Natural zeolite as a chromium VI removal agent in tannery effluents

This study concerns Cr(VI) removal using zeolites in a batch system for tannery effluent. In the initial stage, natural zeolite (ZN) and synthetic zeolite (ZS) were characterized, obtaining a Si/Al ratio of 4.64 and 1.60, and with predominant clinoptilolite and faujasite phases, the surface area of 9.34 and 25.82 m²/g and cation exchange capacity of 84.05 and 188.72 meq/100 g, respectively. Subsequently, ZN and ZS were activated with HCl and NaOH. Through preliminary tests, with a solution of K₂Cr₂O₇, it was determined that the highest Cr(VI) removal for both, ZN and ZS, was with NaOH activation, obtaining 82 and 56% removal, respectively. According to Ecuadorian regulations, the Cr(VI) concentration exceeds the maximum permissible limits on the tannery effluent. For this effluent, it was determined that the highest Cr(VI) removal, 45%, is obtained with 1 g of ZN activated with NaOH and 100 mL of effluent. With ZN-NaOH, removal tests were carried out in a fixed bed with 5, 10, and 20 g of natural zeolite. The natural zeolite also has chromium removal capacity in the bed system, achieving similar removals to those obtained in the batch experiments, but decreasing the treatment time. Thus, both natural and synthetic zeolites can remove Cr(VI) in tannery effluents, achieving this effluent with permissible limits.