Study Of The Mechanism Of Arsenic Oxidation-immobilization And Arsenic Release Mediated By Microbes In Paddy Soils

Arsenic(As)is a toxic and carcinogenic metalloid,which is widely distributed in the environment.In recent decades,As pollution becomes a worldwide problem.Because flooded conditions in paddy soils lead to As mobilization,large scale of paddy soil in southeast of China were contaminated with As,which affects the health of people.In paddy soil,microbial activity and iron redox reaction play a key role in arsenic migration and transformation.Microbial-mediated arsenite[As(Ⅲ)]and Fe(Ⅱ)oxidation coupled with denitrification has great potential for attenuating As mobility in anoxic As-contaminated environments;Dissimilatory Fe(Ⅲ)-reducing Bacteria(DIRB)-mediated reduction and dissolution of iron oxide promotes arsenic release in paddy soil.This study focuses on the soil Fe(Ⅱ)-Fe(Ⅲ)cycle,and explores the mechanism of microbial activity on the Fe transformation and As environmental behavior.The main results are as follows:(1)In this study,we collected As-contaminated paddy soil from Hechi city,Guangxi province.The microcosms were constructed to simulate the nitrate reduction-coupled As(Ⅲ)and Fe(Ⅱ)oxidation systems in the paddy soil.This work strongly suggested that the genus Noviherbaspirillum belonging to β-proteobacterial,were mainly responsible for the oxidation of As(Ⅲ)and Fe(Ⅱ)during the anoxic biological treatment of either NO3-or NO3-plus lactate amended soil microcosms.A novel strain belonging to the genus Noviherbaspirillum was isolated,named Noviherbaspirillum sp.HC18.(2)Strain HC18 was able to oxidize both As(Ⅲ)and Fe(Ⅱ)under anoxic denitrifying conditions.Genome mining analysis suggest that both Aio and Arx are present in the genome of strain HC18.The presence of the aio and arx genes in the genome of strain HC18 suggests that both genes may be responsible for anaerobic As(Ⅲ)oxidation,which drive the As cycle in the flooded paddy soils.We studied the kinetic process of As(Ⅲ)and Fe(Ⅱ)oxidation coupling nitrate reduction.The oxidation of aqueous Fe(Ⅱ)resulted in the precipitation of Fe(Ⅱ)minerals,which were characterized using scanning electron microscope coupled with energy dispersive spectrometer(SEMEDX),X-ray photoelectron spectroscopy(XPS)and powder X-ray diffraction(XRD)technologies.XRD and XPS analysis indicated that 96%As was bound to the biogenically produced Fe(Ⅲ)oxides such as goethite as a consequence of microbial Fe(Ⅱ)oxidation.(3)A dissimilatory Fe(Ⅲ)-reducing bacteria was successfully isolated from Chenzhou city,Hunan province As-contaminated paddy soil named Desulfovibrio sp.DS-1.It was proved that strain DS-1 could reduce As(Ⅴ)to As(Ⅲ)in anaerobic environment.Strain DS-1 can reduced Fe(Ⅲ)oxides and catalyze the release of As in arsenicbearing Fe(Ⅲ)oxides to further generate secondary minerals such as Vivianite and Parasymplesite.In addition,inoculation of washed cells of strain DS-1 into sterilized soil successfully reproduced arsenic release.