Article pubs.acs.org/IECR
Green Approach for Wastewater TreatmentDegradation and Mineralization of Aqueous Organic Pollutants by Discharge Plasma 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 S Supporting Information *
ABSTRACT: A dielectric barrier discharge reactor was designed and tested for the degradation of a model aqueous pollutant crystal violet (CV). The specific advantage of the present configuration is the effective utilization of reactive species generated in the discharge plasma. The reactive species formed in the discharge, particularly the OH• radicals, may cause strong oxidizing effects in the degradation of CV. Mineralization of the dye was confirmed by TOC analyzer and infrared gas analyzer. It was observed that both degradation efficiency and TOC removal increased with increasing the input energy; however, the energy yield decreased. The highest energy yield achieved was 86.3 (g/kWh). Formation of hydrogen peroxide was quantified and addition of Fe2+ increased the performance of the reactor. The dye degradation followed first-order kinetics.
1. INTRODUCTION Water is an indispensable requirement of life as well as industries. However, wastewater from these industries is an alarming situation that requires immediate attention due to the adverse effects of the discharge pollutants on human and aquatic life. In the past many countries recognized the importance of wastewater treatment, and it is estimated that 17−20% of industrial water pollution comes from textile dyeing and treatment plants. It is well-known that dyes are organic pollutants, designed to be chemically and photolytically stable.1 Dyes are toxic and may contain some nondegradable amines as intermediate products having potential carcinogenicity and mutagenicity.2,3 Remediation methods of dyes, such as adsorption and biodegradation, may simply transfer the pollutant from one phase to the other; hence, not offering a permanent solution.4 Some authors have reported water treatment based on metallic nanoparticals,5 titanium dioxide nanotubes,6 photo-Fenton,7 photocatalytic,8 ultrasonic degradation,9 sonolysis combined with ozonolysis,10 and corona discharges.11 Among these techniques, advanced oxidation processes (AOPs) have unique potential in terms of degradation efficiency. These methods proceed via the generation of a specific oxidant like OH•, O3, UV, etc. Electric discharges at the water−gas interface is one of the advanced oxidation processes that offer specific advantages like generation of multiple oxidants. Electrical discharges generated in water induce different physical and chemical effects like high electric fields, UV radiation, overpressure shock waves, and the formation of chemically active species. The interaction of the high energy electrons created by the discharge with the water molecules produces highly oxidative species. The 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 radicals (such as O•, H•, OH•).11−20 In addition, the energetic electrons transform kinetic energy into potential energy of the excited species by energizing the atoms and molecules. There is an increasing interest in the application of plasma reactors in wastewater treatment; it produce electrons of the high energy, which © 2012 American Chemical Society
scopes beyond the dissociation energy of water (5.16 eV) or even ionization energy of water (12.62 eV).14,21 AOP based on the generation of plasma at the gas−water interface was studied for the degradation of water-bound pollutants like dyes.22 Direct photo oxidation of dye in water is very limited,23 among the active species; hydroxyl radical, atomic oxygen, ozone, and hydrogen peroxide are the most important ones for the removal of dyes in water.24 OH• radical, one of the most important oxidants, has a very short lifetime and are mainly generated from the direct dissociation of water molecules in the plasma region.25 This active chemical species offer many interesting applications for degradation of dyes. This paper investigates the feasibility of designed reactor for the degradation of crystal violet (CV) in water. It is generally used in paper and inkjet printers. It is also used to color diverse products such as fertilizers, antifreezes, detergents, and leather jackets. The NTP-DBD reactor was bubbled with zero air. Zero air was supplied by a local vendor and the absence of CO2 was confirmed by cross checking with an infrared COX analyzer. The effect of various parameters like applied voltage, gas flow rates, concentrations of dye, addition of Fe2+ or H2O2, and change in pH was studied to the synergistic effect during degradation.
