July 2, 2008
Oxidation of Polyvinylpyrrolidone and an Ethoxylate Surfactant in Phase-Inversion Wastewater
By Loraine, Gregory A
ABSTRACT: In this paper, components of an industrial wastewater that cause operational problems during biological treatment were oxidized by UV light and hydrogen peroxide (UV/H^sub 2^O^sub 2^). Preoxidation of wastewater was shown to remove polyvinylpyrrolidone (PVP) and ethoxylate surfactant and increase overall biodegradability. Several UV intensities and hydrogen peroxide concentrations were tested to find optimal conditions for the complete depolymerization of PVP in a synthetic wastewater composed of high concentrations of hydroxyl radical scavengers. To compare treatment options, absorption isotherms for PVP on granular activated carbon (GAC) in water and in the synthetic phase- inversion wastewater matrix were determined. The data were extrapolated to estimate the cost of using UV/H^sub 2^O^sub 2^, GAC, or off-site treatment. It was found that UV/H^sub 2^O^sub 2^ pretreatment was economically viable. Incomplete oxidation of an ethoxylate surfactant increased foaming tendency and foam stability; however, extended oxidation (>90 minutes) destroyed the foam. Water Environ. Res., 80, 373 (2008).KEYWORDS: UV/H^sub 2^O^sub 2^, oxidation, biodegradation, phase inversion, hollow fiber membrane.
The use of advanced oxidation processes (AOPs) to increase the overall biodegradability of a waste stream has been demonstrated previously (Kaludjerski and Gurol, 2004; Scott and Ollis, 1995). However, the use of AOP treatment to oxidize a specifie compound in a complex mixture is generally not attempted. This approach was taken to oxidize soluble polymers (polyvinylpyrrolidone [PVP]) and surfactants in a high-strength industrial wastewater.
Phase inversion, in which a spin dope-a mixture of a polymer, solvent, nonsolvent, and additives-is injected through a spinneret and is then immersed into an aqueous bath, is used to make several types of polymer membranes, such as amides, polysulfones, cellulose acetates, and others (Han and Nam, 2002; Qin et al, 2001; Wallace et al., 2006). The polymer is selected depending on the desired properties to the finished membranes. The solvent used in the spin dope must have strong solvent properties, yet be easy to handle (i.e., low toxicity and low flammability). N-methylpyrrolidone (NMP) is often used as a solvent for membrane formation (Qin et al., 2001; Shieh and Chung, 1998; van't Hof et al., 1992; Wallace et al., 2006). Water or alcohols are commonly used as nonsolvents (Wallace et al., 2006). Additives, such as polyvinylpyrrolidone, glycerin, and surfactants, are also frequently used to modify phaseseparation properties and viscosity (Faria et al., 2002; Han and Nam, 2002; Qin et al., 2005; van't Hof et al., 1992). Numerous examples of membrane spin dopes used in phase-inversion processes have been published. While the exact composition and concentrations vary, they generally fall within the following ranges: dimethyl formamide (DMF), 60 to 90% (w/w); NMP, 40 to 85%; glycerin (Gly), 1 to 10%; and PVP, 1 to 20% (Faria et al., 2002; Han and Nam, 2002; Qin et al., 2001; van't Hof et al., 1992). The most commonly used quench material is water (Wallace et al., 2006). Thus, the phase-inversion process produces an aqueous waste stream with high concentrations of monomers, solvents, and additives.
These waste streams are typically high in total organic carbon (TOC) and chemical oxygen demand (COD) and are often treated in biological treatment systems (Huh et al., 2004). Treatment of these waste streams is complicated by additives, such as PVP and ethoxylated surfactants, which are poorly biodegraded and can create fouling and foaming problems in biological treatment systems (Trimpin et al., 2001). Selective removal of these problematic compounds is often difficult, as the high aqueous solubility of PVP (>100g/L) and surfactants make coagulation ineffective.
Surfactants and PVP have been shown to be amenable to oxidation using AOPs (Adams and Kuzhikannil, 2000; Horikoshi et al., 2001; Kaczmarek et al., 1998; Kitis et al., 1999; Lin et al., 1999; Ledakowiczi and Gonera, 1999). An AOP is a treatment process that generates highly oxidative hydroxyl free radicals (OH^sup .^) to remove water pollutants that are difficult to treat by more conventional means. Various AOPs have often been applied as a pretreatment step before biological degradation. Combined chemical oxidation and biological treatment most often led to increased degradation of recalcitrant compounds (Kaludjerski and Gurol, 2004; Scott and Ollis, 1995).
