Catalytic Plasma Reactor for Degradation and

Catalytic Plasma Reactor for Degradation and Mineralization of Pharmaceuticals and Personal Care Products P. Manoj Kumar Reddy and Ch. Subrahmanyam* Energy and Environmental Research Laboratory, Department of Chemistry, Indian Institute of Technology (IIT) Hyderabad, 502205, Andhra Pradesh, India

Abstract: Electrical discharges generated at water-gas interface in a dielectric barrier discharge reactor have been tested for the degradation and mineralization of a model pharmaceutical compound sulfamethoxazole (SMX). Nonthermal plasma degradation of the pollutant proceeds via in-situ generation of active species like hydroxyl radical (HO•), hydrogen peroxide and ozone. It has been observed that degradation and mineralization of SMX was enhanced on addition of ZrO2/CeO2 catalyst to the plasma reactor. Typical results indicated that SMX degradation followed first-order kinetics.

Keywords: Advanced oxidation processes, Pharmaceuticals and personal care products, first order kinetics, Degradation and Mineralization.

Introduction The presence of a large number of micro pollutants such as pesticides, pharmaceutical and personal care products (PPCPs) and dyes in surface waters have a negative impact on the environment and ecology (13). Even traces of PPCPs in water are of significant concern. PPCPs can enter the environment in their original form, as metabolites and/or as the degradation products. Conventional oxidation technologies are not effective for their mineralization (4). In this context, advanced oxidation processes (AOPs) appear to be promising for their removal (5, 6). In this direction, there is an increasing interest in an application of nonthermal plasma (NTP), as one of the AOPs for wastewater treatment (5, 7-11). Recent studies have demonstrated that electrical discharges in water can effectively treat/mineralize organic pollutants present in water (12-16). In general, application of AOPs for water treatment proceeds via generation/utilization of several strong oxidants that are capable of mineralizing the target pollutant. During the NTP operation, the active species present in the discharge interacts with water molecules and transfer the necessary energy to either ionize/ dissociate them, leading to the generation of various reactive intermediates. Electrical breakdown in water produces UV radiation, shock wave, ions (H+, H3O+, O+, H-, O-, OH-), molecular species (H2, O2, H2O2) and most importantly reactive oxygen species (ROSs) (such as O, H2O2, OH radical, etc) (15, 17-20). In *Corresponding author; E-mail address: [email protected] ISSN 1203-8407 © 2015 Science & Technology Network, Inc.

NTP-DBD (dielectric barrier discharge) ozone is one of important oxidants formed, whose in-situ decomposition may produce even more powerful oxidant atomic oxygen. As a result, several research groups have studied the application of NTP in combination with solid catalysts for the removal of aqueous organic pollutants (21-23). Of all the reactive species produced in the discharge in liquid phase, the hydroxyl radical is a very powerful and non-selective oxidant that initiates the radical reactions and has the potential to mineralize the target compounds into final products carbon dioxide and water (20, 24). Sulfamethoxazole (SMX) represents an important class of sulfonamide antibiotics, which is widely used in human and veterinary pharmaceuticals. Objective of this research is to evaluate the effectiveness of catalytic NTP-DBD reactor for the mineralization of SMX. The studies were conducted under various operational conditions in order to achieve the best degradation efficiency.

Materials and Methods Materials The concentration of the SMX solution in all experiments was varied between 50 and 100 mg/L. A 1000 mg/L stock solution of SMX (Sigma Aldrich Reagent-gradeand used without further purification) was prepared by dissolving a 1000 mg of SMX in 1000 ml of water and filtered through a 0.45 μm filter. The water was purified with a Milli-Q water ionexchange system (Millipore Corporation) to give a resistivity of 18MΩ.cm. Appropriate dilutions were made to obtain the desired concentration. J. Adv. Oxid. Technol. Vol. 18, No. 1, 2015

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P. Manoj Kumar Reddy and Ch. Subrahmanyam

