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Mineralization of paracetamol in aqueous solution with advanced oxidation processes a

a

a

a

Murat Torun , Özge Gültekin , Dilek Şolpan & Olgun Güven a

Department of Chemistry, Hacettepe University, Beytepe, 06800 Ankara, Turkey Accepted author version posted online: 29 Sep 2014.Published online: 21 Oct 2014.

Click for updates To cite this article: Murat Torun, Özge Gültekin, Dilek Şolpan & Olgun Güven (2015) Mineralization of paracetamol in aqueous solution with advanced oxidation processes, Environmental Technology, 36:8, 970-982, DOI: 10.1080/09593330.2014.970585 To link to this article: http://dx.doi.org/10.1080/09593330.2014.970585

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Environmental Technology, 2015 Vol. 36, No. 8, 970–982, http://dx.doi.org/10.1080/09593330.2014.970585

Mineralization of paracetamol in aqueous solution with advanced oxidation processes Murat Torun∗ , Özge Gültekin, Dilek Solpan ¸ and Olgun Güven Department of Chemistry, Hacettepe University, Beytepe, 06800 Ankara, Turkey

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(Received 19 March 2014; final version received 24 September 2014 ) Paracetamol is a common analgesic drug widely used in all regions of the world more than hundred tonnes per year and it poses a great problem for the aquatic environment. Its phenolic intermediates are classified as persistent organic pollutants and toxic for the environment as well as human beings. In the present study, the irradiation of aqueous solutions of paracetamol with 60 Co gamma-rays was examined on a laboratory scale and its degradation path was suggested with detected radiolysis products. The synergic effect of ozone on gamma-irradiation was investigated by preliminary ozonation before irradiation which reduced the irradiation dose from 5 to 3 kGy to completely remove paracetamol and its toxic intermediate hydroquinone from 6 to 4 kGy as well as increasing the radiation chemical yield (Gi values 1.36 and 1.66 in the absence and presence of ozone, respectively). The observed amount of formed hydroquinone was also decreased in the presence of ozone. There is a decrease in pH from 6.4 to 5.2 and dissolved oxygen consumed, which is up to 0.8 mg l−1 , to form some peroxyl radicals used for oxidation. Analytical measurements were carried out with gas chromatography/mass spectrometry and ion chromatography (IC) both qualitatively and quantitatively. Amounts of paracetamol and hydroquinone were measured with gas chromatography after trimethylsilane derivatization. Small aliphatic acids, such as acetic acid, formic acid and oxalic acid, were measured quantitatively with IC as well as inorganic ions (nitrite and nitrate) in which their yields increase with irradiation.

Keywords: pharmaceutical wastes; paracetamol; advanced oxidation processes; mineralization; gamma-irradiation

∗ Corresponding

author. Email: [email protected]

