Cationic Dye Removal (Methylene Blue) from Aqueous Solution Using the Ecologically Friendly Alginate/Hydroxyapatite/Graphene Oxide Nanocomposite Hydrogel Beads

In this study, the alginate/hydroxyapatite/graphene oxide (AHGO) nanocomposite hydrogel beads (nhb) were designed, synthesized by an ionotropic gelation technique, and studied as an efficient, environment-friendly adsorbent for cationic dyes. The adsorptive capacities of AHGO nanocomposite toward methylene blue (MB) as a model dye solution were investigated through batch adsorption experiments in which the effects of initial dye concentration, adsorbent dosage, pH, temperature, ionic strength, and contact time on MB removal efficiency were examined. To explore the adsorption mechanisms, adsorption kinetics, isotherm analyses, XRD, FTIR, SEM–EDS, and point of zero charge (pH PZC ) were performed. The results showed that AHGO-nhb had a maximum adsorption capacity of 311.81 mg/g for MB. This suggests that AHGO could be a good adsorbent for getting dyes out of water. Adsorption kinetics measurements proved to be closely correlated with both first and pseudo-second order (PSO) kinetics at the start, with the PSO taking control after 75 min, whereas the Sips model best described the MB adsorption isotherm process on AHGO. Based on the thermodynamic values ΔG° , ΔS° , and ΔH° , the process was spontaneous and exothermic in nature for cationic dyes. The mechanism underlying MB removal by AHGO is primarily a surface phenomenon involving electrostatic interaction, n-π interaction, and π-π interaction without intercalation. The process was shown to be a sequence of film diffusion followed by intra-particle diffusion, as demonstrated by the Weber-Moris and Boyed models. The adsorbent still maintains its adsorption ability for up to five cycles and could be a good alternative for treating wastewater. Graphical abstract