Optimization of Arsenate Adsorption over Aluminum-Impregnated Tea Waste Biochar Using RSM–Central Composite Design and Adsorption Mechanism

Abstract

Aluminum-impregnated tea waste activated carbon (Al-TC) synthesized for adsorption of arsenate at optimum conditions. The Al-TC produced through slow pyrolysis of tea waste followed by the impregnation of anhydrous aluminum. The operating conditions (initial metal concentration, pH, contact time, and adsorbent dose) were optimized using response surface methodology–central composite design statistical technique, with percentage arsenate removal as a targeted response. The response surface analyses and analysis of variance reveals an excellent fit of experimental data into the quadratic model, and the interactive effect of pH and initial concentration was highly significant on arsenate removal capacity. The obtained optimal conditions corresponding for good percentage arsenate removal were at 100 μg/L, pH 6.0, adsorbent dose 1.0 g, and contact time 1 h. The synthesized mesoporous adsorbent possesses a high surface area of 396 m2/g and an excellent uptake capacity of 99.6 μg/g. The probable mechanism for adsorption proposes that electrostatic forces and hydrogen bonding are responsible for adsorption of arsenic species (H2AsO4− and H3AsO4) at pH 2.0, the arsenic species, including HAsO4−2 and H2AsO4− are removed through adsorption and ion exchange at pH 6.0, and weak adsorption of AsO43− through ion exchange at pH 10.0. Coexisting PO43− and SiO32− ions negatively affect the arsenate adsorption whereas CO32−, SO42−, Ca2+, and Mg2+ ions do not have a significant impact on the adsorption process. The arsenate’s using Al-TC obeyed the Langmuir isotherm model and the kinetic data best fits into the pseudo-second order model. The adsorption process has proven to be spontaneous and endothermic. The produced Al-TC can be used very effectively for the treatment of arsenate containing aqueous media.