Zero-valent manganese nitrate synergistic soil remediation agent and application thereof in remediation of arsenic pollution in rice field and inhibition of greenhouse effect

Abstract

The invention relates to the technical field of remediation for synergistic treatment of rice field soil arsenic and greenhouse gas emission reduction, in particular to a zero-valent manganese nitrate synergistic soil remediation agent and application thereof to remediation of rice field arsenic pollution and inhibition of greenhouse effect. The invention provides a technical scheme for synergistically repairing arsenic pollution in the rice field and inhibiting the greenhouse effect based on zero-valent manganese / nitrate, an electron transfer shunt network driven by manganese circulation is constructed, a slow release reaction-dynamic regeneration-electron shunt three-in-one system is constructed, and synergistic interaction of arsenic fixation and greenhouse gas emission reduction is realized. According to the technical scheme, the potential regulation and control contradiction between arsenic fixation aerobic and carbon reduction anaerobic is successfully broken, and an innovative solution is provided for multi-target remediation of rice field soil. According to the self-sustaining circulating system, the specific surface area of the manganese oxide is stabilized at 50m < 2 > / g to 120m < 2 > / g, and the regeneration rate is gt; 95%. Experiments show that compared with a single technology, the synergistic treatment group improves the arsenic fixing efficiency to 100%, and the comprehensive emission reduction intensity of greenhouse gas reaches 76%.

Description

Technical Field

[0001] The present invention relates to the field of repair technology for synergistically controlling arsenic in paddy soil and reducing greenhouse gas emissions, and in particular to a zero-valent manganese nitrate synergistic soil repair agent and its application in repairing arsenic pollution in paddy fields and inhibiting the greenhouse effect. Background Art

[0002] Arsenic contamination of rice paddies in my country has intensified in recent years. Environmental geological surveys indicate that the average soil arsenic concentration in my country has reached nearly 12.6 mg / kg (2.1 times the global average). In particular, arsenic levels in rice paddies surrounding mining areas in Hunan and Yunnan can reach as high as 709 mg / kg. Under flooded conditions, As(V) in the soil is easily reduced by microorganisms to the highly mobile and more toxic As(III), which is absorbed by rice roots and accumulated in the grains. In some contaminated areas, arsenic levels in rice exceed national standards by 3 to 5 times. Arsenic contamination has led to chronic poisoning incidents in numerous locations. Furthermore, wastewater irrigation not only exacerbates arsenic contamination but also significantly increases greenhouse gas emissions of methane (CH4) and nitrous oxide (N2O) by altering soil microbial communities and redox states. Experiments have shown that wastewater-irrigated rice paddies increase CH4 and N2O emissions by 33% and 170%, respectively, compared to paddy fields irrigated with fresh water. CH4 and N2O are the second and third most powerful greenhouse gases under the Kyoto Protocol, after CO2, with global warming potentials 34 and 298 times greater than CO2, respectively. Therefore, there is an urgent need to develop a rice paddy remediation technology that can simultaneously achieve in-situ stabilization of arsenic contamination and synergistic greenhouse gas emission reduction. This technology can overcome the trade-off between arsenic resistance and carbon emissions in traditional technologies, while simultaneously meeting the dual goals of food security and carbon neutrality.

[0003] Although traditional arsenic pollution remediation technologies (such as soil import method and chemical passivation) can reduce the biological effectiveness of arsenic, they may change the soil microbial environment and lead to increased greenhouse gas emissions. For example, the application of iron oxides can fix arsenic, but the iron reduction process will promote the generation of CH4. Furthermore, although the combined application of nitrate nitrogen and ammonium nitrogen fertilizers can reduce and weaken N2O emissions to a certain extent, it also aggravates the activation and release of arsenic forms to a certain extent. Furthermore, by changing agronomic production management and regulating water management measures (such as intermittent irrigation), CH4 and N2O emissions can be appropriately reduced, but the alternation of dry and wet soils may promote the activation of arsenic, forming a contradictory effect of "emission reduction-increased toxicity". Currently disclosed patent technologies mostly focus on the control of single pollutants, and there is still a gap in the coordinated regulation technology for "arsenic-greenhouse gas".

