Chemical Engineering Journal 263 (2015) 336–345

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Chemical Engineering Journal

journal homepage: www.elsevier .com/locate /ce j

Predicting pharmaceutical degradation by UV (MP)/H2O2 processes:A kinetic model

http://dx.doi.org/10.1016/j.cej.2014.10.1011385-8947/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author at: KWR Watercycle Research Institute, Groningenhaven7, 3430 BB Nieuwegein, The Netherlands. Tel.: +31 (0)30 60 69 604; fax: +31 (0)3060 61 165.

E-mail address: bas.wols@kwrwater.nl (B.A. Wols).

B.A. Wols a,b,c,⇑, D.J.H. Harmsen a, E.F. Beerendonk a, C.H.M. Hofman-Caris a

a KWR Watercycle Research Institute, Groningenhaven 7, 3430 BB Nieuwegein, The Netherlandsb Delft University of Technology, The Netherlandsc Wetsus, Centre for Sustainable Water Technology, P.O. Box 113, Leeuwarden 8900 CC, The Netherlands

h i g h l i g h t s

� (Photo)kinetic model of polychromatic (MP) UV advanced oxidation processes.� Degradation pathways via direct photolysis, OH radical reactions and carbonate radical reactions.� Validation of model with large group of pharmaceuticals.� Validation of model in different water matrices.� Sensitivity analysis for the chemical reactions involved, including nitrate reactions.

a r t i c l e i n f o

Article history:Received 21 August 2014Received in revised form 27 October 2014Accepted 29 October 2014Available online 13 November 2014

Keywords:UVAdvanced oxidation processHydrogen peroxidePharmaceuticalsWater treatmentModelling

a b s t r a c t

A kinetic model for the degradation of organic micropollutants (OMPs) by MP UV lamps has been devel-oped. The model includes (photo)chemical reactions of hydrogen peroxide and nitrate; reactions with themost important water matrix components; changes in pH; and transient radical concentrations. Threepathways of organic micropollutant degradation are involved: direct photolysis, hydroxyl radical reac-tions and carbonate radical reactions. The model was validated using collimated beam experiments fora group of 35 pharmaceuticals in synthetic and natural water matrices. For MP UV with 10 mg/L H2O2,good agreement between the modelled and measured degradation rates was found. For MP UV withoutH2O2, most of the pharmaceuticals were well predicted, except for the sulphonamides. A sensitivity anal-ysis on different water matrix constituents was performed for MP UV with H2O2, NO�3 and both H2O2 andNO�3 . Also, the most important chemical reactions for the degradation of OMPs are identified for differentwater matrices.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

The increasing amount and concentrations of organic micropol-lutants (OMPs) found in water sources is a major concern [23] andputs high demands on the treatment of (waste)water streams. UVadvanced oxidation processes (AOPs) are known to be effective bar-riers against OMPs [43,47]. Applications of UV/H2O2 can be found intreatment of wastewater, purification of drinking water and treat-ment of different industrial (waste)water streams. Monochromaticlow-pressure (LP) and polychromatic medium-pressure (MP) UVlamps are most often used as UV sources. Both types of lamps are

used in AOP systems. Examples of the use of MP UV lamps in effec-tively removing OMPs can be found in [36,32,22,48,21,40]. Com-pared to LP lamps, the direct photolysis of OMPs is more effectivesince the MP lamp covers more wavelengths in the UVC region thanthe 254 nm LP UV lamp, and more photons can be absorbed at lowerwavelengths. Also the photolysis of hydrogen peroxide, responsiblefor the formation of highly reactive hydroxyl radicals (�OH), is moreeffective at wavelengths between 200 and 240 nm (although thewater absorption is also higher).

Many kinetic models for mainly LP UV conditions have beenproposed [13,11,26,41,1,16,46]. For MP UV conditions, the modelsneed to capture the polychromatic UV radiation distribution. Innitrate (NO�3 ) containing waters, also photolysis of both nitrateand nitrite (NO�2 ) needs to be incorporated. The importance ofnitrate in the (photo)chemistry of polychromatic UV sources hasbeen stipulated by [25,24,37–39,50,19]. The formation of radicals

Fig. 1. Molar absorptions, quantum yields, �OH and CO��3 reaction rate constants of the organic micropollutants.

B.A. Wols et al. / Chemical Engineering Journal 263 (2015) 336–345 337

Table 1Water matrices used in the experiments for model validation.

