Nowadays, a high number of organic contaminants are resistant to conventional chemical and biological treatments. Therefore, the sustainability approach in water sources management includes enhanced demands on new wastewater treatment technologies in order to reduce the negative impacts on the water bodies and to facilitate recycling and reuse of wastewater .
Advanced oxidation processes (AOPs) constitute a collection of established treatment technologies which rely on the formation of hydroxyl radical to affect the destruction of emergency pollutants [2–4]. During the last ten years, heterogeneous photocatalytic AOP is placed in the forefront of most researches due to its high efficiency in total degradation of pollutants, non-selectively and generation of benign products [3, 5, 6]. Photocatalytic oxidation processes deal with photoactivated metal oxides as semiconductors to remove contaminants in aqueous environment .
The photocatalytic mechanism begins when a photon with energy hv matches or exceeds the band gap energy of the semiconductors. Conduction electrons are promoted from the valance band into the conduction band (CB), leaving a hole behind. The hole can either oxidize a compound directly or react with electron donors like water to form OH radicals, which in turn react with pollutants [8–10].
TiO2 is the main semiconductor used due to its ready availability, low cost, activity under a wide range of pH, efficiency and long-term stability [3, 11]. The main drawback of TiO2 is based on the economy of the extensive use for large-scale facilities, and, in some cases, on the low mineralization level achieved, requiring a final polishing stage . There is renewed effort to study for more reliable semiconductors [1, 13].
Transition metal oxides have proved to be active in the photocatalytic reactions of the elimination of chlorophenols and its derivatives [14–16]. In this study, NiO was selected as the alternative semiconductor. Since NiO is an important transition metal oxide widely used as a catalyst with extraordinary electrical, thermal, catalytic, and redox properties [17, 18]. Most attracting features of NiO are excellent durability and electrochemical stability. Reported bandgap energy value for the nickel oxide is in the range of 3.4-3.8 eV. This suggests that the optical transition in NiO takes place through direct inter-band transition . Also, NiO can act as a promoter for the generation of OH radicals .
Chlorinated phenols are listed as priority pollutants since they are toxic, recalcitrant, and suspected carcinogen and mutagen to living, chlorophenols (CPs) posing serious ecological problems . Environmental issues with these chemicals occur from industrial wastewater, such as petroleum refining, and production of pesticides, paint, plastic, resin, textile, iron, solvent, pharmaceutics and wood preserving chemicals [22–26]. 4-chlorophenol (4-CP) has attracted interests owing to its direct relevance to environment and informative photochemical behaviors .
Different AOPs removing phenolic compounds have been reported, such as UV/H2O2, UV/catalyst , photo-Fenton process , MW/NiO , UV/H2O2/TiO2[32, 33], O3/ZnO  and UV/O3/TiO2. Despite extensive research, robust and economical treatment of the 4-CP has still to be implemented, and, there is a continued need to develop effective systems. The major objective of this study was to examine the efficiency of UV/H2O2/NiO system. The effects of pH, concentration of H2O2, and the amount of NiO photocatalyst, on 4-CP degradation rate were also evaluated.