Congo Red–Functionalized Maize Stalk for Fe3+, Cr3+ and Mn2+ Adsorption: Multi-Analytical Characterization of Interaction Mechanisms

This study examines the adsorption and interaction mechanisms of Congo red (CR) immobilized onto maize stalk (MS) to form MS-CR material, used for the removal of Fe3+, Cr3+, and Mn2+ (Mn+) from aqueous media. Initially, the MS was functionalized with CR, achieving adsorption capacities between 41.4 and 48.0 mg/g across pH 2–10, confirming the formation of hydrogen bonding and aromatic interactions, as indicated by the shift of the OH band from 3338.91 to 3335.54 cm−1 and the appearance of characteristic azo–aromatic peaks (1601–1506 cm−1) in the FTIR spectra. Stability tests showed that CR remains anchored to the lignocellulosic matrix even under 2 M HCl/NaOH. Subsequently, adsorption experiments revealed a strong pH dependence: at pH 10, removal efficiencies reached 93% for Mn2+, 89% for Fe3+, and 72% for Cr3+ at 2 mg/L, driven by surface deprotonation and enhanced electrostatic attraction. Increasing the initial metal concentration (1–10 mg/L) led to maximum adsorption capacities of 2.00 mg/g for Fe3+, 1.64 mg/g for Cr3+, and 1.46 mg/g for Mn2+. Desorption experiments identified 0.5 M HCl as the optimal regenerating agent, achieving 90–97% metal release. FTIR analysis of MS-CR–Mn2+ showed the disappearance of the 1243 cm−1 carboxyl band and the emergence of a metal–oxygen vibration at 559.37 cm−1, confirming adsorption via coordination to deprotonated carboxyl and phenolic groups. TG/DSC/DTG analysis demonstrated improved stability of MS-CR compared to native MS. SEM/EDX confirmed the presence of S, Na, and Mn+. The combined spectroscopic, microscopic, and thermal evidence demonstrates that MS-CR operates as a robust, multifunctional adsorbent capable of Mn+ retention, offering a sustainable solution for water treatment.