2. EXPERIMENTAL SECTION A schematic of the reactor and other equipment used for these experiments is shown in Figure 1. The electrical discharge was produced in a parallel plane type coaxial NTP-DBD reactor by a high-voltage 0−40 kV AC source transformer (Jayanthi transformers). The reactor is a transparent quartz cylinder with an inner diameter of 19 mm and wall thickness of 1.6 mm. Silver paste painted on the outer surface of the quartz tube acts Received: Revised: Accepted: Published: 11097
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spectrophotometric quantification of the yellow color pertitanic acid formed when Titaniumsulphate reacts with hydrogen peroxide at 418 nm.27 Titaniumsulphate was prepared by dissolving 1 g of anhydrous titanium powder with 100 mL of hot sulfuric acid. The resulted solution was kept at 150 °C for 20 h, cooled to room temperature, filtered, and kept in a reagent bottle. Total organic carbon (TOC) in the aqueous solution was measured by using a TOC-VCPH (Shimadzu, Japan) analyzer, which works on combustion method. The total carbon content TC is defined as the sum of the total organic carbon (hydrocarbons) and total inorganic carbon (TIC; carbonate, bicarbonate, and dissolved carbon dioxide). TOC determination involves purging an acidified sample with carbon-free air to remove all inorganic carbon prior to measurement. The sample is injected into a combustion chamber, and the temperature was raised up to 680 °C. Here, all the carbon reacts with oxygen, which leads to carbon dioxide, which is then flushed into a cooling chamber and finally into the nondispersive infrared spectrophotometer detector.
Figure 1. Experimental setup.
as the outer electrode, whereas a cylindrical stainless steel rod served as the inner electrode. The discharge length was 20 cm, and the discharge gap was around 3.5 mm. The applied voltage during the present study was varied between 14 and 18 kV. The applied voltage, reactor current, power, and waveform were measured using an oscilloscope (Tektronix TDS 2014B, 100 MHz, 1.0GS/s) and a HV probe (Agilent 34136A HV 1/1000). Discharge power (W) in the plasma reactor was measure by using a voltage (V)−charge (Q) Lissajous method (discharge power was calculated by multiplying the area of the V−Q Lissajous figure with frequency) as reported.12,26 A voltage− charge (V−Q) Lissajous figure with and without dye solution is given in the Supporting Information (Figure S1). The gas flow rate was controlled by a mass flow controller from AALBORG flow instruments (GFC 17). The gas after passing through the solution was connected to the CO-COX analyzer. The energy yield of the degradation was calculated.13 Y (g/kWh) =
C(g/L) × V (L) × 1/100 × conv(%) P(kV) × t (h)
Figure 2. Concentration of H2O2 formed as a function of time with different applied voltage.
(1)
Where C is initial dye concentration, V is the volume of the solution, P is power, and t is time. It has been observed that increasing applied voltage increases the power and decreases the energy yield.
3. METHODS AND ANALYSIS Aqueous dye solution was prepared by dissolving the solid dye 500 mg/L in Millipore water. The experimental solutions (50−100 ppm) were obtained by diluting the stock solution in accurate proportions. Dye solution was taken from the NTP-DBD reactor at regular time intervals; the residual concentration was continuously monitored. Degradation of the CV was measured by using a double beam UV spectrophotometer (Shimadzu) at absorption maximum of 588 nm (Supporting Information Figure S2). The degradation percentage was calculated by using C − Ct degradation percentage (%) = o × 100 Co (2) Co and Ct are the initial and the final concentrations of CV solution, respectively. Hydrogen peroxide produced by the discharge in water was quantified following a reported procedure, which involves the
Figure 3. Variations of pH during plasma treatment at 18 kV applied voltage, 100 mL/min flow rate, and 100 ppm CV concentration. 11098
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organic contaminants.29 H2O2 may form by the following reaction;14,30,31 Figure 2 shows the concentration of H2O2 as a function of time at different applied voltages for 100 mL flow rate of zero air.
The amount of CO and CO2 released during the reaction was measured in an automated device called the CO-CO2 and NOX Analyzer from Siemens instruments (ultramat 23) equipped with an IR detector. The progress of the reaction was followed with a gas chromatograph with mass spectrometer (GC-MS) (thermo scientific).
