochar means that more sites for fixing arsenic can be exposed on the Fe on biochar, which is more conducive to the combination of Fe-As. In the absence of biochar, Fe itself aggregates and reacts with arsenic to form larger Fe-As particles (>2 nm, as shown in (b) in the following Figure 2As shown in (c), the efficiency of Fe immobilizing As is greatly reduced compared to the case of forming small Fe-As nanoparticles on the surface of biochar, because without biochar, the aggregation of Fe itself makes all of its sites for immobilizing As not be used. In addition, it is worth noting that even without biochar, the formed Fe-As particles (>2 nm) still have strong mobility, while in the presence of biochar, even though the formed Fe-As particles are smaller (<1 nm), but because they are formed on the surface of biochar, the particle size of biochar in the present application is much larger than nanometer scale (the particle size of biochar in the present application is preferably >8 mm; the particle size of biochar is generally in millimeter scale, and the present application preferably uses large particles, mainly to prevent clogging), taking 8 mm as an example, the mobility of 8 mm biochar is significantly reduced relative to 2 nm Fe-As particles. It can be seen that the promoting effect of biochar in the method of the present application mainly has two aspects, on the one hand, biochar can promote the dispersion of Fe, and improve the efficiency of As immobilization on Fe; on the other hand, the Fe-As particles formed after As is immobilized on Fe can also be immobilized on biochar, which further reduces the mobility of Fe-As particles, and thus also reduces the mobility of As.

[0021] (2) The preparation temperature of biochar used in the present application is 600-800℃, at this temperature, the dissolved organic matter on the surface of biochar can be significantly reduced, and the biochar prepared at 600℃ and above can remove most of the dissolved organic matter, which can significantly inhibit the removal of arsenic, because the dissolved organic matter can compete with the removal of arsenic on Fe (animal, microbial source of biochar, high content of dissolved organic matter, which can compete with the formation of arsenic adsorption and precipitation on iron oxide, and the input of dissolved organic matter can easily cause secondary pollution of arsenic groundwater treatment, therefore, the present application is suitable for using biochar from agricultural and forestry waste). At the same time, the biochar used in the method of the present application corresponds to non-straw agricultural and forestry waste (for example, it can be forestry waste produced by bamboo, oak, camphor tree, eucalyptus, etc.), and the silicon content of the prepared biochar is required to be not more than 3%, which can avoid the release of excessive silicate by biochar in the reaction (because silicate can compete with the fixation process of arsenic on Fe(II) and Fe(III), which reduces the removal efficiency of arsenic in arsenic-containing groundwater). And the preparation temperature of 600℃ and above can significantly increase the porosity and specific surface area of biochar, which is beneficial to the formation of nano-Fe. When the preparation temperature of biochar exceeds 800℃, the structure of biochar collapses, greatly reducing the porosity and specific surface area of biochar, and reducing the formation of nano-Fe 2+ The precipitation and oxidation rate on biochar, and thus reduces the removal efficiency of arsenic. That is, by using biochar prepared at a pyrolysis temperature of 600-800℃, the present application has high porosity and specific surface area, which is helpful to promote the dispersion of Fe2+ precipitation (70% precipitation) and oxidation (30% oxidation), and can retain most (70%) of the precipitated Fe(II) from being oxidized, ultimately removing arsenic in groundwater through the synergy of biochar, Fe(II) and Fe(III).

[0022] The present application can quickly remove Fe 2+ adsorbed on the surface and pore size of biochar, so that it is not oxidized by oxygen, forming biochar-Fe(II)-As to fix arsenic. The non-porous region on the biochar can quickly oxidize ferrous iron to Fe(III) through the electron transfer and surface catalysis of biochar, and Fe(III) forms Fe(III)-As precipitate, further promoting the removal of arsenic. In addition, the micropores and surface organic matter of biochar limit the nucleation growth of Fe(II) / Fe(III) oxides formed into crystalline iron oxides, forming nano-Fe(II) / Fe(III) oxides, which hinder the transformation of Fe(II) / Fe(III) oxides into crystalline minerals. The nano-Fe(II) / Fe(III) oxides have significantly more arsenic fixation sites than the crystalline Fe(II) / Fe(III) oxides. If biochar is not present, the Fe(II) / Fe(III) oxides formed will inevitably undergo crystalline transformation, and the Fe(II) / Fe(III) oxides that fix arsenic may release adsorbed arsenic again during the transformation process, causing potential environmental risks. The present application utilizes the cooperation of biochar and ferrous salt to not only remove arsenic but also further ensure the stability of the material after iron fixes arsenic.