[0004] Microbial remediation technology is considered an important direction for soil pollution control due to its environmental friendliness and economic advantages. Existing research shows that single microbial regulation technology can only achieve significant control effects in specific pollution scenarios: in terms of arsenic fixation, denitrifying bacteria can achieve an arsenic oxidation and fixation efficiency of over 90% through nitrate reduction coupled with As(III) oxidation mechanism; in terms of greenhouse gas control, methane oxidizing bacteria can reduce CH4 emissions by 73% through the catalytic action of methane monooxygenase. However, the coordinated control of arsenic fixation and carbon reduction still faces multiple technical bottlenecks. For example, arsenic-type denitrifying bacteria (such as Pseudomonas and Azoarcus) can detoxify As(III) to As(V) through nitrate reductase (Nar) during As(III) oxidation. However, the activity of nitrous oxide reductase (Nos), the terminal of the denitrification electron transport chain, is lost, leading to a surge in N2O emissions. Meanwhile, methane-oxidizing bacteria (such as Methylocystis) catalyze CH4 to CO2 through methane monooxygenase (MMO), which simultaneously activates the Fe(III) mineral reduction process, resulting in the reduction of adsorbed As(V) to highly mobile As(III). The arsenic concentration in pore water can increase by 3-5 times. This cascade effect of "methane oxidation-iron reduction-arsenic release" has led to a decrease in the efficiency of soil arsenic fixation and the risk of secondary pollution. Therefore, there is an urgent need to develop microbial metabolic pathway decoupling technologies to break the trade-off between pollutant control and carbon emission reduction in traditional remediation. Summary of the Invention

[0005] The purpose of the present invention is to overcome the shortcomings and deficiencies of the existing technology and to provide a zero-valent manganese nitrate synergistic soil remediation agent and its application in repairing arsenic pollution in rice fields and inhibiting the greenhouse effect.

[0006] The technical solution adopted by the present invention is as follows: a zero-valent manganese / nitrate synergistic soil remediation agent, comprising zero-valent manganese powder and nitrate.

[0007] Preferably, the nitrate is calcium nitrate, and the mass ratio of the zero-valent manganese powder to calcium nitrate is 1:10 to 1:20. Further preferably, the mass ratio of the zero-valent manganese powder to calcium nitrate is 1:10 to 1:15.

[0008] Preferably, the zero-valent manganese is prepared by reducing a manganese sulfate solution with sodium borohydride, followed by deoxidation, washing, and freeze-drying.

[0009] Preferably, the method for preparing zero-valent manganese comprises the following steps: injecting a manganese sulfate solution into a sodium borohydride solution at a constant flow rate, stirring the reaction by magnetic drive during the injection process, continuing to stir after the injection is completed, and allowing to settle; washing the product with deoxygenated deionized water to completely remove impurity ions, maintaining anaerobic conditions during the process, and then transferring the product to a freeze dryer for drying to obtain zero-valent manganese powder, which is then sealed and stored away from light.

[0010] Preferably, the concentration of the manganese sulfate solution is 0.15 to 0.25 mol / L.

[0011] Preferably, the concentration of sodium borohydride is 0.35 to 0.45 mol / L.

[0012] Preferably, the injection rate of the manganese sulfate solution is 8 to 12 mL / min.

[0013] Preferably, the stirring reaction temperature is 35-45° C., and the stirring speed is 700-900 rpm.

[0014] Preferably, the continuous stirring duration is 8 to 12 minutes, and the static sedimentation time is 18 to 22 minutes.

[0015] Preferably, the washing times are 4 to 6 times; and the drying conditions are vacuum freeze drying at -40°C to -20°C, with a vacuum degree ≤10Pa.

[0016] The invention relates to an application of the zero-valent manganese / nitrate synergistic soil remediation agent as described above for repairing arsenic pollution in rice fields and inhibiting the greenhouse effect.

[0017] Preferably, the zero-valent manganese / nitrate synergistic soil remediation agent is mixed with the paddy soil to be treated and then cultured under light-proof and anaerobic conditions.

