Synthesis of a high-purity zeolite A from bagasse fly ash: optimization, characterization, and application as a low-cost adsorbent for cadmium

Abstract

The perpetual demand for sugar led to expansion of the sugar industry creating large quantities of liquid and solid waste materials. Bagasse fly ash (BFA) is one such particulate residue collected from flue gas effluent system of sugarcane bagasse-fired boiler units. The BFA is transformed into a zeolite with better adsorption capacity by optimization of synthesis parameters Si/Al, NaOH/BFA, temperature, and time. HCl pretreatment significantly enhanced the zeolite-forming components of BFA, namely SiO₂, Al₂O_3, and Na₂O from initial 77.85 to 84.24% while simultaneously reducing oxide impurities. The pretreated BFA was fused at 550 °C for 90 min followed by hydrothermal treatment under optimized conditions, including a well-adjusted Si/Al ratio of 1.1, NaOH/BFA ratio of 1:0, synthesis temperature of 100 °C, and a 6-h synthesis duration. The resulting zeolite A possessed high crystallinity of 91.89%. The synthesized zeolite A comprehensively validated through advanced characterization techniques, including X-ray diffraction (XRD), X-ray fluorescence (XRF), scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET), and Fourier transform infrared spectroscopy (FTIR) confirming its structural integrity and enhanced material properties. Analyses confirmed zeolite had a surface area of 126.24 m²/g and a cation exchange capacity (CEC) of 145.33 meq/mol. The ability of synthesized zeolite to adsorb heavy metals (HMs) was tested using Cd⁺² ions, with optimized parameters, namely, pH, adsorbent dosage, concentration, and contact. Under the optimized conditions, the zeolite achieved a maximum Cd⁺² removal of 118.20 mg/g, aligning with pseudo-second-order kinetics and the Langmuir isotherm model. Successive adsorption–desorption studies demonstrated high efficiency, with a slight reduction in subsequent cycles, highlighting the potential of BFA as a zeolite for HM removal. Thermodynamic studies confirmed the adsorption process is endothermic and spontaneous, with a positive entropy change signifying increased randomness at the solid–liquid interface. Cost analysis demonstrated that the cost of BFA-synthesized zeolites was significantly less compared to commercially available zeolites. The zeolite A derived from BFA addresses the issues of waste minimization and metal contamination alleviating environmental burden.