Natural Volcanic Tuff-Based Geopolymer for Zn Removal: Adsorption Isotherm, Kinetic, and Thermodynamic Study

Natural volcanic tuff was used for the synthesis of geopolymer and then for the removal of Zn+2. The characteristics of the natural volcanic tuff and the synthesized geopolymer were determined by X-ray diffraction (XRD), X-ray fluorescence (XRF), Fourier transform infrared (FTIR), and scanning electron microscopy (SEM). Results referred that the synthesized geopolymer had a higher efficiency uptake of 97.7 % as against 78.5 % for the natural volcanic tuff. The uptake capacity of geopolymer for Zn+2 adsorption increased with increasing temperature in the studied range of 25–45 °C, contact time up to 30 min, pH up to 7, and initial concentration up to 160 ppm, while it decreased with an increase in geopolymer dosage. The isotherm study showed best fit on Langmuir and Radlich-Peterson models. The maximum uptake capacity obtained from Langmuir model increased from 14.7 to 17.63 mg/g as the temperature increased from 25 to 45 °C. The pseudo-second-order model showed the best fitness for the experimental data followed by intraparticle diffusion model. The adsorption process can be characterized as endothermic, homogeneous, spontaneous, irreversible, physical, and a high adhesion of the ions to the geopolymer surface. The results obtained buttressed the feasibility and applicability of producing geopolymer from natural volcanic tuff for the removal of heavy metals.