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Sonocatalytic degradation oftetracycline antibiotic in aqueoussolution by sonocatalysisMohammad Hoseinia, Gholam Hossein Safaria, Hossein Kamania,Jalil Jaafaria, Marjan Ghanbaraina & Amir Hossein Mahviabc

a Department of Environmental Health Engineering, School ofPublic Health, Tehran University of Medical Sciences, Tehran, Iranb Center for Solid Waste Research, Institute for EnvironmentalResearch, Tehran University of Medical Sciences, Tehran, Iranc National Institute of Health Research, Tehran University ofMedical Sciences, Tehran, IranPublished online: 29 Apr 2014.

To cite this article: Mohammad Hoseini, Gholam Hossein Safari, Hossein Kamani, Jalil Jaafari,Marjan Ghanbarain & Amir Hossein Mahvi (2013) Sonocatalytic degradation of tetracyclineantibiotic in aqueous solution by sonocatalysis, Toxicological & Environmental Chemistry, 95:10,1680-1689, DOI: 10.1080/02772248.2014.901328

To link to this article: http://dx.doi.org/10.1080/02772248.2014.901328

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Sonocatalytic degradation of tetracycline antibiotic in aqueous

solution by sonocatalysis

Mohammad Hoseinia, Gholam Hossein Safaria, Hossein Kamania, Jalil Jaafaria, Marjan

Ghanbaraina and Amir Hossein Mahvia,b,c*

aDepartment of Environmental Health Engineering, School of Public Health, Tehran University ofMedical Sciences, Tehran, Iran; bCenter for Solid Waste Research, Institute for EnvironmentalResearch, Tehran University of Medical Sciences, Tehran, Iran; cNational Institute of HealthResearch, Tehran University of Medical Sciences, Tehran, Iran

(Received 24 December 2013; accepted 28 February 2014)

Tetracycline (TC), one of the most common antibiotics, is often poorly bio-degraded inconventional wastewater treatment plants. In this study, the sonocatalytic degradation ofTC was investigated using TiO2 nano-particles as catalyst. The effect of pH, initial TCconcentrations, reaction times, and H2O2 concentrations were evaluated. The efficacy ofultrasonic irradiation alone in the removal of this pollutant was negligible but removalefficiency increased upon addition of TiO2 up to 250 mg L�1; increase of pH and initialTC concentration attenuated TC degradation. Addition of H2O2 raised the removalefficiency so that complete removal of TC was achieved within 75 min.

Keywords: H2O2; nano-TiO2; tetracycline; ultrasound; water pollution

Introduction

In recent years, pharmaceutical residues in the numerous environmental matrices have

been detected in various environmental matrices, including groundwater, surface water,

and municipal wastewater (Dalm�azio et al. 2007; Bottoni, Caroli, and Caracciolo 2010).

Specifically, tetracyclines (TCs) are a large group of antibiotics widespread used in

human and veterinary medicine and account for approximately 29% of total antibiotic

use (Khetan and Collins 2007; Wammer et al. 2011). They have been shown to disrupt

microbial soil respiration (Boleas et al. 2005), nitrification (Halling-Sørensen 2001), and

phosphatase activities.

Conventional water and wastewater treatment processes have failed to destruct TC

antibiotics effectively (Maroga Mboula et al. 2012). Due to their antibacterial nature, the

removal of TC antibiotics by traditional biological methods is generally incomplete

(Reyes et al. 2006). The conventional physical or physical–chemical methods such as

coagulation, sedimentation, filtration, and chlorination may control the mobility and

spread of TC, but further treatment processes are required (Zhang et al. 2009).

In general, advanced oxidation processes (AOPs) are the most efficient technology for

the degradation of various refractory pollutants in aqueous solutions and during the last

few decades, they have been widely applied for the removal of many organic pollutants

such as pesticides (Bazrafshan et al. 2007), phenol (Maleki et al. 2007; Mahvi and Maleki

2010), humic substances (Mahvi et al. 2009), etc. In recent years, AOPs have been dem-

onstrated to be an effective technology for the removal of pharmaceuticals, including TC

