Application of natural earth-based materials as adsorbents for the treatment of chromium (VI)-contaminated tannery wastewater: Box-Behnken and fixed-bed column optimization

This study focuses on the application of earth materials (gravel, clay, zeolite, lime rock, sea shell (Diplodonta)) as adsorbents for the removal of chromium (VI) from tannery wastewater. The removal efficiencies and adsorption capacities of chromium (VI) followed this order: pristine lime rock (63.59 %, qe: 65.5 mg/g)> pristine gravel (48.88 % and 50.4 mg/g)> pristine sea shell (39.03 %, qe: 40.2 mg/g)> pristine zeolite (38.5 %, qe: 39.7 mg/g) > pristine clay (33.55 %, qe: 34.5 mg/g). Lime rock and gravel gave the highest chromium (VI) adsorption capacities and hence, were used in subsequent experiments. Employing Box-Behnken design through response surface methodology, gravel exhibited the highest adsorption capacity (40.7 mg/g) at pH 4, and lime rock (56.1 mg/g) at pH 2, with initial chromium (VI) concentration of 150 mg/L and 11-hour contact time. Scanning electron microscopy and Brunauer–Emmett–Teller analysis showed a lack of pores on both adsorbents suggesting electrostatic attraction as the primary removal mechanism. Best-fitted models were Dubinin-Radushkevich and Temkin isotherms for lime rock and gravel, respectively, while pseudo-first-order kinetics suited both. Thermodynamics calculations showed adsorption is exothermic for lime rock and endothermic for gravel. Application of lime rock in fixed-bed column study showed the highest removal percentage and adsorption capacity (16.64 %, qe: 2.3 mg/g). Applying optimum conditions from column study to synthetic tannery wastewater achieved 63.61 % removal and adsorption capacity (qe: 12.7 mg/g) at 150 mg/L initial chromium (VI) concentration. Regeneration of the adsorbents after 3 cycles maintained strong adsorption capacity. The cost-effective, easily-prepared adsorbent underscores its potential for large-scale wastewater treatment.