Adsorption and eliminating of diquat herbicide using layer double hydroxide enclosed in double layer hydrogel beads of carboxymethyl cellulose and alginate: Synthesis, characterization, adsorption isotherm, kinetics, thermodynamics and optimization via box-behnken design

This study involved the creation of AlCu-layered double hydroxide (LDH) encapsulated in carboxymethyl cellulose (CMC) and alginate (Alg), which was then crosslinked with epichlorohydrin to form hydrogel beads (AlCu-LDH/CMC-Alg hydrogel beads) used for the removal of diquat (DQ) herbicide. The resulting material, AlCu-LDH/CMC-Alg hydrogel beads, underwent a comprehensive analysis of its properties using XRD, FT-IR, XPS, EDX, N₂ adsorption/desorption isotherm, and FESEM to determine its textural characteristics. The AlCu-LDH/CMC-Alg hydrogel beads was analyzed using nitrogen adsorption/desorption isotherms to assess its textural properties. The hydrogel beads of AlCu-LDH/CMC-Alg showed a surface area of 114.22 m²/g, a pore volume of 0.35 cc/g, and a pore radius of 3.62 nm, indicating a mesoporous structure with a notable adsorption capacity. Following the adsorption of DQ, these measurements decreased to 65.145 m²/g, 0.2 cc/g, and 2.4 nm, respectively, suggesting that the DQ had filled or blocked the pores. This study also analyzed the impact of dose, pH, temperature, and initial concentration on the adsorption process. Equilibrium and adsorption kinetics were used to examine the adsorption characteristics. The process followed the pseudo-second-order and Langmuir isotherm models. The primary adsorption mechanism identified was chemisorption, with an adsorption energy of 29.6 kJ.mol⁻¹. The rise in DQ absorption at higher temperatures suggests an endothermic and spontaneous adsorption process. The optimal adsorption parameters, as determined by the Box-Behnken design software, are a pH of 8, a dosage of 0.02 g of AlCu-LDH/CMC-Alg hydrogel beads per 25 mL, and an adsorption capacity of 302.6 mg/g for the DQ solution. Through careful testing and utilization of the Box-Behnken design and response surface technique in the Design-Expert software, significant enhancements were made to the adsorption process. The stability of the adsorbent was assessed by conducting six successive cycles of adsorption and desorption, revealing that its reusability remained steady with no noticeable decline in removal efficiency. Furthermore, it preserved its original chemical makeup both before and after being reused, demonstrated steady effectiveness, and kept consistent X-ray diffraction (XRD) results.