Contaminant fate in Searsia Lancea woodlands on acid mine drainage in the Witwatersrand Basin Goldfields

Joubert, Maxine Kelly
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There has been increasing research into plants used for phytoremediation, specifically for phytoextraction and phytostabilisation of heavy metals in soil. There has been little research on trees for their large biomass, especially field studies. There is also a lack of research on trees in South Africa specifically. This study evaluated the fate of contaminants (Na, Mg, Al, S, Cl, Cd, Cr, Mn, Fe, Cu, Zn, and Pb) in Searsia lancea, a tree native to South Africa, planted in woodland trials for phytostabilisation and hydrological control on AngloGold Ashanti mining properties, at the base of tailings storage facilities as part of the Mine Woodlands Programme – a collaboration between the University of the Witwatersrand and AngloGold Ashanti. Trees of average height were harvested from three out of four S. lancea plots at four different sites; two sites at the West Wits mining operations (Madala and Redsoils), and two sites at the Vaal River mining operations (West Complex and Mispah). One site at each mining operation had nutrient-poor soil, and one site had nutrient-rich soil for plant growth. Harvesting of above-ground biomass took place first, in which the tree compartments were separated into wood (stems), twigs, and leaves. These were bagged and weighed, and then dried naturally. Sub-samples of wood, twigs, and leaves were taken after weighing the bulk samples. These sub-samples were washed, freeze-dried, and ground using ceramic burr grinders for analysis. Tree roots were excavated using a backacter (TLB), which then proceeded to dig soil pits roughly 2.5 x 3 x 3 m for soil sample collection. Sub-samples of coarse roots and fine roots were taken, but roots were bagged and weighed together. Sub-samples of roots were also washed, freeze-dried, and ground using a ceramic burr grinder for analysis. Soil samples were taken at certain depths within the pits (0-2, 2-5, 5-10, 10-15, 15-20, 20-30, 40-50, 50-60, 60-70, 70-80, 90-100, 120-130, 145-155, 170-180, 190-210, 240-260 and 280-300 cm). These were bagged and sent for analysis of pH, Electrical Conductivity (EC), and Reduction-Oxidation Potential (Eh). All samples were analysed by X-Ray Fluorescence (XRF). The Mann-Whitney U Test and a non-parametric analysis of variance (Kruskal-Wallis) were used to test for significant differences in contaminant distribution. Post-hoc pairwise comparisons were performed using Dunn’s procedure with a Bonferroni correction for multiple comparisons to test for specific differences between soils (sites), tree compartments and soil using IBM SPSS statistics 24. Bioconcentration Factors (BCF) and Translocation Factors (TF) were calculated to assess the phytostabilisation and phytoextraction abilities of S. lancea. The fate of contaminants was found to be different for different contaminants. Sulphur and Mn were highest in the leaf compartment; Chlorine, Cu, and Zn were highest in the twig compartment; no elements were found to be highest in the wood compartment; Mg and Fe were highest in the coarse roots; and Ca was highest in the fine root compartment. It was also found that S. lancea is an accumulator of S, Cl, and Ca with levels of 2 508.92, 2 500.96 and 16 733.46 mg/kg respectively. Searsia lancea appears to be a Al, Fe, and Cr stabiliser with TFs < 1 and translocates metals in the sequence Ca > Na > Fe > Mg > Zn > S > Mn > Pb > Cu > Al > Cl > Cr. BCF results show that S. lancea is more of an accumulator than a stabiliser as BCF root: soil pattern was found to be Cl > S > Cu > Cr > Zn > Mn > Mg > Na > Pb > Ca > Al > Fe; while BCF shoot: soil pattern was found to be > 1 in the sequence Cl > S > Ca > Na > Mg > Zn > Cu > Mn > Pb > Cr > Fe > Al, with Al, Cr, Fe, and Pb higher in soil compared to shoot concentrations. This study demonstrates that certain indigenous tree species are capable of phytoremediation of contaminated sites and that larger biomass species can take up great elemental masses of certain elements. Key words: phytoremediation, phytostabilisation, phytoextraction, native trees, mine pollution, Searsia lancea
A dissertation submitted in fulfilment of requirements for the degree Master of Science to the Faculty of Science, University of the Witwatersrand, Johannesburg , October 2017.
Joubert, Maxine Kelly (2017) Contaminant fate in searsia lancea woodlands on acid mine drainage in the Witwatersrand Basin Goldfields, University of the Witwatersrand, Johannesburg,