4. RESULTS 4.1. Characteristics of the DBD Reactor. The power distributed in the NTP-DBD reactor was determined by measuring the voltage across the capacitor of 2.2 μF connected series to the ground electrode. The voltage across the capacitor multiplied by its capacitance (2.2 μF) corresponds to the charge dissipated in the NTP-DBD reactor.28 The discharge power was can be calculated by multiplying the area of the voltage−charge (V−Q) Lissajous curve by the operating frequency (50 Hz). NTP reactors when operated under aerated conditions produce ultraviolet light.11,17,22 Hence, it is expected that the UV light may promote the oxidation of aqueous
H 2O + e− → H• + OH• + e−
(3)
OH• + OH• → H 2O2
(4)
2H 2O → H 2O2 + H 2
(5)
Ozone is one of the major active species in an NTP-DBD reactor. The main ozone generation reaction is given in eq 6. Ozone is a well-known and widely applied strong oxidizing agent for the treatment of wastewater. Ozone reacts almost indiscriminately with all organic compounds present in the reacting system.11,15,16,20,24 Ozone reacts with wastewater compounds directly via molecular and indirectly through
Figure 5. Effect of variation in initial concentration and applied voltages on CV degradation after 25 min of plasma treatment for 100 mL/min flow rate.
Figure 4. First-order kinetics of dye degradation in DBD reactor.
Table 1. Changing the Parameters (Percent of Degradation, Initial TOC, Final TOC, Percent of TOC Decreased, k1 (min−1), and R2 Value) during the Experiment at 25 min of Plasma Duration flow rate
C0 (ppm)
voltage (kV)
initial TOC (ppm)
final TOC (ppm)
TOC decrease in %
% of degradation at 25 min
rate constant (min−1)
R2
100 mL/min
50
14 16 18 14 16 18 14 16 18 14 16 18 14 16 18 14 16 18
36.85
33.12 32.17 30.76 47.23 45.4 43.87 62.13 59.7 57.6 31.92 30.02 29.23 45.93 44.04 42.36 60.87 58.47 55.89
10.12 12.70 16.52 14.54 17.85 20.62 15.69 18.99 21.84 13.37 18.53 20.67 16.89 20.31 23.35 17.40 20.66 24.165
93.8 94.9 96.3 92.4 94 94.7 89 93 93.2 95.1 96 97 93 93.9 94.6 90.6 93.3 94.2
0.99 0.111 0.116 0.94 0.105 0.111 0.91 0.97 0.101 0.114 0.123 0.129 0.106 0.117 0.121 0.99 0.109 0.102
0.95 0.93 0.96 0.99 0.99 0.98 0.97 0.97 0.94 0.96 0.95 0.95 0.98 0.95 0.94 0.97 0.98 0.97
75
100
200 mL/min
50
75
100
55.27
73.7
36.85
55.27
73.7
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degradation. Discharges in water may change the pH of the solution significantly,32 which may be due to the formation of various acids.32,33 The formation of various acids is a result of a series of reactions initiated by the discharge as shown in eq 11−1630,31
radical type chain reactions. Both reactions occur simultaneously. Simplified reaction mechanisms of ozone are given in eq 7−10.
O + O2 → O3
(6)
3O3 + H 2O → 2•OH + 4O2
(7)
N2 + e− → 2N• + e−
(11)
(8)
O2 + e− → 2O• + e−
(12)
(9)
N• + O• → NO
(13)
(10)
NO + O → NO2
(14)
NO2 + OH• → HNO3
(15)
NO2 + H 2O → HNO2 + HNO3
(16)
•
H 2O2 + 2O3 → 2 OH + 3O2 •
•
O3 + HO2 → OH + O2 + O2 •
•
−
H 2O2 + O3 → OH + O2 + HO2
these active specious are responsible for CV degradation. The major degradation intermediates are identified by using a gas chromatograph with mass spectrometer (GC-MS) and a plausible degradation mechanism is given in Supporting Information Figure S3. 4.2. Change in pH. Various parameters have been studied to arrive at the best efficiency conditions during CV
For 100 ppm of CV solution, at 18 kV, after 25 min of the plasma treatment decreased the pH from 7.5 to 2.1. Figure 3 Table 2. Energy Yield (g/kWh) during the Experiment at 10 min Plasma Treatment flow rate 100 mL/min
C0 (ppm) voltage (kV) yield (g/kWh) 50
75
100
200 mL/min
50
75
100
Figure 6. Effect of chemical additives on enhancement of the dye degradation at 14 kV applied voltage, 100 mL/min flow rate, and 100 ppm CV concentration.
