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WA T E R R E S E A R C H 4 2 ( 2 0 0 8 ) 3 8 6 – 3 9 4

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Photocatalytic degradation and drug activity reduction ofChloramphenicol

A. Chatzitakisa, C. Berberidoua, I. Paspaltsisb, G. Kyriakouc, T. Sklaviadisb, I. Pouliosa,�

aLaboratory of Physical Chemistry, Department of Chemistry, Aristotle University of Thessaloniki, 54124 Thessaloniki, GreecebPrion Disease Research Group, Laboratory of Pharmacology, School of Pharmacy, Aristotle University of Thessaloniki, 54124 Thessaloniki,

GreececLaboratory of Inorganic Chemistry, Department of Chemical Engineering, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece

a r t i c l e i n f o

Article history:

Received 1 May 2007

Received in revised form

26 June 2007

Accepted 18 July 2007

Available online 27 July 2007

Keywords:

Photocatalysis

Chloramphenicol

Photo-oxidation

Drugs

ZnO

TiO2

nt matter & 2007 Elsevie.2007.07.030

hor. Tel.: +30 2310 997785oulios@chem.auth.gr (I.

a b s t r a c t

The photocatalytic degradation of Chloramphenicol, an antibiotic drug, has been

investigated in aqueous heterogeneous solutions containing n-type oxide semiconductors

as photocatalysts. The disappearance of the organic molecule follows approximately a

pseudo-first-order kinetics according to the Langmuir–Hinshelwood model. It was observed

that, with TiO2 P-25 as photocatalyst, quantitative degradation of the organic molecule

occurs after 4 h of illumination. During this time, the dechlorination of the substrate is

complete, while the organic nitrogen was recovered in the form of nitrate and ammonium

ions. The effect of temperature on the degradation rate of Chloramphenicol shows similar

apparent activation energies for both TiO2 P-25 and ZnO photocatalysts. The initial

apparent photonic efficiency (z0) of the photo-oxidation and the mineralization under

various experimental conditions have been calculated, while the Kirby–Bauer disc diffusion

method showed a 100% reduction of the drug activity after 90 min of photocatalytic

treatment.

& 2007 Elsevier Ltd. All rights reserved.

1. Introduction

Although pharmaceuticals have been consumed for many

decades, only during the last few years their fate and release

in the aquatic environment have been recognized as one of

the most urgent questions of environmental chemistry.

Current drinking water standards do not even require testing

for any of the more than 7000 pharmaceutical compounds

being prescribed. Pharmaceutically active compounds such as

analgestics, antibiotics, steroid, hormones, etc., have been

detected in several public water systems in Europe, USA and

Australia as a result of human, animal and plant activities

(Ajit et al., 2006; Kolpin et al., 2002). Mass balance of the input

and output of pharmaceuticals in sewage treatment plants

indicates that during sewage treatment not all pharmaceu-

r Ltd. All rights reserved.

; fax: +30 2310 997784.Poulios).

ticals are removed quantitatively (Heberer, 2002), and in

surface and underground water, they are detected at times.

Their environmental presence gains importance because they

are related abnormal physiological processes in reproduction,

increased incidences of cancer, development of antibiotic-

resistant bacteria and potential increased toxicity of chemical

mixtures. For many substances, the potential effects on hu-

mans and aquatic ecosystems are not clearly understood

(Halling-Sorensen et al., 1998). As a consequence, several efforts

are being made to find out ways of inactivating or eliminating

this class of substances in surface or wastewater.

Among the so-called advanced oxidation processes, homo-

geneous and heterogeneous photocatalytic oxidation have

shown, recently, great promise in the treatment of industrial

wastewater, groundwater and contaminated air.

ARTICLE IN PRESS

WAT E R R E S E A R C H 4 2 ( 2 0 0 8 ) 3 8 6 – 3 9 4 387

The photocatalytic decomposition of organic compounds of

environmental concern (e.g. pesticides, dyes, etc.) has been

studied extensively during the last 20 years and it has been

demonstrated that heterogeneous photocatalysis using TiO2

(anatase) as catalyst can be an alternative to conventional

methods for the removal of organic pollutants from water and

air (Blake, 2001). Additionally, an advantage of the photo-

catalytic process is its mild operating conditions and the fact

that it can be powered by sunlight, thus reducing significantly

the electric power required and, therefore, operating costs

(Blanco and Malato, 2003). Recently, a number of research

groups have dealt with the homogeneous and heterogeneous

photocatalytic decomposition of various types of drugs in the

presence of near-UV (UV-A) or solar light with very encoura-

ging results (Andreozzi et al., 2003; Ravina et al., 2002).

