缺氧条件下土壤砷的形态转化与环境行为研究

Speciation Transformation and Behavior of Arsenic in Soils Under Anoxic Conditions

采集张士污灌区0～100 cm深的土壤并在实验室里负载低浓度的砷,采用不加硫和加硫对比研究了厌氧条件下土著微生物对土壤中砷的形态转化、环境行为影响及其机制.结果表明,在不外加硫酸盐条件下厌氧培养8 d后,微生物还原作用造成砷的大量还原和释放,释放的砷70%以上是以As(Ⅲ)形式存在,尤其20～40 cm深度土壤砷的释放量明显高于其它层土壤,As(Ⅲ)和As(T)分别达到892.8μg.L-1和1 240.6μg.L-1.与非生物对照相比每层土中盐酸可提取的砷总量都大大降低,且盐酸提取的As(T)几乎全部转化为As(Ⅲ).伴随砷的释放,铁发生还原和释放,溶解态的亚铁基本都在40 mg.L-1以上,不同土层固相中亚铁离子的量都在9.0～13.4 g.kg-1范围内,固相盐酸可提取态总铁中亚铁离子所占的比例基本都在50%以上,说明微生物还原作用造成固相中铁氧化物发生还原性溶解和矿物结构转化.当体系中添加10 mmol.L-1的硫酸盐时,每层土的生物培养体系中铁的释放几乎完全被抑制,砷和铁浓度也减少了50%.与不加硫生物培养体系相比,固相中盐酸可提取的砷量减少了50%,一部分砷被转化为稳定的硫化物As2S3而固定.可见在硫酸盐不足条件下微生物还原作用可造成砷被还原、活化和释放,而补充土壤中硫酸盐的量可促使微生物还原/活化的砷转化成更加稳定的形态,稳定的硫化物矿物As2S3是土壤微生物固定砷的重要途径.

The soil of 0-100 cm depth was collected from the wastewater-irrigated farmland in Zhangshi, Shenyang and was amended with low concentration of arsenic. Microbial-mediated speciation transformation and environmental behavior of arsenic were investigated with and without addition of sulfate. The results showed that without addition of sulfate, arsenate was significantly reduced and released into soil solution after eight days of anaerobic incubation. Above 70% of arsenic presented as arsenite. More arsenic was released from the soil of 20-40 cm depth with arsenite and total arsenicconcentration of 892.8 μg·L-1 and 1240.6 μg·L-1 respectively. Compared with abiotic control, the amount of arsenic dissolved in hydrochloric acid decreased greatly in each layer of soil, moreover, almost all of arsenic was reduced to arsenite. Ferric iron was also significantly reduced to ferrous and released into soil solution simultaneously. The concentration of ferrous iron in soil solution was above 40 mg·L-1 in solution and was 9.0-13.4 g·kg-1 in soil solid. Above 50% of the iron dissolved in hydrochloric acid was presented as ferrous.This indicates that microbial reduction leads to reductive dissolution of iron oxides and transformation of mineral configuration in soil solid. The release of arsenic and iron was notably suppressed after addition of 10 mmol·L-1 sulfate, with the concentration reduced by 50%. The amount of HCl-dissolvable arsenic in soil solid decreased by 50%, compared to the treatment without sulfate addition, which probably due to precipitation of arsenic sulfide. It is obvious that microbial reduction leads to reduction and release of arsenic when sulfate is absent. In the presence of sulfate, microbes may transform mobile arsenic into more stable species. Formation of arsenic sulfide mineral was probably the mechanism for arsenic fixation in soil by microbes.