The effectiveness of electro-remediation of aged, metal-contaminated sediment in relation to sequential extraction of metals

Abstract

Soil pollution is a universal environmental issue, and the clean-up of contaminated soils can be costly and time consuming. Traditional methods often do not provide an effective solution when it comes to fine-grained and highly impermeable soils and/or immobile contaminants. Electro-remediation, however, is a promising technique. The aim of this study was to relate electrokinetic removal of heavy metals from a contaminated, aged sediment sludge to the metal speciation in the sludge. Metal speciation was determined by a Tessier-based sequential extraction method. Two electro-remediation experiments were conducted in the laboratory in order to investigate removal of metals. The studies were done in a cell designed for a predictive laboratory test of feasibility of electro-remediation of soil. Maximum removal percentages during the experiment using a constant current of 1 mA/cm2 over ∼180 h with a corresponding potential gradient of 0.5 V/cm were larger than 15 % for Cd (21 %), Cu (15 %), and Zn (23 %); Ni and Pb showed removal less than 10 %; Cr demonstrated no removal. Removal of non-contaminant metals Mn and Ca exceeded that of heavy metals. The removal rates were compared to metal speciation in the non-treated sludge, measured by the sequential extraction. It was observed that the electrokinetic removal did not directly relate to the metal distribution over the fractions within the sludge. The aging of metals and other geochemical characteristics of the contaminated sludge affected the removal of heavy metals. Therefore, sequential extraction alone does not provide a predictive tool for the effectiveness of electro-remediation. Additional geochemical information is needed.