Advanced Oxidation Processes - Science topic

Joe Tylczak thanks for the reply. Could you name some of free resources.

With such a limited infomation it is impossible even guess. More details.

Well it depends on the potential that we applied. Positive potential gives oxidation reaction and negative potential gives reduction potential. If working electrode is in oxidation state then counter/ auxiliary electrode get reduced. It also depends on chemical behaviour of the electrolyte and assays that you studied.

Hi, Magnetic stirrer and RB flask are enough to reach out TiO2-RGO composite (https://www.sciencedirect.com/science/article/abs/pii/S1004954118308759)

If you want methodology in a detail, kindly let me know.

High-pressure membranes, such as nanofiltration or reverse osmosis, have been extremely effective at removing PFAS. Reverse osmosis membranes are tighter than nanofiltration membranes.

The flow diagram

thanks for this information

Hi Federico Verdini,

For the said industrial wastewater, determine or (repeat) the COD by varying dilution factors

(1:10, 1:50, 1:100, 1: 200). After that you may get average range of COD.First confirm intial value of given sample. For more accuracy you can correlate with BOD value.

However you can more details about interference and its limitations in APHA Standard method 5220, SECTION 5-11)

Reg

Prashant. B.B

Amir Loghmani i do agree with the expert comments above but I only have to say about that there are many studies available and reports various kinetic modelling of ethane oxidative dehydrogenation under an oxygen free atmosphere employing a catalyst of various percentage and sometimes it might be based on the weight and can from 5-12% wt % of VOx supported on ...... and one can see these studies in the links shared

https://pubs.acs.org/doi/abs/10.1021/ie303305c#:~:text=This%20study%20reports%20kinetic%20modeling,c%2DAl2O3.&text=On%20the%20basis%20of%20the,to%20the%20catalyst%20oxidation%20extent.

Dear Prof.

Looking forward to hear if any special issue under your editorship.

Thanks

Siddhartha

Almost all quantum chemistry codes support applying external electric field in calculations. My recent work about ultrastrong regulation effect of electric field on various properties of cyclo[18]carbon is a typical example: https://doi.org/10.1002/cphc.202000903.

Try using electrodes with high OER and HER overpotentials. Such materials include MMO and Pt. This will reduce the overall gas evolution, thereby reducing the froth formation.

Thanks for your invitation. We can contribute if publication charges are waived.

In the paper, "Heterogeneous photo–Fenton treatment for degradation of indigo carmine dye ". Cosme-Torres, et al., MRS Advances © 2019 Materials Research Society DOI: 10.1557/adv.2019.451 . 23 de febrero 2020. https://www.cambridge.org/core/terms,

you can read about this topics

For L-B-L technique you need strong binder and pH control, maybe you try with chitosan. You can also try screen printing or dip coating method with TiO2 paste. Ref:

Fabrication of screen-printing pastes from TiO2 powders for dye-sensitised solar cells

So sorry Stjepan!..in fact my answer was directed to the person who asked the question in the first place (@Michael Wierer)

Best regards

That is a really awesome question, which has troubled me for quite a long time. To be honest, we have paid too much attention to the cathodic H2O2 accumulation and unquestionably, it is crucial in the electro-Fenton process. In fact, electro-Fenton comprises two Fenton’s reagents: one is H2O2 and another is VALID Fe2+. The role of the valid Fe2+ is important as well since Fe3+/Fe2+ is the rate-limiting step in the classical Fenton reaction. We could notice as we pave our way to improve H2O2 accumulation to fabricate various cathodes, among which gas diffusion electrodes (GDEs) are regarded as one of the most efficient ones for H2O2 formation, however, hydroxyl radical generation in the GDEs-based electro-Fenton is not that promising. Because iron precipitation happened near-cathode vicinity as H+ was consumed too much by an oxygen reduction reaction (Journal of Power Sources, 2020, 466: 228342). As a result, we need to balance H2O2 formation and Fe3+ reduction to maximize the radical generation if one single cathode used for both reactions.

Regarding Fe3+ reduction, in my view, we need to consider it from two aspects.

One is about its chemical nature before it migrates to the cathode surface, I mean, iron tend to complex with various ions (like -OH) or chelating agents with function groups (-O, -N) such as humic substances polycarboxylates, etc, commonly presented in the aqueous environment. Those ligands donate a pair of electrons to the Fe atom and then redox-properties of Fe3+/Fe2+ were modified.

