Jianqiao Zhang

Comment on Electrolytic manipulation of persulfate reactivity by iron electrodes for TCE degradation in groundwater.

Iron Electrodes for TCE Degradation in Groundwater I a recent study, Yuan et al. developed an effective approach to manipulate the reactivity of persulfate in situ for trichloroethylene (TCE) degradation with iron electrodes. Their study is interesting and provides options to resolve the intrinsic drawbacks of persulfate in applications. Their paper attempted to elucidate the mechanism of TCE degradation in persulfate/iron electrodes system. As interested readers of their paper, we would like to raise our doubts on their discussion about the mechanism of TCE degradation in persulfate/iron electrodes system. Was •OH Contribution to TCE Degradation More Significant than That of SO4 •−? Activation of persulfate has attracted a great deal of attention not only because persulfate is relatively stable for its slow reaction kinetics with organics but also because of the generation of SO4 •− and/or •OH. SO4 is an more selective radical for electron transfer reactions than •OH which can undergo reaction rapidly by hydrogen abstract and/or addition. Additionally, SO4 •− is a more stable radical than •OH for its longer half-life. Owing to the difference of SO4 •− and •OH, it is important for researchers to identify the primary reactive oxidants and to differentiate the contribution of SO4 •− and •OH in the activation of persulfate. In this work, the authors stated that •OH contribution to TCE degradation was more significant than that of SO4 •− in persulfate/iron electrodes system. However, this statement is not sufficiently supported by their experimental results. In their radical scavenging studies, TBA (kSO4 = 4.0 × 10 5 M−1 s−1, k•OH = 6.0 × 10 8 M−1 s−1) and methanol (kSO4 = 3.2 × 10 M−1 s−1, k•OH = 9.7 × 10 8 M−1 s−1) were selected to evaluate the relative contribution of SO4 •− and •OH. However, it should be noted that the reaction rate constants of TCE with SO4 •− and •OH are about kSO4 = 1.7 × 10 9 M−1 s−1 (obtained by means of competition kinetics (mixtures of benzoic acid and TCE) in UV/PDS system) and k•OH = 4.0−4.3 × 10 M−1 s−1, 8 respectively. Here, the value of ck (c is the concentration of the probes, k is the reaction rate constant of the probes with SO4 •−

ABTS as an Electron Shuttle to Enhance the Oxidation Kinetics of Substituted Phenols by Aqueous Permanganate

In this study, it was, interestingly, found that 2,2-azino-bis(3-ethylbenzothiazoline)-6-sulfonate (ABTS), a widely used electron shuttle, could greatly accelerate the oxidation of substituted phenols by potassium permanganate (Mn(VII)) in aqueous solutions at pH 5-9. This was attributed to the fact that these substituted phenols could be readily oxidized by the stable radical cation (ABTS(•+)), which was quickly produced from the oxidation of ABTS by Mn(VII). The reaction of Mn(VII) with ABTS exhibited second-order kinetics, with stoichiometries of ∼5:1 at pH 5-6 and ∼3:1 at pH 7-9, and the rate constants varied negligibly from pH 5 to 9 (k = (9.44 ± 0.21) × 10(4) M(-1) s(-1)). Comparatively, the reaction of ABTS(•+) with phenol showed biphasic kinetics. The second-order rate constants for the reactions of ABTS(•+) with substituted phenols obtained in the initial phase were strongly affected by pH, and they were several orders of magnitude higher than those for the reactions of Mn(VII) with substituted phenols at each pH. Good Hammett-type correlations were found for the reactions of ABTS(•+) with undissociated (log(k) = 2.82-4.31σ) and dissociated phenols (log(k) = 7.29-5.90σ). The stoichiometries of (2.2 ± 0.06):1 (ABTS(•+) in excess) and (1.38 ± 0.18):1 (phenol in excess) were achieved in the reaction of ABTS(•+) with phenol, but they exhibited no pH dependency.