[0023] (3) The method of the present application is particularly suitable for arsenic-containing groundwater with a pH of 5.5-7.5, because under this pH condition, ferrous salt usually exists in the form of Fe 2+ , and will not produce a large amount of H + (As shown below Figure 9 ) through hydrolysis like Fe 3+ . Further, the biochar used in the present application can also act as a buffer. The pH of the groundwater treated by the method of the present application does not change significantly and does not need to be adjusted by additional reagents.

[0024] The arsenic-containing groundwater suitable for the present application has a pH in the range of 5.5-7.5. If the pH of the groundwater is >7.5, Fe 2+ will rapidly precipitate and aggregate and oxidize, resulting in low arsenic fixation efficiency of iron oxides; and when the pH is <5.5, Fe 2+The precipitation and oxidation rate is slow, usually needs several days, based on the characteristics of large amount of arsenic-containing groundwater, long hydraulic retention time, low treatment efficiency, can not meet the removal requirements of arsenic in arsenic-containing groundwater. In addition, the arsenic concentration of arsenic-containing groundwater can be 0.1-20mg / L.

[0025] (4) In the method of the application, the mass ratio of Fe / As is 2-5, which greatly reduces the addition amount of Fe in the repair of arsenic-containing water. In the prior art, the Fe / As in the treatment of arsenic-containing groundwater is usually several hundred or even thousands, and excessive iron can easily cause secondary pollution of water body and acidification of water body. Excessive iron will cause acidification of water body, often requiring additional pH alkalization unit. After pH alkalization, a large amount of harmful sludge is produced, increasing the treatment process and economic cost. At the same time, excessive addition of iron can easily cause mineral crystal transformation (amorphous to crystalline mineral transformation, and the fixed site of arsenic is less, such as Figure 2 (b) shows that the needle-shaped substance is goethite, Figure 2 (a) shows amorphous iron oxide. As can be seen from the EDS diagram, the more iron added, the less arsenic fixed), which will reduce the arsenic fixation efficiency. In addition, the addition amount of biochar in the method of the application can be 0.2g / L-2g / L, which can uniformly disperse Fe on the surface of the adsorption of biochar, form nano Fe(II) / Fe(III) oxide, and promote the fixation efficiency of arsenic.

[0026] (5) Furthermore, the application greatly simplifies the conventional arsenic-containing groundwater treatment process. By using a small amount of ferrous salt and the adsorption and buffering capacity of biochar, the pH alkalization unit, sludge treatment unit and flocculation and precipitation unit in the treatment process are reduced, greatly simplifying the treatment process and reducing the treatment cost.

