Biomass and Activated Carbon from Zea mays Stems as Potential Adsorbents of Acid Green 25 from Aqueous Solutions—Kinetics, Isotherms, Thermodynamics, and Theoretical Study

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In the present research, Zea mays stem (ZMS) agricultural waste and its corresponding activated carbon (AC-ZMS) were successfully utilized as alternative low-cost adsorbents for the removal of Acid Green 25 (AG25), a highly resilient and toxic contaminant textile dye. The evaluation was systematically conducted using synthetic wastewater precisely prepared in the laboratory by dissolving the commercial analytical-grade dye in distilled water to eliminate external chemical interference. The ZMS powder exhibited a fibrillar morphology characteristic of lignocellulosic materials, and AC-ZMS a porous morphology. The removal experiments showed that at an initial concentration of 50 mg/L, pH = 2, an adsorbent dose of 20 g/L, and 20 °C, both materials reached equilibrium in 30 min, achieving removal efficiencies of 87.57% (ZMS) and 91.43% (AC-ZMS). The equilibrium data were fitted to the Langmuir isotherm model, resulting in maximum adsorption capacities (qmL) that increased with temperature, reaching maximum values of 12.61 mg/g for ZMS and 13.56 mg/g for AC-ZMS at 40 °C. Thermodynamic and mechanism analyses justified the process of preparing AC-ZMS from ZMS for the removal of AG25, since ZMS powder would exhibit predominantly weak and reversible physical adsorption (ΔH0 = 15.94 kJ/mol), driven by pH-sensitive electrostatic attraction; in contrast, the adsorption of AG25 onto AC-ZMS is consistent with a chemisorption regime (ΔH0 = 86.64 kJ/mol). This change would prevent the degradation of active sites, allowing AC-ZMS to retain a remarkable 80.66% of its initial capacity after 5 consecutive regeneration cycles, whereas untreated ZMS suffers a severe drop in performance to 49.25%. The theoretical study demonstrated that the defect and oxygenated sites present in AC-ZMS attract and immobilize AG25, which causes it to be efficiently removed from synthetic solutions. In conclusion, AC-ZMS is a promising adsorbent in synthetic solutions; further work needed for real wastewater and reuse.