Sorption Equilibria of Cr(VI), Mo(VI), Se(IV), and Re(VII) Oxo Ions with TiO2||C Oxide–Graphene Composite

Abstract

The equilibrium mechanism of sorption interaction of CrO₄^2–, MoO₄^2–, SeO₃^2–, and ReO₄^– ions with surface functional centers of the graphene–oxide composite TiO₂||C was established. The anion sorption isotherms are described by the Langmuir model for a monoenergetic sorbent with the Henry region at concentrations below 1 μM. The pH dependences of the anion distribution coefficient K_d are dome-shaped with a maximum at pH in the range 3–4. At pH in the range 2–6, the composite exhibits the properties of a collective action sorbent for the oxo anions studied with logK_d [mL/g] ≈ 2–5. A mechanism of oxo anion sorption based on the 2-pK model of surface complexation of anions by the sorption sites of the composite is proposed. The sorption mechanism suggests that the affinity of the nonprotonated anion for sorption sites in the pH range studied significantly exceeds that of the protonated species. As a result, the parameters of the surface complexation of anions K_M(0,1,2) are independent of the protonation constants of anions in the solution and remain constant, but different for the sorption sites Ti–OH₂⁺, Ti–OН, and Ti-O^– of the composite. Based on the modeling results, a predictive calculation of the sorption isotherm of the Sb(OH)₆^– anion by the composite as a function of pH was performed. Qualitative agreement of the calculation results with published data was demonstrated.