Synthesis, characterization, and adsorptive removal of methyl blue using combustion-synthesized Al₄B₂O₉ and Al₅BO₉

This study investigates the removal process of methyl blue (MeB), an anionic dye, using synthesized Al₄B₂O₉ and Al₅BO₉ as adsorbent materials. Al₄B₂O₉ and Al₅BO₉ were synthesized via the combustion method. The structural, morphological, textural, thermal and surface-charge properties of the synthesized borate materials were evaluated using FTIR, XRD, SEM, BET, TGA, and zeta potential measurements. The adsorption behavior of Al₄B₂O₉ and Al₅BO₉ was assessed by varying solution pH, contact duration, initial MeB concentration, adsorbent loading, and temperature. Kinetic, equilibrium, and thermodynamic analyses were carried out to clarify the adsorption behavior and possible interaction pathways between MeB and the synthesized borate surfaces. Kinetic modeling showed that the PSO equation most accurately describes MeB removal by both borate materials, as indicated by the high R ² values. The thermodynamic parameters showed that MeB adsorption was favored by increasing temperature and proceeded spontaneously, as confirmed by the positive ΔH° and negative ΔG° values, respectively (Al₄B₂O₉: ΔH = 16.827 kJ mol; Al₅BO₉: ΔH = 17.130 kJ mol^-1). The positive ΔS° values indicate that the adsorption process was accompanied by increased interfacial disorder during the transfer of MeB molecules from solution to the borate surface (Al₄B₂O₉: ΔS = 80.222 J (mol^-1 K^-1); Al₅BO₉: ΔS = 82.940 J (mol^-1 K^-1)). Equilibrium modeling indicated that the Freundlich equation more suitably described MeB uptake by both Al₄B₂O₉ and Al₅BO₉, suggesting adsorption on heterogeneous surface sites. According to the Freundlich model, the K _F values were determined as 21.146 for Al₄B₂O₉ and 27.072 for Al₅BO₉. Although the Freundlich model provided the best fit, the Langmuir model was used to estimate the maximum monolayer adsorption capacities, which were calculated as 70.547 mg g^-1 for Al₄B₂O₉ and 105.785 mg g^-1 for Al₅BO₉. Overall, the results demonstrate the potential of combustion-derived Al₄B₂O₉ and Al₅BO₉ as inorganic borate adsorbents for removing anionic MeB from aqueous media.