Kinetics, isotherm, thermodynamic, and adsorption mechanism studies of La(OH)3-modified exfoliated vermiculites as highly efficient phosphate adsorbents

Abstract

La(OH)3-modified exfoliated vermiculites were fabricated, characterized, and investigated for phosphate removal in batch tests for the first time. The BET surface area of the La5EV adsorbent, which was synthesized in the solution consisting of 5.00mmol/g La/exfoliated vermiculite (EV), was significantly increased, accompanied with a larger pore diameter and greater total pore volume, as compared with the unmodified EV. The phosphate adsorption capacity of La5EV was approximately one order of magnitude higher than that of EV. Effects of initial phosphate concentration, contact time, temperature, pH, and co-existing ions on the adsorption capacity of La5EV were investigated in detail. The experimental equilibrium data were fitted better by using the Langmuir model (maximum adsorption capacity of 79.6mg P/g) than the Dubinin–Radushkevich or the Freundlich model, suggesting that the adsorption feature be monolayer. Meanwhile, the phosphate adsorption kinetics could be well described by the pseudo-second-order model, and the adsorption process might be controlled by boundary layer (film) diffusion. ΔG°, ΔH° and ΔS° were also determined, in which it was found that the phosphate adsorption onto La5EV was spontaneous and exothermic in nature. The phosphate adsorption of La5EV was pH-dependent; that it exhibited a high adsorption capacity in the pH range of 3.0–7.0. The presence of 0.1M CO32- caused a reduction in phosphate adsorption capacity by 54.3%. On the other side, the addition of F−, Cl−, NO3-, and SO42- had neglectable effects on its phosphate removal capacities. In the synthetic secondary treated wastewater with a low phosphate concentration of 2mg P/L, 97.9% of its final adsorption capacity reached in the first 10min and the phosphate concentration dramatically decreased below 50μg P/L. The spent La5EV could be regenerated and reused in phosphate adsorption; that could remove more than 70% phosphate in the 3rd adsorption–desorption cycle.