The adsorption of HCrO4\u2212 on activated carbon of date pits and its photoreduction on the hetero-system ZnCo2O4/TiO2

Abstract

The adsorption properties of activated carbon were successfully tested toward the elimination of hazardous HCrO4− (30, 50, 70 and 100 mg L−1). The material was prepared from Algerian date pits by physical and chemical activations of ZnCl2/CO2 in the goal to develop the microporous volume. The characterization by N2 adsorption at 77 K, the mercury intrusion porosimetry and scanning electron microscopy showed important textural properties. CO2 increases the specific surface area (1192 m2 g−1) and pore volume (0.96 cm3 g). The HCrO4− 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 KL (0.12 L g−1). The remaining HCrO4− concentrations (8, 23, 32.5 and 43.5 mg L−1) were photocatalytically reduced on the hetero-system ZnCo2O4/TiO2 down to 5 mg L−1. The spinel ZnCo2O4, prepared by co-precipitation from the nitrates precursors and characterized by photoelectrochemistry, gives a conduction band of − 1.49 VSCE, more cathodic than the HCrO4− level (0.53 V). Therefore, the photoelectrons transfer toward HCrO4− species is achieved through TiO2 located midway between the spinel and chromate levels. The photocatalysis is investigated by varying the catalyst dose and HCrO4− 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 (HCrO4− 23 mg L−1, pH ~ 7, ZnCo2O4/TiO2 50/50%) and follows a first-order kinetic with a rate constant of 3.86 × 10−3 min−1.