SnCl2 Treated Red Mud as an Effective Adsorbent for the Removal of Toxic Molybdate Ions from Industrial Wastewater

Abstract

Molybdate removal from wastewaters from industries, mining and nuclear power stations, is an important task in the water remediation methodologies. This oxyanion of molybdenum is difficult to be removed and its presence in aquatic environment is detrimental to aquatic life and endogenous human beings due its high toxicity and bio-accumulation nature. In present investigation, SnCl2 treated red mud (TRM) and zirconium alginate beads doped with TRM (TRM-Zr.alg) are observed to have good affinity for molybdate ions. Hence, these materials were investigated as adsorbents and various extraction conditions viz., pH, contact time, dosage of TRM and TRM-Zr.alg, initial molybdate concentration and temperature are optimized for the maximum removal of molybdate ions. The optimized conditions were pH: 2.5; dosage: 2.0 g/L for TRM and 2.25 g/L for TRM-Zr.alg; contact time: 45 min for TRM and 60 min for TRM-Zr.alg; temperature: 30 ± 1 ºC and rpm: 300. Interferences of co-ions in the solutions were investigated and the spent TRM and TRM-Zr-alg were also nvestigated for their regeneration and reuse. The endothermic and spontaneity of sorption process can be inferred from the negative ΔG and positive ΔH values and further, the high values of ΔH indicate the sorption mechanism is not ‘physisorption’ but more inclined towards ion-exchange and/or some sort of surface complex formation between molybdate and adsorbents. The sorption process was analyzed and observed that Freundlich model well explains, indicating the heterogeneous surface and multilayer adsorption. The kinetics of adsorption is explained by pseudo-second order model. The methodology developed was successfully applied to treat industrial and mining effluents samples. The novelty of the present investigation is that red mud, a waste material, is explored for its inherent potentials as adsorbent for extracting the highly toxic molybdate ions from wastewater.