c 2014 Taylor & Francis 

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Environmental Technology Introduction Pharmaceutical compounds are widely consumed in modern societies as drugs and personal care products which were detected in surface waters, sewage effluents, surface and groundwaters as well as drinking water.[1–5] Contamination of water with such compounds arise from industrial applications during production, direct disposal of surplus drugs into domestic wastes and excretion by living organisms after drug administration.[4,6–9] The presence of pharmaceuticals in the aquatic environment has some adverse effects on human health which include aquatic toxicity, development of pathogenic bacteria, genotoxicity and endocrine disrupter.[10,11] Paracetamol (acetaminophen, N -(4-hydroxyphenyl)acetamide)) is one of the most used analgesic drugs formulated in various dosage forms as tablets and syrups in both the single-ingredient and multicomponent ones.[12] It is effective for patient’s pain, including that of children, elders and pregnant women.[13] It is also used as an intermediate in the manufacturing of azodyes and photographic chemicals.[14] Paracetamol is assigned as one of the top three prescribed drugs, with more than 400 tonnes per year.[15] The solubility of paracetamol is 17.39 g in 1kg water at 30◦ C [14] which is higher than most of the organic compounds. The amount of paracetamol in sewage treatment plants has been detected as 6 µg l−1 [16] and various concentrations up to 65 µg l−1 in natural waters, [17,18] which is expected to be stable in water depending on conditions since the aromatic ring resists biodegradation or photodegradation. Under sunlight and in the presence of halogens, paracetamol can be transformed into persistent halogenated phenols. When paracetamol in wastewater and pure water is chlorinated with hypochlorite, the major products detected are more toxic and persistent 1,4-benzoquinone, N -acetyl-p-benzoquinone imine, mono- and dichlorinated paracetamol.[19] Paracetamol has some hazardous effects on cell cultures by increasing EC50 values from environmental assessment.[20,21] Hazard quotient effects of paracetamol were determined to be greater than unity suggesting potential ecological risks [22] and it is persistent on hydrolytic or microbial degradation.[23] Paracetamol is removed from water by several ways which include ozonation,[24] H2 O2 /UV system,[24, 25] photocatalytic degradation,[26–30] electrochemical methods,[31–33] pulsed corona discharge [34] and chlorination.[19] Paracetamol is mainly removed from water using such methods but persistent toxic intermediates are still present. Increase in treatment time or chemicals may solve this problem which is not economically viable. Other conventional treatment methods remove organic compounds such as paracetamol from one phase to another, [35] which brings another waste disposal problem. Radiation technology is an efficient method to remove organic pollutants from water. It is environmentally friend since no chemical is introduced into the aquatic system.

Radiation technology for water treatment is used in some regions in the world and became popular in many developed countries. High-energy irradiation of water gener• ates hydrated electrons (e− aq ), hydroxyl radicals ( OH) and • hydrogen radicals (H ) in water that interact with a wide range of pollutants.[36–38] Main products of water radiolysis are [39,40]: • + − H2 O  • H, e− aq , OH, H2 , H2 O2 , H3 O OH .

(1)

In the presence of oxygen, water radiolysis products undergo the following reactions: H + O2 → HO•2

k = 1.9 × 1010 l mol−1 s−1 k = 1.2 × 1010 l mol−1 s−1

[39,41,42]

•− e− aq + O2 → O2

k = 2 × 1010 l mol−1 s−1 k = 1.9 × 1010 l mol−1 s−1

[39,42,43]



H+

HO•2  H+ + O•− 2

pK = 4.8

(2)

(3) (4)

[44,45]

k = 1.2 × 1010 l mol−1 m s−1 k = 9.7 × 107 l mol−1 s−1

HO•2 + O•− 2 → H2 O2 + O2

[45,46] (5)

2H+

HO•2 + HO•2 −→ H2 O2 + O2

k = 3.7 × 106 l mol−1 s−1 k = 8.3 × 107 l mol−1 s−1

[44,46] (6)

2H+

•− • O•− 2 + O2 −→ H2 O2 + O2

2k < 10 l mol−1 s−1 2k = no reaction

[44,47]

(7)

Ozone is a powerful oxidizing agent used in a technical scale for purification of drinking and wastewaters.[48–51]. Ozone may react with solutes either by direct oxidation by attacking on the electron-rich site or by unselective attacking of hydroxyl radicals formed after ozonation. Hydroxylfree radicals can also react with ozone to produce oxygen and hydroperoxyl radicals.[52] Reactions of ozone in water are O3 + OH− → HO− 2 + O2 •− O3 + HO− 2 → HO2 + O3 •− O3 + O•− 2 → O3 + O2

• + O•− 3 + H → HO3

k = 7 × 101 l mol−1−1

(8)

[53]

k = 2.2 × 106 l mol−1 s−1

k = (1.6 ± 0.2) × 109 l mol−1 s−1

k = (5.2 ± 0.6) × 1010 l mol−1 s−1

(9)

[53]

[54] (10)

[55] (11)

+ HO•3 → O•− 3 +H

HO•3 → HO• +O2

k = (3.7 ± 0.3) × 104 s−1

[55]

k = (1.009 ± 0.06) × 105 l mol−1 s−1

(12) [55] (13)

O3 + HO• → HO4

k = 2 × 109 l mol−1 s−1

HO•4 → HO•2 + O2

k = (2.8 ± 0.3) × 104 l mol−1 s−1

(14)

[56] [56]

(15)

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M. Torun et al.