Water matrix HCO�3 NO�3 NPOCa pH H2O2 Irr.b

mg/L mg NO�3 /L mg C/L mg/L mW/cm2

MilliQ+NO�3 – 20.5 0 – 0 2.545MilliQ+NO�3 – 20.2 0 – 10 2.471NWG 281 1.1 2.0 8.2 0 2.375NWG 279 1.0 2.0 8.1 10 2.429Meuse 175 17.1 4.0 8.0 0 2.346Meuse 174 16.8 4.0 8.0 10 2.453

a Non-purgable organic carbon.b Irradiance (summed over 200–300 nm) entering the water.

Fig. 2. Wavelength dependent variables: (upper panel) lamp output (solid line) andsensor sensitivity (relative towards 254 nm), (middle panel) molar absorption (solidlines) and quantum yield (dash-dotted lines) of H2O2, NO�3 and NO�2 and (lowerpanel) absorptions of the different water matrices without (solid lines) and withH2O2 (dash-dotted lines).

338 B.A. Wols et al. / Chemical Engineering Journal 263 (2015) 336–345

during the photolysis of nitrate becomes substantial due to highabsorption of nitrate below wavelengths of 240 nm.

Modelling photolysis of nitrate by MP UV lamps has been pro-posed using steady-state hydroxyl radical reactions [36,34,19]. Sofar, no extensive (photo)kinetic models have been developed forpolychromatic UV lamps, that include changes in pH, transient rad-ical concentrations and nitrate reactions. In this work, we elabo-rated on a kinetic model developed for LP UV lamps [46] andincorporated the polychromatic photolytic reactions in the model.The model includes a complex reaction scheme caused by the rad-ical reactions after photolysis of nitrate and hydrogen peroxidewithout the use of quasi steady-state conditions. Degradation ofOMPs occurs via direct photolysis, hydroxyl radical (�OH) reactionsand carbonate radical (CO��3 ) reactions. The model is validated for alarge number of pharmaceuticals in different synthetic and naturalwater matrices.

2. Materials and methods

2.1. Experimental conditions

Experiments were performed in a collimated beam apparatus. Apolychromatic (MP) lamp (Philips HOK 2 kW) was installed (lampoutput is shown in Fig. 2, upper panel). Water samples were irra-diated in a 100 mL Petri-dish (optical path length of 1.61 cm) andthe distance from the lamp to the Petri-dish was 80 cm. The lampoutput was measured using a UV sensor (IL 1700 radiometerSED240w), the spectral sensitivity is shown in Fig. 2 (upper panel).The UV dose, summed over the 200–300 nm region, was calculatedfrom the Bolton spreadsheet [4]. A large group of 35 pharmaceuti-cals was used in different water types for model calibration andvalidation. The pharmaceuticals, previously reported in [48], areshown in Fig. 1. More details on these pharmaceuticals, analysismethod and limits of detection can be found in [48] and in the sup-porting information. UV absorbance of water samples was mea-sured by a spectrophotometer (Thermo Spectronic UnicamUV500) at a path length of 1 cm over the wavelengths 200–300 nm. Molar absorptions were also determined by spectropho-tometry. Each compound was dissolved separately in MilliQ water.Since the molar absorption may vary largely over the 200–300 nmregion, three dilutions were made (targeted concentration of:1 mg/L, 0.1 mg/L and 0.01 mg/L). The absorbance of each dilutionwas measured in triplicates by spectrophotometry at a path lengthof 10 cm. Only the measured absorbances (as a function of wave-length) of each dilution that were in the accurate range of the spec-trophotometer (range of absorbances between 0.01 and 6) werekept. The molar extinction was calculated for each dilution bydividing the measured absorbances with the analysed concentra-tion of OMPs and path length of 10 cm. Finally, at each wavelengththe measured values were averaged to obtain the molar extinctionat the 200–300 nm region for each OMP (Fig. 1).

An overview of the water types is shown in Table 1. The experi-ments in NWG (tap water from the city of Nieuwegein) and Meuse

water were reported before in [48]. A large group of pharmaceuti-cals was spiked at concentrations between 1 and 10 lg/L (spikedconcentration of the pharmaceuticals can be found in [48]. The rel-evant kinetic parameters (molar absorption, quantum yield(254 nm and averaged over 200–300 nm), �OH and CO��3 reactionrate constant) of the pharmaceuticals are shown in Fig. 2. Quantumyields over the 200–300 nm region were fitted from the previously

Table 2Reaction scheme used in the kinetic modelling. The question mark indicates that the produced compounds are unknown.