There are several methods used to generate OH^sup .^ radicals, as follows (Ferry and Glaze, 1998; Loraine and Glaze, 1999; Rosenfeldt et al., 2006; Scott and Ollis, 1995):
(1) Ozone (O^sub 3^),
(2) Ozone/hydrogen peroxide (O^sub 3^/H^sub 2^O^sub 2^),
(3) Ozone/UV light (O^sub 3^/UV),
(4) UV/H^sub 2^O^sub 2^,
(5) Fenton's reagent (H^sub 2^O^sub 2^/Fe^sup 2+^/Fe^sup 3+^), and
(6) Peroxide and a heterogeneous catalyst (TiO^sub 2^/ H^sub 2^O^sub 2^).
Naturally, the characteristics of the waste stream make selection of the AOP to be applied an important decision. Application of ozone gas would exacerbate foaming problems, so the first three methods were rejected. Fenton's reagent generates large amounts of sludge, so it also was not investigated. Thus, the investigation focused on using UV/ H^sub 2^O^sub 2^. In addition, removal of PVP by activated carbon adsorption was also evaluated.
Horikoshi et al. (2001) proposed a mechanism for the OH^sup .^ radical oxidation of PVP. The reaction is initiated by radical attack at the N-C^sub 2^ bond, resulting in opening of the lactam ring (see Figure 1). This is followed by subsequent OH^sup .^ radical reactions at the N-C bond at the polymer backbone and leads to the formation of methylamine and propionic acid. As oxidation progresses, the length of the polymer is reduced, decreasing its affinity for the membrane and increasing its biodegradability (Horikoshi et al., 2001; Kaczmarek et al., 1998).
In the oxidation of ethoxylate nonionic surfactants, the hydrophobic chains are shortened, and the addition of -OH groups increases the polarity (Adams and Kuzhikannil, 2000; Ledakowiczi and Gonera, 1999). Thus, foam stability, which depends on surface tension and the formation of bilayers, is changed (Beneventi et al., 2001; Patist et al., 2002; Pilon et al., 2001).
In this work, the application of UV/H^sub 2^O^sub 2^ oxidation as a pretreatment step before biodegradation was examined. The focus of the work was on removal of the relatively low-concentration additives, which can cause operational problems during biological treatment-PVP and surfactants. The efficacy of UV/H^sub 2^O^sub 2^ oxidation to depolymerize PVP and increase biodegradability was examined. The reduction of the foaming tendency of an ethoxylate surfactant was examined separately.
Oxidation. Based on literature reports and consultation with manufacturers, it was decided that, because of the variable nature of the process waste stream, a simulated wastewater would be used for experimental investigation of treatment options for phase- inversion wastewater (Faria et al., 2002; Han and Nam, 2002; Qin et al., 2001; van't Hof et al., 1992). A synthetic waste consisting of DMF, NMP, glycerin, and PVP was developed. In the foaming-tendency experiments, a tetrafunctional block copolymer of propylene oxide and ethylene oxide on ethylenediamine (Tectronic 1307, BASF Corporation, Mount Olive, New Jersey) was substituted for PVP. The concentrations in Table 1 were used to simulate an average waste (all samples were made in deionized [DI] water, Milli-Q A10, Millipore Corporation, Billerica, Massachusetts).
Oxidation reactions were done in a 3-L quartz batch reactor in a Rayonet photoreactor (Southern New England Ultraviolet Company, Branford, Connecticut) equipped with either 8 or 16 "ozone-free" low- pressure mercury vapor lamps. These lamps emit UV primarily at 253.7 nm; the 185-nm wavelength was absorbed by the lamp material. While hydrogen peroxide absorbs UV at this wavelength, none of the waste components did. This was verified by measuring the absorbance at 254 nm of the synthetic waste solution; PVP solutions (0.19 g/L) in deionized water showed an Abs = 0.007 +- 0.002, DMF (22.0 g/L in deionized water) Abs = 0.030 +- 0.002, and the synthetic wastewater without H^sub 2^O^sub 2^ had an Abs = 0.057 +- 0.002. The UV intensity was measured by chemical actinometery (iodide-iodate method) (Rahn et al., 1999). The experimental conditions used in the oxidation experiments are given in Table 2. In experiment 4, hydrogen peroxide was added in three doses of 0.04 M at times 0, 60, and 120 minutes. The reaction matrix was either PVP in deionized water or PVP (experiment 1 in Table 2) or surfactant in DMF-NMP-GIy at the concentrations listed in Table 1 (experiment 2, 3, and 4 in Table 2).
