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H2O2 preoxidation on the aerobic biodegradability of quaternary amine surfactants
PII: S0043-1354(99)00186-4
Wat. Res. Vol. 34, No. 2, pp. 668±672, 2000 # 1999 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/99/$ - see front matter
www.elsevier.com/locate/watres
RESEARCH NOTE EFFECTS OF UV/H2O2 PREOXIDATION ON THE AEROBIC BIODEGRADABILITY OF QUATERNARY AMINE SURFACTANTS C. D. ADAMS*M and J. J. KUZHIKANNIL Environmental Engineering Program, Department of Civil Engineering, University of Missouri-Rolla, 202 Civil Engineering Building, Rolla, MO 65409, USA (First received 1 December 1998; accepted in revised form 1 April 1999) AbstractÐThe purpose of this study was to examine the eect of UV/H2O2 advanced oxidative pretreatment on the biodegradability of two major classes of quaternary amine surfactants. In this study, the biodegradability of unoxidized and oxidized solutions of quaternary amines were determined using BAS assays. UV/H2O2 preoxidation eectively enhanced the biodegradability of alkyldimethylbenzyl ammonium chlorides (Barquats), but had little eect on the biodegradability of dioctyl-dimethyl ammonium chloride (Bardac LF). # 1999 Elsevier Science Ltd. All rights reserved Key wordsÐhydrogen peroxide, UV, advanced oxidation technologies, biodegradation, quaternary amine surfactant, Barquat, Bardaq
INTRODUCTION
Industries worldwide discharge a wide range of surfactants, or surface active agents, to their wastewater treatment facilities. Biodegradation is generally considered to be the most economical means of treating biodegradable surfactants. Some classes of surfactants, however, are biorecalcitrant due to enzymatic de®ciencies or to their toxicological properties. Cationic surfactants containing a quaternary ammonium (e.g. R4N+; where R=alkyl chain and N=quaternary nitrogen), often have a strong biocidal nature (Baleux and Caumette, 1977; Swisher, 1987). Generally, the biodegradability of quaternary amines decreases with the number of non-methyl alkyl groups (i.e. R4N+ < R3MeN+ < R2Me2N+ < RMe3N+ < Me4N+; where Me=methyl radical) (Swisher, 1987). Additionally, replacement of a methyl group in a quaternary amine with a benzyl group can decrease biodegradability further (i.e. RBzMe2N+ < R2Me2N+) (Swisher, 1987). Two common classes of quaternary amines were examined in this study, speci®cally: (1) alkyldimethylbenzyl ammonium chlorides (Barquats) and (2) dioctyldimethyl ammonium chloride (Bardac LF) (Lonza Chemical). Advanced oxidation technologies (AOTs) (as well
as ozonation) have been shown to be highly eective at enhancing the rate and extent of biodegradability of many surfactants and other organic compounds (Gilbert, 1987; Calvosa et al., 1991; Adams et al., 1994, 1996, 1997; Hu and Yu, 1994; Kitis et al., 1998). The eciency of AOTs and ozonation is a strong function of the treatment objective desired, the inorganic and organic constituents in solution and the nature of the chemical oxidants. AOTs are processes which use the hydroxyl radical ( OH) as a primary oxidant. The hydroxyl radical is a strong, non-selective oxidant with rate constants often in the order of 109 l/mol s and a standard electrode potential (relative to the hydrogen electrode) of 2.8 V compared with 2.07, 1.78 and 1.36 V for ozone, hydrogen peroxide and chlorine, respectively (Pontius, 1990). In the UV/H2O2 AOT, hydroxyl radicals are generated and react with pollutants (P) and scavengers (S) by the overall reactions: H2 O2 UV 4 2 OH
1
OH T 4 Products
2
OH S 4 Byproducts
3
where T and S are target compounds and scavenger concentrations, respectively. The hydroxyl radical is
