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Oxidative degradation of sulfathiazole by Fenton andphoto-Fenton reactionsMarizú Velásquez a , I. Paola Santander a , David R. Contreras a , Jorge Yáñez a , ClaudioZaror b , Ricardo A. Salazar c , Montserrat Pérez-Moya d & Héctor D. Mansilla aa Facultad de Ciencias Químicas , Universidad de Concepción , Casilla , Concepción , Chileb Facultad de Ingeniería , Universidad de Concepción , Casilla , Concepción , Chilec Facultad de Química y Biología , Universidad de Santiago de Chile, Casilla , Correo ,Santiago , Chiled Departament d’Enginyeria Química, EUETIB , Universitat Politècnica de Catalunya ,Barcelona , SpainPublished online: 12 Feb 2014.

To cite this article: Marizú Velásquez , I. Paola Santander , David R. Contreras , Jorge Yáñez , Claudio Zaror , Ricardo A.Salazar , Montserrat Pérez-Moya & Héctor D. Mansilla (2014) Oxidative degradation of sulfathiazole by Fenton and photo-Fenton reactions, Journal of Environmental Science and Health, Part A: Toxic/Hazardous Substances and EnvironmentalEngineering, 49:6, 661-670, DOI: 10.1080/10934529.2014.865447

To link to this article: http://dx.doi.org/10.1080/10934529.2014.865447

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Journal of Environmental Science and Health, Part A (2014) 49, 661–670Copyright C© Taylor & Francis Group, LLCISSN: 1093-4529 (Print); 1532-4117 (Online)DOI: 10.1080/10934529.2014.865447

Oxidative degradation of sulfathiazole by Fentonand photo-Fenton reactions

MARIZU VELASQUEZ1, I. PAOLA SANTANDER1, DAVID R. CONTRERAS1, JORGE YANEZ1, CLAUDIOZAROR2, RICARDO A. SALAZAR3, MONTSERRAT PEREZ-MOYA4 and HECTOR D. MANSILLA1

1Facultad de Ciencias Quımicas, Universidad de Concepcion, Casilla, Concepcion, Chile2Facultad de Ingenierıa, Universidad de Concepcion, Casilla, Concepcion, Chile3Facultad de Quımica y Biologıa, Universidad de Santiago de Chile, Casilla, Correo, Santiago, Chile4Departament d’Enginyeria Quımica, EUETIB, Universitat Politecnica de Catalunya, Barcelona, Spain

This article presents experimental results on 47 µmol L−1 sulfathiazole (STZ) degradation by Fenton and photo-Fenton reactionsusing multivariate analysis. The optimal experimental conditions for reactions were obtained by Response Surface Methodology(RSM). In the case of the Fenton reactions there were 192 µmol L−1 ferrous ions (Fe(II)) and 1856 µmol L−1 hydrogen peroxide(H2O2), as compared with 157 µmol L−1 (Fe(II)) and 1219 µmol L−1 (H2O2) for photo-Fenton reactions. Under these conditions,around 90% of STZ degradation were achieved after 8 minutes treatment by Fenton and photo-Fenton reactions, respectively.Moreover, a marked difference was observed in the total organic carbon (TOC) removal after 60-min treatment, achieving 30% and75% for the Fenton and photo-Fenton reactions, respectively. Acetic, maleic, succinic and oxamic acids could be identified as mainFenton oxidation intermediates. A similar pattern was found in the case of photo-Fenton reaction, including the presence of oxalicacid and ammonia at short periods of irradiation with UV-A. The calculated values of Average Oxidation State (AOS) corroboratethe formation of oxidized products from the initial steps of the reaction.

Keywords: Antibiotics, Fenton, photo-Fenton, RSM, sulfathiazole.

