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Chemistry of UV-Oxidation

UV-Oxidation in comparison
Ranges of different processes in relation of COD-concentration and volume-rate

a.c.k. specializes in „advanced oxidation processes“ (AOP) for industrial applications. For that we apply two different process groups for treatment:
- Enviolet®: UV-Oxidation of highly concentrated solutions in homogeneous state (H2O2/UV-process)
- UV Deep-Clean: UV-Oxidation of solutions in heterogeneous state to obtain very low final concentrations (UV/catalyst-process)

Basics of UV-Oxidation

The requirement for a photoreaction of a molecule R is the absorption of light of that molecule (Calvert und Pitts, 1966). The amount of this absorbed light A(λ) is described by the Lambert-Beer law, whereby I(0) is the light intensity prior to absorbance of light and I is the remaining light intensity past absorbance through the solution. The light absorption depends on the concentration C of the layer thickness d and decadal extinction coefficient ε(λ).

By absorption of light with sufficient energy a molecule R* excited to a more energetic state is generated from the molecule R, which energetic state is elevated by the amount of photon energy input. In principle this absorption is a basic requirement for any type of photoreaction. This means, initially, even very dark solutions may be treated by UV-Oxidation if reactors are adequately designed and built.

R* may continue its reaction from this state, either directly or reverting to its initial state via reactive intermediates, or physically to photo products. Due to subsequent chemical processes, radicals, radical ions, ions or stabile fragments are generated in aqueous solutions, which may continue their reactions through thermal processes.
In presence of oxidizing agents additional reactions occur:
For instance, with light of proper wavelength hydrogen peroxide (H2O2) can be transformed into highly reactive hydroxyl radicals by means of photolysis (UV/H2O2-process), which react quickly with organic and inorganic water compounds (LAMING et al. 1969, BAXTON and WILMARTH 1963, HOCHNADEL 1962):

Such hydroxyl radicals (OH-radicals) are not only generated with the least amount of chemicals (Legrini et al. 1993) but also with the most economical energy input (Bolton and Cater 1994) by the UV/H2O2-process. For this reason AOP is also at high target-concentrations very well suited for effective treatment of pollutants in aqueous solutions such as, highly contaminated wastewater, electroplating baths and even ultrapure process water. However, for treatment at ppb-level catalysts are generally used instead of H2O2 (see UV-Deep-Clean).   The degradation of organic compounds via OH-radicals is initiated through hydrogen abstraction (HABER und WILLSTÄTTER 1931):

With the presence of Olefins an electrophilic addition of OH-radicals follows:

These initiated reactions follow various reaction possibilities of the generated radicals. In presence of oxygen an organic peroxyl-radical is formed:

Furthermore, various competitive reactions can occur:

 

In general, polymerization is not a favored reaction because polymerized products may lead to precipitation issues on the UV lamp surface. For this reason it is important to avoid such polymerization by proper process control design and UV reactor construction. Additional process equipment may be necessary in such cases to achieve optimal reaction conditions for an efficient oxidation process.
Also the peroxyl-radical (RHO2•) may for instance continue its reaction as follows:

The resulting Aldehydes respectively Ketones will oxidize into carboxylic acids through the subsequent reaction process, which are subject to either a thermic or photo-chemical decarboxylation (WEEKS und MATHESON 1955).

UV-Oxidation causes qualitative and quantitative changes

Change of relevant parameters during a typical UV-Oxidation
Chart illustrating typical degradation of TOC, COD, BOD and increase of bio-availability as function of the UV-oxidation progress, irradiation time respectively.

It is recognized that many various reactions are leading ultimately to a complete mineralization (CO2 and H2O) and the intermediate stages contain already a substantial increase on oxygen-driven groups. The intermediaries, which contain one or several –OH, =O und COOH – groups, are often considerably less toxic than the original compounds and show already a good bio-availability. Therefore, Enviolet®-UV-Oxidation can often be combined with a biological degradation process.
The described degradation mechanisms lead also to mineralization of organically bound halogens, e.g. forming of chlorides. From hetero-atoms such as sulfur, nitrogen and phosphor, sulfate, nitrate and phosphate is formed.

