Speciation And Electrokinetic Remediation Of Heavy Metals In Municipal Sludge

The presence of heavy metals in municipal sludge restricts its use for agricultural purposes. In this paper, the bioavailability and eco-toxicity of heavy metals in municipal sludge was evaluated, taking into consideration both the speciation of metals and the local environmental characteristics. The dewatered municipal sludge samples were collected from five sewage plants in Changsha, Zhuzhou and Xiangtan respectively, which are representative cities with characteristics of the middle-south region of China. Some agricultural significant parameters and total metal concentrations in the sludge were determined, and the metal speciation was studied by using BCR sequential extraction procedure. The effects of controlling acidity of cathode pond solution and adding complex agents on Cd electrokinetic processes in actual sewage sludge were investigated. It was found experimentally that:In general the municipal sludge collected from the five sewage plants was rich in organics, N and P. Except that the sludge from Xia Wan Sewage Treatment Plant showed higher concentrations of heavy metals, the sludge from other plants all showed a low total content of heavy metals with only Cd slightly exceeding the permitted values of the national application standard of acid soil in China (GB18918-2002).The sequential extraction results showed that Cu and Zn were principally distributed in the oxidizable fraction, which meant a high potential toxicity, but the bioavailability of Zn might be overestimated to the soil of Hunan. Pb was mainly in the residual fraction. The distribution of Cd showed no obvious characteristic. The electrokinetic remediation results showed that adding EDTA and citric acid not only changed the quantity of current and electroosmotic flow, but also affected the speciation of Cd significantly: The fraction of Cd which was immobile and strongly bound was transformed to mobile and weakly bound fraction, and was moved out of sewage sludge by electrokinetic action. Therefore, the removal efficiency of electrokinetic processes was improved and the maximum reached 70.1%.The promoting effect of EDTA and citric acid to electrokinetic processes mainly depended on the acidity of system, in which the removal ratios was 66.4%, 7.1%, 62.3%, 66.5% respectively at pH 4 and 8, near 2~3 times higher those at pH 6. Compared with citric acid, EDTA was more efficient under high pH condition, due to the stronger ability of complexation, and citric acid had a wider pH range of applying.