electrochemical degradation of pharmaceutical ...

Environmental Engineering and Management Journal

March 2012, Vol.11, No. 3, 627-634

http://omicron.ch.tuiasi.ro/EEMJ/

“Gheorghe Asachi” Technical University of Iasi, Romania

ELECTROCHEMICAL DEGRADATION OF PHARMACEUTICAL EFFLUENTS ON CARBON-BASED ELECTRODES Sorina Motoc1, Florica Manea1, Aniela Pop1, Rodica Pode1, Carmen Teodosiu2 1

“Politehnica” University of Timisoara, 2 P-ta Victoriei, 300006 Timisoara, Romania ”Gheorghe Asachi” Tehnical University of Iasi, 71 Prof. Dr. Doc. D. Mangeron Blvd., 700050 Iasi, Romania

2

Abstract This study aimed to characterize the electrochemical behaviors of three types of carbon-based electrodes, i.e., boron-doped diamond (BDD), glassy carbon (GC), and multiwall carbon-nanotubes-epoxy composite (CNT) electrodes in the presence of ibuprofen (IBP), which was chosen as a model of the pharmaceutical pollutant from water. Cyclic voltammetric results informed that CNT electrode allowed to reach the best oxidation current signal for IBP electrooxidation, which can be more enhanced by the presence of chloride in the supporting electrolyte. Under chronoamperometry (CA) and multiple-pulsed amperometry (MPA) applying conditions, CNT electrode exhibited the best performance in terms of the degradation and the mineralization efficiency. However, higher charge consumption was recorded in comparison with BDD electrode, which denoted a limited practical application for CNT electrode. Key words: carbon-based electrode, electrochemical degradation, pharmaceutical effluent Received: September, 2011; Revised final: February, 2012; Accepted: March, 2012

1. Introduction Over the past century the evolution in the sciences has led to major breakthroughs that have expanded and vastly improved human life in the area of exploration and development of pharmaceuticals. Pharmaceuticals and endocrine disrupting compounds (EDCs) are a structurally diverse class of emerging contaminants that have been detected throughout the environment, especially in wastewater that impacted surface water, groundwater, estuarine water, and drinking water. The widely used nonsteroidal anti-inflammatory drugs: ibuprofen, naproxen, diclofenac and some of their metabolites are very often detected in sewage, surface and ground water, drinking water, tap water, ocean water, sediments and soil. Although the amount of these pharmaceuticals in the aquatic environment is low, its continuous input may constitute in the long term a 

potential risk for aquatic and terrestrial organisms. Therefore, over the past few years they are considered to be an emerging environmental problem (Fent et al., 2006; Hernando et al., 2005; Klavarioti et al., 2009; Kumar et al., 2010; Miege et al., 2009; Nikolaou et al., 2007; Ter Laak et al., 2010). Ibuprofen (IBP) is the most consumed medicines all over the world, having an estimated annual production of several tons e.g. 345 t in Germany in 2001 (Fent et al., 2006). Industrial pollution occurs because of the release of untreated effluents from the pharmaceutical companies (IBP is a non-prescription, non-steroidal drug, widely used as an anti-inflammatory and antipyretic drug especially prescribed for the treatment of fever, migraine, muscle aches, arthritis and tooth aches), whereas the domestic contamination is due to the use of IBP as a medicinal drug by human beings and animals.