*Author to whom all correspondence should be addressed. 668
Research Note
capable of eciently oxidizing both target species and scavengers in a solution. Many other inorganic and organic radicals area also generated in the UV/ H2O2 AOT [e.g. superoxide ( Oÿ 2 ), perhydroxyl ( HO2), organic radicals ( R), etc.]. The relative importance of individual radical species on the overall reaction kinetics and pathways, however, was beyond the scope of this study. Depending upon the relative concentration and kinetic rate constants for the reaction of hydroxyl radical with the target species vs scavengers, the fraction ( f ) of the oxidant being used eciently (i.e. reacting with the target species) is described by reaction rate with target species total reaction rate koxidant,target OH T koxidant,target T or f all species all species X X k OH i k i OH,i
i
OH,i
i1
The purpose of this study was to examine the UV/H2O2 AOT for the pretreatment of biorecalcitrant (or partially biodegradable) quaternary amines. For each study compound, oxidative pretreatment was followed by aerobic batch activated sludge (BAS) bioassays. Ozone and ozone-based AOTs were also examined initially but, due to formation of excessive and rigid foam upon ozonation, only the only UV/H2O2 was used for chemical oxidation in this research. MATERIALS AND METHODS
Chemicals
f
i1
669
4
i
where T is the target species and i are individual organic and inorganic constituents in the system. Therefore, the relative reaction of OH with the target compound vs scavengers is of paramount importance with regard to process eciency. The goal for advanced oxidative treatment of biorecalcitrant compounds is often not to mineralize the compounds (i.e. convert to CO2 and H2O), but rather to convert biorecalcitrant compounds to byproducts which are readily biodegradable in conventional biological treatment processes. The purpose of coupling chemical and biological processes is to allow mineralization of a target pollutant using a minimum (or optimum) amount of the more costly chemical oxidation, followed by a relatively inexpensive (often existing) biological process such as activated sludge or ®xed-®lm biological treatment process.
The study compounds were all common quaternary amines. The quaternary amines examined were three alkyldimethylbenzyl ammonium chlorides (Barquat MX, Barquat MB, and Barquat OJ), and a dioctyl-dimethyl ammonium chloride (Bardac LF) (Lonza Chemical) (Fig. 1). The compounds were used as received. The Barquats are mixtures of dimethylated benzyl ammonium chlroides with varying normal-alkyl chain lengths from C12 to C18 (Fig. 1). All other chemicals used in the study were at least of reagent grade. Chemical oxidation procedure The quaternary amines were oxidized using the UV/ H2O2 advanced oxidation process. Initial solutions contained 1000 mg/l as COD of surfactant and 8 mM sodium phosphate. The initial H2O2 concentration and pH were adjusted to 1000 mg/l and 8, respectively. The UV/H2O2 oxidations were conducted in a 1.2-l batch photoreactor (Ace Glass and Kimble Scienti®c) using a low-pressure UV lamp with a principle wavelength of 254 nm (Pen Ray). The energy output of the UV lamp was determined using H2O2 actinometry to be 0.62 W corresponding to 0.56 W/l for the reactor used. Additional reactor ports were provided for a thermometer and for sample extraction. A magnetic stirrer provided continuous mixing during the reaction. All oxidations were conducted at a temperature of 22 (21)8C. The UV lamp was allowed to warm up and stabilize for 10 min prior to initiating an experiment.
Fig. 1. Structures of Barquats and Bardac examined in study.
