The Effects Of Redox Potential And Organic Matter On Arsenate Immobilization In Contamiunated Soil By Biochar-Supported Zero Valent Iron

Arsenic(As)is a common toxic metalloid in wastewater and soil.In wastewater,As mainly exists in the form of inorganic As,including trivalent arsenic(As(?))and pentavalent arsenic(As(?)).In soil,in addition to inorganic arsenic,As also exists in the form of organic arsenic,including monomethylarsenate(MMA),dimethylarsenate(DMA),tetramethylarsenic ion,and trimethylarsenic oxide.Iron-based materials are widely used as adsorbents of As,especially zero-valent iron(ZVI)materials.ZVI has a large specific surface area and high reactivity.Therefore,it can remove As in water through adsorption,redox,and co-precipitation mechanisms.In soil environment,As was precipitated and thus immobilized by corrosion products of ZVI.However,ZVI tends to aggregate with a chain-like structure and is easily passivated by water and oxygen,hindering its electron transfer and weakening the reduction ability of As(?).This paper aimed to utilize biochar as support material of ZVI to enhance its removal capacity toward As(?).The pyrolysis temperature has a certain influence on the redox ability and electron transfer ability of biochar and its composites.Herein,biochar and its composites were prepared at different pyrolysis temperatures to remove As(?)from aqueous solution for screening suitable materials,and then explore the removal mechanisms of As(?)in water solution.Combined with soil incubation experiments,the effects of water management,and organic matter on remediation effectiveness of As-contaminated soil by biochar and biochar-supported ZVI were explored.The main results obtained are as follows:(1)In order to explore the effect of pyrolysis temperature on As(?)removal by biochar and biochar-supported ZVI,pine wood biomass and hematite were used to prepare materials via pyrolysis at 700,800,900,and 1000 ?,respectively.Results showed that ZVI/BC materials prepared at 700 ? at pH=3 had the best removal ability and reduction capacity of As(V).The removal capacity and reduction capacity were 439.80 and 309.05 mg/(g ZVI),respectively.Results of electrochemical analysis showed that the electrical conductivity of the material increases with the pyrolysis temperature,but ZVI/PBC700 has a higher electron donating capacity(EDC).(2)The adsorption characteristics and removal mechanism of As(?)in water were investigated using PBC700 and ZVI/PBC700 as adsorbents.The kinetic results showed that the removal of As(?)by the two materials is more in line with the pseudo-second-order kinetic model,indicating that As(?)adsorption was mainly chemical adsorption and the maximum adsorption amounts was 22.20 and 161.73 mg/g by PBC700 and ZVI/PBC700,respectively.The experimental results of pH effect showed that the removal capacities of PBC700 and ZVI/PBC700 to As(?)decreased with the increase in pH.Desorption experiments and X-ray photoelectron spectroscopy(XPS)results showed that the removal mechanisms of As(?)by the two materials were mainly involved reduction and accompanied by precipitation.The results of Zeta potential showed that electrostatic adsorption was also the removal mechanism of As(?)by these two materials.(3)PBC700 and ZVI/PBC700 were selected as test materials to explore the effects of water management on remediation effectiveness of As-contaminated soils.The results showed that under three water management strategies(wet irrigation,moisture,and flooding),the proportion of As in soil was the following:amorphous Fe-Al oxide bound As>non-specifically adsorbed As>residual As>crystalline Fe-Al oxide bound As>specifically adsorbed As.After 30 days of incubation,DB_ZBC700 treatment had the best effect on immobilization of available As in soil.(4)The effects of organic matter on remediation effectiveness of As-contaminated soils by PBC700 and ZVI/PBC700 were explored.Results showed that after subtracting organic matter from soil,soil pH increased,which promoted the release of free iron oxides and amorphous ironoxides in soil,facilitating the release of available As in soil.It was confirmed that the adsorption site of soil As was the complex of iron minerals and organic matter.The results of microbial analysis showed that the application of PBC700 and ZVI/PBC700 decreased the richness and diversity of microbial community in soil under DB,F2B,and F2B_OMR management,and increased the richness and diversity of microbial community in soil under DB_OMR managenment.In comparison to control,the relative abundance of Bacillus was lower and that of Pseudomonas was higher after application of ZVI/PBC700,indicating that As mainly existed in the form of arsenite(As(III))in soil.