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

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 (Igeo) 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 Igeo 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.