14 16 18 14 16 18 14 16 18 14 16 18 14 16 18 14 16 18
42.19 30.5 24.5 36.4 26.9 22.6 34.9 26.5 21.04 86.3 51.6 29.4 76.7 47.9 27.4 71.17 43.7 25.75
% of degradation at 10 min 84.3 86.6 87.0 72.8 76.4 79.3 70.1 75.2 76.3 86.3 87.7 89.3 76.7 81.4 83.3 71.2 74.4 78.1
Figure 7. (a) CO and CO2 released during degradation (b) percent of degradation and TOC decrease percent as a function time at 18 kV applied voltage, 200 mL/min flow rate, and 100 ppm CV concentration. 11100
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Figure 8. (a) Energy yield and percent of dye degradation as a function of time during plasma treatment for 100 ppm CV at 14 kV with 100 mL flow rate. (b) Energy yield as a function of time for various initial concentrations at 14 kV applied voltage and 100 mL flow rate. (c) Energy yield as a function of time for various applied voltages for 75 ppm initial CV concentration and 100 mL/min flow rate. (d) Energy yield as a function of time for two flow rates at 16 kV applied voltage with 75 ppm initial CV concentration.
Table 3. Energy Efficiencies of Various Types of Electric Discharge Processes S. no.
discharge type
pollutants
initial concentration (ppm)
source gas
yield (g/kWh)
ref
1 2 3 4 5 6 7 8
dielectric barrier corona dielectric barrier dielectric barrier dielectric barrier corona discharge pulsed discharge dielectric barrier
crystal violet methylene blue acid red 88 acid red 88 pentoxifylline methylene blue/methyl orange methyl orange methylene blue
100 150 50 25 100 10 80 50
air oxygen air oxygen oxygen air oxygen oxygen
86.3 5 4.56 11.14 16 4.5 8.86 57
present work 13 17 17 40 41 42 43
concentration and applied voltage during the degradation of the CV, concentration varied at 50−100 ppm in the applied voltage range 14−18 kV and the results are shown in Figure 5 and Table 1. It was observed that the percent degradation increased with the applied voltage as shown in Table 1 and Figure 5. At higher voltage, more reactive species may form that increases the rate of degradation. As seen in Figure 5, removal of low concentration is beneficial and the performance of the technique decreases with increasing concentration of the dye. 4.5. Effect of Chemical Additives. As explained earlier, electric discharges produce various reactive species. One of the ways of improving the performance is by adding suitable additives that may improve the efficiency. The chemical additives for enhancement of the degradation efficiency of the dye wastewater are one of the advocated methods.35,36 Since the H2O2 formation was confirmed, the addition of Fe2+ may be
shows the variation of pH as a function of time during the plasma treatment. 4.3. Kinetics of CV Degradation. Kinetics of CV degradation was followed for 50, 75, and 100 ppm concentrations. As shown in Figure 4, ln(C/C0) are proximately linear with the degradation time and the degradation obeys first-order kinetics. The first-order integral rate equation
ln(C /C0) = −k1t
(17)
where C, C0, and k1 are the concentration of CV for a given reaction time, initial concentration, and first-order rate constant (min−1), respectively.4,29,34 Table 1 shows the rate constant and R2 values for different conditions. The coefficient indicated the linearity and confirms the first-order kinetics behavior. 4.4. Effect of Initial Concentration and Applied Voltage. In order to understand the influence of dye 11101
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100 to 200 mL/min, yield increased from 42.2 to 86.5 g/kWh for 50 ppm initial concentration even at 14 kV applied voltage. The energy efficiencies of various types of electric discharge processes are given in Table 3.
