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

Abstract

S.M., 2022. Bioavailability of heavy metals in the soil of Doustbaglu region (Meshginshahr) and determining toxic species by sequential extraction and Visual Minteq software. Environmental Sciences. 20(1): 41-56. 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 (SO 4 ) 22- , Pb 2+ and PbSO 4(aq) ; copper as CuSO 4(aq) and Cu 2+ ; nickel as NiSO 4 (aq) , Ni 2+ and NiSO 4 ; antimony as Sb(OH) 3 , Sb(OH) 2+ and Sb(OH) 61- ; zinc as Zn(SO 4 ) 22- , ZnSO 4(aq) and Zn 2+ ; arsenic as H 3 AsO 3 and H 2 AsO 4- ; cadmium as Cd(SO 4 ) 22- , and Cd 2+ . The predominant species of chromium were CrSO 4+ , CrOHSO 4(aq) , and HCrO 4- . 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: Cd 2+ (1.49%), CrOHSO 4(aq) (25.20%), Cu 2+ (10/38%), Pb 2+ (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.