Adsorption of chromium(VI) and crystal violet onto granular biopolymer-silica pillared clay composites from aqueous solutions

Abstract

The present study deals with the adsorption of chromium oxyanions (Cr(VI)) as a metallic pollutant and Crystal Violet (CV) dye from an aqueous solution. Silica Pillared Clay (SPC) was prepared using the sol–gel method by SiO2 pillaring agent and cationic surfactant template. The main results of the pillaring process were the significant increase in specific surface area (40 to 600 m2/g), pore volume (0.13 to 1.43 cm3/g), and the formation of a mesopore structure. The powdered SPC was granulated by a combination of biopolymers (alginate and chitosan), which led to a decrease in SBET to 455 m2/g and the addition of functional groups (amine and carboxyl). The best pH for Cr(VI) adsorption was 3; however, CV adsorption varied slightly by pH. GBP-SPC's maximum monolayer adsorption capacities for Cr(VI) and CV were 66.33 and 208.9 mg/g, respectively. The kinetic studies showed the higher initial sorption rate of CV (29 mg/g.h) compared to Cr(VI) (18 mg/g.h). The adsorption of Cr(VI) was exothermic with an activation energy of 22.08 kJ/mol; in contrast, the adsorption of CV was endothermic with an activation energy of 13.95 kJ/mol, both in the range of physical adsorption. The higher affinity of GBP-SPC towards CV is related to the same behavior in the raw clay and SPC. Due to their permanent negative charge structure, these samples showed higher attraction towards cationic dye. Binary solution adsorption revealed that the presence of CV had a negligible negative effect on Cr(VI) uptake, whereas CV adsorption was significantly improved in the presence of Cr(VI).