HO•4 → HO•4 → H2 O2 + 2O3

k = 5 × 109 l mol−1 s−1

[56] (16)

k = 5 × 109 l mol−1 s−1

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HO•4 + HO•3 → H2 O2 + O3 + O2

[56] (17)

Advanced oxidation processes (AOPs) are referred as methods that generate hydroxyl-free radicals for the effective decomposition of pollutant.[57] AOPs are characterized by the production of oxidative hydroxyl radicals at ambient temperatures to decompose various organic wastes until complete mineralization with the formation of CO2 , H2 O and mineral acids. Various types of AOPs have been developed for the aqueous waste treatment as well as pharmaceuticals [58–61] by a single process or by combination of ozonation, UV irradiation with or without catalysts, electron beam irradiation, gamma-irradiation, the Fenton process, ultrasound irradiation and hydrogen peroxide. When ozonized water is exposed to gamma irradiation, the radicalic reactions are Propagation [52] − O3 + O− 2 → O3 +O2 ,

(18)

O− 3

(19)

+

+ H → OH + O2 , •

OH + O3 → HO2 + O2 ,

(20)

HO•2 → H+ + O− 2.

(21)





Termination [52] HO•2 + • OH → H2 O + O2 (Dominant at low pH), (22) •

OH + • OH → H2 O2 (Dominant at pH > 4).

Experimental Standards and reagents Analytical-grade paracetamol, catechol, p-aminophenol, resorcinol, hydroquinone, p-nitrophenol, pyrogallol, acetic acid, succinic acid, malonic acid, maleic acid, oxalic acid, citric acid and ammonium acetate were purchased from Sigma-Aldrich. N -Methyl-N -trimethylsilyltrifluoroacetamide (MSTFA), p-bromophenol, phenol, propionic acid, valeric acid, c,c-muconic acid and t,t-muconic acid were obtained from Fluka. Formic acid, sodium nitrate, sodium nitrite, glutaric acid, formaldehyde, acetyl acetone and methyl alcohol were purchased from Merck. Highpurity nitrogen gas (99.99%) was used to remove humidity from analytes. Aqueous solutions were prepared in deionized water produced from an mpMINIpure DESTup water purification system with a conductivity of 0.0547 µS cm−1 . All chemicals were used as received without any further purification. Sample preparation A paracetamol stock solution of 50 mg l−1 was prepared daily and stored in a refrigerator. Phenolic compounds and some carboxylic acids were dissolved in MSTFA for silyl esterification. Silyl esters of paracetamol and standards were injected into a gas chromatography/mass spectrometry (GC/MS) system one by one to examine the retention time and mass spectra of standards, and to check for any contamination. Aliphatic acid standards were dissolved in deionized water and injected into the ion chromatography (IC) system one by one for detection of retention time and to check the purity. Deionized water was tested as a blank for possible contamination.

(23)

The present study focuses on the degradation of aqueous solutions of paracetamol by using gamma-irradiation and ozonation for the application of AOPs constituted by high-energy irradiation and ozonation. Paracetamol is one of the most used pharmaceuticals and resists biodegradation. Because of its high prescription, this compound is selected for the application of AOPs treatment in the aquatic environment. The amount of paracetamol in surface or wastewaters is expected to be constant or increasing since there is a constant release to the aquatic environment. There are some investigations about the degradation of paracetamol which do not include radiation technology. An effective method is necessary to overcome adverse effects of paracetamol and its metabolites in the aquatic environment. The aim of this study is to examine the effect of high-energy irradiation on paracetamol mineralization as well as to remove its toxic phenolic intermediates. Mineralization of paracetamol and its degradation path have been investigated by using a gamma-irradiation/ozonation process in this study.