Reactions Rate constants (M�1 s�1) References

1 H2O2 + hv ! 2 �OH Fig. 2 [44]2 NO�3 + hv ! ONOO� Fig. 2 [25]3 NO�3 þ H2Oþ hv ! NO�2 þ OH� þ OH� Fig. 2 [15]4 NO�2 þ H2Oþ hv ! NO� þ OH� þ OH� Fig. 2 [15]5 Mi + hv ! ? Fig. 1 lit./meas.

1 H2O2 () HO�2 + H+ (pKa = 11.6) 1 � 1010;2:51 � 10�2 [31]

2 HO�2 () O��2 + H+ (pKa = 4.8) 1 � 1010;1:58 � 105 [31]

3 H2CO3 () HCO�3 + H+ (pKa = 6.35) 1 � 1010;4:5 � 103 [26]

4 HCO�3 () CO2�3 + H+ (pKa = 10.35) 1 � 1010;4:5 � 10�1 [26]

5 H2O () OH� + H+ (pKa = 16.0 )⁄ 1 � 1010;1:14 � 10�6

6 ONOOH () ONOO� + H+ (pKa = 6.6) 1 � 1010;2:5 � 103 [10]

7 ONOO� () NO� + O��2 0:017s�1;5:0 � 109 [10]

8 NO�2 þ NO� () N2O3 1:1 � 109;8:4 � 104 s�1 [10]

9 2NO�2 () N2O4 4:5 � 108;6:9 � 103 s�1 [10]

1 �OH + HO�2 ! HO�2 + OH� 7:5 � 109 [9]

2 �OH + O��2 ! O2 + OH� 7:0 � 109 [2]

3 �OH + HO�2 ! H2O + O2 6:6 � 109 [35]

4 �OH + �OH ! H2O2 5:5 � 109 [6]

5 �OH + H2O2 ! HO�2 + H2O 2:7 � 107 [6]

6 HO�2 + O��2 ! HO�2 + O2 9:7 � 107 [3]

7 HO�2 + HO�2 ! H2O2 + O2 8:3 � 105 [3]

8 HO�2 + H2O2 ! �OH + H2O + O2 3.0 [20]9 O��2 + H2O2 ! �OH + OH� + O2 0.13 [45]10 �OH + CO2�

3 ! CO��3 + OH� 3:9 � 108 [6]

11 �OH + HCO�3 ! CO��3 + H2O 8:5 � 106 [6]

12 CO��3 + HO�2 ! CO2�3 + HO�2 3:0 � 107 [12]

13 CO��3 + H2O2 ! HCO�3 + HO�2 8 � 105 [30]

14 CO��3 + OH� ! products 3:0 � 109 [11]

15 CO��3 + O��2 ! CO2�3 + O2 6:5 � 108 [30]

16 CO��3 + CO��3 ! products 3:0 � 107 [17]

17 �OH + DOC ! ? 2:0 � 108 [18]

18 CO��3 + DOC ! ? 3:4 � 106 [8]

19 ONOOH! 0:7HNO3 þ 0:3OH� þ 0:3NO�2 0:8 s�1 [10]20 N2O3 þ H2O! 2Hþ þ 2NO�2 2 � 103 s�1 [10]

21 N2O4 þ H2O! 2Hþ þ NO�2 þ NO�3 1 � 103 s�1 [10]

22 N2O3 þ ONOO� ! NO�2 þ 2NO�2 3 � 108 [10]

23 OH� þNO�2 ! NO�2 þ OH� 1 � 1010 [10]

24 NO�2 þ O��2 ! O2 + NO�2 4:5 � 109 [10]

25 NO�2 þ HO�2 ! O2 + NO�2 + H+1:8 � 109 [10]

26 ONOO� + �OH ! O��2 + NO�2 + H+4:8 � 109 [10]

27 CO��3 + NO�2 ! NO�3 + CO2 1�109 [14]

28 Mi + �OH ! x DOC + ? kOH lit./meas.29 Mi + CO��3 ! x DOC + ? kCO3

lit./meas.

⁄ Calculated from a pKw of 14.2 (at 20 �C) after correction with concentration of H2O (55.6 mol/L).

B.A. Wols et al. / Chemical Engineering Journal 263 (2015) 336–345 339

reported experimental results in MilliQ water with MP lamps, theother kinetic parameters were reported before in [46]. Additionalexperiments were performed for the validation of the nitrate reac-tions in the model. Therefore, water samples of MilliQ water (Milli-pore Academic A10) with 20 mg/L NO�3 were irradiated in thecollimated beam. The experiments were conducted without andwith 10 mg/L H2O2. UV doses of 0, 20, 50, 100 and 200 mJ/cm2 wereapplied in duplo. The values of the overall fluence-based degrada-tion rate constants (ktot) were determined from the degradation(C=C0) as an exponential function of fluence, obtained by fitting ofthe measured data points (minimising the least-squares errors).

