Plasma Acid: Water Treated by Dielectric Barrier Discharge

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1 Full Paper Plasma Acid: Water Treated by Dielectric Barrier Discharge Natalie Shainsky,* Danil Dobrynin, Utku Ercan, Suresh G. Joshi, Haifeng Ji, Ari Brooks, Gregory Fridman, Young Cho, Alexander Fridman, Gary Friedman It is demonstrated that direct exposure of deionized water to a dielectric barrier discharge (DBD) plasma creates an acid ( 2) and is in fact, a strong oxidizer (providing, e.g., peroxidation of a cell membrane). This study addresses the question: which acid is created in water by plasma treatment. Two major possibilities are considered: nitric/nitrous acid and ), which, for the lack of a better term, we call plasma acid. The presence of nitric/nitrous acid in the water after plasma treatment in air is confirmed, although the observed 2 cannot be completely explained by the production of nitric acid. Moreover, experiments with oxygen-plasma treatment of water also lead to high acidity, without production of nitrogen based acids at an acid which consist of a hydrogen cation (H þ ) and a superoxide anion (O 2 all. Therefore, O 2, the conjugate base of the plasma acid, is at least partially responsible for both lowering of the and the increase in the oxidizing power of the solution. Experiments indicate that peroxides such as H 2 O 2 and O 2, together with an acidic environment are likely to be responsible for the oxidation properties of the plasma treated water. This plasma acid remains stable for at least a day, depending on the gas where plasma is generated, but the effect is temporal. Existence of a temporal and stable oxidizer created using the plasma treatment of pure water not only raises interesting scientific questions and possibilities, but is also likely to provide many applications in situations where direct plasma treatment may be difficult to achieve Oxygen 5.5 Argon Plasma treatment time (min) N. Shainsky, D. Dobrynin, G. Friedman Electrical and Computer Engineering Department, College of Engineering, Drexel University, Philadelphia, Pennsylvania, USA natalie.shainsky@gmail.com U. Ercan, G. Fridman School of Biomedical Engineering, Science and Health Systems, Drexel University, Philadelphia, Pennsylvania, USA S. G. Joshi, A. Brooks Department of Surgery, Collage of Medicine, Drexel University, Philadelphia, Pennsylvania, USA H. Ji Department of Chemistry, College of Art and Science, Drexel University, Philadelphia, Pennsylvania, USA Y. Cho, A. Fridman Department of Mechanical Engineering and Mechanics, College of Engineering, Drexel University, Philadelphia, Pennsylvania, USA ß 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com DOI: /ppap

2 N. Shainsky et al. 1. Introduction Ozonation and advanced oxidation processes utilizing effects of oxidative species and UV have been used to oxidize organics in solution phase. [1 8] It has also been widely documented that the exposure of organic molecules dissolved in water to various forms of plasma discharges leads to their oxidation. [2 4,9 11] These oxidation phenomena find applications ranging from environmental remediation [9,11 14] to water cleaning [10,15] and sterilization. [1,10] Oxidation also plays an important role in many industrial processes such as the conversion of methanol to formaldehyde. [16 18] Although ozone is a good oxidizer, direct exposure of deionized water to plasma, which contains not only ozone but also charged species, reactive oxygen species, and radicals, [19] creates a much stronger oxidizer than ozone. [20 23] It has been reported that water treated by various plasma discharges becomes acidic, [9 11,20 24] which leads to antimicrobial effects. [9 11,24 27] The important question is which acid is created in water treated by plasma. Oehmigen et al. [28] suggested that the acid created in water is nitric/nitrous acid and the antimicrobial properties observed are the result of synergetic action between H 2 O 2 and nitric/nitrous acid. Similar results were obtained by. [10] Our goal was to analyze plasma-treated deionized water without any organic compounds to find out the source of acidity and the oxidative ability of such water. 