Dolochar Derived Zeolites: Optimal Synthesis and Adsorptive Removal of Water Pollutants

Abstract

Water contamination by different sources is a major environmental issue because of adverse ecological impacts and detrimental health conditions. Adsorption is a highly favoured method due to its versatility and high efficacy in wastewater. Hence, the present work aims to develop low-cost, highly effective adsorbent-dolochar derived nanoporous zeolite to purify contaminated water. Highly versatile an efficient zeolites (X, modified X, and Y) have been synthesized from low-cost material, dolochar. Dolochar is a byproduct generated from the sponge iron industry during the direct reduction of iron. Doochar consists of silica, alumina, and other trace elements, thus it can be used for the zeolite synthesis through alkali activation. The zeolites were synthesized using conventional hydrothermal treatment and non-conventional, ultrasound-assisted hydrothermal treatment. The synthesized zeolites were subsequently used for the wastewater treatment, in removing heavy metals, dyes, and pharmaceuticals. The batch study for conventional adsorption and sono-assisted adsorption were performed, adsorption isotherms, kinetics, and thermodymanics were investigated and the maximum adsorption capacity of the zeolites were evaluated. The synthesized zeolites were characterized using different characterization techniques such as FESEM, TEM, XRD, FTIR, BET, TG-DTA, Raman spectroscopy, and Zeta potential. The zeolite synthesis and the adsorption of pollutants were optimized using Design of Experiments, particularly RSM-BBD. The synthesis parameters such as NaOH/Dolochar ratio, crystallization temperature, and crystallization time were optimized using RSM-BBD to maximize the crystallinity of the synthesized zeolite X (ZX). The optimum conditions obtained include NaOH/Dolochar ratio 1.375, crystallization temperature of 100℃ and crystallization time of 11 h. ZX had a crystallinity of 87.23%, and average crystallite size of 0.79μm. ZX had a pore size of 3.316 nm, specific surface area of 583.117 m2/g, micropore area of 567.226 m2/g, and pore volume of 0.311 cc/g, and ZX was stable at high temperatures up to 800℃. ZX was used for the batch adsorption study of Cd(II) and Pb(II). The optimum parameters for Cd(II) and Pb(II) adsorption includes 6.5 pH, concentration of 50mg/l, adsorbent dosage of 0.25 g/l and 80 min. The maximum adsorption capacity was 714.825 and 738.017 mg/g respectively, following Freundlich isotherm and pseudo-second order kinetics. The ZX was used for the ciprofloxacin (CIP) removal and resulted in 37.786% removal, as zeolite has lesser affinity towards the anionic pollutants. Hence, the surface ZX was modified using Fe(III) to enhance the CIP removal. FeZX was synthesized from ZX using ion-exchange method by varying the Fe(III) concentration from 0.05-0.25M. As observed from SEM and XRD, the iron incorporation didn’t alter the morphology or the crystallinity of the parent material. The CIP removal was maximized using RSM-BBD, the pH, time, initial concentration, and adsorbent dosage were chosen as the input parameters. A removal of 97.974% was obtained at 8.06 pH, time of 59.422 min, concentration of 17.117 mg/l, and adsorbent dosage of 0.478 g/l. Freundlich isotherm and pseudo-second kinetics best fits the experimental data. The nano zeolite X (NaX) was synthesized using ultrasound-assisted hydrothermal method. This technique yields smaller crystals with better efficiency and reduces the time. The synthesized NaX had a cubic octahedral crystals with 88.948% crystallinity. The pore size, pore volume, specific surface area, and micropore area were 3.072nm, 0.354 cc/g, 617.079 m2/g, and 604.782 m2/g. NaX was employed for the sono-assisted removal of dyes such as Rhodamine B (RB), Bismarck brown (BB), and Malchite green (MG). A maximum removal of RB, BB,and MG was 98.841%, 98.543%, and 97.912% respectively, achieved under 8 pH, time of 20 min, adsorbent dosage of 0.35 g/l and initial concentration of 15 mg/l. The Freundlich isotherm and pseudo-second order kinetics well suited the experimental data. Nano zeolite Y (NaY) was hydrothermally synthesized for the sono sorption of sulfamethoxazole (SMX). NaY exhibited 87.178% crystallinity, with cubic octahedral crystals, having pore size of 2.834 nm, pore volume of 0.208 cc/g, specific surface area of 543.661 m2/g, and micropore area of 528.117 m2/g. The SMX removal was maximized using RSM- BBD, the input parameters were pH, sonication time, initial SMX concentration, and adsorbent dosage. A removal of 98.417% was achieved under optimum conditions of 6.123 pH, 56.729 min of sonication time, 11.977 mg/l of SMX concentration, and 0.466 g/l of adsorbent dosage. The Langmuir isotherm and pseudo-second order kinetics were the best fit, with maximum adsorption capacity of 90.909 mg/g. The relevance of the results highlights the importance of using this dolochar-derived nanoporous zeolite as an adsorbent to effectively treat wastewater containing heavy metals, dyes, and pharmaceuticals. Therefore, the concurrent utilization of hazardous solid waste like dolochar to produce value-added materials, for the application in wastewater treatment processes, brings us closer to achieving environmental sustainability.