Effect of nonionic surfactants on the oxidation of carbaryl by anodic Fenton treatment

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Effect of nonionic surfactants on the oxidation of carbaryl by anodic Fenton treatment Lingjun Kong, Ann T. Lemley Graduate Field of Environmental Toxicology, TXA, MVR Hall, Cornell University, Ithaca, NY 14853-4401, USA

ar t ic l e i n f o

abs tra ct

Article history:

As a potentially promising technology, anodic Fenton treatment (AFT) has been shown to

Received 4 December 2006

be very successful in pesticide removal. However, the influence of other constituents in the

Received in revised form

pesticide formulation, such as nonionic surfactants, has not been addressed. In this study,

27 February 2007

the effect of Triton X (TX) on the degradation kinetics and pathways of carbaryl undergoing

Accepted 2 March 2007

AFT was investigated in an effort to facilitate its practical application. The presence of

Available online 24 April 2007

Triton X-100 was found to slow down the carbaryl degradation rate. This result can be

Keywords:

attributed to the consumption of hydroxyl radicals (dOH) by surfactants and the formation

Carbaryl

of a carbaryl?TX?Fe3+ complex, resulting in the unavailability of carbaryl to dOH attack.

Surfactant

The modified AFT kinetic model previously developed in this laboratory shows an excellent

Degradation Fenton reaction Kinetics

fit to the carbaryl degradation profile (R240.998), supporting the formation of a carbaryl?TX?Fe3+ complex. The carbaryl degradation rate decreased as Triton X-100 concentration increased from 20 to 1000 mg L1. Both dOH consumption by surfactants and complex formation are responsible for the degradation rate reduction below the critical micelle concentration (CMC), whereas the complex and micelle formation becomes a more dominant factor above the CMC. The effect of ethylene oxide (EO) numbers of a given nonionic surfactant mainly lies in the consumption of hydroxyl radicals, which increases with the length of the EO chain, but does not significantly affect the formation of the carbaryl?TX?Fe3+ complex. Based on the GC–MS and LC–ESI–MS results, no evidence was found that the carbaryl degradation pathway was affected. Carbaryl was typically oxidized to 1-naphthol and 1,4-naphthoquinone similar to what is observed in the absence of surfactants. Triton X-100 was degraded via the breakdown of EO chains and o-oxidation of the terminal methyl group, which resulted in the production of a series of ethoxylate oligomers. & 2007 Elsevier Ltd. All rights reserved.

1.

Introduction

Pesticides have been extensively used in modern agriculture. Due to heavy application, there have been serious problems generated by pesticide wastes which could eventually endanger water resources and human health (Muller et al., 2002; Felsot et al., 2003). As a consequence, the treatment of pesticide wastewater generated by the manufacturing Corresponding author. Tel.: +1 607 255 3151; fax: +1 607 255 1093.

E-mail address: [email protected] (A.T. Lemley). 0043-1354/$ - see front matter & 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2007.03.001

industry or agriculture-related activities has been the focus of researchers and regulators. Most of the published work to date has been focused on the removal of the active ingredients (AIs). However, actual pesticide wastewater contains not only the pesticide, but other adjuvants (i.e. surfactants, organic solvents, etc) in the pesticide formulation (Cross and Scher, 1987; Knepper et al., 2003). The presence of surfactants in pesticide wastewater has

