Oxidation Mechanism And Influencing Factors Of Iron-Arsenic Coupling Under Flooding Conditions

Arsenic is one of the main elements of heavy metal pollution in farmland with high cancer risk.Toxicity and migration are closely related to its morphology.As（III）toxicity and migration are usually greater than As（V）.Dissimilatory iron reduction is a microbial-mediated enzymatic reaction and one of the core driving forces of soil key ecological processes.The morphological transformation of arsenic is closely related to the dissimilatory iron reduction coupling.Paddy soil creates a suitable environment for microbial-driven dissimilatory iron reduction with its unique flooding anaerobic conditions.In this study,the paddy soil was used as the object to simulate the dynam-ic changes of total iron,total arsenic,Fe²⁺,As（III）and As（V）in paddy soil under the irrigated environment under simulated flooding and anaerobic conditions,and Wenzel The characteristics of the content of the extracted state and the key microbial commu-nities that play a leading role in the process of dissimilar iron,explore the relationship between arsenic and total iron and ferrous iron in different types of minerals and soil solutions,and clarify the dissimilar iron reduction drive of paddy soil.The intrinsic dynamic mechanism of arsenic speciation.The research results will provide a theoret-ical basis for reducing the absorption of arsenic in paddy soil.The main research con-tents and results are as follows:（1）Under natural conditions,the process of microbial dissimilatory iron reduc-tion occurred during the flooding anaerobic process of paddy soil.As the culture pro-gresses,arsenic is gradually reduced,and the increase of Fe²⁺in the paddy soil is cou-pled with the transformation of arsenic species.In the process of flooding anaerobic,the ratio of As（III）to total arsenic in paddy soil gradually increases and finally reach-es equilibrium.During the flooding culture process,the pH value gradually became neutral,and the Eh value gradually decreased to below-30 mV as the culture pro-gressed,and the equilibrium was strong.With the increase of culture time,the pro-duction of Fe²⁺and As（III）in paddy soil increased gradually to equilibrium,indicat-ing that arsenic reduction occurred simultaneously in the iron reduction process in paddy soil.Compared with the purification reaction of the sterilization group,the biggest difference between the microbial-mediated dissimilatory iron reduction pro-cess is the reduction of F-leaching and the acceleration of SO₄^2-,NO₃^-reduction.（2）During the flooding anaerobic process,the application of exogenous goethite and amorphous iron oxyhydroxide to the paddy soil resulted in the arsenic reduction process,and the non-specific adsorption state of arsenic（F1）to the obligate adsorp-tion state（F2）.And the conversion of crystalline iron-manganese oxide bound state（F4）reduces the mobility.The content of Fe²⁺in 0.1M HCl increased with the in-crease of culture time.The reduction rate of the first 10d was the highest.At 10d,the content of Fe²⁺reached 577.413mg/kg,and the average reduction rate was3.213mg/（kg·d）.The rate of reduction gradually decreases over time.The application of goethite reduces the iron reduction rate and the equilibrium content.The higher the applied concentration,the more effective the effect of inhibiting the equilibrium Fe²⁺content.The application of amorphous iron oxyhydroxide can inhibit the reduction of HCl extracted Fe3+and the production of Fe²⁺.There is no significant difference in the effect of different applied amounts on the inhibition of the equilibrium Fe²⁺con-tent.（3）Adding exogenous arsenic to paddy soil during flooding anaerobic process can inhibit the process of dissimilatory iron reduction in paddy soil.The higher the concentration,the stronger the inhibition effect,and the arsenic form undergoes the process of first reduction and oxidation.The logistic equation can be used to better fit the dissimilatory iron reduction rate curve under anaerobic conditions,and the fitting correlation coefficient（R2）is 0.935-0.984.Under this condition,the maximum reduc-tion potential of iron is about 993.11-1132.53 mg/kg.The higher the concentration of exogenous arsenic,the lower the reduction potential of iron.When the exogenous As enters the soil,it rapidly