will react with water the reaction steps presented here are just a part of the reaction sequence, see reference for more details Currently there is no consensus on the detailed mechanisms in Part 3, but researchers have cast light on the processes of initial attacks in Part 2. In essence, ·OH is a radical species and should behave like a highly reactive electrophile. Thus two type of initial attacks are supposed to be Hydrogen Abstraction and Addition. The following scheme, adopted from a technical handbook and later refined, describes a possible mechanism of the oxidation of benzene by ·OH. Scheme 1. Proposed mechanism of the oxidation of benzene by hydroxyl radicals The first and second steps are electrophilic addition that breaks the aromatic ring in benzene (A) and forms two hydroxyl groups (–OH) in intermediate C. Later an ·OH grabs a hydrogen atom in one of the hydroxyl groups, producing a radical species (D) that is prone to undergo rearrangement to form a more stable radical (E). E, on the other hand, is readily attacked by ·OH and eventually forms 2,4-hexadiene-1,6-dione (F). As long as there are sufficient ·OH radicals, subsequent attacks on compound F will continue until the fragments are all converted into small and stable molecules like H2O and CO2 in the end, but such processes may still be subject to a myriad of possible and partially unknown mechanisms. Advantages AOPs hold several advantages in the field of water treatment: They can effectively eliminate organic compounds in aqueous phase, rather than collecting or transferring pollutants into another phase. Due to the reactivity of ·OH, it reacts with many aqueous pollutants without discriminating. AOPs are therefore applicable in many, if not all, scenarios where many organic contaminants must be removed at the same time. Some heavy metals can also be removed in forms of precipitated M(OH)x. In some AOPs designs, disinfection can also be achieved, which makes these AOPs an integrated solution to some water quality problems. Since the complete reduction product of ·OH is H2O, AOPs theoretically do not introduce any new hazardous substances into the water. Current shortcomings AOPs are not perfect and have several drawbacks. Most prominently, the cost of AOPs is fairly high, since a continuous input of expensive chemical reagents is required to maintain the operation of most AOP systems. As a result of their very nature, AOPs require hydroxyl radicals and other reagents