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the potential to significantly impact the efficiency of pesticide removal. Surfactants are a class of amphipathic chemicals possessing hydrophobic moieties (mainly hydrocarbon chains of appropriate length) and hydrophilic groups (i.e. ethylene oxide chains, sulfate, carboxylate, etc). Due to their amphiphilic characteristics, surfactants are widely used in pesticide formulation to enhance the solubility, adsorption, penetration and translocation of AIs into the target (Foy, 1993; Cserha´ti and Forga´cs, 1997; Haas et al., 2001; Krogh et al., 2003). Among these surfactants, nonionic surfactants, such as alkylphenol ethoxylates (APEOs), alcohol ethoxylates (AEOs) and alkylamine ethoxylates (ANEOs), are most widely used due to their relatively low toxicity (Krogh et al., 2003). The significance of surfactants in environmental pollution control and treatment has long been recognized. Surfactantenhanced aquifer remediation (SEAR), based on micelle solubilization and interfacial tension (IFT) reduction, has become an increasingly popular technology in subsurface remediation in an effort to change the mobility of contaminants in the aquifer or their availability to bacteria (Sa´chezCamazano et al., 1995; Pennell et al., 1997; Saxe et al., 2000; Mata-Sandoval et al., 2001; Zheng and Obbard, 2002; Ahmad et al., 2004). Nevertheless, the impact of surfactants, which are ubiquitous in wastewater including pesticide wastewater, has rarely been addressed. The limited available reports are mainly focused on the effect of surfactants on the photocatalytic degradation of contaminants, which is one of the most popular advanced oxidation processes (AOPs). Bianco Prevot et al. (1999) observed an inhibitory effect of surfactants including N-hexadecyl-N,N,N-trimethylammonium bromide (HTBA), sodium dodecylsulfate (SDS) and polyoxyethylenedodecylether (Brij 35) on carbaryl (1-naphthyl-N-methylcarbamate) degradation by photooxidation. A similar observation was made by Barrios et al. (2005) that the photocatalytic degradation of naphthalene was reduced in the presence of Triton X-100. Several other researchers, however, found that the presence of nonionic surfactants could either enhance (i.e. Span 80, Tween 80, etc.) or suppress (i.e. Triton X, Tween 20, etc.) the photodegradation of Azadirachtin-A (Johnson and Dureja, 2002). The reason that most of the research addressing surfactant effect on organic-contaminated wastewater was focused on photodegradation is most likely due to the fact that surfactants not only impact the degradation process by micelle solubilization but also impact photosensitization (Johnson and Dureja, 2002; Cho et al., 2004). As a potentially promising AOP technology, anodic Fenton treatment (AFT), developed in our laboratory, removes pesticides in a batch reactor by using electro-generated ferrous ion (Fe2+) and continuously delivered hydrogen peroxide (H2O2) (Wang and Lemley, 2001). This treatment technology enables the Fenton reaction to occur in selfdeveloped optimal acidic conditions (pH 3.0). In addition, it overcomes the disadvantage of handling easily oxidized ferrous salts. AFT has been applied to the degradation of a wide variety of pesticides in the absence of surfactants and has shown great success with a removal efficiency great than 99% within 10 mins in most cases (Wang and Lemley, 2001, 2002, 2003a; Wang et al., 2004). The AFT degradation products have been found to become more biodegradable as evidenced

CH3 H3C

C

CH3 CH2

CH3

C

O

CH2CH2O

CH3

n

H

Fig. 1 – Structure of Triton X nonionic surfactants.

by an increase in the 5-day biochemical oxygen demand to chemical oxygen demand ratio (BOD5/COD) to 40.4, indicating a completely biodegradable solution (Wang and Lemley, 2003a; Friedman et al., 2006). In other work, a toxicity assay shows that the fatal toxicity of carbofuran to earthworms can be totally removed after the AFT process (Wang and Lemley, 2003b). To facilitate the practical application of AFT in pesticide wastewater treatment, the effect of surfactants, especially nonionic surfactants, is one of the key issues to be examined prior to a large-scale application. In this study, research was carried out to investigate the influence of nonionic surfactants on the pesticide degradation kinetics and pathways by AFT. Carbaryl (a carbamate insecticide) and Triton X (TX) series surfactants (octyl phenol ethoxylates, Fig. 1) were selected as representative pesticide and nonionic surfactants, respectively. The specific objectives of this research were to (1) investigate the degradation kinetics of carbaryl in the presence of Triton X-100; (2) examine the effect of concentration of Triton X-100 on the degradation kinetics of carbaryl; and (3) determine the effect of the length of the ethoxylate (EO) chain of TX including Triton X-100, X-45, and X-405 on carbaryl degradation; and (4) investigate the degradation pathways of carbaryl and Triton X-100 by AFT.

