The Adsorption of Arsenate and Arsenite Ions on Oxidic Substrates Prepared with a Variable-Charge Lithological Material

The adsorption of As(V) and As(III) (0.01-1 mM) on a calcined oxidic lithologic material substrate with pH-dependent surface variable charges, chemically modifiable, was investigated. The substrate was prepared via thermal treatment using a natural lithologic material rich in amphoteric oxides of Fe, Al, Mn and Ti. The calcined substrate was treated with acid media (HCl 0.1) to homogenize the positive charge density on the oxide surface via oxide protonation so that anion adsorption would be favored. A batch experiment was performed on the acid-treated substrate (activated) and non-activated substrate. L-type isotherms were obtained, which fit the Freundlich model. Isotherm constants showed that there was a greater affinity between the activated substrate and As(V) (<i>K</i> = 10.58) compared to As(III) (<i>K</i> = 5.45). The adsorption capacity of the activated substrate was two times greater than that of the non-activated substrate, As(V) (<i>K_act</i> = 10.58 and <i>K_noact</i> = 5.45) vs. As(III) (<i>K_act</i> = 5.45 y <i>K_noact</i> = 2.44), which was due to the greater positive charge density on the activated surface, created by the protonation of the surface oxides. Protons were liberated during the adsorption reaction (As(V): 2.17 × 10^-3 and As(III): 0.96 × 10^-3 mmol/mL). The forms H₂AsO₄^- and H₃AsO₃ deprotonated when adsorbed by the surface groups M-OH2+ (M: Fe, Al). Kinetic data showed a second-order process for As(V) adsorption and a first-order process for As(III) adsorption. The adsorption rate on the activated substrate was two times greater compared with the non-activated substrate: As(V) (<i>k_act</i> = 3.78 × 10^-5 L/mg·min and <i>k_noact</i> = 2.16 × 10^-5 L/mg·min) vs. As(III) (<i>k_act</i> = 0.055 h^-1 and <i>k_noact</i> = 0.027 h^-1). The tested substrate is potentially useful as a low-cost natural material for arsenic removal from contaminated water.