Single and Binary Adsorption of Methyl Orange and Phosphate onto Magnesium Oxysulfite/Sulfate: Equilibrium, Kinetics, and Surface Interactions

The coexistence of dyes and phosphate in industrial effluents creates a treatment challenge because both contaminants can occur as negatively charged species while interacting differently with adsorbent surfaces. To explore a potential solution, this study evaluated magnesium oxysulfite/sulfate (MgOS) for methyl orange (MO) and phosphate adsorption under single- and binary-solute conditions. MgOS was characterized by complementary physicochemical and surface analyses, and adsorption equilibrium, kinetics, thermodynamics, pH effects, and competitive behavior were examined. MgOS exhibited a mesoporous, chemically heterogeneous structure and maintained effective adsorption over a broad pH range with an apparent pH-regulating effect. Adsorption capacities reached 50.6 mg g−1 for MO and 16.3 mg g−1 for phosphate in single-solute systems. MO reached equilibrium within 7–45 min and was generally described by pseudo-first-order kinetics. Phosphate required 150–360 min and showed concentration-dependent kinetic behavior, with PSO and Elovich models providing the best fits depending on the fitting approach and initial concentration. MO adsorption was spontaneous and exothermic, while phosphate adsorption was spontaneous and endothermic. Surface characterization supported electrostatic interactions, hydrogen bonding, and Mg-site coordination for MO, whereas phosphate removal involved ligand exchange, inner-sphere coordination, and Mg-O-P/Mg-phosphate-like species. Phosphate reduced MO adsorption by 11.9–30.7% in binary systems, while phosphate adsorption remained comparatively stable. MgOS therefore showed potential for simultaneous MO and phosphate removal under competitive conditions.