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Remediation of Heavy Metal(loid)s Contaminated Soils – To Mobilize or To Immobilize?
Bolan, N., A. Kunhikrishnan, R. Thangarajan, J. Kumpiene, J. Park, T. Makino, M. Kirkham, AND K. Scheckel. Remediation of Heavy Metal(loid)s Contaminated Soils – To Mobilize or To Immobilize? D. Aga, W. Choi, A. Daugulis, G. Li Puma, G. Lyberatos, and J. Hwa Tay (ed.), JOURNAL OF HAZARDOUS MATERIALS. Elsevier B.V., Amsterdam, Netherlands, 266:141-166, (2014).
With greater public awareness of the implications of contaminated soils on human and animal health there has been increasing interest among the scientific community in the development of technologies to remediate contaminated sites. This is especially necessary since traditional methods of soil removal and replacement of clean soil is often cost prohibitive. For diffuse distribution of metal(loid)s, remediation options generally include amelioration of soils to minimise the metal(loid) bioavailability. Bioavailability can be minimised through chemical and biological immobilization of metal(loid)s using a range of inorganic compounds, such as lime and phosphate (P) compounds , and organic compounds, such as ‘exceptional quality’ biosolids. The more localized metal(loid) contamination found in urban environments is remediated by metal(loid) mobilization processes that include bioremediation (including phytoremediation) and chemical washing. Removal of metal(loid)s through phytoextraction (i.e., phytoremediation) techniques and the subsequent recovery of the metal(loid)s or their safe disposal are attracting research and commercial interests. However, when it is not possible to remove the metal(loid)s from the contaminated sites by phytoextraction, other viable options, such as in-situ immobilization should be considered as an integral part of risk management. An overview of the sources of some of the common heavy metal(loid)s input to soils, their interactions and bioavailability in soils, and the remediation of metal(loid) contaminated soils through manipulating their bioavailability using a range of conventional and advanced soil amendments will be presented in this review.
Unlike organic contaminants, metal(loid)s do not undergo microbial or chemical degradation and persist for a long time after their introduction. Bioavailability of metal(loid)s plays a vital role in the remediation of contaminated soils. In this review, the remediation of heavy metal(loid)s contaminated soils through manipulating their bioavailability using a range of soil amendments will be presented. Mobilizing amendments such as chelating and desorbing agents increase the bioavailability and mobility of metal(loid)s. Immobilizing amendments such as precipitating agents and sorbent materials decrease the bioavailabilty and mobility of metal(loid)s. Mobilizing agents can be used to enhance the removal of heavy metal(loid)s through plant uptake and soil washing. Immobilizing agents can be used to reduce the transfer to metal(loid)s to food chain via plant uptake and leaching to groundwater. One of the major limitations of mobilizing technique is susceptibility to leaching of the mobilized heavy metal(loid)s in the absence of active plant uptake. Similarly, in the case of the immobilization technique the long-term stability of the immobilized heavy metal(loid)s needs to be monitored.