2.

Materials and methods

2.1.

Chemicals

Carbaryl was purchased from Chem Service (West Chester, PA). Triton X-100 was purchased from SPI Supplies (West Chester, PA). Triton X-45, X-405 (70%) and H2O2 (30%) were purchased from Sigma-Aldrich Chemicals (Milwaukee, WI). Acetonitrile (ACN) high-performance liquid chromotography (HPLC grade), methanol, phosphoric acid (85%), sodium chloride, and water (HPLC grade) were purchased from Fisher Scientific (Fair Lawn, NY). All reagents were used without further purification except where specifically indicated. Deionized water (DI water) was obtained from a Barnstead Nanopure system with an electric resistance of the effluent water 418.1 MO cm1. All solutions were prepared from DI water.

2.2.

Degradation of pesticides by AFT

The AFT apparatus consisted of two 250 mL customized glass half cells (anodic and cathodic half cell) separated by an anion exchange membrane (Electrosynthesis, Lancaster, NY) with an electric resistance of 8 O cm2 in 1 M NaCl. A pure iron plate (2 cm  10 cm  0.2 cm) and a graphite rod (1 cm

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(i.d.)  10 cm (length)) were used as anode and cathode, respectively. The electric current was supplied by a BK Precision DC power supply 1610. Approximately 200 mL of carbaryl solution (with or without surfactants) and 0.08 M NaCl solution were added to the anodic and cathodic half cells, respectively. The solution in each half cell was mixed by a magnetic stirring bar during the AFT process. H2O2 was delivered into the anodic half cell using a Stepdos peristaltic pump (Chemglass Inc., Vineland, NJ) at a flow rate of 0.50 mL min1. Unless specified otherwise, the concentration of carbaryl was approximately 100 mM, the electric current was kept at 0.050 A, and the corresponding H2O2 concentration was 0.0311 M, which resulted in an H2O2 to Fe2+ delivery ratio (H2O2:Fe2+) of 5:1, and the NaCl concentration ratio in the anodic and cathodic cell was 1:4. The DC power supply was turned on and the electric current was adjusted to 0.050 A once the first drop of H2O2 entered the pesticide solution in the anodic half-cell. Over a given time period, 1.0 mL of pesticide solution was taken out by a 1000 mL Eppendorf micropipetter at certain time intervals and immediately transferred to a 1.5 mL HPLC-vial containing 0.1 mL methanol, which served as a quencher of hydroxyl radicals. The collected pesticide degradation solution was hand shaken and ready for pesticide concentration or degradation product analysis. All experiments were performed at room temperature (22.071.0 1C). Each treatment was conducted in triplicate.

2.3.

Concentration analysis of pesticides and H2O2

The pesticide concentration was analyzed by a reverse-phase HPLC system equipped with a Restek ultra C18 (5 mm) column (4.6  150 mm) and a diode-array (DAD) UV/vis detector (Series 1100, Agilent Technology). The DAD wavelength was chosen at 280720 mm. The mobile phase consisted of ACN and water with an ACN:water ratio of 60:40. The pH of the water phase was adjusted to 3.0 using phosphoric acid. The retention time of carbaryl under the described analytical conditions was 6.0 min. Concentration of H2O2 was determined by potassium permanganate titration (Huckaba and Keyes, 1948).

2.4. GC–MS and LC–MS analysis of carbaryl and Triton X-100 degradation products The degradation products used for GC–MS and LC–MS analysis were obtained after 5 and 10 min of AFT treatment. GC–MS analyses were carried out on an Agilent 6890N network GC system equipped with Agilent 5973 Network mass selective detector and Agilent 7683 series injector. A 30 m  0.25 mm (i.d.) fused silica capillary column with 0.25 mm film thickness (HP19091S-433) and a carrier gas of helium (10.50 psi, 1.0 mL min1) were used. A column gradient temperature increase was employed, which started at 55 1C, stayed for 1 min, and then gradually increased to 275 1C at a rate of 10 1C min1, and finally increased to 300 1C at a rate of 5 1C min1. The temperature of the injector port was 220 1C. The HPLC system for the LC–MS analysis was an Agilent 1100 microbore system. The column used was a Vydac C18 (5 mm) column (1.0 mm  150 mm), coupled to a Brownlee 1.0 mm  10 mm C18 guard column. The mobile phase con-

