The adsorption of HCrO4− on activated carbon of date pits and its photoreduction on the hetero-system ZnCo2O4/TiO2

The adsorption properties of activated carbon were successfully tested toward the elimination of hazardous HCrO 4 − (30, 50, 70 and 100 mg L −1 ). The material was prepared from Algerian date pits by physical and chemical activations of ZnCl 2 /CO 2 in the goal to develop the microporous volume. The characterization by N 2 adsorption at 77 K, the mercury intrusion porosimetry and scanning electron microscopy showed important textural properties. CO 2 increases the specific surface area (1192 m 2 g −1 ) and pore volume (0.96 cm 3 g). The HCrO 4 − adsorption is described by a pseudo-first-order kinetic model, and the equilibrium data are fitted by the Langmuir model with a maximal adsorption capacity of 46.72 mg/g within 30 min and a constant K L (0.12 L g −1 ). The remaining HCrO 4 − concentrations (8, 23, 32.5 and 43.5 mg L −1 ) were photocatalytically reduced on the hetero-system ZnCo 2 O 4 /TiO 2 down to 5 mg L −1 . The spinel ZnCo 2 O 4 , prepared by co-precipitation from the nitrates precursors and characterized by photoelectrochemistry, gives a conduction band of − 1.49 V SCE , more cathodic than the HCrO 4 − level (0.53 V). Therefore, the photoelectrons transfer toward HCrO 4 − species is achieved through TiO 2 located midway between the spinel and chromate levels. The photocatalysis is investigated by varying the catalyst dose and HCrO 4 − concentration. ZnCo2O4 has a gap of 1.82 eV and the best reduction efficiency (82%) was obtained under visible light (50 mW cm −2 ) and optimal conditions (HCrO 4 − 23 mg L −1 , pH ~ 7, ZnCo 2 O 4 /TiO 2 50/50%) and follows a first-order kinetic with a rate constant of 3.86 × 10 −3 min −1 .