Ozonation process Ozone was generated from dried atmospheric air by an Opal OPL 20 (Turkey) model ozone generator with an ozone output of 10 g h−1 . The paracetamol solution (200 ml) was ozonized in a 500-ml cylindrical pyrex reactor by bubbling ozone/air mixture into the solution through a sinterized glass filter with a pore size of 50–80 µm. Irradiation process The paracetamol solution in open pyrex bottles was irradiated in the presence and absence of ozone in a Gammacell 220 type γ -irradiator with a dose rate of 0.03 kGy h−1 at ambient temperature (23◦ C) for various doses. The dose rate of 60 Co gamma-source was determined by a Fricke dosimeter. Extraction and derivatization processes In all, 20–200 ml of samples was extracted by solidphase extraction with a Dionex Autotrace 280 automatic concentrator in C18 cartridges. C18 cartridges were first

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Environmental Technology conditioned with hexane and methyl alcohol before analyte elution. C18 cartridges were dried under pure nitrogen and analytes were obtained by eluting with methyl alcohol into vials. After evaporation of methyl alcohol by nitrogen bubbling, residual samples in closed vials were dissolved in MSTFA and heated at 60◦ C in a water bath for 10 min for derivatization. Silyl esters were cooled for 2 min at ambient temperature and injected into the GC/MS system immediately. MSTFA was injected into the GC/MS system alone to check for contamination before analysis. Percent conversion of paracetamol and aromatic intermediates from water in C18 cartridges was more than 93%, which was determined experimentally. Silylation in MSTFA is achieved with more than 96% conversion. The direct derivatization method in MSTFA is followed rather in solution since it is observed that silylation in solvents such as acetonitrile is successful for phenolic compounds although the conversion yield is low for paracetamol. pH measurements pH changes were measured with an ISTEK NeoMet pH200L model pH-meter. The pH-meter reading was verified with standards before each measurement and calibrated daily. Dissolved oxygen measurement Change in dissolved oxygen was measured with a WTW Microprocessor OX˙I 3000 Oxygen Meter, which contains a membrane-type dissolved oxygen electrodes using a galvanic cell. Calibration of the oxygen-meter is controlled daily.

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CA, USA). Heat of injector valve was set at 270◦ C connected to the VF5-MS chromatographic column (30 m × 0.25 mm i.d.) with a 0.25-µm film thickness conditioned at 80◦ C for 1 min, then at 150◦ C with a heating rate of 7◦ C min−1 that holds for 5 min and 200◦ C with a heating rate of 7◦ C min−1 that holds for 5 min. Helium (99.99%) with a flow rate of 1 ml min−1 was used as a mobile phase. Transfer line and ion trap temperatures were 280◦ C and 210◦ C, respectively. The analysis was performed with a filament through a multiplier with a 4.0-min delay. Automatic gain control was activated with a target of 20,000 counts. Ionization filament was set at 10 µA generating electrons with 70 eV energy in the ionization mode. The axial modulation amplitude voltage was 4.0. Spectra of possible compounds were compared with the NIST library and our own library was constructed with standards.

Analytical instrumentation and operating conditions for ion chromatography The ion chromatographic determinations of aliphatic acids, nitrate and nitrite ions were performed by using the Dionex DX-3000 model IC system equipped with an ultra-anion self-regenerating suppressor model ASRS 300 (4 mm) and conductivity detector (100 mA) set at 35◦ C. Separation of aliphatic acids and inorganic ions was performed by a 4 × 250 mm analytical Ionpac AS11-HC column at 30◦ C. In all, a sample of 100 µl was introduced into the IC system by an autosampler. The standards and analytes were separated in the column with the mobile-phase potassium hydroxide solution generated from the Dionex EGC II KOH eluent generator cartridge that gradually changes from 0 to 28 mM with a flow rate of 1.300 ml min−1 .