sisted of A (0.1% formic acid in water) and B (0.1% formic acid in ACN). A step gradient method was used in the analysis, i.e. 20% B for 10 min, stepped to 55% B for 30 min at a flow rate of 0.1 mL min1. The wavelength of the UV detector was chosen at 260 nm. A post column splitter was employed with 50% of the effluent diverted to waste. The mass spectrum was obtained by using a Bruker Esquire LC (Bruker Daltonics, MA) fitted with standard electrospray source (ESI). Data acquisition was done in the positive ion mode.

3.

Results and discussion

3.1. Degradation kinetics of carbaryl in the presence of Triton X-100 by AFT The AFT kinetic model (Eq. (1)) has been successfully applied to simulate the degradation process of a wide variety of pesticides by AFT. The detailed derivation of this model has been published elsewhere (Wang and Lemley, 2001). The governing relationship is dC ¼ Klpon20 tC, dt

(1)

where C is the pesticide concentration (mM), t is the treatment time (min), K ¼ kk1, where k1 and k are reaction rate constants of the Fenton reaction and the pesticide degradation reaction (mM1 min1), respectively, p and l are constants related to average life of ferrous ion (Fe2+) and hydroxyl radicals (dOH), respectively, and o is a parameter related to the H2O2:Fe2+ delivery ratio and to the consumption of H2O2. n0 is referred to as the delivery rate of Fe2+ (mM min1). Upon integration of Eq. (1), the form of the AFT model can be obtained as follows: ln

Ct 1 ¼  Klpon20 t2 , 2 C0

(2)

where Ct and C0 represent the concentration of pesticide at time t and 0, respectively. The degradation of carbaryl by AFT in the absence of surfactants has been found to follow the AFT kinetic model (R2 ¼ 0.999) (Wang and Lemley, 2002). However, in the presence of Triton X-100, the AFT kinetic model cannot provide a good simulation of the observed carbaryl degradation profile. In contrast, a previously developed modified AFT model fits the experimental data very well (R2 ¼ 1.000) (Fig. 2). A brief description of the modified AFT model can better explain this phenomenon (Wang et al., 2004). Compared to the original AFT model, the modified model used the uncombined pesticide concentration, Cfree, which was defined as the pesticide concentration available for degradation by hydroxyl radicals, instead of the total pesticide concentration, C, as shown in Eq. (3). dC ¼ Klpon20 tCfree . dt

(3)

It was suggested that the formation of a weak complex between the pesticide and ferric ion (C    Fe3+) could cause unavailability of some pesticide to degradation by hydroxyl radicals. Hence, C ¼ Cfree þ ½C    Fe3þ ,

(4)

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Normalized Concentration, C/C0

1.2 without Triton X-100 with Triton X-100

1.0 0.8 0.6 0.4 0.2 0.0

0

5

10

15

20

Treatment Time, t (min) Fig. 2 – Carbaryl (100 lM) degradation kinetics in the absence and presence of Triton X-100 (100 mg L1) (symbols represent experimental data, dashed and solid lines represent AFT and modified AFT fit, respectively).

where [C    Fe3+] is referred to as the concentration of the weak complex (mM). Substitution of Eqs. (4) into (3) yields, dC ¼ Klpon20 tðC  ½C    Fe3þ Þ. dt

(5)

It is assumed that an instantaneous equilibrium is achieved between the complex and pesticide with an equilibrium constant, KCFe3þ (mM1), ½C    Fe3þ  ¼ KCFe3þ , Cfree ½Fe3þ 

dC C ¼ Klpon20 t . dt 1 þ KCFe3þ ½Fe3þ 

(7)