Introduction

A large amount of pharmaceuticals, such as antibiotics,are daily consumed worldwide to control human and ani-mal diseases. Most of these compounds are metabolized bytarget organisms; however, a significant fraction of thesechemical or their by-products are discharged to the envi-ronment, with long-term undesirable ecological and humanhealth effects.[1] Those contaminants could be found in lowconcentrations in groundwaters and surface waters. Healthrisks include allergic reactions and alterations of the im-mune system, increasing the resistance to antibiotics andleading to the use of more powerful formulations to treatcurrent infections.[2]

Conventional water treatment systems based on biolog-ical processes have proven to be unable to efficiently re-move antibiotics, due to their antimicrobial nature. On the

Address correspondence to Hector D. Mansilla, Faculty of Chem-ical Sciences, University of Concepcion, P.O. Box 160-C, Con-cepcion 4030000, Chile; E-mail: hmansill@udec.clReceived August 9, 2013.Color versions of one or more of the figures in the article can befound online at www.tandfonline.com/lesa.

other hand, advanced physicochemical technologies suchas membrane filtration, or advanced oxidation processes(AOPs) have been identified as suitable alternatives. In par-ticular, AOPs has been used to readily achieve partial orcomplete oxidation of a wide variety of organic pollu-tants.[2–5]

AOPs are based on physicochemical advanced proce-dures capable to induce deep changes in the chemical struc-ture of contaminants. This concept was earlier introducedby Glaze and co-workers,[6] that define the AOPs as pro-cesses involving the generation and use of transient specieswith high oxidant power, primarily the hydroxyl radical(E = 2.8 V v/s SHE),[7] which react with organic moietieswith rate constant in the range of 107–1010 Lmol−1s−1. Thereaction, mainly conducted by hydroxyl radicals allows theoxidation and eventually, if the reaction time is adequate,the mineralization of organic compounds.[8–10]

Radicals could be generated via photochemical pro-cedures or by combination of oxidants and electromag-netic radiation. Unfortunately, one of the main draw-backs of AOPs is the high operating costs, involved inenergy and chemicals consumption. Several options havebeen proposed to reduce costs, such as process inte-gration, use of homogeneous reaction systems avoiding

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662 Velasquez et al.

the catalyst recovering step, or using solar light as pho-ton source.[11] In this context, Fenton and photo-Fentonreactions have been identified as suitable alternatives fora wide range of organic structures, including antibiotics,due to their high treatment efficiency, and short treatmenttimes.[11–14] Fenton’s system feature relatively low operat-ing costs, simpler reactor designs and no mass transferlimitations.[15]

The well-known Fenton system involves the reaction be-tween ferrous ion (Fe(II)) and hydrogen peroxide (H2O2),at acidic pH (around 3), leading to the formation of hy-droxyl radicals (•OH)[3,8,16,17] (Eq. (1)). The high activityfor Fenton reaction has been found at pH around 3 be-cause values over 5 provoke the iron precipitation. On theother hand values lower to 2.5 induce the formation of[Fe (II)(H2O)]2+ complexes that react slowly with hydrogenperoxide.[18–20]

Fe(II) + H2O2 → Fe(III) + HO− +• OHk = 76

[Lmol−1s−1] (1)

Additionally, ferric ions (Fe(III)) could further react withH2O2 at low pH, regenerating Fe(II) ions, through the fol-lowing Fenton-like reactions.[9,21]

Fe(III) + H2O2 → Fe − OOH2+ + H+

Keq = 3.1 × 10−3 (2)

Fe − OOH2+ → HO•2 + Fe(II)

k = 0.01 − 0.02[Lmol−1s−1] (3)

HO•2 + Fe(III) → Fe(II) + O2 + H+

k < 2 × 10−3 [Lmol−1s−1] (4)

In the case of photo-Fenton, Fe(III) photoreductionto Fe(II) occur, promoting the generation of new hy-droxyl radicals.[22–25] At pH 2-3 the predominant speciesare [Fe(OH)(H2O)5]2+ and [Fe(OH)]2+. Such speciesare photosensitive to electromagnetic radiation com-prised in the range 280–405 [nm] allowing the use ofsunlight:[8,11,12,16, 26,27]

