焦化废水处理过程外排污泥中重金属形态特征及其潜在环境风险

焦化废水处理过程所排放污泥中重金属的含量及化学形态是否构成环境风险将直接影响污泥处置方法的选择,为此,实验采用BCR顺序提取法分析了焦化废水处理站外排污泥中重金属(Cd、Hg、Pb、Cr、As、Ni、Zn、Cu和Mn)的形态特征,并采用地累积指数(Igeo)和潜在生态危害指数(RI)评价了重金属对土壤的潜在环境风险。研究结果表明:除Ni主要以可氧化态存在外,焦化废水外排污泥中其他几种重金属元素主要存在于残渣态,重金属元素的含量低于《城镇污水处理厂污染物排放标准(GB18918—2002)》中的控制限值;与城市污泥相比,焦化废水外排污泥具有低Pb、Cr、Zn、Cu含量,而高Cd、Hg、Mn含量的特点;基于Igeo和RI的评价结果,Cd和Hg是外排污泥中具有一定环境风险的元素,需要考虑其下游去向。焦化废水处理外排污泥中主要存在残渣态重金属成分,不表现为很高的环境风险,其处置应重点考虑其中有机污染物特别是POPs。

States distribution of heavy metals in coking wastewater sludge and its potential environmental risks

The selection of disposal methods for the sludge from a coking wastewater treatment plant is directly dependent on whether the concentrations and chemical forms of heavy metals contained in the sludge have environmental risks. Therefore, sludge samples from the coking wastewater treatment plant were analyzed to determine the chemical forms of heavy metals (Cd, Hg, Pb, Cr, As, Ni, Zn,Cu and Mn) using the BCR (European Community Bureau of Reference) sequential extraction procedure. The index of geo-accumulation (I_geo) and potential ecological risk (RI) were used to evaluate their potential environmental risks to the soil environment. The results showed that the heavy metals including Cd, Hg, Pb, Cr, As, Ni, Cu and Mn were mainly in residual form, and Ni was mainly in oxidation fraction. The concentrations of these heavy metals in the sludge were lower than the levels recorded in the Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant (GB18918-2002). The concentrations of Pb, Cr, Zn and Cu were lower, but the concentrations of Cd, Hg and Mn were higher as compared to those in relation to the heavy metals in the municipal sludge. The results of I_geo and RI for Cr and Hg suggested that they posed certain environmental risks, and thus concerns should be raised when considering their downstream migration. It can be concluded that the environmental risks of the sludge in the coking wastewater treatment plant are not high because the heavy metals existed are mainly in residual form. Therefore, more attention should be paid to the distribution and environmental risk assessment of organic pollutants especially POPs contained in the coking wastewater sludge when selecting the methods for its disposal.