铬是一种银白色的坚硬金属.主要以金属铬、三价铬和六价铬3种形式存在.所有铬的化合物都有毒性,其中六价铬(Cr(VI))毒性最大.我国河流中的Cr(VI)绝大多数是由铬盐、铁合金、皮革、电镀等生产工厂将没有处理的废液和废渣直接排入河流的,对流域亲水岸生态环境造成极大的破坏.
湘江是流经长株潭的第一大河流,流域范围内有近4 000万人口.其下游长株潭段,位于东经111°53′~114°15′,北纬27°51′~28°41′,四季分明,天然生态环境良好.该河段自20世纪50年代开始,沿岸逐步建有涉铬钢铁、冶炼、化工、轻工、机械等行业的重点废水污染源507个.每年排放工业含Cr(VI)废水约284万t.以长沙河西铬盐厂为例,每年将约15万t含Cr(VI)废水直接排入湘江.资料显示,2002年该铬盐厂排放的废水中Cr(VI)最高超标值是11.9倍,年均超标4.6倍;2003年的废水中Cr(VI)含量为5.159 mg/L,超标9.3倍.过量的(超过10 mg/L)Cr(VI)对水生动植物有致死作用[1].Cr(VI)废水迁移对河流和亲水岸环境造成摧残性的破坏,人类和动物长期饮用和接触含Cr(VI)的废水,体内的组织和器官细胞会发生器质性病变.因此,开展对湘江长株潭段Cr(VI)废水污染对生态环境影响的研究,对治理湘江污染、建设两型社会提供有效参考.
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