Sustainable and rapid pillared clay synthesis with applications in removal of anionic and cationic dyes

Abstract

Adsorbents that remove both anionic and cationic dyes in aqueous solutions are essential in wastewater treatment. To address this need, chromium metal polycations are intercalated in bentonite clay and the efficacy of the resulting adsorbent for anionic as well as cationic dyes removal are explored in this study. The problems to be addressed are ensuring permanent porosity in the raw clay which tends to swell upon contact with water, and identifying optimal conditions for high capacity of the adsorbent. Further the time-consuming steps involved in preparation of the pillaring solution and its intercalation in the clay need to be expedited. Pillared clays with different chromium contents were synthesized using an economical and sustainable method. Ultrasonication at 35 kHz frequency intensified chromium polycations synthesis and their intercalation into the clay by reducing the time from 24 h to 40 min. Cr pillared clays with high basal spacing, surface area, total pore volume, and micropore volume were synthesized. The residual Cr polycations solution after intercalation could be reused in two additional stages, minimizing the disposal of spent Cr solution into the environment. The pillared clay's point of zero charge was reduced considerably with increasing Cr content while the basal spacing increased. Cr was stable in calcined pillared clay matrix and was non leaching. Cationic Basic Orange 2 dye and anionic Acid Blue 74 dye was used for adsorption studies with Cr pillared clays. Effects of dosage, pH, initial concentration of the dyes, and temperature on the adsorbent loading were analyzed. The maximum adsorbent loading for Acid Blue 74 and Basic Orange 2 dyes were 81 mg/g and 170.9 mg/g respectively. Gibbs free energy, enthalpy and entropy changes were estimated. Cr pillared clay adsorbents could successfully adsorb both cationic and anionic dyes. Ultrasound accelerated the Basic Orange 2 dye desorption time from the clay by 48 times and enabled its reuse over four more cycles without an appreciable drop in capacity. Basic Orange 2 dye could still adsorb on pillared clay which was permanently saturated with Acid Blue 74 dye. The ultrasound aided rapid chromium pillared clay adsorbent synthesis process proposed in this study enables the reuse of the intercalating solution over three cycles without discharge into the environment and led to a non-leachable as well as reusable adsorbent. This adsorbent had adjustable point of zero charge depending on its chromium content and revealed a high capacity for anionic as well as cationic dyes.