Synergistic co-adsorptive removal of crystal violet and chromium(vi) from water by pozzolan-charcoal based geopolymer composites

Abstract

In this study, the geopolymer composites GP₀, GP_7.5-CP, and GP_10-CP were synthesized using mixtures of pozzolan (Pz) and waste charcoal powders (CP) as precursors with CP mass contents of 0, 7.5 and 10%, respectively. The geocomposites obtained were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Brunauer–Emmett–Teller (BET) surface analysis and scanning electron microscopy analysis (SEM). The sequestration performance for crystal violet (CV) and chromium VI (Cr VI) was evaluated in mono- and bi-component systems. The incorporation of 7.5% CP (GP_7.5-CP) resulted in a ∼19% increase in the specific surface area and densification of the functional groups. Incorporation of CP increased the adsorption capacity due to (1) increased surface areas (2) increased active functional group density and distribution, and (3) shielding effect of CP on acidic sites improving CV adsorption. The adsorption capacities in binary systems were 45 and 59% higher than in single solute systems for CV and Cr(VI), respectively. The R_q > 1 and 1/n > 1 implied synergistic and cooperative adsorption, denoting adsorption of Cr(VI) creates new binding sites for CV adsorption. Ultrahigh adsorption density of 2803 and 3659 mg g⁻¹ in single solute systems was achieved for CV and Cr(VI), respectively. The adsorption mechanism is multi-mechanistic involving the reduction of HCrO₄⁻ to Cr³⁺ by the donor groups of geocomposites, ion exchange, electrostatic and hydrogen bonding interactions. Modification of geopolymers with coal ash stupendously ameliorates their adsorption capacity for simultaneous adsorption of dyes and heavy metals in water.