Ind. Eng. C h e m . R e s . 1987,26, 125-128
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Use of Graphite Fiber-Polymer Matrix Composite Electrodes for the Electrochemical Disinfection of Latex Solutions Paul N. Natishan,* George L. Cahen, Jr., and Glenn E. Stoner Applied Electrochemistry Laboratory, Department of Materials Science, University of Virginia, Charlottesville, Virginia 22901
An ac electrochemical disinfection unit that produces a disinfecting agent, C12, in situ was evaluated as an alternative to present latex disinfection practices that dilute the latex and lower its percent solids content. The ac electrochemical reactor used in this investigation was in a parallel plate configuration and used graphite fiber-epoxy matrix composites as electrode material. The relative disinfection efficiencies and reactor stability were determined to be functions of the chloride concentration, ac frequency, and the current density. Two latexes designated for their pH values of 5.5 and 8.5 were used in this investigation. The ac electrochemical disinfection unit was found to be effective in disinfecting contaminated volumes of latex 8.5 and maintaining a noncontaminated volume of latex 5.5 in that state. After latex is produced, it is often held in storage tanks, awaiting sales and shipment. During this time, the latex is exposed to air and soilborne bacteria which can infect and grow in the latex. Presently, the most widely used method for disinfecting a contaminated latex before shipment is the addition of hypochlorous acid, HOC1. Although effective, the volume of HOCl added to the latex dilutes it and, thereby, lowers its percent solids content. Since the applications of a latex are determined, in part, by its percent solids content, dilution results in loss of applicability in certain processes and ultimately in a loss of sales. Also, HOCl presents a chemical hazard when improperly handled or spilled. In this study, an ac electrochemical disinfection system (Stoner, 1973) that produces a disinfecting agent in situ was considered as an alternative to present disinfection practices. The system is capable of electrochemically generating a disinfecting agent such as Clzby oxidizing C1(Stoner, 1973) and/or directly oxidizing bacteria at the anode (Cahen, 1976). Chloride, if not already present in the latex, can be added in the form of a salt such as NaCl which does not dilute the latex. The purpose of this investigation was to evaluate the ac electrochemical disinfection system for the treatment of contaminated latexes. The ac disinfection system has been found to be effective in treating sewage (Hendricksen, 1978) and other contaminated waters (Stoner et al., 1982, Srinivasan et al., 1977).
Experimental Section Solutions. Two latexes designated for their pH values of 5.5 and 8.5 were used in this investigation. Latex 5.5 had a 50% solids content and a pH of 5.5. Latex 8.5 had a 48% solids content, was ammonia stabilized, and had a pH of 8.5. Due to the proprietory nature of these latexes, the above was the only compositional information available to the authors. Attempts to determine the presence and concentration of chloride in the as received latex were unsuccessful. The latexes were naturally contaminated. A bacterial growth medium was prepared by mixing Difco bacto-dehydrated nutrient broth (8 g), NaCl (8 g), Difco Bacto-agar (15 g), and distilled water (1 L). The solution was autoclaved, allowed to cool, and poured into *Present address: US Naval Research Laboratory,Washington, DC.
