Preparation and performance of anatase-loaded porous carbons for water purification

Fine particles of photoactive anatase-type TiO2 were loaded on activated carbon and carbon spheres through the hydrolysis of titanium oxysulfate under autogenous hydrothermal conditions. Their photoactivity and adsorptivity were evaluated separately by the measurements of relative concentration of methylene blue (MB) and iminoctadine triacetate (IT) remained in the solution either in the dark or under UV irradiation. Activated carbon could adsorb a large amount of MB but only a small amount of IT, and carbon spheres could adsorb only a small amount of MB. Loading of anatase under hydrothermal conditions was found to occur preferentially at the entrance of pores on the surface of substrate carbon in the beginning of deposition and to result in the reduction of specific surface area and consequently of the adsorption rate of MB. Further hydrothermal treatment was found to give homogeneous coverage of the surface of activated carbon by anatase fine particles. By UV irradiation after the saturation of adsorption in the dark, a marked decrease in concentration of MB and IT was observed, which recovered a high adsorptivity of activated carbon. Through repetitive measurements of concentration decrease in the solution in the dark and under UV irradiation, it was proved that MB and IT, which were adsorbed in the dark, could be decomposed under UV irradiation.

Hybridization of photoactivity with adsorptivity is expected to open new applications of both photocatalyst anatase-type TiO2 and adsorptive porous carbons. In the present work, anataseloaded carbons were successfully prepared through autogenous hydrothermal treatment of substrate carbons in an aqueous solution of titanium oxysulfate. On these samples, changes in relative concentration of MB and IT remained in the solution and were measured in the dark and under UV irradiation; in the dark only adsorption occurred and under UV irradiation both adsorption and photodecomposition were expected to occur. By selecting substrate carbons, AC and oxidized carbon spheres, and also the water pollutants, MB and IT, it was experimentally proved that these pollutants adsorbed onto carbon substrates in the dark were able to be decomposed under UV irradiation. On these hybrid materials, anatase-loaded porous carbons in the present work and carboncoated anatase particles reported in our previous works [9,10], two functions, i.e., photoactivity and adsorptivity, are working. The separated evaluation of these two functions is required in order to control these functions, but practically quantitative evaluation is not easy. These two functions in these hybrids are reasonably supposed to be governed by the diffusion of pollutant molecules in carbon, which may depend strongly on pore structure of substrate carbon and coated carbon as well as the size and nature of pollutant molecules. Therefore, studies on a wide range of molecules using a wide range of porous carbons are desirable. The present results seem to propose the possibility of application of these hybrid materials for water purification, for example, adsorption of pollutants at night and their decomposition during the day. For practical application, the control of rates of adsorption and photo-decomposition is desirable to get the appropriate combination of these two functions.