Hydrogen peroxide concentrations were measured using the potassium iodide/thiosulfate method. This method is not specific for hydrogen peroxide and is a measure of overall oxidant concentration. However, the initial hydrogen peroxide concentrations were much higher than the expected concentrations of secondary oxidants, such as organic free radicals. The method of Levy and Fergus (1953) was modified to measure PVP. The chromophore formed is a complex of triiodide (I^sub 3^^sup -^) and two vinylpyrrolidine units. Thus, color intensity corresponds to the length of intact polymer. In this way, oxidation of PVP could be measured in a matrix of high TOC. Samples of PVP were diluted, and potassium triiodide (0.006 N) was added. The resulting colored complex was measured at 500 nm by spectrophotometry. The original method was modified, so that dilutions were made in deionized water rather than citric acid; this increased color stability in the presence of hydrogen peroxide.
Biodegradability Tests. The rate and extent of biodegradability of the initial and oxidized solutions were assessed by respirometric tests measuring carbon dioxide formation and oxygen consumption as a function of time. This method was developed by Kaludjerski and Gurol (2004) to quantify the effectiveness of oxidation in improving biodegradability. Oxygen and carbon dioxide were measured using an Oxymax respirometer (Columbus Instruments, Columbus, Ohio) (Kaludjerski and Gurol, 2004). A sample (25 mL) was seeded with a mixed culture of microorganisms (25 mL of activated sludge, 800 mg/ L total suspended solids), and the consumption of oxygen and the production of carbon dioxide were measured automatically, every 3 hours, over the course of 5 days. The more oxygen consumed and carbon dioxide produced, the more complete the biodegradation. Residual hydrogen peroxide was quenched with catalase before seeding the test solutions; this was found to slightly increase the consumption of oxygen and the production of carbon dioxide (see Figures 2 and 3).
Foaming Tendency. The foaming tendency, as defined by foam height and foam stability, was measured using a modified Bikerman method (Beneventi et al., 2001). This is a simple test, in which a steady stream of air is bubbled through 50 mL of solution, until stable foam is generated (approximately 10 minute). The foam height was measured, and the air flow shut off. The height was measured over time, and the time to reach one-half of the initial foam height was taken as the foam half-life. Thus, a comparable measure of how much foam was generated and how long that foam lasted could be made.
Carbon Isotherms. Solutions of PVP (190 mg/L) were made in either buffered water (pH = 8.0) or the DMF-NMP-GIy matrix buffered to pH 8.0. Aliquots (100 mL) were transferred to vials containing 0, 0.25, 0.5, 1, 2, 5, or 10 g activated carbon (WPH Pulverized, Calgon Carbon Corporation, Pittsburgh, Pennsylvania). The vials were mixed on a shaker table at room temperature (25[degrees]C) for 5 days. The samples were then centrifuged, and TOC and PVP concentrations were measured.
Results and Discussion
Oxidation and Biodegradation of Polyvinylpyrrolidone Solutions. The oxidation of PVP and phase-inversion wastewater by W/H^sub 2^O^sub 2^ oxidation was examined under several conditions (Table 2). In experiment 1, 190 mg/L PVP in deionized water was oxidized with an initial hydrogen peroxide concentration of 0.04 M and a UV intensity of 2.53 J/s-L. The PVP was completely removed within 30 minutes (Figure 4, circles). In the presence of the other waste components (experiment 2), however, oxidation at the same UV intensity and hydrogen peroxide concentration removed only 35% of the PVP before the hydrogen peroxide was depleted. In experiment 3, UV intensity and hydrogen peroxide concentration were doubled, but the PVP removal was nearly unchanged.
Comparing the relative rates of the reaction of PVP and DMF, NMP, and glycerin from the reaction rate constants and concentrations in Table 1 and using an initial hydrogen peroxide concentration of 0.04 M and a reaction rate constant (k^sub H2O2^) of 2.3 x 10^sup 7^ L/ mol^sup -1^ s^sup -1^, it can be seen that only approximately 2.0% of the available OH^sup .^ radicals would react with PVP in the presence of other components under initial conditions (Rosenfeldt et al., 2006).
Thus, it would take approximately 50 times longer to oxidize PVP in the DMF-NMP-Gly matrix than it would to treat PVP alone (Figure 4). This was observed during the oxidation experiments, where the rate of removal of PVP in deionized water was much greater than in the synthetic wastewater.
Analysis of hydrogen peroxide concentrations in experiments 2 and 3 showed that, when the concentration fell below approximately 0.02 M H^sub 2^O^sub 2^, the removal of PVP stalled. To overcome this, in experiment 4, hydrogen peroxide was added in three periodic additions of 0.04 M every 60 minutes throughout the course of photolysis, and complete removal was achieved.