670
Research Note
Oxidation times of 0, 30 and 120 min were used for most solutions. Residual H2O2 was measured using a chemical titration method (described below) and any residual H2O2 was immediately quenched using a slight stoichiometric excess of sodium sul®te. Samples were stored at 58C prior to COD analysis, BAS bioassay and DOC analysis. BAS bioassay procedure BAS bioassays were performed on 200 ml solutions at an initial concentration of 300 mg/l as COD of oxidized or 2+ unoxidized surfactant, added nutrients (e.g. NH+ , 4 , Ca 3+ 2+ 2+ 2+ 2+ 2+ 2+ and Mo ) and Fe , Co , Zn , Mg , Mn , Cu a mixed liquor suspended solids (MLSS) concentration of 1500 mg/l. Control measures included bioassaying: (1) each ¯ask in duplicate, (2) duplicate blanks (with no surfactant solution) and (3) duplicate positive controls containing an easily degradable substrate (ethylene glycol). Tests were also performed to assure that volatilization and sorption losses were minimal for the surfactant solutions. All BAS bioassays were run at 22 (21)8C. Samples were shaken at 180 rpm on a Labline shaker table to provide mixing and aeration. 10-ml samples were removed from the ¯asks at predetermined sample times. The samples were then centrifuged at 5000 rpm for 5 min to remove biomass prior to dissolved organic carbon (DOC) analysis. A separate 12-l sequencing batch reactor (SBR) was operated exclusively to generate biomass for the BAS bioassays. The SBR was fed nutrients as well as a mixture of synthetic organic chemicals including: ethylene glycol, polyethylene glycol, Alkumuls 0±14, catechol, resorcinol, hydroquinone, trihydroxybenzene and butyl ether. The solids retention time (SRT) and hydraulic retention time (HRT) for the SBR were 6 and 1 days, respectively. The solids wasted from the SBR on a daily basis were either discarded or used in the BAS bioassays. The DOC values for each sample removed from the shaker ¯asks were plotted vs BAS bioassay time from which the ultimate DOC removal (UDR, percent) was determined. The UDR provided an estimate of the biodegraded fraction of the oxidized or unoxidized solutions and an estimate of the non-biodegraded fraction (100 minus UDR, percent). All BAS bioassays were conducted in duplicate with the average coecient-of-variation for UDR between bioassays of 14%. Analytical procedures COD was measured with Hach COD ampoules (0± 150 mg/l range) with absorbance measured on a Spectronic 20D (Milton Roy). DOC was measured via Standard Method 5310B using a Model TOC-5050A Analyzer (Shimadzu) with ASI 5000A autoloader. pH was measured using a Model 320 pH meter (Corning) and probe. Hydrogen peroxide concentrations were measured using the potassium iodide (KI) method with starch indicator, and dropwise titration with sodium thiosulfate (Hach HYP-19 H2O2-Test Kit). MLSS was measured in accordance with Standard Method 2540D (Greenberg et al., 1992). RESULTS AND DISCUSSION
Foamability tests Initially, it was planned to compare direct ozonation, the H2O2/O3 AOT and the UV/H2O2 AOT for preoxidation prior to the BAS bioassays. Initial experiments, however, showed that the quaternary amines formed a rigid foam that would not disperse in the ozonation reactor or in the bubble trap
Fig. 2. Bioassay plots for Barquats MX (top), OJ (middle) and MB (bottom).
above the reactor, even after an extended period of time. Therefore, only the UV/H2O2 AOT was technically feasible for preoxidation of the quaternary amines in these experiments. Biodegradability enhancement results All of the unoxidized quaternary amines studied were relatively biorecalcitrant which is consistent with the biocidal nature of this class of surfactant (Figs 2 and 3). Advanced oxidation for 30 and 120 min (0.28 and 1.1 W h/l) resulted in a signi®cant increase in the biodegradation rate and extent for all three alkyldimethylbenzyl ammonium chlorides (Barquats) (Fig. 2). The highest UV dosage resulted in COD removals of between 20 and 30% for each Barquat. The resulting ultimate DOC removals for the oxidized Barquats in the BAS bioassays were approximately 90% in all cases (Fig. 2). Speci®cally, it appears that radical intermediates (e.g. OH, Oÿ 2 , etc.) resulting from the photolysis
Research Note
Fig. 3. Bioassay plot for Bardac LF.