Since the total iron is generated by the electrolysis of iron in the AFT, it can be assumed that the Fe3+ concentration is proportional to the Fe2+ delivery rate and treatment time, which has been validated in previous measurements (Wang and Lemley, 2001), and ½Fe3þ  ¼ Zn0 t,

(8) 3+

where Z is the ratio of Fe concentration to total iron ion concentration during the AFT. Thus, Eq. (7) can be written as dC C ¼ Klpon20 t . dt 1 þ KCFe3þ Zn0 t

(9)

That is   Ct a a t  , ¼  2 ln a þ bt b C0 b

the ferric ion. However, the presence of TX causes the carbaryl degradation to follow the modified AFT model, implying that it is the presence of TX as well as the ferric ion that is affecting its availability to hydroxyl attack. It is well known that ferric ion has empty electron orbitals and that oxygen has extra electron pairs. In this scenario, ferric ions could interact with the oxygen (O) in the EO groups on TX and form a weak complex. Other researchers have observed the formation of a complex between Fe3+ and an alcohol exthoxylate nonionic surfactant (Brij 35) via Fe3+ and O bonding (Cho et al., 2004). We ran an AFT control experiment with Triton X-100 (100 mg L1) in the absence of carbaryl. The degradation profile of Triton X-100 follows the modified AFT kinetic model rather than the original model (data not shown). This result supports the formation of a complex between Triton X-100 and ferric ion. At the same time, carbaryl can also interact with TX molecules or micelles via van der Waals attraction, and could form a weak complex (carbaryl?TX?Fe3+), which would make the carbaryl unavailable to hydroxyl radical attack. This would explain the fact that the modified AFT kinetic model provides an excellent fit to the carbaryl degradation profile in the presence of nonionic surfactants. In order to make a quantitative estimate of this interaction, one needs to consider the fitting parameter, ZKCFe3þ . The Z could be estimated via the solubility product (ksp) of Fe(OH)3 (ksp ¼ 1.10  1012 mM4) by knowing the pH of the system (3.8), which is approximately 0.05 (Wang et al., 2004). The equilibrium constant of the complex (KCFe3þ ) can then be calculated from the fitting parameter, ZKCFe3þ , giving a KCFe3þ value of 0.44, which indicates a weak bonded complex.

(6)

where [Fe3+] is the concentration of ferric ion (mM). Therefore, Eq. (5) can be written as

ln

2797

(10)

where a ¼ 1=ðKlpon20 Þ and b ¼ ZKCFe3þ =ðKlpon0 Þ. This modified AFT model was applied to triazine and triazinone pesticide degradation data, where the assumption was made and supported that a complex was formed between the ferric ion and the nitrogen on the ring of the pesticide. Since carbaryl degradation follows the original AFT model in aqueous solution, it does not form a complex with

3.2. Effect of Triton X-100 concentration on carbaryl degradation To examine the effect of the nonionic surfactant concentration on carbaryl degradation, Triton X-100 was added to the carbaryl solution at various concentration levels (20, 50, 100, 150, 200, 500, and 1000 mg L1). Based on previous control experiments, the sorption of carbaryl on the wall of the reactor and reduction of carbaryl at the anode can be neglected. At the same time, no carbaryl degradation was observed in the absence of either or both of the Fenton reagents (Kong and Lemley, 2006). As shown in Fig. 3a, the higher the Triton X-100 concentration, the longer the time required to achieve complete removal of carbaryl and the slower the carbaryl degradation rate. As a key factor in the Fenton reaction, the pH profile during the degradation of carbaryl in the absence and presence of Triton X-100 was monitored. No significant difference was found (P ¼ 1.00 at a confidence level of 95% by ANOVA), indicating that the presence of surfactant did not affect the degradation rate due to pH changes and the effect of pH can be neglected. Therefore, the decrease of carbaryl degradation rate in the presence of Triton X-100 can have two explanations: (1) as active amphiphilic compounds, surfactants can compete with pesticides for hydroxyl radicals during the AFT process, reducing the opportunities for carbaryl to be degraded and (2) the formation of an Fe3+?TX?carbaryl complex can reduce the availability of carbaryl for hydroxyl radical attack.

