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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 (Qe, mgAB62/gadsorbent) 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 Qe 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.

Constructing appropriate heterojunction systems has been considered the most promising among different methods for achieving effective charge transfer and separation to improve photocatalytic performance. Herein, SnS2/g-C3N4 composites consisting of SnS2 nanosheets loaded on a porous g-C3N4 matrix were successfully prepared via direct calcination from the precursors of tin (IV) chloride and thiourea. Under visible light, all the composites outperformed pure SnS2 and g-C3N4 in terms of Rhodamine B (RhB) photodegradation, and the highest removal efficiency after 6 h of irradiation was 92.22%. Their photocatalytic performance was significantly enhanced because of the positive synergistic relationship between semiconductors (SnS2 and g-C3N4) and the rational Step-scheme charge transfer mechanism. Thus, these composites contributed to the strong redox power, high migration efficiency, and extended lifetime of photogenerated carriers.