[0027] In summary, the application utilizes the fixation and oxidation promoting effect of biochar on ferrous salt, and fully utilizes the characteristics of non-dissolved organic matter and large porosity of biochar, promotes the adsorption of Fe(II), and prevents about 70% of Fe(II) from being oxidized to form adsorbed Fe(II). Fe(II) has higher adsorption efficiency than Fe(III), which promotes the adsorption of arsenic in water body. In addition, Fe 2+The present application forms nano Fe(II) / Fe(III) oxide on the biochar, reduces the Fe / As mass ratio, and improves the arsenic fixation efficiency of Fe. By rapidly oxidizing Fe(II) to Fe(III) (about 30%), Fe(III)-arsenic precipitate is formed, which further promotes the fixation of arsenic; the biochar, the adsorbed state Fe(II) and Fe(III) converted from ferrous iron, and the nano Fe(II) / Fe(III) oxide formed by multiple effects improve the arsenic removal efficiency, achieve the purpose of rapid and efficient removal of arsenic in arsenic-containing groundwater, and the arsenic treatment reaction time can be within 5-20 minutes, greatly improving the arsenic fixation efficiency, and being suitable for the treatment of arsenic in large water volume arsenic-containing groundwater. The present application changes the traditional water treatment technology which improves the arsenic treatment efficiency by increasing the amount of reagents, and realizes the synergistic removal of arsenic in groundwater by low-dose ferrous salt and the characteristics of biochar. Moreover, the materials used are economically available materials, the operation is simple, and the secondary pollution to the environment in the process of arsenic treatment is minimized. The present application utilizes the synergistic rapid removal of arsenic in groundwater by biochar and ferrous salt, and provides a rapid, economical, efficient and green treatment method for the removal of arsenic in groundwater. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1 The present application is a flowchart for the process of rapidly removing arsenic in groundwater by biochar and ferrous salt.

[0029] Figure 2 The TEM-EDS diagram of the reaction solid product after adding different Fe / As mass ratios and not adding biochar to fix arsenic for 20 minutes; wherein, Figure 2 (a) in the figure corresponds to the addition of biochar (2g / L) system and ferrous salt, and the Fe / As mass ratio before reaction is 5:2, Figure 2 (b) in the figure corresponds to the addition of biochar (2g / L) system and ferrous salt, and the Fe / As mass ratio before reaction is 20:1, and (c) corresponds to the addition of biochar, and the Fe / As mass ratio before reaction is 5:2. In addition, Figure 2 (a) in the figure, the TEM scale from left to right represents 500nm, 50nm, the EDS scale of C, As, Fe, and O represents 200nm; Figure 2 (b) in the figure, the TEM scale from left to right represents 200nm, 10nm, and the EDS scale of As, Fe, and O represents 50nm; Figure 2 (c) in the figure, the TEM scale from left to right represents 50nm, 20nm, and the EDS scale of As, Fe, and O represents 50nm.

[0030] Figure 3A comparison of the efficiency of biochar in synergistic removal of arsenic by ferrous salt in arsenic-containing groundwater at different pH values; where the pH values ​​are 3.0, 5.5, 6.8, 7.5 and 9.0.

[0031] Figure 4 For the removal of arsenic by biochar and ferrous salt in both systems and biochar-free systems, the dissolved Fe(II)- form (Fe) is obtained. 2+ ), Fe(II)-solid and Fe(III)-solid content variation graphs; among which, Figure 4 (a) in the text corresponds to the biochar-assisted ferrous salt arsenic removal system. Figure 4 (b) in the text corresponds to the biochar-free system.

[0032] Figure 5 A comparative diagram of the kinetics of arsenic removal by ferrous salts from bamboo biochar (i.e., bamboo charcoal) prepared at different temperatures; the preparation temperatures of the biochar were 300℃, 600℃, 800℃ and 900℃.

[0033] Figure 6 The kinetics of arsenic removal from rice husk biochar prepared at different temperatures were investigated; the biochar preparation temperatures were 300℃, 600℃ and 900℃.

[0034] Figure 7 A comparison of the efficiency of arsenic removal by synergistic effect of different biochar addition amounts with ferrous salt; where the biochar addition amounts are 0, 0.1, 0.2, 1 and 2 g / L.

[0035] Figure 8 A comparative diagram showing the effects of biochar, oxygen, ferrous salt, and ferric salt on arsenic removal; including 6 reaction systems: ① biochar + ferrous salt under anaerobic conditions, ② biochar + ferric salt under anaerobic conditions, ③ ferrous salt under anaerobic conditions, ④ oxygenated biochar, ⑤ oxygenated ferric salt, and ⑥ oxygenated biochar + ferrous salt.