2.2. Model

The model described in [46] for LP UV lamps forms the basis ofthe kinetic model for MP UV lamps. The model is summarisedbelow, more details can be found in [46].

2.2.1. PhotolysisThe spectral distribution needs to be incorporated in the model.

Photons of different wavelengths are emitted by the MP UV lampand these can be absorbed by compounds in the water. Theabsorbed photons may result in a reaction, the ratio of absorbedphotons resulting in a reaction is defined as the quantum yield.Both absorption and quantum yield are wavelength dependent.The photolysis rate equation of compound M in a small volumeof water [49,27] comprises a summation over all wavelengths:

d½M�dt¼ � lnð10Þ

Xk

UMðkÞeMðkÞ½M�E0pðk; xÞ ð1Þ

where UMðkÞ represents the wavelength dependent quantum yield[mol/einstein], eMðkÞ the wavelength dependent molar absorptioncoefficient of the compound [m2/mol], and E0pðk; xÞ the fluence ratein a small water volume at a certain wavelength [Einstein/m2/s]at position x. Often an averaged quantum yield over the 200–300 nm region is used, since the wavelength dependent quantum

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Fig. 3. Measured versus modelled fluence-based reaction rate (ktot) of pharmaceuticals in MilliQ water with 20 mg/L NO�3 , without H2O2 (upper panel) and with 10 mg/L H2O2

(lower panel). The R2 shows the overall coefficient of determination of all pharmaceuticals. The measured and modelled degradation as a function of UV dose is shown in thesupporting information for each pharmaceutical.

340 B.A. Wols et al. / Chemical Engineering Journal 263 (2015) 336–345

yield is difficult to measure and its variation over the wavelength isnot as large as for the molar absorption coefficient. In the collimatedbeam the fluence rate is also wavelength dependent and can bewritten as a function of depth (z):

E0pðk; zÞ ¼ E0pðk;0Þ10�aðkÞz Lp

Lp þ z

� �2

ð2Þ

where Lp is the distance from the lamp to the water surface, aðkÞ thewavelength dependent absorption of the water, and E0pðk;0Þ the flu-ence rate entering the water at a certain wavelength. The fluencerate entering the water is determined from the radiometer reading(which is a summation over all wavelengths). The radiometer signalis also corrected for the reflectance at the free surface (reflectionfactor), radiation distribution over the surface area of the Petri dish(Petri factor) and sensor sensitivity at different wavelengths (sensorfactor, Fig. 2), see [4]. The radiometer reading is redistributed overthe wavelengths using the relative photon flow (determined fromthe spectral power distribution of the MP UV lamp, Fig. 2). Sincethe water in the Petri dish is stirred, the depth-averaged fluencerate is calculated by (numerically) averaging of Eq. (2). The total flu-ence in the MP system is obtained by summation over the 200–300 nm wavelength region.

2.2.2. Advanced oxidationHighly reactive radicals are formed when H2O2 or NO�3 is pho-

tolysed, resulting in a complex reaction scheme (Table 2). Thewavelength dependent quantum yield for NO�3 and NO�2 wasobtained from [15], whereas for H2O2 the 254 nm quantum yield

of 0.5 was used for all wavelengths [44]. The wavelength depen-dent molar absorptions were obtained from [28,24].

2.2.3. ImplementationFrom the reactions in Table 2, rate equations are constructed for

each compound, resulting in a system of ordinary differential equa-tions that are solved simultaneously as a function of time (the sto-chiometrics matrix derived from Table 2 is shown in thesupporting information). Initial conditions were set according tothe concentrations of the compounds in the water matrix. For com-parison with the measured data, the same UV doses as in theexperiments were simulated, and the degradation rate constantswere determined in the same way.

