Bioprospecting of plants species with potential to uptake inorganic chemical species from gold mine tailings and acid mine drainage

Abstract

In South Africa, mining of gold and coal has huge negative impacts on the environment. This is due to the exposure of sulphide-bearing minerals such as pyrite (FeS2) to oxygenated water during and after the extraction of ore deposits which results in the formation of acid mine drainage (AMD). Acid mine drainage is characterised by low pH ranging between 1-4, high concentration of sulphate and chemical species which degrade the water quality and threaten the life of terrestrial and aquatic organisms. This study aims at bio-prospecting plant species with potential use in phytoremediation of AMD and gold mine tailings. First batch of samples of plants, water leachates and mine tailings were collected from Crown Mine within Witwatersrand gold fields in Johannesburg, South Africa while the second batch of samples of plants, water and sediments were collected from the mine water discharging points and within the AMD impacted wetlands streams in eMalahleni, Mpumalanga Coal Fields. In the first part of the study deals with the characterization of gold mine tailings and acidic mine leachates collected from crown mines in Johannesburg and further identification of plants species with potential for use phytoextraction of inorganic chemical species from the acidic leachates and gold mine tailings. The physicochemical parameters of leachates such as pH, electrical conductivity (EC), redox potential (Eh), total dissolved solids (TDS), dissolved oxygen (DO) and salinity were measured in the field using pH multi-meter. Elemental composition of mine tailings were analysed x-ray fluorescence (XRF) and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) techniques. Leachates samples were analysed for metals contents using Inductively Coupled Plasma Spectrometry (ICP-MS) and anions using Ion Chromatography. Plant samples were digested using aqua-regia method and analysed with ICP-MS for elemental composition. The results showed that the mine tailings are mainly composed of SiO2, Al2O3, Fe2O3, MgO and K2O as major oxides and Cr, Co, Cu, Ni, Zn, Tb, Ta, Tm, Mo, Eu, Lu, Ho and Cs as trace elements. The pH, EC, TDS of the leachates were found to be ranging from 3.31 to 5.21, 3 857 to 5 517 mS/cm and 1930 to 2704 mg/L, respectively. The leachate samples were characterized by higher concentrations of Mn, As, Cr, Al, Pb, Ca, Na, K and Fe. The most dominating anions within the leachates were Cl- (135.97-201.28 mg/L) and SO42- (59.39-62.65 mg/L). Cortaderia selloana and Populus alba accumulated high concentration of Mn, Zn, P, Mg, K and B in their leaves than other parts of the plant. Translocation factor (TF) reflected that Cortaderia selloana plant species has the potential to translocate all the chemical species from the roots to the shoots except Se. Cortaderia selloana showed bioconcentration factor greater than 1 for chemical species such as B, Mn, Zn, Sr, Ca, Mg, P, and Si. Similarly, Populus alba showed the bioconcentration greater than 1 for Mn, Ni, Cu, Sb, Na and Mg. In addition, Populus alba showed bioconcentration factor greater than 10 showing hyperaccumulation ability for species such as B, Co, Zn, Ca, Cd, K, P and Sr. These plants can be used for the phytoremediation of mine tailings. Second part of the thesis focused on characterising mine water discharging from abandoned mine shafts and assessing the change in water quality from the discharging point, within retention ponds, upstream to downstream of the wetland. Furthermore, evaluating the role of sediments and native plants species in the remediation of acid mine drainage. The physicochemical characteristics such as pH, EC, TDS, Eh, DO and Salinity were found to be ranging from 2.53 to 3.6, 1066 to 2285 μS/cm, 610 to 5230 mg/L, 194 to 256.8 mV, 2.94 to 8.24 mg/L and 0.82 to 6.09 psu, respectively. The concentration of SO42-, Cl-, NO32- and F- were found to be ranging from 992.90 to 12580.38, 19.63 to 160.61, 1.77 to 23.56, and 4.76 to 14.95 mg/L, respectively. The dominant inorganic chemical species in water were found to be Ca, K, Mg, Na, Si, Al, Fe, Zn and Mn. The sediments collected along the streams showed higher concentration of Fe, Ca, Al, K, Mg, Na, P, Si, Zn, Mn, and V as compared to concentration in water. This implied that sediments are adsorbing chemical species from acid mine drainage and hence improve the quality of water. The concentration of metals in plants tissues are in the following order: Fe > Mg > Al > Mn > Zn > Cr > Ni > Cu > Co > Pb > As > Cd. Amongst all the native plant species, Cyperus esculentus had higher translocation factor (greater than 1) in Cr, Mn, Ni, Zn, Ca, K, Mg, Na, P and higher bioconcentration factor (greater that 1) in Cr, Co, Ni, Cu, and Zn, and a bioconcentration factor of greater than 10 in Cd, Ca, K, Mg, Na and P. This implied that Cyperus esculentus have the potential in phytostabilization of Cr, Co, Ni, Cu, Zn, and Cd, and Cd, Ca, K, Mg, Na and P and phytoextraction of Cd, Ca, K, Mg, Na and P. In nutshell, the results from this study showed all the plants species identified in this study (Cortaderia selloana, Populus alba, Cyperus esculentus, Phragmites mauritianus, Cynodon dactylon, Typha capensis and Juncus effussus and Juncus lomatophyllus) has potential for use in the phytoextraction of inorganic contaminants from the AMD and gold mine tailings. Therefore, the study recommends detailed studies optimizing the species application in phytoremediation of AMD and gold mine tailings in a pilot scale project.