distributes to the morphological transformation of each level,and the reaction is most rapid within the first 15 days.The arsenic culture was carried out by applying exogenous arsenic to the paddy soil.The arsenic in the paddy soil experienced the process of reduction after oxidation and finally pentad arsenic.Ex-ogenous arsenic has strong selectivity to microorganisms,promotes the abundance of Betaproteobacteria,facilitates nitrate-dependent Fe²⁺oxidation,and promotes the in-crease of abundance of specialized iron-reducing bacteria Geobacteraceae,but on iron The reduction of Clostridia,which plays an important role,is strongly inhibited,and the combined action of exogenous arsenic on microorganisms inhibits the progress of dissimilatory iron reduction.Exogenous arsenic was added to paddy soil through an-aerobic culture,and arsenic was first transformed into F1 and F2 forms in multi-stage morphology,and its component content increased significantly.The most affected an-ions in the microbial-mediated dissimilatory iron reduction process are NO₃^-,SO₄^2-,which may be due to the microbial-mediated reduction reaction,and the content of NO₃^-and SO₄^2-is reduced,which is much lower than that of the control group.（4）Adding different concentrations of nitrate to the paddy soil during flooding anaerobic process accelerates the decrease of Eh,accelerates the formation of reduc-ing environment,and makes the time of As（III）production reach equilibrium.The ar-senic form undergoes the process of reduction after oxidation..Nitrogen application under flooding anaerobic conditions can significantly inhibit the process of iron re-duction.The higher the concentration,the stronger the inhibition.At the same time,the morphological transformation process of arsenic is also inhibited.The lower the concentration,the stronger the inhibition.The most affected anions in microbi-al-mediated dissimilatory iron reduction were NO₃^-and SO₄^2-,which showed that NO₃^-decreased with incubation time and SO₄^2-content was much lower than that of the control group.Exogenous nitric oxide has strong selectivity to microorganisms,while the abundance of Clostridia and Geobacteraceae,which play a major role in the reduction of dissimilatory iron,is strongly inhibited,thereby inhibiting the process of dissimilatory iron reduction.The reduction of iron and arsenic in the sterilization group was less than that in the bacterial group,indicating that iron-arsenic-coupled redox is a biochemical process and not a separate chemical process.（5）Humic acid has a great influence on the reduction of dissimilatory iron in paddy soil.The higher the concentration of humic acid applied in paddy soil under flooding conditions,the lower the total amount of dissimilatory iron reduction,which in turn affects the reduction process of pentavalent arsenic in paddy soil,and thus has a certain inhibitory effect on the morphological transformation of arsenic.The appli-cation of exogenous humic acid can be used as a carbon source for microbial metabo-lism.The abundance of most microorganisms including Anaerolineaceae is increased,while Geobacteraceae is not affected much,but the abundance of Clostridia and Be-taproteobacteria,which play a major role in the reduction of dissimilatory iron,is in-hibited.,so slowed down the process of dissimilatory iron reduction.The most af-fected anions in microbial-mediated dissimilatory iron reduction are NO₃^-,NO₂^-and SO₄^2-,because the applied humic acid is used as a carbon source for growth metabo-lism by soil microorganisms,resulting in increased abundance of bacteria;The sharp increase will cause the various oxidizing substances in the soil to be rapidly depleted,so that the reduction process is accelerated,so that NO₃^-and SO₄^2-are reduced,and the content of NO₂^-is increased.Correlation analysis showed that the Fe²⁺content in the control group was significantly positively and negatively correlated with pH and Eh,respectively（p<0.05）.The addition of humic acid treatment group was also the same conclusion but not significant,and the treatment group and the control group.The Fe²⁺content has a negative correlation with As（III）.It also shows that exogenous humic acid inhibits the reduction of soil dissimilatory iron and affects the morpholog-ical transformation of arsenic in paddy soil.