Analysis SMX was analyzed by using a high performance liquid chromatograph (Waters 515, HPLC) equipped with a C18 column (4.6 mm×250 mm) and a PDI detector at 254 nm (mobile phase: acetonitrile /water = 70:30 (V/V)HPLC-grade were purchased from Merck, flow rate: 1 ml/min). Total organic carbon (TOC) was monitored as a function of time by a Shimaduz TOC-VCPH analyzer. The % degradation was calculated as follows: C -C Degradatio n percentage (%)  o t x 100 Co

(1)

Cois initial concentration and Ct is the concentration at timet, respectively. The TOC removal was calculated from Eq-2 TOC reduction (%) = [(TOCi – TOCt)/ TOCi] x100 (2) where, TOCi= the initial total organic carbon of SMX and TOCt= the total organic carbon at time t. H2O2 formation was confirmed by as reported previously (4).

Experimental Setup The experimental set-up was described in detail elsewhere (4), briefly, the DBD reactor consisted of a transparent quartz cylinder with an inner diameter of 19 mm and wall thickness of 1.6 mm. The outer surface of the quartz tube (200 mm) covered with silver paste that acts as the outer electrode, whereas a cylindrical stainless steel rod of 12 mm diameter served as the inner electrode. The discharge length was 200 mm and the discharge gap was around 3.5 mm. The applied voltage was kept constant of 18 kV (peak-to-peak), which was generated by a high voltage transformer (Jayanthi Transformer) that operates at 50 Hz frequency. The air flow rate was regulated at 200 ml/min by using a mass flow controller. The gas at the outlet was analyzed with a CO-CO2 analyzer. The corresponding experimental setup is shown in Figure 1. Ozone can degrade aqueous pollutants via direct reaction or undergoes decomposition through chain reactions leading to the formation of a secondary oxidant hydroxyl radicals (Equations 3 to 9), which is a highly reactive and nonselective oxidant. Hence insitu decomposition of ozone on a suitable catalyst may facilitate the formation of atomic oxygen. Numerous studies have shown that metal oxide catalyst can accelerate the ozone decomposition (13-16). 162

J. Adv. Oxid. Technol. Vol. 18, No. 1, 2015

Figure 1. Schematic diagram of DBD reactor used in present study.

O3 + H2O 2HO• 2O3 + H2O2 O3 + HO• H2O2 + H2O O3 + HO23O3 + H2O

O2 + 2HO• H2O2 2 HO• + 3O2 HO2- + O2 HO2- + H3O+ 2 HO• + O2 + O22 HO• + 4O2

(3) (4) (5) (6) (7) (8) (9)

Power and Energy Yield Calculation The voltage-charge (V-Q) Lissajous method was used to determine the power dissipated in the reactor, where the charge Q was recorded by measuring the voltage across a capacitor (100 nF) connected series to the ground electrode (18). The applied voltage was measured by using a 1000:1 voltage probe (Agilent 34136A HV). A Tektronix (TDS 2014B) digital oscilloscope was employed to obtain the voltage and charge waveforms and plotted to get typical V-Q Lissajous diagrams. The discharge power (W) was calculated by multiplying the area of the V-Q figure with the frequency. The energy yield (g/kWh) of the degradation was calculated using Eq-10.

where C is initial concentration of the target compound, V is volume of the solution, P is power and t is time.

Results and Discussion Kinetics of SMX Degradation The degradation of SMX was found to follow first order kinetics. From the curve fitting of the degradation profile for each variable, the apparent 1st order kinetic constant was calculated. Figure 2 shows the first-order plots for the removal of the SMX in the plasma reactor under catalytic conditions. The firstorder kinetics equation is expressed as Eq-11.

P. Manoj Kumar Reddy and Ch. Subrahmanyam Table 1. % degradation, decreasing in TOC%, energy yield values, and kinetic parameters during the SMX degradation at 18 kV applied voltage.

