Bioavailability of heavy metals in topsoil of Doustbaglu area (Meshginshahr) and determining toxic species by sequential extraction and Visual Minteq software

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 around 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 such as As, Cd, Cu, Cr, Pb, Sb, Ni and Zn were determined.

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, bound to carbonate, bound to iron and manganese oxides, bound to organic matter and residual phase) and using of the Visual Minteq thermodynamic software.

The results of the sequential extraction method show that the highest concentration of the total concentration of all 8 heavy elements studied were 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 has 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 (Pb) as Pb (SO4)22-, Pb2+ and PbSO4(aq); copper as CuSO4 (aq) and Cu2+; nickel as NiSO4 (aq), Ni2+ and NiSO4; antimony as Sb(OH)3, Sb(OH)2+ and Sb(OH)61-; zinc as Zn(SO4)22-, ZnSO4(aq) and Zn2+; Arsenic as H3AsO3 and H2AsO4-; Cadmium as Cd(SO4)22-; and Cd2+ The predominant species of chromium were CrSO4+, CrOHSO4(aq) and HCrO4-. In general, the free water-soluble species of these elements are more mobile than other species; instead, the concentration of these species is very low in comparison, and most of these elements are more present in the form of complexes with low mobility.

 Based on sequential extraction results; all 8 studied elements show 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%), CrOHSO4(aq)(25.20%), Cu2+(10/38%), Pb2+(1/37%), ZnSO4(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.