Study on the surface speciation of Fe-pillared montmorillonite and mechanism of its photocatalytic effect on degradation of ionic dye rhodamine-B

Graphical abstract: - Highlights: • The surface protonation constants of Namt and Femt were obtained. • The content of >FeOH₂⁺ is correlated with the catalytic decolorization rate for the dye. • The mechanism of >FeOH₂⁺ for the photocatalytic degradation of rhodamine-B was proposed. - Abstract: The surface protonation constants of Na-montmorillonite (abbreviated as Namt) and Fe-pillared montmorillonite (abbreviated as Femt) were obtained from experimental determination and then fitted with Protfit 2.1 software. The values of pK_a1, pK_a2 and N_t as well as the iron content of Femt are higher than those of Namt. The surface speciation of the sample presents lagging performance as the pH changes. The adsorption amount and catalytic decolorization rate of Femt for rhodamine-B are higher than that of Namt at the same pH. When the pH value increases, the adsorption amount and catalytic decolorization rate of Femt for rhodamine-B decline. The content of >FeOH₂⁺ on the surface of Femt is positively correlated with the adsorption amount and catalytic decolorization rate for the dye. The mechanism of >FeOH₂⁺ for the photocatalytic degradation of rhodamine-B may be interpreted as follows: after >FeOH₂⁺ effectively captures hydrogen peroxide and photoelectrons in the valence band of >FeOH₂⁺, hydroxyl radicals are produced. Hydroxyl radicals are also produced by electron holes on the valence band of >FeOH₂⁺ absorbing OH–. The interrupted electrostatic field produced by >FeOH₂⁺ on the surface of Femt can prevent the electron–hole recombination, which improves the catalytic efficiency of the Femt. Rhodamine-B is photocatalytically degraded by hydroxyl radicals.