*Corresponding author. Email: ahmahvi@yahoo.com

� 2014 Taylor & Francis

Toxicological & Environmental Chemistry, 2013

Vol. 95, No. 10, 1680–1689, http://dx.doi.org/10.1080/02772248.2014.901328

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antibiotics, from water (Maroga Mboula et al. 2012). Ultrasonic (US) irradiation is an

AOP which has emerged to remove lower levels of pollutants in water and wastewater

(Mahvi 2009). US mediated by suitable catalysts is an emerging technology which has

been investigated by some researchers for the removal of various pollutants and, to some

extent, pharmaceuticals from water. The main objectives of the present study were to

investigate the efficacies of US irradiation, US/TiO2, and US/TiO2/H2O2 methods in

removing TC antibiotic from aqueous solutions. The influences of operational variables

(pH and concentrations of TC, TiO2, and H2O2) on TC removal efficiency were explored.

Materials and methods

Tetracycline hydrochloride (C22H24O8N2.HCl) with a purity of over 95% was purchased

from Sigma–Aldrich (Munich, Germany) and was used without further purification.

Aqueous solutions of TC at concentrations of 25, 50, 75, and 100 mg L�1 were prepared

by dissolving TC in de-ionized water. Solutions of H2O2 at the four concentrations, 20,

50, 75, and 100 mg L�1, were prepared by diluting 30% of H2O2 (Merck Co., Darmstadt,

Germany) and the concentrations were verified by standard iodometry. Titanium dioxide

(TiO2, P25, Degussa AG, D€usseldorf, Germany) with a specific surface area of 50 �15 m2 g�1, primary particle diameter of 21 nm, and a crystal distribution of 80% anatase

and 20% rutile was used as catalyst in the range of 100–500 mg L�1. Adjustments of pH

were done with 0.1 mol L�1 NaOH and HCl (Merck Co.) using a pH meter (E520,

Metrohm, Tehran, Iran). Sonication of the solution in a 200-mL glass reactor was per-

formed with a US generator at a frequency of 35 kHz and power of 500 W (Elma, Singen,

Germany).

Since the laboratory method has been described in detail previously (Hou, Zhang, and

Xue 2012), it is only briefly outlined here. To achieve adsorption–desorption equilibrium,

at first TiO2 with the concentration of 250 mg L�1 was added to 200 mL of TC solution

(concentration of 75 mg L�1) in a 250-mL glass reactor and stirred with a mechanical stir-

rer. After 30 min, H2O2 at concentrations of 20, 50, 75, and 100 mg L�1 was added and

sonication was performed by immersing the reactor into the US bath. All experiments

were conducted at constant temperature (25 �C) by cooling with water. The position of

the reactor in the US bath was always kept the same. At time intervals of 15, 30, 45, 60,

90, and 120 min, 2-mL aliquots were taken, filtered through 0.22 mm membrane filter,

and mixed with 2-mL methanol to quench the reaction between OH� radicals and the

remaining TC before analysis.

For high performance liquid chromatography (HPLC)-determination of TC, an HPLC

instrument (LC-20 AB pump, Shimadzu, Kyoto, Japan) equipped with a reversed-phase col-

umn (VP-ODS-C18 4.6 mm � 250 mm, 5 mm, Shim-Pack, Kyoto, Japan) and ultraviolet

detector (UV-1600 spectrophotometer, Shimadzu) was used. Injection volume was 20 mL,mobile phase was acetonitrile and 0.01 mol L�1 oxalic acid solution (31:69, v/v) at a flow

rate of 1.0 mL min�1, and detection was at 359 nm. The retention time of TC was 3.6 min.

Results and discussion

Removal of TC under different oxidation systems

The removal efficiency of TC at an initial concentration of 75 mg L�1 (Figure 1) was slow

with US alone. This result is similar to the results reported by Hou, Zhang, and Xue

(2012), who studied the application of 20 kHz US on the degradation of TC. Similarly,

