Recently, the photocatalytic degradation technique with titanium dioxide (TiO2) has been widely applied for the degradation of humic acid (HA) from aqueous solution due to its ability to achieve complete mineralization of organic contaminants. Because TiO2 is the most commonly used semiconductor photocatalyst, efforts on the modification of TiO2 in order to improve catalyst efficiency were presented in this review manuscript. The key photoreactor operation parameters such as TiO2 loading, pH, temperature, oxygen concentration, concentration and nature of HA, light wavelength, light intensity, the presence of inorganic ions and mechanistic pathway for pollutant removal, and the formation of the intermediates and their effects on the mineralization and disinfection of the photo-process were also assessed. Although we can see an increase in the number of papers that have been published in this area, further progress is needed to improve the understanding of the dynamic interactions between TiO2 photocatalytic oxidation process and HA, as well as to suggest possible future developments in this promising field.

This research is the first report on the purification of domestic wastewater in respect to the application of strontium-doped TiO₂ coated on porous ceramic filter media (Sr-TiO₂/PCFM) in eight laboratory-scale vertical flow constructed wetlands under irradiation of UV light. All the major pollutant parameters of wastewater were investigated. The online parameters showed that the decrease of dissolved oxygen (DO) reached 68.89% and in the constructed wetlands group which had the Sr-TiO₂/PCFM material and had a certain anti-clogging ability. By the presence of photocatalyst Sr-TiO₂/PCFM material, the removal efficiency of COD reached over 80% and eventually stabilized at a higher level (over 71%). Besides that, the Sr-TiO₂/PCFM material in constructed wetlands improved the average removal efficiency of total phosphorus (TP), total nitrogen (TN), NH₄⁺-N, total suspended solids (TSS), and turbidity by 60.89%, 34.89%, 41.87%, 21.10%, and 36.85%, respectively. With Sr-TiO₂/PCFM material, under the irradiation of UV light, the overall removal efficiency of organic matter removal in terms of dissolved organic carbon (DOC) was 52% in 4 h, which was the best compared with other constructed wetland groups. The results of this study may in turn support improved testing and better optimization of constructed wetland systems for wastewater treatment.

A photocatalyst comprising Strontium doped TiO2 coated on porous ceramic filter media (Sr-TiO2/PCFM) was prepared by the heat-treated process. The main objective of this study is to investigate the effects of catalyst dosage, initial HA concentrations, pH, and temperature on the adsorption and degradation of humic acid (HA) in the solution containing Sr-TiO2/PCFM under irradiation of UV light. The removal efficiency for HA was achieved 84.25 % under an experimental condition for a Sr-TiO2/PCFM dosage of 80 g at an initial concentration of 15 mg/L in a period of 8 h. Higher degradation was found for HA in the acidic environment and at higher operating temperatures. The rate of the adsorption reaction followed the pseudo second-order kinetics with the sorption isotherm well fitted to the Freundlich and the Langmuir isotherm models. The oxidation rate constants of HA were evaluated by using pseudo-first-order kinetics, which can describe the photodegradation process. Furthermore, the photocatalytic stability of Sr-TiO2/PCFM was performed with three cycles reused. These findings suggest that the Sr-TiO2/PCFM was found to be an effective and promising approach to eliminate HA in water resource.