Another one is what you have mentioned is the nature of the cathode itself once iron approaches to the cathode. Research about it is quite limited and as far as I knew, cathode surface modified with electron-donating moieties, like phenolic -OH, quinoid -C=O, carbon-centered persistent free radicals favors Fe3+ reduction.

Dear Djamel Ghernaout ,

The relation between these two parameters is what i m seeking indeed. During my research to chose the main parameters to study in the electrochemical reactor, the fact that i don't have an interesting idea about the stirring degree rang makes me wonder about finding the appropriate domain basing on the bulk proprieties to get an idea about the mass transfer in this later, and i ended up with the Zeta potential.

As you mentioned, i think the Zeta potential will be influenced by the hydrodynamic regime and so the mass transfer, thus, it will be interesting if we can express such relation.

Thank you for the clarification and your support.

Thanks for sharing, I appreciate your help.

Dear Noor

To understand the position of Bi in TiO2 you need to first perform Neutron scattering to get a high resolution Data, Next you should perform Rietveld refinement using the TiO2 cif file. However, you will have to perform solid state NMR to understand the coordination number of Bi in the structure.

Using these methods, you will be able to develop a model for your structure which will tell you the exact occupation of your Bi.

There is another method which is called first principle method. in this method you will be able to optimize your geometry based on your primary guess for occupation of Bi. This is an excellent method in which you can even calculate band gap energy using HUMO and LUMO difference and by density of states.

Were your experiments done with ozone generated from air or from oxygen? If from air, you may find other molecules formed in the generator, are interfering with your results.

In answer to question 1, I would guess that it is the same as why ozone is not more used in Medicine i.e. there have been lo large scale trials. The reason for the lack is the same. There is no way of protecting your investment in a trial nor large money to be made.

It certainly is a cheap biocide. Even the equipment needed is cheap.

I have had saline ozone IVs and my condition did improve although it's back to where it was prior to treatment 2yrs ago.

It depends. It might be good because the degradation rate does not decrease in the course of reaction

If it is homogeneous Fenton's process, two options could be used: (i) removing H2O2 by adding small quantity of sodium sulfite (Na2SO3) (conc. of around 30 mM) should be enough and (ii) precipitating the catalysts by adding strong base. The aim of both method is to remove one of the Fenton's reagents, which will immediately halt the reaction.

When your dyes are sulfur ones, the choice of activated carbon as an adsorbent is not too good. It is well known, that efficiency of sulfur dyes reduction by activated carbon is rather low. Please see for example Thai Anh Nguyen, Ruey-Shin Juang (2013): Treatment of waters and wastewaters containing sulfur dyes: A review, Chemical Engineering Journal 219:109–117:

"The effectiveness of the activated carbon adsorption process is different with various dyes [ O. Marmagne, C. Coste, Color removal from textile plant effluents, Am. Dyest. Rep. 85 (1996) 15–21. ]. High removal rates (over 90%) are achieved using activated carbon for cationic, mordant, and acid dyes. For direct, sulfur, dispersed, and reactive dyes, the efficiency is moderate (over 40%); it can be improved using massive dosages of activated carbon. For vat dyes, color removal is very low (under 20%). In the case of sulfur dyes, the COD removal is 45.5% and the color removal is 40.9%"

Best regards

Vit

Hi,

I agree with Dr. Frank ... You need to give more informations about your operating conditions (gas pressure and flow, T°, pH of your liquid, ... )

There are many factors to be considered in your experiment to optimize the O3 solubility. First at all, try to increase the height of your sample ...

Kind regards

Chemical treatment of photocatalytic degradation (semiconductor based materials like ZnO and TiO2 ), has been used extensively due to its excellent catalytic activity, eco-friendly as well as low secondary pollution.

Unfortunately metal oxide nanomaterials have no efficient absorption in the visible light region and require activation/irradiation in the UV region.

Metal nanoparticles, on the other hand exhibit strong absorption and excellent catalytic activity in the visible range and gained much attention due to its potential application in degradation and removal of organic pollutants under irradiation of visible light.

Keep in mind the energy consumption of AOPs (ozone generation, UV lamps, production of H2O2 etc)!

In the case of hospital discharges, where the contents must be of nano grams, the use is indicated even if it costs relatively expensive but the obligation to remedy renders the technique valabale

The choice of the most suitable treatment technique depends upon pollutant concentration. When free phase of products is present, the first step should be physical treatment followed by.............. It depends upon quality of pollutants present in the effluent.

