Assessing Physicochemical Technologies for Removing Hexavalent Chromium from Contaminated Waters—an Overview and Future Research Directions

Heavy metal contamination of groundwater is one of the most recent and serious environmental issues. Chromium (VI), also called hexavalent chromium, is one such metal. It poses a critical threat to the environment because of its high toxicity and carcinogenic nature. Chromium compounds are extensively used in industries like metallurgies, tanneries, refectories, electroplating, and steel, which in turn releases chromium (VI) containing effluent into the environment. Being a heavy metal, it is non-biodegradable and thus remains persistent in the ecosystem, contaminating water and soil. Before discharge of chromium (VI)–containing effluent, bringing it to a permissible level as per statutory norms requires a remediation process. The World Health Organization recommended a level of 0.1 mg/L of chromium in water for human consumption. Numerous approaches can be used to address these issues. Among the different approaches available, the traditional method involves chemical precipitation, filtration, solvent extraction, electrocoagulation, catalytic reduction, membrane filtration, biological treatment, and ion exchange. The later advancements incorporate the use of nanotechnology and photo-catalytic reduction. Conventional methods are, however, associated with certain drawbacks such as high sludge formation, which requires licence disposal, high chemical/energy requirements, and low removal efficiency. In the recent past, newer, innovative, and more economical technologies like photocatalysis and nanotechnology have been investigated and found to be more efficient even at low metal ion concentrations. The major purpose of this article is to provide up-to-date information on extensively used techniques, with an emphasis on the significant issues that are experienced while removing hexavalent chromium from water.