Bioavailable Heavy Metal Fractions in Industrially Impacted Soils and the Imperative for Advanced Risk Assessment Models: A Critical Review

Heavy metal contamination of soils, particularly near industrial corridors, represents a pervasive global environmental challenge. Traditional risk assessments, reliant solely on total metal concentration, inherently overlook the critical determinant of metal toxicity: bioavailability. This review synthesizes current understanding of heavy metal speciation and fractionation in soils, focusing on the principles and applications of sequential extraction protocols (e.g., BCR, Tessier) to quantify bioavailable pools. We critically evaluate the limitations of total concentration approaches and demonstrate how fractionation data transforms ecological risk assessment, providing a more realistic estimation of hazard. The paper further explores the interplay of soil parameters (pH, organic matter, clay content) on metal mobility and reviews emerging remediation strategies that target bioavailable fractions. Finally, we identify key research gaps, including the need for standardized fractionation methods and the integration of bioavailability into regulatory frameworks, arguing that a paradigm shift from total to bioavailable metal assessment is essential for accurate risk diagnosis and effective remediation of contaminated sites.