Toxicological & Environmental Chemistry 1681

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they found that the concentration of TC was hardly decreased when US alone was applied

to the TC solution and they reported that it could be due to the reason that a limited

amount of OH� is formed in the bulk solution at low frequency of US. In another study

performed by Hou et al. (2012), the researchers have found that the removal of sulfameth-

oxazole antibiotic from aqueous solution was negligible in the presence of US alone due

to the limited production of OH� in the bulk solution at low frequency of US, and this

fact that the reaction under US process occurred mainly at the interface of cavitation bub-

bles through a radical reaction rather than inside of the bubbles through a pyrolytic reac-

tion. When TiO2 nano-particles were added into solution simultaneously with US

irradiation, an improvement in the removal efficiency was observed; however, no signifi-

cant enhancement was achieved. It can be due to the fact that only a small amount of

active radicals is formed during the catalytic ultrasound reactions in the absence of oxi-

dant (Hou, Zhang, and Xue 2012). Integration of H2O2 with ultrasound and TiO2 nano-

particles significantly increased the removal efficiency. The removal efficiency was

90.62% in the US/TiO2/H2O2 process after the reaction time of 15 minute which was

much higher than those the US/TiO2 process (by 21.3%). It is attributed to this fact that

H2O2 can be decomposed under the influence of sonication to reactive hydroxyl radicals,

thus promote TC degradation (Isariebel et al. 2009). Table 1 gives an overview on TC

degradation by some AOPs as well as sonocatalysis.

The effect of TiO2 concentrations on TC sonodegradation

Figure 2 shows the removal of TC at different concentrations of TiO2 at neutral pH, US

35 KHz, and TC concentration of 75 mg L�1. TC removal increased with increasing the

TiO2 dosage and then reached a plateau at a TiO2 concentration of 250 mg L�1, after

which it remained approximately constant. This can be due to the fact that when all TC

molecules are adsorbed on the nano-particles, any further increase would not have an

effect on degradation efficiency.

The increase in TC degradation with increasing the density of TiO2 nano-particles can

be due to increased number of active sites, higher formation of OH�, and more effective

Figure 1. The removal of TC with initial concentration of 75 mg L�1 under different oxidationsystems at acidic conditions (C0= 75 mg L�1; US 35 KHz; US/TiO2: TiO2 ¼ 250 mg L�1; US/TiO2/H2O2: TiO2 ¼ 250 mg L�1, H2O2 ¼ 100 mg L�1).

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Table1.

Rem

ovalofTCbydifferentAOPs.

TCinitialconcentration

Matrix

AOPfeatures

Operationconditions

Summaryofresults

Reference

[TC]¼

10–50mgL�1

Distilled

water

UV/TiO

2MPUV125W

TiO

2

(100%

anataseor

anatase/rutile¼

4/1);

1and0.4gL�1

More

than

98%

TC

degradationafter120min

Addam

oetal.(2005)

[TC]¼

40mgL�1

De-ionized

water

UV/TiO

2UV(254nm,365nm);

TiO

2;05–1.0gL�1

50%

and100%

degradation

(UV365and254nm,

respectively);0.5gL�1

TiO

2;after120min

Reyes

etal.(2006)

[TC]¼

10mgL�1

Distilled

water

Photoelectrocatalytic

process

TiO

2photoanode;UV

(254nm);pH¼

5.5

More

than

80%

of

tetracyclinedegradation

after180min

Liu

etal.(2009)

[TC]¼

25–100mgL�1

De-ionized

water

US/TiO

2/H

2O2

35KHzUS;Suspended

DegussaTiO

2;

0.1–0.5gL�1;H2O2

20–100mgL�1

100%

TCdegradationafter

75min

Thisstudy

Toxicological & Environmental Chemistry 1683

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interaction with the TC molecules. It has been found that the addition of particles of

appropriate amount and size into the liquid system under the US irradiation results in

increase in the acoustic noise and a rise in temperature in the irradiated liquid (Tuziuti

et al. 2005). Due to high surface energy, nano-particles tend to aggregate, while US irradi-

ation favors dispersion of the catalyst particles. The increase in TiO2 dosage in the solu-

tion leads to the formation of more nucleation sites for cavitation bubbles at their surface

(Anju et al. 2012). This will result in decrease in the cavitation thresholds responsible for

the increase in the number of bubbles in the liquid under the US irradiation and increase

the pyrolysis of water and the sonocatalytic TC degradation. However, when the density

of TiO2 nano-particles exceeds a certain value, the catalyst particles may agglomerate

resulting in lack of significant increase in TC removal efficiency (Wang et al. 2005).

