Sulfur-replaced Fenton systems: can sulfate radical

3 downloads 0 Views 686KB Size Report
Sep 25, 2014 - system mediated by polycarboxylic acids (PCOA) for the production of HO• (blue) and a photo-Fe(III)/Fe(II)–sulfite system for production of SO4.
Perspective Received: 24 June 2014

Revised: 20 August 2014

Accepted article published: 27 August 2014

Published online in Wiley Online Library: 25 September 2014

(wileyonlinelibrary.com) DOI 10.1002/jctb.4525

Sulfur-replaced Fenton systems: can sulfate radical substitute hydroxyl radical for advanced oxidation technologies? Danna Zhou,a* Hui Zhangb* and Long Chenc Abstract Sulfate radicals (SO4 •− ) and hydroxy radicals (HO• ) are the major radicals used in advanced oxidation technologies (AOTs) for the removal of contaminants. Although SO4 •− reacts with organic or inorganic compounds with rate constants comparatively lower than that of HO• , AOTs based on SO4 •− (abbreviated as SR-AOTs) have gained lots of attention due to the selective oxidation and non-pH-dependence. A series of systems using persulfate (PS) or peroxymonosulfate (PMS) instead of H2 O2 is designated as a sulfate radical-Fenton system or sulfur-replaced Fenton system (SR-Fenton). Comparisons and analogies between Fenton (Fenton-like) systems and SR-Fenton systems are made and some new SR-AOTs systems without PS or PMS are introduced. The possibility for the substitution of HO• by SO4 •− for AOTs is discussed. Most likely in the future, efforts will be concentrated on product-oriented AOTs with the purpose of recovery of chemical products rather than mineralization of organic contaminants, producing greenhouse gas CO2 . Moreover, such SR-Fenton system may be more atomically economical. © 2014 Society of Chemical Industry Keywords: advanced oxidation technologies; sulfate radical; Fenton process; hydroxy radical; mineralization

INTRODUCTION It is well-known that conventional advanced oxidation technologies (AOTs) were based on HO• , namely HR-AOTs. In the past decade, AOTs based on sulfate radicals (SO4 •− ) (here abbreviated to SR-AOTs), have attracted much attention owing to their capability for oxidative removal of pollutants from wastewaters.1 Among the reagents for SR-AOTs, persulfate (PS) and peroxymonosulfate (PMS) have been widely used as oxidants as sources of SO4 •− . Both strategies of chemical catalysis and energy assistance have been applied to enhance the production of SO4 •− . Transition-metal ions such as iron, copper, and cobalt may be used as catalysts. UV irradiation, thermolysis, microwave irradiation, and sonic irradiation may be utilized as assistant physical methods.2 Many different systems have been derived from the simple oxidant PS/PMS. This situation resembles that of peroxide, H2 O2 , which is used in various systems based on Fenton reactions (reaction (1)). We propose a series of systems that use PS/PMS instead of H2 O2 , that is, a sulfate radical-Fenton system or sulfur-replaced Fenton system (both abbreviated as SR-Fenton system). Reactions (2) and (3) represent the processes of ferrous iron-catalyzed decomposition of PS/PMS to produce SO4 •− . It is interesting that HO• can be produced by the reaction of SO4 •− with H2 O or OH− (reactions (4) or (5)), but at low rate. Therefore, Ahmed and Chiron termed PS/Fe(II)/UV-Vis system a ‘photo-Fenton like’ system.3 From this perspective, we can make an analogy between systems containing H2 O2 and PS/PMS (see Scheme 1). There have been some publications reporting comparison of the oxidative removal of organic pollutants by HO• and SO4 •− . 4

J Chem Technol Biotechnol 2015; 90: 775–779

(1)

(2)

Fe2+ + HSO−5 → Fe3+ + SO•− + OH− 4

(3)

SO•− + H2 O → HO• + SO2− + H+ 4 4

(4)

SO•− + OH− → HO• + SO2− 4 4

(5)

ANALOGY OF FENTON (LIKE) SYSTEMS AND SR-FENTON SYSTEMS As Scheme 1 shows, some novel systems from a basic routine of substitution have been proposed; for example, polycarboxylic acid (PCOA) complexes with Fe(III) to form a photo-Fenton-like system without added H2 O2 . H2 O2 and HO• are produced through photolysis of Fe(III)-PCOA complexes in the presence of dissolved oxygen. Similarly, sulfite complexes with Fe(II) and Fe(III) form a photo-Fe(II/III)/sulfite system and produce SO4 •− without