Author to whom all correspondence should be addressed: e-mail: [email protected]; Phone: +40256-403-071; Fax: +40256-403-069

Motoc et al./Environmental Engineering and Management Journal 11 (2012), 3, 627-634

Various treatment processes for ibuprofen removal from wastewater have been reported, e.g., activated carbon (Mestre et al., 2009), ultrafiltration (Jermanna et al., 2009), biological (Marco-Urrea et al., 2009), photo-Fenton (Mendez-Arriaga et al., 2010), supramolecular solid-phase extraction (Costi et al., 2008), photolytic (Castell et al., 2008), gamma irradiation (Zheng et al., 2011). The selection of the treatment methods have to take into account the process performance in relation with process efficiency and economic aspects. Electrochemical methods represent a very promising way to achieve the total degradation of wastewaters containing persistent pollutants. Recent studies have been conducted on the electrodegradation of pharmaceutical pollutants (Ai et al., 2011; Ciriaco et al., 2009; Dermentzis et al., 2011; Garrido et al., 2007; Isarain-Chavez et al., 2011). These are based on direct electrolysis, where pollutants are removed by direct electron transfer at the electrode or indirect electrolysis, where pollutants are destroyed in the bulk solution by active species generated at the electrode. In general the electrochemical methods exhibit several advantages, e.g., environmental compatibility, since the used reagent is the electron, versatility, because pollutants can be treated using different reactors and electrode materials of distinct forms and configurations. In addition, processes can be easily scaled from laboratory to industrial plant, security, due to the mild operative conditions employed and the small amounts of generally innocuous chemicals used, and energy efficiency, since electrochemical processes are operated at low pressure and temperature (environmental conditions), using low cost equipment and simple operations. The key of the electrochemical performance is given by the electrode material. Carbon, in many respects, represents an ideal electrode. Its attractive features include access to a wide anodic potential range, low electrical resistance and residual currents and a reproducible surface structure. A large variety of carbon forms suitable for the degradation of pharmaceuticals by electrochemical degradation have been reported, e.g., glassy carbon (GC) (Hegde et al., 2009; Ozkan et al., 2004; Patil et al., 2011; Shamsipur et al., 2010; Tabeshnia et al., 2010), carbon pastes (CP) (Shahrokhian et al., 2004), boron doped diamond (BDD) (Brillas et al., 2010, Dominguez et al., 2010; Garrido et al., 2007; Klavarioti et al., 2009; Murugananthan et al., 2008; Sires et al., 2006; Zhao et al., 2009), and carbon nanotubes (Heli et al., 2009; Li and Xing, 2009; Tang et al., 2006; Yang et al., 2008). Recently, the use of carbon nanotubes (CNTs) as a catalyst has recently attracted growing attention for environmental applications, including the oxidation of organic compounds present in polluted waters by catalytic wet oxidation or electrooxidation. The aim of this study is to characterize the electrochemical behaviors of three types of carbon-

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based electrodes, i.e., boron-doped diamond (BDD), glassy carbon (GC), and multiwall carbon-nanotubesepoxy composite (CNT) electrodes in the presence of ibuprofen (IBP), which was chosen as a model of the pharmaceutical pollutant from water. The electrochemical characterization and applications of the electrode for IBP degradation and mineralization were performed using cyclic voltammetry (CV), chronoamperometry (CA) and multiple-pulsed amperometry (MPA) techniques. The electrochemical performances of the electrodes were assessed in terms of degradation and mineralization process efficiency and electrochemical efficiency. Also, the effect of chlorine/hypochlorite oxidation agent generated in-situ was investigated to improve the electrochemical performance of CNT. 2. Experimental 2.1. Electrochemical characterization application of the electrodes

and

The electrochemical behavior of the BDD, GC and CNT electrodes in the presence of Ibuprofen (IBP) were studied by cyclic voltammetry (CV). Subsequently, an electrochemical pretreatment by three repetitive cycling between -0.5 V to 1.5 V vs. Ag/AgCl in 0.1M Na2SO4 and respective, 0.075M Na2SO4 and 0.025M NaCl supporting electrolytes was performed. Prior to use, the working electrodes were gradually cleaned, first polished with abrasive paper and then on a felt-polishing pad by using 0.5 μm alumina powder (Metrohm, Switzerland) in distilled water for 5 minutes and rinsing with distilled water. The electrochemical experiments were carried out at room temperature (22-25 °C) using an undivided cell of 0.1 dm3 volume. IBP mineralization was checked by TOC and COD parameters, which were analyzed using a Shimadzu TOC analyzer and respective, standard method. Thus, the degradation of IBP was calculated based on IBP concentration determined by absorbance recorded at 220 nm wavelength. The IBP degradation degree was determined using the Eq. (1): 