2. Experimental Section The test setup consisted of a glass chamber with a plastic cap that contained a quartz-protected copper electrode (0.5 mm thick quartz); and the power supply used for these experiments were previously described by authors. [29] Self-healing rubber, which allowed adjustment of the electrode height, relative to the water surface while maintaining air-tight environment, was used between the electrode and the dish containing the water. Next to the electrode, four small self-sealing holes were made into the chamber with a syringe for introduction of water and for degassing. The bottom of the chamber was a Pyrex glass Petri dish with an inner diameter of 37 mm and outer diameter of 40 mm. The dish was placed on a grounded aluminum plate, which had four screws that were attached to the chamber cap, providing both sealing of the chamber and adjustment of the gap between the copper electrode and liquid. The liquid surface acted as a second electrode (see Figure 1). In this case we can say that we used direct plasma treatment as defined by ref. [30] meaning that ions and electrons created by plasma were able to reach the liquid surface. This system is essentially different from the systems used by refs. [10,28] which can be referred to as indirect systems, according to the definition in ref., [30] meaning that only neutral and exited species were able to reach the liquid surface. The chamber was filled with 100 ml to 10 ml of liquid. The distance between the copper electrode and the liquid surface was set to 1.5 mm. The pulsed voltage was 17 kv with 1.7 khz pulse frequency. Treatment time was 10 s to 15 min, depending on water volume. Electrode diameter was 33 mm (25 mm copper, 33 mm with insulator), while the inner diameter of the container was 37 mm. When experiments were done with a gas other than air, we degased the liquid for 12 h, using the following setup. A tall glass test tube ( mm 2 ) was sealed with rubber stopper. Two needles were inserted through the seal. The gas tank was connected to the long needle (to bubble air through the water). A shorter second needle was inserted above the liquid to let the gas escape. The gas tank was then connected through a flow meter allowing a gas flow rate of 30 bubbles s 1. Before starting the next experiment, the empty chamber was degassed in the same method for 3 h, and the degassed liquid was transferred to the container using a syringe through above mentioned holes located near the electrode. This procedure allowed us to minimize the concentration of nitrogen dissolved in water and present in the air. In order to avoid contaminations of pure deionized water (18 MV, Millipore), both glass container and tube (degassing and storage) were cleaned with sulfuric acid and then thoroughly washed with deionized water. The of each container was checked before any liquid was poured to make sure that no residuals of sulfuric acid were left. For measurements, a micro electrode (Lazar esearch Laboratories, Shelfscientific Model PH-146 micro electrode, 1 mm tip and 2.8 mm diameter body) was used. Overnight cultures of E. coli were diluted using phosphate buffered saline (PBS, 1X), to get a concentration of colony forming units per milliliter (cfu ml 1 ). The number of viable microorganisms (cfu ml 1 ) was estimated by the surface-spread plate count method using aliquots of serial dilutions of microorganism suspensions. Detection limit of this procedure is about 1 cfu ml 1. Inactivation kinetics of microorganisms is shown in Figure 1. Experimental Setup DBD electrode is placed in a closed glass chamber. The distance between the electrode and the liquid is 1.5 mm. 2 ß 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: /ppap

3 Plasma Acid: Water Treated by Dielectric Barrier Discharge Oxygen semi-logarithmic plots. If the number of microorganisms fell below the detection limit, i.e., no viable microorganisms were found, for clarity these values in the graphs are shown as 1 cfu ml esults Plamsa treatment time [s] Figure 2. esults of versus DBD treatment time in air (100 mlof water). Figure 2 shows values of deionized water as a function of the plasma treatment. There was a rapid increase in the acidity of deionized water from the initial of to after 90 s of treatment. The level of 2 was observed for all sample volumes treated (100 ml to 10 ml), after exposure times of 1.5 to 15 min, depending on the sample volume (data not shown). Figure 3 presents dependence on treatment time for three different gasses: air, oxygen, and argon. While no Time after plasma treatment (hours) Figure 4. esults of changes of deionized water as a function of time after plasma treatment. change was observed for argon, treatments in air and oxygen resulted in similar changes. In both air and oxygen water acidity increased to the minimum level of 2 with 15 min exposure to the discharge (10 ml of water). We assessed the stability of plasma acid by following change after plasma treatment. Figure 4 presents variations of over 70 h waiting time after the plasma treatment in air and in oxygen. Deionized water treated in air, shows stable values over time of approximately ¼ 2, while water treated in oxygen