of hydrogen peroxide were highly eective at modifying the chemical structure of the Barquats to yield readily biodegradable oxidation byproducts. It is apparent that advanced oxidation pretreatment is eective at enhancing the biodegradability of the alkyldimethylbenzyl ammonium chlorides. The dioctyl-dimethyl ammonium chloride (Bardaq LF) also exhibited signi®cant biorecalcitrance in its unoxidized form. However, even after 120 min (i.e. 1.1 W h/l) of UV/H2O2 oxidation, the biodegradability of the Bardaq LF was only slightly improved (Fig. 3). Therefore, advanced oxidation pretreatment appears ineective at enhancing the biodegradability of the dioctyldimethyl ammonium chloride, Bardaq LF. Hydroxyl radical rate constants are in the order of 109 to 1010 for compounds related to both the Barquats and Bardaq. For example, benzene and several alkylated benzenes and the corresponding non-benzylated alkanes, have hydroxyl radical rate constants ranging from 1(109) to 8(109) (Hickel, 1975; Sehested and Holcman, 1979; Rudakov et al., 1981; Buxton et al., 1988; Geto, 1991). Substitution of an amine group on aromatic or aliphatic compounds tends to raise hydroxyl radical rate constants only slightly if at all (Geto and Schwoerer, 1973; Hickel, 1975; Neta and Schuler, 1975; Sehested and Holcman, 1979; Rudakov et al., 1981; Solar et al., 1986; Buxton et al., 1988; Roder et al., 1990; Geto, 1991). Absolute reaction rate constants for Barquats and Bardaqs were not determined. However, due to the aforementioned similarity in hydroxyl radical rates constants for related compounds, and due to similar COD removal rates, it is hypothesized that the Barquats and Bardaq both compete eectively with scavengers in solution for hydroxyl radicals (Eqs. (2)±(4)). It is further hypothesized that the signi®cant dierence in the biodegradability enhancement after oxidation observed for the benzylated vs non-benzylated quaternary amines is, therefore, related directly to the structure and biodegradability of the oxidation byproducts themselves. It was, however, beyond the scope of this study to determine the identities of these oxi-
671
Fig. 4. Ultimate DOC removal (UDR) vs dosage for study compounds
dation byproducts. Furthermore, primary process parameters (e.g. temperature, pH, alkalinity, organic and inorganic background constituents) have a signi®cant eect on oxidation pathways and resulting byproducts. The concentration of the substrate in these experiments was relatively high (i.e. 1000 mg/l COD) and other process conditions and substrate concentrations may result in a dierent byproduct mixture. The ultimate DOC removals achieved for each study compound are summarized in Fig. 4. It is apparent that UV/H2O2 advanced oxidation pretreatment was highly eective at enhancing the aerobic biodegradability of the alkyldimethylbenzyl ammonium chlorides (Barquat MX, Barquat MB and Barquat OJ) yet ineective at enhancing the biodegradability of dioctyldimethyl ammonium chloride (Bardaq LF). These results point to the fact that because the eectiveness of advanced oxidation pretreatment on biodegradability enhancement is highly dependent on the nature of the species being treated and other process parameters, laboratory- or pilot-scale studies should be conducted prior to design of integrated chemical±biological treatment processes for wastewaters containing mixed surfactants. AcknowledgementsÐThis work was funded by the National Science Foundation (Grant number BES9696011). All experiments were conducted in the Environmental Research Center at the University of Missouri-Rolla (UMR). C. D. A. is Mathes Professor of Environmental Engineering and Associate Professor of Civil Engineering at the UMR. J. J. K. was a graduate research assistant earning an M.Sc. Environmental Engineering degree at the time of this study.
REFERENCES
Adams C. D., Cozzens R. and Kim B. (1997) Eects of ozonation on the biodegradability of substituted phenols. Water Research 31(10), 2655±2663. Adams C. D., Scanlan P. A. and Secrist N. D. (1994) Oxidation and biodegradability enhancement of 1,4dioxane using hydrogen peroxide and ozone. Environ. Sci. Technol. 28(11), 1812±1818.
672
Research Note