IBP0  IBP   100 (%) IBP0

(1)

The electrochemical efficiency for oxidation was determined based on Eq. (2): E IBP 

IBP0  IBP   V (mg / C·cm2) QS

IBP

(2)

where IBP0-IBP represent the change in the IBP concentration determined spectrophotometrically , during experiments for a charge consumption of Q corresponding to various electrolysis time, V is the sample volume (30 cm3) and S is the area of the electrode surface (cm2).

Electrochemical degradation of pharmaceutical effluent on carbon-based electrodes

The mineralization degree was assessed also based on Eq. (1) using COD and TOC parameters instead of IBP concentration determined spectrophotometrically. The electrochemical efficiency for IBP mineralization defined as an overall efficiency for complete oxidation to CO2 was determined based on Eq. (2) modified as 2’ and 2” taking into consideration the change in TOC and COD measurements during experiments, determining TOC0 – TOC and COD0-COD, respectively: ETOC 

TOC0  TOC   V

ECOD 

COD0  COD   V (mg / C·cm2)

QS

(mg / C·cm2)

QS

In according with the literature (Canals, 2009), the products of IBP oxidation could be 2-(4isobutylphenyl)-2-hydroxypropionic acid and 1-(1hydroxyethyl)-4-isobuylbenzene.

(2’) (2”)

All measurements were carried out using an Autolab potentiostat/galvanostat PGSTAT 302 (Eco Chemie, The Netherlands) controlled with GPES 4.9 software and a three-electrode cell, with a Ag/AgCl electrode reference electrode, a platinum counter electrode and BDD, GC and CNT as working electrodes. Standard solution of 1 gL-1 Ibuprofen was prepared from analytical reagent from BASF SE, (Ludwigshafen, Germany) using distilled water and 0.1 M NaOH solution. 0.1 M Na2SO4, and mixture of 0.075M Na2SO4 and 0.025M NaCl solutions as supporting electrolytes for the characterization and application of electrodes materials were freshly prepared from Na2SO4 and NaCl of analytical purity (Merck) with distilled water.

a)

b)

3. Results and discussions a) Cyclic voltammetric measurements Fig. 1 a-c shows cyclic voltammograms (CVs) recorded in 0.1 M Na2SO4 supporting electrolyte in the presence of 10 and 30 mgL-1 IBP on GC, BDD and CNT electrodes. Based on the voltammograms shapes, it can be noticed that the direct IBP oxidation process occurred on each electrode, and the voltammetric parameters are gathered in Table 1. For each electrode, the anodic oxidation current increased with IBP concentration increasing, and the best oxidation signal was achieved using CNT electrode. In order to generate in-situ hypochlorite by chloride oxidation, the electrochemical behavior of CNT electrode in the presence of chloride is investigated. In Figure 2 are presented CVs recorded in 0.1 M Na2SO4 supporting electrolyte and in the mixture of 0.075 M Na2SO4 and 0.025 M NaCl in the presence of 50 mgL-1 IBP. The CV shape is modified in the presence of Cl-, an oxidation peak occurred at the potential value of about + 1 V/Ag/AgCl that could be attributed to the chloride oxidation. Also, a very clear oxidation peak of IBP is evidenced in the presence of chloride, and the IBP oxidation signal is strongly improved (235 A in comparison with 37 A in absence of chloride, for 50 mgL-1 IBP).

c)

Fig. 1. Cyclic voltammograms recorded in 0.1 M Na2SO4 supporting electrolyte (curve 1) and in the presence of various IBP concentrations (curve 2-10 mgL-1 IBP, curve 3-30 mgL-1 IBP) at: a) GC electrode; b) BDD electrode; c) CNT electrode; potential scan rate: 0.05Vs-1; potential range: -0.5 to +1.5V/Ag/AgCl

During the reverse scanning no cathodic peaks appeared that informed about the irreversibility of the IBP oxidation process. CVs recorded in the presence of various IBP concentrations informed that the anodic current recorded at about +1.2 V vs. Ag/AgCl increases direct proportionally with increasing IBP concentrations in solution (the results are not shown here), which could inform about diffusion controlled IBP oxidation process.

