Sustainable Process for Producing Alginate-Encapsulated Activated Carbons from Almond Waste: Impact of Activation Temperature on Dye Adsorption

This study developed a sustainable and cost-effective method for producing alginate-encapsulated activated carbon hydrogel beads from almond shell waste biomass, aimed at the efficient removal of methylene blue (MB) dye from aqueous solutions. The activated carbons were developed by heating biomass to different temperatures (500, 600, and 700 °C) and then mixing them with a calcium alginate matrix biopolymer to make composite hydrogel beads labeled AC500@Alg, AC600@Alg, and AC700@Alg. Zeta potential measurement, SEM, EDS, and FTIR analyses were carried out to evaluate the structural, morphological, chemical, and surface properties of the beads. Adsorption experiments showed that raising the activation temperature greatly improved porosity, surface carbon content, and adsorption performance. Among the adsorbent beads, AC700@Alg hydrogel beads had the best ability to adsorb MB, with a maximum Langmuir monolayer capacity of 316.46 mg/g. The pH of the solution and the charge on the surface had a great effect on the adsorption process. The best removal was achieved at alkaline pH due to the electrostatic attractions. The pseudo-second-order model best explained the kinetic data, which meant that surface interactions controlled the adsorption process. Thermodynamic analysis verified that MB adsorption was spontaneous and endothermic. Also, AC700@Alg beads were reusable, keeping their removal efficiency at over 80% after four cycles of adsorption and desorption. These results show that alginate-encapsulated activated carbon made from agricultural waste could be a good, eco-friendly, and reusable adsorbent for cleaning up wastewater.