Practical Relevance of the UV-Oxidation

However, in praxis complete mineralization by UV-oxidation is seldom object of treatment because already in early stages of the treatment process required limits like e.g. COD (chemical oxygen demand) are achieved or detoxification is finished. This is shown in a strongly increased bio-availability, which is generated from the presence of easily degradable compounds such as carboxylic acids, alcohols and aldehydes.Picture 2 illustrates typical behavior of essential parameters during the Enviolet® UV-process. These are total organic carbon (TOC), chemical oxygen demand (COD), biological oxygen demand (BOD) as well as the resulting bio-availability.

With the aid of this diagram practical significance of the above described mechanisms during the H2O2/UV process is clearly demonstrated. The degradation process of the present values starts by oxidizing the carbon of organic molecules, whereby oxygen-containing functional groups are formed. However, no carbon dioxide is formed at first, which is why TOC reduction is rather insignificant at the beginning. But, since C-atoms of the molecules already have obtained a higher oxidation number, less oxygen is needed for a complete oxidation to carbon dioxide and, therefore, COD is reduced rather quickly. With COD reduction (=feeding molecules with oxygen) the bio-availability also is improving which is noticeable with an increasing BOD. This is clearly recognizable as well when the changes of COD and BOD are described as ratio of BOD to COD (=bio-availability in %). 

Example:

Ethane -> Ethanol -> Acetaldehyde -> Acetic Acid -> Methanol and Carbon Dioxide
It is noticeable the first 4 molecules in a solution still result with the same TOC level, whereas the COD is already clearly reduced. Only with the dissociation of CO2 the TOC level is also reduced.

A ratio of approximately 3 can typically be expected between COD/TOC in an untreated aqueous solution containing various organic compounds. Of course, this is also depending on the amount of bound hetero-atoms such as sulfur, nitrogen or phosphor, which will also be oxidized without contributing to TOC reduction. With increasing oxidation level the COD/TOC ratio is typically reduced by the UV/H2O2 process to approximately 2, as shown in the diagram. Also shown is the growing bio-availability as a result of increasing oxidation level of solutions containing refractory or toxic compounds. A bio-availability of 60% can be considered “good” and between 40 – 60% it is typically good enough for the water to be discharged to a sewage treatment plant.

A Practical Example of UV-Oxidation

Wastewater from a chemical production plant was batch treated and many parameters were analyzed at various treatment times. The wastewater containing mainly phthalic acid-derivatives and biologically not treatable was treated with the Enviolet® UV-process.

Diagram 1 shows the summary parameters. The bio-availability arises from COD and BOD and the resulting COD after a biological phase. In addition the degradation of aromatic compounds during the UV/H2O2-Oxidation is included, which are recorded as sum of all aromatic compounds. Concentration trends of the carboxylic acids are illustrated on the diagram 2.

Succinic acid, acetic acid and formic acid are substances, which are at first generated faster from the degradation process of complex molecules than being degraded itself. Therefore their concentrations show an increase in the beginning of the UV/H2O2-Oxidation process until degradation of the original substances is widely completed and concentrations of these oxidation products are reduced as well. For this wastewater original substances are described by the sum of aromatic compounds.

Even though aromatic compounds are still present the bio-availability is elevated after relatively short irradiation time. As soon as aromatic compounds are no longer detectable the bio-availability is elevating to nearly 80%. At this point the COD concentration is reduced by 50%.

UV-Oxidation von organischen Verbindungen
Verlauf einer UV-Oxidation

Visible Change of UV-Oxidation on Water Quality

Illustration of typical color changes during the treatment process of an industrial wastewater with the Enviolet® UV-Oxiation-process.

The efficient degradation by UV-Oxidation can be clearly seen by looking to the optical chenge: a black-brown broth becomes clear water.

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We not only understand the theory of advanced oxidation processes, we also know how to convert the theory in an efficient, tailor-made commercial unit. 

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