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Table 1. Voltammetric parameters determined by cyclic voltammetry recorded in 0.1 M Na2SO4 supporting electrolyte and in the presence of IBP; potential scan rate of 0.05 Vs-1; potential range: -0.5 to +1.5V/Ag/AgCl IBP conc / mgL-1 10 30

Electrode material GC BDD CNT GC BDD

Eoxidation starting / V 1.38 1.25 1.20 1.38 1.25

b) Chronoamperometry measurements To obtain information about electrode fouling and the electrochemical activity of GC, BDD and CNT electrodes used for IBP oxidation at the anodic potential values of +1.5 V and respective, 2 V vs. Ag/AgCl, chronoamperometric measurements were performed. Chronoamperometry is very useful to simulate the electrooxidation application under potentiostatic conditions to determine the optimum operating electrolysis conditions. Fig. 3 a and b presents the chronoamperograms recorded at BDD and GC electrodes in 0.1 M Na2SO4 supporting electrolyte for the time duration of 2 h, and each electrode exhibited a different behavior in relation with the applied potential value.

Eoxygen evolution / V 1.45 1.50 1.30 1.45 1.50

Useful signal / µA 5.76 2.92 15.34 12.74 2.20

evolution had just started, while for GC and CNT electrodes the oxygen evolution occurred. In contrast, at the applied potential value of +2 V vs. Ag/AgCl, BDD electrode exhibited the activated electrode surface for the IBP electrooxidation and the current increased. This effect was not noticed for GC, for which a slight current decreasing was recorded. Even if the chloride presence led to a higher current recorded at CNT electrode, no electrode surface activation was noticed (Fig. 4).

a)

Fig. 2. Cyclic voltammograms recorded at CNT electrode in: 1-0.1 M Na2SO4 supporting electrolyte; 2- 0.075 M Na2SO4 and 0.025M NaCl supporting electrolyte; 3-50 mgL-1 IBP in 0.1 M Na2SO4 supporting electrolyte; 4-50 mgL-1 IBP in 0.075 M Na2SO4 and 0.025M NaCl supporting electrolyte; potential scan rate: 0.05Vs-1; potential range: -0.5 to +1.5V/Ag/AgCl

Also, the chronoamperograms recorded comparatively at CNT electrode in 0.1 M Na2SO4 supporting electrolyte and the mixture of 0.075 M Na2SO4 and 0.025 M NaCl supporting electrolyte are shown in Fig. 4. At the applied potentials, the electrolysis is under mass-transport control and the secondary reactions of oxygen evolution are involved with a decreasing current efficiency (Bebeselea et al., 2009). Thus, during the chronoamperograms running at the potential value of +1.5 V vs. Ag/AgCl the recorded current decreased because of the electrode fouling due to the oxidation products adsorption on the electrode surface. At this potential value, this effect is more pronounced for BDD electrode because the oxygen 630

b) Fig. 3. Chronoamperograms (CAs) recorded at E=+1.5V vs. Ag/AgCl (curve 1) and E=+2V vs. Ag/AgCl in 0.1 M Na2SO4 supporting electrolyte and in the presence of 10 mgL-1 IBP concentration at a) BDD electrode, and b) GC electrode

The determination of IBP concentrations after chronoamperometry running under the potentiostatic conditions were performed by spectrophotometric, classical COD and TOC methods to assess the process degradation efficiency () and the electrochemical efficiency (E).


