Transformation of heavy metal fraction distribution in contaminated river sediment treated by chemical-enhanced washing

Abstract

Mobility and toxicity of heavy metals in river sediments seldom relate to their total concentrations, but to a greater extent depend on their distribution of speciation fractions. Ex situ washing of dredged sediment can effectively reduce the total metal contents, but it may transform metals into more mobile species. This study investigated the changes of the metal fraction distribution and the mobility risk of chemical washed river sediment. Batch chemical-enhanced washing experiments with different agents, including inorganic acids and salt solutions, chelating agents, and oxidizing agents, were conducted on the heavy metal-contaminated sediment collected from Shenzhen River in South China. The removal of Cr, Cu, Pb, and Zn from the sediment was evaluated, and the corresponding changes of metal fraction distribution were examined by using a sequential extraction procedure. The mobility of the remaining metals in the washed sediment was assessed by leaching extractability and compared to those in the original sediment. The results showed that inorganic acid washing effectively removed the majority of heavy metals, but the transformation of Cr, Cu, and Pb from the reducible fraction to the acid-extractable fraction significantly enhanced the mobility of the remaining metals in the washed sediment. Chelating agents such as EDTA and EDDS could efficiently extract Pb and Zn from the sediment, but to a less extent with Cu and Cr. Chelating agents also reduced the relative portions of Cr, Cu, and Pb associated with reducible fraction. Cu and Cr extraction would require the use of oxidizing agents such as H2O2 and Na2S2O8 that reduced their relative portions associated with the oxidizable fraction. Citric acid was only effective for Cr removal through dissolution of Fe oxides/hydroxides into which Cr(III) incorporates. The feasibility of acid washing for sediment remediation should be reconsidered due to high mobility of residual metals, while H2O2 could be a promising additive for sediment remediation. However, a balance should be established between removing labile metal species and minimizing metal transformation.