[0036] Figure 9 For Fe 2+ and Fe 3+ Speciation at different pH levels; among which, Figure 9 (a) in the text corresponds to Fe 2+ , Figure 9 (b) in the text corresponds to Fe 3+ . Detailed Implementation

[0037] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0038] like Figure 1 As shown, this invention utilizes biochar and ferrous salt to rapidly and efficiently remove arsenic from arsenic-containing groundwater. The method can be divided into three steps: adding biochar, adding ferrous salt, and an aeration reaction. By performing these steps in sequence, rapid and efficient removal of arsenic from groundwater can be achieved. The reason for adding biochar first and mixing it thoroughly before adding ferrous salt is that if ferrous salt is added first, ferrous iron is easily oxidized to Fe(III). Fe(III) has much lower dispersibility than ferrous iron, making it prone to agglomeration and difficult to form nanoparticles on the biochar, thus affecting treatment efficiency.

[0039] The following are specific examples, in which the ferrous solution is prepared with ferrous sulfate (FeSO4·7H2O), the Fe(III) solution is prepared with ferric chloride (FeCl3), and the pH is adjusted using NaOH solution (of course, in actual engineering applications, Ca(OH)2 solution, etc., can also be used):

[0040] Example 1: Preparation of Biochar

[0041] The biochar used in this invention all comes from bamboo planting areas in Longyan, Fujian Province. After being collected, the bamboo is broken into small pieces of 1-3cm, rinsed several times with deionized water to remove impurities, and then dried in an electric heating drying oven for 12 hours (80℃). After cooling to room temperature, it is sealed in a self-sealing bag for later use. Then, 100g of bamboo is weighed and placed in a crucible and covered (to isolate oxygen), and then placed in a muffle furnace at 600℃ for 1 hour of lower oxygen pyrolysis (no gas is introduced during the pyrolysis process). After cooling to room temperature, the biochar is obtained and stored for later use. This biochar is denoted as BC600.

[0042] Using the same moso bamboo, but varying the muffle furnace treatment temperature, the biochar was processed at 300℃, 800℃, and 900℃ respectively, and the resulting biochar was denoted as BC300, BC800, and BC900.

[0043] In addition, rice husks were used as the raw material for biochar preparation. The rice husks used in this invention came from a farm in Suizhou, Hubei Province. After being washed several times with deionized water to remove impurities, they were dried in an electric heating drying oven for 12 hours (80°C). After cooling to room temperature, they were sealed in a self-sealing bag for later use. Then, 100g of rice husks were weighed and placed in a crucible with a lid (to isolate oxygen). The crucible was then placed in a muffle furnace at 600°C with limited oxygen for 1 hour (no gas was introduced during the pyrolysis process). After cooling to room temperature, the resulting biochar was stored for later use. The obtained biochar is denoted as rice husk biochar and is designated as RBC600.

[0044] Similarly, rice husk was used, only the temperature of muffle treatment was changed, and the treatment was carried out at 300℃ and 900℃, respectively, and the obtained biochar was recorded as RBC300 and RBC900, respectively.

[0045] In addition, the content of silicon in the biochar prepared from different raw materials and at different temperatures was detected, and the results are shown in the following table.

[0046] Table 1. Content of silicon in BC and RBC at different preparation temperatures

[0047]

[0048] Example 2

[0049] 500 mL of arsenic-containing groundwater was taken in a brown glass bottle (the arsenic-containing groundwater was from a certain mine in Lengshuijiang City, Hunan, containing arsenic groundwater, pH = 6.8; the concentration of pentavalent arsenic element in the arsenic-containing groundwater was 2 mg / L; in addition, the arsenic-containing groundwater also contained a small amount of Ca and Mg ions), 0.5 g of biochar BC600 was added, air was continuously introduced, and it was placed on a magnetic stirrer for continuous stirring at a speed of 600 r / min, and ferrous solution was added to make the corresponding Fe 2+ concentration in the reaction system 5 mg / L, and the pH of the system was maintained at 6.8, and samples were taken at the preset time points (0, 5, 10, 15, 20, 30, 60 and 120 min), and arsenic, Fe(II) and Fe(III) in the system were analyzed and determined by atomic fluorescence instrument and ultraviolet spectrophotometer.

