I am doing a photocatalytic performance evaluation by RhB dye degradation. I am facing a problem which is:
After achieving the absorption -desorption equilibrium, I checked the concentration and marked it as C0. I repeat the same process to check the performance of my sample after every 30minutes. And I can clearly observe the decrease in concentration, even though, the degradation of RhB is very slow. However, after some time, the concentration suddenly increased to a specific number. For example, 1, 2, 3, and 4th sample concentration is 1.88, 1.65, 1.35, and 1.10 respectively. but in the 5th sample, it will be like ~2.15. What is the reason of facing such a problem and how to counter this?
Hi ! If i have rightly perceived form your question posted here, the very reason for increase in absorbance is the desorption process. May be you will have to initially optimize the dosage and then go for rest of the evaluation on RhB dye degradation.
Shivamurthy Ravindra Yashas Sir, "Optimizing dosage" means I have to add the right amount of sample in the RhB solution. Right?
YES! First, you can decide on the required time/duration of the photocatalytic run, i.e optimizing the contact time. Then, you can optimize the dosage of the catalyst.
Shivamurthy Ravindra Yashas Sir, I have taken all these parameters into consideration. For example; I have added 10mg RhB into 1-liter water and then taken 50 ml of RhB solution, added my sample into the solution, and put it on stirring in dark for 30 minutes. After all these steps, I exposed the solution to the light and then taken out the 4ml of the solution, centrifuge it, and then finally, checked the concentration. I kept repeating this process until the degradation to a certain point. The concentration is supposed to keep decreasing and it happens. But after testing 2 or 3 samples, the concentration suddenly starts increasing. I still couldn't anticipate the reason.
If I get your question right - you expect decolorization of RhB under irradiation, and it happens, and e.g. in the 5th sample you notice an increase in concentration (absorption probably). Am I right? I faced this problem once and in my opinion in the first step of photocatalytic reaction the reduction of RhB to the leuko form takes place, then the colour reappears and its intensity increases gradually probably due to the oxidation by the oxygen from the air. The behavior is linked to the pH and surface charge of your semiconductor.
Please find the article beneath concerning very similar phenomena:
(PDF) Study of the Discoloration Rate of Rhodamine B as a Function of the Operating Parameters at Pt and BDD Electrodes (researchgate.net)
And remember that photocatalytic processes are redox processes.
If i m right and up to know your question.
Heating effect is also responsible for enhancing the concentration. And other onr is that after two or three cycle catalyst does not show stability and by the fact of temperature it might be increased. So my opinion is that fix the temperature through out the test.
Hamza Majeed
Check the degradation intermediates and by-products formed during the process. May be the increase in absorbance after 3-4 sample intervals is due to their interference.
Anna Jakimińska
I have observed same phenomenon with organic contaminants other than dyes also. Can it be due to phenomenon other than redox-processes?
Aarif Shah
I observed this behavior only with rhodamine dyes, but not with crystal violet or methyl orange, and only with a catalyst with specific properties. The other contaminants I have worked with act differently. In the case of RhB, I believe this is a redox reaction, but if you are working with e.g. carboxylic acids - the concentration changes determined by the absorbance measurements can be attributed to dimer formation or other processes. So yes - you can explain the changes not only as redox processes, but probably not in the case of RhB in this experiment. To answer your question more properly I should know which kind of organic contaminants you examine.
Best regards,
Anna
Anna Jakimińska
Thankyou for the wonderful explanation.
I work with drugs like anti-pyretics, analgesics, antibiotics etc.
Warm Regards
Aarif Shah
You're welcome.
In case of antipyretics - when it contains benzoic acid unit or something similar, probability of dimers formation is very high. Drugs can also form charge-transfer complexes with semiconductor (via hydroxyl or carboxyl groups) which can also affect the changes in concentration. It depends of course on the photocatalyst type, and most often it is included in adsorption-desorption equilibrium of the contaminant.
Anna Jakimińska
Doesn't that mean spectrophotometry is not an efficient technique to monitor degradation process during photocatalysis
Aarif Shah
Spectrophotometry won't answer all the questions such as: what are the intermediate products (sometimes more stable intermediates can be observed), how many processes are taking place simultaneously and what kind they are, but it is helpful for the estimation of photocatalysts activity or determination of the products of photocatalytic processes and it is quite simple and available technique, therefore it is widely used.
Anna Jakimińska
Actually i asked about the reliability of this technique in view of the problem faced by the person in determining the degradation of samples (4th sample beyond). So i was wondering how will he determine the time at which 100 % degradation occurs, as 5th sample shows increased absorbance rather than decrease.
@Aarif Shah
Determination of RhB degradation in this case is impossible using only UV-Vis spectrophotometry. Other techniques have to be applied to confirm the degradation. In described case occuring processes are rather complex and may take place at the same time. Remember that decolorization of the dye and decrease in the peak around 553 nm are not equal to RhB degradation.
Anna Jakimińska
Yes, decolorization & decrease in peak don't directly correlate with RhB degradation. I believe Hamza Majeed needs to use GC-MS/LC-MS or HPLC-PAD to monitor the dye degradation as well as formation of intermediates and by-products.
Aarif Shah
Yes, of course I agree with your statement. Also TOC can be helpful in terms of confirmation of the dye mineralization.
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