Removal of hexavalent chromium from contaminated synthetic groundwater via functionalized carbon nanomaterials modified with zinc and potassium

Chromium has been shown to be a significant contributor to water pollution, leading to cancer. This study aimed to investigate the potential of various functionalized carbon nanomaterials for removing Cr(VI) from synthetic groundwater. Functionalized carbon nanomaterials with layered and tube-like structures were designed on the basis of thermal methods (KOH-activated micrographite sheets) and impregnation methods by anchoring K and Zn on carbon nanotubes (CNTs), respectively, for the removal of Cr(VI) from contaminated synthetic groundwater. Several experimental parameters, including the concentration of the solution, pH, adsorbent dosage, and duration of contact, were systematically varied in a series of batch experiments. The optimal conditions were determined to be a 30 mg/20 mL adsorbent dosage (after the dosage test was 5-50 mg), a pH of 2 (tested at 1-240 min), and a constant agitation speed for all adsorbents. The maximum Cr(VI) removal by K-CNTs (83.04%) occurred at pH 4 and 25 °C and at a constant agitation speed. Zn-CNTs achieved up to 79% removal efficiency at pH 2. However, at neutral pH (8), only K-CNTs maintained a high removal capacity (97%), whereas the other adsorbents exhibited an approximately 53% decrease in removal efficiency. The Cr(VI) removal efficiency reached 95% and 32.7% after five adsorption/desorption cycle tests for K and Zn-CNTs, respectively, at pH = 8, and K-CNTs achieved 91% removal after six cycles, which is particularly noteworthy. Fourier transform infrared (FTIR) spectroscopy was employed to observe the adsorption processes in the polluted water samples, both before and after treatment. The results indicated an increase in the specific surface area and beneficial development of adsorption sites on the CNTs. To elucidate the adsorption mechanisms, the experimental data were analyzed via various isotherm models, namely, the Langmuir, Freundlich, Temkin, and Sips models. Additionally, the adsorption kinetics were examined via pseudo-first-order and pseudo-second-order kinetic models to characterize the temporal aspects of the adsorption process.