EVALUATION OF CURRENT TREATMENT TECHNOLOGIES FOR Cr(VI) REMOVAL FROM WATER SOURCES AT SUB-PPB LEVELS

Hexavalent chromium [Cr(VI)] has long been recognized as a potential carcinogen, via the inhalation route while nowadays, there is an increasing concern that Cr(VI) is also carcinogenic by the oral route of exposure. According to this, governments apply more stringent regulations for chromium species. There are several discussions and intentions to lower the current maximum permissible concentration limit of chromium in drinking water from 50 down to 5-10 mu g/L. Fortunately, the remarkable growth in economics and living standards has accelerated the development of appropriate water and wastewater purification technologies. The motivation of this study was to evaluate the technologies available for the efficient removal of Cr(VI) from contaminated water sources (mostly groundwater) at sub-ppb levels in order to become safe for drinking purposes. Several treatment technologies have been developed to remove chromium from water or wastewater matrices. Common methods include chemical reduction with co-precipitation, adsorption, ion exchange, membrane separation, electro-dialysis, electro-coagulation, phyto-remediation, flotation and solvent extraction. Although there is extensive scientific literature on the aforementioned methods for the treatment of Cr(VI) species in wastewaters, only few works refer to their use in Cr(VI) removal at sub-ppb levels and far too little for the full-scale remediation of Cr(VI) in water treatment. It is therefore necessary to define certain criteria for a precise evaluation of the available technologies, starting with the feasibility of achieving residual concentrations of Cr(VI) at sub-ppb levels. An initial assessment of available current technologies shows that most of these treatment methods, such as membrane separation, electro-dialysis, electro-coagulation, phyto-remediation, flotation and solvent extraction cannot be used for the removal of Cr(VI) from water sources, as they do not meet the selected criteria. Chemical reduction with co-precipitation, is found to be a rather effective method for the removal of Cr(VI), although it requires the development of an efficient process for the treatment of residual sludge. Adsorption could also be considered as an effective treatment method, however there is not enough evidence that it can be used for the removal of Cr(VI) from drinking water. Removal of Cr(VI) through ion exchange seems also promising, but the labor cost for handling the regeneration solution along with the relevant fixed cost are too high to be ignored. It turns obvious that there are several methods that can be used for the remediation of Cr(VI), but its removal to sub-ppb levels requires answering several questions and further detailed investigation.