| BiOCl, is a new photocatalyst, have been widely used for removal of organic pollutants such as dyes, pharmaceuticals and personal care products (PPCPs) since it has a very high photocatalytic activity both under UV and visble light irradiation. Recently, most of the reported researches believe that the high removal rate of organic compounds is due to the photocatalytic effect of BiOCl while very little research are focused on its adsorption property. In this paper, six dyes, three PPCPs and several small organic molecules were selected as the target pollutants, the adsorption/photocatalytic efficiency and mechanism of such organic pollutiants by BiOCl was studied. In particular, the adsorption mechanism of typical dyes and PPCPs by BiOCl was deeply studied. BiOCl microspheres were prepared by solvothermal method, then it was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), N2 adsorption-desorption, UV-visible reflectance spectroscopy and etc. Results show that the as-prepared BiOCl microsphere has a diameter of 3-5 μn, and each microsphere is composed of numerous of nanoparticles with diameter～20 nm. The BET surface area and the band gap of the BiOCl microsphere is 16.31 m2/g and 3.38 eV, respectively.It has been found that the photocatalytic oxidation efficiency of BiOCl is highly adsorptivity dependent, with higher photocatalytic rate at higher adsorption capacity. Moreover, there is no photocatalytic activity without adsorption capacity. For the selected organic pollutants, the proportion of adsorption in the photocatalytic efficiency by BiOCl under UV and visible light irradiation was about 80%-95% and about 30% -90%, respectively. It is therefore confirmed thatthe real high photocatalytic efficiency of BiOCl for decomposition of organic compounds in water is actually contribute from the high adsorption ability of BiOCl.BiOCl itself can be hydrolyzed in the aqueous solution to form O-Bi-OH, H+ and Cl-, the OH" then can adsorb on the bare surface of Bi. Organic compounds will adsorb on the surface of BiOCl with the competition of Cl-,OH-. Outer electron layer of Bi can coordinate conjugated π-bond on organic matters. This mechanism was further proved by infrared spectroscopy (IR) and X-ray photoelectron spectroscopy (XPS). It was found that carboxyl group can be attached to Bi during the adroptionprocess. In addition, the adsorption rate between BiOCl and organic compounds was also influenced by the steric hindrance after the Cl dissociation, which was proved by a large number of small organic molecules adsorption experiments.The selective removal of organic compounds by BiOCl can be well explained by our adsorption mechanism. The adsorption mechanism can also help to understand and the traditional BiOCl-based photocatalysis mechanism. Finally, it offers new methods and new materials for removing dyes, PPCPs and other organic pollutants. |