The current study provides results describing the hetero-

geneous photocatalytic oxidation of the antibiotic Chloram-

phenicol, also known with the commercial name

Chloromycetin over semiconducting powders, such as TiO2

and ZnO, under various experimental conditions.

Chloramphenicol is a broad-spectrum antibiotic exhibiting

activity against both Gram-positive and Gram-negative bac-

teria, as well as other groups of microorganisms. It exerts its

action through microorganism protein synthesis inhibition

and it is effective in the treatment of several infectious

diseases. This, together with its low cost and ready avail-

ability, has made it extensively used since the 1950s in the

treatment of domestic animals all over the world (Ajit et al.,

2006). However, Chloramphenicol is, in certain susceptible

individuals, associated with serious toxic effects in humans

including bone marrow depression, particularly severe in the

form of fatal aplastic anemia (Woodward, 2004). Since this

condition is dose independent, Chloramphenicol has been

banned for use in food-producing animals in many countries

including EU. As a consequence, Chloramphenicol is included

in Annex IV of Council Decision 2077/90 (EEC, 1990), which

comprise the drugs with an established zero-tolerance level

in edible tissues. Its structure is presented below

O2N CH2O CH

NHCOCHCl2

CH2OH

Chloramphenicol, CAS 56-75-3.

2. Experimental

2.1. Materials

Chloramphenicol (C11H12O5N2Cl2, 2,2-dichloro-N-[2-hydroxy-

1 (hydroxyl-methyl)-2(4-nitrophenyl)ethyl]acetamide, MW

322.96) is a product of Sigma Chemie GmbH company and

was used as received.

As catalysts, for all photocatalytic experiments, TiO2 P-25

(anatase/rutile ¼ 3.6/1, surface area 56 m2 g�1, nonporous),

TiO2 (A) (Tronox McGee, BET 10.5 m2 g�1, 100% anatase), TiONa

(100% anatase) and ZnO (Merck, BET 10 m2 g�1) were used. All

other chemicals were purchased through commercial com-

panies and were used without further purification.

2.2. Procedures

Experiments were performed in a closed Pyrex cell of 300 mL

capacity (6 cm diameter and 15 cm height). In all cases during

the experiments, 250 mL of the Chloramphenicol solution

containing the appropriate quantity of the semiconductor

powder was magnetically stirred, before and during the

illumination, while the solution was purged with CO2� free

air. At specific time intervals samples of 4 mL were with-

drawn. To remove the TiO2 particles, the solution was filtered

through a 0.45mm filter (Schleicher and Schuell). The suspen-

sion was left for 15 min in the dark in order to achieve

maximum adsorption of the drug onto the semiconductor

surface. The reaction vessel was fitted with a central 9 W

lamp Osram Dulux S 9W/78 and had inlet and outlet ports for

bubbling the desired (CO2 free) gas under which the reaction

was taking place. The spectral response of the irradiation

source according to the producer, ranged between 320 and

400 nm with a maximum at 365 nm, a wavelength suitable for

the activation of the semiconductors.

The photon flow per unit volume of the incident light was

determined by chemical actinometry using potassium fer-

rioxalate (Braun et al., 1991). The initial light flux, under

exactly the same conditions as in the photocatalytic experi-

ments, was evaluated to be 1.12�10�4 Einstein min�1.

2.3. Analytical procedures

Changes in the concentration of Chloramphenicol were

observed from its characteristic absorption band at 276.5 nm

using a UV–Visible spectrophotometer (Perkin-Elmer, Lamba

3, UV–VIS Spectrophotometer). Since a linear dependence

between the initial concentration of the drug and the

absorption at 276.5 nm is observed, during the experimental

procedure the photodecomposition was monitored spectro-

photometrically at this wavelength.

In order to determine the reduction of the dissolved organic

carbon, a TOC analyzer (Shimandzu 5000) was used to

measure the organic content.

The ions were detected using a DIONEX–DX 100 ANION-

S–CATIONS Analyzer (Dionex, CA, USA) equipped with cation

and anion micromembrane suppressors and a DIONEX

electrical conductivity detector. The anions were analyzed

using an anion pre-column (Dionex AG4A-SC), an anion

separator column (Dionex AS4A-SC), a supressor column

(Dionex ASRS-ULTRA II) and a conductivity detector (Su-

pressed Conductivity meter). The mobile phase (flow rate

2 mL min�1) was 1.8 mM Na2CO3–1.7 mM NaHCO3 and the

suppressor solution was 12.5 mM H2SO4. The cations were

analyzed using a cation pre-column (Dionex CG 12), a cation

separator column (Dionex AS4A-SC), a supressor column

(Dionex CSRS-ULTRA) and a conductivity detector (Supressed

Conductivity meter). The mobile phase (flow rate 1 mL min�1)

was 18 mM methanesulfonic acid and the suppressor solution

was 12.5 mM H2SO4.

