Modified coal fly ash as a low-cost, efficient, green, and stable adsorbent for heavy metal removal from aqueous solution

Abstract

The treatment of an industrial effluent contaminated with a high concentration of Ni(II) and Cu(II) was executed using naturally available biomasses. Low-cost adsorbents such as wheat bran (WB), calcium bentonite (CB), orange peel (OP), and fly ash (FA) were synthesized by different protocols and performed further modifications to remove Ni(II) and Cu(II). The highest removal efficiency for both metals was achieved by modified fly ash with zeolite (FA-Z). Physio-chemical characterization studies were performed using field emission scanning electron microscopy (FESEM), Fourier transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS), and Brunauer–Emmett–Teller (BET). The Barrett–Joyner–Halenda (BJH) results have indicated the adsorbent had dominant mesopores with a BET surface area of 346.62 m2/g. The effect of various operating factors on the metal removal by FA-Z was investigated and optimized using the central composite design (CCD) matrix of response surface methodology (RSM). The highest removal efficiency of Cu(II) and Ni(II) was 99.04% and 97.73%, respectively, at optimal conditions in the batch study. The highest removal efficiency for Cu(II) and Ni(II) was noticed at a bed height of 5 cm, initial concentration of 100 mg/L, and flow rate of 5 mL/min, which is equal to 99% and 96%, respectively in the column study. The Langmuir adsorption isotherm holds the best for Ni(II) with a maximum adsorption capacity of 60.69 mg/g, indicating monolayer adsorption is favorable, whereas, for Cu(II), Freundlich was found to be the best fit with a maximum adsorption capacity of 96.42 mg/g, shows the adsorbent surface is heterogeneous, and uptake of adsorbate as multi-layered. The kinetic study represented the pseudo-second-order (PSO) model was the best fit for both Cu(II) and Ni(II) sorption, suggesting a process to show chemisorption. The efficiency of modified fly ash with zeolite in removing Ni(II) and Cu(II) in multi-metal and real effluent was ~ 60% and ~ 95%, respectively. The FA-Z desorption was obtained up to 4 cycles, and the desorption efficiency of Cu(II) and Ni(II) metal ions achieved 95% and 93%, respectively.