Formaldehyde measurement Formation of formaldehyde is one of the most important stages, which is also the last stage of degradation of organic compounds. Its detection after ozonation and/or irradiation is of great importance. Formaldehyde amounts in samples were measured by the Hantzsch method.[62] Acetyl acetone in acetic acid/ammonium acetate buffer was mixed with the sample and heated at 57◦ C for 5 min in a water bath. The yellow complex formed at 412 nm is recorded for quantitative determination using a Varian Cary 100 model UV–vis spectrophotometer. The same procedure is applied for acetaldehyde in which no complex was formed. It is verified that only formaldehyde forms this complex at 412 nm for quantitative examinations. Analytical instrumentation and operating conditions for gas chromatography/mass spectrometry system Paracetamol and its intermediates were converted into silyl derivatives and measured with GC-3900 gas chromatography equipped with a Saturn 2100 T ion trap mass spectrometer from Varian Instruments (Sunnyvale,

Results and discussion Paracetamol and standard characterization with GC/MS and IC Paracetamol and possible aromatic intermediate standards were derived with MSTFA to obtain more volatile and sensitive trimethysilyl (TMS) derivatives for GC/MS analysis. Derivatization of paracetamol and other possible intermediates with MSTFA is shown in Figure 1. TMS derivative of p-bromophenol was injected as an internal standard for quantitative analysis. m/z at 295 is a molecular ion peak of paracetamol di-TMS (M+ ), fragmentations at m/z 280 indicate the loss of methyl group (M − CH3 )+ and at m/z 206 the loss of oxotrimethylsilane (M − OSiCH3 )+ for paracetamol di-TMS. Previously, some possible aliphatic acids were injected into the IC system with an autosampler one by one to determine the retention times of aliphatic acids. Calibration curves for each aliphatic acid were constructed for quantitative analysis by the software. Muconic acid has a special place among other aliphatic acids since it is one of the most

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M. Torun et al.

Figure 1. Derivatization reaction of paracetamol and possible functional groups with MSTFA.

important products of aromatic ring opening [36,63] and it has three species with retention times of 10.51, 20.13 and 20.93 min for c,t-muconic acid, c,c-muconic acid and t,tmuconic acid, respectively. The analysis with IC showed that both c,c-muconic acid and t,t-muconic acid form a new compound observed at 10.54 min which is attributed to c,t-muconic. Isomerization of c,c-muconic acid and c,tmuconic acid in aqueous solution was observed by several authors.[64,65] c,t-muconic acid cannot be commercially made available and peak at 10.54 min is attributed to the conversion of muconic acid isomers in water, which is not a prepared standard. Possible aliphatic acid standards were derived to TMS derivatives for GC injection for qualitative and quantitative analyses but among them only TMS derivatives of malonic acid, fumaric acid, adipic acid, tartaric acid, ascorbic acid and glutaric acid showed reasonable results in the GC column with retention times of 17.65, 20.24, 16.73, 17.69, 20.18 and 16.62 min, respectively. TMS derivatives of c,c-muconic acid and t,t-muconic acid were observed in the GC column with a retention time of 18.48 minute for both acids that overlaps and isomers of muconic acid were separated with IC more precisely. Degradation of paracetamol A paracetamol solution of 50 ppm in open pyrex bottles was irradiated at various doses with a dose rate of 0.03 kGy h−1 in the presence and absence of 30 s preliminary ozonation with a rate of 10 g O3 h−1 . Degradation of paracetamol with dose is shown in Figure 2 with a radiation chemical yield value (Gi ). G value is the number of decomposed molecules per 100 eV absorbed energy and Gi value is the initial G value.[66,67] Paracetamol is completely removed from water at 5 kGy irradiation dose and preliminary ozonation decreases the irradiation dose value to 3 kGy. The number of decomposed molecules per

Figure 2. Change in the amount of 50 ppm paracetamol with irradiation and ozonation/irradiation (dose rate: 0.03 kGy/h).

100 eV absorbed energy (Gi ) in the presence and absence of ozone is 1.66 and 1.36, respectively. The efficient degradation in the presence of ozone is due to the selective attack of molecular ozone to the double bonds of aromatic ring of paracetamol that results in ring opening and unselective attack of hydroxyl radicals from both radiolysis product of water and ozone decomposition. Some less reactive water radiolysis species can be converted to highly reactive oxidants in the presence of ozone, which enhances the degradation efficiency. The reaction rate constant of paracetamol with hydroxyl radical (. OH) is determined as 5.6 × 109 mol−1 dm3 s−1 ,[68] (9.8 ± 0.4) × 109 mol−1 dm3 s−1 , [69] 1.7 × 109 mol−1 dm3 s−1 [36] and 2.2 × 109 mol−1 dm3 s−1 [24] by several methods. Paracetamol reacts with hydrated electron (e− aq ) at a rate constant of (2.5 ± 0.3) × 108 mol−1 dm3 s−1 [69] and

