Low-valent manganese active sites: Insight into reinforced interaction with sulfonated anthraquinone dye and kinetic adsorption studies over iron-modified cryptomelane

This study presents a facial co-precipitation method to enrich low-valent manganese sites for iron-doped cryptomelane. Fourier-transform infrared spectroscopy exhibits a noticeable enhancement of both vibrations at 1041 and 1116 cm^-1 ascribed to Mn³⁺-OH bond over as-prepared materials. X-ray diffraction, scanning electron microscopy, Raman spectroscopy, the temperature-programmed desorption of oxygen and inductively coupled plasma-mass spectrometry results all verify the increase in oxygen vacancies on iron-doped cryptomelane. The vital role of Mn³⁺-OH sites for adsorptive removal of acid blue 62 (AB62) was experimentally evident when adsorption capacity (Q_e, mg_AB62/g_adsorbent) increased from 54 ± 1.3 mg/g (for non-doped cryptomelane) to 161 ± 6.7 mg/g (for Fe-0.15) at initial pH 5.7. The decrease of Q_e from 313 mg/g (for initial pH 3.70) to 67 mg/g (for initial pH 9.95) over Fe-0.15 suggests protonation in acid media and deprotonation in basic media, reflecting efficient Mn³⁺-OH sites for reinforced interaction with sulfonate groups. The disappearance of sharp bands at 1041 and 1116 cm^-1 after adsorption and the replenishment of a broad band at ∼1250 cm^-1 over Fe-0.15 demonstrate the displacement of sulfonate groups by -OH species (from Mn³⁺-OH sites). Moreover, the deterioration of two stretching modes for O=S=O at 1187 and 1230 cm^-1 after adsorption reveals the formation of a monodentate or bidentate complex. Kinetic studies confirm the compatibility of AB62 chemisorption over Fe-0.15 with the pseudo-second-order kinetic, Elovich, and Langmuir isotherm models. The current findings first support evidences for the AB62 chemisorption on iron-doped cryptomelane and a Fe-0.15-feasible adsorbent for removal of sulfonated anthraquinone dye.