increases almost linearly after reaching a local minimum at t ¼ 5h. Figure 5 gives an absorption spectra measured using UV/ VIS Spectrophotometer (Perkin-Elmer Model: Lambda 2) of nitric acid and two dielectric barrier discharge (DBD) plasma-treated water samples in air and oxygen. Nitric Plasma treatment time (min) Oxygen Argon Intensity (a.u.) Wave length (nm) Oxigen Nitric acid Figure 3. Variations of of deionized water after plasma treatment in three different gases: air, oxygen and argon (10 ml of water). Figure 5. Absorption spectra measured by UV VIS spectrometer of nitric acid ( 2.0) and water samples treated with plasma in air ( 2.07) and in oxygen ( 2.01). ß 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 3

4 N. Shainsky et al. H 2 O 2 (mg/l) acid was diluted with deionized water to have a of 2.0, plasma treated water in air had ¼ 2.07 and plasmatreated water in oxygen had ¼ Although the absorption peak at 300 nm, which is specific to nitric acid, was registered in all liquid samples, the intensity in the water sample treated in oxygen was significantly lower than the other two liquid samples. Figure 6 shows the changes in hydrogen peroxide (H 2 O 2 ) concentration in plasma treated deionized water in air as a function of time after the treatment. Immediately after the plasma treatment the concentration of H 2 O 2 was mg L 1, which dropped to 250 mg L 1 at t ¼ 5 min. During the next 5 min, the H 2 O 2 concentration in the plasma-treated water dropped to 10 mg L 1. Figure 7 shows changes of Escherichia coli concentration as a function of plasma treatment time for two different E. Coli Bacteria Concentration [cfu/ml] Time after plasma treatment (min) Figure 6. esults of hydrogen peroxide concentration measurements after plasma treatment. E.coli 3e3 [cfu/ml] E.coli 3e5 [cfu/ml] Plasma treatment time [s] Figure 7. esults of E. coli inactivation by plasma treated water show that 60 s treatment was enough to inactivate cfu ml 1 E. coli. E. Coli Bacteria Concentration (cfu/ml) 1.6x x x x x x x x initial concentrations of and cfu ml 1. For this experiment, 25 mlofe. coli was dried on a glass slide (to prevent an interference of organics that might have been present in E. coli medium) then mixed with plasma treated water, after 2 min (holding time) it was plated and counted following a 24-h incubation. In both cases, the bacteria were fully inactivated by plasma treated water for treatment time of 90 s. Figure 8 shows E. coli inactivation by various acids with acidity of ¼ 2 with the same holding time. Almost no inactivation was observed for all acids. 4. Discussion before treatment after treatment HNO3 HCl H2SO4 H3PO4 Figure 8. esults of E. coli inactivation by various acids at ¼ 2. Experimental results presented above show that when pure water (deionized water with an electrical resistivity of 18 MV) is treated with DBD plasma it becomes acidic (Figure 2); a similar effect was previously reported by ref. [9 11,28] Plasma treated water also becomes a strong oxidizer, whose oxidizing properties are demonstrated (as an example) in Figure 7 for the case of E. coli inactivation. These two properties of plasma treated water (acidity and oxidation) are indicative of plasma acid s potential application in industrial and medical setting for surface cleaning or inactivation of bacteria, proteins, etc. Although the low values of water after plasma treatment has been shown before, [9,10,20 23,28] it is still not clear what the source of acidity was. It was previously suggested that the acidity of plasma treated water might be the result of nitric and nitrous acid formation. [9 11,28,31] However, as it is shown in Figure 3, drops in plasma treated liquid not only in air but also in oxygen environment: nitric acid formation cannot explain the acidity of plasma treated water in oxygen. In order to explain the differences between the results obtained by ref. [10,28] and the results reported in this 4 ß 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: /ppap

