Bioavailability of heavy metals in soils: definitions and practical implementation—a critical review

• Published in: Environmental geochemistry and health Vol. 37; no. 6; pp. 1041 - 1061
• Main Authors: Kim, Rog-Young, Yoon, Jeong-Ki, Kim, Tae-Seung, Yang, Jae E, Owens, Gary, Kim, Kwon-Rae
• Format: Journal Article
• Language: English
• Published: Dordrecht Springer Netherlands 01-12-2015; Springer Nature B.V
• Subjects: Adsorption; Bioavailability; cadmium; calcium chloride; chromium; Contaminants; Contaminated sediments; Copper; Earth and Environmental Science; Environment; Environmental Chemistry; Environmental Health; Geochemistry; Heavy metals; Hydrogen-Ion Concentration; Land pollution; lead; Metals, Heavy - chemistry; Metals, Heavy - metabolism; Models, Theoretical; nickel; nitric acid; Original Paper; Plants - metabolism; polluted soils; Pore water; Public Health; Remediation; Risk assessment; Soil - chemistry; Soil contamination; Soil Pollutants - chemistry; Soil Pollutants - metabolism; Soil Science & Conservation; Solubility; Terrestrial Pollution; zinc; Mobility; Bioaccessibility; Specific adsorption; Complex formation; Phytoavailability; Free metal ion activity
• ISSN: 0269-4042; 1573-2983
• DOI: 10.1007/s10653-015-9695-y

Worldwide regulatory frameworks for the assessment and remediation of contaminated soils have moved towards a risk-based approach, taking contaminant bioavailability into consideration. However, there is much debate on the precise definition of bioavailability and on the standardization of methods for the measurement of bioavailability so that it can be reliably applied as a tool for risk assessment. Therefore, in this paper, we reviewed the existing definitions of heavy metal bioavailability in relation to plant uptake (phytoavailability), in order to better understand both the conceptual and operational aspects of bioavailability. The related concepts of specific and non-specific adsorption, as well as complex formation and organic ligand affinity were also intensively discussed to explain the variations of heavy metal solubility and mobility in soils. Further, the most frequently used methods to measure bioavailable metal soil fractions based on both chemical extractions and mechanistic geochemical models were reviewed. For relatively highly mobile metals (Cd, Ni, and Zn), a neutral salt solution such as 0.01 M CaCl₂ or 1 M NH₄NO₃ was recommended, whereas a strong acid or chelating solution such as 0.43 M HNO₃ or 0.05 M DTPA was recommended for strongly soil-adsorbed and less mobile metals (Cu, Cr, and Pb). While methods which assessed the free metal ion activity in the pore water such as DGT and DMT or WHAM/Model VI, NICA-Donnan model, and TBLM are advantageous for providing a more direct measure of bioavailability, few of these models have to date been properly validated.