ARTICLE IN PRESS

0 20 40 60 80 1000

10

20

30

40

50

C (

mgL

-1)

Illumination time (min)

Fig. 1 – Photodegradation of 50 mg L�1 Chloramphenicol as a

function of irradiation time in the presence of (’) 1 g L�1

TiO2 P-25, (K) 1 g L�1 TiO2 (A), (m) 1 g L�1 ZnO, (.) 1 g L�1

TiONa and (E) without catalyst.

WA T E R R E S E A R C H 4 2 ( 2 0 0 8 ) 3 8 6 – 3 9 4388

All photocatalytic experiments were carried out at an initial

pH value of 5.170.2 and the reaction temperature was kept

constant at 2570.2 1C.

Some photocatalytic experiments were repeated three

times in order to check the reproducibility of the experi-

mental results. The accuracy of the optical density values was

within 75%, while the corresponding ones of TOC and

inorganic ions were within 710%.

2.4. Initial apparent photonic efficiency

The photonic efficiency (z) describing the effectiveness of

light conversion to product in heterogeneous, light scattering

systems, is directly proportional to the quantum yield, and

may be considered to be the lower limit of this more

commonly used measure of light efficiency (Muneer and

Bahnemann, 2002).

In our case of photo-oxidation, the following equation was

used for the calculation of the initial apparent photonic

efficiency:

z0ð%Þ ¼ r0ð323Ihv103Þ�1� 100, (1)

where r0 is the initial reaction rate of the photo-oxidation

(mg L�1 min�1), 323 is the molecular weight of Chloramphe-

nicol and Ihv the incident photon flux (mol photons

L�1 min�1¼ Einstein min�1).

For the mineralization process, the initial apparent photo-

nic efficiency, zDOC, per carbon atom can be calculated using

the following equation (Muneer and Bahnemann, 2002):

zDOCð%Þ ¼ rDOCð12 Ihv 103Þ�1� 100, (2)

where rDOC is the initial rate of dissolved organic carbon

reduction (mg L�1 min�1), while for the inorganic nitrogen

production, expressed as a sum of nitrogen in NO�3 ; NHþ4 , the

initial apparent photonic efficiency per nitrogen atom, zInN,

can be calculated using the following equation:

zInNð%Þ ¼ rInNð14Ihv103Þ�1� 100, (3)

where rInN is the initial rate of inorganic nitrogen production

(mg L�1 min�1).

In a similar way, the initial photonic efficiency zCl� (%) of

the Cl� release can be calculated using

zCl�ð%Þ ¼ rCl�ð35:5Ihv103Þ�1� 100. (4)

2.5. Antimicrobial susceptibility test

In order to determine the inhibition of Chloramphenicol’s

antibiotic activity caused by the photocatalytic treatment, the

Kirby–Bauer disk diffusion method was used (Bauer et al.,

1966). About 50 mg L�1 of the antibiotic was degraded photo-

catalytically in the presence of 1 g L�1 TiO2. Samples of 1 mL

were taken in time intervals during the photocatalytic

treatment, they were added on 25 mm diameter disks

(0.45mm Millipore filters) and were dried at 37 1C for 1 h.

The plates were prepared as following: 75 mL of LB agar (5 g

NaCl, 5 g yeast extract, 10 g tryptone and 15 g agarose per 1 L)

was added in Petri dishes (145�20 mm, Greiner). They were

then left to cool down at room temperature and stored at 4 1C.

E. coli top 10 bacteria were used as the test organism. Briefly,

150mL from an overnight culture were added in 6 mL of top

agarose (10 g tryptone, 5 g yeast extract, 5 g NaCl and 7 g of

agarose for 1 L) which was kept at 42 1C. After a short stir, the

mixture was added on top of the agar plates and was allowed

to cool for 5 min at room temperature. The filters containing

the test substrates were placed on top of the plate, which was

incubated overnight at 37 1C. A titration curve was prepared

by measuring the zones of inhibition of the bacterial growth

caused by known amounts of Chloramphenicol.

3. Results and discussion

3.1. Photodecomposition kinetics of Chloramphenicol

Results of the photolysis of a 50 mg L�1 (1.55�10�4 M)

Chloramphenicol solution containing 1 g L�1 TiO2 P-25, TiO2

(A), TiONa and ZnO are shown in Fig. 1. The amount of the

organic molecule present in the supernatant is plotted as a

function of the irradiation time.

