Manganese Oxides And Sulfate Reduction Influencing Arsenic Speciation And Bioavailability In Paddy Soils

Arsenic(As)is a toxic and carcinogenic metalloid,which is widely distributed in the environment.In recent decades,large areas of paddy soil around the word,especially in soutli and southeast Asia,are contaminated with As due to mining,irrigation,smelting with high-As groundwater,and the use of As-containing agrochemicals.As contamination of paddy soils can reduce rice yield and increase As accumulation in rice grain.Rice,the staple food for more than half of world’s population,is a major source of dietary As for people in south and south east Asia.The predominant chemical species of As in the environment are inorganic arsenite[As(Ⅲ)]and arsenate[As(Ⅴ)].Under the flooded conditions,As has a higher bioavailability due to three reasons.First,Fe(oxyhydr)oxsides are reduced and solubilized,releasing the sorbed As into the solution phase.Second,As(V)sorbed on the solid phase is reduced to As(Ⅲ),and the latter is less strongly sorbed than As(V)and therefore has a greater tendency to partition into the solution phase.Hence,to reduce arsenic entering the food chain requires effective measures to control the bioavailability of arsenic in paddy soils.Previous research mainly focused on how to reduce the As mobilization or toxicity in the pour medium,but little in the paddy soil.Here,we used HPLC-ICP-MS analysis of porewater arsenic speciation and synchrotron radiation X-ray absorption near-edge structure(XANES)to analyze As speciation in the soil solid phase.At the same time,a seven-step sequential extraction method was used to determine the occurrence of arsenic in solid phase The main aim of this thesis was to explore the factors that affect arsenic mobilization in paddy soils.Quantitative molecular biology techniques such as quantitative PCR and high-throughput sequencing were combined with chemical analysis methods to study the influences of Mn oxides and sulfate reduction on As mobilization.In addition,pot experiments were carried out to investigate the effects of adding Mn oxides and sulfate on arsenic accumulation by rice.Key results are presented as follows:(1).We tested eight different soils which were contaminated by arsenic in southern China.The soils were incubated under flooded conditions for up to 30 days.The concentrations of As,Fe and Mn,and As speciation in the porewater were determined.Arsenic fractions and speciation in the solid phase were quantified using sequential extraction and XANES.After 30 days of anaerobic flooded incubation,porewater As concentration varied greatly among different soils(76±4.2-15007±906 μg L-1).The proportion of porewater As in soil total arsenic varied from 0.079±0.005%to 2.82±0.047%.As(Ⅲ)accounted for 20.0±1.1-88.8 ±7.3%of the porewater As.The soil from Zhangping(ZP)had the lowest As mobilization capacity but 2-3 times higher manganese content than the other 7 soils.In contrast,soils from Shangyu(SY)and Dayu(DY)had the highest As mobilization capacity.The results from Sequential extraction showed that ZP soil had a higher proportion of As in the Mn oxides fraction than other soils.During flooded incubation,the As fractions in Acid volatile sulfides,very poorly crystalline Fe(hydr)oxides and Poorly crystalline Fe(hydr)oxides decreased significantly in SY and DY soils,whereas the Sulfides fraction increased significantly.XANES results showed that proportion of As(Ⅲ)in solid phase was only 19%,much less than other soils.Besides,As mobilization in porewater was found to correlate with the content of ammonium oxalate extractable As,suggesting that As sorbed on amorphous iron(oxy)hydroxides represents a potential mobilizable pool of As under anaerobic conditions and combined the sulfate reduction could alleviate the As toxicity.Furthermore,the indigenous manganese oxides also play an important role in influencing the mobilization of As in paddy soil.(2).We chose two soils with similar total arsenic content as that in ZP soil and investigated the effects of addition of synthetic manganese oxides(mainly Hausmannite,1200 mg Mn kg-1 soil)on As mobilization.The results show that addition of Mn oxides delayed the decrease of soil Eh and increased the porewater Mn content.However,the porewater iron content and pH were not influenced significantly by the addition of Mn oxides.Addition of Mn oxides significantly reduced porewater As(Ⅲ)concentration,but increased As(Ⅴ)concentration.In addition,addition of Mn oxides significantly decreased As accumulation in both grain and straw in a pot experiment.(3).A strain of manganese oxidizing bacteria(MOB)FJ-6 was isolated from ZP soil.Strain FJ-6 belongs to Bacillus and has a strong manganese oxidation ability.In the L2 medium with an initial Mn(Ⅱ)concentration of 2 mM and a pH of 7.0,48.7±5.0%of Mn(Ⅱ)was oxidized by strain FJ-6 within x hours.XRD and XPS analysis showed that the manganese oxidation product produced by FJ-6 was mainly Mn(Ⅲ)and Mn(IV)mixture In addition,the manganese oxidation product had a strong oxidation and adsorption function for As(Ⅲ),and could remove more than 90%of As(Ⅲ)in the medium,but only 33.2±2.9%of As(Ⅲ)was oxidized to As(V).(4).We carried out soil incubation experiments and greenhouse pot experiment with SY and DY soils.The results show that additions of sulfate significantly reduced As(Ⅲ)and iron content in soil porewater.Sequential extraction showed that the arsenic content in the solid-phase F4(Acid volatile sulfides,very poorly crystalline Fe and A1(hydr)oxides)and F5(Poorly crystalline Fe and A1(hydr)oxides)increased significantly.The effects of sulfate additions were driven by sulfate-reducing bacteria(SRB),because the effect was completely abolished when molybdate,a SRB inhibitor,was added to the soils.At the same time,additions of sulfate significantly increased the abundance of SRB and changed its community structure.However,pot experiments showed no significant effect of sulfate additions on As accumulation in rice.In summary,the concentration of arsenic under flooding conditions is different from regions.Except iron oxides,soluble organic matter,Eh and other common influencing factors,this study confirmed that manganese oxides and sulfate reduction also can reduce the mobility and bioavailability of arsenic in paddy soils,and can effectively reduce arsenic accumulation in rice grains.The main mechanism for Mn oxides reduce the As bioavailability is oxidation of As(Ⅲ)to As(Ⅴ).Adsorption and copreciption maybe the main mechanism of sulfate reduction to allevate the As bioavailability in paddy soils.