Enhanced adsorption and desorption of Cr(VI) from aqueous solution using hydrous Ce_1–xZr_xO₂: Isotherm, kinetics and thermodynamic evaluation

Hydrous Ce_1–xZr_xO₂ (x = 0.25; 0.5; 0.75) mixed oxides were used as potential Cr(VI) adsorbents, expecting to take an advantage of benefits of their corresponding single oxides (e.g. high adsorption capacity) and to eliminate their weaknesses (e.g. limited reusability, long adsorption time). Chemical incorporation of ZrO₂ into CeO₂ lattice significantly changed the texture and the morphology of hydrous single oxides, enhancing their adsorptive and desorptive properties. Cr(VI) removal efficiency depended on Ce/Zr molar ratio, increasing in the following order: CeO₂0.75Zr_0.25O₂0.5Zr_0.5O₂< ZrO₂0.25Zr_0.75O₂. Hydrous Ce_0.25Zr_0.75O₂ showed the highest adsorption capacity (ca. 35.5 mg g⁻¹), desorption efficiency (ca. 34.5 mg g⁻¹), short time needed to attain adsorption equilibrium (30 min) and chemical stability (90% of original Cr(VI) adsorption capacity in the third cycle). Cr(VI) adsorption on hydrous Ce_1–xZr_xO₂ oxides obeyed the Langmuir isotherm model and followed a pseudo-second-order kinetic model. The values of thermodynamic parameters indicated exothermic and spontaneous adsorption. For hydrous Ce–Zr oxides, adsorption was governed mainly by physisorption and ion exchange, making possible effective and fast regeneration. For hydrous CeO₂ and ZrO₂, Cr(VI) adsorption was not completely reversible. Reduction of Cr(VI) to Cr(III), on CeO₂ surface, and formation of ≡Zr–OH₂⁺–HCrO₄⁻ complex on ZrO₂ surface, were also considered as suppliers of Cr(VI) removal.

Successive Cr(VI) adsorption/desorption capacity on hydrous Ce_1–xZr_xO₂