Evaluation of low-cost geo-adsorbents for As(V) removal

Four low-cost iron-bearing geo-adsorbents were tested for As(V) removal without pretreatment: Montanit300® (M), diatomite (D), pumice (P) and black sand (BS). The solids were carefully characterized by different analytical techniques (SEM-EDX, TPD-pyridine, N₂ Physisorption, XRD and point of zero charge). The adsorption of As(V) was evaluated through isotherms and kinetic studies (bottled water matrix, pH₀=8,25 g/L of solids), and experiments addressing pH effect (pH₀=3.6,7.5,11) and the presence of interfering anions (Cl⁻, SO₄²⁻, NO₃⁻, PO₄³⁻). Experimental results were fitted to the Freundlich and Langmuir sorption isotherms. Under the employed conditions, P showed negligible adsorption, M and D presented adsorption capacities around 0.02 mg/g and, in spite its lower surface area, BS displayed the highest value (0.045 mg/g), which relates to a higher density of Fe species, M and D samples were easily regenerated (80%–100% desorption) through a basic treatment (0.01 M NaOH) and presented fast adsorption kinetics (1 h for D and seconds for M). BS showed a slow adsorption kinetic (24 h) and poor regeneration (only 50% desorption). Sample M (natural zeolite) resulted a promising option due to its remarkably fast adsorption kinetic, easy regeneration and adsorption capacity suitable for systems with relatively low As concentrations.