Sequential batch contactors for removal of chromium (VI) from wastewater using sheep's wool

Abstract

The effluent streams of various industries contain heavy metals like chromium. The hexavalent form of it is highly toxic, which needs specific intensive treatment before disposal. Sheep’s wool, as a keratinous adsorbent, has been found to be able to reduce the chromium concentration below the permissible limits thus eliminating the requirement for any further treatment before disposal. In this work, sheep’s wool was used to treat synthetic wastewater containing hexavalent chromium in sequential batch contactors. The optimum parameters of dosage, time, pH, shaking speed and temperature were established by batch studies to be 8 g/L, 25 min, 2, 150 rpm and 25 oC respectively. Sequential batch contactors ensured complete removal of hexavalent chromium. With three sequential batch contactors, the Cr(VI) concentration was reduced from 100 mg/L to 0.06 mg/L, which is less than the permissible discharge limit for Cr(VI) in water bodies. Desorption studies were carried out to determine the optimum parameters for effective desorption using potassium chloride. The optimum parameters in terms of time, concentration and temperature were concluded to be 25 minutes, 1.0 M and 50 oC. The removal efficiency of Cr(VI) after the third cycle was determined to be 82 ± 2 percent which is similar to the removal efficiency of Cr(VI) in the first cycle that is 83 ± 2 percent. The adsorption-desorption cycles dictate that wool can be regenerated several times without any pronounced change in its removal efficiency. The lab scale experiments were performed for 100 mL and then were scaled to 1000 mL of sample laboratory synthesized chromium wastewater. The experimental results obtained from the scale up experiment are comparable to the results obtained from 100 mL of sample laboratory synthesized solution without any significant change in removal efficiency of Cr(VI) and total chromium. Thus, this method has the potential to be scaled up to pilot plant, and eventually, industrial scale utilization for chromium removal from industrial wastewater streams.