2.2.4. Sensitivity studyThe degradation of pCBA was used to perform a sensitivity

analysis on the different water matrix components. Syntheticwater matrices were simulated using different combinations ofMilliQ with H2O2, NO�3 , HCO�3 and DOC. Concentrations were cho-sen in the same order of magnitude as they occur in natural watertypes (a DOC concentration of 2.5 mg/L, a HCO�3 concentration of150 mg/L and a NO�3 concentration of 20 mg/L). Also a concentra-tion of 3 mg/L HCO�3 is used, representing MilliQ with CO2 dis-solved from the air. For H2O2, a commonly used value of 10 mg/L is used (similar as in the experiments). The kinetic parametersfor pCBA are shown in Fig. 1. The molar absorptions was obtainedfrom [42], quantum yield from [33], �OH reaction rate constantfrom [29] and CO��3 reaction rate constant from [46]. In thesensitivity study, a similar set-up as in the collimated beam

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Fig. 4. Measured versus modelled fluence-based reaction rate (ktot) of pharmaceuticals in NWG water, without H2O2 (upper panel) and with 10 mg/L H2O2 (lower panel). TheR2 shows the overall coefficient of determination of all pharmaceuticals. The measured and modelled degradation as a function of UV dose is shown in the supportinginformation for each pharmaceutical.

B.A. Wols et al. / Chemical Engineering Journal 263 (2015) 336–345 341

experiments was simulated. For the water matrices with NO�3 andboth NO�3 and H2O2, also a more in-depth study is performed con-sidering the contribution of each chemical reaction. The relativecontribution of each reaction on the pCBA degradation is visual-ised. For each reaction, the percentual difference in degradationrate is shown when this particular reaction would be omitted(positive change means that this reaction hampers pCBA degrada-tion, whereas a negative change means that this reaction pro-motes pCBA degradation).

3. Results

3.1. Validation for nitrate reactions

The model was validated for a synthetic water matrix consistingof MilliQ with 20 mg/L NO�3 . The measured and modelled overallfluence-based degradation rate constants are shown in Fig. 3 with-out and with the addition of 10 mg/L H2O2, the measured and mod-elled degradation as a function of fluence is shown in thesupporting information. Degradation rate constants in water with-out H2O2 are substantial higher than with H2O2, both in the mea-sured data and in the modelled data. In general there is a goodagreement between measured and modelled data, although a smalloverestimation of the predicted degradation rate constants can beobserved for almost all compounds.

3.2. Validation for natural water matrix

Validations in natural water matrices were performed forMeuse water and NWG tap water (Table 1). In NWG water without

H2O2, some compounds are overestimated by the model (Fig. 4).Paroxetine and the sulphonamides were most pronouncedly over-estimated by the model. A better agreement was found when H2O2

was added. All compounds, except paroxetine and the sulphona-mides again, showed a good agreement here (sotalol could notbe accurately measured here).

In Meuse water without H2O2 (Fig. 5), the model, contrary to theNWG tap water, predicted smaller degradations than measured forsome specific compounds. Degradations of clenbuterol, diclofenac,prednisolone, propranolol, sotalol and the sulphonamides weresubstantial lower in the model. When H2O2 was added, the pre-dicted degradation rates were more in line with the measured data,although the compounds that were underestimated during photol-ysis remained slightly underestimated.

3.3. Sensitivity study

pCBA degradation in MilliQ water (MQ) with NO�3 , H2O2 and bothof them was simulated for different concentrations of HCO�3 /CO2�

3

and DOC (Fig. 6). Considering MilliQ without NO�3 or H2O2 therewas only direct photolysis resulting in small degradation rates.HCO�3 /CO2�

3 and DOC had no influence on the degradation here. InMilliQ with NO�3 ;

�OH are formed upon the photolysis of NO�3 result-ing in substantial degradation rates. Addition of HCO�3 /CO2�

3 resultsin a largely increased degradation rate, whereas addition of DOC orboth DOC and HCO�3 /CO2�

3 results in reduced degradation rates. Con-sidering the contribution of each reaction, Fig. 7 (upper panel), thisfirst photolysis reaction of NO�3 has a negative contribution to thedegradation of pCBA, since ONOO- may scavenge �OH (reaction26), whereas the second photolysis reaction of NO�3 has a positive

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Fig. 5. Measured versus modelled fluence-based reaction rate (ktot) of pharmaceuticals in Meuse water, without H2O2 (upper panel) and with 10 mg/L H2O2 (lower panel). TheR2 shows the overall coefficient of determination of all pharmaceuticals. The measured and modelled degradation as a function of UV dose is shown in the supportinginformation for each pharmaceutical.

MQ MQ+NO3 MQ+H2O2 MQ+NO3+H2O20

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m2 /m

J]

HCO3−=3 mg/L, DOC=0 mg/L

HCO3−=150 mg/L, DOC=0 mg/L

HCO3−=3 mg/L, DOC=2.5 mg/L

HCO3−=150 mg/L, DOC=2.5 mg/L

Fig. 6. Modelled degradation rate of pCBA for different water types: MilliQ (MQ),MilliQ with 20 mg/L NO�3 (MQ+NO3), MilliQ with 10 mg/L H2O2 (MQ+H2O2), MilliQwith 20 mg/L NO�3 and 10 mg/L H2O2 (MQ+NO3+H2O2).

