LAB-SCALE STUDIES ON THE REMOVAL OF CHROMIUM FROM INDUSTRIAL WASTES

Abstract

Chromium is one of the major toxic pollutants discharged from plating, electroplating and chrome tanning processes. It is also found in the effluents of dyes, paint pigments, etc., manufacturing units. Chromium exists in aqueous systems in both trivalent (Cr+3) and hexavalent (Cr+6) forms. Hexavalent chromium is carcinogenic and also toxic to aquatic life. Trivalent chromium is less toxic than the hexavalent chromium. This study was undertaken to investigate the total chromium removal from industrial effluents by chemical means in order to achieve the Pak NEQS level of I mg/L. For this purpose reduction and precipitation were employed. Research was conducted in the following four steps: Preliminary studies; finding the cheapest chemical Reduction of hexavalent chromium in synthetic waste Precipitation of trivalent chromium in synthetic waste Application to the actual Industrial wastes Steps 1,2 and 3 were conducted with synthetic samples of chromium prepared in the laboratory while step 4 was performed on actual industrial wastes from chrome tanning and electroplating industry. Step-I was just an optimization study to select best pH and cost-effective reducing agent among four popular commercial chemicals for the reduction process to be ii employed in step-2. Optimum pH of 2.(), and sodium metabisulfite were found to be the most cost-effective pH and reducing agent, respectively. Results of step-2 experiments showed that low dosages of sodium metabisulfite were sufficient to obtain 100% efficiency to reduce hexavalent chromium to trivalent chromium for different chromium concentrations. It was also found that reaction control time had no significance on the reduction of chromium as it was spontaneous. Most importantly, a design graph for the hexavalent chromium reduction was established. This curve provides dosages of sodium metabisulfite for required residual Cr(VI) concentration at a known initial Cr(VI) concentration. The design graph provides a good tool tor the industry to treat Cr(VI) wastes. Results of step-3 studies indicated that a pH value of 8.5() was best for the precipitation of trivalent chromium to chromium hydroxide by using lime. An efficiency of I ()()% was achievable and a settling time of 3() minutes gave clear sample. Results of step-4 studies on the waste sample from chrome tanning industry indicated that precipitation at a pH value of 12.0 would give 100% efficiency at a settling time of 30 minutes. This was a deviation from the result obtained in step-2 which may be due to the highly acidic nature of waste and the presence of different metals in the waste. The results on the electroplating industry waste showed negligible deviation from the anticipated results of step-2. The overall results of this study show that for Pakistan, chemical means of reduction and precipitation is a feasible and viable solution for treating chromium wastes from industries.