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Environmental Technology Table 1. Observed radiolysis products of paracetamol in this study. Compound

Analytical technique

Retention time (min)

Molecular peak (TMS) (M+ ) (m/e)

GC/MS

17.74

342

Gamma-irradiation, ozonation

GC/MS

11.85

254

Gamma-irradiation, ozonation, ozonation/gammairradiation

CH3

GC/MS

17.28

223

Gamma-irradiation, ozonation/gammairradiation

CH3

GC/MS

21.74

296

Ozonation, ozonation/gammairradiation

GC/MS

22.43

295

Gamma-irradiation, ozonation, ozonation/gammairradiation

GC/MS

9.45

244

GC/MS

9.06

194

Gamma-irradiation, ozonation, ozonation/gammairradiation Gamma-irradiation, ozonation

GC/MS

16.76

282

Gamma-irradiation, ozonation

GC/MS IC

10.13 16.97

262

Gamma-irradiation

GC/MS IC

(c,c),(t,t) (18.48) (c,t) 10.51 (c,c) 20.13 (t,t) 20.93

286

Gamma-irradiation, ozonation, ozonation/gammairradiation

GC/MS IC

9.93 18.64

259

Gamma-irradiation, ozonation, ozonation/gammairradiation

Structure

Trihydroxy benzene (other than pyrogallol)

OH HO

Process

OH

Hydroquinone

OH

HO

p-Acetylminophenol

O C NH2

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HO

p-Acetaminophenol oxy-

O C NH2

HO

HO

1,4-Benzene-dicarboxylic acid

O

HO O

O

Succinic anhydride

O

O

Benzoic acid

O

O

OH

4-Hydroxy benzoic acid

O

OH

HO

Succinic acid

O HO OH O

Muconic acid

O HO OH O

Maleic acid

HO

O O

OH

(Continued)

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M. Torun et al. Table 1. Continued Compound

Structure

Levulnic acid enol

OH

Analytical technique

Retention time (min)

Molecular peak (TMS) (M+ ) (m/e)

GC/MS

9.63

260

Ozonation, ozonation/gammairradiation

GC/MS IC

10.86 20.24

260

Ozonation

IC

5.56

Gamma-irradiation, ozonation, ozonation/gammairradiation

IC

6.87

IC

11.69

Gamma-irradiation, ozonation, ozonation/gammairradiation Gamma-irradiation, ozonation, ozonation/gammairradiation

IC

15.48

Gamma-irradiation, ozonation, ozonation/gammairradiation

IC

17.06

Gamma-irradiation, ozonation, ozonation/gammairradiation

IC

17.65

Gamma-irradiation, ozonation, ozonation/gammairradiation

IC

6.17

Gamma-irradiation, ozonation, ozonation/gammairradiation

IC

20.10

Gamma-irradiation, ozonation, ozonation/gammairradiation

OH

Process

O

Fumaric acid

O HO OH O

Acetic acid

O

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OH

Formic acid

HO

Nitrite

O

N O–

O

Nitrate

O N –

Malic acid

+

O



O

O OH HO OH

Malonic acid

O

O

O

HO

OH

Propionic acid

O

OH

Oxalic acid

O OH HO O

5 × 108 mol−1 dm3 s−1 [68] although e− aq reacts with oxy10 gen at a rate constant of 1.2 × 10 mol−1 dm3 s−1 [39,42, 43] in which e− aq is scavenged by dissolved oxygen for

aerated solutions that makes the presence of e− aq unimportant for degradation of paracetamol in aerated solutions. Reaction of paracetamol with molecular ozone is reported

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as pH dependent and it is favourable at high pH values with a range of 1.41 × 103 ± 1.55 × 103 mol dm3 to 1.32 × 105 ± 1.63 × 104 mol dm3 at pH 2 and 5.5, respectively. Molecular ozone attacks on double bonds on paracetamol selectively depending on the medium and forms ozonides through a Criegee mechanism as an electrophile which results in bond cleavage.[70–72] On the other hand, the ozone intermediates HO•3 and O•3− are very strong oxidants and hence play an important role in the decomposition of organic pollutants.[36] In all, 0.5-min ozonation of 200 ml 50 ppm paracetamol solution oxidized 18% of paracetamol to ozonolysis products.

