Adsorption kinetics, thermodynamics, and equilibrium of α-toluic acid onto calcium peroxide nanoparticles

The present work addresses the kinetics, thermodynamics, and equilibrium of α-toluic acid adsorption onto calcium peroxide (CaO2) nanoparticles. CaO2 nanoparticles were synthesized by chemical precipitation method and characterized using XRD, TEM, and FT-IR. Characterization results confirmed that CaO2 nanoparticles were in the size range of 5–15 nm. The rate of α-toluic acid adsorption was determined by fitting the batch adsorption experimental data with pseudo first order, pseudo second order, Elovich, intra-particle diffusion, fractional power, and Bangham kinetic models. Amongst all models, the pseudo second order model showed good correlation with a rate constant, k2 = 8.53 × 10−5 g mg−1 min−1. Thermodynamic parameters (ΔG°, ΔH°, and ΔS°) showed that adsorption is more favorable at low temperature, and an exothermic process. The equilibrium data were analyzed with Langmuir, Freundlich, Temkin, Toth, and Radke-Prausnitz isotherms, and the best fit was observed with Langmuir isotherm suggesting monolayer and chemisorption of α-toluic acid. This was also confirmed from error analysis. On quantitative basis, the adsorption capacity of CaO2 nanoparticle adsorbent was found to be around 30% higher as compared to the conventional CaO2. Based on FT-IR analysis, a mechanism for α-toluic acid removal from aqueous solution using CaO2 nanoparticles was proposed.