Strategies for Removal of Hexavalent Chromium from Contaminated Water in the Mining Area of Sukinda Valley

Abstract

Hexavalent Chromium, Cr(VI) is a highly toxic form of chromium metal mainly used in different industrial applications because of the important properties like resistance to corrosion, oxidation, and hardenability. In metallurgical industry, chromite is used for manufacturing low and high carbon, Ferro chrome and charge chrome which are used as an ingredient in making stainless steels and special alloy steel. For this vast use of chromite, the chromite mining is an important process. However, there is always a possibility of chromium contamination of water bodies by the utilization industries.

Chromium exists in two stable states, i.e. hexavalent chromium (Cr VI) and trivalent chromium (Cr III) of which Cr(VI) is the most toxic form. Some of the common ailments associated with the drinking of water contaminated with Cr(VI) are gastrointestinal bleeding, peptic ulcer, tuberculosis, and asthma. Infertility, birth defects, and stillbirths and have also be noticed in a few cases.

Sukinda Valley in the state of Odisha is a well-known chromite hub in the world and hosts more than 95% of country’s chromite resources. The extraction of chromite in this region is being mostly carried out by opencast mining and at some places by underground mining. Mining is known to create various environmental impacts during operation and after closure. Opencast chromite mining activity leads to various environmental problems due to the release of hexavalent chromium. A lot of solid wastes have been generated in the form of rejected minerals, overburden material/waste rocks and sub-grade ores, that that has the potential to cause environmental degradation, mainly causing lowering of the water table as well as deterioration in surface and groundwater quality. During mining, a large quantity of wastewater is also generated which is contaminated with Cr(VI). These along with the surface runoff generated during monsoon are pumped outside to provide a free face for the working. This water is supposed to be treated before being released outside. However, it has been observed that some of the mines still do not have effluent treatment plants for the treatment of Cr(VI). As a result toxic Cr(VI) pollutant introduced into natural water bodies. Studies carried out by various researchers, regulatory bodies, and NGOs; have pointed out the contamination of water bodies by Cr(VI) due to mining activities in this region, and have suggested stringent measures to be adopted by the mining companies. Removal of hexavalent chromium from water is essential in order to avoid its adverse impacts. Since the substantial quantity of water is required to be treated in a mining area, compared to any other industry, development of a cost-effective material for treatment is essential.

In this research, therefore, a thermally activated carbon prepared from biomass of P.putida and another novel adsorbent called coal modified by lanthanum (La-Coal); prepared by coal and rare earth element lanthanum was used as a low-cost adsorbent for the removal of Cr(VI) from aqueous solution. Laboratory studies were carried out to know the removal efficiency of activated carbon and La-Coal for the removal of Cr(VI) from the aqueous solutions by the batch method. The experiments were conducted as a function of adsorbent dose, contact time, temperature, pH, initial Cr(VI) concentration. The kinetics, thermodynamics, and isotherm behavior were also studied. The characteristics of activated carbon and La-Coal before and after adsorption were studied with the help of instrumental techniques like SEM-EDX, TEM, XRD, FT-IR, TGA-DTA, and chemical methods. Adsorption study was carried out with the initial concentration of 5mg/L, 10 mg/L and 20 mg/L of Cr(VI) solution. Adsorption kinetics results showed that both the adsorbent follow the pseudo-second-order model. The adsorption data were best fitted to Langmuir isotherm model which suggests the monolayer adsorption pattern for La-Coal and for activated carbon. The adsorption data were best fitted to both Langmuir isotherm and Freundlich isotherm models. The maximum adsorption capacity for activated carbon and La-Coal was found to be 57.09 mg/L and 51.81 mg/L respectively. The above adsorbents i.e. activated carbon showed 93.17% removal, whereas the La-Coal showed 89.19% removal in the groundwater of Sukinda mines. The thermodynamic study suggested that the adsorption was spontaneous and endothermic in nature. The intra-particle diffusion model suggested that it is not the sole limiting factor. The regeneration and reusability study was carried out to assess the applicability of the adsorbent material. The findings of this study suggest that both the adsorbents are excellent materials for the removal of Cr(VI) from the water. This study demonstrates a method which utilized the easily available material like coal which can be converted to a valuable resource for adsorption of hexavalent chromium.