Facile Synthesis of Novel Rare-Earth Elements-Modified SiO2 Films for Effective Cr(VI) Removal from Electroplating Effluent

The rare-earth elements (La, Ce, and Y)-modified SiO2 films for effective Cr(VI) removal from electroplating effluent were synthesized by a convenient sol–gel method successfully. X-ray diffraction, Fourier transform infrared spectra (FT-IR), scanning electron microscope, energy-dispersive system, and X-ray photoelectron spectroscopy (XPS) were applied to characterize their physicochemical properties. The adsorption isotherms showed that the adsorption capacity of SiO2 modified by La3+ (64.43 mg g–1) is the best, followed by Ce3+ (53.45 mg g–1) and Y3+ (47.83 mg g–1) because of the different ion-exchange capacities of La/Ce/Y–(OH), whereas that of SiO2 film without modification is only 38.54 mg g–1. Their adsorption processes could be well depicted by the pseudo-second-order kinetic model. The thermodynamics study showed that the adsorption process of the La/SiO2 film is spontaneous. The adsorption mechanism includes the ion exchange and electrostatic effect. Furthermore, the conversion of Cr(VI) to Cr(III) during the in situ reduction process was verified by both macro (optical photographs) and micro (FT-IR and XPS) levels. Importantly, the La/SiO2 film showed high adsorption selectivity and high removal efficiency of 97.4% toward Cr(VI) from initial concentration of 41.7 mg L–1 to residual concentration of 1.1 mg L–1 in the real electroplating effluent, indicating its potential industrial value for pretreatment of heavy metal wastewater.