Correspondence to: Danna Zhou, Faculty of Material Sciences and Chemistry, China University of Geosciences, Wuhan 430074, P. R. China. e-mail: [email protected] and Hui Zhang, School of Resources and Environmental Science, Wuhan University, Wuhan 430079, P. R. China. e-mail: [email protected]

a Faculty of Material Sciences and Chemistry, China University of Geosciences, Wuhan, 430074, P. R. China b School of Resources and Environmental Science, Wuhan University, Wuhan, 430079, P. R. China c Department of Chemical and Environmental Engineering, University of California, Riverside, Riverside, California, 92521, USA

www.soci.org

© 2014 Society of Chemical Industry

775

Fe2+ + H2 O2 → Fe3+ + OH− + HO•

Fe2+ + S2 O2− → Fe3+ + SO2− + SO•− 8 4 4

www.soci.org

D Zhou, H Zhang, L Chen

Scheme 1. Analogy of Fenton (like) systems based on HO• and sulfur-replaced Fenton systems based on SO4 •− . As an example, a photo-Fenton like system mediated by polycarboxylic acids (PCOA) for the production of HO• (blue) and a photo-Fe(III)/Fe(II)–sulfite system for production of SO4 •− (red) are presented.

added PS/PMS.5 Sulfite can replace PS/PMS in SR-Fenton systems since sulfite is usually cheaper and more environmentally friendly than PS/PMS. Other sulfuric species (S2 O5 2− , S2 O6 2− , and S2 O7 2− etc.) may also be used for SR-Fenton systems activated by iron ions. In Fenton-like systems and SR-Fenton systems, the production processes of HO• or SO4 •− include radical chain reactions, which are more complicated than that of the original Fenton process. Reactive oxygen species (ROS) and reactive sulfur species (RSS) are involved in Fenton-like systems and SR-Fenton systems, respectively. These two series of systems can be operated identically at circumneutral pH values. However, such aforementioned SR-Fenton systems achieve very limited mineralization of organic pollutants in terms of total organic carbon (TOC) removal. This limitation poses an important question: can SO4 •− substitute HO• for AOTs?

COMPARISON BETWEEN ROS AND RRS

776

HO• is the most important ROS. Three ROS, HO• , O2 •- /HO2 • , and H2 O2 can interconvert. Likewise, SO4 •− is the most important RSS for AOTs. Three RSS, SO4 •− , SO3 •− , and SO5 •− can also interconvert. Since oxygen and sulfur are from the same family in the periodic table, ROS and RSS are similar in transformation behaviors. Both are strongly affected by the content of dissolved oxygen in waters. Table 1 shows that the approximate values of standard reduction potentials (SRP) of ROS and RRS are close to each other (i.e. H2 O2 vs K2 S2 O8 , O2 •- vs SO3 •− , HO2 • vs SO5 •− , and HO• vs SO4 •− ), although SRPs for them are generally pH-dependent. By using the reported rate constants from the NDRL/NIST Solution Kinetics Database on the Web,12 a comparison of rate

wileyonlinelibrary.com/jctb

Table 1. Comparison of standard reduction potential between ROS and RRS ROS H2 O2 /H2 O O2 •- /H2 O HO2 • /H2 O HO• /H2 O

E0 (V) vs NHE 1.7636 0.64 (pH 7)7 > 1.09 1.8–2.710

RRS K2 S2 O8 /SO4 2− SO3 •− /SO3 2− SO5 •− /SO5 2− SO4 •− /SO4 2−

E0 (V) vs NHE 1.966 0.73 (pH 7)8 1.183 (pH 3)8 2.5–3.111

constants of second-order reactions between chemicals and HO• (kHO ) or SO4 •− (kSO4 ) is made in Fig. 1. It shows that most rate constants of HO• are higher than those of SO4 •− considering the 60 data as a whole data set; kHO is about 1.6 times kSO4 according to the linear correlation equation. Among 60 chemicals, 40% of them have the ratio kHO /kSO4 of 2–10, 32% of them have the ratio kHO /kSO4 of 10–100. This indicates that HO• reacts with chemicals generally faster than SO4 •− when HO• has the same concentration as SO4 •− . When it comes to multiple pollutants in real wastewaters, HO• exhibits higher universality than SO4 •− . In addition, ROS could be cleaner than RSS: any RSS will increase the salinity of waters. For example, PS/PMS increases the concentration of sulfate ions in the waters after oxidation treatment. Sulfate ion of high concentration is considered one of the common inorganic contaminants. Other RSS may have the same drawbacks for water treatment. SO4 •− can react with water and result in pH decrease through reaction (4). Oxidation with HO• normally produces acidic products only through reactions with organic substrates.