90- X 15-mm sterile, disposable Petri dishes. The amount of NaCl added to the latex was based on the weight that would be added to a volume of water (not to the latex) to produce a given weight percent. In this paper the symbol ?& will be used when referring to the amount of NaCl added to the latex to distinguish it from an actual weight percent. Electrochemical Reactor. Randomly oriented graphite fiber-epoxy matrix composites were used as electrode material in a parallel plate reactor configuration. The length of an electrode was 20 cm, and the width was 6.35 cm. Gaskets, 20 X 0.424 X 0.015 cm, were placed lengthwise on either side of the electrodes, and there were manifolds at the inlet and outlet of the reactor. The exposed geometric surface area of a composfe electrode was 110 cm2,and the electrode separation was 0.15 cm. Current densities reported are based on the exposed geometric surface area. General Procedures. A schematic diagram of the experimental setup for this investigation is presented in Figure 1. Bacterially contaminated latex was pumped from a 25-L holding tank through a surge tank to the ac electrochemical reactor where it was treated and then returned to the holding tank. There was a branch hose on the effluent side of the reactor for sample collection. A Sorenson SRL 20-12 power supply was coupled to a polarity reversal unit, PRU, that controlled the power duty cycle as well as the cell polarity. The PRU was coupled to the treatment reactor and an oscilloscope. The oscilloscope was used to monitor the voltage response to an alternating current that had a square waveform. The cell resistance, iR,portion of the voltage waveform depends on the active electrode surface area, so that this component of the waveform could be used to monitor changes in the electrochemically active surface area (Stafford, 1980). Immediately after a latex sample was taken, a Petri smear and a DPD test were performed. The chemical was used to DPD (N,N-diethyl-p-phenylenediamine) qualitatively determine the presence of a chlorine residual. The DPD was effective in qualitatively determining the presence of Clz when the added C1- concentration was 0.56% or greater. The two tests were repeated on the original sample after a period of time, usually 'Iz h. Petri smears were placed in an oven at 37 "C for 48 h to incubate, and the degree of bacterial growth for each smear was ranked visually according to Dow latex method 45. This method requires a visual comparison of the experimental
0888-588518712626-Ol25$01.50/0 0 1987 American Chemical Society
126 Ind. Eng. Chem. Res. Vol. 26, No. 1, 1987 11 1
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Figure 1. Apparatus scheme.
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Figure 4. Relative disinfection rating vs. time for electrochemically treated (T76)and control volumes (control) of latex 8.5.
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Figure 2. Relative disinfection ranking system.
Figure 3. Efi”ect of standing time on the degree of disinfection.
Petri smears to a set of photos of standard ranked Petri smears (Dow Chemical Co., 1980). See Figure 2 for photos of four standard ranked Petri smears. The standard rankings range from 1 to 10, with a ranking of 1 being bacteria free. A ranking of 10 indicates a very high bacteria level and represents the highest (worst) possible ranking in this system. Samples were taken before and after electrochemical treatment. In order to determine if the electrochemical treatment damaged the latex, visual inspections as well as the following test were performed. A thin film of latex was smeared on a glass slide and allowed to dry overnight at room temperature. Blemishes would appear in the dried film if the latex was damaged (Irons, 1980).
Results The following examples demonstrate some of the characteristics and limitations of the ac electrochemical disinfection unit when applied to latex emulsions. Latex 8.5. Figure 3 shows three Petri smears taken during a disinfection experiment. In this test, the current density was 27 mA/cm2, the frequency was 0.25 Hz, the flow rate was 0.69 L/min, and the added chloride con-
Figure 5. Change in iR with time for two electrochemical treated volumes of latex 8.5 (T76had 0.50% added chloride and T78 had no added chloride).
centration was 0.56%. Petri smear 380, taken before electrochemical treatment, was ranked as an 8, and the DPD test was negative. Petri smear 389 was taken immediately after sample 389 was collected. It was ranked as a 5, and the PDP test was positive, indicating the presence of C12or HOC1. Petri smear 390 was taken from sample 389 40 min after it had been collected. The rank was 1, and it had a positive DPD test. These results, confirmed in subsequent tests, demonstrated that a residual disinfecting agent was produced during the electrochemical treatment and was effective in disinfecting the latex. Figure 4 presents the results of test T76. The current density was 27 mA/cm2, the frequency was 0.33 Hz, the flow rate was 0.69 L/min, and the added chloride concentration was 0.50%. A control volume was taken from the holding tank prior to electrochemicaltreatment. The electrochemical treatment completely disinfected the volume of latex in 23 h. The electrochemical treatment continued for an additional 141 h, and there was no bacterial reinfection noted. In contrast, the