[Fe(OH)(H2O)5]2+ + H2O + hν → [Fe(H2O)6]2+

+ •OH (5)Fe(III) + H2O → [FeOH]2+ + H+ (6)

[FeOH]2+ Fe(II) + •OHk = 3, 3 × 10−6 Lmol−1s−1 (7)

The use of photo-Fenton reaction to remove antibi-otics from wastewaters has been previously reported.[27,28]

Sulfonamide-family antibiotics, amply used for mam-malians infection treatments, are an issue of concern be-cause to their negative effect on aquatic ecosystems. Ithas been proved their carcinogenic effect and bacterialresistance in humans.[29,30] They are frequently found inwastewater in concentrations ranged between µgL−1 tongL−1.[31–32] Sulfathiazole belongs to the sulfonamide fam-ily which has been described as harmful for human life and

refractory to oxidation.[33] Recently, Batista and Nogueiraperformed the degradation of sulfadiazine and sulfathia-zole, using Fe(III)-oxalate complex comparing it to freeFe(III),[34] founding that Fe(III)-complex improve the oxi-dation of the antibiotics. In addition, ferric ions coated onhumic acid completely degrade and mineralize STZ in 1 hand 6 h, respectively.[35]

The aim of the present research is to assess the effectsof Fe(II) and H2O2 concentrations on the degradation ofSTZ, both in Fenton and photo-Fenton reaction systems.A factorial design is used to optimize those variables.STZ degradation is monitored by high performanceliquid chromatography (HPLC) analysis, TOC, COD, Gaschromatography-mass spectrometry (GC-MS) and UV-Visspectrophotometer system coupled to a rapid mixing“stopped flow.” A possible STZ degradation pathway inthe reactions of Fenton and photo-Fenton is proposed.

Materials and methods

Chemicals

Sulfathiazole sodium salt (C9N3S2O2H8Na, purity ≥99%) was purchased from Sigma-Aldrich (St. Louis, MO,USA). Ferrous sulfate heptahydrate (FeSO4 × 7H2O, pu-rity 99.5%), hydrogen peroxide (30%) and sulfuric acid(H2SO4, 97–98%) were obtained from Merck (Darm-stadt, Germany). Fenton and photo-Fenton reactions werestopped with a solution of sodium bisulfite and sodiummetabisulfite (NaHSO3 and Na2S2O5 mixture, Sigma-Aldrich).[14]

For the HPLC analysis of carboxylic acids, 98% sul-furic acid (Merck) and bidistilled water were used in thepreparation of the mobile phase. The mobile phase was fil-tered and purged with 99.995% extra pure helium. Oxalic(p.a.), maleic (for synthesis) and acetic acid (glacial, 100%)standards were purchased from Merck. Succinic (≥99.9%)and oxamic (>98%) acids were obtained from Sigma-Aldrich.

Acetonitrile (LiChrosolv, Merck), methanol (LiChro-solv, Merck) and anhydrous acetic acid (proanalysis,Merck) were used in the preparation of mobile phase forSTZ analysis.

Fenton and photo-Fenton reactions

A 250-mL sodium sulfathiazole 47 µmol L−1 solution wasprepared and acidified to pH 3.0 with sulfuric acid, fol-lowed by stoichiometric addition of Fe(II). After homoge-nization, hydrogen peroxide was also added to the solution.The Fe(II) and H2O2 doses were determined by factorialdesign. The reaction setup consists in a borosilicate cylin-drical flask magnetically stirred.

During reaction runs, samples were taken at differentintervals of time. The pH was measured at different pe-riods of time, indicating an increase of around 0.3 units

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Oxidative degradation of sulfathiazole by Fenton reactions 663

of pH, until the end of the experiment. The reaction wasstopped with 23 µL sodium bisulfate 20% p/v, which re-acts with residual hydrogen peroxide. Merckoquant teststicks (Merck) were used to verify the absence of hydrogenperoxide.