© 2014 Society of Chemical Industry

J Chem Technol Biotechnol 2015; 90: 775–779

Rate constants of hydroxyl radicals (M-1 s-1)

Sulfur-replaced Fenton systems

1.6x10

10

1.2x10

10

www.soci.org y = 1.572x

y=x

Ratio

8.0x10

9

4.0x10

9

19 (32%)

24 (40%)

0-1 1-2 2-10 10-100 100-900

5 (8.3%) 4 (6.7%) 8 (13%)

0.0 0.0

4.0x109

8.0x109

1.2x109 -1

1.6x1010

-1

Rate constants of sulfate radical (M s )

Figure 1. Comparison of rate constants of second-order reactions between chemicals and HO• or SO4 •− . Red line is the boundary of the identical rate constants (equation y = x). Blue line is the linear fitted curve (equation y = 1.572 x, R = 0.6498, n = 60). Inset is the distribution pie of the ratio of rate constants (y/x).

CAN SO4 •− SUBSTITUTE HO• FOR AOTS?

J Chem Technol Biotechnol 2015; 90: 775–779

Far more case studies on HR-AOTs, especially Fenton (like) and photo-Fenton (like) systems, can be found than that on SR-AOTs. Most lab or pilot experiments of SR-AOTs have been conducted with simulated wastewaters. A relatively small number of SR-AOTs have been reported treating some real wastewaters, such as domestic wastewater,3,14 landfill leachate,15,22 and petroleum-hydrocarbon contaminated groundwater.23 Although SR-AOTs can treat many different types of contaminants in simulated wastewaters, they must overcome the drawbacks of RSS facing real wastewaters. Many more results from applied research on SR-AOTs in real wastewater treatment are needed for comprehensive assessment of their applicability and effectiveness. Hydroxyl radical technologies have been used as a pre-biological treatment or a post-biological treatment process of wastewaters.24,25 Although such technologies aimed at increasing the mineralization efficiency, they can be considered negligible contributors of CO2 in the overall treatment process. Maybe SR-Fenton cannot completely substitute Fenton system in terms of mineralization. But it’s very possible to combine both of them, in series reaction, i.e. SR-Fenton reaction first and then Fenton reaction. In this way, large molecules can be degraded selectively into small molecules first, and the generated and unmineralized products can be further degraded to the required extent. Therefore, the present SR-Fenton technology is suitable for the pretreatment of contaminants, and a subsequent process such as biological treatment is necessary for polishing the effluent of SR-Fenton oxidation. Although SO4 •− normally could not completely replace HO• to mineralize contaminants, utilizing such incomplete oxidation may create new functions for AOTs, i.e. producing commercial industrial chemicals. Then AOTs are product-oriented rather than waste-oriented.

PRODUCT-ORIENTED AOTS FOR POLLUTION PREVENTION Reduction of pollutants at their sources is known as pollution prevention (P2), which is in the highest hierarchy and has been applied extensively in many kinds of industries. One of the activities of P2 is to redesign products to cause less pollution during manufacture, use, or disposal.26 For wastewater treatment plants (WWTPs), it is better to get commercially useful products besides water rather than just transform the organic pollutants into CO2 . Product-oriented environmental management system (POEMS) 27 and product-oriented pollution prevention28 are very common terms used among product-oriented technologies. WWTPs should develop along the direction of product-oriented pollution prevention by providing chemical products (especially energy chemicals) and reducing the loads of

