Sustainable Hydrogel Composite of Alginate and Opuntia ficus-indica Mucilage for Neutral Red Dye Adsorption in Synthetic Water

Wastewater treatment remains a pressing global challenge due to the continuous discharge of recalcitrant pollutants from anthropogenic activities. Among these, synthetic dyes are particularly problematic due to their persistence and toxicity, requiring advanced and sustainable removal strategies. In this study, a novel bio-based hydrogel was synthesized via a green process, avoiding the use of harmful chemicals. The hydrogel, composed of alginate and mucilage extracted from Opuntia ficus-indica, was applied as an adsorbent for the removal of Neutral Red dye from a synthetic aqueous solution. Characterization revealed a rough surface morphology, mesoporosity, and a specific surface area of 0.14 m2/g. The material exhibited a point of zero charge at pH 9.0, and its surface was rich in functional groups, including hydroxyl, deprotonated carboxyl, and ether groups, which contributed to dye adsorption through non-electrostatic mechanisms. Optimal removal was achieved at acidic pH, with a maximum adsorption capacity of 26.6 mg/g at pH 5.0. The adsorption kinetics followed a pseudo-second-order model, reaching equilibrium within 200 min, with a maximum uptake of 24.8 mg/g after 720 min. Thermodynamic analysis indicated a spontaneous, exothermic, and reversible adsorption process, with maximum capacities of 1420, 1432, and 1308 mg/g at 25, 35, and 45 °C, respectively. Competitive adsorption studies showed a decrease in capacity in the presence of electrolytes (KCl, NaCl, and MgCl2). The hydrogel demonstrated excellent reusability, retaining over 90% efficiency after five consecutive cycles, and showed no toxicity in lettuce seed germination tests. These results highlight the potential of alginate-Opuntia ficus-indica hydrogel as a sustainable, non-toxic, and efficient adsorbent for dye removal in water treatment applications.