Adams C. D., Spitzer S. and Cowan R. L. (1996) Ethoxylated surfactant biodegradation and the eects of oxidative pretreatment. J. Environ. Eng. 122(6), 477± 483. Baleux B. A. and Caumette P. (1977) Biodegradation of some cationic surfactants. Water Res. 11, 833± 841. Buxton G. V., Greenstock C. L., Helman W. P. and Ross A. B. (1988) Critical review of rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals (OH/O±) in aqueous solution. J. Phys. Chem. Ref. Data 17, 513±886. Calvosa L., Monteverdi A., Rindone B., et al. (1991) Ozone oxidation of compounds resistant to biological degradation. Water Res. 8, 985±993. Geto N. (1991) Radiation- and photoinduced degradation of pollutants in water. A comparative study. Radiat. Phys. Chem. 37, 673±680. Geto N. and Schwoerer F. (1973) Pulse radiolysis of ethyl, n-propyl, n-butyl and n-amyl amine in aqueous solutions. Int. J. Radiat. Phys. Chem. 5, 101±111. Gilbert E. (1987) Biodegradability of ozonation products as a function of COD and DOC elimination by example of substituted aromatic substances. Water Res. 21, 1273±1278. Greenberg A. E. Clesceri L. S. and Eaton A. S. (eds.) (1992) Standard Methods for the Examination of Water and Wastewater, 18th edn. American Public Health Association, Washington, DC. Hickel B. (1975) Absorption spectra and kinetics of methyl
and ethyl radicals in water. J. Phys. Chem. 79, 1054± 1059. Hu S. T. and Yu Y. H. (1994) Preozonation of chlorophenolic wastewater for subsequent biological treatment. Ozone Sci. Eng. 16, 13±28. Kitis M., Adams C. D., Daigger G. (1999) The eects of Fenton's Reagent pretreatment on the biodegradability of non-ionic surfactants. Water Res. 33 (11), 2561±2568. Neta P. and Schuler R. H. (1975) The mode of reaction of O-radical with aromatic systems. Radiat. Res. 64, 233± 236. Pontius F. W. (1990) Complying with the New Drinking Water Quality Regulations. J. Am. Water Works Assoc. 82, 32±52. Roder M., Wojnarovits L. and Foldiak G. (1990) Pulse radiolysis of aqueous solutions of aromatic hydrocarbons in the presence of oxygen. Radiat. Phys. Chem. 36, 175±176. Rudakov E. S., Volkova L. K. and Tret'yakov V. P. (1981) Low selectivity of reactions of OH-radicals with alkanes in aqueous solution. React. Kinet. Catal. Lett. 16, 333±337. Sehested K. and Holcman J. (1979) Radical cations of ethyl-, isopropyl- and tert-butylbenzene in aqueous solution. Nukleonika 24, 941±950. Solar S., Solar W. and Geto N. (1986) Resolved multisite OH-attack on aqueous aniline studied by pulse radiolysis. Radiat. Phys. Chem. 28, 229±234. Swisher R. D. (1987) Surfactant Biodegradation, 2nd edn. Marcel Dekker, New York.
Wat. Res. Vol. 34, No. 2, pp. 668±672, 2000 # 1999 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/99/$ - see front matter
www.elsevier.com/locate/watres
RESEARCH NOTE EFFECTS OF UV/H2O2 PREOXIDATION ON THE AEROBIC BIODEGRADABILITY OF QUATERNARY AMINE SURFACTANTS C. D. ADAMS*M and J. J. KUZHIKANNIL Environmental Engineering Program, Department of Civil Engineering, University of Missouri-Rolla, 202 Civil Engineering Building, Rolla, MO 65409, USA (First received 1 December 1998; accepted in revised form 1 April 1999) AbstractÐThe purpose of this study was to examine the eect of UV/H2O2 advanced oxidative pretreatment on the biodegradability of two major classes of quaternary amine surfactants. In this study, the biodegradability of unoxidized and oxidized solutions of quaternary amines were determined using BAS assays. UV/H2O2 preoxidation eectively enhanced the biodegradability of alkyldimethylbenzyl ammonium chlorides (Barquats), but had little eect on the biodegradability of dioctyl-dimethyl ammonium chloride (Bardac LF). # 1999 Elsevier Science Ltd. All rights reserved Key wordsÐhydrogen peroxide, UV, advanced oxidation technologies, biodegradation, quaternary amine surfactant, Barquat, Bardaq
INTRODUCTION
Industries worldwide discharge a wide range of surfactants, or surface active agents, to their wastewater treatment facilities. Biodegradation is generally considered to be the most economical means of treating biodegradable surfactants. Some classes of surfactants, however, are biorecalcitrant due to enzymatic de®ciencies or to their toxicological properties. Cationic surfactants containing a quaternary ammonium (e.g. R4N+; where R=alkyl chain and N=quaternary nitrogen), often have a strong biocidal nature (Baleux and Caumette, 1977; Swisher, 1987). Generally, the biodegradability of quaternary amines decreases with the number of non-methyl alkyl groups (i.e. R4N+ < R3MeN+ < R2Me2N+ < RMe3N+ < Me4N+; where Me=methyl radical) (Swisher, 1987). Additionally, replacement of a methyl group in a quaternary amine with a benzyl group can decrease biodegradability further (i.e. RBzMe2N+ < R2Me2N+) (Swisher, 1987). Two common classes of quaternary amines were examined in this study, speci®cally: (1) alkyldimethylbenzyl ammonium chlorides (Barquats) and (2) dioctyldimethyl ammonium chloride (Bardac LF) (Lonza Chemical). Advanced oxidation technologies (AOTs) (as well
