Reaction Rate Constants of Hydroxyl Radicals with ...

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Journal of Advanced Oxidation Technologies 2018; 21(1) Article ID- 20170075

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Reaction Rate Constants of Hydroxyl Radicals with Micropollutants and Their Significance in Advanced Oxidation Processes Suyash Mandal University of Duisburg-Essen, Universitätsstr. 5, 45141 Essen, Germany [email protected] Abstract Advanced oxidation processes (AOPs) have gained popularity and are being extensively used for the degradation of micropollutants. The importance of 2nd order reaction rate constant of hydroxyl radicals with micropollutants (k˙OH) in AOPs is undisputable. The degradability of a micropollutant by an AOP can be predicted from its k˙OH along with other physicochemical properties, and hence are important for the design of water treatment chains. Furthermore, a micropollutant with known k˙OH value can be used as a probe compound to assess AOPs. In this work, the k˙OH of 212 micropollutants are reviewed. An overview is also provided on the generation of hydroxyl radicals, significance of k˙OH, and the typical methods used to determine k˙OH. Keywords Contaminants; Pesticide Removal; Pharmaceutical Degradation; Second Order Reaction Rate Constants; Water Technology; OH Radicals Received: July 17, 2017; Revised: October 31, 2017; Accepted: November 11, 2017

Introduction The increase in use of pharmaceuticals, personal care products, pesticides, and industrial solvents has led to creation of challenges as well as opportunities for water scientists across the globe. In industrially developed and developing countries, at several occasions, researchers have found traces of micropollutants in water higher than the permissible limits. Cho et al. [1] detected 29 out of 35 pollutants investigated in Korean river waters, and found links to their origin as industrial discharge. In fact, some banned pollutants (organochlorine pesticides) were also detected. They were reportedly being produced as intermediates during industrial processes. In a study by Nam et al. [2], 12 micropollutants were detected out of 14 investigated in the influent of a water treatment plant (WTP), and 11 of them were found in ppt levels in the effluent. The treatment chain of the plant was conventional, and included coagulation, flocculation, sedimentation, followed by sand filtration and chlorination. Rodriguez-Mozaz and co-workers [3] in their study measured the occurrence of several micropollutants in river water and ground water, which served as source water for a Spanish WTP. They observed that the concentrations of pesticides in river water were related to their seasonal application in agriculture. Conversely, during the period of study, no significant change was observed in concentrations of several micropollutants detected in the ground water, indicating slower degradation rate in ground water (compared to that in river water). Recent developments in science and engineering have brought about various cutting edge technologies for the removal of micropollutants in water. Alsbaiee et al. [4] in their work investigated the adsorption rates of a variety of micropollutants by high-surface-area, mesoporous polymer of β-cyclodextrin, which they prepared by

Journal of Advanced Oxidation Technologies 2018; 21(1) DOI: 10.26802/jaots.2017.0075

www.jaots.net

Journal of Advanced Oxidation Technologies 2018; 21(1)

crosslinking β-cyclodextrin with rigid aromatic groups. In another recent study, Su and co-workers [5] showed that asymmetric Faradaic cells, which can suppress water reduction and enhance ion separation due to redox functionalization of both cathode and anode, can be used to target micropollutants. However, these new technologies, have their reservations, and till today, industrially accepted and implemented technologies for micropollutant treatment are mostly based on advanced oxidation processes (AOPs) [6]. AOPs are those processes that are associated with the generation of hydroxyl radicals (˙OH) [7]. AOPs are relatively expensive when compared to conventional water treatment technologies and thus, are still very new in various parts of the world. The plausibility of degradation of a micropollutant by hydroxyl radicals can be estimated from the 2nd order reaction rate constant of the micropollutant with ˙OH. Several researchers in their reports had discussed degradation of various micropollutants in water by ˙OH oxidation. The degradation of micropollutants by different AOPs, including UV/H2O2 treatment [8-14], ozonation, O3/H2O2 [15-28], Fenton process [29-33], photo catalysis [3437] and many others had been extensively studied. The reaction rate constant of hydroxyl radical with a micropollutant is usually in the order of 109 M-1s-1 [38]. This paper presents the reaction rate constants of ˙OH with 212 micropollutants, that were determined by several researchers using different methods, some of which are discussed in the text. Hydroxyl Radical Generation and Scavenging Advanced Oxidation Processes for Water Treatment and Hydroxyl Radical Generation Hydroxyl radicals are very strong oxidants with oxidizing potential of 2.8 V, and as they are non-selective oxidants, they react with most organic compounds [36]. Selective oxidants such as O3˙- react only with a few electron-rich organic moieties, whereas ˙OH react with almost all organic moieties [38]. Some of the widely studied AOPs for the degradation of micropollutants are presented in Table 1. The degradation of a micropollutant during ozonation occurs by the two oxidants, ozone and hydroxyl radicals (that are produced during the process) [18]. During Fenton process, hydroxyl radicals are generated when Fe2+ reacts with H2O2 [39]. In UV based technologies, along with the reactive species, UV photolysis also plays a major role in the degradation of micropollutants, and the degradation varies depending on the type of UV lamp, due to the difference in their emission wavelengths. The most commonly used lamps are mercury based UV lamps, which may be monochromatic (low pressure lamps emitting light at 254 nm) or polychromatic (medium pressure lamps emitting light in the wavelength range of 200-400 nm) [40]. Due to developments in UV-LEDs, they are also gaining popularity and are seen as the future for UV based processes [41]. Photolysis of H2O2 produces hydroxyl radicals, which is a pH dependent process. The molar absorption coefficient of H2O2 at 254 nm is 18.6 M-1cm-1. The addition of H2O2 in ozonation experiments also enhances the hydroxyl radical production. The reason behind this is the formation of ˙OH during the decomposition of ozone, thus, H2O2 plays a role in hydroxyl radical generation [42]. TABLE 1 SOME WIDELY STUDIED ADVANCED OXIDATION PROCESSES Advanced oxidation process Ozonation (O3) O3/H2O2 or peroxone process O3/UV UV/TiO2 UV/TiO2/O3 UV/H2O2 Fe2+/H2O2 (Fenton process) Photo-Fenton reaction Ultrasonic irradiation