The results regarding the electrode performance for the electrooxidation of IBP are gathered in Table 2.

Fig. 4. Chronoamperograms (CAs) recorded at E = +1. 5 V vs. Ag/AgCl in the presence of 10 mgL-1 IBP at CNT electrode in: 1-0.1 M Na2SO4 supporting electrolyte and 20.075M Na2SO4 and 0.025M NaCl

It can be seen that CNT electrode exhibited the best performance regarding the process degradation efficiency (75% vs. 50% for BDD and respective, 45 % for GC). Even if this electrode was operated only at +1.5 V vs. Ag/AgCl, higher charge consumption was recorded and thus, the electrochemical degradation efficiency decreased. The similar situation was noticed for the removal of organic load expressed as COD parameter and corresponding to IBP presence. The best performance in relation with the process efficiency was reached for CNT electrode but for the electrochemical efficiency point of view the BDD electrode allowed to achieve the best results. Also, using CNT electrode the best mineralization degree was reached but with higher charge consumption in comparison with BDD electrode. Moreover, the presence of chloride in the supporting electrolyte improved the process efficiency in terms of IBP degradation and especial of mineralization. This could be explained by the chlorine/hypochlorite attack over IBP and byproducts in addition of the direct oxidation on the electrode surface. c) Multiple-pulsed amperometric measurements It is known that a simple method for in-situ renewing the electrode surface is based on the anodic treatment. This relies on selecting the working electrode potential high enough to oxidize the insulating film and making it water soluble without damaging the electrode. Multiple-pulsed amperometry (MPA) has proved to be extremely sensitive for in-situ cleaning and reactivating the electrode surface during the electroanalytical oxidation (Bebeselea et al., 2009). Two different schemes for MPA applying for IBP electrodegradation were tested based on the CV shape in the presence of IBP. The first scheme consisted of three potential levels for IBP oxidation (E=+1.5 V/Ag/AgCl for time duration of 50 ms), the electrode reactivation by advanced oxidation

(E=+1.75 V/Ag/AgCl for time duration of 50 ms) and electrode conditioning (E=-0.5 V/Ag/AgCl for time duration of 50 ms) and the second one consisted of two potential level, for IBP oxidation (E=+1.5 V/Ag/AgCl for time duration of 50 ms) and for electrode reactivation by the advanced oxidation (E=+1.75 V/Ag/AgCl for time duration of 50 ms). This last scheme was tested also in the presence of chloride in the supporting electrolyte. The multiplepulsed amperograms recorded for each scheme are presented in Figs. 5 and 6, and no electrode fouling was noticed for MPA applying under all proposed scheme. The results regarding the electrode performance for the electrooxidation of IBP in relation with the process degradation efficiency () and the electrochemical efficiency (E) are gathered in Table 3. It can be noticed that in comparison with CA, a slight improvement of the electrode performance for IBP electrodegradation and mineralization was achieved by MPA applying. Also, the best results in relation with both the process degradation efficiency () and the electrochemical efficiency (E) were reached by applying the three levels of potential pulses. The presence of chloride in the supporting electrolyte enhanced also the electrode performance especial in relation with the mineralization degree.

Fig. 5. Multiple-pulsed amperograms (MPAs) recorded at CNT electrode in 0.1 M Na2SO4 supporting electrolyte and in the presence of 10 mgL-1 IBP concentration, at 1-Eoxidation = +1. 5V, Ereactivation=+ 1.75V, Econditioning=-0.5V vs. Ag/AgCl; 2- Eoxidation = +1. 5V, Ereactivation= +1.75V vs. Ag/AgCl

The strategy based on the combination of the IBP oxidation at low potential, advanced oxidation and reduction in alternating mode was suitable for the IBP electrodegradation and mineralization under the above-mentioned conditions without electrode fouling. The best performance of the CNT electrode was achieved for MPA applying under the working conditions of two potential pulses for IBP oxidation at low potential (+1.5 V vs. Ag/AgCl) and for reactivation by advanced oxidation at higher potential value (+1.75 V vs. Ag/AgCl) in the presence of chloride.