Photo-Fenton reaction system was irradiated placing acommercial solarium (Philips HB 311, Eindhoven, TheNetherlands) arranged with 6 × 20 W lamps (365 nm)parallel to the cylindrical flask separated by 10 cm. Irra-diance (3.5 mW cm−2, photon flux 106.6 µmol m−2 s−1)was measured with a UVP digital radiometer model UVX(Jena, Germany).

Optimization and modeling of response surface

A Central Composite Circumscribed design (CCC) wasused in order to optimize the concentration of Fe(II) andH2O2. It consists of a 2k factorial design, where in thelevels are encoded as +1 and −1. For two variables, k =2. Three central points and four star points provide thestatistical consistency and expand the dominium of themodel, respectively. Finally, 11 experiments were performedto complete the design.

The concentrations of Fe(II) and H2O2 for the Fentonand photo-Fenton reactions ranged from 47 to 188 µmolL−1 for Fe(II) and from 469 to 1970 µmol L−1 forH2O2, considering the values usually reported in litera-ture.[21,23,27,36] It is important to consider that the experi-mental values of star points exceed the variables range. Dataanalysis was performed with the software Modde 7.0TM

(UMETRICS Inc./MKS Instruments, San Jose, CA, USA)and the statistical validation was determined by ANOVAwith a confidence level of 95%.

Analytical determinations

Sulfathiazole was quantified injecting a 20-µL sample toa HPLC Knauer model SmartLine (Advanced ScientificInstruments, Berlin, Germany) equipped with a Smart-

line 2500 UV detector (set at 284 nm) and a RP-18LiChroCART R© 100 column (5–10 µm, 0.46 × 15 cm,Merck, Chile), maintained at 30◦C.The mobile phase wasinitially made of 2.5: 86: 11.5 of acetonitrile: 0.1% aceticacid: methanol. After 20 min of elution gradient the com-position of mobile phase was 15: 72: 13 at flow rate of1.5 mL min−1.

The initial degradation profile of STZ in the Fenton re-action was also followed in a UV-Vis Agilent 8453 spec-trophotometer (Agilent Technologies, Santa Clara, CA,USA) coupled to a stopped flow of rapid mixing (RX2000)recording the UV-Vis spectrum each 0.5 sec.

For determination of carboxylic acids, 50 µL of samplewas injected in a modular HPLC Waters 625 LC System(Waters, Milford, MA, USA) equipped with a photodiodearray detector (Waters 2996 fitted at 210 nm). Mobile phaseof 4 mmol L−1 H2SO4 was continuously bubbled with he-lium. A Biorad HPLC column Aminex HPX-87H (9 µm,300 × 7.8 mm, Hercules, CA, USA) was maintained at 30◦Cat a flow rate of 0.6 mL min−1.[37,38] The corresponding cal-ibration curves were performed using pure standards.

Released NH4+ was quantified by ionic chromatography

by injecting 50 µL aliquots in a Waters 600 chromatographequipped with a 600 controller pump and a Waters 431conductivity detector and an IC-Pak C M/D, 150 mm ×3.9 mm (i.d.), cation column at 25◦C and using 0.1 mmolL−1 EDTA/3.0 mmol L−1 HNO3 at 1.0 mL min−1 as mobilephase.

Derivatization of samples for CG-MS analysis was car-ried out on 10 mL sample of STZ after photo-Fentondegradation, which was liophylized and re-dissolved in1 mL methanol. Then, 100 µL of bis-(trimethylsylil)-trifluoroacetamide (BSTFA) was added; mixed in a vortexand heated at 90◦C for 45 min. Derivatized sample was in-jected in an Agilent chromatograph model 7890A with a5975C MS detector, equipped with a capillary column VA-5(30 m, 0.25 mm i.d.; 0.25 µm). TOC determination was per-formed in Elementar equipment (vario TOC cube, Hanau,

Table 1. Experimental results from factorial design of sulfathiazole degradation by Fenton and photo-Fenton reactions.