Though degradation by ˙OH oxidation takes place during AOPs, there may be other reactive species involved as well, such as O3 ˙, O2 ˙, HO3˙, HO2˙ and H˙ [12, 32, 43-47, 37, 48-51]. In natural waters, photolysis by sunlight, or UV based AOPs may generate reactive species such as singlet oxygen (1O2) and natural organic matter triplet excited state (3NOM*) along with ˙OH, that participate during indirect photolysis of micropollutants [52]. Wang and coworkers [53] reported in a study that the contribution of ˙OH (28.7-31%) towards the indirect photolysis of


2,2',4,4',5,5'-hexabrominated diphenyl ether (BDE-153) was greater than that by 1O2 (12.9-14.9%). However, this may vary depending on the target micropollutant and the water matrix. Pulse Radiolysis for Hydroxyl Radical Generation Pulse radiolysis is a technique to study fast kinetic processes generated by radicals that are generated by irradiating an aqueous solution by highly accelerated electrons. The radiolysis of water can be described by equation (1) [54-57], where the numbers in the braces indicate the G-values (in mol J-1). H2O → eaq {0.27} + ˙H {0.06} + ˙OH {0.28} + H2 {0.05} + H2O2 {0.07} + H+ {0.27}

(1)

The ˙OH generation is enhanced by pre-saturating the solution with N2O, which converts hydrated electrons and hydrogen atoms to hydroxyl radicals. These radicals are then quantified using a detector, usually employing absorption detection metho [58-62]. Several researchers had used pulse radiolysis to produce hydroxyl radicals for the determination of 2nd order reaction rate constant of micropollutants with hydroxyl radicals (k˙OH). Mezyk et al. [63]determined the ˙OH rate constant of four sulfa drugs in water. The absolute rate constants of ˙OH with five common artificial sweeteners were measured by Toth et al. [62]. An et al. [56] used this method to find the absolute rate constants for ciprofloxacin with ˙OH, N3˙ and eaq , and also found that SO4˙ did not react with the pharmaceutical. In the study by Song et al. [55], ˙OH were generated by pulse radiolysis and k˙OH of three βblockers, viz., atenolol, metoprolol and propranolol were determined. Elliot and Simsons [64] determined the ˙OH rate constants of ferrocyanide, thiocyanate, iodide, formate ions, 2-propanol and tert-butanol over the temperature range of 19 to 79C. Scavenging of Hydroxyl Radicals In a study by Brezonik and Fulkerson-Brekken, it was shown that dissolved organic carbon (DOC) is the principal scavenger for ˙OH in natural waters, and the DOC concentration can be used to estimate the dissolved organic matter (DOM) sink. Carbonate and bicarbonate play a smaller role, but can be a major cause of scavenging when the DOC concentration is low in high alkalinity waters [65]. The rate constants of ˙OH with water matrix components are presented in Table 2, and the OH radical scavenging rate in natural waters denoted by k'scav (s-1), can be calculated from equation (2) [10, 66]. In a study by Lee et al. [67], 8 different wastewater effluent samples on ozonation showed that ˙OH consumption by effluent organic matter ranged from 70% to 86%, and ˙OH consumption by HCO3 /CO32- ranged from 9% to 30%. TABLE 2 SECOND ORDER REACTION RATE CONSTANTS OF HYDROXYL RADICALS WITH WATER MATRIX CONSTITUENTS k˙OH 2.5 x 104 L mg-1 s-1 8.5 x 106 M-1s-1 3.9 x 108 M-1s-1

DOC HCO3 CO32-

Reference [91] [60] [60]

