Arsenic Mobility During Flooding Of Paddy Soil:the Effect Of Feammox Process

Arsenic contamination that causes arsenic enrichment in rice poses serious threats to human health.A large area of domestic agricultural land is contaminated by arsenic,which severely limited the safety of rice production.Quantities of researches have shown that flooding treatment during rice cultivation is a key process contributing to arsenic release in paddy soil and enrichment of arsenic in rice.Amorphous iron oxide serves as the most important sorbent for arsenic in paddy soil and thus its reductive dissolution during flooding proves to be one of the main mechanisms leading to arsenic release.Ammonium has often been used as fertilizer in rice cultivation.Previous studies have demonstrated the process of ferric iron reduction that couples ammonium oxidation in flooded and anaerobic paddy soil,which is termed ‘Feammox’.Considering that redution of iron,particularly amorphous iron oxide,had significant impacts on arsenic release and reduction,it is of significance to study the effects of ammonium amendment on arsenic transportation and transformation in paddy soil.In this study,arsenic contaminated paddy soil was used as research material and soil microbes were enriched and isolated using enrichment solution.Release of arsenic from arsenic containing ferrihydrite induced by Feammox was discussed and on the basis of this,iron mineral characterization was conducted.Additionally,the effects of ammonium of different concentration on amorphous iron oxide bound arsenic was discussed.Results show that:（1）In terms of arsenic adsorption,ferrihydrite exhibited higher adsorption capacity for arsenate than arsenite.With the addition of ferrihydrite,the extent of decrease in solution arsenate was larger than that of arsenite.Therefore,Feammox could drive arsenic release by reducing ferric iron mineral.In particular,blank+2m M NH₄⁺-N treatment had the most obvious promotion effects on Feammox,under which total arsenic,arsenate and arsenite concentration reached 0.628,0.353 and 0.275 μmol/L,respectively.FTIR spectra showed the adsorption peak of As-O,indicating that ferrihydrite reacted with arsenic and enhanced both arsenic adsorption and desorption.XRD spectrogram showed that compared with nontreated ferrihydrite,NH+-N treated ferrihydrite exhibited characteristic peaks of iron-arsenic compounds.SEM characterization revealed the regular surface morphology of ferrihydrite treated by NH₄⁺-N,with surface particles scattering orderly and the porous,coherent surface of ferrihydrite was observed.（2）After NH₄⁺-N treatment,paddy soil Eh decreased and p H increased.With the consumption of NH₄⁺-N,continous increase in NO3--N,NO2--N,HCl-extractable ferrous iron and total arsenic was observed.Evident increase in soil solution total arsenic were observed under medium NH₄⁺-N concentration treatment compared with that under low NH₄⁺-N concentration treatment,the extent of increase reaching a percentage of 66.48%.Additionally,paddy soil after medium NH₄⁺-N concentration treatment showed lower concentration of HCl-extractable ferrous iron and F3（amorphous iron oxide bound arsenic）（p<0.05）,which was 0.497 mg/kg and 13.89 mg/kg,respectively.This provided evidence that Feammox process could drive arsenic release by inducing reductive dissolution of amorphous iron oxide in paddy soil.（3）Dissimilatory iron reducing bacteria play an important role in bioreduction of iron,such as Geobacter,Shewanella,Myxobacter.They can utilize organic matter as electron donor and reduce Fe（III）to Fe（II）.In Feammox reaction some iron reducing bacteria reduce Fe（III）to Fe（II）using ammonium as electron donor.Previous studies have listed some Feammox related iron reducing bacteria,eg.Acidimicrobiaceae,Shewanella,Myxobacter,the abundance of some species of which showed significant correlation with nitrogen dynamics.