Best regards

Vit

In general, you have to answer these questions in order:

1. What is the design capacity of your plant (m3 wastewater/day)?

2. How much energy is required to process 1 m3 of watstewater (J/m3)?

3. How much one solar cell can generate (J/cell)?

It depends on solar cell design capacity, efficiency, local solar data (solar isolation, air, temp.,...). You can take ready solar modules from industry, and weather data from RETScreen if not available locally.

4. How many solar cells are required (cells)?

Hope this helps,

Abdo,

Dear Saeed,

This is one of the main concerns for students who must determine their own research topic. So, please do not be frustrated. I know that you are looking for a new, yet efficient, way to refine one of the most complex sewage. But please note that your time is only one year, and this time is not enough to carry out the number of tests. My suggestion is to combine two of them, such as oxidation and biological methods, and then focus only on them and all the effective parameters. I can tell without any information and only on the basis of my experience that there are still thousands of experiments that can be done on their combination that nobody has done yet. So you do not have to create a new method, but just do experiments that others have not done. I'll give you a simple example. Changes in temperature, pH, flow rate and feed concentration! That is, even if another researcher has checked the parameter effect, you can change the parameter check range. This is also a new experiment. Please do not drown yourself in a world of papers and methods of purification. What I've done before.

Good Luck

Hi Ayushi,

Our research group found that applying pulsed current could drastically enhance the H2O2 production in EF process, especially for porous cathode materials, such like graphite felt, carbon felt, carbon foam, etc.

Below are links of our recent research, hope they are helpful to you:

Many options are available.

1. KI,

2. NaOH can be used.

3.We can use MnO2 but it may interfere with TOC analysis.

4. We can safely use inorganic scavengers- Catalase is the best as it will not interfere with the analysis.

5. Tert-butanol or tert-butyl alcohol can be used but may interfere in analysis.

6.Even Chelating agents like EDTA can also be used for the purpose.

7. One of my students had used Sodium Thiosulfate Na2S2O3 without pH adjustment successfully.

Answers given by Niels and Soliu are very right and recommended by me

Hydrogen peroxide and UV light from the sun together catalyzes the reaction  yielding two free hydroxyl radicals  which are potent oxidizing agents.

H2O2 + hv→ 2OH·

Presence of metal ions like Fe2+ or Cu+  in seawater may assist generation of hydroxyl radical .Iron(II) is oxidized by hydrogen peroxide to iron(III), forming a hydroxyl radical and a hydroxyl ion in the  Fenton's process

Fe2+ + H2O2 → Fe3+ + HO• + OH−

Also, additional source of hydroxyl radicals is the photo-Fenton  process  

H2O2 + hν → HO• + HO•

Fe3+ + H2O + hν → Fe2+ + HO• + H+

Thank you very much for the answer

Dear Pounsamy,

It is difficult to answer as long as the question is very broad. Some people define AOPs solely as processes that work through HO· reactions. In this case it is difficult to find a process that has a variable reactivity to inorganic ions. If you take the broader definition you can get more selectivity from reactions that involve sulfate radicals (UV/PS, figure 3 in Maria Antoniou paper), ClO₂, ozone and possible Cl· from UV/Cl₂. 

I think a novel and very interesting process for selective reactions is the hydrated electrons generated by UV/SO₃^2- and gamma radiation. 

You can find very fine work on both sulfate radicals and hydrated electrons in Prof. Sayeds recent publications.

pKa of BHT (Butylated hydroxytoluene) is 12.3. No ideas about pKa of phenolic group on the surface of antioxidant. BHT is an antioxidant because of the weak O-H bond. No relationship with pKa.

We have several full-scale case studies where ozone was used to removed various dyes from textile plant effluent.  Ozone is not technically an AOP and is much less expensive compared to AOP's and chemical removal using coagulants and flocculants.  I agree with Mohamed that biological is best for pretreatment prior to chemical oxidation.  If you have a vat or batch process perhaps it is possible to use SBR or MBBR.

In photo Fenton process, hydrogen  ions H+ are produced through reduction of Fe3+ ions under UV light: 

Fe3+ + H2O+ hν → Fe2++ HO•+H+

Also, some organic  dyes  show changes in   pH  of their solution under UV irradiation

Yes, I approve the answer of Patrick! time depends on many parameters. The performer needs to know about (weight) of the parameters in the experiment.

