水环境中天然有机质对砷光化学行为的影响机制研究

Arsenic pollution in water, particularly in drinking water causes great damages to environment and human health. Study on arsenic photochemical transformation is an important foundation of assessment of arsenic pollution in water, prediction of its final fate and optimization of drinking water treatment. However, there is a lack of relevant cognition about arsenic transformation in the photoreactive system which is triggered by NOM. This work plans to investigate on arsenic speciation transformation which initiated by NOM through photoreaction and its influence factors and mechanism. Mechanisms of arsenic photoreaction are explored from the point of compounds of humus-arsenic and compounds of humus-iron(oxide)-arsenic. Arsenic-contaminated water will be sampled from reservoir water of high-arsenic coal district in Xingren county, Guizhou province. Simulation experiments will be carried on and photo-transformation of arsenic in natural water will be confirmed. The results will help to understand the speciation and environmental behavior of arsenic, gain insight into the effect law of NOM on the speciation transformation of arsenic and reveal the contribution of photochemical reaction to the abiotic transformation and remediation of arsenic-contamination. Research on the transformation of arsenite to arsenate might be a positive contribution to the assessment and remediation of arsenic pollution as well as optimization of drinking water treatment.

水体砷污染，尤其是饮用水源的砷污染危害环境与人体健康。水体中砷的光化学转化过程的研究是评估水体砷污染状况、预测砷的最终归趋及优化饮用水处理的重要基础。但是目前对天然水体中以天然有机质为中心的光活性体系对砷的化学转化缺乏相关的认识。本项目拟研究天然有机质引发的砷形态光化学转化过程、影响因素与机制，从腐殖质-砷、腐殖质-铁(氧化物)-砷的复合物角度对砷的光化学转化机制进行研究，并以砷污染严重的贵州省兴仁县高砷煤矿区水库水体为研究对象进行模拟实验，确定天然水体中砷的光化学转化过程。本项目可以深化人们对天然水体中砷的化学行为的认识，深入了解天然有机质对砷形态转化的影响规律，揭示光化学反应对水体中砷的非生物转化和污染修复的贡献。尤其是对毒性和运动性都相对较强的三价砷转化为低毒性、弱运动性的五价砷的研究对评估水体中砷污染和修复，以及优化饮用水处理具有积极的作用。

该项目以三价砷(As(III))为目标污染物，主要研究了单独腐殖质（腐殖酸和富里酸）体系以及含铁腐殖酸体系中As(III)的光化学氧化动力学和机理。研究发现，在紫外灯(λmax = 365 nm)照射下单独腐殖酸体系中As(III)的光氧化过程均符合准一级动力学。As(III)的光氧化速率随着pH值的升高而迅速降低。主要原因是在低pH条件下腐殖质吸收光辐射产生的氧化As(III)的活性氧化物种高于高pH条件。不同分子量范围的腐殖质对As(III)的光氧化能力不同，这主要与腐殖酸中酚基含量成正相关。ESR实验表明，腐殖酸(富里酸)在光照条件下能够产生∙OH、∙O2-和1O2，腐殖质可以作为As(III)光氧化的光敏剂。通过自由基捕获实验发现，As(III)的光氧化主要通过三重态腐殖质和∙OH的氧化作用实现。在UV-365nm照射下，在低pH时外加Fe(III)对As(III)在腐殖质体系中的光氧化表现出促进或抑制作用。加入低浓度的Fe(III)时，Fe(III)的主要以铁-腐殖质二元络合物的形式存在，该二元络合物仅具有弱的光活性或无光活性，从而抑制了As(III)的光氧化。加入高浓度的Fe(III)则有利于As(III)的氧化， Fe(III)以高光活性的Fe(OH)2+络合物形式存在，促进了As(III)的氧化。另外在UV-365nm照射下的单独铁氧化物体系中As(III)的去除主要通过空穴和表面羟基的氧化和吸附去除。在铁氧化物体系中加入小分子羧酸后，促进了液相中活性氧化物的生成量，从而有利于As(III)的去除。