Under the given experimental conditions, ZnO followed by

TiO2 P-25 appears to be the best catalysts leading after 90 min

of illumination to almost complete (�90%) degradation of the

antibiotic. The photocatalytic oxidation of Chloramphenicol,

on the other hand, in the presence of the commercial anatase

forms, TiO2 (A) and TiONa, is a slower process and, after

90 min of illumination, only 37% and 23%, respectively, of the

drug have been degraded. The superiority of TiO2 P-25 may be

attributed to the morphology of crystallites, which was

proposed to be one of the most critical properties for the

photocatalytic efficiency of P-25 among various grades of

TiO2. Recently, Hurum et al. (2003) had shown that in the case

of TiO2 P-25 a transfer of the photogenerated electrons from

rutile to anatase particles takes place leading to stabilization

of the charge separation and, therefore, lowering of recombi-

nation of the photogenerated carriers, which determine the

efficiency of most photocatalysts. Additionally, the small size

of the rutile particles in this formulation and their close

ARTICLE IN PRESS

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.50.0

0.2

0.4

0.6

0.8

1.0

r 0 (m

g L

-1 m

in-1

)

Fig. 2 – Dependence of the initial reaction rate, r0, on the

concentration of the TiO2 P-25 for constant Chloramphenicol

concentration (50 mg L�1).

0 20 40 60 800.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

0.00 0.02 0.04 0.06 0.08 0.100.40.60.81.01.21.41.61.82.0

r 0 (m

g L

-1 m

in-1

)

Ceq

(mg L-1)

Fig. 3 – Plot of r0 vs. Ceq at different Chloramphenicol

concentrations for constant catalyst concentration at 1 g L�1,

(’) TiO2 P-25 and (K) ZnO. Inset: linear transform of r0�1 vs.

WAT E R R E S E A R C H 4 2 ( 2 0 0 8 ) 3 8 6 – 3 9 4 389

proximity to anatase particles are crucial to enhance the

catalyst activity. The respective initial photonic efficiencies

(z0) of degradation, under the above-mentioned conditions,

are presented in Table 1.

The blank experiments for either illuminated Chloramphe-

nicol solution (see Fig. 1) or the suspension containing TiO2

and Chloramphenicol in the dark showed that both, illumina-

tion and the catalyst, are necessary for destruction of the

drug.

TiO2 dosage in slurry photocatalytic processes is an

important factor that can influence strongly the degradation

of the organic compound. The optimum quantity depends on

the nature of the organic compound, as well as the photo-

reactor’s geometry (Parra et al., 2002). The effect of varying the

quantity of TiO2 P-25 on the observed initial reaction rate of

the Chloramphenicol degradation is presented in Fig. 2. As

the TiO2 concentration increases from 0.25 to 4 g L�1, the

initial reaction rate increases by a factor of 2. The curve is

reminiscent of a Langmuir-type isotherm, suggesting that the

r0 of the photo-oxidation reaches a saturation value at higher

TiO2 concentrations, as has also been reported in similar

cases.

The influence of the initial concentration of the solute on

the photocatalytic degradation rate of most organic com-

pounds is described by a pseudo-first-order kinetics, which is

rationalized in terms of the Langmuir–Hinshelwood model,

modified to be valid for reactions occurring at a solid–liquid

interface (Cunningham et al., 1994)

r0 ¼ �dCdt¼

krKCeq

1þ KCeq, (5)

where r0 is the initial rate of disappearance of the organic

substrate and Ceq is the equilibrium bulk-solution concentra-

tion. K represents the equilibrium constant for adsorption of

the organic substrate onto the semiconductor and kr reflects

the limiting rate constant of reaction at maximum coverage

under the given experimental conditions. This equation can

be used when data demonstrate linearity plotted using:

1r0¼

1krK

1Ceqþ

1kr

. (6)

The effect of the initial equilibrium concentration of

Chloramphenicol on the initial reaction rate (r0) of photo-

decomposition is shown in Fig. 3.

The r0 values were independently obtained by a linear fit of

the Ceq�t data in the range 10–80 mg L�1 initial Chloramphe-

Table 1 – Apparent photonic efficiency (f) of complete oxidatio

Conditions z0 (%)

1 g L�1 TiO2 P-25 2.3270.22

1 g L�1 TiO2 P-25+400 mg L�1 H2O2 3.6070.14

1 g L�1 TiO2 (A) 1.4770.11

1 g L�1 TiO2 (A)+400 mg L�1 H2O2 –

1 g L�1 ZnO 3.5070.63

DOC reduction, chloride and inorganic nitrogen release under different e

nicol concentration. Only the experimental data obtained

during the first 20 min illumination were used in calculating

the initial reaction rates, in order to minimize variations as a

result of the competitive effects of the intermediate products

and pH changes.

n of the antibiotic Chloramphenicol

zDOC (%) z�Cl (%) zInN (%)

12.6572.16 1.79670.096 1.80570.167

13.9170.37 270.12 2.2170.084

1.0470.07 0.3170.0025 0.62470.085

– 0.15370.050 0.70170.072

6.9270.52 – –

xperimental conditions.

