Sequential extraction to assess metal retention and mobility in acid mine drainage-impacted sediments from the Witkranz legacy mine, South Africa

Authors: Malatji, M; Makono, T; Sekiba, K; Vadapalli, V; Madzivire, G; Fosso-Kankeu, E; Malenga, E

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DOI https://doi.org/10.36487/ACG_repo/2615_85

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Malatji, M, Makono, T, Sekiba, K, Vadapalli, V, Madzivire, G, Fosso-Kankeu, E & Malenga, E 2026, 'Sequential extraction to assess metal retention and mobility in acid mine drainage-impacted sediments from the Witkranz legacy mine, South Africa', in AB Fourie, G Boggs, J Heyes & M Tibbett (eds), Mine Closure 2026: Proceedings of the 19th International Conference on Mine Closure, Australian Centre for Geomechanics, Perth, pp. 1-19, https://doi.org/10.36487/ACG_repo/2615_85

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Abstract:
A sequential extraction procedure was conducted to evaluate metal retention and mobility in sediments collected along the channel transporting acid mine drainage (AMD) from the Witkranz legacy mine discharge point into the Boesmanspruit stream in Carolina, Mpumalanga, South Africa. The AMD is characterised by low pH (3.1) and elevated metal (22 mg/L Fe and 30 mg/L Mn) and sulphate concentrations (1098 mg/L). Contaminated waters ultimately flow through the Boesmanspruit stream into the Boesmanspruit and Nooitgedacht dams, posing risks to downstream ecosystems, human health and aquatic life. Sequential extraction evaluates the physico-chemical forms of mobile metals rather than their concentration, hence its use in this study. The results indicated that Mn is mostly associated with the exchangeable fraction, making it highly mobile in the area. Fe is associated with both the exchangeable and reducible fractions, giving it moderate mobility. Ca is mainly associated with the residual fraction but can become available due to its significant association with the exchangeable fraction (42%). Cr is largely associated with the reducible fraction, remaining stable under oxidising conditions; however, it can become mobile under reducing conditions. Mg is mostly associated with F3 and the residual fractions, remaining stable and immobile. The general order of mobility is Mn > Fe > Ca > Cr > Mg. Overall, these findings show that the sediments are not only a sink for metals but also a potential long-term secondary source of contamination under changing physico-chemical conditions.

Keywords: contaminated water, elevated metals, sulphate concentrations, physico-chemical conditions

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