淹水-落干循环驱动下土壤砷结合形态的动态转化

Dynamic Transformation in Arsenic Fractions Driven by Flood-drying Cycles in Soils

外源砷在土壤中的毒性/迁移性主要受控于其结合形态,探明土壤砷结合形态的动态变化是准确评价其生态风险的前提。通过室内土柱淹水-落干循环模拟野外土壤氧化还原环境的交替性转变,利用连续提取技术分析了土壤砷各结合形态的变化规律及相互转变过程,结果表明:土柱的淹水-落干循环致使不同层位土壤氧化还原电位值(oxidation reduction potential,ORP)在-114~615 mV范围内振荡变化并驱动着土壤氧化环境的交替变化。此外,ORP的振荡变化提高了与砷迁移性密切相关的氧化铁晶形化。在120 d的试验周期内,非专性吸附态和残渣态砷含量总体减少,而非晶质和晶质氧化物结合态砷含量增加。水溶性砷在水动力作用下向上层土壤迁移,导致非专性吸附态砷含量表现为上层土壤>底层土壤,其他形态则相反。微生物对土壤矿物的氧化/还原性溶解、微孔扩散、氧化物的吸附包裹等可能是土壤砷结合形态相互转化的主要驱动机制。对比特定氧化/还原条件,氧化还原条件的交替转变明显延缓了土壤砷的“老化”并使其在各形态的(重)分配变得更为复杂。

The migration/toxicity of exogenous arsenic in soil is mainly controlled by its fractions.So,understanding the dynamic change in arsenic fractions of soil is a prerequisite for the accurate evaluation of its ecological risk.The alternating transformation of soil redox environment in the field was simulated by the cycle of flooding and drying of columns in the study,and the sequential extraction procedure was used to investigate the changes and transformation processes of arsenic fractions in soil.The results show that the redox potential(ORP)is oscillating at the range of-114~615 mV in different depth of soil,which drives alternating change of redox environment in soils,resulting from the flood-drying cycle of columns.Moreover,the crystallization of iron oxide is improved on account of the oscillating ORP values,which is closely related to arsenic migration.The contents of the non-specifically bound and residual arsenic are decreased,while the contents of amorphous and crystalline oxide bound arsenic are increased during the 120 days.The watersoluble arsenic is moved the upper soil by the hydrodynamic force,resulting in that the content of arsenic in the nonspecific fraction showing upper soil>subsoil,while the other fractions perform oppositely.The oxidation/reducing dissolution of soil minerals by microorganisms,micropore diffusion,adsorption and encapsulation of oxides may be the main driving mechanisms for the interconversion of arsenic fractions in soil.It is concluded that the alternating of redox condition significantly delays the aging of arsenic in soil,which complicates distribution of arsenic in different fractions,compared to the specific oxidation/reduction conditions.