[0050] As Figure 3 shown, when the biochar prepared at 600℃ was used, the pH of the reaction system was 6.8, the initial Fe / As of the reaction was 5:2, and at 5 min of the reaction, the removal rate of arsenic reached 75.2%, and at 20 min of the reaction, the removal rate of arsenic reached 98.3%.

[0051] As Figure 4 shown in (a) thereof, the added Fe(II)-dissolved state was converted into Fe(II)-solid (67.1%) and Fe(III)-solid (26.9%) at 5 min, and at 20 min of the reaction, the Fe(II)-dissolved state was converted into 69.9% of Fe(II)-solid and 30.1% of Fe(III)-solid.

[0052] Example 3

[0053] This example is basically the same as Example 2, except that the pH of the arsenic-containing groundwater in Example 2 is adjusted to 3.0.

[0054] As Figure 3As shown, when the pH of the arsenic-containing groundwater is 3.0, the arsenic removal efficiency is 25.8% after 5 minutes and 49.5% after 20 minutes.

[0055] Example 4

[0056] This embodiment is largely the same as Embodiment 2, except that the pH of the arsenic-containing groundwater in Embodiment 2 is first adjusted to 5.5.

[0057] like Figure 3 As shown, when the pH of the arsenic-containing groundwater is 5.5, the arsenic removal efficiency is 31.5% after 5 minutes and 80.2% after 20 minutes.

[0058] Example 5

[0059] This embodiment is largely the same as Embodiment 2, except that the pH of the arsenic-containing groundwater in Embodiment 2 is first adjusted to 7.5.

[0060] like Figure 3 As shown, when the pH of the arsenic-containing groundwater is 7.5, the arsenic removal efficiency is 79.1% after 5 minutes and 97.7% after 20 minutes.

[0061] Example 6

[0062] This embodiment is largely the same as Embodiment 2, except that the pH of the arsenic-containing groundwater in Embodiment 2 is first adjusted to 9.0.

[0063] like Figure 3 As shown, when the pH of the arsenic-containing groundwater is 9.0, the arsenic removal efficiency is 15.2% after 5 minutes and 27.9% after 20 minutes.

[0064] Example 7

[0065] This embodiment is largely the same as Embodiment 2, except that BC300 is used (that is, the only difference is that the preparation temperature of biochar in Embodiment 2 is adjusted to 300°C).

[0066] like Figure 5 As shown, when using biochar prepared at 300℃, the arsenic removal efficiency was 23.3% after 5 minutes and 43.2% after 20 minutes.

[0067] Example 8

[0068] This embodiment is largely the same as Embodiment 2, except that BC800 is used (that is, the only difference is that the preparation temperature of biochar in Embodiment 2 is adjusted to 800°C).

[0069] likeFigure 5 As shown in Table 2, when the biochar prepared at 800℃ is used, the removal efficiency of arsenic is 88.1% at 5 min and 99.6% at 20 min.

[0070] Example 9

[0071] This example is basically the same as Example 2, except that BC900 is used (that is, the only difference is that the preparation temperature of the biochar in Example 2 is adjusted to 900℃).

[0072] As shown in Table 3, when the biochar prepared at 900℃ is used, the removal efficiency of arsenic is 45.5% at 5 min and 52.3% at 20 min. Figure 5 Example 10

[0073] This example investigates the effect of biochar with different silicon contents (corresponding to different biochar raw materials) on the method of the present application. This example is basically the same as Example 2, except that rice husk biochar is used, i.e., RBC600, RBC300 and RBC900 are used respectively.

[0074] As shown in Table 4, when RBC600 is used, the removal efficiency of arsenic is 35.5% at 5 min and 58.7% at 20 min; when RBC300 is used, the removal efficiency of arsenic is 18.5% at 5 min and 37.8% at 20 min; and when RBC900 is used, the removal efficiency of arsenic is 34.6% at 5 min and 42.1% at 20 min.

