Metakaolin-based geopolymer for mercury removal from model wastewater

Mercury is recognized as a serious hazard, nature bioaccumulative and persistent to the environment. Developing a low-cost, effective, eco-friendly adsorbent for removing mercury constitutes an urgent task. In this study, metakaolin-based geopolymer (MKG) was synthesized and applied to remove mercury from model wastewater by adsorption. The samples were characterized by X-ray fluorescence (XRF), Brunauer-Emmett-Teller (BET), X-ray diffraction (XRD) pattern, Fourier transform infrared (FTIR), and scanning electron microscope (SEM). They confirmed the synthesis of geopolymer due to the mass loss (LOI), crystallinity, peak changes, increase in surface area, and pore size. The highest removal efficiency recorded was 65.1%, with a corresponding adsorption capacity of 38.1 mg/g for MKG in 50 mg of dosage, 50 mg/L initial concentration, at a temperature of 25°C, contact time of 5 h and optimal pH of 3. The kinetic studies showed that mercury removal increased with the contact time until equilibrium after an hour. The adsorption equilibrium data were best described by Freundlich, whereas the pseudo-second-order model best fitted the adsorption kinetics. This may be indicative that chemisorption controlled the adsorption process. The stability studies of MKG adsorbent within the four reagents revealed that MKG has good stability in water, followed by 10% HCl, 3M NaCl, and 5% NaOH. In contrast, the desorption revealed the MKG represented the highest desorption efficiency in 10% HCl, followed by 3M NaCl, deionized water and 5% NaOH, respectively. The highest desorption efficiency was 96.83% for 10% HCl, followed by 91.83% for 3M NaCl, 82.55% for H₂O, and 50.72% for 5% NaOH. PRACTITIONER POINTS: A low-cost metakaolin-based geopolymer (MKG) was synthesized and applied as an adsorbent for removing Hg(II). The most dominant mechanism involved in the removal process of Hg(II) was chemisorption. The maximum adsorption capacity obtained in this study was 38.1 mg/g. Hg(II) removal is influenced by pH zeta potential and temperature. MKG shows good potential for the effective removal of mercury from modeled wastewater.