a logical approach from the viewpoint of Fenton type reactions. Hence, the effect of Fe2+ has been investigated on the degradation of CV. 4.5.1. Effect of Hydrogen Peroxide. In order to understand whether 60 ppm of H2O2 formed in the DBD reactor is sufficient or not, the reaction was followed with the addition of hydrogen peroxide. The reactivity of hydroxyl radicals is extremely high. In contrast to other active species, these are considered to be relatively high reaction rates.37 The prominent increase of degradation most probably appears due to the reactions of dyes with hydroxyl radicals formed by UV-induced homolytic fission of the O−O bond in H2O2 which attacks the organic pollutants to initiate oxidation. H2O2 may increase the concentration of active OH• and thus accelerate the degradation rate.29 H 2O2 + hν → 2OH•
5. CONCLUSIONS This study demonstrated the development of a new DBD reactor that can be used to oxidize dyes from water. It was observed that the major reactive species involved in the degradation process are hydroxyl radical, hydrogen peroxide, and ultraviolet. Formation of H2O2 was confirmed, and it increases with treatment time. The degradation process of the model dye, CV, obeys first-order kinetics, and the rate of degradation was strongly increased by the addition of Fe2+ due to the catalytic formation of hydroxyl radicals via Fenton type reactions. The specific advantage of the present process is the mineralization of CV, the highest energy yield up to 86.3 g/kWh.
(18)
2+
4.5.2. Effect of Fe . The presence of iron significantly enhanced the rate of CV degradation. The addition of iron increases the formation of OH• radicals by the decomposing of H2O2 in Fenton-type reactions and provides an additional source of hydroxyl radicals. Fe2+ [0.5 g/L] was added to the reaction solution, and the formation of active species are expected as per eq 19.38 Fe 2 + + H 2O2 → OH• + OH− + Fe3 +
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ASSOCIATED CONTENT
S Supporting Information *
V−Q Lissajous figure, decrease in absorption maxima as a function of time, and degradation mechanism figures. This material is available free of charge via the Internet at http:// pubs.acs.org.
■
(19)
2+
It was found that addition of Fe in smaller doses was more successful in degradation of CV by utilizing hydroxyl radical are shown in Figure 6. 4.6. Mineralization of Dye. As mentioned in the Introduction, mineralization by avoiding the formation of the unwanted products is the best way to remove the organic compounds, the total organic carbon (TOC) is the amount of carbon bound in an organic compound and is often used as a nonspecific indicator of water quality. A different way to measure mineralization of the target compound is to determine the carbon content. Due to oxidation, the carbon skeleton of an organic compound is gradually chopped into shorter carbon chain molecules. The TOC level of the oxidation product mixture decreases by release of oxides of carbon (mineralization)39 and gaseous intermediates. The amounts of CO and CO2 released during the reaction are shown in Figure 7a and percent of degradation and TOC with respect to time is shown in Figure 7b. From Table 1, it can be seen that percent TOC removal increases as a function of applied voltage, concentration, and flow rate. The highest removal percent was observed at 100 ppm, 18 kV, and 200 mL flow rate at 25 min and shown in Table 1 that increased to 48% at 60 min. 4.7. Energy Efficiency. The dye degradation efficiency may be better illustrated by the amount of CV decomposed per unit of energy (yield). The energy yield depends on the type of discharge reactor, initial concentration, and nature of the compound.13 During the present study, ∼ 80% degradation within 10 min was observed, and Table 2 presents a detailed data of energy yield under various conditions. It was observed that as a function of time the energy yield decreases and percent degradation increases as shown in Figure 8a and change in yield with voltage, initial dye concentration, and gas flow rates are shown in Figure 8b, c, and d, respectively. It is clear with increasing applied voltage from 14 to 18 kV, the yield decreased from 42.2 to 24.5 g/kWh for 50 ppm, from 36.4 to 22.5 g/kWh for 75 ppm, and from 34.9 to 21.04 g/kWh for 100 ppm at 100 mL/min flow rate. With increasing flow rate from
AUTHOR INFORMATION
Corresponding Author
*Phone: +91-40-23016050. Fax: +91-40-2301 6032. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors greatly acknowledge MOEF (Ministry of Environment and Forests), India, for Financial support. The authors also thank our colleagues who participated in this work.
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REFERENCES
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dx.doi.org/10.1021/ie301122p | Ind. Eng. Chem. Res. 2012, 51, 11097−11103
