Arsenate and arsenite adsorption onto iron-coated cork granulates

There is a growing demand for low-cost, effective adsorbents for arsenic removal from water intended for human consumption in affected rural areas. This work presents a novel adsorbent based on the coating of cork granulates with iron (oxy)hydroxides for the removal of As(III) and As(V) from aqueous matrices. A 2^6-3 fractional factorial design was used to determine the optimal conditions for the iron coating procedure. The optimal adsorbent was produced by coating low-density cork granulates with iron (oxy)hydroxides precipitated from a 0.05 mol L^-1 FeCl₃ solution at pH 7, 20 °C temperature and 20 g L^-1 S/L ratio, in a single coating cycle. Arsenic adsorption was found to be dependent on pH, with inverse trends for As(III) and As(V). The iron leaching from the adsorbent was also taken into account to select the optimum pH, which was pH 9 for As(III) and pH 3 for As(V). Adsorption kinetics were better described by the pseudo-second-order model for As(III) and the Elovich model for As(V). Equilibrium was reached in 16 h for As(III) at pH 9 and 48 h for As(V) at pH 3. The isotherm models indicated different adsorption behaviours for As(III) and As(V), with better fits by Langmuir and Freundlich models, respectively. The Langmuir maximum adsorption capacity of iron-coated cork adsorbent for As(III) at pH 9 was 4.9 ± 0.3 mg g^-1. However, at low equilibrium concentrations, As(V) adsorption was higher than As(III) (e.g. 2.1 ± 0.2 mg g^-1 in equilibrium with 0.16 ± 0.03 mg L^-1). Speciation studies and XPS analyses indicated that no substantial oxidation of As(III) to As(V) occurred during the adsorption process. The study shows that iron coating can enhance both arsenate and arsenite adsorption capacity of cork materials, leading to an innovative natural adsorbent with high resilience and stability, with possible application in arsenic remediation.