Initial measurements of the biodegradability of nonoxidized PVP, DMF, DMF-NMP-Gly, and PVP-DMF-NMP-Gly (Figures 2 and 3) showed that PVP was not biodegradable; DMF degraded slowly; and the mixtures degraded faster (possibly because of the presence of glycerin, which is readily biodegradable [APHA et al., 1995]). The endogenous lines are the carbon dioixde produced and oxygen consumed by the seed organisms alone. The PVP lines fall nearly on top of the endogenous lines.
After 30 minutes of UV/H^sub 2^O^sub 2^ treatment, the biodegradability of the PVP in deionized water solution was examined using the respirometer. Figures 5 and 6 show that oxidation of PVP increased its biodegradability. After 120 hours, oxygen consumption and carbon dioxide production in the oxidized sample were 1.6 times greater than in the nonoxidized sample. Figures 7 and 8 show the carbon dioxide production from nonoxidized waste and the postoxidation samples from experiments 3 and 4 (both of these matrices were synthetic wastewater). Both the experiment 3 (partial removal of PVP) and experiment 4 (100% removal) samples showed increased biodegradation compared with the nonoxidized waste. Even incomplete oxidation of PVP increased the overall biodegradability of the synthetic wastewater. Although carbon dioxide production of the more oxidized solution (experiment 4) was higher from 48 to 96 hours, by 120 hours, the experiment 3 solution showed greater respiration.
As can be seen from Figures 5 through 8, the 100% oxidized PVP in deionized water generated approximately 4000 [mu]g CO2, and the matrix produced 45 000 to 51 000 [mu]g CO2. Thus, the carbon dioxide contribution of oxidized PVP was overshadowed by the degradation of the other components.
Foaming. Phase-inversion waste containing surfactants may generate high volumes of stable foam during aerobic wastewater treatment (Pilon et al., 2001). While an in-depth study of foaming characteristics is beyond the scope of this paper, an investigation of the effects of oxidation on the foam stability of a surfactant in the synthetic wastewater was undertaken. There are several factors that affect foaming; surfactant concentration, solvent polarity, viscosity, and surface tension (Beneventi et al., 2001; Patist et al., 2002; Pilon et al., 2001). Therefore, it is more accurate to measure foaming empirically (Pilon et al., 2001).
Solutions of Tectronic 1307 (BASF Corporation) at two concentrations (500 and 2000 mg/L in the DMF-NMP-Gly matrix) were oxidized using the lower intensity UV (2.53 J/s-L) and an initial hydrogen peroxide concentration of 0.04 M. Figure 9 shows the hydrogen peroxide concentration and the COD concentrations measured throughout the 500-mg/L Tectronic experiment. Despite the photolysis of the hydrogen peroxide, no significant removal of COD was observed. Figure 10 shows the foam height (centimeters) plotted against the oxidation time, for both surfactant experiments. The addition of hydrogen peroxide alone increased foam height, as seen in Figure 10. Before the addition of hydrogen peroxide, the height for the 500-mg/L surfactant concentration was 4 cm, and, after 0.04 M H^sub 2^O^sub 2^ was added, the foam height doubled, to 8.0 cm. After 15 minutes of oxidation, the height had dropped to below 4 cm, although the foaming tendency was not significantly decreased until after 60 minutes (38% of initial height). After 90 minutes, the foam height was below the 0.5-cm detection limit.
At the higher surfactant concentration (2000 mg/L), it can be seen that foaming tendency actually increased with oxidation and did not fall until after 90 minutes of treatment. However, by 120 minutes, the solution hardly foamed at all, and the foam height was 0.5 +- 0.5 cm. This indicates a nonlinear decrease in foam stability with UV/ H^sub 2^O^sub 2^ oxidation and that incomplete oxidation would increase operational problems. There appears to be a treatment threshold that must be reached before the foaming tendency is broken.
In a surfactant bubble, a thin layer of aqueous solution is sandwiched between an interior and an exterior air phase. The walls of the bubble are made of monolayers of surfactant. The surfactant monomers are in dynamic equilibrium between the micelles, dissolved monomers, and the bubble walls. The more stable the micelles, the lower the monomer flux into the aqueous phase, and the fewer monomers available to partition to the air-water interface. This results in less stable foams.