control volume reached a maximum level of contamination, as determined by a ranking of 10, in 75 h. The iR component of the voltage waveform increased and at 164 h was 1.8 V higher than the initial value; see curve T76 in Figure 5. Since i was held constant and R has a reciprocal relationship with A, the electrochemically active surface area, the increase in the iR portion of the waveform would indicate a decrease in electrochemicallyactive surface area, surface masking. The results of the next experiment designated T78 are presented in Figure 6. The current density was 14 mA/cm2, the frequency was 0.50 Hz, the flow rate was 0.69 L/min, and no chloride was added to the latex. Again, a control volume was taken from the holding tank prior to electrochemicaltreatment. The electrochemicaltreatment completely disinfected the volume of latex in 20 h. The latex showed signs of reinfection at 96 h, indicating a re-
Ind. Eng. Chem. Res. Vol. 26, No. 1, 1987 127
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T73 _
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and HOCl can be found elsewhere (White, 1972). The latexes were found to be sensitive to the chloride concentration, the current density, and the ac frequency. When one of these parameters was above a critical limit, the latex was readily damaged and the reactor fouled. Chloride concentrations greater than 1% , current densities greater than 36 mA/cm2, and low frequencies less than 0.05 Hz quickly damaged the latex and fouled the reactor. In a practical reactor system, one that would function for 200 h without maintenance (Irons, 1980), the upper operational limits would, in general, be 27 mA/cm2 or less for the current density, 0.50% or less for the added chloride concentration, and less than 0.25 Hz for the ac frequency. The parameters are interdependent at or below the operational limits. The effects of these parameters on the performance and life of the reactor are discussed below. The major mode of reactor failure during the electrochemical treatments were surface masking. Stafford (1980), working with graphite fiber-polymer matrix composited electrodes in aqueous salt solutions, demonstrated that under dc conditions the anode was subject to fiber oxidation and electrolyte absorption. This process continued until there was a loss of fiber/fiber contact and resulted in a surface layer that was durable but electrochemically inactive. The inactive layer acted as a barrier to mass transport. A composite electrodes stability was influenced by the rate of formation of the inactive surface layer, its porosity, and its durability. In ac electrochemical studies, Stafford demonstrated that during the cathodic phase hydrogen production provided sufficient interfacial pressure to cause the inactive surface layer of a brittle composite, such as a graphite fiber-epoxy matrix composite, to ablate. The ablation provided a fresh electrochemically active surface. In contrast, the more ductile composites accommodated the stress by deforming plastically and retained the electrochemically inactive surface layer. The voltage waveform response of the more ductile composite electrodes showed a large increase in iR with time. Again, since i was held constant and R has a reciprocal relationship with A , the electrochemically active surface area, the increase in iR indicated a loss of electrochemically active surface area, masking. The voltage waveforms of the brittle composite showed a decrease in their iR components, indicating roughening (Stafford, 1980). In the latex studies, the normal brittle behavior of the graphite fiber-epoxy matrix composite electrodes, as determined by the iR component of the voltage waveform, was replaced by the behavior Stafford noted for the more ductile composites. It appears that during electrochemical treatment, latex adhered to the electrode surface and formed a latex-epoxy-graphite fiber composite, LEG. Since the latex was fairly ductile, the LEG layer was durable. Also, a secondary layer of damaged latex adhered to the LEG layer. This secondary layer grew rapidly and blocked the reactor flow channel when the parameters were above the critical limits. The inactive layers were observed visually and by the increase in the iR component of the voltage waveform. The secondary layer of latex was easily removed by gently rubbing it under running water, whereas the inactive layer was durable and required polishing with 600-grit S i c paper to remove it. It is believed that both layers interfere with the electrodes ability to ablate and clean its surface. In dc tests the damaged latex was shown to adhere only to the anode. The formation of the LEG and the secondary layers that ultimately lead to reactor failure appears to be related to
128 Ind. Eng. Chem. Res. Vol. 26, No. 1, 1987
