Fixed-bed column system for Cd²⁺ uptake from aqueous solution by sodium- and thiourea-modified clinoptilolite-rich...

Two kinds of clinoptilolite-rich tuffs from the state of Sonora (Mexico), previously treated with NaCl solution and then modified with thiourea, were investigated to remove Cd²⁺ from aqueous solution in a fixed-bed column system. Experiments were conducted with a 30 mg L^-1 Cd²⁺ solution at a pH value of 5 and three-bed heights of sodium- and thiourea-modified clinoptilolite-rich tuffs. The experimental data were examined with the bed depth service time model (BDST), the Thomas model, and a mass balance approach. According to the BDST model, the dynamic sorption capacity (No) for the thiourea-modified clinoptilolite-rich tuff was 0.0357 Kg L^-1. This value corresponded to 1.2 times higher than that for sodium-modified clinoptilolite-rich tuff (0.0287 Kg L^-1). The experimental data obtained for sodium- modified clinoptilolite-rich tuff (ZSGNa) with 3 cm of bed height, fitted to the Thomas model, with R² = 0.9679 and Cd sorption capacity of 35.4 mg g^-1, while for the thiourea-modified clinoptilolite-rich tuff (ZSGThio) with 4.5 cm of bed height, was 36.52 mg g^-1 with R² = 0.9368. The parameters calculated with the mass balance model describe a favourable process where the system with ZSGThio (6 cm bed height) had the best performance for sorption capacity up to the breakthrough point, q = 22.35 mg g^-1. The zeolite modified with thiourea exhibited higher capacity in Cd²⁺ removal than the one modified with NaCl, in the column with 6 cm of bed height. To investigate the mechanisms of removal of Cd²⁺ by the modified zeolites, Na⁺ was measured to determine the milliequivalents of cadmium removed.