Micelle stability can be measured as relaxation time, the average lifetime of the micelle. Patist et al. (2002) found that the relaxation times of polyoxylethylene alkyl ethers were much higher than for ionic surfactants because of the absence of ionic repulsion between the head groups. Increasing the polarity of the surfactant decreased the relaxation time, which increased the number of monomers in solution. These monomers were then available to partition to the air-water interface and stabilize foam formation. Because the expected products of OH^sup .^ radical oxidation of polyethoxylatepolypropylene polymers are alcohols, aldehydes, and acids (Gallet et al., 2002; Sherrard et al., 1996), the ionic repulsion in the micelles can be expected to increase. Thus, it is possible that, as the oxidation progresses, the observed foam stability first increases as partially oxidized monomers destabilize the micelles, then decreases as the monomers lose their hydrophobic character as they are further oxidized. Carbon Adsorption. Figure 11 shows the isotherm for solutions of PVP (190 mg/L) in buffered water (pH = 8.0) and the synthetic matrix buffered to pH 8.0. These data were fit to the Freundlich equation.
x/m = weight of PVP (in mg) adsorbed per gram of granular activated carbon (GAC),
c = concentration of PVP in solution after equilibrium (mg/L), and
k and n = constants.
The k values in buffered deionized water and in the synthetic wastewater were found to be 12.7 and 13.0 and the n values were 126 and 161, respectively. The PVP is poorly absorbed by GAC, but is not significantly affected by the matrix conditions. The TOC removal from both matrices (Figure 12) shows that approximately 700 mg/L TOC/ g GAC was removed from the matrix, while only 5 mg/L TOC/g GAC was removed in deionized water. This can be extrapolated to show that only 5 mg/L of the 700 mg/L removed in the matrix was the result of PVP removal.
Pretreatment Cost Estimates. Estimates of the pretreatment costs for phase-inversion waste containing PVP using UV/H^sub 2^O^sub 2^ oxidation, GAC, and off-site treatment were calculated (Table 3). A volume of 10 million L/y (27 397 L/d) was assumed. The costs of reactor manufacture or rental were not included. The figure for the annual costs for off-site biological treatment was obtained from a membrane manufacturer.
The figure for GAC is for the cost of the carbon only. From the Freundlich equation, at an initial PVP concentration of 190 mg/L, 3.45 mg PVP would be adsorbed per gram of GAC or 55 g of GAC would be required per liter of wastewater. Assuming a flowrate of 27 397 L/ d, this would require 1507 kg GAC/d. At a price of $807/metric ton for GAC, the cost of carbon would be $0.039/L, or $3,900,000 annually. Often, column breakthrough is reached before equilibrium conditions are reached, so additional carbon may be required. One. additional area of concern would be the surfactants. It is possible that, if a solution containing surfactant were passed through an activated carbon column that had previously been used for PVP removal, PVP could be resolublized.
The price of hydrogen peroxide (50%) is based on a price of $0.73/ L (U.S. Peroxide, Atlanta, Georgia) and a dose rate of 8.2 mL/L waste water. The energy costs are based on the kilowatt-hours required by the lamps to remove 100% of the PVP at a cost of $0.05/ kWh. Capital expenses are the cost of the lamps divided by 2000 hours of lamp lifetime divided by 1142 L/h. The UV lamp capital costs were calculated based on the experimental system, with an assumed lamp price of $60 per lamp, based on prices obtained from retail vendors, and therefore are somewhat conservative. Table 3 shows that UV/H^sub 2^O^sub 2^ oxidation before biological treatment would be more economical than GAC or off-site treatment.
Oxidation before biodegradation leads to more complete biodegradation for many industrial wastewaters (Kaludjerski and Gurol, 2004; Scott and Ollis, 1995). This appears to hold true for phase-inversion wastewater also. Oxidation by UV/H^sub 2^O^sub 2^ was shown to remove PVP and increase the biodegradability of the wastewater. In addition, it was seen that a minor component in a complex mixture, such as PVP (
The application of UV/H^sub 2^O^sub 2^ to reduce foam formation was more problematic. Partial oxidation of the surfactant solutions resulted in more stable foam rather than less stable. While there are several studies in the literature in which AOPs have been used to oxidize surfactants (Adams and Kuzhikannil, 2000; Kaczmarek et al., 1998), there is surprisingly little on how oxidation affects foaming tendency, considering how important foams are in many applications (Patist et al., 2002). These results indicate that oxidation, particularly using hydrogen peroxide, may create additional problems when used to treat surfactants.
Submitted for publication February 4, 2007; revised manuscript submitted November 5, 2007; accepted for publication November 9, 2007.
The deadline to submit Discussions of this paper is July 15, 2008.
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Gregory A. Loraine*
Dynaflow Inc., Jessup, Maryland.
* Dynaflow Inc., 10621 Iron Bridge Rd, Jessup, MD 20794; e-mail: [email protected]
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