Investigation of the Thermodynamics for the Removal of As(III) and As(V) from Water Using Synthesized ZnO Nanoparticles and the Effects of pH, Temperature, and Time

Abstract

In the present study, the removal of both As(III) and As(V) from aqueous solutions using synthesized ZnO nanomaterials was achieved. The ZnO nanomaterial was synthesized using a precipitation technique and characterized using XRD, SEM, and Raman spectroscopy. XRD confirmed the ZnO nanoparticles were present in the hexagonal wurtzite structure. SEM of the particles showed they were aggregates of triangular and spherical particles. The average nanoparticle size was determined to be 62.03 ± 4.06 nm using Scherrer’s analysis of the three largest diffraction peaks. Raman spectroscopy of the ZnO nanoparticles showed only ZnO peaks, whereas the after-reaction samples indicated that As(V) was present in both As(V)- and As(III)-reacted samples. The adsorption of the ions was determined to be pH-independent, and a binding pH of 4 was selected as the pH for reaction. Batch isotherm studies showed the highest binding capacities occurred at 4 °C with 5.83 mg/g and 14.68 mg/g for As(III) and As(V), respectively. Thermodynamic studies indicated an exothermic reaction occurred and the binding of both As(III) and As(VI) took place through chemisorption, which was determined by the ΔH values of −47.29 and −63.4 kJ/mol for As(V) and As(III), respectively. In addition, the change in Gibbs free energy, ΔG, for the reaction confirmed the exothermic nature of the reaction; the spontaneity of the reaction decreased with increasing temperature. Results from batch time dependency studies showed the reaction occurred within the first 60 min of contact time.