Lee and von Gunten [38] in their work had shown that for micropollutants containing electron-rich moieties, the competition with water matrix components for selective oxidants remain until the electron-rich moieties present in the matrix are consumed, whereas the competition with non-selective oxidants (˙OH) prevails throughout the reaction.

k 'scav  k





OH ,DOC

2

[DOC ]  k  OH ,HCO  [HCO 3 ]  k  OH ,CO 2  [CO 3 ] 3

3

(2)

In some AOPs, though H2O2 is added to enhance the ˙OH production, at higher concentrations of H2O2, it itself may act as scavenger of hydroxyl radicals. The second order reaction rate constants of hydroxyl radicals with H2O2 given by Coddington et al. [68], Buxton et al. [60], and Thomas [58] are mentioned with their respective equations– (3) and (4). H2O2 + ˙OH → H2O + H+ + ˙O2 H2O2 + ˙OH → H2O + HO2˙

(k˙OH,H2O2=2.7 x 107M-1s-1) [68] (k˙OH,H2O2=2.7 x 107M-1s-1) [60]

(3) (4)


(k˙OH,H2O2=2.25 x 107M-1s-1) [58] The scavenging of ˙OH by H2O2 was discussed in the works of various researchers including Sharpless et al. (2003), Hislop and Bolton (1999), and Zhang et al. (2003) [49, 69-71]. Significance of k˙OH in AOPs The second order reaction rate constant of a micropollutant with hydroxyl radical is a good indicant of the susceptibility of the micropollutant towards oxidation by hydroxyl radicals [72], and in most AOPs contribution of ˙OH oxidation towards the degradation process is considerably fair. Ozone Based AOPs The degradation of a micropollutant ‘P’ during ozonation in deionized water can be represented by equation (5) [18, 67], where [O3]dt and *˙OH+dt are the ozone and hydroxyl radical exposures, respectively.

 [ P]    kO3 ,P  [O3 ]dt  k  OH ,P  [  OH ]dt  ln   [ P]o 

(5)

For a reaction where the absolute rate constants of the probe compound (P) with ozone and ˙OH are known, measuring the decrease in concentration of ‘P’, and calculating the integral of ozone concentration versus time will give the hydroxyl radical exposure of the reaction. The process parameter Rct was introduced by Elovitz and von Gunten [73] which is the ratio of ˙OH exposure to O3 exposure. In their experiments, they had shown that the Rct can be calculated using a probe compound like pCBA (4-chlorobenzoic acid). pCBA was used as the probe compound in their experiments as it is a compound which degrades readily by hydroxyl radicals (k˙OH=5 x 109 M-1s-1 [60]) but has a very low kO3 of ≤0.15 M-1s-1 [74]. For pCBA, neglecting the term kO3,pCBA[O3]dt from equation (5) and substituting *˙OH+dt/[O3]dt with Rct, equation (6) can be obtained.

Rct  

ln [ pCBA] /[ pCBA]o  k  OH , pCBA  [O3 ]dt

(6)

Thus, Rct can be calculated using experimentally measured degradation of the probe compound and ozone [73]. Furthermore, Elovitz and von Gunten tested Rct for different water types and found it to be constant for majority of the reaction, and hence Rct for a given water can also be written as Rct=[˙OH+/*O3]. The kO3 and k˙OH of atrazine and its degradation products during ozonation and peroxone process were determined by Acero et al. [75], who reported that at 11C, kO3 was 4 M-1s-1, and k˙OH was 2.7 x 109 M-1s-1. Similar rate constants were obtained for the degradation products. They had shown that the degradation of atrazine obtained by ozonation in 30 minutes was same as that obtained by O3/H2O2 process in 2 minutes (H2O2 is added to ozonation reactions to enhance ˙OH production). Thus, showing the effect of increase in hydroxyl radicals and indicating that k˙OH value directly gives an idea about the degradability of a compound by hydroxyl radicals. Lee and co-workers [76] determined the percentage of NDMA oxidation during ozonation (initial [O3] = 160µ M). In their experiments, 25% of NDMA was oxidized without adding H2O2, and when H2O2 was added ([O3]o/[H2O2]o=2), 55% of NDMA was oxidized. Lee et al. [67] in a study involving ozonation of 10 wastewater effluent samples showed the relative contribution of ˙OH (f˙OH) and ozone (fO3) towards the degradation of micropollutants. It was shown that f˙OH for the degradation of tris (2-carboxyethyl) phosphine (TCEP) in all samples was 100%, and that for the degradation of primidone, meprobamate and atrazine was 100% in almost all the water samples. For N,N-Diethyl-meta-toluamide (DEET) and ibuprofen, f˙OH was about 75% and 85% respectively in all the water samples. Whereas degradation of triclosan, diclofenac, carbamazepine, bisphenol-A, sulfamethoxazole, trimethoprim and naproxen were attributed mainly


towards oxidation by ozone, where fO3 was about 85-90% in most of the water samples. These f˙OH and fO3 correspond to the k˙OH and kO3 of the compounds, which were 6.7 x 109 M-1s-1 and