Ceq�1 according to Eq. (6).

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0 100 200 300 400 500 600 7000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

initi

al r

eact

ion

rate

, ro

(mg

L-1

min

-1)

WA T E R R E S E A R C H 4 2 ( 2 0 0 8 ) 3 8 6 – 3 9 4390

In both cases, the curves are reminiscent of a Langmuir-

type isotherm, for which the rate value of photodecomposi-

tion first increases and then reaches a saturation value at

higher concentrations of Chloramphenicol.

As indicated in the inset of Fig. 3, the plot of the reciprocal

initial rate r0 as a function of the reciprocal of the initial

concentration Ceq yields for constant concentration of TiO2

P-25 and ZnO at 1 g L�1 a straight line. The kr and K values

calculated according to Eq. (6) from the slope of each straight

line and from the intercept with the 1/r0 axis were,

respectively, in the case of TiO2 P-25, 1.34 mg L�1min�1

(4.15�10�6 M min�1) and 0.057 mg�1 L (1.76�104 M�1) and,

respected in the case of ZnO, 1.87 mg L�1 min�1

(5.79�10�6 M min�1) and 0.050 mg L�1 (1.55�104 M�1).

Fig. 4 – Effect of H2O2 concentration on the initial rate of

photodegradation of 50 mg L�1 Chloramphenicol in the

presence of 1 g L�1 TiO2 P-25.

3.2. Effect of H2O2 on the photodecomposition ofChloramphenicol

The addition of other powerful oxidizing species, such as

hydrogen peroxide and potassium peroxydisulfate, to TiO2

suspensions is a well-known and extensively studied proce-

dure and in many cases leads to an acceleration of the rate of

the photocatalytic degradation (Wolfrum and Ollis, 1994; Sioi

et al., 2006).

In our case the photocatalytic oxidation of 50 mg L�1

Chloramphenicol in the presence of 1 g L�1 TiO2 P-25 has

been studied at different H2O2 concentrations. The reaction

kinetics were similar to those observed without the oxidants.

H2O2 is considered to have two functions in the process of

photocatalytic degradation. It accepts a photogenerated

electron from the conduction band and thus promotes the

charge separation

H2O2 þ e� ! OH� þ dOH; (7)

and it also forms dOH radicals according to

H2O2 þO�2 ! OH� þ dOHþO2. (8)

However, a possible reaction between the H2O2 with the

photogenerated intermediates cannot be excluded. In the

presence of excess H2O2, it may act as a hole or dOH scavenger

or react with TiO2 to form peroxy compounds, which are

detrimental to the photocatalytic action. In addition, it can

also compete with the organic compound for the adsorption

sites on the semiconductor’s surface, resulting in a ‘‘chroma-

tographic peaking effect’’ of the pollutant concentration in

the solution during the initial stages of the photocatalytic

process (Sauer et al., 2002; Dionysiou et al., 2004). This

explains the need for an optimal concentration of H2O2 for

the maximum effect.

The influence of the H2O2 concentration on the initial rate

of photo-oxidation of Chloramphenicol is shown in Fig. 4. The

photocatalytic efficiency increases as the concentration of

H2O2 increases and it reaches the optimum in the area of

300–400 mg L�1. Consequently, it decreases as the concentra-

tion of H2O2 increases beyond the optimum. Under these

conditions, the presence of H2O2 at an initial pH value of 5.1,

and at concentrations of 50–400 mg L�1, results in a small

increase in the rate of the reaction (�1.5 of the initial one),

quite lower than that observed in other cases (Sioi et al., 2006).

3.3. Effect of temperature on the photocatalytic process

Although the temperature in a chemical reaction plays an

important role, very little information concerning the tem-

perature effect on the heterogeneous photocatalytic degrada-

tion of pollutants in aqueous solutions is available. This is due

to the fact that such reactions are usually not very tempera-

ture sensitive, because the forbidden band-gap energy of the

semiconductive catalysts is too high (�3.0 eV) to be overcome

by the thermal activation energy (kT ¼ 0.026 eV at room

temperature). Increasing the reaction temperature may

increase the oxidation rate of organic compounds at the

interface between the catalyst and the solution, but it also

reduces the adsorptive capacities associated with the organ-

ics and dissolved oxygen (Herrmann, 1999).

