Enhanced adsorption and desorption of Cr(VI) from aqueous solution using hydrous Ce1- x Zr x O2: Isotherm, kinetics and thermodynamic evaluation

Hydrous Ce1- (x) Zr (x) O-2 (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: CeO2<Ce0.75Zr0.25O2<Ce0.5Zr0.5O2< ZrO2<Ce0.25Zr0.75O2. Hydrous Ce0.25Zr0.75O2 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 Ce1- (x) Zr (x) O-2 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 equivalent to Zr-OH2 (+)-HCrO4 (-) complex on ZrO2 surface, were also considered as suppliers of Cr(VI) removal.