Hydroxyl radical scavenging effect. http://pubs.acs.org/doi/full/10.1021/ez4000862 http://thesis.library.caltech.edu/972/5/04_Chap4_CDV.pdf http://iopscience.iop.org/0963-0252/20/3/034009

Dear Ciro Bustillo-Lecompte,

I did not get your point. Can you please tell how hydroxyl radical scavengers help to increase TOC value during UV photolysis of PFOA/PFOS in water? The attached document referred to your answer doesn't have any information/explanation on hydroxyl radical scavenging effect on TOC values. Furthermore, I can't see a connection between hydroxyl radical scavengers and TOC values. Can you please kindly explain about it?

There are two possibilities I can think of: (1) the deionized water is leaching carbon out of tubing and fittings in your experimental set-up [as the pH of deionized water can be less than 6] or more likely (2) the UV is breaking the longer chain PFCs (C6-C9 and more into shorter chain length PFCs (C4 or less) which are then more easily quantified/measurable by UV/persulfate oxidation in your TOC analyzer (which most water samples are analyzed with). Consider using combustion TOC. I'm not sure if it will work for long chain PFCs though. TOC analyzers are constrained by the settings used (sparge time, etc.). Hope this helps.

Thank you William for your views on the point! Let me make the point more clear. I am using glass reactor operated in batch mode. Additionally, two glass tubes are immersed into reaction solution (from the top) for sampling and N2 sparging, and a small round-shaped commercial diffuser (it appears being made of stone/cement material) is adjusted at the lower end of the sparge tube.

The increased TOC is observed not with UVC photolysis alone, but with samples from electrolytic decomposition experiments also. I am using combustion TOC analyzer (Shimadzu), not persulfate oxidation TOC analyzer. After repeated occurrence of the phenomenon, I thought that not all the PFOA/PFOS molecules in the samples before UVC photolysis/electrolytic oxidation were oxidized by TOC analyzer, and then I increased its (TOC analyzer) combustion temperature from 680 oC to 750 oC. Moreover, I measured TOC values of PFOA/PFOS standard samples, and the measured TOC values were very close to the theoretical/calculated values. Then, I concluded that partial oxidation of PFOA/PFOS molecules in the samples (before UVC photolysis/electrolytic oxidation started) may not be the reason.

Although, I thoroughly washed all the components with methanol and then ultrapure water before conducting a test, I am not pretty sure if organic carbon leached into the reaction solution through the glass tubes and sparger as you mentioned.

The analysing of the TOC over combustion is normaly detected with IR-sensors. Most of the ir-sensors interfere with gases like methane or other substances with related ir- spectrums. It may be possible, that the combustion of your PFOA/PFOS is not complete and you get a cross-detection with other gases, which are able to cause a higher optical transmission. In the normal case the TOC analyser have oxidated all substance to CO2 and is calibrated for this case.

Thank you Ingo Schreiber for your opinion! I am not sure about interference of gases (other than CO2) to IR-sensors in my TOC analyzer. As the tests were carried out in ultrapure water, the reactor including its other components were thoroughly washed with methanol followed by ultrapure water and generation of any such gases (e.g., methane) are not known during UV photolytic/electrochemical degradation of PFOA/PFOS, I think the chances of such interference are unlikely.

I also in the beginning suspected of incomplete combustion of PFOA/PFOS in the TOC analyzer. Then, I raised combustion temperature from 680 oC to 750 o C. Moreover, I used pure oxygen as carrier gas instead of air (used before) to enhance combustion. But, the changes did not have any impact on the measurement results. Then, I came to the conclusion that incomplete combustion may not be the reason for the problem.

The problem is persistent with low dose UVC photolysis while it is not seen in case of VUV photolysis. I think this is an important reference point for identification of the problem. As William Anderson suggested earlier, the sparger tube and diffuser in my setup could be a source of PFOA/PFOS resulting in their increasing concentrations in the reaction solution during the tests. In case of UVC photolysis, the rate of leaching could be higher than the rate of depletion as low dose UVC is very inefficient to degrade the compounds. However, the rate of leaching could be significantly smaller than the rate of depletion in case of VUV photolysis as VUV is very effective in degrading the compounds.

I really face the same problem in photocatalysis experiments. Did you find a possible explanation? Rabindra Raj Giri

Have you find out any reason for high TOC after degradation in photocatalysis experiments? Mohamed Gar Alalm ?