Abstract
The adsorptive performance of the natural Saudi Arabian volcanic ash (VA) and its NaOH-modified form (VA-NaOH) for Malachite green (MG) uptake from water were evaluated using response surface methodology (RSM) modeling technique and mechanistic aspects studies. The characterization analyses showed that both materials possess abundant functional groups (O-H, Si-OH, Si-O and Al-O), mineral salts (SiO2, Al2O3, CaO, Fe2O3, MgO), better surface characteristics leading to favorable adsorption of MG dye from water. The MG removals and the uptake capacities data were well described by cubic polynomial models with optimal condition at 25 degrees C, initial MG concentration of 120 mg/L, and pH 10 for VA and VA-NaOH. The kinetics of MG adsorption was best explained by pseudo-second order, while the Redlich-Peterson model fitted the equilibrium isotherms. The maximum Langmuir uptake capacity of 152 mg/g (VA) and 158 mg/g (VA-NaOH). The adsorption mechanism was speculated to be endothermic and involving both mono and multi-layer surface adsorption comprising chemisorption process (Delta G degrees = -27.6 to -33.9 kJ/mol), with increasing randomness scale at the liquid/solid interface without considerable entropy changes. The excellent regeneration ability, after successive cycles of desorption-adsorption, further indicates the high potential of the natural Saudi volcanic ash as excellent adsorbent for dyes removal from aqueous systems.