Fenton's process should be an appropriate method for the treatment of industrial wastewaters which are characterized by a high COD content, so it it is a bit strange that only a poor COD removal is experimentally observed. In the Fenton process is recommended as a typical range for

However, in your description above, nothing was mentioned about the initial pH value of your wastewater. Remind that the Fenton process is usually inefficient when the solution under treatment has a pH value higher than 5. When the pH value of the wastewater is to high, Fe³⁺ ions willl tend to precipitate as iron(III)hydroxide, and hydrogen peroxide will decompose in H₂O₂ and O₂. This will result in a significantly lower production of OH* radicals, and hence a lower process efficiency.

In the most ideal case, the pH value should be adjusted to pH = 2. You can use sulphuric for doing this pH correction. At the optimal pH value of 2, the optimal dose Fe²⁺ / H₂O₂ of is approximately 1:3.(on weight basis). When you have a very high COD concent I also would recommended you to use 5 parts of Fe²⁺ for one part of COD (also on weight basis).

One possibility could be to use electrodialysis and separate the nitrate ions (smaller) from the humic acids. It also depends on the feed you have and if you only want to separate nitrates, because in this case other anions could be separated too (like chlorides, for example).

Thank you again Mr Manohar Sehgal for your clear explication.

If your organic contaminants are known then use UV/visible spectroscopy

Yes, I converted Rutile to Anatase phase by multiple shock treatment for the first time, on subsequent shock treatment Anatase converted back to Rutile structure. I am in the process of finding the exact thermodynamic condition to convert Rutie TiO2 to Anatase TiO2 phase. However I am presenting a research paper on reversible phase transformation from Rutile to Anatase and back to Rutile (controllable ratio of mixed phase) in SAMPE 2016 symposium on 24th May 2016 at Long beach CA, USA.    

Dear Jonathan,

In my opinion, the volume and initial concentration of p-nitrosodimethylanyline (RNO) depend on your system. For example, if you generate hydroxyl radicals by an electrochemical system using large electrodes, you need a larger reactor. If your method has a high rate of generation of OH radicals using a low concentration of the RNO, you will see complete bleaching with little reaction time. You could start with 10 mg/L.

You only need to perform this bleaching method in the presence and absence of your thiol in the same conditions in order to compare.

See J Appl Electrochem (2011), 41: 599-607 and Journal of Photochemistry and Photobiology A: Chemistry (2010) 216:244-249.

Hope this helps.

Best regards.

As for the oxidation of a-Si:H HOMO plays an important role. the higher energy level of HOMO is , the faster oxidation rate is if hydrogen densies are same . So you should investigate the differene of HOMO energy between n-type and p-type a-Si:H  

Dear Ali Farhat,

How well chlorine works in water, depends on what the pH level is. At a p^H of 8.0, chlorine is only 20% effective! That means green and cloudy water and dangerous bacteria. The higher the pH, the less effective chlorine is. At a pH between 7.4 – 7.6, chlorine works best.

Chlorine is a gas. It does not have a pH value. However, if the chlorine gas is dissolved in water, the following occurs: 

Cl₂ + H₂O → HOCl + HCl 

hypochlorous acid (HOCl) and hydrochloric acid are formed. Obviously these are acid solutions, and the pH will be below 7.00. The actual figure will depend on the concentrration.

 At p^H 0, Chlorine in water is present as Chlorine gas – Cl₂. With rising pH, Chlorine reacts with water to form Hypochloric acid - HOCl. In the range of pH 2-6, this is the prominent compound. At higher pH values, Hypochloric acid is neutralized to form its corresponding salt ion, the Hypochlorite ion – ClO-. At approx. pH 9, the neutralisation is complete. The disinfection activity of Hypochlorite is only 1% of that of the Hypochloric acid. As a result of its reaction with water, Chlorine remains in solution for several hours, depending on temperature, circulation etc. 

Regards,

Prem Baboo

There may be several factors that contribute to the enhanced activity. First of all the ZnO may have altogether different vacancy states when it was synthesized and kept for years. It could easily come across the atmospheric oxygen, moisture and other gases to have some chemistry which can modify the properties easily. Did you check the XPS of the sample just after synthesis and after keeping it for 2  years? That may provide some fruitful information.

