To resolve the role of DOM in regulating arsenic environmental behavior,here we examine As desorption from ferrihydrite-coated sands presorbed with As(Ⅴ)and its redox transformation influenced by the different fractions of DOM extracted from sludge mediated by the dissimilatory iron reducing bacterium Shewanella decolorationis S-12.The influence different fractions of DOM on the microbial reductively dissolution of ferric(hydr)oxides and release of arsenic to the aqueous phase could be seen during the incubation time,the degree and extend were enhanced significantly when S12 inoculum concentration increased from 2%(V/V)to 20%(V/V)or after addition of 0.1mMAQS and 2mM EDTA.However,in parallel abiotic experiment,the obvious dissolution of ferric(hydr)oxides and release of arsenic can only be noticed in reaction system containing 0.1mM AQS.Similar to that of the total arsenic,the tendency of As(Ⅴ)release from presorbed ferrihydrite-coated sands was strengthened by the addition of 0.1mMAQS and 2mM EDTA,while compared to abiotic control experiment,the ability of the release enhancement by addition of 0.1mMAQS and 2mM EDTA was slim.As for As(Ⅲ),whether in biotic or abiotic experiments,the obvious release can only be seen in reaction system containing 0.1mM AQS.In high S12 inoculum concentration,the elevated release of As(Ⅲ)could be happened influenced by fractions of DOM except eluted fraction within 300 hours incubation,as for As(Ⅴ),the enhancement of the release by different fractions of DOM could obviously noticed in whole incubation time.