as ozonation) have been shown to be highly eective at enhancing the rate and extent of biodegradability of many surfactants and other organic compounds (Gilbert, 1987; Calvosa et al., 1991; Adams et al., 1994, 1996, 1997; Hu and Yu, 1994; Kitis et al., 1998). The eciency of AOTs and ozonation is a strong function of the treatment objective desired, the inorganic and organic constituents in solution and the nature of the chemical oxidants. AOTs are processes which use the hydroxyl radical ( OH) as a primary oxidant. The hydroxyl radical is a strong, non-selective oxidant with rate constants often in the order of 109 l/mol s and a standard electrode potential (relative to the hydrogen electrode) of 2.8 V compared with 2.07, 1.78 and 1.36 V for ozone, hydrogen peroxide and chlorine, respectively (Pontius, 1990). In the UV/H2O2 AOT, hydroxyl radicals are generated and react with pollutants (P) and scavengers (S) by the overall reactions: H2 O2 UV 4 2 OH
1
OH T 4 Products
2
OH S 4 Byproducts
3
where T and S are target compounds and scavenger concentrations, respectively. The hydroxyl radical is
*Author to whom all correspondence should be addressed. 668
Research Note
capable of eciently oxidizing both target species and scavengers in a solution. Many other inorganic and organic radicals area also generated in the UV/ H2O2 AOT [e.g. superoxide ( Oÿ 2 ), perhydroxyl ( HO2), organic radicals ( R), etc.]. The relative importance of individual radical species on the overall reaction kinetics and pathways, however, was beyond the scope of this study. Depending upon the relative concentration and kinetic rate constants for the reaction of hydroxyl radical with the target species vs scavengers, the fraction ( f ) of the oxidant being used eciently (i.e. reacting with the target species) is described by reaction rate with target species total reaction rate koxidant,target OH T koxidant,target T or f all species all species X X k OH i k i OH,i
i
OH,i
i1
The purpose of this study was to examine the UV/H2O2 AOT for the pretreatment of biorecalcitrant (or partially biodegradable) quaternary amines. For each study compound, oxidative pretreatment was followed by aerobic batch activated sludge (BAS) bioassays. Ozone and ozone-based AOTs were also examined initially but, due to formation of excessive and rigid foam upon ozonation, only the only UV/H2O2 was used for chemical oxidation in this research. MATERIALS AND METHODS
Chemicals
f
i1
669
4
i
where T is the target species and i are individual organic and inorganic constituents in the system. Therefore, the relative reaction of OH with the target compound vs scavengers is of paramount importance with regard to process eciency. The goal for advanced oxidative treatment of biorecalcitrant compounds is often not to mineralize the compounds (i.e. convert to CO2 and H2O), but rather to convert biorecalcitrant compounds to byproducts which are readily biodegradable in conventional biological treatment processes. The purpose of coupling chemical and biological processes is to allow mineralization of a target pollutant using a minimum (or optimum) amount of the more costly chemical oxidation, followed by a relatively inexpensive (often existing) biological process such as activated sludge or ®xed-®lm biological treatment process.
The study compounds were all common quaternary amines. The quaternary amines examined were three alkyldimethylbenzyl ammonium chlorides (Barquat MX, Barquat MB, and Barquat OJ), and a dioctyl-dimethyl ammonium chloride (Bardac LF) (Lonza Chemical) (Fig. 1). The compounds were used as received. The Barquats are mixtures of dimethylated benzyl ammonium chlroides with varying normal-alkyl chain lengths from C12 to C18 (Fig. 1). All other chemicals used in the study were at least of reagent grade. Chemical oxidation procedure The quaternary amines were oxidized using the UV/ H2O2 advanced oxidation process. Initial solutions contained 1000 mg/l as COD of surfactant and 8 mM sodium phosphate. The initial H2O2 concentration and pH were adjusted to 1000 mg/l and 8, respectively. The UV/H2O2 oxidations were conducted in a 1.2-l batch photoreactor (Ace Glass and Kimble Scienti®c) using a low-pressure UV lamp with a principle wavelength of 254 nm (Pen Ray). The energy output of the UV lamp was determined using H2O2 actinometry to be 0.62 W corresponding to 0.56 W/l for the reactor used. Additional reactor ports were provided for a thermometer and for sample extraction. A magnetic stirrer provided continuous mixing during the reaction. All oxidations were conducted at a temperature of 22 (21)8C. The UV lamp was allowed to warm up and stabilize for 10 min prior to initiating an experiment.
