Effect of pH on arsenite (As3+) sorption on goethite: Kinetic and equilibrium experiments

Sorption on minerals significantly influences the fate and transport of arsenic (As) in subsurface environmental systems. In the study, goethite is widely utilized to remove both arsenate (As(V)) and arsenite (As(III)) species, was the focus. The objectives were to investigate the mechanism and reactions governing the transport of As(III) from the solution to the goethite surface and to evaluate the effects of different pH levels (4, 7, and 10) on its sorption. The sorption experiment evaluated various As(III) concentrations at different pH levels to determine the equilibrium time, while the kinetic experiment involved observing As sorption on goethite at these pH levels until equilibrium was reached. Physiochemical parameters, including pH, contact time, and initial As(III) concentration sorption capacity, were analyzed using Langmuir and Freundlich isotherms, along with pseudo-first and pseudo-second sorption kinetic models. Results showed that the maximum sorption capacity for As(III) was 2.17 mg/g at pH 7. The data fitted better with the Freundlich isotherm than with the Langmuir isotherm model across all pH levels (R2 = 0.99 at pH 7). Although the separation factor (RL) of the Langmuir model was between 0 and 1, indicating favorable sorption. The optimum contact time for As(III) was observed to be 360 mins at all tested pH levels. Kinetic data from As(III) sorption fitted well with the pseudo-second-order model (R2 = 0.99), with the equilibrium rates (k2) being 8.63, 6.08 and 6.48 g/mg⋅min−1 for pH levels 4, 7, and 10, respectively. In conclusion, this study revealed that a pH level of 7 is optimum for the sorption of As(III) from aqueous systems, positioning goethite as a viable As(III) sorbent for further investigation in acidic or alkaline environments.