Highly effective sequestration of Ni(II) and Cr(III) ions from aqueous solution using foamed and non-foamed metakaolin based geopolymer.

Heavy metals are considered one of the most hazardous materials, which can cause serious threats to the environment and living organisms. Their toxicity, persistence, and bio accumulative nature make them a prime environmental concern. The present study aims to develop novel adsorbents fabricated from 80% metakaolin and 20% fly ash for producing foamed and non-foamed geopolymer composites, tailored to remove Ni(II) and Cr(III) from an aqueous solution. The synergistic effects emanating from this unique amalgamation of materials present a promising avenue for the effective remediation of Ni(II) and Cr(III) contamination in aqueous environments. The synthesized adsorbents before and after adsorption were characterized by SEM, FTIR, BET, and XRD to provide a better insight into the adsorption process. The experimental data indicated that the maximum adsorption capacities of the non-foamed geopolymer for Ni(II) and Cr(III) were determined to be 28.34 mg/g and 29.81 mg/g, respectively. While, in the case of the foamed geopolymer, the maximum adsorption capacities of 38.46 mg/g and 39.42 mg/g were recorded for Ni(II) and Cr(III), respectively, at an initial concentration of 200 mg/L. Compared to other commonly used adsorbents reported in the literature, such as activated carbon (17.24 mg/g), natural zeolites (7.90 mg/g), and metakaolin-based geopolymer (10.2 mg/g), it was observed that the foamed geopolymer developed in this study exhibits markedly superior adsorption performance for both Ni(II) and Cr(III) ions. According to the obtained results, the porous geopolymer achieved a maximum removal efficiency of 91.67% and 97.21% at 25 °C, pH 5.0, 120- and 180-min contact time, 0.1 g/L adsorbent dosage, and a preliminary concentration of 5 mg/L for Ni(II) and Cr(III), respectively. The adsorption kinetics followed the pseudo-second-order kinetics, whereas the adsorption data fitted well with the Freundlich isotherm model, which indicated that the sorption mechanism is mainly chemisorption. The negative values of ΔG° indicated the spontaneous nature of the adsorption process, and the positive values of enthalpy (ΔH°) values confirm that the adsorption is endothermic. The FTIR analysis confirmed shifts and intensity changes in functional groups such as OH and Si-O-T after adsorption, indicating chemical interaction with heavy metals. Additionally, SEM images revealed morphological differences in both sorbents, suggesting the entrapment of Ni(II) and Cr(III) onto the sorbent after adsorption. The findings suggest that the prepared geopolymer proves to be a promising cost-effective adsorbent for the remediation of heavy metal contamination in industrial wastewater.