Natural organic matter (NOM) is defined as a complex matrix of organic materials present in all surface, ground, and soil waters [1]. The presence of NOM, mainly humic substances (HS) in surface waters, constitutes the main route for the absorbance of solar energy in aquatic systems through which a series of photophysical and photochemical processes occur. Humic acid (HA) accounts for a significant fraction of the HS.The presence of HA in aqueous solutions is not directly toxic but could lead to organic disinfectant by-products (DBPs), which are undesired and hazardous products in water treatment after disinfection [2][3]. Furthermore, the high HA concentration in drinking water has the potential for some serious diseases, such as stomach cancer [4][5]. Thus, the presence of HA can be the cause of many problems, and their removal from water is a priority task [6] and has been attempted in different ways. Several processes have been attempted to remove HA from water such as coagulation [7-9], electrocoagulation (EC) [4,[10][11][12][13][14][15], electro-oxidation (EO) [16], ion exchange [17], membrane filtration [18-20], activated carbon adsorption and advanced oxidation processes [21-29].
AbstractRecently, the photocatalytic degradation technique with titanium dioxide (TiO 2 ) 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 TiO 2 is the most commonly used semiconductor photocatalyst, efforts on the modification of TiO 2 in order to improve catalyst efficiency were presented in this review manuscript. The key photoreactor operation parameters such as TiO 2 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 TiO 2 photocatalytic oxidation process and HA, as well as to suggest possible future developments in this promising field.