In the present paper, the Chloramphenicol degradation was

studied as a function of the reaction temperature under the

same operating conditions for both illuminated TiO2 P-25 and

ZnO systems. The results concerning the dependence of the

rate constant k of the photocatalytic degradation of Chlor-

amphenicol from the reaction temperature, in the range

3–57 1C, are shown in Fig. 5. By increasing the temperature, an

increase of the apparent reaction rate constant k is achieved

for both photocatalytic systems until 45 1C, while tempera-

tures higher than 45 1C lead to a decrease of the photocata-

lytic activity. A possible explanation in the case of the

temperatures above 45 1C could be the lower saturation value

of oxygen, a factor of great importance because it regulates

the photocatalytic mechanism by capturing the photogener-

ated electrons, as well as the increase of the desorption of the

reactants from the catalyst surface (Mills and Le Hunte, 1997;

Chen and Ray, 1998; Herrmann, 1999). The higher tempera-

ture dependence in the case of ZnO could be a result of its

higher donor density (lower resistivity) in comparison to TiO2

P-25. Rising the temperature then leads to higher amounts of

photogenerated carriers reaching the surface of the catalyst

and a reaction occured.

By plotting the natural logarithm of k as a function of the

reciprocal absolute temperature (T), a linear behavior was

ARTICLE IN PRESS

270 280 290 300 310 320 3300.00

0.01

0.02

0.03

0.04

0.05

3.1 3.2 3.3 3.4 3.5 3.6 3.7

-5

-4

-3

initi

al r

eact

ion

cons

tant

, k (

min

-1)

reaction temperature, T (K)

Fig. 5 – Initial reaction constant of 50 mg L�1 Chloram-

phenicol degradation vs. reaction temperature, (K) 0.5 g L�1

TiO2 P-25 and (’) 0.5 g L�1 ZnO. Inset: Dependence of the

natural logarithm of the reaction’s rate constant k from the

inverse of the absolute temperature.

0 50 100 150 200 2500

5

10

15

20

DO

C (

mgL

-1)

Illumination time (min)

Fig. 6 – Dissolved organic carbon (DOC) reduction as a

function of irradiation time during the photocatalytic

degradation of 50 mg L�1 Chloramphenicol: (’) 1 g L�1 ZnO,

(K) 1 g L�1 TiO2 P-25, (m) 1 g L�1 TiO2 (A) and (.) 1 g L�1 TiO2

P-25+400 mg L�1 H2O2.

WAT E R R E S E A R C H 4 2 ( 2 0 0 8 ) 3 8 6 – 3 9 4 391

obtained for both TiO2 P-25 and ZnO catalytic systems as

shown in the inset of Fig. 5. The activation energy (Ea)

determined according to Arrhenius equation (Eq. (9)) gives

33.375.4 and 27.274.4 kJ mol�1 for TiO2 P-25 and ZnO,

respectively:

ln kr ¼ ln A�Ea

R1T

. (9)

3.4. Photodegradation products

The photodecomposition of Chloramphenicol, like all the

other organic pollutants, is a complicated process with many

intermediary products. These intermediates are of great

significance in water treatment because they can be proved

more toxic than the parent compounds. Thus, it is of vital

importance to succeed complete mineralization of the

organic pollutants via the photocatalytic method. For this

reason, the extent of mineralization of Chloramphenicol was

studied under various experimental conditions.

The overall equation, valid after a long irradiation time in

the presence of excess oxygen, which describes the photo-

catalytic mineralization of Chloramphenicol, is

C11H12O5N2Cl2 þ 13:5O2 þ hvo400 nm

�!TiO2

11CO2 þ 2HNO3 þ 2HClþ 4H2O. ð10Þ

General description of heterogeneous photocatalysis under

artificial or solar irradiation is presented in several excellent

review articles (Fujishima et al., 2000; Hoffman et al., 1995;

Blanco and Malato, 2003). Therefore, the various stages of the

photocatalytic process will not be further presented here.

Fig. 6 shows the extent of the dissolved organic content

(DOC) reduction vs. illumination time of an air-saturated

solution containing 50 mg L�1 Chloramphenicol in the pre-

sence of 1 g L�1 TiO2 P-25, TiO2 (A), ZnO, as well as in the

presence of 1 g L�1 TiO2 P-25+400 mg L�1 H2O2, while in Table 1

the initial apparent photonic efficiencies (zDOC) of complete

oxidation of the antibiotic, calculated according to Eq. (2), are

given.