Run Fe2+ (µmol L) H2O2 (µmol L) YF exp. (%) YpF exp. (%) YF calc. (%) YpF calc. (%)

11 47(−1) 469(−1) 27.8 41.6 26.3 37.38 188(+1) 469(−1) 74.3 88.9 69.1 89.59 47(−1) 1970(+1) 39.7 76.3 45.9 71.14 188(+1) 1970(+1) 81.7 95.4 84.1 94.9

10 18(−1.4) 1219(0) 25.6 32.4 22.4 38.23 217(+1.4) 1219(0) 77.6 93.1 79.7 92.06 117(0) 158(−1.4) 44.6 65.7 49.4 67.47 117(0) 2281(+1.4) 79.4 92.1 73.9 95.25 117(0) 1219(0) 71.7 95.0 69.6 93.42 117(0) 1219(0) 69.2 93.2 69.6 93.41 117(0) 1219(0) 67.9 91.9 69.6 93.4

Codified values for variables are shown in parentheses. Response factor Y is defined as the STZ percentage degradation after 8-min reaction underFenton (YF) and photo-Fenton (YpF) conditions for 47 µmol L−1 of STZ.

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Fig. 1. Response surface for (a) Fenton and (b) photo-Fentonreactions, showing the dependence of the degradation extent withFe2+ and H2O2 concentrations.

Germany). A HACH reactor (Digital Reactor Block 200,Loveland, CO, USA) was used for COD analyses.

Results and discussion

Optimization procedure

Table 1 describes experiments performed for optimizationpurposes in STZ degradation. Columns 2 and 3 representthe real and coded values (in parentheses) for Fe(II) andH2O2 concentrations, respectively. Columns 4 and 5 indi-cate the percentage of STZ degradation (47 µmol L−1) after

0 2 4 6 8 10

0.0

0.2

0.4

0.6

0.8

1.0

1.2

C/C

0

Time (min)

Fig. 2. Profile of Sulfathiazole removal for Fenton (◦) and photo-Fenton (•) reactions. Controls with H2O2 (×), UV-A (♦) andH2O2/UV-A (�) are also depicted.

8 minutes reaction for Fenton and photo-Fenton reactions,respectively. For analytical reasons, STZ initial concentra-tion exceeds the concentration levels found in natural waterand wastewater.[31,32] The reaction time (8 min) chosen forfactorial design was determined after preliminary experi-ments.

The results of Table 1 were fitted to a polynomial modelgenerated by multiple linear regressions (MLR) with the

0.0

0.5

1.0

1.5

2.0

2.5

0 100 200 300 400 500 600

Abs

orba

nce

λ (nm)

0 100 200 300 400 500 6000,3

0,4

0,5

0,6

0,7

0,8

0,9

Abso

rban

ce

Time (sec.)

284 (nm.)

Fig. 3. Sulfathiazole degradation followed by fast kinetics in theFenton reaction. The inset shows the decay profile in the firstminutes at 284 nm.

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Oxidative degradation of sulfathiazole by Fenton reactions 665

0 20 40 60 80 100 1200.0

0.2

0.4

0.6

0.8

1.0

TO

C/T

OC

0

Time (min)

Fig. 4. STZ mineralization by Fenton (◦) and photo-Fenton (•)reactions.

aim of obtaining polynomials to predict the response ofthe system under study. These results match with the the-oretically calculated responses shown in columns 6 and 7,respectively. The slight difference with the respective exper-imental results shows that the model is correct in predictingSTZ degradation.

Polynomials, YF and YPF depicted in Eqs. (8) and (9)represent the percentage of STZ degradation after 8 minof reaction for Fenton and photo-Fenton, respectively. Thevalues shown in parentheses in the polynomials representthe standard deviation of each coefficient associated withthe variable. The cases in which the standard deviation isgreater than the coefficient value, it was eliminated from

0 10 20 30 40 50 60

-4

-2

0

2

4

AO

S

Time (min)

Fig. 5. Average oxidation state (AOS) of STZ solutions duringFenton (◦) and photo-Fenton (•) reactions.

the polynomial. Validation of these polynomials was per-formed by ANOVA (95% confidence). In the Fenton reac-tion the variable Fe (II) is the most significant since it isresponsible for catalyzing the decomposition of hydrogenperoxide. The increase of the H2O2 concentration also playsa positive effect on STZ oxidation.

