Dear Monsuru,

to my knowledge not so much literature is available about UV/PAA advanced oxidation process, in particular I know the following 3 papers (1 from our group and it is related to sunligth/PAA and I also attached the file) and are related to wastewater disinfection:

Moreover, we are investigating the effect of UV/PAA process on contaminants of emerging concern in wastewater and I trust we will publish the early results in the next months (you can follow the updates in researchgate to this link: https://www.researchgate.net/project/Water-and-wastewater-treatment-disinfection-by-homogeneous-photocatalytic-processes).

hello

this link could help you

https://www.hindawi.com/journals/ijp/2015/860845/

regards

Hi dr, I apologize for not answering my specialty so greetings

Article Water and Wastewater Disinfection with Peracetic Acid and UV...

I appreciate you all. Thank you.

Let's be honest and very clear here: UV/PAA won't ever be an efficient AOP and this is established by photochemistry principles. In order for an AOP to take place and be efficient, something has to be efficiently photolized under the applied type of light. UV, at 253.7 nm, won't be able to efficiently photolyze PAA since the molar absorption coefficient of PAA at 253.7 nm is very small. Moreover, by adding PAA you are also adding acetic acid (AA), which can efficiently scavenge OH*. As a result, the overall efficiency of UV/PAA as an AOP is very low.

Thank you for your contribution dear Santoro.

According to Domenico (Santoro) answer, I think is important to explain that the expected results about UV-C/PAA efficiency depends on the target water/wastewater and contaminants (including initial concentration). If you wish to investigate UV-C/PAA process either in the treatment of high polluting loading wastewater (e.g., industrial wastewater) or in the removal of high concentrations of contaminants of emerging concern (CECs) (some tens mg/L), I agree with Domenico that process efficiency will be really poor. But if you wish to investigate performances of UV-C/PAA as tertiary treatment method of urban wastewater for bactaeria inactivation and CECs removal, the low formation of hydroxyl radicals may be still sufficient to remove them, particularly because CECs occur at really low concentrations (from ng/L to fractions of microg/L) in the effluent of biological process. UV-C has also found effective in the degradation of some CECs, so its combination with PAA can improve the removal of CECs (we are finalizing some experiments and we should publish the results in the next months).

Hi Luigi, I agree with you that matrix effects are key when discussing the applicability of any AOPs. Here the issue I was trying to highlight is different. Before we bring in water matrix effects, we should look on how efficiently hydroxyl radicals can be formed based on photochemistry principles. These apply throughout, irrespective of whether you are dealing with atmospheric chemistry or water chemistry (to state very extreme scenarios). The intrinsic limitation of UV/PAA as an AOP relies, as I said, on the very low absorption and quantum yield efficiency of photolysis of PAA under 253.7 nm. If you want to make hydroxyl radicals, just go with H2O2 and UV. H2O2 has a quantum yield of 0.5 (primary) and an overall one of (1) when you take into account the fact that for every mole of H2O2 photolyzed makes 2 moles of OH*. One mole of photolized PAA would produce 1 mole of OH*. So, UV/PAA is a dead end irrespective of the matrix being targeted.

So, my statement applies to any kind of water matrix, and even more so to tertiary wastewater. In fact, I could argue that the cleanest the water matrix the bigger the difference in efficiency between UV/PAA and UV/H2O2 will be. This is because, on relative basis, UV/H2O2 will perform better than UV/PAA.

Another point (when I say PAA, I mean PAA alone, not PAA + H2O2 as you will find in any commercial mixture of PAA). Those investigators who claim an AOP effect with UV/PAA should really work hard to make sure the AOP effect is not coming from UV/H2O2, as H2O2 is there anyway. But this is a different story.

Now, when it comes to UV alone degrading micropollutants, that's a completely different story and no AOP is involved there. It's just direct photolysis of the targeted molecule by light.

Sorry to be a bit passionate in clarifying all the above but I have recently seen hypotheses around on UV/PAA that are just impossible based on first principles. And readers should be aware of this before wasting their time in trying something impossible. Note that PAA was the topic of my PhD, so if I arrive to say today that UV/PAA is a dead end, this is not without sadness since I really like that molecule and the chemistry associated with it.

Best,

Domenico

Hi Domenico. First I'd like to clarify that I did not wrote in my previous comment that UV-C alone is an AOP. If you read my comment, I've just told that UV-C process has found to be effective in the removal of some CECs (and for sure this is the effect of a direct photolysis mechanism).

Moreover, we have investigated the effect of UV-C/PAA on a model antibiotic (at high initial concentration (25 mg/L) to better evaluate the formation of possible oxidation intermediates) and we also added catalase to the solution to remove H2O2 from the commerial PAA solution to evaluate the effect of PAA only. The efficiency of UV-C/PAA with catalase drastically decreased compared to UV-C/PAA without catalase, thus confirming that the main contribution to radicals formation is from H2O2 combination with UV-C. But UV-C/PAA with catalase was better than UV-C alone.

Taking into account that tertiary treatment of urban wastewater is also useful to make effluent suitable for reuse in agricoltural applications and that UV-C/H2O2 process may result in some toxicity to crops because of residual H2O2, it is of interest to investigate alternative options (such as UV-C in combination with peracids). So, in spite of UV-C/PAA is less effective than UV-C/H2O2, it can be still effective in the removal of low concentrations of CECs and result in a less toxic effluent (we have not yet investigated toxicity so far) compared to UV-C/H2O2.

kind regards

Luigi

Great discussion! Sounds like we are in agreement on the mechanistic interpretation of certain phenomena.

If you want to remove H2O2, just have a very it go through a very short carbon filter (5 min will do) and then you can have a very efficient AOP (UV/H2O2) with no H2O2 residual at the end. I personally am very doubtful that H2O2 (at the concentration we are talking about) can be toxic for plants, as soil will quench H2O2 in no time. The problem with PAA is the cost as well. As you know, the cost of PAA is 5x the one of NaClO, and 3x the one of H2O2. So, it's hard to imagine for me a situation where PAA could make sense as a AOP if you can do a better job with H2O2.

Sorry for having misinterpreted your statement on UV-C as an AOP. Many people have put that in writing in top journal papers and I wanted to make sure we don't propagate this misunderstanding. But for the same reason, we should not compare UV-C and UV/PAA (cit from your previous message: "UV-C/PAA with catalase was better than UV-C alone").

I look forward to catching up in person with you soon and have this discussion continued in front of a beer, or even better a good (real) mozzarella from Salerno :-) :-)

Best,

Domenico

Actually, we do not want to remove H2O2 from PAA solution (the process is expected to work with commercial PAA solution, so also including H2O2). We used catalase just for experimental needs, to understand the contribution of PAA as AOP in absence of H2O2 (like a control test). The same for UV-C test, as you know when you investigate a photo-driven AOP, you are also expected (reviewers will ask you) to evaluate the contribution of the single agents (UV-C and PAA, respectively in our case), as control tests.

You are welcome in Salerno ... to taste "our" mozzarella :)

see you

Luigi

And you (all) are welcome to London ON (Canada). In fact, the call for paper for ecoSTP18 is now open...please consider submitting an abstract on this very interesting topic!

www.ecostp2018.com

All are welcome.

Domenico

Thank you all. Great minds!