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

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.