Advanced oxidation process

AOPs rely on in-situ production of highly reactive hydroxyl radicals (·OH) or other oxidative species for oxidation of contaminant. These reactive species can be applied in water and can oxidize virtually any compound present in the water matrix, often at a diffusion-controlled reaction speed. Consequently, ·OH reacts unselectively once formed and contaminants will be quickly and efficiently fragmented and converted into small inorganic molecules. Hydroxyl radicals are produced with the help of one or more primary oxidants (e.g. ozone, hydrogen peroxide, oxygen) and/or energy sources (e.g. ultraviolet light) or catalysts (e.g. titanium dioxide). Precise, pre-programmed dosages, sequences and combinations of these reagents are applied in order to obtain a maximum •OH yield. Researchers are also exploring doped metal-oxide catalysts and carbon-based materials to boost radical yields and broaden pH operating windows. In general, when applied in properly tuned conditions, AOPs can reduce the concentration of contaminants from several-hundreds ppm to less than 5 ppb and therefore significantly bring COD and TOC down, which earned it the credit of being one of the most effective "water treatment processes of the 21st century". Current research also focuses on reducing energy demand and minimizing byproducts produced during the process—such as bromate—to improve the viability of AOP implementation in different industries. The AOP procedure is particularly useful for cleaning biologically toxic or non-degradable materials such as aromatics, pesticides, petroleum constituents, and volatile organic compounds in wastewater. Additionally, AOPs can be used to treat effluent of secondary treated wastewater which is then called tertiary treatment. The contaminant materials are largely converted into stable inorganic compounds such as water, carbon dioxide and salts, i.e. they undergo mineralization. A goal of the wastewater purification by means of AOP procedures is the reduction of the chemical contaminants and the toxicity to such an extent that the cleaned wastewater may be reintroduced into receiving streams or, at least, into a conventional sewage treatment. Although oxidation processes involving ·OH have been in use since late 19th century (such as Fenton's reagent, which was used as an analytical reagent at that time), the utilization of such oxidative species in water treatment did not receive adequate attention until Glaze et al. and a low temperature (30 °C - 50 °C), in a safe and efficient way, using optimized catalyst and hydrogen peroxide formulations. ==Chemical principles==

Generally speaking, chemistry in AOPs could be essentially divided into three parts: • Formation of ·OH; • Initial attacks on target molecules by ·OH and their breakdown to fragments; • Subsequent attacks by ·OH until ultimate mineralization. The mechanism of ·OH production (Part 1) highly depends on the sort of AOP technique that is used. For example, ozonation, UV/H2O2, photocatalytic oxidation and Fenton's oxidation rely on different mechanisms of ·OH generation: • UV/H2O2: :H2O2 + UV → 2·OH (homolytic bond cleavage of the O-O bond of H2O2 leads to formation of 2·OH radicals) • UV/HOCl: :O3 + HO− → HO2− + O2 (reaction between O3 and a hydroxyl ion leads to the formation of H2O2 (in charged form)) :O3 + HO2− → HO2· + O3−· (a second O3 molecule reacts with the HO2− to produce the ozonide radical) :O3−· + H+ → HO3· (this radical gives to ·OH upon protonation) :HO3· → ·OH + O2 :the reaction steps presented here are just a part of the reaction sequence, see reference for more details • Fenton based AOP: Fe2+ + H2O2 → Fe3++ HO· + OH− (initiation of Fenton's reagent) Fe3+ + H2O2 → Fe2++ HOO· + H+ (regeneration of Fe2+ catalyst) H2O2 → HO· + HOO· + H2O (Self scavenging and decomposition of H2O2) the reaction steps presented here are just a part of the reaction sequence, see reference for more details • Photocatalytic oxidation with TiO2: 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. Optimized AOPs often also achieve > 99% contamination removal yields, in addition to > 90% mineralization of organic carbon yield which successfully converts pollutants fully to CO2 and H2O and other individual elements found within the treatment batch. • 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 simultaneously with contamination removal which makes these AOPs an integrated solution to some water quality problems that require sanitization. • Since the complete reduction product of ·OH is H2O, AOPs theoretically do not introduce any new hazardous substances into the water but can produce undesired byproducts if the process is incomplete. ==Current shortcomings==

AOPs are not perfect and have several drawbacks. • Most prominently, the cost of AOPs is fairly high priced at US$3–4 million and upwards for 0.42 ML/h production at use in industrial treatment due to specialized materials and high intensity UV required to run the process. • Some techniques require pre-treatment of wastewater to ensure reliable performance, which could be potentially costly and technically demanding. For instance, presence of bicarbonate ions (HCO3−) can appreciably reduce the concentration of ·OH due to scavenging processes that yield H2O and a much less reactive species, ·CO3−. ==Future==

Since AOPs were first defined in 1987, the field has witnessed a rapid development both in theory and in application. So far, TiO2/UV systems, H2O2/UV systems, and Fenton, photo-Fenton and Electro-Fenton systems have received extensive scrutiny. Despite the technological advancements made, there are still many key challenges and obstacles revolving around AOPs. These challenges such as catalyst fouling, energy consumption of UV lamps, large scale reactant distribution within reactors, and control of partial-oxidation by-products all require further research on existing AOPs options to allow for the integration of efficient and advanced AOPs systems. Modified AOPs promise to be more efficient and economical. Doped TiO2 can show enhance the photocatalytic activity; and implementation of ultrasonic treatment could promote the production of hydroxyl radicals. Additionally further efforts to harness visible-light photocatalysts, such as doped graphitic carbon nitride (g-C3N4)—aim to reduce reliance on UV sources (lamps, lights, etc.) which typically need replacement every few years. Modified AOPs such as Fluidized-Bed Fenton has also shown great potential in terms of degradation performance and economics. ==See also==