Fig. 1. Structures of Barquats and Bardac examined in study.
670
Research Note
Oxidation times of 0, 30 and 120 min were used for most solutions. Residual H2O2 was measured using a chemical titration method (described below) and any residual H2O2 was immediately quenched using a slight stoichiometric excess of sodium sul®te. Samples were stored at 58C prior to COD analysis, BAS bioassay and DOC analysis. BAS bioassay procedure BAS bioassays were performed on 200 ml solutions at an initial concentration of 300 mg/l as COD of oxidized or 2+ unoxidized surfactant, added nutrients (e.g. NH+ , 4 , Ca 3+ 2+ 2+ 2+ 2+ 2+ 2+ and Mo ) and Fe , Co , Zn , Mg , Mn , Cu a mixed liquor suspended solids (MLSS) concentration of 1500 mg/l. Control measures included bioassaying: (1) each ¯ask in duplicate, (2) duplicate blanks (with no surfactant solution) and (3) duplicate positive controls containing an easily degradable substrate (ethylene glycol). Tests were also performed to assure that volatilization and sorption losses were minimal for the surfactant solutions. All BAS bioassays were run at 22 (21)8C. Samples were shaken at 180 rpm on a Labline shaker table to provide mixing and aeration. 10-ml samples were removed from the ¯asks at predetermined sample times. The samples were then centrifuged at 5000 rpm for 5 min to remove biomass prior to dissolved organic carbon (DOC) analysis. A separate 12-l sequencing batch reactor (SBR) was operated exclusively to generate biomass for the BAS bioassays. The SBR was fed nutrients as well as a mixture of synthetic organic chemicals including: ethylene glycol, polyethylene glycol, Alkumuls 0±14, catechol, resorcinol, hydroquinone, trihydroxybenzene and butyl ether. The solids retention time (SRT) and hydraulic retention time (HRT) for the SBR were 6 and 1 days, respectively. The solids wasted from the SBR on a daily basis were either discarded or used in the BAS bioassays. The DOC values for each sample removed from the shaker ¯asks were plotted vs BAS bioassay time from which the ultimate DOC removal (UDR, percent) was determined. The UDR provided an estimate of the biodegraded fraction of the oxidized or unoxidized solutions and an estimate of the non-biodegraded fraction (100 minus UDR, percent). All BAS bioassays were conducted in duplicate with the average coecient-of-variation for UDR between bioassays of 14%. Analytical procedures COD was measured with Hach COD ampoules (0± 150 mg/l range) with absorbance measured on a Spectronic 20D (Milton Roy). DOC was measured via Standard Method 5310B using a Model TOC-5050A Analyzer (Shimadzu) with ASI 5000A autoloader. pH was measured using a Model 320 pH meter (Corning) and probe. Hydrogen peroxide concentrations were measured using the potassium iodide (KI) method with starch indicator, and dropwise titration with sodium thiosulfate (Hach HYP-19 H2O2-Test Kit). MLSS was measured in accordance with Standard Method 2540D (Greenberg et al., 1992). RESULTS AND DISCUSSION
Foamability tests Initially, it was planned to compare direct ozonation, the H2O2/O3 AOT and the UV/H2O2 AOT for preoxidation prior to the BAS bioassays. Initial experiments, however, showed that the quaternary amines formed a rigid foam that would not disperse in the ozonation reactor or in the bubble trap
Fig. 2. Bioassay plots for Barquats MX (top), OJ (middle) and MB (bottom).