342 B.A. Wols et al. / Chemical Engineering Journal 263 (2015) 336–345

contribution as it forms directly �OH. The main degradation path-ways of pCBA are direct photolysis (reaction 5) and reaction with�OH (reaction 28) and with CO��3 (reaction 29). When HCO�3 /CO2�

3 isadded to the water, remarkably the �OH scavenging reactions withHCO�3 /CO2�

3 have a positive contribution to the degradation of pCBA(reaction 10 and 11) due to the formation of CO��3 . Since there is littlecompetition for CO��3 reactions in this water type, degradation ofpCBA by CO��3 becomes substantial. This is further explained in thediscussions section (Eq. (4)). However, when DOC is also presentin the water, this CO��3 degradation pathway is cut off, because theDOC scavenges the CO��3 (reaction 18). In MilliQ with H2O2 (Fig. 6),the highest degradation occurs in the absence of scavengers likeHCO�3 /CO2�

3 or DOC. When these compounds are added, the degrada-tion rates decrease. In MilliQ with both H2O2 and NO�3 ;

�OH areformed by photolysis of both compounds. As can be seen from theindividual reactions (Fig. 7, lower panel), H2O2 is also an importantscavenger for both �OH (reaction 5) and CO��3 (reaction 13). The majorincrease in pCBA degradation by adding HCO�3 /CO2�

3 in MilliQ withonly NO�3 disappears, since the carbonate radicals are scavengedby H2O2. The overall effect of nitrate in the UV/H2O2 process resultsin a bit lower degradation rates, caused by the formation of hydroxylradical scavengers such as nitrite and peroxynitrite.

4. Discussions

4.1. Model validation

The good agreement between measured and modelled data inMilliQ with NO�3 showed that the photolysis of nitrate and

consecutive nitrate reactions were well captured by the model.Also, the substantial higher degradation without H2O2 is well pre-dicted by the model. This higher degradation can be explained byhydroxyl radical scavenging by H2O2. Although the photolysis ofH2O2 will also result in the formation of �OH, the total �OH scaveng-ing capacity of the water is much more increased by the addition ofH2O2, so that the �OH concentrations become lower. The modelled�OH concentrations (time averaged during irradiation) indeed

−100 −50 0 250 5008, [H2O2] [HO2.]

7, [HO2.]

6, [O2.−] [HO2.]

5, [H2O2] [OH.]

4, [OH.]

3, [OH.] [HO2.]

2, [OH.] [O2.−]

1, [HO2−] [OH.]

9, [NO2.]<−> [N2O4]

8, [NO2.] [NO.]<−> [N2O3]

7, [ONOO−]<−> [O2.−] [NO.]

6, [ONOOH]<−> [H+] [ONOO−]

5, [H2O]<−> [H+] [OH−]

4, [HCO3−]<−> [CO3−2] [H+]

3, [H2CO3]<−> [HCO3−] [H+]

2, [HO2.]<−> [O2.−] [H+]

1, [H2O2]<−> [HO2−] [H+]

5, [pCBA] + hv

4, [NO2−] + hv

3, [NO3−] + hv

2, [NO3−] + hv

1, [H2O2] + hv

percent−100 −50 0 250 500

29, [CO3.−] [pCBA]

28, [OH.] [pCBA]

27, [CO3.−] [NO2.]

26, [OH.] [ONOO−]

25, [HO2.] [NO2.]

24, [O2.−] [NO2.]

23, [OH.] [NO2−]

22, [ONOO−] [N2O3]

21, [H2O] [N2O4]

20, [H2O] [N2O3]

19, [ONOOH]

18, [CO3.−] [DOC]

17, [OH.] [DOC]

16, [CO3.−]

15, [O2.−] [CO3.−]

14, [OH.] [CO3.−]

13, [H2O2] [CO3.−]

12, [HO2−] [CO3.−]

11, [OH.] [HCO3−]

10, [OH.] [CO3−2]

9, [H2O2] [O2.−]

percent

pH=8, H2O2=0mg/L, NO3−=20mg/L, HCO3

−=3mg/L, DOC=0mg/L

pH=8, H2O2=0mg/L, NO3−=20mg/L, HCO3

−=150mg/L, DOC=0mg/L

pH=8, H2O2=0mg/L, NO3−=20mg/L, HCO3

−=3mg/L, DOC=2.5mg/L

pH=8, H2O2=0mg/L, NO3−=20mg/L, HCO3

−=150mg/L, DOC=2.5mg/L

−100 −50 0 250 5008, [H2O2] [HO2.]