© 2014 Society of Chemical Industry

wileyonlinelibrary.com/jctb

777

It is believed that SO4 •− has characteristics different from HO• . First, SO4 •− has more opportunities to react with organic pollutants, as it has a much longer lifetime (30–40 μs) compared with HO• (20 ns). Moreover, pH has an insignificant effect on the oxidation ability of SO4 •− , whereas the oxidizing power of HO• is pH-dependent. For example, SO4 •− has a little higher SRP than that of HO• at neutral pH.13 Its stronger ability to degrade organic compounds such as 2,4-dichlorophenol in Fenton systems is mainly attributed to its higher redox potential. At low pH, SO4 •− and HO• radicals demonstrate similar redox potentials, but SO4 •− is generally a more selective oxidant than is HO• . HO• is more likely to attack organic compounds via hydrogen abstraction or addition reactions, whereas SO4 •− reacts more selectively with organic compounds through electron transfer. Matta et al.14 reported that SO4 •− yielded faster degradations of carbamazepine compared with HO• in aqueous solution and real urban wastewater. They believed that SO4 •− could be more selective than HO• for the oxidation of carbamazepine and may represent an interesting alternative in AOTs for real urban wastewater. Deng and Ezyske15 indicated that SR-AOTs appeared to be advantageous over HR-AOTs because HO• almost does not oxidize ammonia. Moreover, compared with Fenton treatment of the same batch leachate sample, thermal persulfate oxidation achieved higher COD removal (91%, S2 O8 2− : 12COD0 = 2, pH 4) at an identical chemical dose to that of Fenton treatment (40%, H2 O2 : 2.125COD0 = 2, pH 3). We observed great enhancement of the toluene degradation efficiency and lower degradation efficiency for sodium dodecyl sulfate (SDS, commonly used in soil flushing) in the SR-Fenton process (Fe2+ /PS) compared with those in the Fenton process. It should be noticed that the comparison was made under optimal conditions obtained from the Fe2+ /PS process while the same amount of PS was replaced by H2 O2 in the Fenton process. Despite this point, the difference of toluene removal between SR-Fenton and Fenton processes was more pronounced with increasing SDS concentration. Therefore, we believe that selective oxidation of contaminants such as toluene could be used in surfactant recovery from flushing effluents mixed with toluene and surfactant.16 However, the SR-Fenton process cannot achieve satisfactory mineralization of contaminants. This problem is similar to that encountered in the classical Fenton process, in which TOC

removal generally cannot exceed 60%. Consequently, a very high PS-to-contaminant mole ratio or energy assistance (usually in the form of UV radiation), higher activator concentration, and very long reaction time are needed to obtain higher TOC removal. For example, 20 − 30% TOC removal was achieved in the Co(II)/PMS system at high concentrations of PMS (about 100 − 1000 times higher than that of the substrate).1 A higher TOC removal of phenol (c. 70%) in water was achieved with UV light activation with high ratio of PS/phenol 84/0.5 (mmol L).17 Further studies have been focused on the improvement of the SR-Fenton process so that mineralization of contaminants could be carried out at relatively mild conditions. Applications of HR-AOTs in the treatment of many different kinds of real wastewaters have been extensively investigated.18 – 21

www.soci.org

Table 2. Second-order rate constants for the reactions of alkyl alcohols with HO• and SO4 •− in water Rate constant (M−1 s−1 ) Alkyl alcohols

with HO•

Methanol10 Ethanol10 2-Propanol10 l-Butanol10 l-Hexanol35 1-Heptanol35 l-Octanol35

9.7 × 108 1.9 × 109 1.9 × 109 4.2 × 109 7.0 × 109 7.4 × 109 7.7 × 109

with SO4 •− 34 1.1 × 107 4.3 × 107 7.9 × 107 8.1 × 107 1.6 × 108 2.2 × 108 3.2 × 108

organic pollutants, which could also help to reduce the treatment costs overall. Typical examples include hydrogen and methane production from wastewater using microbial electrolysis cells,29 biofuel production from algae or seaweed,30 polyhydroxyalkanoates production by mixed microbial consortium technology.31 For SR-Fenton, the desired degree of oxidation of organic contaminants could be obtained under certain conditions by utilizing the selective oxidation ability of SO4 •− . For example, alcohols, aldehydes/ketones and carboxylic acids are known to be the common by-products of organic pollutants treated by HR-Fenton32,33 or SR-Fenton.23 By-products are dependent on the extent of oxidation, which are decided by the dosage of oxidants. However, due to the lower rate constants of the reactions of alkyl alcohols with HO• (see Table 2), SR-Fenton may facilitate the oxidation merely to produce alkyl alcohols of low molecular weight. Therefore, methanol or ethanol as energy chemicals may be obtained from wastewater treatment by SR-Fenton. Additionally, SO4 •− might be a better alternative to specific organic materials (e.g. sulfate adducts5,36 ) for product-oriented technologies for the treatment of organic pollutants. Mineralization of organic pollutants with HO• causes increased CO2 emissions, which has been regarded as a debatable deficiency of AOTs.