YF = 69.6 (±3.2) + 20.3 [Fe(II)] (±1.9)+8.6[H2O2] (±1.9) − 9.3 [Fe(II)]2 (±2.3) (8)

YpF = 93.4 (±2.6) + 19.0[Fe(II)] (±1.6)

+9.8[H2O2] (±1.6) − 14.1[Fe(II)]2 (±1.9)−6.0[H2O2]2 (±1, 9) −7.1[Fe(II)H2O2] (±2.2) . (9)

A similar behavior is observed in the photo-Fenton re-action (Eq. (9)) where both variables positively improvethe reaction, being Fe(II) concentration the most signifi-cant. The negative first-order interaction between variablesobserved in the photo-Fenton reaction is indicative that avariable gain interferes negatively. This could be explainedbecause in the photo-Fenton reaction the production ofhydroxyl radical (•OH) increases, therefore increasing thequenching reactions promoted by Fe or peroxide, accord-ing to Eqs. (10) and (11).[39] Moreover, the second ordernegative coefficients for Fenton and photo-Fenton reac-tions predict maximum values for the variables Fe(II) andH2O2 where the reaction reaches a maximum degradationextent.

•OH + H2O2 → H2O + HO•2

k = (1.2 − 4.5) × 107 [Lmol−1s−1] (10)

•OH + Fe(II) → Fe(III) +− OHk = 3.2 × 108 − 4.3 × 108 [

Lmol−1s−1] (11)

These results allow us to make a three-dimensionalgraphic representation or response surface representing theSTZ degradation in the range of the variables studied (Figs.1a and 1b). It can be observed that degradation of STZ(47 µmol L−1) requires 192 µmol L−1 and 1856 µmol L−1

of Fe(II) and H2O2, respectively, achieving an 84% degra-dation in 8 min of reaction in the Fenton system (Fig. 1a).In comparison, at the same reaction time, a degradationof about 95% was reached in the photo-Fenton reactionusing 157 µmol L−1and 1219 µmol L−1of Fe(II) and H2O2,respectively (Fig. 1b). It is observed that the photo-Fentonreaction requires a lower concentration of reagents to ob-tain a higher degradation rate than the Fenton reaction.This is indicative that the photo-reduction of iron is crucialto increase the reaction yield. On the other hand it is im-portant to consider that the formation of carboxylic acidintermediates can increase yields of homogeneous catalysissince they form complexes with Fe(III) increasing photo-reduction to Fe(II) with visible light (Eq. (12)).[12] For exam-ple, in the presence of oxalic acid, Fe(III) forms a complexwhose quantum efficiency of generation of Fe(II) increases

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666 Velasquez et al.

0 5 10 15 20 25 30-10123456789

101112131415

Con

cent

ratio

n (m

gL-1

)

Time (min)

Fig. 6. Evolution of oxamic (�), succinic (◦), acetic (� ) andmaleic (�) acids in the Fenton reaction.

from 0.14 to 1.24 at 300–313 nm (Eqs. (13) and (14)).[40–41]

[Fe(III)(O2CR)]2+ + hν → Fe(II) + CO2 + R• (12)Fe(III)(C2O4)3−

3 + hν → Fe(II) + 2C2O2−4

+C2O•−4 (13)

Fe(III) (C2O4)3−3 + C2O•−

4 → Fe(II) + 3C2O2−4

+2CO2 (14)

STZ degradation profiles in Fenton and photo-Fentonreactions under optimized conditions were performed byHPLC-UV (Fig. 2). It can be seen that 90% of STZ wasremoved in 5 min in photo-Fenton reaction, whereas in theFenton reaction the same result was obtained after 10 min.Control reactions do not induce STZ degradation.

