Removal of tetracycline from aqueous solutions by adsorption on raw Ca-bentonite. Effect of operating conditions and adsorption mechanism

Abstract

• Bentonite had the highest capacity for adsorbing tetracycline (TC) than other clays. • Bentonite presented a maximum adsorption capacity towards TC of 283.5 mg/g. • Adsorption capacity of bentonite was comparable to those of carbon-based materials. • TC adsorbed on the external surface and in the interlayer space of bentonite. • Adsorption of TC was due to cation exchange and electrostatic attraction. This study aims to investigate the adsorption of tetracycline (TC) from water onto several clays with different structural characteristics such as bentonite, sepiolite, vermiculite, halloysite, phlogopite and kaolinite. Among these, raw bentonite (Bent) presented the highest adsorption capacity due to its swelling characteristic, interlayer cations nature, and TC molecular size. The Bent and Bent equilibrated with TC were characterized by SEM, XRD, FT-IR, TGA, N 2 physisorption and Zeta potential techniques. The importance of the pH, ionic strength and temperature upon Bent adsorption capacity towards TC was studied thoroughly, and the reversibility of TC adsorption onto Bent was also analyzed. The results showed that the TC adsorption occurred on the external surface and in the interlayer space of Bent, and both physical and chemical mechanisms governed the adsorption. The basal space (0 0 1) at pH = 7 increased from 1.50 to 1.62 nm, while raised from 1.50 to 1.89 nm at pH = 3. The TC maximum uptake was 283.5 mg/g at pH = 3, relating to cationic exchange between interlaminar cations of Bent and the cationic TC species (TCH 3 + ), and electrostatic attraction among the negative surface of Bent and TCH 3 + . At pH = 7, there are weak electrostatic attractions between the Bent cationic sites and the positively charged dimethylamino group of the zwitterionic TC species (TCH 2 ± ) and the formation of outer-sphere coordination complexes between the TC negative species (TCH - ) and the internal hydration sphere of the interlayer Ca 2+ cation. Additionally, the TC adsorption on Bent was partially reversible and endothermic.