Besides, deactivation of the activated TiO2 through collisions with ground-state catalysts

may occur in these conditions. Higher density of the catalyst may also disturb the trans-

mission of US in solution. Therefore, no further increase in the degradation of the TC

was observed beyond the optimum TiO2 dosage. There are inconsistent results regarding

the influence of nano-particles in sonochemical reactions. Hartmann et al. (2008) have

found that the addition of TiO2 particles did not cause an increase in the H2O2 formation

in the sonication of drug diclofenac, while under the same conditions of Hartmann’s

study, Keck, Gilbert, and K€oster (2002) have reported an increase in the formation of

H2O2. The results of our study, however, are in accordance with the results reported by

other researchers who studied the effect of TiO2 nano-particles on the degradation of

other organic pollutants in various AOPs (Mahvi et al. 2009; Dobaradaran et al. 2010;

Hoseini et al. 2013). Therefore, as the results showed, the optimum amount of TiO2 cata-

lyst was 250 mg L�1.

The effect of initial pH on sonocatalytic TC degradation

It has been found that pH can play an important role in the sonochemical degradation of

pollutants (De Bel et al. 2009). To investigate the influence of initial pH on the removal

of TC, the pH values were varied as 4, 5.5, 7.5, and 9.0. As shown in Figure 3, TC

removal efficiency increased with the decrease of pH. The removal efficiencies were

Figure 2. Degradation of TC at different TiO2 concentrations (C0 ¼ 75 mg L�1, US ¼ 35 KHz, pH ¼7.5).

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64.3%, 52.3%, 41.5%, and 32.6% at pHs 5.5, 9, 7.5, and 4, respectively. Similar results

were obtained by De Bel et al. (2009) who found that degradation of antibiotic ciprofloxa-

cin at pH 3 was almost four times faster than at pH 7. The results are also in accordance

with the results of G€uyer and Ince (2011). The pH value influences the adsorption and dis-

sociation capacity of compounds, the charge distribution on the catalyst surface, and the

oxidation potential of the valence band of catalyst. Besides, the pH modifies physical

properties (including charge and hydrophobic enrichment) of molecules with ionizable

functional groups (De Bel et al. 2009). However, it is difficult to interpret the influence of

pH value on sonocatalytic process and also other AOPs. The widely accepted method of

interpretation of the role of pH on these processes is the point of zero charge of TiO2,

which influences the ionization rate of the TiO2 surface (Zhang et al. 2010). It has been

found that the pH point of zero charge of TiO2 P-25 was about 6 (Bizani et al. 2006). It

means that below this value, the TiO2 surface is positively charged, while above the

value, it is negatively charged. On the other hand, it has been found that TC predomi-

nantly existed as positively charged molecule at acidic pH (Wang, Yap, and Lim 2011).

Therefore, because both the TiO2 surface and TC molecule are positively charged at

acidic pH, the TC removal must be theoretically low at acidic pH. However, in this study,

the removal efficiency increased with the decrease of pH. This can be due to this fact that

the positively charged TC molecules at this pH will accumulate at the negatively charged

liquid–bubble interface of solution in which the concentration of reactive radicals and the

temperature are higher (Watmough et al. 1992; De Bel et al. 2009). This could be the rea-

son why the degradation efficiency of TC is higher at acidic pH.

In addition, in acidic conditions, aggregation of TiO2 nano-particles is lower than in

neutral conditions, resulting in increased surface area of the catalyst and enhanced sono-

catalytic degradation of TC.

The effects of reaction time and initial TC concentration on TC degradation

The removal efficiency increased with increasing the reaction time from 15 to 90 min and

then remained approximately stable (Figure 4). The results also showed that the removal

Figure 3. The effect of initial pH on the removal of tetracycline (C0=75 mg L�1, US=35 KHz,TiO2 ¼ 250 mg L�1.