Degradation profiles

With the purpose of monitoring the onset of Fenton reac-tion in short time intervals, under optimized conditions forFe(II) and H2O2 concentrations the reaction was followedin a fast kinetics UV-Vis spectrophotometer. Figure 3 showsthe STZ absorption spectra, recorded at 0.5 s intervals. Therapid decay of the STZ absorption peak at 284 nm indi-cates that STZ oxidation begins almost instantaneously atthe very moment reactants are mixed.

The analysis of Chemical Oxygen Demand (COD) givesus information regarding the oxidation susceptibility of sul-fathiazole under optimized conditions of Fe2+ and H2O2.As seen in Fig. 4, the extent of mineralization in photo-Fenton reaction was greater than in Fenton reaction, asillustrated by 75% and 30% mineralization achieved byphoto-Fenton and Fenton reactions, respectively, after 60-min reaction.

Additionally, it was observed that in the photo-Fentonreaction COD declined faster, by about 90% after 15 min,

0 5 10 15 20 25 30-1

0

1

2

3

4

5

6

7

8

9

Con

cent

ratio

n (m

gL-1

)

Time (min)

Fig. 7. Evolution of oxamic (�), succinic (◦), acetic (� ), maleic(�) and oxalic (×) acids in the photo-Fenton reaction.

as compared with only 40% for the Fenton reaction (datanot shown). Although HPLC analysis shows that 90% STZdisappears in the photo-Fenton reaction after 5 minutes,COD and TOC reductions were less than 25%, indicatingthat reaction by-products were more refractory to oxida-tion and mineralization processes, respectively remaining insolution even after long irradiation times. To determine theextent of the oxidation process, the average oxidation state(AOS) was estimated, from COD and TOC concentrations,using Eq. (15):[42]

AOS = 4 × (TOC − COD)TOC

(15)

TOC and COD values are expressed in mole L−1 of car-bon and oxygen, respectively. Figure 5 shows AOS evolu-tion in both reaction systems. Clearly, photo-Fenton reac-tion reaches higher carbon oxidation states than Fenton

0 10 20 30 40 50 600.10

0.15

0.20

0.25

0.30

0.35

NH

4+ (mgL

-1)

Time (min)

Fig. 8. Ammonia evolution during the photo-Fenton reaction.

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Oxidative degradation of sulfathiazole by Fenton reactions 667

N

H

H

S

O

O

N

H

NS

OH OH OH

OH S N

O

O

H

NS

4- hydroxy-N-(1,3 thiazol) benzenesulfonamide

OH

-NH4+

OH

O

OH

O

Succinic acidMaleic acid

+OH

O

OH

O

O

NH2 O

OH

+

Oxamic acid

CO2

O OH

OH O

Oxalic acid

OH

CH3COOH

Acetic acid

OH

-NH4+

OHOH

OH

OH

Fig. 9. Possible route for the oxidation of sulfathiazole during photo-Fenton reaction.

reaction during the initial stages of reaction. After 1 h,both systems converge to AOS values around 3. Charac-teristic AOS reported in the literature for various oxidizedspecies range from -1 for benzene, zero for formaldehydeand acetic acid, +2 for formic and oxalic acids, to +3 foroxamic acid.[18,43,44] Results reported in Figure 5 indicate

that after 30 min reaction products are carboxylic acidswith oxidation states between +2 and +3.5 for Fenton andphoto-Fenton reactions, respectively.

Indeed, as seen in Fig. 6 and 7, maleic, oxamic, suc-cinic and acetic acids were identified as STZ oxidation by-products, using ion exclusion chromatography. The time

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evolution of these acids during the course of the Fentonand photo Fenton reactions are shown in Figures 6 and7, respectively. Interestingly, oxalic acid was only found inthe photo-Fenton reaction system. However, acetic acidconcentrations were quite low as compared to other acids,peaking at 0.76 mg L−1 at 2 min, and totally disappearingafter 8 min. In the Fenton system, oxamic acid concen-tration reached 5.3 mg L−1 after 15 min, and remainedconstant afterwards, showing a high stability in the Fentonsystem. Succinic and maleic acids concentrations peakedat 13.5 mg L−1and 4.6 mg L−1, respectively, at 2 min,and then decreased sharply to stabilize at low levels after12 min.

