Elimination of Ni(II) from wastewater using metal-organic frameworks and activated algae encapsulated in chitosan/carboxymethyl cellulose hydrogel beads: Adsorption isotherm, kinetic, and optimizing via Box-Behnken design optimization

Abstract

The study investigated the enhancement of stability and efficacy in the removal of bivalent nickel ions (Ni(II)) by utilizing a cerium metal-organic framework (Ce-MOF) encapsulated within a food-grade algal matrix. This composite material is integrated into a dual-layer hydrogel containing chitosan and carboxymethyl cellulose. The enhancement of structural integrity in the final product can be attributed to the cross-linking process with epichlorohydrin, leading to the development of Ce-MOF-FGA/CMC-CS hydrogel beads. A comprehensive characterization of the adsorbent was conducted utilizing various analytical methods. These included X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectroscopy (EDX), nitrogen adsorption/desorption isotherm analysis, and field emission scanning electron microscopy (FESEM), all aimed at clarifying the textural characteristics of the material. The investigation also explored the effects of multiple variables, including dosage, pH, temperature, and initial concentration, on the adsorption process. The study conducted equilibrium analyses alongside kinetic evaluations to assess the adsorption characteristics. The results demonstrated that the adsorption behavior aligned with the principles of pseudo-second-order kinetics and was suitably described by the Langmuir isotherm model. The data showing heightened metal adsorption as temperatures increase suggests that the adsorption process is characterized by both endothermic properties and spontaneity. In order to determine the most favorable conditions for adsorption, the Box-Behnken design software was utilized. The findings indicate that the optimal conditions include a pH of 5, a dosage of 0.02 g of Ce-MOF-FGA/CMC-CS hydrogel beads for every 25 mL of solution, and an adsorption capacity of 301.05 mg/g specifically for the Ni(II) solution. To optimize the performance of the adsorbent in the removal of Ni(II) during water purification, it is essential to take into account several key variables. The adsorption efficiency was significantly improved by conducting a series of methodically planned experiments, employing the Box-Behnken design alongside response surface methodology, as supported by Design-Expert software. An in-depth evaluation of the adsorbent's reusability carried out over six successive cycles of adsorption and desorption, indicates a significant stability in its removal efficiency.

Keywords: Adsorption isotherm; Box-Behnken design; Ce-MOF-FGA/CMC-CS hydrogel beads; Kinetics; Reusability.