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The Triton X-100 degradation in the evaluated system will be discussed in detail in a later section. The carbaryl degradation profiles in the presence of different concentrations of Triton X-100 follow the modified AFT kinetic model with a R2 value X0.998. The lumped parameter Klpou20 can be obtained from fitting parameter ‘‘a’’, which is defined as a ¼ 1=ðKlpon20 Þ. In this lumped parameter, K, o and n0 are invariant under the evaluated experimental conditions, so the variables in Klpon20 are l and p, which relate

Normalized Concentration, C/C0

1.2

0 mg L-1 20 mg L-1 50 mg L-1 100 mg L-1 150 mg L-1 200 mg L-1 500 mg L-1 1000 mg L-1

1.0 0.8 0.6 0.4 0.2 0.0

0

10

20

30

40

Treatment Time, t (min)

Kλπων02 (min-2) or ηKC-Fe3+ (μM-1)

0.25 Kλπων02 (min-2)

ηKC...Fe3+ (μM-1)

0.20 0.15 0.10 0.05 0.00

0

200

400

600

800

Concentration of Triton X-100 (mg

1000

L-1)

Fig. 3 – Effect of Triton X-100 concentration (20–1000 mg L1) on carbaryl (100 lM) degradation kinetics: (a) modified AFT kinetic model fit to experimental data (symbols represent experimental data, solid lines represent modified AFT fit); (b) fitting parameters Kkpxm20 and gKCFe3þ as a function of Triton X-100 concentration.

to the lifetime of hydroxyl radicals and ferrous ion, respectively. From the Klpon20 values, it can be observed that Klpon20 decreases with increasing Triton X-100 concentration when that concentration is below 150 mg L1, whereas it shows little change with concentration when above 150 mg L1 (Fig. 2b). The concentration of Triton X-100 corresponding to the minimum Klpon20 value (150 mg L1) is consistent with its critical micelle concentration (CMC) in water (Table 1), indicating that the average lifetime of hydroxyl radicals and/or ferrous ion in the system drops with increasing surfactant concentration below the CMC, whereas the change is relatively insignificant above the CMC. It is well known that the number of surfactant monomers increases with increasing surfactant concentration below the CMC but remains constant above the CMC due to the formation of micelles. Therefore, the trend of the Klpon20 value as a function of surfactant concentration is obviously similar to that of the surfactant monomer numbers, suggesting that the surfactant monomers are most likely responsible for the competition of Triton X-100 with carbaryl for hydroxyl radicals. In the fitting parameter ZKCFe3þ , the equilibrium constant KCFe3þ is a function only of temperature; thus, the only variable in the parameter is Z, defined as the ratio of ferric ion to total iron in the system. As shown in Fig. 3b, ZKCFe3þ increases with increasing Triton X-100 concentration over the entire evaluated range, implying a higher ferric ion concentration in the system and providing a greater potential for ferric ion to combine with surfactant/pesticide, thus reducing the availability of carbaryl to attack by hydroxyl radicals. In addition, the formation of surfactant micelles could ‘‘wrap’’ the carbaryl molecule and result in a relatively more stable complex. Therefore, the degradation rate of carbaryl decreases with increasing Triton X-100 concentration above the CMC regardless of the fact that the consumption of dOH by Triton X-100 rarely changes (i.e. Klpon20 rarely varies with increasing concentration above the CMC). Based on the above discussion, it could be suggested that the fitting parameter Klpon20 is most likely related to the competitive consumption of dOH between carbaryl and surfactants, and ZKCFe3þ reflects the formation of a carbaryl?TX?Fe3+ complex. Overall, the competition between carbaryl and Triton X-100 for hydroxyl radicals and the formation of the weak complex Fe3+?TX?carbaryl determine the carbaryl degradation rate at low surfactant concentration (oCMC), whereas, the complex and micelle

Table 1 – Relevant properties of Triton X series nonionic surfactants used in this study Name Triton X-45 Triton X-100 Triton X-405 a

Formula

Molecular weight (MW)a

Average EO number (n)a

CMC in H2O (mg L1)a

C14H22O(C2H4O)n

404 625 1966

4, 5 9, 10 40

44 150 1228

Data were provided by manufacturers.

