Heavy metal removal from industrial effluent using bio-sorbent blends

There is extensive literature on the use of adsorbents derived from waste to treat industrial effluents containing heavy metal ions. However, there is limited information on the use of adsorbent blends. This is applicable for treating effluents which contain a number of heavy metals, so any one single adsorbent may not be suitable for achieving high percent removal of all ions. The present work employs adsorbent blends to treat an electrochemical effluent in batch mode. This work aims to provide valuable insights on the interaction of heavy metals with the adsorbents in blends. Effluent from an electrochemical industry was treated with blends of calcium bentonite, fly ash and wheat bran in different compositions to remove heavy metal ions (Fe, Ni, Cu, As, Zn, Cd) from an aqueous effluent solution. The optimal set of conditions identified were pH, 5-7; contact time, 60-90 min; agitation speed of 200 rpm; adsorbent dosage of 1g/50 mL; and particle size of 150-300 mu. Metal ions arsenic, zinc and cadmium were completely removed. The percentage removal of (Fe, Ni, Cu) metal ions was in the order Fe(II)(96.73%)>Ni(II)(74.03%)>Cu(II)(70.70%) at optimum conditions in a short equilibrium time of 90 min. Batch experiments were conducted to determine the factors such as adsorbent composition, temperature, pH, agitation speed, dosage, contact time and particle size affecting adsorption, and all these parameters were found to strongly influence the adsorption. Experimental data were analyzed for Langmuir and Freundlich isotherms. Langmuir isotherm shows the maximum adsorption capacities were in order Fe(II)146.1 mg/g>Ni(II) 115.9mg/g>Cu(II) 74.5mg/g. Freundlich parameter n' was found to be greater than 1 which showed that the adsorption is feasible for all three metals. Freundlich isotherm fit the data comparatively better, but neither of the two isotherms satisfactorily explained the adsorption, which indicated heterogeneity of adsorption. Kinetic studies were performed and analyzed for pseudo-first- and pseudo-second-order kinetics and the Weber-Morris intraparticle diffusion model. The adsorption data fit accurately to pseudo-second-order kinetic model with coefficient of correlation values greater than 0.99 for all three heavy metals. The kinetic constants were found to be 8.49 * 10(-3),5.82 * 10(-3),5.27 * 10-(3)g/mg min for Fe(II), Ni(II) and Cu(II), respectively. From the intraparticle diffusion model, it was inferred that there were different rate-limiting steps during the duration of the adsorption process, including intraparticle diffusion and boundary layer diffusion.