On the other hand, in the photo-Fenton system, oxamicacid reached a peak concentration of around 5 mg L−1

at 6 min, decreasing steadily at longer irradiation times.Apparently, maleic and succinic acids, formed during theearly stages, come from the direct opening of benzenerings and oxamic acid from the oxidation of the thiazolring.

As indicated previously (Eqs. (12)–(14)), the formationof oxalate-iron complexes increases significantly the reduc-tion of Fe(III) enhancing the production of hydroxyl rad-icals and promoting in consequence the mineralization toCO2.[45]

Mineralization was further confirmed following the am-monia (NH4

+) concentration. This compound was foundonly in the photo-Fenton reaction. As seen in Fig. 8, themaximum ammonia concentration occurred at 60 min,which is consistent with the decrease in oxamic acid con-centration in the photo-Fenton system. This result wouldsuggest that hydroxyl radicals attacked the amino groupin oxamic acid thereby generating NH4

+. This is reflectedin the decrease in oxamic acid concentration during thecourse of photo-Fenton reaction and the generation of ox-alic acid. The maximum detected NH4

+concentration was0.32 mg L−1, which corresponds to 16% of the initial N.On the other hand the oxamic acid remaining at 60 minaccount for only 11% of the initial N and no nitrates weredetected in solution, suggesting the formation of nitrogenvolatile species as NOx or N2.

Reaction pathway

Results obtained by GC-MS in the photo-Fenton reactionwould suggest that hydroxyl radicals substitute the aminogroup in the benzene ring forming a phenol derivative(m/z = 328, rt = 6.7 min), after 30 s. This compoundwas identified as their thrimethylsilyl derivative by GC-MSobtained by reaction with BSTFA.

Based on GC-MS, HPLC-ion exclusion and NH4+ de-

terminations, we propose the oxidation pathway for STZdegradation by the photo-Fenton reaction illustrated inFig. 9. STZ degradation would start with the attack of •OHradicals to benzene ring, with the formation of 4-hydroxy-N-(1,3-thiazol)-benzenesulfonamide, releasing NH4

+ in

solution. In turn, this compound would initiate the ringopening generating a complex mixture of carboxylic anddicarboxylic acids. In the case of photo-Fenton reaction,oxamic acid under hydroxyl attack would promote the for-mation of oxalic acid and ammonia. Additionally, it is pos-sible that oxalic acid would come from decarboxylationof succinic, acetic and maleic acids, explaining why thoseacids were found at lower concentrations than in the Fentonsystem.

Conclusions

STZ could be degraded more efficiently by photo-Fentonreaction using fewer reagents and reaching higher degrada-tion rates than Fenton reaction. Thus, multivariate designis an adequate tool to find optimal concentrations of Fe(II)and H2O2. Sulfathiazole disappears completely from solu-tion after 5 minutes in the photo-Fenton reaction, twiceas faster than in Fenton system. This improvement in ef-ficiency results from the Fe2+ photo-regeneration and theformation of Fe(III) and Fe(II) complexes with carboxylicacids, especially for the formation of complexes with ox-alate. Although both processes fail to remove completelythe intermediate products by conversion to CO2, photo-Fenton reaction proved to be more effective, with 75% TOCreduction in 60 min.

In the photo-Fenton system, oxamic acid decay to formoxalic acid and ammonia due to hydroxyl radical attack. Incontrast, in the Fenton reaction oxamic acid remains stableafter long periods of reaction.

Funding

This work has been developed under financial supportof Fondecyt (Grants 1080230, 1110641 and 1130502),Anillo PIA (ACT-130) and Conicyt/FONDAP/15110019Projects.

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