7, [HO2.]

6, [O2.−] [HO2.]

5, [H2O2] [OH.]

4, [OH.]

3, [OH.] [HO2.]

2, [OH.] [O2.−]

1, [HO2−] [OH.]

9, [NO2.]<−> [N2O4]

8, [NO2.] [NO.]<−> [N2O3]

7, [ONOO−]<−> [O2.−] [NO.]

6, [ONOOH]<−> [H+] [ONOO−]

5, [H2O]<−> [H+] [OH−]

4, [HCO3−]<−> [CO3−2] [H+]

3, [H2CO3]<−> [HCO3−] [H+]

2, [HO2.]<−> [O2.−] [H+]

1, [H2O2]<−> [HO2−] [H+]

5, [pCBA] + hv

4, [NO2−] + hv

3, [NO3−] + hv

2, [NO3−] + hv

1, [H2O2] + hv

percent−100 −50 0 250 500

29, [CO3.−] [pCBA]

28, [OH.] [pCBA]

27, [CO3.−] [NO2.]

26, [OH.] [ONOO−]

25, [HO2.] [NO2.]

24, [O2.−] [NO2.]

23, [OH.] [NO2−]

22, [ONOO−] [N2O3]

21, [H2O] [N2O4]

20, [H2O] [N2O3]

19, [ONOOH]

18, [CO3.−] [DOC]

17, [OH.] [DOC]

16, [CO3.−]

15, [O2.−] [CO3.−]

14, [OH.] [CO3.−]

13, [H2O2] [CO3.−]

12, [HO2−] [CO3.−]

11, [OH.] [HCO3−]

10, [OH.] [CO3−2]

9, [H2O2] [O2.−]

percent

pH=8, H2O2=10mg/L, NO3−=20mg/L, HCO3

−=3mg/L, DOC=0mg/L

pH=8, H2O2=10mg/L, NO3−=20mg/L, HCO3

−=150mg/L, DOC=0mg/L

pH=8, H2O2=10mg/L, NO3−=20mg/L, HCO3

−=3mg/L, DOC=2.5mg/L

pH=8, H2O2=10mg/L, NO3−=20mg/L, HCO3

−=150mg/L, DOC=2.5mg/L

Fig. 7. Sensitivity of each reaction to the degradation of pCBA. For each reaction, the (percentage) difference in log degradation of pCBA if this reaction would be omitted isshown. The colours show MilliQ water with different concentrations of background components.

B.A. Wols et al. / Chemical Engineering Journal 263 (2015) 336–345 343

344 B.A. Wols et al. / Chemical Engineering Journal 263 (2015) 336–345

reduce from 6:4 � 10�12 M to 4:8 � 10�12 M when adding H2O2. Theeffect is dependent on the H2O2 concentration, but at other concen-trations also a reduction is observed: at H2O2 concentrations of1 mg/L and 3 mg/L the averaged OH radical concentrations become5:7 � 10�12 M and 5:1 � 10�12 M, respectively. A similar effect wasobserved by [38], where the production of NO�2 is increased whenH2O2 is added, indicating that less OH radicals are available toreduce NO�2 . Moreover, we also observed that during the photolysisof NO�3 , NO�2 is being formed which is a strong hydroxyl radicalscavenger, so that the concentration of hydroxyl radicals substan-tially reduces during the process resulting in smaller degradationrates of pCBA. As a result of NO�2 production, there is a point whereadding H2O2 becomes beneficial, but the overall effect remainednegative at the UV doses applied here, as the averaged OH radicalconcentrations already indicated. This shows that using a steady-state �OH concentration may lead to inaccurate results.

