Rate constants and mechanism of reaction of sulfate radical anion with aromatic compounds

Abstract

Absolute rate constants for reaction of Sod.-with substituted benzenes and benzoates have been determined by pulse radiolysis. The values are found to range from about 5 X lo9 M-' s-I for anisole to < I O 6 M-' s-I for nitrobenzene. A correlation of the rate constants with the Hammett substituent constant u gave p = -2.4 for both series of compounds. It is concluded that the reaction takes place by an electron transfer from the ring to so4*-.The reaction of the sulfate radical anion, SO4.-, with several aromatic compounds has been shown by ESR to result in the production of hydroxycyclohexadienyl radicak2 In the case of methoxy derivatives radical cations have been observed3 and in the case of several carboxy derivatives decarboxylation has been found to take place, resulting in the production of phenyl radicah4q5 These observations can be explained by various mechanisms. The hydroxycyclohexadienyl radicals can be produced by addition to the ring followed by hydrolysis as suggested by Norman et al.? or by electron transfer from the ring to the SOY-, as suggested by several authors,6-8 followed by hydroxylation with water or OH-. The radical cations from anisoles can also be produced by direct electron transfer3 or via addition followed by elimination. Decarboxylation can occur through direct oxidation of the carboxyl group, as is the case with aliphatic carboxylic acids,2s8 or again following oxidation of the ring.5 It appeared that examination of the effect of substituents on the rate constant for reaction of s04.-with aromatic compounds might shed some light on the mechanism.Absolute rate constants for reactions of SO4--radical can be determined by kinetic spectrophotometric pulse radiolysis as described p r e v i o u~l y .~-~~ The pseudo-first-order decay rate of the SO4.-absorption is measured in the presence of increasing concentrations of the aromatic compound and the second-order rate constant calculated from a linear plot of the rate vs. the concentration. The rate constants were found to vary from 5 X lo9 M-I s-l for anisole down to <lo6 M-I s-I for nitrobenzene. A Hammett type plot of log k vs. the substituent constant u gives a value of p = -2.4, much larger than that observed for reactions of OH radicals. Experimental Sectionby reaction of eaq-with peroxydisulfate anion.Sulfate radical anions were produced in irradiated aqueous solutionsThis reaction is very rapid ( k = 1.1 X 1Olo M-I s -' )~ and at the concentrations of peroxydisulfate used (0.01 -0.05 M) was virtually complete at the end of the radiation pulse. The decay Of SO4.-in the absence and presence of various solutes was followed at 450 nm. In several cases, where the aromatic product radicals absorb in this region, the decay could be observed more clearly at slightly higher wavelengths. The pulse radiolysis setup for these observations was that described previously,' I with slight modifications.I2 Pulses of 2.8 MeV electrons from the Van de Graaff accelerator were approximately of 1-L(S duration and supplied energy for production of 2-3 X ...

Advanced Oxidation Processes (AOPs) in Wastewater Treatment

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Effect of chloride and sulfate ions on the advanced photo Fenton and modified photo Fenton degradation process of Alizarin Red S

“…Persulfate ions can generate sulfate and hydroxyl free radicals and they can provide free radical mechanism similar to hydroxyl radical pathways as generated in the Fenton's chemistry. Sulfate radical is one of the strongest oxidizing species in aqueous media with a redox potential of 2.6 V, which is next only to the hydroxyl free radical whose redox potential is 2.8 V. The sulfate radical anions produced in the case of APS shows various possible reaction mechanism in the process of mineralization which are as follows: (i) abstraction of hydrogen atom from the saturated carbon, (ii) The ions are capable of adding to the unsaturated compounds, (iii) It can remove an electron from the anions and neutral molecules [18,19]. These attributes combine to make the persulfate ions a viable option for the chemical oxidation of a broad range of contaminants.…”

Photo-degradation of di azo dye Bismarck Brown by advanced photo-Fenton process: Influence of inorganic anions and evaluation of recycling efficiency of iron powder

“…Persulfate anion can also generate free radicals like sulfate and hydroxyl radicals which provide free radical mechanism similar to hydroxyl radical pathways generated in the Fenton's chemistry. Sulfate radical is one of the strongest oxidizing species in aqueous media with a redox potential of 2.6 V which is next only to the hydroxyl free radical whose redox potential is 2.8 V. The sulfate radical anions produced in the case of APS show various possible reaction mechanism in the process of mineralization: (i) abstraction of hydrogen atom from the saturated carbon; (ii) capable of adding to the unsaturated compounds; (iii) it can remove an electron from the anions and neutral molecules (Neta et al, 1977). These attributes combine to make persulfate a viable option for the chemical oxidation of a broad range of contaminants.…”