Simulation Study On The Influence Of Sulfate On Arsenic Migration And Transformation

The bioavailability,toxicity,migration and transformation process of arsenic?As?are strongly controlled by its speciation,in addition the sulfur is one of the factors that significantly affect the speciation of arsenic,and biotic and abiotic redox reactions mediated by sulfate have a good regulation toward the migration and transformation process of arsenic.Considering the lake Wuliangsuhai as the research object,the laboratory simulation experiment was carried out,the effects of sulfate at the low concentration on arsenic speciation migration and transformation during the ice-bound and non-ice bound periods were discussed respectively,and through contrast experiments,the influencing mechanism of microorganisms on arsenic migration and transformation during ice-bound and non-ice bound periods was simulated.The continuous change of arsenic speciation in sediment was investigated by applying the Eight-step continuous extraction method,and involving the structural equation model,as well as the migration and transformation pathways of arsenic based on the speciation in the sediment-water system were explored through simulation analysis,in order to reveal the differential effects of arsenic migration and transformation as well as speciation redistribution in the sediment-water system under different conditions,explain the biotic and abiotic redox action mediated by sulfate on the transformation of arsenic speciation,and provide the basic data enable to clarify the mechanism of arsenic migration and transformation at the sediment-water interface in lake Wuliangsuhai.The conclusions are as follows:?1?Although the concentration of each compound releasing into the overlying water during the non-ice bound period was obviously higher than that during the ice-bound period,while the total arsenic concentration during the ice-bound period in the overlying water was up to 14.8?g/L at the highest and the maximum releasing rate was up to 106?g?m^-2?d^-1,which indicates that the weak microbial activity during the ice-bound period could induce the arsenic able to release at some extent and there is still great ecological risk.?2?By contrast,under the biotic action mediated by the sulfate at the low concentration,the releasing concentration and rate of arsenic was faster in the early stage during the whole experiment,while in these groups arsenic concentration in the overlying water was obviously lower than that in the sterilized groups in the later stage.This indicates that microorganisms promoted the release of arsenic in the early stage,but re-immobilized arsenic in the later stage.?3?In the cultivation period,S6 and S7?As oxides and As coprecipitated with silicates,As coprecipitated with pyrite and amorphous As₂S₃?generally presents the tendency of decreasing,as well as S1,S2 and S3?ionically bound As,strongly adsorbed As,As coprecipitated with AVS,carbonates,Mn oxides,and very amorphous Fe oxyhydroxides?gradually climbed,which showed the process of the stable state transformed into the active state and it could be observed that S6appeared to“rising-decreasing-rising again”and the increase of S6 content in the later stage also proved the biotic reaction mediated by sulfate at the low concentration could immobilize the arsenic.?4?The Pathway analysis shows that S6 and S7?As oxides and As coprecipitated with silicates,As coprecipitated with pyrite and amorphous As₂S₃?could be directly transformed into S0?As?T?in the overlying water?in the sterilized groups during the ice-bound period,while in the microbial groups S6 and S7 could be unable to be directly transformed into the overlying water in turn though the S1 and S2?ionically bound As and strongly adsorbed As?indirectly influencing S0,which demonstrates there is still existing arsenic releasing into the overlying water and the weak microbial activity could promote transformation of S1,S2 into S0.During the non-ice bound period,S1,S2,S6 and S7 could be directly transformed into S0 releasing to the overlying water in the sterilized groups,while only S5?As coprecipitated with crystalline Fe oxyhydroxides?could be transformed into S0 in the microbial groups.The reduction of transformation pathways meant that the microorganisms could induce more bound-arsenic through the adsorption etc.and then immobilized in the sediment,which resulting in the lower arsenic content in the overlying water than that in the sterilized groups during the non-ice bound period.