[0075] Figure 6 As can be seen from the comparison of Example 2 and Example 10, and

[0076] it can be known that the method of the present application can especially use forestry wastes such as bamboo and oak as the raw material source of biochar. If other agricultural and forestry wastes with high silicon content such as rice straw, wheat straw and corn straw are used, since the biochar prepared therefrom can release silicate, which competes with arsenic in the adsorption and precipitation process on iron oxide, the removal efficiency of arsenic in arsenic-containing groundwater is reduced. Figure 6 Figure 5 Example 12

[0077] This example is basically the same as Example 2, except that no biochar is added.

[0078] As shown in Table 5, when no biochar is added, the removal efficiency of arsenic is 2.3% at 5 min and 10.6% at 20 min.

[0079] As shown in Table 5, when no biochar is added, the removal efficiency of arsenic is 2.3% at 5 min and 10.6% at 20 min. Figure 7 ​​

[0080] In addition, in the control system without biochar, the ferrous salt was converted to 8.9% Fe(III) in 5 min (as shown in (b) of Figure 4 In 20 min, it was converted to 19.86% Fe(III). From Figure 4 The control of Example 2 and Example 12 shows that biochar significantly promotes the conversion of dissolved Fe 2+ to Fe(II) solid and Fe(III) solid, and biochar maintains the non-oxidation of Fe(II) solid, and the mixture of Fe(II) and Fe(III) formed has a stronger efficiency in arsenic fixation.

[0081] Example 13

[0082] This example is basically the same as Example 2, except that the amount of biochar added is 0.1 g / L.

[0083] As shown in Figure 7 , when the amount of biochar added is 0.1 g / L, the removal efficiency of arsenic is 5.4% in 5 min, and the removal efficiency of arsenic is 17.8% in 20 min.

[0084] Example 14

[0085] This example is basically the same as Example 2, except that the amount of biochar added is 0.2 g / L.

[0086] As shown in Figure 7 , when the amount of biochar added is 0.2 g / L, the removal efficiency of arsenic is 25.6% in 5 min, and the removal efficiency of arsenic is 65.4% in 20 min.

[0087] Example 15

[0088] This example is basically the same as Example 2, except that the amount of biochar added is 2.0 g / L.

[0089] As shown in Figure 7 , when the amount of biochar added is 2.0 g / L, the removal efficiency of As is 85.4% in 5 min, and the removal efficiency of As is 99.8% in 20 min.

[0090] Example 16: Explore the effect of ferrous salt on arsenic fixation (no oxygen + ferrous salt)

[0091] Take 500 mL of arsenic-containing groundwater in a brown glass bottle (the concentration of pentavalent arsenic element in the arsenic-containing groundwater is 2 mg / L, and the pH is 6.8), and remove oxygen by nitrogen gas blowing for 1 h, then put it into a high-purity nitrogen atmosphere glove box, and continuously stir at a speed of 600 r / min on a magnetic stirrer, then add ferrous salt solution to make the corresponding Fe2+ The concentration was 5 mg / L. The pH of the system was maintained at 6.8. Samples were taken at preset time points (0, 2, 5, 10, 20, 40, 60 and 120 min). As and Fe in the system were analyzed and determined by atomic fluorescence spectrometry and ultraviolet spectrophotometry.

[0092] like Figure 8 As shown, arsenic was not detected to be removed even after 120 minutes of reaction, indicating that under these conditions, ferrous salts (added Fe) are not effectively removed. 2+ That is, dissolved Fe(II) does not have a fixing effect on arsenic.

[0093] Example 17: Investigating the effect of oxygen on the synergistic effect of biochar and ferrous salt on arsenic fixation (anaerobic + biochar + ferrous salt)

[0094] In Example 16, ferrous salt (the corresponding Fe in the reaction system before the reaction) was added. 2+ Add biochar BC600 (1g / L) to the base of 5mg / L.