above the reactor, even after an extended period of time. Therefore, only the UV/H2O2 AOT was technically feasible for preoxidation of the quaternary amines in these experiments. Biodegradability enhancement results All of the unoxidized quaternary amines studied were relatively biorecalcitrant which is consistent with the biocidal nature of this class of surfactant (Figs 2 and 3). Advanced oxidation for 30 and 120 min (0.28 and 1.1 W h/l) resulted in a signi®cant increase in the biodegradation rate and extent for all three alkyldimethylbenzyl ammonium chlorides (Barquats) (Fig. 2). The highest UV dosage resulted in COD removals of between 20 and 30% for each Barquat. The resulting ultimate DOC removals for the oxidized Barquats in the BAS bioassays were approximately 90% in all cases (Fig. 2). Speci®cally, it appears that radical intermediates (e.g. OH, Oÿ 2 , etc.) resulting from the photolysis
Research Note
Fig. 3. Bioassay plot for Bardac LF.
of hydrogen peroxide were highly eective at modifying the chemical structure of the Barquats to yield readily biodegradable oxidation byproducts. It is apparent that advanced oxidation pretreatment is eective at enhancing the biodegradability of the alkyldimethylbenzyl ammonium chlorides. The dioctyl-dimethyl ammonium chloride (Bardaq LF) also exhibited signi®cant biorecalcitrance in its unoxidized form. However, even after 120 min (i.e. 1.1 W h/l) of UV/H2O2 oxidation, the biodegradability of the Bardaq LF was only slightly improved (Fig. 3). Therefore, advanced oxidation pretreatment appears ineective at enhancing the biodegradability of the dioctyldimethyl ammonium chloride, Bardaq LF. Hydroxyl radical rate constants are in the order of 109 to 1010 for compounds related to both the Barquats and Bardaq. For example, benzene and several alkylated benzenes and the corresponding non-benzylated alkanes, have hydroxyl radical rate constants ranging from 1(109) to 8(109) (Hickel, 1975; Sehested and Holcman, 1979; Rudakov et al., 1981; Buxton et al., 1988; Geto, 1991). Substitution of an amine group on aromatic or aliphatic compounds tends to raise hydroxyl radical rate constants only slightly if at all (Geto and Schwoerer, 1973; Hickel, 1975; Neta and Schuler, 1975; Sehested and Holcman, 1979; Rudakov et al., 1981; Solar et al., 1986; Buxton et al., 1988; Roder et al., 1990; Geto, 1991). Absolute reaction rate constants for Barquats and Bardaqs were not determined. However, due to the aforementioned similarity in hydroxyl radical rates constants for related compounds, and due to similar COD removal rates, it is hypothesized that the Barquats and Bardaq both compete eectively with scavengers in solution for hydroxyl radicals (Eqs. (2)±(4)). It is further hypothesized that the signi®cant dierence in the biodegradability enhancement after oxidation observed for the benzylated vs non-benzylated quaternary amines is, therefore, related directly to the structure and biodegradability of the oxidation byproducts themselves. It was, however, beyond the scope of this study to determine the identities of these oxi-
671
Fig. 4. Ultimate DOC removal (UDR) vs dosage for study compounds
dation byproducts. Furthermore, primary process parameters (e.g. temperature, pH, alkalinity, organic and inorganic background constituents) have a signi®cant eect on oxidation pathways and resulting byproducts. The concentration of the substrate in these experiments was relatively high (i.e. 1000 mg/l COD) and other process conditions and substrate concentrations may result in a dierent byproduct mixture. The ultimate DOC removals achieved for each study compound are summarized in Fig. 4. It is apparent that UV/H2O2 advanced oxidation pretreatment was highly eective at enhancing the aerobic biodegradability of the alkyldimethylbenzyl ammonium chlorides (Barquat MX, Barquat MB and Barquat OJ) yet ineective at enhancing the biodegradability of dioctyldimethyl ammonium chloride (Bardaq LF). These results point to the fact that because the eectiveness of advanced oxidation pretreatment on biodegradability enhancement is highly dependent on the nature of the species being treated and other process parameters, laboratory- or pilot-scale studies should be conducted prior to design of integrated chemical±biological treatment processes for wastewaters containing mixed surfactants. AcknowledgementsÐThis work was funded by the National Science Foundation (Grant number BES9696011). All experiments were conducted in the Environmental Research Center at the University of Missouri-Rolla (UMR). C. D. A. is Mathes Professor of Environmental Engineering and Associate Professor of Civil Engineering at the UMR. J. J. K. was a graduate research assistant earning an M.Sc. Environmental Engineering degree at the time of this study.
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