In the presence of TiO2 P-25, 88% of the initial carbon

content of Chloramphenicol was converted to CO2 after

120 min of illumination, while at the same time the decom-

position was almost complete, showing the presence of

intermediates. Full oxidation under the given experimental

conditions requires longer illumination times. The addition of

400 mg L�1 H2O2 in a suspension of TiO2/Chloramphenicol

does not alter significantly the efficiency of the photominer-

alization process and this agrees with the results of the

degradation experiments in the presence of various concen-

trations of H2O2 (Fig. 4).

In the case of TiO2 (A), as in the degradation experiments

(Fig. 1), the photomineralization proceeds with lower effi-

ciency compared to the one when TiO2 P-25 has been used

and after 240 min of illumination only 50% of the initial DOC

in the Chloramphenicol molecule has been converted to CO2.

In the case of ZnO despite the fact that it appears to be a

better catalyst than TiO2 P-25 (Fig. 1), same is not the case

with complete mineralization of the organic content of the

Chloramphenicol molecule. After 120 min of illumination,

44% of the initial carbon content of the antibiotic is remaining

in the solution followed by a retardation of the photominer-

alization rate toward a steady-state value. The dissolution as

well as the photodissolution of ZnO as a result of the self-

oxidation of the semiconductor by the photogenerated holes

could be a possible explanation for the decrease of its

photocatalytic activity (Pleskov and Gurevich, 1986; Evgen-

idou et al., 2005).

In Figs. 7 and 8, the conversion of organic chlorine and

nitrogen in the Chloramphenicol molecule to the respective

inorganic ions is shown, while in Table 1 the initial photonic

efficiencies of these conversions, calculated according to

Eqs. (3) and (4) respectively, under various experimental

conditions, are given.

The organic chlorine conversion to Cl� is an efficient

process in the case of TiO2 P-25, with and without the

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0 50 100 150 200 2500

20

40

60

80

100

Cl- r

elea

se (

%)

illumination time (min)

Fig. 7 – Cl� formation as a function of irradiation time during

the photocatalytic degradation of 50 mg L�1 Chloram-

phenicol: (’) 1 g L�1 TiO2 P-25, (K) 1 gL�1 TiO2 P-25+

400 mgL�1 H2O2, (m) 1 g L�1 TiO2 (A) and (.) 1 g L�1 TiO2 (A)+

400 mgL�1 H2O2.

0 50 100 150 200 2500

20

40

60

80

100

tota

l ino

rgan

ic n

itrog

en (

%)

illumination time (min)

Fig. 8 – Total inorganic nitrogen formation as a sum of

nitrogen in NHþ4 and NO�3 ions vs. irradiation time during

the photocatalytic degradation of 50 mg L�1 Chloram-

phenicol: (’) 1 g L�1 TiO2 P-25, (K) 1 gL�1 TiO2 P-25+

400 mgL�1 H2O2, (m) 1 g L�1 TiO2 (A) and (.) 1 g L�1 TiO2 (A)+

400 mgL�1 H2O2.

WA T E R R E S E A R C H 4 2 ( 2 0 0 8 ) 3 8 6 – 3 9 4392

addition of H2O2. After 2 h of illumination, it was possible to

completely remove chlorine from the molecule of Chloram-

phenicol in the presence of 1 g L�1 TiO2 P-25, as well as by the

synergetic effect of 1 g L�1 TiO2 P-25 and 400 mg L�1 H2O2,

while in the presence of TiO2 (A) only 28% of the expected

value of Cl� has been released.

Moreover, the organic nitrogen conversion to the respective

inorganic ions ðNO�3 ; NH�4 Þ is presented in Fig. 8. Again TiO2

P-25 appears to have better photocatalytic properties than

TiO2 (A) and after 3 h of illumination, the organic nitrogen has

been completely released in the form of NO�3 and NH�4 ions.

In photocatalytic processes involving nitrogen-containing

organic compounds, the final oxidation state of nitrogen or

the NHþ4 =NO�3 ratio, when mineralization of the organic

carbon has occurred, may depend on several factors, the

most significant being the nature and the concentration of

the initial organic compound, the concentration of molecular

oxygen, the pH of the solution, the loading and nature of the

catalyst, as well as the intensity of the irradiation source (Low

et al., 1991).

In the present case, the organic nitrogen conversion to the

respective inorganic ions under the given experimental

conditions, as presented in Fig. 8, is a very efficient process.