At 90 min of plasma treatment C0 (ppm)

Feeding gas

Catalyst

50

Air

75

Rate EE/O Energy (kWh/m3/order) constant (min-1) yield (g/kWh)

R2

% of degradation

% of TOC removal

----

94.22

9.3

1.72

21.8

0.032

0.977

Air

----

75.68

14.6

2.07

33.2

0.021

0.969

100

Air

----

70.10

19.3

2.58

41.1

0.017

0.981

100

Oxygen

----

78.36

23.5

2.87

30.3

0.023

0.951

100

Argon

----

47.03

4.6

0.72

99.8

0.007

0.982

100

Air

Plasma/Fentons

78.59

21.4

2.88

31.7

0.022

0.972

100

Air

ZrO2/CeO2

92.76

28.6

3.40

24.1

0.029

0.983

Figure 3. Influence of initial concentration on degradation of SMX at 18 kV applied voltage.

Figure 2. First order kinetics of SMX degradation.

where C0, Ct, k, and t are the initial concentration of SMX, concentration at a given reaction time, the rate constant (min-1), and reaction time (min), respectively. The rate constants of SMX under oxygen bubbling condition were higher than that of argon bubbling (Table 1).

than zero air and argon bubbling. The first-order rate constant of SMX degradation also followed the order: oxygen> zero air > Argon > (0.017, 0.023, and 0.007 min-1 respectively). It is worth meaning that H2O2 formation is also effected by changing the gas. H2O2 during the present study was 68, 60 and 29 ppm, respectively with oxygen, zero air and argon gases after 30 min of discharge.

Effect of Feed Gas

pH Variation

Feed gas may also influence the degradation of the pollutants in plasma reactors, as it may affect the formation of oxidants, positive and negative charged ions. On formation, these primary species react either with the pollutant or transform into secondary oxidants. During the present study oxygen, zero air and argon were bubbled through the SMX solution. As shown in Figure 4 oxygen bubbling showed better decomposition

Feed gas may alter the pH of the solution due to generation of various organic acids/bases. During the present study while using zero air, the pH of the solution dropped rapidly (Figure 5) when compared to other two feed gases. This decrease in pH may be due to the formation of nitrogen based acids like nitrous acid and nitric acid and some organic acids. Formation of nitric acid was ensured by acid-base titration

ln(Ct/C0) = -kt

(11)


163


Figure 4. Effect of feeding gas on degradation of 100 ppm SMX at18 kV applied voltage.

Figure 5. Variations of pH during plasma treatment at 18 kV applied voltage, with different feeding gases.

(1.65×10-3 M at 18 kV applied voltage and zero air as a feed gas), the decrease of pH for argon may be due to the formation of organic acids.

Addition of the Catalyst A combination of catalyst and NTP appears to be the best choice for the removal of SMX used in this study. Degradation and mineralization of the target compounds may be favored by the addition of suitable catalysts. During the present study at 18 kV, 60 ppm of H2O2 was confirmed. In order to facilitate Fenton type reactions, 100 mg of Fe2+ was added. As seen from Figure 6, addition of Fe2+increased the degradation to 78% against 70% with plasma alone. In a similar manner, formation of 330 ppm ozone at 18 kV was observed under air bubbling. In order to facilitate ozone decomposition, 10% ZrO2/CeO2 was prepared and added to SMX solution (20). The improved degradation for catalytic plasma reactor is shown in Figure 6, which confirms the degradation of 92% against 70% for plasma alone degradation. The better performance of the plasma catalytic technique may be due to in-situ decomposition of ozone to more active species in presence of catalyst. The best performance of 10% ZrO2/CeO2 may be due to its ozone decomposition capacity that may lead to the formation of atomic oxygen, which is even a stronger oxidant than ozone. In order to ensure this observation, ozone concentration was measured with ZrO2/CeO2 that conformed the decrease of ozone to 167 ppm.

TOC Reduction Mineralization is the conversion of organic compounds to the final products CO2 and H2O. CO2 formation was identified by a COx analyzer, whereas TOC analyzer was used to estimate the mineralization. 164


Figure 6. Effect of additives on enhancement of SMX degradation at 18 kV applied voltage.

