pH-Responsive biopolymer/nano-clay hydrogel beads for dual dye removal: adsorption kinetics, isotherms, and reusability

Abstract

This study presents the synthesis and evaluation of a new, environmentally friendly cross-linked hydrogel bio-composite (NIP-Clay/Kc-GG-Ca) for the effective removal of anionic and cationic dyes from aqueous solutions. The hydrogel was fabricated by copolymerizing κ-carrageenan (Kc) and guar gum (GG), modified with N-isopropyl acrylamide (NIP) and reinforced with vermiculite clay, followed by ionic cross-linking using calcium ions (Ca²⁺). Several formulations were prepared and optimized to enhance the structural, swelling, and adsorption properties of the hydrogel. The composite was comprehensively characterized using various techniques: field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) to examine surface morphology and microstructure; energy-dispersive X-ray spectroscopy (EDX) for elemental analysis; X-ray diffraction (XRD) to determine crystallinity; Fourier transform infrared spectroscopy (FT-IR) for identifying functional groups; thermogravimetric analysis (TGA) for assessing thermal stability; and Brunauer–Emmett–Teller (BET) surface area analysis to evaluate surface area and porosity. Swelling behavior was studied at pH levels of 2, 4, 7, and 10 over 24 h, with maximum swelling observed at pH 7. Optimal formulation parameters were determined to be a KC/GG ratio of 1:3, NIP at 0.55 g/10 mL, clay at 0.1 g/10 mL, and 4% CaCl₂. Under these conditions, the hydrogel showed excellent adsorption capacities for both Congo Red (693.86 mg/g at pH 5) and Maxilon Blue GRL (947.87 mg/g at pH 7). Adsorption followed a pseudo-first-order kinetic model, while equilibrium data best fit the Freundlich isotherm model. The mechanism of adsorption involved π–π stacking, hydrogen bonding, and electrostatic interactions. Thermodynamic analysis confirmed that the adsorption process was endothermic and spontaneous. Furthermore, the hydrogel retained high adsorption performance and structural stability over six regeneration cycles, demonstrating its practical potential as a reusable and sustainable adsorbent for wastewater treatment applications.