Adsorption of Benzoic Acid by Hydrotalcites and Their Calcined Products

摘要

Benzoic acid (BA) is a common pollutant in industrial wastewater. It has high toxicity and is a threat to human health and the ecosystem. Mg-Al-CO3 hydrotalcites (HTs) and their calcined products (CHTs) are good ion exchangers/adsorbents for removal of toxic anions from contaminated water. In this study, HTs with different Mg/Al molar ratios were prepared by coprecipitation method, and CHTs were obtained by calcining at 500°C. Adsorption of BA by HTs and CHTs at different pH values was first investigated. Taking HT and CHT with an Mg/Al molar ratio of 2:1 as an example, adsorption kinetics and adsorption isotherms of BA by HT and CHT were analyzed, and impacts of adsorbent dosage and adsorption temperature on equilibrium adsorption quantities were studied. Results indicated that the maximum adsorption of HTs and CHTs could be obtained when the initial pH value of BA solution was 4.5, and equilibrium adsorption quantities significantly decreased with increase of pH value. Kinetic data of the adsorption of BA by HT and CHT with an Mg/Al molar ratio of 2:1 under an ambient temperature of 293 K could be fitted by pseudo-second-order equation. It was found that adsorption isotherms obeyed the Freundlich equation. Equilibrium adsorption quantities of BA by HT and CHT gradually decreased with increase of adsorbent added. Values of thermodynamic parameter (ΔH°) indicated that the adsorption of BA was endothermic on HT and was exothermic on CHT. Different adsorption mechanisms of BA on HT and CHT were revealed. Results showed that CHT could be used as an effective adsorbent to remove BA from waste water. This not only provides a new way to remove BA, but also enlarges the application field of CHT.