School of Chemistry
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Item Attenuation of pollution arising from acid mine drainage by a natural wetland on the Witwatersrand(Academy of Science of South Africa (ASSAf), 2017-01) Humphries, M.S.; McCarthy, T.S.; Pillay, L.Wetlands are well known to be efficient at sequestering pollutants from contaminated water. We investigated metal accumulation in the peats of the Klip River, a natural wetland that has received contaminated water from gold mining operations in Johannesburg for over 130 years. Previous work conducted in the downstream portion identified the wetland as an important system for sequestering metals. We focused on the upstream section of the wetland, more proximal to the source of acid mine drainage, to provide a better understanding of the pollutant sources and the role of the wetland in pollutant attenuation. Geochemical and mineralogical analyses of peat cores revealed considerable metal enrichments in the peat ash, particularly in Co, Ni, Zn, Pb, Cu and U. Metal concentrations are typically between 4 to 8 times higher than those previously reported for the downstream, more distal portion of the wetland. The distribution of metal accumulation within the peat profiles suggests that contamination arises from a combination of sources and processes. Elevated concentrations in the shallow peat are attributed to the input of contaminated surface water via tributaries that drain the Central Rand Goldfield, whereas enrichments in the deeper peat suggest significant sub-surface inflow of contaminated water through the underlying dolomitic rocks. Metal immobilisation occurs through a combination of mechanisms, which include the precipitation of gypsum, metal sulfides, Fe-Mn oxyhydroxides and phosphates. Our study highlights the environmental and economic importance of natural wetland systems which have the ability to accumulate large quantities of metals and thus remediate polluted waters.Item The effect of structural properties of Cu2Se/polyvinylcarbazole nanocomposites on the performance of hybrid solar cells(Hindawi Publishing Corporation, 2016) Govindraju, S.; Ntholeng, N.; Ranganathan, K.; Sikhwivhilu, L.M.; Moloto, N.; Moloto, M.J.It has been said that substitution of fullerenes with semiconductor nanocrystals in bulk heterojunction solar cells can potentially increase the power conversion efficiencies (PCE) of these devices far beyond the 10% mark. However new semiconductor nanocrystals other than the potentially toxic CdSe and PbS are necessary. Herein we report on the synthesis of Cu2Se nanocrystals and their incorporation into polyvinylcarbazole (PVK) to form polymer nanocomposites for use as active layers in hybrid solar cells. Nearly monodispersed 4 nm Cu2Se nanocrystals were synthesized using the conventional colloidal synthesis. Varying weight % of these nanocrystals was added to PVK to form polymer nanocomposites. The 10% polymer nanocomposite showed retention of the properties of the pure polymer whilst the 50% resulted in a complete breakdown of the polymeric structure as evident from the FTIR, TGA, and SEM. The lack of transport channels in the 50% polymer nanocomposite solar cell resulted in a device with no photoresponse whilst the 10% polymer nanocomposite resulted in a device with an open circuit voltage of 0.50 V, a short circuit current of 7.34 mA/cm2, and a fill factor of 22.28% resulting in a PCE of 1.02%.Item The effect of temperature and methanol-water mixture on pressurized hot water extraction (PHWE) of anti-HIV analogoues from Bidens pilosa(BioMed Central Ltd., 2016-06) Gbashi, S.; Njobeh, P.; Steenkamp, P.; Tutu, H.; Madala, N.Background: Pressurized hot water extraction (PHWE) technique has recently gain much attention for the extraction of biologically active compounds from plant tissues for analytical purposes, due to the limited use of organic solvents, its cost-effectiveness, ease-of-use and efficiency. An increase in temperature results in higher yields, however, issues with degradation of some metabolites (e.g. tartrate esters) when PHWE is conditioned at elevated temperatures has greatly limited its use. In this study, we considered possibilities of optimizing PHWE of some specific functional metabolites from Bidens pilosa using solvent compositions of 0, 20, 40 and 60 % methanol and a temperature profile of 50, 100 and 150 °C. Results: The extracts obtained were analyzed using UPLC-qTOF-MS/MS and the results showed that both temperature and solvent composition were critical for efficient recovery of target metabolites, i.e., dicaffeoylquinic acid (diCQA) and chicoric acid (CA), which are known to possess anti-HIV properties. It was also possible to extract different isomers (possibly cis-geometrical isomers) of these molecules. Significantly differential (p ≤ 0.05) recovery patterns corresponding to the extraction conditions were observed as recovery increased with increase in methanol composition as well as temperature. The major compounds recovered in descending order were 3,5-diCQA with relative peak intensity of 204.23 ± 3.16 extracted at 50 °C and 60 % methanol; chicoric acid (141.00 ± 3.55) at 50 °C and 60 % methanol; 4,5-diCQA (108.05 ± 4.76) at 150 °C and 0 % methanol; 3,4-diCQA (53.04 ± 13.49) at 150 °C and 0 % methanol; chicoric acid isomer (40.01 ± 1.14) at 150 °C and 20 % methanol; and cis-3,5-diCQA (12.07 ± 5.54) at 100 °C and 60 % methanol. Fitting the central composite design response surface model to our data generated models that fit the data well with R2 values ranging from 0.57 to 0.87. Accordingly, it was possible to observe on the response surface plots the effects of temperature and solvent composition on the recovery patterns of these metabolites as well as to establish the optimum extraction conditions. Furthermore, the pareto charts revealed that methanol composition had a stronger effect on extraction yield than temperature. Conclusion: Using methanol as a co-solvent resulted in significantly higher (p ≤ 0.05) even at temperatures as low as 50 °C, thus undermining the limitation of thermal degradation at higher temperatures during PHWE.Item Leachability of metals from gold tailings by rainwater: An experimental and geochemical modelling approach.(South African Water Research Commission, 2016-01) Grover, B.P.C.; Johnson, R.H.; Tutu, H.Mine leachates from gold tailings impoundments usually contain elevated concentrations of metals and sulphates that impact negatively on water quality. This study was aimed at assessing the leachability of such metals from tailings by rainwater. Oxidised and unoxidised tailings were leached experimentally and through simulations using the PHREEQC geochemical modelling code. The results revealed that the majority of readily leachable metals were held in secondary mineral phases, mainly sulphates. A good agreement between experimental and modelling techniques was obtained, indicating the potential use of geochemical modelling in future metal release studies for the site. A list of reactive minerals for the tailings material was compiled. These minerals may or may not be present in the tailings; however, the list provides a means of estimating future reactivity or bulk metal release from the tailings.