Radiolysis products of paracetamol Radiolysis products of paracetamol after irradiation were determined with GC/MS and IC, as quantitatively and qualitatively given in Table 1. Hydroquinone is one of the most important radiolysis intermediates observed at all stages of irradiation. It is persistent on degradation in wastewaters.[73] Han et al. investigated that catalyst was not efficient to decompose hydroquinone under UV or Vacuum UV (VUV) at different wavelengths.[74] The attack of hydroxyl radical on paracetamol forms hydroquinone [52] and oxidation in the presence of oxygen with other reactive radicals forms peroxyl radicals caused ring opening to form muconic acid which is transformed into small aliphatic acids and formaldehyde. Change in the amount of hyroquinone with irradiation dose is shown in Figure 3. The amount of hydroquinone increased with irradiation dose and further irradiation completely removed toxic and persistent radiolysis product hydroquinone. The formed amount of hydroquinone was lower for preliminary ozonized paracetamol solutions, which is completely

Figure 4. Change in the amount of acetic acid with irradiation and ozonation/irradiation of 50 ppm paracetamol (dose rate: 0.03 kGy/h).

Figure 5. Change in the amount of formic acid with irradiation and ozonation/irradiation of 50 ppm paracetamol (dose rate: 0.03 kGy/h).

Figure 3. Change in the amount of hydroquinone with irradiation and ozonation/irradiation of 50 ppm paracetamol (dose rate: 0.03 kGy/h).

removed at 4 kGy although it is 6 kGy for ozone-free paracetamol solutions. It is reported that the reaction of phenol with ozone produces hydroxyl radical to form dihydroxy benzene [64] as well as the reaction of hydroquinone with ozone may produce hydroxyl radical to form trihydroxy benzene observed with GC/MS in this study. Ozone and hydroxyl radicals with oxygen form ozonides and peroxyl radicals on the double bond of aromatic ring (paracetamol and other aromatic intermediates), respectively, which results in bond opening to form mainly muconic acid. Muconic acid is one of the important parameters that confirm bond opening and transforms into small aliphatic acids, such as succinic, maleic, levulinic, fumaric, acetic, formic, malic, malonic, propionic and oxalic acids that

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were detected with both GC/MS and IC. Muconic acid reacts directly with ozone more favourably than hydroxyl radical with a stoichiometric ratio of 1 and a rate constant of 1.6 × 104 M s−1 and 1.4 × 105 M s−1 at pH 3 and 7, respectively [75] to produce small aliphatic organic acids such as, acetic acid, formic acid and oxalic acid, observed at each irradiation dose shown in Figures 4–6. These small aliphatic acids are the last stage of paracetamol mineralization to CO2 and H2 O by ozone and hydroxyl radicals with rate constants given in Table 2. Small aliphatic acids react more favourably with hydroxyl radical than ozone with a first-order reaction rate constant and preliminary ozonation of paracetamol solution produced small amounts of aliphatic acids.

Figure 6. Change in the amount of oxalic acid with irradiation and ozonation/irradiation of 50 ppm paracetamol (dose rate: 0.03 kGy/h).

Table 2. Reaction rate constants of observed carboxylic acids with ozone [76] and hydroxyl radical.[42] kO3 kOH (M−1 s−1 ) (M−1 s−1 )

Carboxylic acid Acetic acid Formic acid Oxalic acid

Dissociated Non-dissociated Dissociated Non-dissociated Dissociated HA− Dissociated A2− Non-dissociated

10−5 10−5 100 5 10−2 10−2 10−2

8.5 × 107 1.6 × 107 109 109 7.7 × 106 4.7 × 107 1.4 × 106

Figure 7. Change in the amount of formaldehyde with irradiation and ozonation/irradiation of 50 ppm paracetamol (dose rate: 0.03 kGy/h).