5 Plasma Acid: Water Treated by Dielectric Barrier Discharge manuscript, it is important to note that the systems used in the experiments [10,28] were indirect systems, where only neutral species and radicals are able to reach the liquid surface. In this case, the only source of the change in the liquid is a chemical change HNO 3 and HNO 2 that are created by the plasma in the air are reaching the liquid surface and becoming dissolved there. In our experiments, a significant role in the change can be attributed to positive and negative charges created in the DBD that reach the water surface. It is likely that in the direct treatment positive charge (M þ ), which reach the liquid surface exchanges charge with the water, resulting in the creation of Hþ ions (the charge exchange mechanism): M þ þ H 2 O! H 2 O þ þ M (1) H 2 O þ þ H 2 O! H þ ðh 2 OÞþ OH (2) where M þ is any plasma ion. Figure 5 indicates that in air we are able to measure the presence of Nitric/Nitrous acid. But when comparing the UV Vis signal obtained from a nitric acid (Nitric Acid (65% w/w/certified)), it shows that the nitric acid signal is weaker for nitric acid then plasma treated water in air. Moreover, water treated by plasma in oxygen shows no signal attributed to Nitric and Nitrious acid, though the values of plasma treated water in air and oxygen are similar (Figure 3). This result suggests that there is another conjugate base present in plasma acid in air besides NO 3 and NO 2 in both air and oxygen, a good candidate for the conjugate base is superoxide, O 2. To verify the presence of O 2 we added superoxide dismutase enzyme (SOD) to plasma acid, similar experiment performed by ref., [10] and we saw the H 2 O 2 concentration increase and the increase to 4 (results not shown). e þ O 2! O 2 (3) O 2 þ 2Hþ! SOD H 2 O 2 (4) Although the presents of O 2 was also confirmed by ref., [10] the source of the O 2 is likely to be different due to the system difference. In our direct system, O 2 is produced in plasma and reaches the liquid surface in addition to the O 2 that is produced in the indirect system. In the indirect system such as in ref., [10] the only source of O 2 is the reverse reaction in Equation (4). [10] In other words, in a direct system, the production of H þ ions can be a result of ion exchange mechanisms and the main conjugate base of the plasma treated water in oxygen is the superoxide radical, which is also present to some extend in plasma treated water in air. To explain the oxidation properties of plasma acid it is not enough to discuss its acidity. As indicated in Figure 8 the acidic environment by itself usually does not have bacteria inactivation properties, and in general does not have the oxidation properties. Under the same conditions both weak acids (H 3 PO 4 ) and strong acids (HCl, H 2 SO 4 ) resulted in poor bacteria inactivation, consistent with the previous findings. [10,28] This shows that an oxidizer, which may have no influence on acidity, is present in the liquid treated by plasma. One of such oxidizers is H 2 O 2 which can be produced in the plasma acid in fairly high concentration (Figure 6). H 2 O 2 is considered a strong oxidizer, especially in an acidic environment. [32] Another oxidizer that is present in plasma acid is O 2 (superoxide anion). Thus plasma acid may consist of H þ ions and O 2 as a conjugate base. Together with H 2 O 2 it has very strong oxidizing properties. 5. Conclusion Strong oxidizing liquid is produced by direct exposure of deionized water to DBD plasma. While oxidizing properties and acidity remain stable for days, these effects are temporal. Plasma acid s oxidizing power may be linked to the significant lowering of its. Although nitric acid can be present in the solution, especially when it is treated in air plasma, the observed low (2.0) is partially caused by the superoxide anion, O 2, which also contributes to the oxidizing power of the solution. Changing the gas used in plasma treatment can influence stability and oxidizing properties of the plasma-treated water. We suggest plasma acid hydrogen cation H þ and superoxide anion O 2 as the cause of water acidification following plasma-treatment. Strong oxidizing properties are attributed to peroxides present in the water such as hydrogen peroxide H 2 O 2 and superoxide O 2 that, together with an acidic environment, results in a strong oxidizing solution. Plasma acid, as a stable oxidizer, is likely to be used in many applications ranging from food sterilization to cleaning of pipes and filters. Plasma acid is easy to use for oxidation of materials on hard-to-reach surfaces or surfaces with complex topology (skin folds and pores, for example). In many of these applications it may be desirable to produce a liquid that has a strong oxidizing potential and is capable of maintaining its oxidative power over a reasonably long time period. At the same time, it may be desirable for this liquid to eventually dissipate its oxidative power in order to safely dispose of the remaining material. eceived: May 6, 2011; evised: September 5, 2011; Accepted: October 4, 2011; DOI: /ppap Keywords: atmospheric pressure plasma; dielectric barrier discharge; disinfection; non-thermal plasma; plasma medicine; plasma treatment of liquids; sterilization ß 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 5

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