[0018] The present invention has the following beneficial effects: It proposes a technical solution for constructing a zero-valent manganese / nitrate-based system for synergistically remediating arsenic contamination in rice paddies and inhibiting the greenhouse effect. By constructing a manganese-driven electron transfer and shunting network, it establishes a three-in-one system of slow-release reaction, dynamic regeneration, and electron shunting, achieving synergistic arsenic fixation and greenhouse gas emission reduction. This technical solution successfully resolves the potential regulation contradiction between "arsenic fixation requiring oxygen" and "carbon reduction anaerobic" and provides an innovative solution for multi-target remediation of rice paddy soils.

[0019] Specifically, the present invention mainly demonstrates effects in the following four aspects:

[0020] (1) Dynamic regeneration mechanism of manganese oxide film: After zero-valent manganese comes into contact with water, Mn(III) / Mn(IV) oxide film (Mn2O3 / MnO2) is rapidly generated with a specific surface area of ​​50m 2 / g~120m 2 / g and effectively reduces the migration of As(III) from the overlying water layer. MnO2 acts as an electron acceptor, driving the conversion of As(III) to As(V) (ΔG° = -152kJ / mol), oxidizing As(III) to the less toxic and more easily adsorbed As(V), while simultaneously forming a stationary phase of manganese arsenate on the mineral membrane surface. This environment not only meets the denitrifying bacteria's need for As(III) oxidation (conversion efficiency increases to 80% to 100%), but also promotes the activity of methane oxidizers (CH4 reduction rate increases to 65% to 80%).

[0021] (2) Synergistic regulation of arsenic fixation by the Mnammox process: The manganese ammonia oxidation (Mnammox) pathway is used to weaken the arsenic migration associated with iron release in the sediment layer, thereby achieving dual-effect fixation of arsenic migration (sediment layer and overlying water layer).

[0022] (3) Carbon reduction pathway: Based on the manganese-based microbial anaerobic methane oxidation process, MnO2 is used as the electron acceptor to competitively inhibit the anaerobic methane oxidation involving Fe(III) / As(V), thereby increasing the CH4 oxidation flux by 4 to 10 times (compared with iron-based minerals).

[0023] (4) Self-sustaining manganese circulation system: The Mn(II) generated by the reaction reacts with nitrate to form a microbial mineralization reaction accompanied by a complete denitrification process. The regenerated MnO2 can realize the recycling of materials. This process stabilizes the specific surface area of ​​manganese oxide at 50m 2 / g~120m 2 / g, the cumulative concentration of N2O decreased to 5% to 15% of the initial value.

[0024] In summary, the zero-valent manganese surface will be rapidly oxidized in an aqueous environment to form a layer rich in Mn 3+ / Mn 4+ Manganese oxide film (such as Mn2O3, MnO2). The film has strong oxidizing ability and can oxidize As(III) into less toxic and more easily adsorbed As(V), and adsorb and fix it on the membrane surface. The oxide film will be partially reduced to Mn(II) during the adsorption or oxidation of As(III). In a nitrate-containing system, the manganese oxide film can inhibit the activity of denitrifying enzymes, so that the added nitrate is preferentially used for microbial-mediated Mn(II) oxidation rather than denitrification to produce N2O. The Mn(II) produced in the process of As(V) oxidation and manganese ammonia oxidation can be further converted into Mn through metal oxidizing bacteria in the soil coupled with nitrate reduction. 3+ / Mn 4+ . Thus, Mn is formed 0 →MnO x →Mn 2+ →MnO xThis cycle not only enhances the efficiency of Mn utilization but also suppresses soil toxicity caused by Mn(II) accumulation. Furthermore, the slow-release reaction of zero-valent manganese enables it to maintain stable reactivity even in long-term flooding conditions in rice paddies, preventing rapid passivation or inactivation of the manganese oxide film.

[0025] Based on the application of the present invention, the following improvement effects can be achieved: arsenic fixation efficiency reaches 90% to 100%, CH4 emission reduction efficiency continues to be 70% to 90%, N2O emission reduction efficiency is 70% to 80%, and CO2 emission reduction efficiency is 35% to 45%. DETAILED DESCRIPTION

[0026] In order to make the objectives, technical solutions and advantages of the present invention more clear, the present invention will be described in further detail below.