Figure 8. Change in the amount of nitrate with irradiation and ozonation/irradiation of 50 ppm paracetamol (dose rate: 0.03 kGy/h).

Figure 9. Change in the amount of nitrite with irradiation and ozonation/irradiation of 50 ppm paracetamol (dose rate: 0.03 kGy/h).

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Figure 10. Change of pH with irradiation and ozonation/irradiation of 50 ppm paracetamol (dose rate: 0.03 kGy/h).

Figure 11. Change of dissolved oxygen with irradiation and ozonation/irradiation of 50 ppm paracetamol (dose rate: 0.03 kGy/h).

Aldehydes are the other radiolysis products of most organic compounds mainly oxidized to carboxylic acids. The formation of formaldehyde is the last stage before mineralization which is oxidized to form both formic acid and carbonates. The formation of formaldehyde is

favoured with irradiation dose although in the presence of ozone, further irradiation accelerated the decomposition of formaldehyde by oxidation from both ozone and hydroxyl radical (Figure 7).

Figure 12. Possible degradation pathway of paracetamol with gamma-irradiation.

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Figure 13. Possible degradation pathway of paracetamol with ozonation/gamma-irradiation.

Carbamic acid is the by-product of paracetamol radiolysis and its mineralization results in the formation of nitrate (Figure 8) and nitrite (Figure 9) ions which are strong evidence for degradation of paracetamol. The amount of nitrate and nitrite increases in the presence of ozone with irradiation dose by the oxidation of ammonium/ammonia species although this process is complex under gammairradiation because of the oxidation–reduction of nitrite– nitrate to each other. Initial pH of 50 ppm paracetamol solution was measured as 6.4 (Figure 10) and 30 s ozonation of paracetamol solution decreased pH value to 6.1. Oxidation of paracetamol and its radiolysis products decreased pH value down to 4 kGy irradiation dose and it is constant for further irradiation that shows major percent of mineralization was completed at 4 kGy irradiation dose. Dissolved oxygen change is another important parameter for radiolysis of organic compounds since it is consumed during oxidation (Figure 11) to form reactive peroxyl radicals. The amount of dissolved oxygen in 50 ppm paracetamol solution was measured as 3.50 mg l−1 and it decreased with the irradiation dose. In the presence of ozone, excess ozone decomposes to yield oxygen that increases the amount of dissolved oxygen in solution and it is consumed under gamma-irradiation that powered oxidation.

Degradation mechanism of paracetamol The degradation path of paracetamol was suggested for gamma-irradiation in the absence and presence of ozone in Figures 12 and 13, respectively. Hydroquinone is the main intermediate as a radiolysis product of paracetamol in the absence of ozone and it oxidizes to aliphatic organic acids up to carbonic acid. By-product carbamic acid forms nitrite and nitrate ions. In the presence of ozone, ozone and hydroxyl radicals are main oxidizers and selective attack of ozone on double bonds breaks down the aromatic ring to form aliphatic acids and carbonic acid.

Conclusions Radiation technology appears to be an efficient method to remove paracetamol and its toxic intermediates from water completely. Hydroquinone is the major aromatic radiolysis product of paracetamol and it is persistent on degradation since more irradiation doses are necessary to remove this compound completely. The formation of small carboxylic acids and inorganic ions is the evidence for mineralization. Further irradiation is necessary to remove small aliphatic carboxylic acids from aqueous solutions completely. Preliminary ozonation of aqueous paracetamol solutions increases the efficiency of degradation of

Environmental Technology paracetamol as well as aromatic organic intermediates. Decrease in pH results in the formation of carboxylic acids as well as oxidation.

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Acknowledgements The authors thank Hacettepe University Scientific Research Projects Coordination Unit for supporting this study under Project number 1089.

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Mineralization of paracetamol in aqueous solution with advanced oxidation processes.

Paracetamol is a common analgesic drug widely used in all regions of the world more than hundred tonnes per year and it poses a great problem for the ...
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