It is well known that direct reactions of ozone are selective and significantly slower in comparison to AOPs. However, ozonolysis is often used for water and wastewater treatment. What are the reasons?
Jan, direct ozone reactions are in fact selective (not as stated above) but I agree their rate constant is lower than AOPs. How would you define ozonolysis?
Hi Jan:
The ozone reaction with organics in water/waterwaters depends on the solution pH. In acidic solutions, direct reaction between the ozone molecule and the organic molecule will happen, ozone molecule attacks unsaturated double bonds such as (-C=C-) easily and selectively. However, ozone is kind of selective oxidants. In basic (neutral to high pH) solutions, ozone undergoes decompostion to generate OH radicals that react unselectively with almost all organics (this process is called indirect ozonation).
Advace oxidation processes (AOPs) based on the generation of OH radicals in situ (as the service life of OH radicals is very short) that react wthi organics unselectively. on this bases, direct ozonation is not AOP, but indirect ozonation is AOP.
As ozone is more selective than OH radicals, it is removing organics (in terms of COD and TOC) in a less efficient way than AOP does.
Hope this answers your question, and please let me know if you need any clarifications
Khalid Zakaria
The speed of the reaction (Kinetic) is important during the selection of oxidation process. Some of the wastewaters may not be treated completely by the fast reactive oxidant; which need reaction at a controlled rate of reaction, this can be achieved by the ozonolysis than other AOP, because in the ozonolysis the O3 need to convert as nacest oxidant for the oxidation of organic pollutant.
So, it is favourable in few wastewater than the other AOPs.
Hi Jan,
Ozone as stated by Chedley is selective and reacts very fast with selected organic (phenolics, a variety of pharmaceuticals, etc.) and inorganic (sufide, cyanide, etc.) compounds. Ozone is also an excellent disinfectant and is used to inactivate even micro-organisms (e. coli, Giardia, etc.). Therefore, ozone is used for disinfection and to remove low level of micro-pollutants from water and wastewater.
I agree with most of the arguments stated above. The choice between ozonation or ozone based AOPs depends on the reactivity of the target compounds towards ozone. Generally, pollutants with second order rate constants exceeding 10^4 M-1s-1 are easily (and almost exclusively) removed by ozone. Below 10^2 M-1s-1, one should think about enhancement of the HO production.
Just another important remark: if the wastewater contains a significant amount of unsaturated or aromatic compounds (such as phenolics, natural organic matter, ...) or secondary and tertiary amines, HO radicals are indirectly produced from electron transfers between these compounds and ozone. These reactions give rise to ozone radicals and/or superoxide radicals, which rapidly lead to HO formation. In those cases (e.g. in the case of municipal wastewater), significant HO production can occur with HO exposures exceeding those generally found in conventional AOPs. So, you don't always need hydrogen peroxide addition or elevated pH to have significant HO production during ozonation. If a complex matrix is present, this issue is worth incorporating in your study.
I will have a publication on this soon.
Hi, here's the publication I promised.
Article Comparison of ozone and HO induced conversion of effluent or...
I just read Wim's remark and I am affraid to say that I do not agree with his statement that phenol and/or NOM will transfer electron to ozone and generate ozone radical becuase of the fact that phenol is not a good electron donor. The electron transfer possibility may exit if WW is contaminated with hydroquinon or dihydroxy-phenol. At any rate one has to look at the reduction potentials of both the possible electron donors and ozone to see if this is thermodynamically allowed or not.
Hi Ali and Wim, maybe mechanisms published by Gulas et al., 1995 could be helpful. The authors present that the presence of organic wastewater solutes which contain C-C double bonds (ligninsulfonic acid in the treated paper-mill effluent and humic acid in the oil reclaiming model wastewater) were shown to yield hydrogen peroxide by the reaction with ozone. Therefore, these wastewaters are efficiently ozonated even without addition of hydrogen peroxide.
Hi, please read the paper attached. It is thermodynamically allowed.
Hi Wim,
Yes, you are correct; electron transfer from phenol to ozone is thermodynamically allowed, however, I do not if electron transfer takes place. I am sure you know that : a)reaction of ozone with phenol is controlled by phenolate ion, and b) in this process O3 transforms to OH-radical first . Is OH-radical formed by a direct electron transfer from phenol to O3? It is not clear (von Sonntag et al., J. Chem. Soc. Perkin Trans. 2001, 264-268). By-the-way, I could not open the paper, obtained phenol reduction potential from P. Wardman (J. Phus. Chem Ref. Data, 18 (1989), 1637) and I would appreciate if you cvould email me a copy.
Best
Indeed, a direct electron transfer is occurring. Ozone firstly wil form the ozone radical, which rapidly transforms into HO. I will send you the pdf of the article I referred to above.
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