The term "activated persulfate" is commonly used in engineering and environmental sciences ecology. However, from chemical point of view it's not clear which molecular entity is called the "activated persulfate." Persulfate itself is very inert, but intermediates formed in the process of catalytic, thermal, photochemical , etc persulfate decomposition are highly reactive. The most common such intermediate is the sulfate anion-radical. Does it make any sense to call this radical as the "activated persulfate?" Persulfate "activation" is basically a chemical reaction with something to produce highly reactive species.

The statement "nitrobenzene and anisole can be used to quantify hydroxyl radical and sulfate radical" is not correct. Both hydroxyl radical and sulfate radicals react with almost everything with the rate constants close to the diffusion controlled.

In opposite, superoxide radical is a strong reductant and does not have a chance to survive in an oxidative environment. I doubt you can identify superoxide in the presence of persulfate

@Yurii Geletii

Thanks for your reply Dr. Geletii. Sorry for the confusion about the word activated persulfate. For persulfate, I mean S2O8 ion. In alkaline conditions, persulfate can be hydrolyzed to form intermediate sulfate radical and superoxide radical. The sulfate radical can further react with hydroxide ion to form hydroxyl radical. This process is often persuflate activation. So when I say activated persulfate, I was referring to this process. And this process has been well documented in many literature.

Second, many studies have used nitrobenzene as a chemical probe to quantify the intermediate hydroxyl radical concentrations in different AOP. Nitrobenzene has a second order rate constant of 10^9 per molar per second with hydroxyl radicals while only 10^5 per molar per second with sulfate radicals. Therefore, if expose nitrobenzene to activated persulfate solution, the degradation of nitrobenzene will be mainly caused by hydroxyl radicals. By measuring the degradation kinetics of nitrobenzene, the steady-state concentration of intermediate hydroxyl radicals can be calculated. Anisole, on the other hand, is reactive to both hydroxyl and sulfate radicals. Combing the degradation kinetics of anisole and nitrobenzene, the steady-state concentration of intermediate sulfate radicals can be calculated.

One example is :"Inactivation of E coli bacteriophage MS2 and B spores under UV H2O2 and UV peroxyl disulfate advanced disinfection conditions". There are many more publications that have used nitrobenzene and anisole to quantify radicals in activated persulfate solution.

What I was asking is to see if there is another chemical probe that is only reactive with superoxide radical so that I can use it to quantify the steady-state concentration of superoxide radicals.

Please correct me if I am wrong with my above statement.

If you say superoxide radical, do you mean HO2, O2- or OH radical.

@Manfred Elend

I meant O2-

I believe HO2 is called hydroperoxyl, and OH is called hydroxyl. Superoxide should be exclusively refering to O2-

Hallo Hang Qi,

to my knowledge the reaction of perdisulfate in solution is this way.

S2O82- + H2O > HSO4- + HSO5-

HSO5- + H2O > H2O2 + HSO4-

No O2- is formed directly, but the HSO5- can oxidize 'the Caro's acid anion'.

Only one oxygen originates from the perdisulfate.

Normally all oxidizing agents which can form H2O2 in water are in equilibrum (Yurii V Geletii wrote before).

Like: O2- + H+ HO2 (pH-dependent)

HO2 in solution can be determined by EPR.

@Manfred Elend

Hello Manfred,

Thank you for your reply. I really appreciate it.

What you mentioned about the perdisulfate reaction in solution is correct. But I believe this reaction mainly takes place at acidic pH based on the book "In Situ Chemical Oxidation for Groundwater Remediation" (by Siegrist et al.), at page 149.

What I was asking is the persulfate activation under alkaline pH. Perdisulfate ion will undergo hydrolysis at alkaline pH.

S2O82- + H2O > HO2- + SO42- + H+ (catalyzed by OH-)

HO2- + S2O82- > SO4.- + SO42- + O2.-

SO4.- + OH- > SO42- + HO.

More info can be found in this article "Mechanism of Base Activation of Persulfate" by Furman et al, 2010, Environ. Sci. Technol

Hello Hang Qi,

you are right, you are working in very alkaline solution.

I found in wikipedia that superoxide (O2-) is also detectable by EPR.

In our Institute we work at about pH 7.4

Hang Qi,

I explained my own opinion concerning the term "activated persulfate." Yes, it has been used in more than thousand papers, but it's confusing. You are dealing with a hydrolysis of persulfate, which is irreversible and results in persulfate decomposition. The mechanism is complex, most of the reaction rate constants are known and you can simulate the reaction conditions. Yes, the reactivity of sulfate anion and hydroxyl radicals are different, sulfate is mostly an electron acceptor, while hydroxyl is an hydrogen atom acceptor.

By adding a radical quencher to the system you completely change your system. You must take into account that one radical produces another radical. HO(.) can react with nitrobenzene to form a radical intermediate, which in its term react with sulfate radical. There is no selective radical scavengers. Superoxide reacts with nitrobenzene. pKa of hydroxyl radical is around 12, meaning that at pH >12, hydroxyl radical is deprotonated.

My answer to your question:

"chemical probe DOES NOT EXIST that is only reactive with superoxide radical so that I can use it to quantify the steady-state concentration of superoxide radicals"

@Manfred Elend

@Yurii V Geletii

Thank you so much for your answers! I really appriecite them!

Hallo Hang Qi,

of course is Yurii V Geletii right, most compounds changes radical concentration, but exploratory measurements are often nesseary to get an idea of the oxidation-process or concentation of the species in solution.

I added a brochure from Bruker-EPR detecting superoxide free radical via spin traps.

@ Manfred Elend

Thank you so much for your help Dr. Elend!

@Yurri V Geletii

Hi Dr. Geletii,

I saw you mentioned that I can simulate the reaction conditions. I actually tried to use Simbiology in Matalab to simulate my reaction conditions to estimate the superoxide concentration. However, I was unable to find the rate constants for the two key steps below.

S2O82- + H2O -> HO2- + SO42- + H+ (catalyzed by OH-)

HO2- + S2O82- -> SO4.- + SO42- + O2.-

Do you have any idea on where I can find the rate constants for the above two equations? If I can get the rate constants, I assume I will not need any probes anymore.

Hang

What is HO2- ?

@ Yurii V Geletii

Sorry, that was a typo. It should be HO2-

The reactions should be:

S2O82- + H2O -> HO2- + SO42- + H+ (catalyzed by OH-)

HO2- + S2O82- -> SO4.- + SO42- + O2.- + H+

I have to look databases. The second reaction is likely to be very slow. The first is not balanced and probably not elementary

Well, the first reaction is actually a combination of two step reactions:

S2O82- + H2O -> SO32- + SO42- + H+ (catalyzed by OH-)

SO32- + H2O -> HO2- + SO42- + H+ (catalyzed by OH-)

These reactions are all based on the paper "Mechanism of Base Activation of Persulfate" by Furman et al. (2010) on Environ. Sci. Technol.

May I ask, which database will you look for? I tried the NIST database (http://kinetics.nist.gov/solution/) but got no results.