Van Huyssteen, C. W.Hansen, R.Bouwer, Mischke2021-05-132021-05-132020-01http://hdl.handle.net/11660/11062The environmental effects of mine waste recently became of greater interest for environmentalists and researchers in South Africa. Therefore, the rehabilitation of mine waste and tailings and the potential uses of treated mine water became of utter importance. One potential use for treated mine wastewater is as irrigation water for crops. Studies on the possibility of irrigating agricultural fields with treated mine wastewater only focused on the behaviour of metalloid cations present in these waters and the possible contamination of these elements. Consequently, inadequate attention was given to the behaviour and potential contamination of metalloid anions such as Arsenic (As) and Selenium (Se) within these environments. This study discusses the impact of As and Se in water on soil material replicating soils found in the Witwatersrand gold mining area as well as in the Karoo. The study used batch extractions and geochemical surface complexation modelling by means of the double layer diffusion model to determine the adsorption potential of As and Se on two artificially prepared (soil) substrates at four pH values, representing typical soils found in the Witwatersrand and Karoo areas of South Africa. The prepared substrates represented the average montmorillonite clay and goethite Fe-oxide content in topsoils found in the Karoo region and Witwatersrand tailings. The modelled results were finally compared with experimental data replicating the modelled conditions. The adsorption of both As (as As5+, AsO4-3 species) and Se (as Se6+, SeO4-2 species) in these substrates decreased with an increase in pH, increasing the possibility of leaching to and contamination of groundwater systems in both mine tailings and irrigated agricultural fields through irrigation with treated mine wastewaters. Due to the high adsorption rates of As within these substrates, the mobility thereof at agronomically high pH values of 7 to 9 in the Witwatersrand substrate or 6 to 9 in the Karoo substrate (or substrates or soils with a similar clay and Fe-oxide content) should not be of major concern. However, this is specific to the conditions and parameters stated in this study. This is also stated without acknowledging the possibility of As accumulation within the mine tailings or soils. Close monitoring should therefore be done to ensure that there is no As accumulation in the soil and that dangerous amounts of As are not taken up by crops or leached into the groundwater systems. Furthermore, it was shown that Se adsorb less than As and that Se started to sorb less at pH values lower than 5 with respect to As. It was therefore concluded that Se would be more mobile than As at agronomic pH values between 5 and 9 in both the Witwatersrand and Karoo substrates (or substrates or soils with a similar montmorillonite clay and Fe-oxides). The adsorption of Se was however much lower in the Karoo than in the Witwatersrand substrates, possibly due to the much lower montmorillonite clay and goethite content of the Karoo substrates, which do not provide as many sorption sites. The adsorption of Se in the Karoo substrate decreased from pH values >5 and from pH values >7 in the Witwatersrand substrates. The low adsorption rates of Se greatly increases the probability and therefore the risk for Se to leach into the groundwater systems or to be taken up by plants. The irrigation of Witwatersrand and Karoo substrates or soils with a similar montmorillonite clay and Fe-oxides is therefore not recommended.enDissertation (M.Sc. (Soil, Crop and Climate Science))--University of the Free State, 2020ArsenicSeleniumSurface complexationSoil substratesMine waste -- South AfricaDissertationUniversity of the Free State