Effect Of Biochar From Different Sources On Arsenic Formation In Contaminated Soil

The problem of soil arsenic pollution is a worldwide problem,which seriously threatens ecological security and human health.Different arsenic forms in soil pose different hazards to ecology and human health,so it is necessary to study the occurrence of arsenic in soil.Due to its wide source,low cost and stable structural properties,biochar has a certain improvement effect on the soil environment.Biochar can reduce the adsorption of soil arsenic,promote the reduction and release of soil arsenic,enhance the effectiveness of arsenic,and have a certain remediation effect on arseniccontaminated soil.In this study,28 kinds of biochar were obtained by pyrolysis of biomass from different sources at different temperatures.The effects of different temperatures and raw materials on the content,composition and distribution of biochar dissolved organic matter(DOM)were explored by SUVA254 and EEMS combined with parallel factor analysis.Peanut shell biochar,rice straw biochar,cow manure biochar and bamboo biochar were selected by pyrolysis at 300°C as experimental materials.Experiments were conducted on simulated arsenic-contaminated soil and actual arseniccontaminated soil to explore the effects of different biochar on soil physicochemical properties and their changes on soil arsenic accumulation forms.The main conclusions are as follows:(1)The influence of different pyrolysis temperatures and different raw materials on biochar and its derived DOM was analyzed.The results shown that pyrolysis temperatures and feedstocks had an important effect on biochar-derived DOM.Among them,the pyrolysis temperature was an important factor affecting the physicochemical properties of biochar and the content and composition of DOM.Under different temperatures,the ratios of H/C,O/C and(O+N)/C in biochar decreased,with the increase of pyrolysis temperature.The contents and properties of DOM extract from different biochar varied with pyrolysis temperatures and feedstocks.The content of DOM obtained from biochar produced at low-temperatures was higher than that of at high pyrolysis temperatures.According to SUVA254 indicated degree of humification,the humification of PSBC,RHBC and BBC were higher at higher temperatures,while the humus component of RSBC,CSBC,WSBC and DMBC had more.Besides,the BIX results showed that PSBC,PSBC,WSBC and BBC had high bioavailability at low pyrolysis temperatures.Correlation results showed that HIX had a positive correlation with C2%,BIX had a positive correlation with C1% and C3%(p < 0.001).Therefore,the DOM was mainly humus-like,the most important of which was fulvic acid-like substances.Humus-like substances decreased with increasing pyrolysis temperatures,and small amounts of protein-like substances appeared at high temperatures.Cause humic acid has a good affinity with heavy metal pollutants,it can effectively repair heavy metal pollution.(2)The results of microculture experiments on simulated contaminated soil with different amounts of biomass char showed that arsenic in soil solution mainly existed in the form of As(V).The application of biochar increased the content of total arsenic in soil solution and increased the proportion of As(III)in solution.The release of arsenic promoted by 5% biochar application is greater than 1% application.The release effect of different biochar was that diary manure biochar was greater than that of the other three biochar.The application of different biochar accelerated the release of arsenic by increasing soil p H,organic matter content,reducing soil Eh,and promoting the reduction of soil iron and manganese oxides.(3)The changes of arsenic formation in soil at different depths after the application of different biochar were studied by soil column experiments.The results showed that arsenic in flooded soil mainly existed in crystalline iron oxide bound state and residual state.The application of biochar would reduce the proportion of F4 and F5 and increase As(III)content in soil solution.Biochar application increases the abundance of arsenicreducing bacteria such as Proteobacteria,Bacteroidetes,Acidobacteria and Desulfobacterota in the soil,thereby increasing the reduction release of arsenic.Due to the different redox environments of soil,the abiotic reduction release mechanism of soil arsenic at different depths is different.The content of As(III)in the overlying water was mainly related to the photooxidation promoted by DOM in the solution.The content of As(III)in pore water in soil tillage layer was mainly controlled by DOM and iron oxide in pore water.In short,biochar application can change the soil environment,increase soil organic matter,promote iron-manganese oxide reduction,increase the abundance of arsenic reducing bacteria in soil,promote the release of arsenic in soil,increase the content of As(III)in flooded soil,and have a good effect on the reduction of soil arsenic content.