Chitosan-nano CuO composite for removal of mercury (II): Box-Behnken design optimization and adsorption mechanism

The study aimed to develop an adsorbent for extracting mercury (II) from water by combining chitosan beads with green copper oxide nanoparticles. This resulted in the synthesis of the CuO NPs@CSC composite sponge, achieved by loading CuO NPs onto citrate-crosslinked chitosan (CSC). Characterization involved X-ray diffraction, X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, and scanning electron microscopy. The BET method confirmed a higher surface area of the adsorbent at 285.55 m2/g, suggesting its potential for effective mercury (II) removal from water. This research aligns with broader efforts in environmental science and nanotechnology to create advanced materials for water purification. The characterization techniques ensure the suitability of the synthesized material for its intended application, and the significant surface area enhances its capacity for contaminant adsorption. The study investigated the impact of adsorbent dosage, pH, and initial Hg (II) concentration on mercury (II) adsorption. Results showed a fit with the pseudo-second-order kinetic model and Langmuir adsorption isotherm model. Using the Dubinin-Radushkevich model (adsorption energy: 22.74 kJ mol-1), chemisorption was identified. Notably, the adsorption process was found to be endothermic, indicating that higher temperatures led to increased removal capacity and related parameters. This temperature influence was explored systematically. Additionally, the study concluded that the adsorption reaction was spontaneous, evidenced by a positive entropy change. This analysis contributes valuable insights into the thermodynamics and kinetics of mercury (II) adsorption in the studied system. The CuO NPs@CSC composite sponge achieved an impressive adsorption capacity of 672 mg/g. Even after five consecutive cycles, it maintained strong adsorption capabilities with 84.5 % removal efficiency. Remarkably, over six reuse cycles, there were no observable changes in chemical composition, and XRD peaks remained consistent before and after each cycle. The study delved into the interaction mechanism between the CuO NPs@CSC composite sponge and heavy metals. Utilizing the Box-Behnken design (BBD), the adsorption process was optimized for enhanced efficiency.