[0027] Example 1 Preparation of Zerovalent Manganese / Nitrate Synergistic Repair Agent:

[0028] 1. Preparation of zero-valent manganese, comprising the following steps:

[0029] (1) Preparation of raw material solution:

[0030] 3.38 g of manganese sulfate monohydrate (MnSO4·H2O) was weighed and dissolved in 100 mL of deoxygenated deionized water. Ultrasonic dissolution (40 kHz, 10 min) was used to obtain a 0.2 mol / L transparent solution. 1.51 g of sodium borohydride (NaBH4) was weighed and dissolved in 100 mL of deoxygenated deionized water to prepare a 0.4 mol / L reducing agent solution.

[0031] (2) Chemical reduction synthesis:

[0032] The manganese sulfate solution was injected into the sodium borohydride solution at a constant rate of 10 mL / min. The reaction system temperature was maintained at 40±1°C (water bath temperature control) and magnetic stirring (800 rpm) was applied throughout the process. After the injection was completed, stirring was continued for 10 minutes and the mixture was allowed to settle for 20 minutes.

[0033] (3) Purification and drying:

[0034] The precipitate was washed five times with deoxygenated deionized water. The washed slurry was transferred to a freeze dryer (-25°C, vacuum ≤10 Pa) and dried for 24 hours to obtain zero-valent manganese powder. The dried zero-valent manganese powder was sealed in a nitrogen-filled glass bottle and stored at 4°C in the dark.

[0035] 2. The zero-valent manganese and nitrate are mixed in a mass ratio of 1:10 to 1:20 to obtain a zero-valent manganese / nitrate synergistic repair agent.

[0036] Test Example 1:

[0037] The zero-valent manganese / nitrate synergistic remediation agent prepared in this invention was applied to simulated paddy soil with severe arsenic contamination. Microcosm experiments were conducted to evaluate the effects of arsenic fixation and greenhouse gas emission reduction. The basic physical and chemical parameters of the paddy soil are shown in Table 1.

[0038] 100 g of paddy soil (air-dried and 2 mm sieved) and 300 mL of simulated paddy overlying water (containing 2078 ± 394 μg / L As(III)) were placed in 500 mL sealed resin containers. The cells were divided into four groups according to the treatment schedule: Group A (control, overlying water only); Group B (nitrate treatment, with 15 g calcium nitrate added); Group C (zero-valent manganese treatment, with 1.0 g zero-valent manganese added); and Group D (synergistic treatment, with 15 g calcium nitrate and 1.0 g zero-valent manganese added). Each group was replicated three times. All reaction vessels were incubated at 30°C in the dark under anaerobic conditions. On the 12th day of incubation, samples were collected to monitor total arsenic and As(III) concentrations in the overlying water and greenhouse gas emissions in the headspace. The results are shown in Table 2.

[0039] Table 1 Basic physical and chemical properties of paddy soils with high ammonia nitrogen and arsenic pollution in flooded experimental areas

[0040]

[0041] Table 2 Removal / reduction rates of arsenic and greenhouse gases in paddy soil after 12 days of treatment compared with the initial incubation period (0 day)

[0042]

[0043] Test Example 2:

[0044] To further validate the remediation efficiency of the zero-valent manganese / nitrate synergistic system described in this invention, paddy field soil contaminated with high ammonia, nitrogen, and arsenic content (physical and chemical properties shown in Table 3) was used for further removal efficiency evaluation. 50 g of air-dried, 2 mm sieved soil was added to 150 mL of simulated overlying water (containing 3027 ± 297 μg / L As(III)) and placed in a 300 mL sealed resin container. The containers were then divided into four groups according to the treatment scheme: Group A (control, overlying water only); Group B (nitrate group, with 7.5 g calcium nitrate added); Group C (zero-valent manganese group, with 0.5 g zero-valent manganese added); and Group D (synergistic group, with 7.5 g calcium nitrate and 0.5 g zero-valent manganese added). Each group was tested in triplicate. All reaction flasks were incubated at 30°C in the dark under anaerobic conditions. On the 16th day of incubation, overlying water and headspace gas samples were collected and analyzed for total arsenic, As(III), N₂O, CH₄, and CO₂ contents.