CONCLUSION SO4 •− cannot substitutes HO• in terms of rate constants of chemicals. SR-AOTs cannot replace HR-AOTs in terms of the mineralization of organic contaminants. Sulfite salts can replace PS or PMS to form new SR-AOTs systems catalyzed by Fe(II/III) with partial oxidation of the organic substrates. This partial oxidation can be used in the synthesis of chemical products. Future AOTs will be product-oriented AOTs without mineralization of organic contaminants to emit CO2 . SR-Fenton rather than HR-Fenton may play a more important role product-oriented AOTs in the future.

ACKNOWLEDGEMENTS This work was supported by the Natural Science Foundation of China (No. 41103066). Comments from anonymous reviewers are also appreciated.

REFERENCES

778

1 Anipsitakis GP and Dionysiou DD, Degradation of organic contaminants in water with sulfate radicals generated by the conjunction of peroxymonosulfate with cobalt. Environ Sci Technol 37:4790–4797 (2003).

wileyonlinelibrary.com/jctb

D Zhou, H Zhang, L Chen 2 Antoniou MG, de la Cruz AA and Dionysiou DD, Degradation of microcystin-LR using sulfate radicals generated through photolysis, thermolysis and e − transfer mechanisms. Appl Catal B - Environ 96:290–298 (2010). 3 Ahmed MM and Chiron S, Solar photo-Fenton like using persulphate for carbamazepine removal from domestic wastewater. Water Res 48:229–236 (2014). 4 Shukla PR, Fatimah I, Wang S, Ang HM and Tadé MO, Photocatalytic generation of sulphate and hydroxyl radicals using zinc oxide under low power UV to oxidise phenolic contaminants in wastewater. Catal Today 157:410–414 (2010). 5 Zhou D, Chen L, Zhang C, Yu, Y, Zhang L and Wu F, A novel photochemical system of ferrous sulfitecomplex: kinetics and mechanisms of rapiddecolorization of acid orange 7 in aqueoussolutions. Wat. Res 57:87–95 (2014). 6 Dean JA, Lange’s Handbook of Chemistry, 15th edn. McGraw-Hill, 8.132–8.134 (1999). 7 Wood PM, The potential diagram for oxygen at pH 7. Biochem J 253:287–289 (1988). 8 Nath Das T, Huie RE and Neta P, Reduction potentials of SO3 •- , SO5 •- , and S4 O6 •3- radicals in aqueous solution. J Phys Chem A 103:3581–3588 (1999). 9 Rao PS and Hayon E, Experimental determination of the redox potential of the superoxide radical O2 •− . Biochem Biophys Res Commun 51:468–473 (1973). 10 Buxton GV, Greenstock CL, Helman WP and Ross AB, Critical review of rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals (• OH/• O− ) in aqueous solution. J Phys Chem Ref Data 17:513−886 (1988). 11 Neta P, Huie RE and Ross AB, Rate constants for reactions of inorganic radicals in aqueous solution. J Phys Chem Ref Data 17:1027−1284 (1988). 12 NDRL/NIST Solution Kinetics Database Resources. [Online]. Available at: http://kinetics.nist.gov/solution [18 September 2014]. 13 Anipsitakis GP and Dionysiou DD, Radical generation by the interaction of transition metals with common oxidants. Environ Sci Technol 38:3705–3712 (2004). 14 Matta R, Tlili S, Chiron S and Barbati S, Removal of carbamazepine from urban wastewater by sulfate radical oxidation. Environ Chem Lett 9:347–353 (2011). 15 Deng Y and Ezyske CM, Sulfate radical-advanced oxidation process (SR-AOP) for simultaneous removal of refractory organic contaminants and ammonia in landfill leachate. Water Res 45:6189–6194 (2011). 16 Long AH, Lei Y and Zhang H, Degradation of toluene by a selective ferrous ion activated persulfate oxidation process. Ind Eng Chem Res 53:1033−1039 (2014). 17 Lin YT, Liang CJ and Chen JH, Feasibility study of ultraviolet activated persulfate oxidation of phenol. Chemosphere 82:1168–1172 (2011). 18 Tambosi JL, Di Domenico M, Schirmer WN, José HJ and Moreira RFPM, Treatment of paper and pulp wastewater and removal of odorous compounds by a Fenton-like process at the pilot scale. J Chem Technol Biotechnol 81:1426–1432 (2006). 19 Miralles-Cuevas S, Oller I, Sánchez Pérez JA and Malato S, Removal of pharmaceuticals from MWTP effluent by nanofiltration and solar photo-Fenton using two different iron complexes at neutral pH. Water Res 64:23–31 (2011). 20 Molina R, Pariente I, Rodríguez I, Martínez F and Melero JA, Treatment of an agrochemical wastewater by combined coagulation and Fenton oxidation. J Chem Technol Biotechnol 89:1189–1196 (2014). 21 Díaz de Tuesta JL, García-Figueruelo C, Quintanilla A, Casas JA and Rodriguez JJ, Application of high-temperature Fenton oxidation for the treatment of sulfonation plant wastewater. J Chem Technol Biotechnol doi: 10.1002/jctb.4494 22 Zhang H, Wang Z, Liu CC, Guo YA, Shan N, Meng CX and Sun LY, Removal of COD from landfill leachate by an electro/Fe2+ /peroxydisulfate process. Chem Eng J 250:76–82 (2014). 23 Liang SH, Kao CM, Kuo YC, Chen KF and Yang BM, In situ oxidation of petroleum-hydrocarbon contaminated groundwater using passive ISCO system. Water Res 45:2496–2506 (2011). 24 Ioannou L, Michael C, Kyriakou S and Fatta-Kassinos D, Solar Fenton: from pilot to industrial scale application for polishing winery wastewater pretreated by MBR. J Chem Technol Biotechnol 89:1067–1076 (2014). 25 Klauson D, Kivi A, Kattel E, Klein K, Viisimaa M, Bolobajev J, Velling S, Goi A, Tenno T and Trapido M, Combined processes for wastewater

