Alkali-Activated Boiler Fly Ash for Ni(II) Removal: Characterization and Parametric Study

In this study, the adsorption behavior of waste boiler fly ash has been explored for purification of nickel heavy metal ion bearing water. The raw boiler fly ash (RBFA) was alkali modified to improve the adsorption characteristics. The modified boiler fly ash (MBFA) was characterized using the SEM, XRF, XRD, BET, and TGA analyzers to confirm the improved textural, mineralogical, porosity and thermal characteristics of the adsorbent. The adsorption studies were conducted in a batch mode by varying different operational parameters like pH, contact time, heavy metal ion concentration, and time. The MBFA showed higher adsorption capacity (~ 86 mg/g) as compared to RBFA (~ 64.8 mg/g) at optimized conditions. The equilibrium data for Ni(II) sorption were analyzed using Langmuir, Sips, and Freundlich isotherm models. Sips model proves to be superior with R² = 0.99. Thermodynamics of Ni(II) removal showed that the process of adsorption is endothermic and spontaneous in nature. Enthalpy calculated was 2.95 and 18.65 kJ/mol for RBFA and MBFA, respectively. The adsorption kinetics of Ni(II) by both RBFA and MBFA were modeled using pseudo-first-order, fractional order, and intra-particle diffusion equations. The results indicate that the fractional order kinetic equation and intra-particle diffusion model were suitable to describe the nickel adsorption.