The bottom line is that you do not have the same structure anymore, although, you didn't do any chemistry but the nature did some chemistry with your sample. 

Dear Hanggara,

To the contrary, a study entitled " Is Methylene Blue An Appropriate Substrate for A Photocatalytic Activity Test? A Study with Visible-Light Responsive Titania" revealed the following: 

An analysis of decomposition of methylene blue (MB) in an aerated aqueous solution by its action spectrum has shown that sulfur-doped titania (S–TiO₂) has activity under visible-light irradiation and that the decomposition mechanism depends on the excitation wavelength. This suggests that photosensitive materials are not suitable as probe chemicals for photocatalytic activity tests, especially those for evaluation of activity under visible light. It is confirmed using acetic acid instead of MB that S–TiO₂, but not ordinary non-doped titania, leads to the photocatalytic reaction under visible radiation below 600 nm, which corresponds to its absorption spectrum.

4. Conclusion

The results indicate that there are two significant problems in the photocatalytic

activity test. One is the inadequacy of MB as a probe molecule for semiconductor

photocatalysis, since the photoinduced reaction by MB photoabsorption may mislead into  believing that a given semiconductor material has visible-light photocatalytic activity. Dyes other than MB [35] should also be examined for their suitability as a probe molecule. The other problem is the photoirradiation systems for the activity test. As above, action spectrum analyses enable us to discriminate the origin of photoresponse by checking the wavelength

dependence. However, when the samples are irradiated in MB solutions, for example, with a set of optical cut-off filters that transmit light of a wavelength longer than a given wavelength, as customarily employed, the results cannot be a proof of visible-light sensitizing ability, i.e., real photocatalytic activity.

In summary, the use of transparent substrates in the region of working wavelengths is strongly recommended, if at all possible, in a photocatalytic activity test for photocatalysts and for action spectrum analysis. By using the preferred test system, the present S-TiO2 has been proved really photoactive in the visible region, at least for the decomposition of AcOH into CO2. However, the present finding does not guarantee the visible-light activity for other frequently-used substrates such as acetone or toluene, which need careful individual examination.

For more details, see attached paper.

Hoping this will be helpful,

Rafik

This is a good question. Standard redox potential does not mean the potential at pH 0, BUT shout be the one as redox couple is at 1 mol/L. To answer this question, please read the original papers by Neta and Huie, or contact the authors directly..Good luck!

Hi Ammar, glucose is easily degraded and easily assayed.

Hi, we just add the powder to a beaker and heat it under continuous stirring. We have done this for all our experiments and I discussed this with other scientists at conferences in past. It goes slow, but it does dissolve. Maybe ultrasonication is dangerous as it may induce molecular changes.

Ok, I will have a look on these reviews.  Thank you.

During heating processes, N₂ and CO₂ are used for two different things:

It's also guaranteed that Cr(VI) will be reduced by Na2SO3

Have a look on my papers. In the course of doctorate I worked with a highly loaded wastewater, maybe these articles are usefull for you.

To correctly interpret whatever results, all details MUST be available. No detail data, no answer. 

Try to find Pourbaix diagram for your systems. There are lines which gives information about water redox stability in the selected pH. I hope it is what you need. Best wishes.

Hi  Jixing Cui,

I would try to explain it in the following lines

Rate Expression

-(dM/dt) = I₀ phi f (1-exp(-A_t))

phi = quantum yield

I₀ = Incident radiation flux

f = Ratio of light absorbed by the M (organic compound) and other components 

A_t = Total absorbance of the solution time a factor 2.3.

Which further reduces to:

r_M = - ln(10) phi epsilon [M] I₀            (Leifer, 1988)

Where

epsilon = Molar extinction co-efficient  (M^-1 cm^-1)

I would recommend you to read following research articles + books as much as you can

Regards

Tipu Sultan

Freeze drying would help. Maybe try to centrifugate first a really low temperature, in fact, if you put your glass in which there is your GO into the fridge, you will see some differences after some days.

Regards

Adrian

I think the answers given are mostly pretty good, except that which reducing agent or chelating agent or scavenger is the best choice clearly depends on the objectives of the experiment. Chelators such as those mentioned are effective, but leave unreacted H2O2. The reducing agents produce new products and may glom up the reaction mixture as far as product characterization is concerned. Scavengers such as ethanol form oxidation products such as acetaldehyde or formaldehyde, which are reactive intheir own rights with many classes of compounds

That can be determined by measuring the zeta-potential of activated carbon as a function of the solution pH. The pH where zero zeta potential is reached is referred as isoelectrical point.

