Characterizing acidic fly ash with and without biomass combustion residue for adsorptive removal of crystal violet with optimization of mixed adsorbent by response surface modeling

Abstract

In this article, removal of crystal violet (CV) by adsorption on acidic fly ash collected from an Indian wet ESP with and without biomass combustion residue (BCR), another industrial waste is reported. Initially, fly ash was characterized by its solid density, slurry pH, particle size, pHPZC, Brauner–Emmette–Teller (BET) surface area and pore volume analyses, scanning electron microscopy, Fourier transform infrared spectroscopy and X-ray diffraction. The pHPZC and BET surface area of fly ash were 3.30 and 8.57 m2/g, respectively. Batch results revealed high removal (98.33 %) with equilibrium time and optimum dose of 90 min and 7 g/50 mL, respectively. Later, experiments were repeated with the mixed adsorbent (1:1 mass ratio of fly ash and BCR) showing the equilibrium time of 10 min with ~100 % removal. The mixed adsorbent enhanced the adsorption capacity from 1.125 to 9.146 mg/g, i.e., almost by 9 times. Investigations were carried out at various mass fractions of fly ash showing maximum removal of 99.16 % at a mass fraction of 0.1. The response surface modeling revealed the optimum mass fraction and initial concentration of CV of 0.5 and 125 mg/L, respectively, to achieve 94.93 % removal. The corresponding experimental removal observed was 95.99 ± 0.3374 % (mean ± SD of five replicates). The mixed adsorbent showed twofold benefits in the proposed use through extensive pH analyses, such as improving the removal efficiency and maintaining the solution pH well within the safe discharge limit. Experimental results followed the Freundlich isotherm and pseudo-second order kinetics with negligible intra-particle diffusion for both acidic fly ash and mixed adsorbent.