Factors affecting formation of deethyl and deisopropyl products from atrazine degradation in UV/H2O2 and UV/PDS

In this study, the formation of deethyl products (DEPs) (i.e., atrazine amide (Atra-imine) and deethylatrazine (DEA)) and deisopropyl product (i.e., deisopropylatrazine (DIA)) from parent atrazine (ATZ) degraded in UV/H2O2 and UV/PDS processes under various conditions was monitored. It was found that SO4˙− displayed a more distinctive preference to the ethyl function group of ATZ than HO˙, leading to the higher ratio of DEPs/DIA in UV/PDS system than that in UV/H2O2 system in pure water. The effects of water matrices (i.e., natural organic matter (NOM), carbonate/bicarbonate (HCO3−/CO32−), and chloride ions (Cl−)) on ATZ degradation as well as formation of DEPs and DIA were evaluated in detail. The degradation of ATZ by UV/PDS was significantly inhibited in the presence of NOM, HCO3−/CO32− or Cl−, because these components could competitively react with SO4˙− and/or HO˙ to generate lower reactive secondary radicals (i.e., organic radicals, carbonate radicals (CO3˙−) or reactive chlorine radicals (RCs)). The yields of these DEPs and DIA products from ATZ degradation were not impacted by NOM or HCO3−/CO32−, possibly due to the low reactivity of organic radicals and CO3˙− toward the side groups of ATZ. Howbeit, the increase of DIA yield companied with the decrease of DEPs yield was interestingly observed in the presence of Cl−, which was attributed to the promotion of Cl− at moderate concentration (mM range) for the conversion of SO4˙− into HO˙. Comparatively, in the UV/H2O2 process, NOM and HCO3−/CO32− exhibited a similar inhibitory effect on ATZ degradation, while the influence of Cl− was negligible. Differing from UV/PDS system, all these factors did not change DEPs and DIA yields in UV/H2O2 process. Moreover, it was confirmed that RCs had a greater selectivity but a lower reactivity on attacking the ethyl function group than that of SO4˙−. These findings were also confirmed by monitoring the degradation of ATZ as well as the formation of DEPs and DIA in three natural waters.

Spectrophotometric determination of peroxymonosulfate anions via oxidative decolorization of dyes induced by cobalt

Dyes can be decolorized by the activation of peroxymonosulfate (PMS) in the presence of cobalt, and four good linear correlations between the depletion of dyes at their maximum absorption wavelengths and the concentration of PMS are demonstrated under the optimum conditions. For four types of samples, sensitive, rapid, and low-cost spectrophotometric methods are developed for the determination of PMS in aqueous solutions, which are based on the oxidative decolorization of the dyes with sulfate radicals generated by the conjunction of PMS and cobalt. The linear ranges and the detection limits are determined to be 0–80 μM and 0.08 μM for acid orange 7, 0–100 μM and 0.1 μM for methylene blue, 0–80 μM and 0.08 μM for methyl violet, and 0–40 μM and 0.04 μM for rhodamine B, respectively. When our proposed methods are used for the determination of the concentration of PMS with the treatment of 4-chlorophenol by the activation of PMS with cobalt, the results are as satisfactory as those obtained by using the classical iodometric method.

Spectrophotometric determination of persulfate anion via oxidative depolarization of methyl orange induced by ferrous ions

AbstractA rapid, simple, and sensitive spectrophotometric method was developed for the determination of persulfate in this study, based on the oxidation decolorization of methyl orange (MO) with sulfate radicals and hydroxyl radicals generated by the conjunction of persulfate with ferrous ions. The depletion of MO at its maximum absorption wavelength (507xa0nm) is in proportion to the concentration of persulfate in water. There was a good linear relationship (above 99%) between the depletion of MO and the concentration of persulfate in the ranges of 0.5–100xa0μmol L−1 with a detection limit of 0.17xa0μmol L−1 under the optimized conditions. The proposed MO method for the determination of persulfate in water was effective to tolerate the interferences of common coexisting foreign species in aqueous solutions. The results were as satisfactory as that obtained by the classical iodometric method used in practical water samples. Additionally, the proposed MO method is rather inexpensive to measure the concentration ...