Toxicological & Environmental Chemistry 1685

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efficiency decreased with the increase of the initial TC concentration. For example, at ini-

tial TC concentration of 25 mg L�1 and contact time of 90 min, 70.1% TC removal was

achieved; whereas for the concentrations of 50, 75, and 100 mg L�1, the TC removal rates

were 65.3%, 58.3%, and 50.0%, respectively. This result was in accordance with the

results found by other researchers who studied degradation of different antibiotics by var-

ious AOPs including sonocatalytic processes. For example, Hou, Zhang, and Xue (2012)

investigated the degradation of TC by Fe3O4-activated peroxydisulfate (Na2S2O8) in the

presence of US irradiation and found that at constant conditions, decreasing the initial

concentration of TC caused an increase in removal rate. The reduction in removal rate

with increasing initial TC concentration can be explained by the fact that under the same

conditions, the amount of OH� radicals formed were equal in all solutions; therefore, the

reaction of TC molecules with OH� radicals becomes more likely at lower TC concentra-

tions (Wang et al. 2005; Elmolla and Chaudhuri 2010).

Effect of H2O2 concentration on sonocatalytic TC degradation

In sonochemical process, H2O2 can enhance degradation rate since it may be decomposed

by US to reactive hydroxyl radicals, thus promoting pollutant degradation (Isariebel et al.

2009). To investigate the influence of H2O2 on the TC removal rate, TC concentration of

75 mg L�1 was exposed to US during five reaction times with the optimized values of

TiO2 (250 mg L�1) and pH value (4). Figure 5 depicts the removal of TC at different

H2O2concentrations. Complete removal of TC was achieved in all concentrations of

H2O2 after 75 min. The results showed that the combination of H2O2 with US and TiO2

nano-particles caused a significant increase in TC degradation rate and the removal effi-

ciency increased quite rapidly up to an H2O2 concentration of 100 mg L�1, and at concen-

trations higher than this, the TC removal rate decreased.

Therefore, although H2O2 acts as a free radical source by the dissociation process, it

can act as a radical scavenger at high concentrations. Therefore, there is a maximum

H2O2 concentration, beyond which H2O2 acts as a radical scavenger, thus leading to

decreased degradation. There are several studies regarding the positive influence of H2O2

Figure 4. Effects of reaction time and initial TC concentration on the TC removal.

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on sonodegradation of different organic pollutants (Svitelska, Gallios, and Zouboulis

2004; Vassilakis et al. 2004; Emery et al. 2005). Isariebel et al. (2009) found that the deg-

radation of drug levodopa under the integration of sonication and H2O2 increased up to

an H2O2 concentration of 948 mg L�1, above which it decreased. The difference in the

optimum H2O2 concentrations can be attributed to the difference in the nature and degra-

dation sites of studied compounds. It has been found that the influence of the integrated

US plus H2O2 oxidation process mainly depends on the utilization of free radicals by the

pollutant molecules, which in turn depends on the efficiency of collision of free radicals

with the pollutant over a specified time period (Isariebel et al. 2009). This is another rea-

son for the difference in the optimum H2O2 concentrations at various studies.

Conclusions

Sonocatalytic degradation of TC was performed under various experimental conditions

including different TiO2 nano-particle dosage, pH, and initial TC and H2O2 concentra-

tions. The results of this study showed that US treatment alone may not be used as an effi-

cient method for the removal of TC antibiotics from aqueous solution. However, removal

efficiency was improved by the addition of TiO2 nano-particles and H2O2 to US process

which resulted in the complete removal of TC after 75-min reaction time. It was found

that decreasing the initial TC concentration caused an increase in the removal rate. The

optimum conditions for TC removal in US/TiO2/H2O2 process was achieved at a TiO2

concentration of 250 mg L�1, an H2O2 concentration of 100 mg L�1, and an acidic pH.

Although the application of US has been shown to be feasible on a small scale, the use of

this technology in large scale treatment process is still a challenge, due to the high energy

requirement of the process. This weaknesses result in the limitation of US technology to

be used widely in a real wastewater treatment plant. As a general conclusion, the results

indicated that US/ TiO2/H2O2 could be a suitable oxidation process for the removal of

refractory pollutants from aqueous solutions on small scales.

Figure 5. Effect of H2O2 concentration on sonocatalytic TC degradation (C0 ¼ 75 mg L�1, US ¼35 KHz, TiO2 ¼ 250 mg L�1, pH ¼ 4).

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Acknowledgments

This research has been supported by Tehran University of Medical Sciences & Health Servicesunder grant no. 92-02-61-20501. The authors express their gratitude to all laboratory staff of theDepartment of Environmental Health Engineering, Tehran University of Medical Sciences, for theirsupport throughout this study.

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