Elimination of malachite green from aqueous and saline water by laterite-derived Na-polyferrosialate and polyferrophosphosialate geopolymers: A comparative study.

In this study, two laterites (LA)-based geopolymers were synthesized using alkalination (GPAL) and phosphoric acid-activation (GPAC) approaches and applied to scavenge malachite green (MG) dye from aqueous (non-saline) and saline water. The precursor (LA) and geopolymers were characterized by XRF, XRD, TG/DTA SEM, and FTIR techniques. Alkalination and acid-activation resulted in morphologically distinguishable geopolymers with different porosity structures. The dissolution of mineral phases and chemical composition was dependent on the activation medium, resulting in Na-polyferrosialate and polyferrophosphosialate geopolymers for GPAL and GPAC, respectively. The adsorption kinetics data were best described by the pseudo-first-order (PFO) model wherein the adsorption rate, denoted by rate constant (k 1), increased with an increase in ionic strength (salinity). The equilibrium data were best modelled by the Sips isotherm. GPAL had a higher maximum adsorption capacity (Q ms) than GPAC in both aqueous (non-saline) and saline water. The adsorption capacities were increased in saline solution relative to aqueous solutions without NaCl, from 12.4 to 54.1 mg/g and from 57.1 to 92.0 mg/g for GPAC and GPAL, respectively. The adsorption mechanism entailed electrostatic interactions, hydrogen bonding and hydrophobic interactions. Salinity decreased the solubility of MG and increased the hydrophobic interactions and affinity of MG, implied by apparent equilibrium constant (K a), resulting in increased adsorption density (Q ms). These results indicate that alkaline-based geopolymers are better candidates for scavenging MG dye from water, especially in saline environments. [ABSTRACT FROM AUTHOR]