A critical review on advances in remediation of toxic heavy metals contaminated solids by chemical processes

This review was designed to present the state-of-the-art advances in chemical processes to remove Potentially Toxic Elements (PTEs) from contaminated solids including sludge, soil, municipal waste incineration fly ash, sediment and industrial waste. Chelation and acid extraction/chemical leaching are the major chemical processes and the PTEs removal efficiency mainly depends on several factors, such as PTEs speciation and their concentration, presence of chemical compounds, initial pH, and acid solution concentration. As chelants, Ethylenediaminetetraacetic acid (EDTA) and N,N-bis(carboxymethyl) glutamic acid (GLDA) demonstrated the promising performances although EDTA is slightly toxic, persistent and slow degradable. EDTA can be completely recycled by acid leaching/electrodialysis treatment, and thus reclaimed-EDTA showed almost similar PTEs removal efficiency as original-EDTA. The order of PTEs selectivity is Cd≥Pb≥Cu≥Zn≥Ni for soil, Pb>Zn≈Ni>Cd>Cu for sewage sludge by EDTA extraction. Ammonium citrate and citric acid showed effective PTEs removal, and the former is preferred for remediation of solids. Application of solvents including ionic liquid, biosurfactant, organic solvent and supercritical fluid on the remediation of solids are continuously growing. However, the employment of biosurfactant and organic solvent are recommended because they cannot only eliminate PTEs but also improve soil bio-physico-chemical properties. Chemical precipitation of PTEs (such as hydroxide compounds) is often applied to remove PTEs from contaminated solids by adding counter-ions. It has been an industrially established PTEs removal technology for hazardous solid waste; however, single utilization of it is not effective in remediation of solids. Finally, chemical extraction followed by precipitation is recommended for PTEs removal and recovery from contaminated solids.