the oxidizing environment atlor near the anode. The highly oxidizing environments that caused latex damage and adherence could result from high concentrations of Clz that were produced when the added C1- concentration was high and/or high current densities. Decreasing the current density and/or decreasing the added chloride concentration decreases the severity of the anode environment and extends the operating life of the reactor. Again, the upper operational limits for the added chloride concentration and current density were 0.50% and 27 mA/cm2, respectively. Another important factor that controlled the formation and growth of the inactive layers was the ac frequency. When the added chloride concentration and the current density were at or below the operational limits outlined above, latex was still oxidized and adhered to the anode. The amount of latex that accumulated was directly related to the ac frequency; the higher the frequency, the less latex that accumulated on the surface. It appears that a t the higher frequencies there was (1)less time for the LEG layer to form and grow, ( 2 ) less time for the secondary inactive layer to form and grow, and (3) less time to the cleaning phase, Le., the cathodic phase where the electrode can ablate. Frequencies of 0.25 Hz or less were effective. In general, strong oxidizing environments seemed to favor the formation and growth of the secondary layer, and mild oxidizing environments and low frequencies favored the formation of the LEG layer. The ac electrochemical treatment system was not able to disinfect heavily contaminated volumes of latex 5.5. Either some component of the latex was inhibiting the production of C1, or once produced the Clz was rapidly oxidizing a component of the latex and therefore was depleted before it could act on the bacteria. However, since one of the modes of disinfection is the direct oxidation of the bacteria at the anode, the disinfection system should be able to hold a noncontaminated volume latex in that state. The values for the three parameters that were used in test T73 were effective in maintaining the latex 5.5 in a noncontaminated state (Figure 6), and the electrodes functioned for 336 h without any apparent instability. Additional testing is required with latex 5.5. The ac disinfection system was very effective in disinfecting contaminated volumes of latex 8.5. However, the latex was sensitive to the ac electrochemical treatment, and there were problems with surface masking. In test T78, no added chloride, the buildup of inactive layers occurred quickly, as determined from the voltage waveform (Figure 5 ) and caused reactor failure in 96 h, as determined by bacterial reinfection (Figure 6). In test T76, 0.50% added chloride, surface masking occurred (Figure 5 ) but the reactor appeared to be functional for 164 h (Figure 4). Although a thin layer of latex accumulated on the surface in test T76, the layer was porous and/or the electrodes were still able to ablate. Both the current density and the added chloride Concentration were at the upper operational limits in this test. Although, no latex 8.5 test has been run longer than 164 h, the results of test T76 were promising, and it appears that by adjusting the chloride concentration,
the current density and/or the ac frequency the amount of latex damage and adherence to the electrodes can be controlled so that the operating life can be extended. The ac electrochemical disinfection system could be effective in treating other latexes. The values for the three parameters would vary with latex composition and sensitivity but should be close to the values used in tests T73 and T76.
Conclusions The ac electrochemical disinfection unit was effectively used to disinfect heavily contaminated volumes of latex 8.5 and maintain a noncontaminated volume of latex 5.5 in that state. The longest test for latex 8.5 was 164 h. The reactor maintained a volume of latex 5.5 in the noncontaminated state for 336 h without reactor maintenance. The ac electrochemical reactor’s performance and operating life were found to be functions of the current density, the added chloride concentration, and the ac frequency. Latex damage, electrode fouling, and reactor failure resulted when the oxidizing atmosphere near or at the anode was too severe and/or the ac frequency was too low. The severity of the oxidizing atmosphere can be decreased by lowering the current density and /or the added chloride concentration. The ac frequency controlled the amount of time in each cycle that the latex-epoxygraphite and the secondary layers could form and grow and the amount of time between cleaning phases. Therefore, by adjustment of these three parameters, the performance and life of the reactor can be regulated. Acknowledgment We acknowledge Dow Chemical U.S.A. for financial support in the form of a fellowship for P. N. Natishan. Also, the US Army Research Office is acknowledged for partial support of this project. Registry No. Clz, 7782-50-5.
Literature Cited Cahen, G. L., Jr. Ph.D. Dissertation, Department of Materials Science, University of Virginia, Charlottesville, 1976. Dow Latex Method 45, Dow Chemical Co., Midland, MI, 1980. Hendricksen, S. C. M.S. Thesis, Department of Civil Engineering, University of Virginia, Charlottesville, 1978. Irons, K. A., Dow Chemical Co., Midland, MI, personal communication, 1980. Srinivasan, s. In Electrochemistry: The Past Thirty and The N e x t Thirty Years; Bloom, H., Gutman, F., Eds.; Plenum: New York, 1977; p 57. Stafford, G. R. Ph.D. Dissertation, Department of Materials Science, University of Virginia, Charlottesville, 1980. Stoner! G. E. US Patent 3 725 226, 1973. Stoner, G. E.; Cahen, G. L., Jr.; Sachyani, M.; Gileadi, E. Bioelectrochem. Bioenerg. 1982, 9(3), 229. White, G. C. Handbook of Chlorination; Van Nostrand Reinhold: New York, 1972.
Received f o r review J u n e 20, 1985 Revised manuscript received May 15, 1986 Accepted July 8, 1986