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formation becomes a more dominant factor influencing the carbaryl degradation rate above the CMC.

3.3. Effect of the length of Triton X EO chain on carbaryl degradation It has been widely accepted that the oxygen in the EO groups (–CH2CH2O–) is most likely to be attacked by hydroxyl radicals during the oxidation process (Calvosa et al., 1991; Brambilla et al., 1997; Brand et al., 2000; Franska et al., 2003). As discussed in the previous sections, the EO groups are also responsible for the formation of the Fe3+?TX?carbaryl complex. Therefore, the length of the EO chain (EO number) can be a significant factor in the carbaryl degradation by AFT in the presence of nonionic surfactants. Three Triton X nonionic surfactants with the same hydrophobic moiety but different numbers of EO groups (Triton X-45, X-100 and X-405) were used to study this hypothesis. The relevant properties of these three surfactants are listed in Table 1. For purpose of comparison, 100 mM carbaryl was mixed with approximately

Normalized Concentration, C/C0

1.2 no Triton X Triton X-45 Triton X-100 Triton X-405

1.0

100 mg L1 of given surfactant in each experiment. As shown in Fig. 4a, in the presence of similar surfactant concentration, the carbaryl degradation rate decreased in the order: Triton X-454X-1004X-405, i.e. the longer the EO chain, the slower the degradation rate of carbaryl. This result could be caused by the fact that the greater the number of EO groups, the more competition for hydroxyl radicals and the greater the opportunity to combine with ferric ion, thus decreasing the carbaryl degradation rate. To better understand the effect of chain length on carbaryl degradation rate, the carbaryl degradation profiles in the presence of different TX surfactants were simulated by the modified AFT model, which fit quite well (R240.999). The value of Klpon20 , which reflects the degradation rate in the context of competition between coexisting constituents, decreases with increasing length of EO chain, implying that TX series surfactants consume more hydroxyl radicals as the number of EO groups increase and this results in a slower carbaryl degradation rate. On the other hand, the obtained ZK3+ C?Fe values show much less change with different TX surfactants, indicating that the formation of the Fe3+?TX? carbaryl complex is most likely very similar for all the surfactants under evaluation and is not a major factor impacting the carbaryl degradation rate (Fig. 4b). In other words, the competition between carbaryl and TX for hydroxyl radicals determines the degradation rate when the nonionic surfactant has a similar hydrophobic group, but different length of EO chain.

0.8

3.4. Carbaryl and Triton X-100 degradation mechanisms by AFT

0.6 0.4 0.2 0.0

0

5

10

15

20

Treatment Time, t (min)

0.20 -1

Kλπων02 (min ) or ηKC...Fe3+ (μM )

2799

2

-2

Kλπων0 (min ) -1

ηKC...Fe3+ (μM )

0.15

-2

0.10

0.05

0.00 TX-45

TX-100

TX-405

Fig. 4 – Effect of the length of ethoxylate chain of Triton X series on carbaryl (100 lM) degradation kinetics: (a) modified AFT kinetic model fit to experimental data (symbols represent experimental data, solid lines represent modified AFT fit); (b) fitting parameters Kkpxm20 and gKCFe3þ of various Triton X surfactants.