After 3 h of illumination in the presence of TiO2 P-25 a 93%

conversion has been achieved, while the respective one in the

presence of TiO2 (A) reaches the lower value of 60%. The

addition of H2O2 enhances slightly the conversion of organic

nitrogen to inorganic anions. In the presence of TiO2 P-25, we

measure a 39% conversion of the organic nitrogen to nitrate,

which corresponds to a 90% conversion of the nitrogroup in

the Chloramphenicol molecule to nitrate. According to the

literature, the nitrogroup in nitroaromatics is a light leaving

group, which can easily be eliminated, favoring the electro-

philic substitution of the dOH radicals at the para position in

the aromatic ring. This group ðNO�2 Þ can then be further

oxidized to NO�3 ions. Taking into account that the amount of

organic nitrogen converted to ammonium ions is more than

the expected one from the aliphatic amid group in the

molecule, we could hypothetize that except the oxidative

mechanism, Chloramphenicol could also participate to a

reductive degradation pathway. Generally, nitrate does not

represent the total nitrogen available from the nitrophenols.

Maurino et al. (1997) reported that the quantity of nitrate ions

reached approximately 70–80% of the total initial organic

nitrogen, whilst Augugliaro et al. (1991) found 60–70% of

nitrate at the end of the photodegradation runs. The

remaining percentages of inorganic nitrogen were usually

due to the formation of ammonium ions (Dieckmann and

Gray, 1996). Maurino et al. (1997) have reported that the direct

reduction of the nitrogroup in nitrophenols can be an

important degradation pathway, even in oxygenated solu-

tions. This means that oxygen and nitrophenol compete for

electrons as confirmed also by the presence of 4-aminophenol

during the initial steps of degradation of 4-nitrophenol

(Maurino et al., 1997; Di Paola et al., 2003). The reaction path

leading to ammonia must involve the nitrogroup before it

forms nitrate ions. Mahdavi et al. (1993) have shown that the

nitrogroup can be reduced to an amine group on the ring and

then is released into the system as ammonia.

On the other hand, the nitrogen mineralization in the

presence of TiO2 (A) is a slower process in comparison to the

TiO2 P-25 one, with a 60% conversion after 4 h of illumination.

This is indicative of the presence of various aliphatic

nitrogen-containing reaction products and is also in accor-

dance with the lower organic carbon reduction in the same

time, leading to the conclusion that longer illumination times

are needed for the complete conversion to inorganic end

products.

3.5. Inhibition of the antimicrobial activity ofChloramphenicol

The results concerning the inhibition of the drug activity of

Chloramphenicol before and after the photocatalytic treatment

ARTICLE IN PRESS

0 20 40 60 80 1000

20

40

60

80

100

drug

act

ivity

inhi

bitio

n (%

)

illumination time (min)

Fig. 9 – Antimicrobial susceptibility test. The initial

antibiotic activity of 50 lg mL�1 Chloramphenicol (100%)

was reduced to 40%, 20% and 0% after 30, 60 and 90 min

photocatalytic treatment, respectively.

WAT E R R E S E A R C H 4 2 ( 2 0 0 8 ) 3 8 6 – 3 9 4 393

in the presence of 1 g L�1 TiO2 P-25 for given irradiation times

are presented in Fig. 9.

The drug activity of either the untreated solution contain-

ing 50 mg L�1 of Chloramphenicol or the photocatalytically

treated solution at different time points was evaluated in

comparison to the activity of standards containing known

amounts of the antibiotic. As shown in Fig. 9 after 30 min of

the treatment, only 40% of the initial antibiotic activity was

detectable according to the Kirby–Bauer disk diffusion test.

Further illumination of the TiO2/Chloramphenicol disper-

sion for 90 min causes a 100% reduction of the initial

antibiotic activity.

4. Conclusions

In the present work the photocatalytic oxidative degradation

of Chloramphenicol, an antibiotic drug, has been demon-

strated. It was observed that, with TiO2 P-25 and ZnO as

photocatalysts, quantitative degradation of the organic mo-

lecule occurs after 4 h. During this time in the presence of

TiO2 P-25, the dechlorination of the substrate is complete,

while the nitrogen in the organic compound is recovered in

the form of nitrate and ammonium ions. The activation

energies for both photocatalysts calculated according to

Arrhenius equation shows that temperature influences

slightly the photodegradation process. The fast reduction of

the drug activity shows that heterogeneous photocatalysis

could be employed as a powerful tool for the deactivation of

this class of materials. The use of a catalyst such as TiO2, and

the possibility of activating it with UV-A or solar light,

combined with the simple technology required for this

method, can offer economically reasonable and practical

solutions to processing of wastewater containing pharma-

ceutically active compounds such as analgestics, antibiotics,

steroid and hormones.

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