Bioavailability of heavy metals in the soil of Doustbaglu region (Meshginshahr) and determining toxic species by sequential extraction and Visual Minteq software

Introduction: Oxidation of sulfide mineralized zones during the weathering processes is intensified by biological and chemical reactions and the resulting Acid Mine Drainage (AMD) causes the release and mobility of toxic and heavy metals from the parent rock and their concentration in soil or water. In this study, soil samples taken from the surroundings of the village of Doustbaglu (northwest of Meshginshahr), which is considered a typical mineralization and alteration area, were studied and chemical species, toxicity, and origin of heavy elements were determined.
Material and methods: In this study, total concentration and bioavailability of heavy elements i.e. As, Cd, Cu, Cr, Pb, Sb, Ni, and Zn in five surface soil samples were evaluated by Tessier sequential extraction method in five phases (exchangeable, connected to carbonate, bound to iron and manganese oxides, bound to organic matter, and residual phase) and using the Visual Minteq thermodynamic software.
Results and discussion: The results of the sequential extraction method showed that the highest concentration of the total concentration of all studied heavy elements was retained in the residual fraction (stabilized in the mineral structure). This indicates the geogenic origin of these elements and can be considered the result of erosion and weathering of rocks in the region. Compared to other elements, Sb had a higher concentration in potentially available fractions (e.g. exchangeable, carbonate-bound, bound to Fe-and Mn-oxides, and/or organic matter) and can be readily available to plants and toxic. The software output delineates that the predominant species in the examined samples were lead as Pb (SO4)₂^2-, Pb²⁺ and PbSO_4(aq); copper as CuSO_{4 (aq)} and Cu²⁺; nickel as NiSO_{4 (aq)}, Ni²⁺ and NiSO₄; antimony as Sb(OH)₃, Sb(OH)²⁺ and Sb(OH)₆^1-; zinc as Zn(SO4)₂^2-, ZnSO_4(aq) and Zn²⁺; arsenic as H₃AsO₃ and H₂As_O4^-; cadmium as Cd(SO₄)₂₂^-, and Cd²⁺. The predominant species of chromium were CrSO4⁺, CrOHSO_4(aq), and HCrO₄^-. In general, the free water-soluble species of these elements were more mobile than other species; instead, the concentration of these species was very low relatively, and most of these elements were more present in the form of complexes with low mobility.
Conclusion: Based on sequential extraction results, all studied elements showed high ecological risk potential and significant pollution in the sediment of waterways and surface soil horizons of the Doustbaglu area. Analysis of the findings of Visual Minteq software indicates that the most active types of elements and related concentrations, among all possible types, include: Cd²⁺(1.49%), CrOHSO_4(aq)(25.20%), Cu²⁺(10/38%), Pb²⁺(1/37%), ZnSO_4(aq) (18.83%), respectively. Since more mobile species have low concentrations and on the other hand, according to the results of sequential extraction, most of the studied elements are present in the remaining phase, so the bioavailability and toxicity of these elements are estimated to be negligible. In general, it can be concluded that only a small percentage of elements are present in bioavailable fractions, and this can alleviate concerns about the possibility of element release by changing environmental conditions and thus accessibility to plants.