Selective adsorption of chromium(VI) in industrial wastewater using low-cost abundantly available adsorbents

The removal of poisonous Cr(VI) from industrial wastewater by different low-cost abundant adsorbents was investigated. Wool, olive cake, sawdust, pine needles, almond shells, cactus leaves and charcoal were used at different adsorbent/metal ion ratios. The influence of pH, contact time, metal concentration, adsorbent nature and concentration on the selectivity and sensitivity of the removal process was investigated. The adsorption process was found to follow a first-order rate mechanism and the rate constant was evaluated at 30 C. In the case of wool, the rate constant was the highest (39.6 x 10(-3) min(-1)) and the cactus leaves gave the lowest value (6.8 x 10(-3) min(-1)). Langmuir and Freundlich isotherms were applicable to the adsorption process and their constants were evaluated. The thermodynamic equilibrium constant and the Gibbs free energy were calculated for each system. The DeltaG(o) for the absorption by wool (-2.26 kJ mol(-1)) and that for the cactus leaves (2.8 kJ mol(-1)) supported the findings that wool was the best among the selected adsorbents for the selective removal of Cr(VI) at pH 2 and an adsorbent concentration of 16 g l(-1) at 30degreesC, for which the removal was 81% out of 100 ppm Cr(VI) after 2 h of stirring. A comparison between a simulated sample containing 100 ppm Cr(VI) and a true wastewater sample containing 100 ppm Cr(VI), 19 ppm Al, 30 ppm Mg, 49 ppm Ca, and 10 ppm B, showed that the adsorption process is satisfactory and selective for Cr(VI). (C) 2002 Elsevier Science Ltd. All rights reserved.