In natural waters, model predictions with H2O2 in generalshowed a good agreement with measured data. The formation ofhydroxyl radicals by the photolysis of H2O2 seems to be well pre-dicted, as well as the scavenging of hydroxyl radicals by back-ground components. Without H2O2, more differences betweenmodel and measurements became apparent. Underpredictionswere found in Meuse water, whereas overpredictions were simu-lated in NWG water. Both mispredictions occured simultaneouslyfor the compounds clenbuterol, diclofenac, furosemide, and thesulphonamides. The overpredictions may be caused by inaccura-cies in the direct photolysis modelling, related to pH effects andthe use of an average quantum yield. The quantum yields andmolar absorptions were determined in MilliQ with a pH of 6,whereas the pH in NWG and Meuse water is around 8. The othercause, using an average quantum yield, may result in small inaccu-racies, as the available photons per wavelength (Epðk; zÞ in equa-tion) may change over the depth due to water absorption. Thesubstantial higher degradation rates measured in Meuse waterwithout H2O2 point at another pathway of degradation than directphotolysis. Since the differences were only found for some partic-ular compounds, the higher degradation in the measured datamust be related to other radicals than the aselective hydroxyl rad-icals. Main differences in the water matrix between NWG tapwater and Meuse water are the concentrations of HCO�3 /CO2�

3 ,DOC and NO�3 . Nitrate photolysis mainly produces �OH, which iswell predicted by the model. (Bi)carbonate (HCO�3 /CO2�

3 ) reactingwith �OH results in CO��3 , that was also captured in the model[46]. So, photolysis of DOC remains as a possible explanation. Asindicated by [7], photolysis of DOC may result in excited tripletstates, 3DOM⁄, that may inhibit degradation of OMPs. The reactionsof these 3DOM⁄ complexes are more selective than those with �OH,and the reactivity also depends on the DOC composition. Sulfon-amide antibiotics that showed higher measured degradation ratesin Meuse water are known to be sensitive to 3DOM⁄ [5,7].

4.2. Involvement of carbonate radicals

The sensitivity study shows the importance of the carbonateradical. The involvement of the carbonate radical can also beunderstood by considering a steady-state carbonate radicalconcentration:

½CO�3�ss ¼k10½OH��½CO2�

3 � þ k11½OH��½HCO�3 �Pkj½Sj�

ð3Þ

where kj½Sj� represents all the scavenging reactions with CO��3 , andk10 and k11 represent the reaction rate constants of �OH withHCO�3 /CO2�

3 (reaction 10 and 11 in Table 2). Next to photolysis, pCBAis degraded by hydroxyl and carbonate radicals. The ratio of thesedegradation pathways can be written as:

rCO3=OH ¼kCO�3 ½CO�3�kOH½OH�� ¼

k10½CO2�3 � þ k11½HCO�3 �P

kj½Sj�kCO�3

kOHð4Þ

This ratio is independent of the hydroxyl radical concentration.

As the ratiokCO�

3kOH

is usually smaller than 0.01, the �OH degradation

pathway will be most dominant. However, when there are largeconcentrations of HCO�3 /CO2�

3 and few carbonate radical scaveng-ers (kj½Sj�), the ratio rCO3=OH increases and the carbonate radicalpathway may become dominant. This occurs in MilliQ with nitrateand bicarbonate, where hydroxyl radicals are formed from nitratephotolysis and carbonate radicals from the reaction of HCO�3 /

CO2�3 with �OH. So, in waters with little carbonate radical scaveng-

ers, the OH radical that reacts with HCO�3 /CO2�3 to form a carbonate

radical may have a larger contribution to organic compound degra-dation than hydroxyl radical, as the scavenging of carbonate radi-cals is much smaller than the scavenging of hydroxyl radicals. Thiswill not occur when hydroxyl radicals are formed from H2O2 pho-tolysis, because H2O2 scavenges CO��3 (reaction 13 in Table 2). Alsoin waters with DOC, the carbonate radicals will be scavenged byDOC.

5. Conclusions

A kinetic model of UV/H2O2 for polychromatic UV sources waspresented. The model incorporates the (photo)chemical reactionsof nitrate and its transformation products. This is confirmed by agood agreement with measured data of pharmaceutical degrada-tion in MilliQ with nitrate. Both hydroxyl radicals and carbonateradicals are included in the model as degradation pathways forOMPs. Most of the pharmaceutical degradations were well pre-dicted by the model. Model predictions in waters without H2O2

were most critical, in particular for the sulphonamides. The modelcould be further improved by the following changes:

� Incorporation of DOC photolysis and reaction 3DOM⁄ complexeswith target compounds.� Inclusion of pH dependent quantum yields and molar

absorptions.� Availability of wavelength dependent quantum yields.

Acknowledgments

This work was performed in the TTIW-cooperation frameworkof Wetsus, centre of excellence for sustainable water technology(www.wetsus.nl) and this work is supported by the joined DutchWater Supply Companies. Wetsus is funded by the Dutch Ministryof Economic Affairs. The authors would like to thank the partici-pants of the research theme ‘clean water technology’ for the fruit-ful discussions and their financial support.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.cej.2014.10.101.

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