Potential toxic elements in the soil of a closed Metalliferous Ore Mine

Abstract

Potential Toxic Elements (PTEs) contamination is a serious environmental concern in soils and sediments. In many ways, mining activities contribute negatively to the environment and its ecosystems with PTE pollution extending far beyond the seizure of mining activity. Some of the negative impact of mining industry includes formation of sinkholes, contamination of groundwater, surface water and as for the scope of this research study, PTE contamination of soils. Understanding PTE bioavailability and mobility in the contaminated soil near mining activity has become a public concern, and research towards the minimization of the PTEs ecological impact is required, thus the aim of this study, i.e. to evaluate the bioavailability and mobility of PTEs from a closed metalliferous ore mine, deemed to be a potential polluter. The three-stage BCR sequential extraction procedure along with multivariate analysis techniques such as XRF and FTIR were employed to evaluate metal chemical fractions and total concentrations of selected PTEs, such as Aluminium (Al), Chromium (Cr), Copper (Cu), Iron (Fe), Manganese (Mn) and Zinc (Zn) in the analysed soil samples. The total organic matter and distribution of particle size were investigated and the affinity of organic matter to grain size was highlighted. The results indicated that higher susceptibility and bioavailability of Cu, Zn and Mn with a strong affinity to the exchangeable fraction – carbonate bounded. Substantial amount of Fe was observed in the reducible fraction which may be due to the stable complexes formed and bound to the Fe-Mn oxides. In the oxidizable fractions, Al showed dominance with Fe and Cu also showing the strong affinity to be embedded in organic matter. Cr was generally the least mobile metal as indicated by the lowest concentration in all fractions. Generally, the mobility of PTEs in the sediments was as follows: exchangeable>reducible and oxidizable. The strong affinity of organic matter to attach to fine grain sized soil was observed with a high fine fraction being observed in samples obtained from the source of pollution. Furthermore, downstream from the source of pollution, there was an indication that the soil had lower organic matter content with coarser particle sized soil. The FITR and XRF results showed a strong association of silicates and quartz minerals, with the soil being constituted predominantly by SiO2, Fe2O3, Al2O3 and traces of MgO, Cr2O3 and MnO. A Risk Assessment Code (RAC) was also used as a criterion to quantitatively evaluate the risks associated with the soil samples and PTEs risk to the environment decreased as follows Cu>Al>Fe>Mn>Zn> with the least risk posed by Cr.