[0045] The results are shown in Table 4. The synergistic group achieved a near-100% removal rate for total arsenic and As(III), significantly outperforming both the nitrate and zero-valent manganese groups. The synergistic group achieved N2O, CH4, and CO2 emission reductions of 41% to 60%, 63% to 76%, and 24% to 36%, respectively. This demonstrates that the zero-valent manganese / nitrate synergistic system of the present invention can effectively and simultaneously fix arsenic and significantly inhibit greenhouse gas production in flooded paddy soils, typically characterized by high ammonia nitrogen and high arsenic content, achieving a "pollution and carbon reduction" effect in paddy soils.

[0046] Table 3 Basic physical and chemical properties of paddy soils with high ammonia nitrogen and arsenic pollution in flooded experimental areas

[0047]

[0048] Table 4 Removal / reduction rates of arsenic and greenhouse gases in paddy soil after 16 days of treatment compared with the initial incubation period (0 day)

[0049]

[0050] Note: The negative sign in the table indicates the efficiency of arsenic solubilization / greenhouse gas emission.

[0051] The above disclosure is merely a preferred embodiment of the present invention and certainly cannot be used to limit the scope of the present invention. Therefore, equivalent changes made according to the claims of the present invention are still within the scope of the present invention.

Claims

1. A zero-valent manganese / nitrate synergistic soil remediation agent, characterized by: Includes zero-valent manganese powder and nitrate.

2. The zerovalent manganese / nitrate synergistic soil remediation agent according to claim 1, characterized in that: The nitrate is calcium nitrate, and the mass ratio of the zero-valent manganese powder to the calcium nitrate is 1:10 to 1:

20.

3. The zerovalent manganese / nitrate synergistic soil remediation agent according to claim 1, characterized in that: The zero-valent manganese is prepared by reducing a manganese sulfate solution with sodium borohydride, followed by deoxidation, washing, and freeze-drying.

4. The zerovalent manganese / nitrate synergistic soil remediation agent according to claim 3, characterized in that: The preparation method of zero-valent manganese comprises the following steps: injecting a manganese sulfate solution into a sodium borohydride solution at a constant flow rate, stirring the solution by magnetic drive during the injection process, continuing to stir the solution after the injection is completed, and allowing the solution to settle; washing the solution with deoxygenated deionized water to completely remove impurity ions, maintaining anaerobic conditions during the process, and then transferring the solution to a freeze dryer for drying to obtain zero-valent manganese powder, which is then sealed and stored away from light.

5. The zerovalent manganese / nitrate synergistic soil remediation agent according to claim 4, characterized in that: The concentration of the manganese sulfate solution is 0.15-0.25 mol / L, and the concentration of the sodium borohydride is 0.35-0.45 mol / L.

6. The zerovalent manganese / nitrate synergistic soil remediation agent according to claim 4, characterized in that: The injection rate of the manganese sulfate solution is 8-12 mL / min.

7. The zerovalent manganese / nitrate synergistic soil remediation agent according to claim 4, characterized in that: The stirring reaction temperature is 35-45° C., and the stirring speed is 700-900 rpm. The continuous stirring duration is 8-12 minutes, and the static sedimentation time is 18-22 minutes.

8. The zerovalent manganese / nitrate synergistic soil remediation agent according to claim 4, characterized in that: The washing times are 4 to 6 times; the drying conditions are vacuum freeze drying at -40°C to -20°C, with a vacuum degree of ≤10Pa.

9. Use of the zero-valent manganese / nitrate synergistic soil remediation agent according to any one of claims 1 to 8 for remediating arsenic contamination in rice fields and inhibiting the greenhouse effect.

10. The use according to claim 9, characterized in that: The zero-valent manganese / nitrate synergistic soil remediation agent is mixed with the paddy soil to be treated and then cultured under light-proof and anaerobic conditions.

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