If you repeat the experiment i suggest that you use your photometer to measure spectra of the reaction mixture at each time point you sample. You can compare the spectra that develop with reference spectra of e.g. phenol and benzoquinol. If the mixture isn't buffered do check if pH changes over the treatment. Decreasing pH will indicate the formation of phenol. 

Ted is right. Be careful...

Water is not enough by itself, and the washing must be extensive.

Therefore, better using a solution of HCl refluxing for several days in a Soxhlet extractor. After this, water must be used in the same conditions until the pH of the rinse if neutral. A bit long, but very efficient.

Alain

Dear collegues!

I would like to note what there are differences between TBARS and MDA. Of course, You could be use manual methods for determination both, but... in ours laboratory for this needs use kit' s produced Cell Biolabs company or (for MDA and 4-HNE) determination kit' s FR 22 (Lubioscience, Switzerland).  the best way, I mean, use microplate reader (for ELISA method). Such plate' s need hiting (till +98C), best equipment for it  - Thermomixer (Eppendorff). If You have really interest, I can send you Cell Biolabs product catalog for 2015. there are possibility to buy  ''training and examination''  kits, and for manual and plate technology. Good luck.

The short answer seems to be yes:

The process patent expired in 2014.

In This Process you inoculate Oxygen with specific Air Pump.

For Concentrate leaching, you should be supply required Oxygen to autoclave.

Best wishes

It should be selective for OH radicals, but I'm not sure if it will not react also with superoxide radical.

Benzoic acid is also good, but you'll get various hydroxybenzoic acids so the detection can be unclear.

I'm using terephtalic acid. It gives only one product, and the detection is with emission spectroscopy.

related to above: For the S compound you have to calculate it when converted to H2SO4 not SO2. If you wantto com-pare to COD measurement assume NH3 is fiormed and not HNO3

Hydrogen peroxide -- as mentioned by others -- if present in solution will contribute to COD.  A mg/L of H2O2 increases COD by about 0.393 mg/L ; this value was obtained employing Milli-Q water spiked with varying concentrations of H2O2; you may do your own measurements employing distilled and deionized water or Milli-Q water, etc.

Addition of catalase also introduces error in COD measurement that has to be accounted for; this can be done by the following relation.

COD_COR = COD_S+C - COD_C

Where COD_S+C is COD of water sample after catalase addition and COD_C is COD of Milli-Q water with the same amount of catalase.

Dear Nidheesh,

In Fenton reaction Fe(II) is oxidized to Fe(III) by H2O2 and increases pH by producing OH- as follow (classic Fenton reaction):

H2O2 + Fe(II) --> Fe(III) + OH (-) + OH (radical)

In other hand some parallel reactions (with lower constant rate) can reduce the pH . For example H2O2 electrogeneration and/or regeneration of Fe(II):

Fe(III) + H2O2 --> Fe(II) + H(+) + HO2

Therefore change in pH will insignificant.

Here is a general guide on converting concentrations:

When measuring ozone in Milligrams per Hour ( mg/hr ), the concentration cannot be determined unless the flow rate itself is known.

1 ug/ml = 1 mg/l = 1 g/m3 = 1 gamma

1 ml is = to 1 cc.

So, to determine the actual concentration if given mg/hr., one first must convert from mg/hr to mg/l. This is done by using the flow rate. If an ozone generator pumps out 250 mg/hr at a flow rate of 1 liter per minute, then:

The ozone generator produces 4.1667 mg/min ( 250 mg/hr divided by 60 ). Since we now know that we have 4.1667 milligrams of ozone each minute, and the flow rate is 1 liter each minute, we now have 4.1667 mg for each liter, or 4.1667 mg/l.

By looking at our chart, we now know that we have an ozone generator that produces 4.1667 ug/ml.

To determine the concentration, we can use the formula given by Saul Pressman:

0.5 % X 1.4 gm/l = 7 ug/ml

Only, let's convert it properly, first, by using the mathematical version with the correct units of measurement:

0.005 X 1400mg/l = 7ug/ml

X times 1400 = 4.1667

X = .00298 or 0.298%

In fact,free radicals like .OH degradate dye molecules.And these radicals may have more life time and concentration in acidic solution.