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Table 2. The electrode performance for the electrooxidation of IBP using chronoameprometry Electrode material BDD

Working conditions

BDD GC GC CNT CNT

Econc (g/C*cm2)

12

39.62

(%) 10

50

9.55

15

(%)

Eox=1.5 V, 0.1 M Na2SO4 Eox=2 V, 0.1 M Na2SO4 Eox=1.5 V, 0.1 M Na2SO4 Eox=2 V, 0.1 M Na2SO4 Eox=1.5 V, 0.1 M Na2SO4 Eox=1.5 V, 0.075 M Na2SO4 and 0.025M NaCl

COD-

conc

TOC

ECOD-Cr (g/C*cm2)

Cr

(%)

ETOC (g/C*cm2)

127.2

1

3.43

44

33.81

7

1.39

6.05

8

12.1

-

-

45

0.44

32

1.32

6

0.065

72

1.43

66.7

4.80

10.85

0.199

75

0.81

69

2.71

27.5

0.278

Table 3. The CNT electrode performance for the electrooxidation of IBP using multiple-pulsed amperometry Supporting electrolyte

0.1 M Na2SO4 0.075M Na2SO4 and 0.025M NaCl

Working conditions Eoxidation=+1.5V Ereactivation=+1.75 Econditioning=-0.5V Eoxidation=+1.5V Ereactivation=+1.75V Eoxidation=+1.5V Ereactivation=+1.75V

conc (%)

Econc (g/C*cm2)

COD-Cr (%)

ECOD-Cr (g/C*cm2)

TOC (%)

ETOC (g/C*cm2)

77

0.45

72.69

1.92

9.76

0.066

74

0.44

69

1.82

9.45

0.064

78

0.21

75

0.73

30.54

0.075

Fig. 6. Multiple pulsed amperograms (MPAs) recorded at CNT electrode recorded in the presence of 10 mgL-1 IBP concentration, at Eoxidation = +1. 5V, Ereactivation= +1.75V vs. Ag/AgCl in: 1- 0.1 M Na2SO4 supporting electrolyte and 20.075M Na2SO4 and 0.025M NaCl supporting electrolyte

4. Conclusions All tested carbon-based electrodes, i.e., borondoped diamond (BDD), glassy-carbon (GC) and carbon nanotubes-epoxy composite (CNT) electrodes exhibited the electrochemical activity for Ibuprofen (IBP) electrooxidation. Cyclic voltammetric results informed that CNT electrode allowed to reach the best current oxidation for IBP electrooxidation, which can be more enhanced by the presence of chloride in the supporting electrolyte. The electrochemical performances of the tested electrodes for IBP degradation and

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mineralization were assessed in terms of process degradation and mineralization efficiency and electrochemical efficiency by chronoamperometry (CA) and multiple-pulsed amperometry (MPA) applying, which simulate the practical electrolysis under potentiostatic conditions. Under all studied conditions, CNT electrode exhibited the best performance in terms of the degradation and the mineralization efficiency. However, a higher charge consumption was recorded, an undesired economic aspect. From the economic point of view, the BDD electrode exhibited the best performance. Based on all experimental results, CNT and BDD electrodes exhibited the practical utility for application in IBP degradation and mineralization and the selection will be performed taking into account the specific practical requirements in relation with an imposed process efficiency or energy consumption. Acknowledgements This work was partially supported by the strategic grant POSDRU/88/1.5/S/50783; POSDRU/21/1.5/G/13798; POSDRU/89/1.5/S/57649, co-financed by the European Social Fund – Investing in People, within the Sectoral Operational Programme Human Resources Development 2007-2013 and partially by the PN II-RU-PD129/2010, PNII-Ideas 165/2011 and PN II 32-125/2008-STEDIWAT.

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