At 18 kV, for 100 ppm SMX initial concentration, plasma alone showed 19.3% TOC reduction that increased to 21.4 and 28.6% respectively (Table 1) on addition of Fenton’s and 10% ZrO2/CeO2 catalyst. In plasma waste water treatment, ozone and •OH are the key oxidants that have the potential to mineralize the organic compound. During the NTP treatment of waste water, it is often believed that the active species drive the degradation and mineralization of pollutant to end products CO2 and H2O. In order to understand the degradation mechanism of SMX, GC-MS was used to identify the major degradation intermediates. Based on the products identified, a •OH initiated degradation mechanism was proposed (Figure 7), where except compound III, all others were identified. Product II, III and IV with m/z of 254, 269, and 255 are a result of substation/addition of •OH to the SMX structure. V and VI were formed through the cleavage

P. Manoj Kumar Reddy and Ch. Subrahmanyam Me O S NH N O O C10H10N2O4S Mol. Wt.: 254.26

HO II



OH

H2N

O S NH N O SMX O C10H11N3O3S I Mol. Wt.: 253.28

HO O S NH O

H2N III

Me N O

C10H11N3O4S Mol. Wt.: 269.28

OH 

Me

OH

O S NH O

H2N IV





OH

SO2NH2

C6H7NO3S Mol. Wt.: 173.19 H2N

SO3H

+

H2N

V

N O

C9H9N3O4S Mol. Wt.: 255.25

OH

Me C6H8N2O2S H2N Mol. Wt.: 172.2

OH

C4H6N2O Mol. Wt.: 98.1

N O VI

VII

C6H7N Mol. Wt.: 93.13

H2N

IIX 

OH

C6H6O Mol. Wt.: 94.11

IX

OH 

C6H6O2 Mol. Wt.: 110.11

OH

H O

OH X 

OH HOOC

COOH O

O

C6H4O2 Mol. Wt.: 108.09



OH



COOH

OH

COOH

HOOC COOH CH3COOH

CH3COOH + CO2 

OH CO2+H2O

HCOOH

Figure 7. Plausible schematic representation of the SMX degradation mechanism.

of the bond between the aminophenylsulfone and the methy lisoxazoleamine moieties. Sulfanilic acid (VII) transforms to hydroquinone (X), which further transformed to benzoquinone. Intermediates would be further decomposed and gradually become degraded to organic acids such as maleic acid and oxalic acid. The final substances are water and carbon dioxide, which has been qualitatively conformed by COx analyzer. These results show that the addition of the catalyst improves the mineralization as well as the performance of the reactor towards the total oxidation.

Energy Efficiency The dye degradation efficiency may be better illustrated by energy efficiency (EE) in kWh/m3, which is a measure of the amount of energy consumed during the pollutant removal. Electrical energy required for the removal of SMX at different conditions has been given in Table 1. As seen in Table 1, on increasing the SMX concentration from 50 ppm to 100 ppm, the EE increases from 21.8to 41.1kWh/m3at 18 kV. For 100 ppm SMX degradation, less energy was consumed under oxygen bubbling when compared J. Adv. Oxid. Technol. Vol. 18, No. 1, 2015

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with air and argon gases. The increase in EE in the presence of Fe2+ may be due to Fenton’s reaction leading to hydroxyl radical formation and whereas ZrO2/CeO2 may be due to the in-situ formation atomic oxygen by ozone decomposition.

(9) (10) (11)

Conclusion As an alternative to the conventional techniques, the SMX degradation in aqueous medium was studied by NTP-DBD reactor. The advantage of NTP-DBD is the ease of operation, energy efficiency and improved mineralization on addition of the catalyst. The SMX degradation followed firs order kinetics and the conversion efficiency decreases on increasing the pollutant concentration. Among the feed gases studied, oxygen bubbling favored the best degradation, probably due to formation of more number of oxygen based active species. The degradation and mineralization efficiency of the NTP reactor increased on addition of the Fe2+, probably due to Fenton reactions. Whereas the best activity observed with ZrO2/CeO2 may be due to in-situ decomposition of O3, leading the formation of atomic oxygen.

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Received for review June 25, 2014. Revised manuscript received November 25, 2014. Accepted November 26, 2014.