Adsorptive removal of As(III) from aqueous solution by waste litchi pericarps

The present study investigated the removal of arsenite anions (AsO₃^3-, referred to as As(III)) from aqueous solutions by waste litchi pericarps (LPs). Influential factors such as the adsorbent dose, contact time, solution pH, and initial As(III) concentration were investigated. The optimum conditions for As(III) adsorption by the LPs occurred at a contact time of 60 min, adsorbent dose of 10.0 g/L, solution pH of 5.0, and initial As(III) concentration of 1 mg/L. A Box-Behnken design with three variables (adsorbent dose, contact time, and solution pH) at three different levels was studied to identify the correlations between the influential factors and the As(III) adsorption; the results showed a significant interaction between the adsorbent dosage and pH. Additionally, adsorption isotherms, kinetics, and thermodynamics were investigated to explore the As(III) adsorption mechanism. Adsorption by the LPs conformed to the Langmuir, Redlich-Peterson, and Koble-Corrigan isotherm models, suggesting that the process proceeds via monolayer, homogeneous adsorption. In addition, the As(III) adsorption could be characterized by a pseudo-second-order mechanism, revealing that the rate-limiting step might be chemisorption. The thermodynamic studies showed that As(III) adsorption by the LPs was spontaneous and endothermic, and disorder at the solid-liquid interface increased in the adsorption process.