Advanced oxidation process

Advanced oxidation processes (abbreviation: AOPs), in a broad sense, refers to a set of chemical treatment procedures designed to remove organic (and sometimes inorganic) materials in water and waste water by oxidation through reactions with hydroxyl radicals (·OH).^[1] In real-world applications of wastewater treatment, however, this term usually refers more specifically to a subset of such chemical processes that employ ozone (O₃), hydrogen peroxide (H₂O₂) and/or UV light.^[2] One such type of process is called in situ chemical oxidation.

Description

AOPs rely on in-situ production of highly reactive hydroxyl radicals (·OH). These reactive species are the strongest oxidants that can be applied in water and can virtually oxidize 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. 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 “water treatment processes of the 21st century”.^[3]

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 waste water.^[4] Additionally, AOPs can be used to treat effluent of secondary treated wastewater which is then called tertiary treatment.^[5] The contaminant materials are converted to a large extent into stable inorganic compounds such as water, carbon dioxide and salts, i.e. they undergo mineralization. A goal of the waste water purification by means of AOP procedures is the reduction of the chemical contaminants and the toxicity to such an extent that the cleaned waste water 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 in Fenton reagent, which, however, was an analytical reagent at that time), the utilization of such oxidative species in water treatment did not receive adequate attention until Glaze et al.^[1] suggested the possible generation of ·OH “in sufficient quantity to affect water purification” and defined the term “Advanced Oxidation Processes” for the first time in 1987. AOPs still have not been put into commercial use on a large scale (especially in developing countries) even up to today mostly because of the relatively high costs. Nevertheless, its high oxidative capability and efficiency make AOPs a popular technique in tertiary treatment in which the most recalcitrant organic and inorganic contaminants are to be eliminated. The increasing interest in water reuse and more stringent regulations regarding water pollution are currently accelerating the implementation of AOPs at full-scale. There are roughly 500 commercialized AOPs installations around the world at present, mostly in Europe and the United States. Other countries like China are showing increasing interests in AOPs.

Chemical principles

Generally speaking, chemistry in AOPs could be essentially divided into three parts:^[6]

1. Formation of ·OH;
2. Initial attacks on target molecules by ·OH and their breakdown to fragments;
3. 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 and photocatalytic oxidation rely on different mechanisms of ·OH generation:

• UV/H₂O₂:^[5]

H₂O₂ + UV → 2·OH (homolytic bond cleavage of the O-O bond of H₂O₂ leads to formation of 2·OH radicals)

• Ozone based AOP:^[7]

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

• Photocatalytic oxidation with TiO₂:^[7]

TiO₂ + UV → e^- + h⁺ (irradiation of the photocatalytic surface leads to an excited electron (e^-) and electron gap (h⁺)

Ti(IV) + H₂O Ti(IV)-H₂O (water absorbs onto the catalyst surface)

Ti(IV)-H₂O + h⁺ Ti(IV)-·OH + H⁺ the highly reactive electron gap 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.^[8]

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 that are unparalleled in the field of water treatment:

• It could effectively eliminate organic compounds in aqueous phase, rather than collecting or transferring pollutants into another phase.
• Due to the remarkable reactivity of ·OH, it virtually reacts with almost every aqueous pollutants without much discrimination. AOPs could therefore be applicable in many, if not all, scenarios where many organic contaminants are expected to be removed at the same time.
• Some heavy metals could also be removed in forms of precipitated M(OH)_x.
• In some AOPs designs, disinfection could also be achieved, leading AOPs to an integrated solution to some of the 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

It should also be aware that AOPs are not perfect and have several drawbacks.

• Most prominently, costs of AOPs are too high, since a continuous input of expensive chemical reagents are required to maintain the operation of most AOPs system.
• Some techniques require pre-treatment of wastewater to ensure reliable performance, which could be potentially costly and technically demanding. For instance, presence of bicarbonate ion (HCO₃^-) can appreciably reduce the concentration of ·OH due to scavenging processes that yield H₂O and a much less reactive species, ·CO₃^-.^[3] As a result bicarbonate must be wiped out from the system or AOPs are compromised otherwise.
• Given the potential costs, AOPs may not individually handle a large amount of wastewater; instead, AOPs should be deployed in the final stage after primary and secondary treatment have successfully removed a large proportion of contaminants.

Future perspectives

Since Glaze first proposed the definition of AOPs in 1987, this realm has witnessed a rapid development both in theory and in application. So far, TiO₂/UV system, H₂O₂/UV system and Fenton system have received extensive scrutiny. Yet there are still many research needs to address the shortcomings of existing AOPs techniques mentioned above.

Another way to fix the shortcomings is the development of new, modified AOPs that are efficient and economical. In fact, there has been some studies that offer constructive solutions. For instance, doping TiO₂ with non-metallic elements could possibly enhance the photocatalytic activity;^[9] and implementation of ultrasonic treatment could promote the production of hydroxyl radicals.^[10]

As the need for safe and clean water increases globally, there is certainly a growing demand for technologies that could fulfill such tasks. A promising future for AOPs awaits.

See also

• List of waste-water treatment technologies
• Fenton reaction
• Process engineering
• Water purification

References

Further reading