© 2014 Society of Chemical Industry

J Chem Technol Biotechnol 2015; 90: 775–779

Sulfur-replaced Fenton systems

26 27 28

29 30

www.soci.org

purification: treatment of a typical landfill leachate with a combination of chemical and biological oxidation processes. J Chem Technol Biotechnol doi: 10.1002/jctb.4484 Phipps E, Pollution Prevention Concepts and Principles, National Pollution Prevention Center for Higher Education • University of Michigan, Ann Arbor MI, (1995). Rocha C and Silvester S, Product-oriented environmental management system (POEMS): from theory to practice – experience in Europe. Delft University of Technology, Delft (2000). Davis GA, Wilt CA, Dillon PS and Fishbein BK, Extended product responsibility: a new principle for product-oriented pollution prevention (EPA530-R-97-009). United States Environmental Protection Agency, Office of Solid Waste (1997). Wagner RC, Regan JM, Oh SE, Zuo Y and Logan BE, Hydrogen and methane production from swine wastewater using microbial electrolysis cells. Water Res 43:1480–1488 (2009). Singh A, Olsena SI and Nigam PS, A viable technology to generate third-generation biofuel. J Chem Technol Biotechnol 86:1349–1353 (2011).

31 Coats ER, Loge FJ, Englund K, Wolcott MP and McDonald AG, Synthesis of polyhydroxyalkanoates in municipal wastewater treatment. Water Environ Res 79:2396–2403 (2007). 32 Ray AB and Selvakumar A, Treatment of MTBE using Fenton’s reagent. Remediation J 10:3–13 (2000). 33 Vilar VJP, Maldonado MI, Oller I, Malato S, Boaventura RAR, Solar treatment of cork boiling and bleaching wastewaters in a pilot plant. Water Res 43:4050–4062 (2009). 34 Clifton CL and Huie RE, Rate constants for hydrogen abstraction reactions of the sulfate radical SO4 − . Alcohols. Int J Chem Kinetics 21:677–687 (1989). 35 Scholes G and Willson RL, 𝛾-Radiolysis of aqueous thymine solutions. Determination of relative reaction rates of OH radicals. Trans Faraday Soc 63:2983–2993 (1967). 36 Das S, Kamat PV, Padmaja S, Au V and Madison SA, Free radical induced oxidation of the azo dye Acid Yellow 9. J Chem Soc Perkin Trans 2:1219–1223 (1999).

779

J Chem Technol Biotechnol 2015; 90: 775–779

© 2014 Society of Chemical Industry

wileyonlinelibrary.com/jctb