Hexavalent chromium removal by polyacrylic acid-grafted Macadamia nutshell powder through adsorption-reduction mechanism: Adsorption isotherms, kinetics and thermodynamics

摘要

Hydrochloric acid-treated Macadamia nutshell (AMM) powder was grafted with acrylic acid monomers at different doses of 0.5, 1.0 and 2.0 M to incorporate the carboxylic groups on its surface. The grafted materials were labeled 0.5 GAM, 1 GAM and 2 GAM signifying the grafting ratios. The incorporation of acrylic acid was demonstrated by an increase in oxygen content from 0.18% to 1.20% in the elemental analysis. The splitting and shifting of the alcohol intermolecular bonded -OH functional groups at 3330 cm(-1) to 3396 and 3321 cm(-1) observed after grafting was further proof of the addition of acrylic acid. Scanning electron microscopy (SEM) images revealed that the materials exhibited stacked sheets with spherical halo openings typical of biomaterials. The maximum conditions for chromium(VI) removal were obtained at pH 2, 180 min contact time, 7.5 g/L adsorbent dosage concentration and a temperature of 40 degrees C. The adsorption mechanism was best described by the monolayer sorption postulated by Langmuir with reported adsorption capacity of 39.21 mg/g. The best fit for the kinetic data was obtained with the pseudo-second-order rate model suggesting a chemisorption nature of interaction. Accordingly, the thermodynamic factors were feasible, spontaneous and endothermic. The acrylic acid-functionalized Macadamia adsorbent showed greater potential compared to other biosorbents in the removal of hexavalent chromium from aqueous solution in the presence of competing anions.