外源氧化剂介导类Fenton效应对稻田砷环境化学行为的影响及其机理研究

Recent studies have shown that rice crop could accumulate arsenic into its straw and grains much easier than other crops. Therefore, scientists pay a high degree of attention to arsenic contamination in rice.It was found that rainfeed rice or periodical flood and dry irrigation schem can dramatically decrease the arsenic absorbtion from soil to rice. However, its mechanism has not been well explored. Hydrogen peroxide （H2O2）existes in common rainwater and may come into contact with Fe(Ⅱ)in paddy soil. The Fenton or Fenton-like chain reactions can take place and generate free radicals whice are even more powerful than H2O2 in terms of oxidative capacity.The objective of this proposal is to discover the role of oxidation from H2O2 or chlorate at micromolar concentration in environment, the effects on environmental behavior and bioavailability change of As in rice paddy.Field investigation and greenhouse experiment will be carried out to examine the above hypothesis. Compartmented rhizo-bag culture systems will be used to study the effects of H2O2 or chlorate when they contact with Fe(Ⅱ) in the oxidized iron sheet around root surface, and check the microbially mediated oxidation process, arsenie fractions in rhizosphere and its bioavailability to rice. The soil chemical/biochemical/microbiological parameters in rice rhizosphere compartments and rhizosphere-soil, root surface, soil pore water and grain As will be measured and quantified. Microbial population in rhizosphere soil and root-surface will be characterized. The mechanism of the relationship of oxidation and arsenic mobility in rhizosphere and mediated by microbial activity will be studied. The results will provide a theoretical basis for management practices to reduce the As concentration in rice.

砷污染稻田和水稻易富集砷及由之引起稻米砷污染的大众关注度日益增加。已有研究发现间歇灌溉或"雨养"水稻能减少砷的吸收积累,而稻田有可能存在由微摩尔级浓度天然过氧化氢在亚铁离子催化作用下产生强氧化活性氧自由基的类Feton效应,导致水稻根际环境和根际微生物的变化,从而影响砷的形态与生物有效性,这可能是稻田砷环境行为被学术界忽视而又可供防控稻米砷污染应用的重要机制之一。本项目结合田间实验,并主要采用能隔离根区的根箱盆栽方法，通过加入含微摩尔浓度过氧化氢模拟雨水或低浓度人工氧化剂（氯酸钾）,加入从水稻土富集得到的铁异化还原菌群,模拟砷污染稻田生态系统,观测稻田土- - 水稻体系中As的环境化学行为,明确类Fenton 反应对水稻根际As与铁氧化膜、根际微环境的生物氧化过程、铁砷交互作用与生物有效性等各种因素影响的规律，为阐明和筛选出稻田降低砷生物累积的科学机理和有效途径提供科学依据。

本项目通过室内模拟实验、温室盆栽及水泥池小区试验相结合的方法，系统研究了微摩尔浓度级过氧化氢介导Fenton效应对水体中砷的沉淀去除效果、对稻田砷污染胁迫下水稻生长、砷吸收及稻田砷的生物地球化学行为的影响，并从防控稻米砷污染角度出发，揭示其潜在重要机制及应用前景；研究了水分管理与H2O2等外源氧化剂输入稻田-水稻系统中砷镉的迁移转化的影响；探讨了过氧化氢介导Fenton效应对典型除草剂降解的影响。主要研究结果如下：. 微摩尔浓度级H2O2介导Fenton效应显著影响了稻田无机砷的环境化学行为。其介导的Fenton效应促进了水体中砷的转化与沉淀去除，且随着外源H2O2与Fe2+配比浓度的增加而作用增强，但水中高浓度腐殖酸HA降低了其对水体砷的去除能力。Fenton效应对水稻根表铁膜形成及其根表铁膜富集形态砷有显著影响，显著减缓了砷对各生育期水稻的毒害作用，促进了水稻生长，并在一定程度上减少了水稻地上部砷的累积，显著降低了稻米中总砷含量。Fenton效应显著提高了土壤微生物群落多样性与土壤铁砷相关菌群的相对丰度，这是影响无机砷在土壤-微生物-水稻间环境行为的重要机制。. 干湿交潜水分管理及模拟降雨 H2O2介导Fenton效应显著影响砷镉复合污染土壤中砷镉在土壤-水稻体系中迁移转化。无论淹水还是干湿处理组，在模拟降雨中80 μM H2O2处理的稻田中米粒及谷壳中砷含量均显著低于对照组，且干湿交替处理组均显著低于淹水处理组，籽粒中镉的含量总体呈增加的趋势。镉浓度为0.3 mg/kg的处理，淹水条件下，Fenton效应的处理其稻米中镉的含量显著高于同浓度的其它处理。. 项目还研究了天然降水中过氧化氢介导Fenton效应对稻田砷的甲基化及砷挥发的影响，并就天然降水中过氧化氢介导Fenton效应对典型除草剂环境行为进行了研究，结果表明该过程对砷挥发和除草剂的促降解作用明显，这为认识和丰富天然水中H2O2等外源氧化剂进入农田影响除草剂和类似有机污染物环境行为与修复机制具有重要意义。