Adsorption and Reduction Transformation Behaviors of Cr(VI) on Mesoporous Polydopamine/Titanium Dioxide Composite Nanospheres

Mesoporous polydopamine/titanium dioxide (PDA/TiO2) composite nanospheres with different polydopamine content were successfully synthesized by a facile organic–inorganic self-assembly method. The synthesized composite materials were characterized by N2 adsorption–desorption isotherm, X-ray photoelectron spectroscopy, X-ray diffraction, transmission electron microscopy (TEM), energy dispersive spectrometry, field emission scanning electron microscopy, Fourier transform infrared spectroscopy, ζ--potential, and cyclic voltammetry. The results showed that the composite material with the most exposure active site exhibits a spherical morphology. The composite material could also be used as a reducing agent, and spheres of the composite material were used as adsorbents for removing hexavalent chromium from aqueous solutions by reducing Cr­(VI) to Cr­(III). The influence of factors, including the initial solution pH, contact time, initial ion concentration, temperature, coexisting ions, and dose of adsorbent, was also studied. The hexavalent chromium adsorption data fit well to a Langmuir isotherm model, and the maximum adsorption capacity was shown to be 244.5 mg/g. The kinetics data follow a pseudo-second-order kinetic model. The study of thermodynamics indicated that the adsorption on the composite nanospheres was a spontaneous and endothermic process. Moreover, a mechanism of adsorption and reduction on the composite spheres was proposed