The chemical equilibria of metal ions between soil solution and solid phases govern the solubility of metals in soil. However, the identity of these controlling phases remains poorly understood in historically polluted environments. This study aimed to identify the dominant mineral phases regulating the activities of Zn2+, Cd2+, Pb2+, and Ni2+ in soils subjected to long-term contamination from sewage sludge, municipal solid waste, river water, and industrial effluents across India. The soil samples were collected from various locations historically polluted by sewage sludge, municipal solid waste, polluted river water and industrial effluents. The free ion activities of Zn2+ (pZn2+), Cd2+ (pCd2+), Pb2+ (pPb2+) and Ni2+ (pNi2+) in soil pore water were estimated using the geochemical speciation model WHAM-VII. The metal ion activities were higher in industrial effluents and solid waste-treated soils as compared to other contaminated soils. The solubility of Zn and Cd in soils contaminated with Zn-smelter effluents was controlled by franklinite (ZnFe2O4) in equilibrium with goethite (α-FeOOH) and otavite (CdCO3), respectively. Identification of minerals further reveals that nickel ferrite (NiFe2O4) in equilibrium with lepidocrocite (γ-FeOOH) governs the activity of Ni2+ in cycle factory effluent-irrigated soils of Sonepat, Haryana. At the municipal solid waste-contaminated site, the Pb2+ activity was controlled by exchangeable Pb in soils, whereas Zn2+ activity was governed by willemite (Zn2SiO4) in equilibrium with quartz (SiO2). These findings provide new insights into mineralogical controls on heavy metal solubility under diverse contamination scenarios. Formation of highly soluble minerals like otavite, willemite, and nickel ferrite suggested the potential ecological risk of Cd, Zn, and Ni, respectively, in polluted soils.
Golui et al. (2026) studied this question.