This study evaluates different methods to determine points of zero charge (PZCs) on five organic materials, namely maple sawdust, wood ash, peat moss, compost, and brown algae, used for the passive treatment of contaminated neutral drainage effluents. The PZC provides important information about metal sorption mechanisms. Three methods were used: (1) the salt addition method, measuring the PZC; (2) the zeta potential method, measuring the isoelectric point (IEP); (3) the ion adsorption method, measuring the point of zero net charge (PZNC). Natural kaolinite and synthetic goethite were also tested with both the salt addition and the ion adsorption methods in order to validate experimental protocols. Results obtained from the salt addition method in 0.05 M NaNO were the following: 4.72 ± 0.06 (maple sawdust), 9.50 ± 0.07 (wood ash), 3.42 ± 0.03 (peat moss), 7.68 ± 0.01 (green compost), and 6.06 ± 0.11 (brown algae). Both the ion adsorption and the zeta potential methods failed to give points of zero charge for these substrates. The PZC of kaolinite (3.01 ± 0.03) was similar to the PZNC (2.9-3.4) and fell within the range of values reported in the literature (2.7-4.1). As for the goethite, the PZC (10.9 ± 0.05) was slightly higher than the PZNC (9.0-9.4). The salt addition method has been found appropriate and convenient to determine the PZC of natural organic substrates.
Water systems in the Witwatersrand Basin are heavily contaminated with pollutants from gold-mine tailing leachates and spillages. Cyanide speciation was studied in relation to the Natalspruit, a natural stream within a gold mining area of Johannesburg. The part of the studied stream was adjacent to an abandoned gold-mine tailings dump. The results of this study pointed to a variation in surface and groundwater chemistry. The groundwater had low pH and contained elevated concentrations of pollutants including secondary cyanide species, e.g. thiocyanate, cyanate, and ammonium. Metal-cyanide complexes that are weak-acid dissociable (CN WAD ) and strong-acid dissociable (CN SAD ) cyanides were found to dominate the speciation of cyanide, with iron cyanide complexes being most prevalent in groundwater. Decreases in the concentration of 63% for Cu CN ð Þ 2À 3 , 83% for Ni CN ð Þ 2À 4 , 69% for Fe CN ð Þ 4À 6 , and 47% for Fe CN ð Þ 3À 6 were observed from groundwater to surface water largely due to dilution and dissociation (in the case of the first two complexes). Speciation modeling was used to estimate the concentrations of CN WAD and CN SAD based on total cyanide (CN T ), total metal, and total anion concentrations. Other cyanide complexes as well as dominant sulfate complexes were also predicted.
Batch sorption studies were conducted to assess the potential of a phosphonated silica polyamine composite (BPAP) to remove metals (Co, Cu, Fe, Mg, Mn, Ni, U and Zn) . Desorption experimental data gave poor results except for U which showed 99.9% desorption.
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