[0095] like Figure 8 As shown, when ferrous salt and biochar are added to the system simultaneously, the arsenic removal rate is 23.4% after 5 minutes of reaction and 40.6% after 20 minutes. This is in contrast to Example 2 (where the arsenic removal rate reached 75.2% after 5 minutes and 98.3% after 20 minutes). Figure 8 The diagram “Aeration BC + Fe(II)” (i.e., aeration biochar + ferrous salt) shows that when there is no aeration in the system, the removal rate of As is significantly reduced, indicating that oxygen or Fe(III) is also a key factor in As removal.

[0096] Example 18: Investigating the effect of Fe(III) on arsenic fixation by biochar (anaerobic + biochar + Fe(III))

[0097] In Example 16, the addition of ferrous salt solution was replaced with the addition of biochar BC600 (1 g / L) and Fe(III) solution (so that the corresponding Fe in the reaction system before the reaction was reduced). 3+ (Concentration is 5 mg / L).

[0098] like Figure 8 As shown, when Fe(III) and biochar were added to the system simultaneously, the arsenic removal rate was 5.7% after 5 minutes of reaction and 15.4% after 20 minutes. The reason why Fe(III) has a lower efficiency in fixing As than Fe(II) is that Fe(III) is more likely to agglomerate under near-neutral water conditions, resulting in a lower arsenic removal rate than ferrous salts (Example 17).

[0099] Example 19: Explore the effect of biochar alone on arsenic fixation (oxygen + biochar)

[0100] This example is substantially the same as Example 2, the only difference is that no Fe is added, only biochar BC600 (1 g / L) is added.

[0101] As shown in Figure 8 , arsenic is not detected to be removed until reaction 120 min, so it is explained that under this condition, only biochar storage has no fixation effect on arsenic.

[0102] Example 20: Explore the effect of Fe(III) alone on arsenic fixation (no oxygen + Fe(III))

[0103] This example is substantially the same as Example 2, the only difference is that no ferrous salt and biochar is added, only Fe(III) solution (so that before the reaction, the corresponding Fe 3+ concentration in the reaction system is 5 mg / L).

[0104] As shown in Figure 8 , the removal rate of arsenic is 9.1% at 5 min of reaction, and the removal rate of arsenic is 20.8% at 20 min of reaction.

[0105] From Examples 16, 17, 18, 19, 20, it is known that when biochar, Fe(II) and Fe(III) coexist, the fixation efficiency of arsenic is the highest, and the addition of biochar and ferrous salt has significant synergistic effect on the removal of arsenic under oxygen exposure. Table 2 calculates the theoretical and actual arsenic treatment efficiency, the synergistic effect makes the removal rate of arsenic increase to 31.1 times at 5 min of reaction, and the removal rate of arsenic increases to 15.8 times at 20 min of reaction.

[0106] Table 2. Synergistic effect of biochar, Fe(II), Fe(III) on As removal

[0107]

[0108] Among them, "theoretical As treatment efficiency" is calculated by calculating the fixation efficiency of arsenic at 5 min and 20 min when only biochar, Fe(II) or Fe(III) exists in the system, and the final As treatment efficiency is obtained by adding the fixation efficiency of each single item to the actual As treatment effect of the three-phase system. The synergistic effect is obtained by comparing the actual As treatment effect of the three-phase system. The specific calculation method is:

[0109] (1) According to Example 19, when only biochar exists in the system, the As treatment amount is 0

[0110] (2) According to Example 16, when only Fe(II) exists in the system, the As treatment amount is 0

[0111] (3) According to Example 20, when only Fe(III) exists in the system, the fixation rate of arsenic per milligram of Fe(III) in 5 min is:

[0112] 9.1% (5 min arsenic fixation efficiency) ÷ 5 mg (Fe(III) adsorbed on BC content) = 1.82% / mg

[0113] The fixation rate of arsenic per milligram of Fe(III) in 20 min is:

[0114] 20.8% (20 min arsenic fixation efficiency) ÷ 5 mg (Fe(III) adsorbed on BC content) = 4.16% / mg

[0115] Therefore, according to the above calculation of the arsenic fixation efficiency per unit of biochar, Fe(II) and Fe(III), and the actual biochar, Fe(II) and Fe(III) present in the three-phase system, the theoretical fixation rate is obtained.

