河套平原第四纪沉积物中砷的赋存形态分析

Modes of occurrence of arsenic in Quaternary sediments of the Hetao Plain

从水-岩相互作用的观点出发,分析沉积物中砷的含量和存在形态,把握砷如何溶于地下水是研究高砷地下水形成机理的重要视点之一。笔者通过对岩、土样品的分析、测试和不同介质(水、酸、碱)的溶出实验,研究了河套平原第四系沉积物中砷的存在形态,认为河套平原第四系沉积物中的砷主要是以非晶质的胶体形式而存在。沉积物中的砷、铁含量呈明显的正相关关系;X射线仪和电子显微镜装着的EDS分析结果表明,在砷污染区,不同深度、不同岩性的钻探岩心样品中均未发现有砷矿物的存在;岩心中的有机质与砷含量不显示有明显的相关关系,且含量很低;沉积物中的砷很容易被碱性水溶液溶解出,其中对砂质沉积物中砷的溶出率为87.5%～98.75%,对粘土类沉积物中砷的溶出率为72.73%～84.21%,而该溶液对硫铁矿矿石(砷含量为87.30mg/kg)中砷的溶出率仅为0.02%。

One of the important methods for studying the formation mechanism of groundwater with high arsenic concentration is to analyze the content and modes of occurrence of arsenic in sediments and to understand how arsenic is dissolved in groundwater from the viewpoint of water -rock interaction. In this paper, the authors mainly concentrated on the study of the modes of occurrence of arsenic in Quaternary sediments of the Hetao Plain, based on the analyses of rock and soil samples and dissolution tests on different media including water, acid and alkali. The results show that arsenic mainly exists in the form of non -crystalline colloid in Quaternary sediments of the Hetao Plain. In the sediments, a positive correlation between the content of arsenic and that of iron in sediments is obviously observed. The analytical results of EDS equipped with X-ray meter and electronic microscope reveal that no arsenic mineral was found in the samples taken from different depths and different lithologic characters in the arsenic contamination area. There is no apparent correlation between the content of organic matter and that of arsenic in sediments. In addition, the content of organic matter is very low. Arsenic in sediments is relatively easily dissolved in alkaline solution. The dissolved ratios are 87.5%-98.75% for arsenic in sandy sediments and 72.73%-84.21% for arsenic in clay sediments. However, for arsenic in pyrite mineral （with arsenic content being 87.30 mg/kg）, the dissolved ratio is only 0.02%.