Cr(VI) contamination of aqueous systems

Abstract

Hexavalent chromium, i.e. Cr(VI), is a potential pollutant species formed due to anthropogenic processes, e.g. leather tanning, Cr(VI) chemical production, stainless steel manufacturing and ferrochrome production. Cr(VI) is of concern since it is toxic to microorganisms, plants and animals, and carcinogenic for humans. Therefore, standard limits for the Cr(VI) contents in air, soil and water have been introduced by different health and legal organisations worldwide. Within the South African context, Cr(VI) water pollution specifically associated with ferrochrome production is of concern, since this is a large industry in South Africa with 14 ferrochrome smelters. Apart from Cr(VI) pollution, wastewater treatment processes applied at ferrochrome smelters could negatively affect water quality in general (e.g. chemical oxygen demand, hardness, pH levels and SO4 2-) if run-off or leakage is allowed. In this study the focus was only the determination of Cr(VI) concentrations and conductivity levels (as a proxy for total dissolved solids). Various analytical methods exist to determine Cr(VI) present in natural water. The method used during this study was ion chromatography coupled with an ultraviolet-visible absorbance detector. Diphenylcarbazide , a post-column colorant, was added to react with the Cr(VI) to form a species that can be detected at 540 nm wavelength. Experimentally the detection limit of this method was determined as 0.9 μg/L, which is slightly lower than the detection limit reported in literature, i.e. 1.0 μg/L. This improvement was achieved by reducing the baseline noise on the chromatographs. Surface- and drinking water samples were collected within the vicinity of 12 ferrochrome smelters for the duration of one year. The water samples collected were analysed for Cr(VI) content, as well as the conductivity and the elemental analysis of the total dissolved solids fraction with scanning electron microscopy incorporated with energy dispersive X-ray spectroscopy. The results obtained for the surface water samples showed that Cr(VI) pollution was mostly not present, with the exception of four sites. Two of these sites had constant Cr(VI) pollution, but with levels lower than the drinking water limit, although such Cr(VI) contamination could still have an impact on the ecological system. The annual means for these two sites were 4.4 and 6.3 μg/L. The other two sampling sites also showed constant pollution, but with a few months in which the values exceeded the drinking water limit (198 and 220 μg/L). For the drinking water sampling sites, there were only three sites where Cr(VI) was detected constantly. Unfortunately, the origin of the water was unknown for two of these polluted sites (with levels lower than the prescribed drinking water limit). For the one site, where the drinking water limit was consistently exceeded, the water originated from a borehole. It was established that the pollution was a result of poor historical waste mismanagement at the nearby ferrochrome smelter. The results obtained from the conductivity and elemental analysis of the total dissolved solids indicated that the surface- and drinking water tested was fit for human consumption. At two smelters where surface water contamination could have been suspected due to run-off, no pollution was detected. At four ferrochrome smelters, the surface water results indicated that these smelters contributed negatively to surface water quality, if conductivity was considered as the only evaluating criteria. Although the surface water quality was affected at these sites, the surface water was not appropriate for human consumption at only one of these sites when taking only conductivity into account. From the results, it could be concluded that deposition emanating from atmospheric emissions contributed less than run-off and/or seepage to the decrease in surface water quality in the proximity of the smelters. The Cr(VI) pollution, conductivity and elemental composition of the total dissolved solids at the different sites were compared and four unique case studies were identified. Three case studies focused on the negative influence of the ferrochrome smelters on the surface water sampling sites, while the fourth case study was selected since the surface water was unpolluted, but the drinking water was contaminated. The surface water pollution was mainly attributed to run-off and/or seepage, while atmospheric deposition contributed less to the pollution at the specific measurement sites. At one of these sites, a high level of Cr(VI) pollution was recorded over a relatively short period. Circumstantial evidence indicated that this spike in Cr(VI) pollution had a significant impact on the population of diatoms. This linkage needs to be confirmed and investigated in greater detail in future.