[0116] The theoretical fixation efficiency of arsenic in 5 min is: 1.82% / mg x 1.3 mg = 2.4%

[0117] The theoretical fixation efficiency of arsenic in 20 min is: 4.16% / mg x 1.5 mg = 6.2%

[0118] Example 21

[0119] This example is generally the same as Example 2, with the only difference being the addition of different amounts of ferrous iron, from Figure 2 As can be seen from (a) in the above table, when the Fe / As mass ratio is 5:2, Fe can be uniformly distributed on the surface of the biochar; when the amount of Fe is added too much, the Fe / As mass ratio is 20:1, and a crystalline goethite is formed, which significantly reduces the fixation sites of arsenic relative to amorphous iron minerals, which will limit the fixation of arsenic. Therefore, in the process of treating arsenic-containing groundwater, the amount of ferrous salt added is not the more the better. In addition, when there is no biochar, we can see that the Fe particles are larger in size, and larger particle size leads to a decrease in arsenic adsorption efficiency. To adsorb the same amount of arsenic, more Fe is needed, so this also explains why existing technologies often need more Fe to adsorb arsenic in water bodies in actual arsenic treatment processes, and the present application breaks this inert thinking.

[0120] The above examples are only examples. In addition to bamboo, based on the method of the present application, other forestry wastes with low silicon content such as oak, camphor tree, eucalyptus, etc. can also be used as raw material sources of biochar, as long as the content of silicon element in the prepared biochar is not more than 3%; and straws with high silicon content are not recommended.

[0121] It is to be understood that the above description is intended to be illustrative and not restrictive. Many other embodiments will be apparent to those of skill in the art upon reading and understanding the above description. The scope of the application should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.

Claims

1. A method for synergistic and rapid removal of pentavalent arsenic from groundwater by biochar in combination with ferrous salt, characterized in that, For the groundwater containing pentavalent arsenic element with pH value of 5.5-7.5, the biochar and ferrous salt are sequentially added to the groundwater, the mass ratio of ferrous ion to pentavalent arsenic element is 2:1-5:1, then the oxygen-containing gas is used for aeration stirring reaction for at least 5 min; after the reaction is completed, liquid-solid separation is carried out, and the obtained liquid is the groundwater after the pentavalent arsenic element is removed; The oxygen-containing gas is air, the aeration stirring reaction time is 5-30 min, every 1 m 3 The corresponding aeration rate of groundwater is 5-20 m 3 / h; the content of silicon element in the biochar is not more than 3wt%; the biochar is obtained by pyrolysis of non-straw agricultural and forestry waste as raw material under the condition of 600-800℃ and oxygen isolation; the raw material is forestry waste, specifically at least one of bamboo, oak, camphor tree and eucalyptus.

2. The method of claim 1, wherein the biochar and ferrous salt are used in combination to rapidly remove pentavalent arsenic from groundwater. The pyrolysis time of the biochar is 60-150 min.

3. The method of claim 1, wherein the biochar and ferrous salt are used in combination to rapidly remove pentavalent arsenic from groundwater. The concentration of the pentavalent arsenic element in the groundwater containing pentavalent arsenic element is 0.1-20 mg / L. The adding amount of the biochar is 0.2 g-2 g per 1 L of the groundwater containing pentavalent arsenic element.

4. The method of claim 1, wherein the biochar and ferrous salt are used in combination to rapidly remove pentavalent arsenic from groundwater. The time of the aeration stirring reaction is 5 min-20 min.

5. The method of claim 1, wherein the biochar and ferrous salt are used in combination to rapidly remove pentavalent arsenic from groundwater. The method further comprises adjusting the pH value of the groundwater containing pentavalent arsenic element to be treated in advance, so that the pH value meets 5.5-7.

5.

6. The method of claim 1, wherein the biochar and ferrous salt are used in combination to rapidly remove pentavalent arsenic from groundwater. The particle size of the biochar is >8 mm.

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