AFT degradation products of carbaryl in the absence of surfactants have been analyzed by GC–MS, and corresponding pathways have been proposed in previously published work (Wang and Lemley, 2002). In this study, degradation products of carbaryl were analyzed by GC–MS in the presence of Triton X-100. Peaks at m=z ¼ 144 and 158, corresponding to products 1-naphthol and 1,4-naphthoquinone, were detected (data not shown). This result is consistent with our previous study and with other research (White and Wood, 1986; Wang and Lemley, 2002). However, no further information could be obtained regarding the possible formation of complexes between carbaryl and Triton X-100 or the formation of Triton X-100 degradation products. It has been well documented that nonionic surfactants have been and can be more easily analyzed by LC–MS due to their high nonvolatility (Ba´n et al., 1992; Kibbey et al., 1996; Cserha´ti and Forga´cs, 1997; Ding and Tzing, 1998; Petrovic and Barcelo´, 2000, 2001; Jonkers et al., 2005). To better understand the degradation pathways of carbaryl and Triton X-100, AFT degradation products of carbaryl in the presence of Triton X-100 were determined by using LC–ESI–MS. Multiple peaks were detected by measuring the total ion current (TIC) of the degradation mixture during the time course of 0–40 min (Fig. 5a). The ESI–MS spectra of two representative peaks at retention times of 3.9 and 28.8 min are shown in Figs. 5b and c. As illustrated in Fig. 5b, one series of mass to charge ratios (m/z) of 217+44n (where n represents the EO number) was observed. The ratios differ from each other by 44 Da, which indicates the

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28.8 min

a

3

3.9 min

2 1 0

0

Intens. x105 1.5

5

15

20 Time

25

30

35

+MS, 3.9 min

b 305.2 349.2 261.1

1.0

393.2 437.3 481.2

217.0

0.5 0.0

10

525.2

100

200

300

400

500

600

700

800

900

m/z Intens. x105

+MS, 28.8 min

c

449.3 493.3 405.3

3 2 1 0

227.1 271.2 130.2 158.1

100

200

361.3 597.4

317.3

300

400

500

600

644.5 685.3

700

800

900

m/z

Fig. 5 – LC-ESI-MS analysis of degradation products after 10 min treatment of carbaryl (100 lM) in the presence of Triton X100 (100 mg/L) by AFT. (a) Total ion current (TIC) of analyzed mixture; (b) MS spectrum of products eluted in 3.9 min; and (c) MS spectrum of products eluted in 28.8 min.

difference of a single oxyethylene unit (EO). The signal at m=z ¼ 217 can be assigned to the sodium (Na) adducts of polyethylene glycols (PEG), indicating that PEGs with 4-11 EO subunits were produced during the degradation process. Similarly, two pronounced series of Na adducts were observed as shown in Fig. 5c: 317+44n and 377+44n, indicating the formation of other EOs. The series of m=z ¼ 317 þ 44n and 377+44n can be identified as the Na adducts of residual octylphenol ethoxylates (APEOs) and dicarboxylated polyethylene glycols (DCPEG), respectively. The first series (m=z ¼ 317, 361, 405, 449, 493 and 537) represents a series complex of APEOs with EO numbers of 2–7, which are lower than those of original Triton X-100 with an average EO number of 9.5, indicating the breakdown of the EO chain during the AFT process. The intensity of these peaks is also much lower than that of the originals. The second series (m=z ¼ 377, 421, 465, 509, 553, 597) can be identified as DCPEG containing EO subunits ranging from 5 to 10. In Fig. 5c, two other series including m=z ¼ 451 þ 44n (451, 495, 539, 583) and 383+44n (383, 427, 471, 515) with relatively lower intensity can also be observed. The series of m=z ¼ 451, 495, 539, and 583 can be identified as the Na complex of monocarboxylate

polyethylene glycols (MCPEGs), corresponding to the EO number of 8–11, whereas the other series of m=z ¼ 383, 427, 471, and 515 is probably the H adducts of APEOs with EO units of 4–7. Mass spectra of other eluants exhibit very similar patterns of m/z series (data not shown). No ion fragment and mass spectrum information was found to suggest that the existence of Triton X-100 changed the carbaryl degradation pathway. Based on the experimental results and literature reports, degradation pathways for carbaryl and Triton X-100 were proposed as shown in Figs. 6a and b. Carbaryl is oxidized by breaking off the carbamate branch to form 1-naphthol, and can be further oxidized to 1,4-naphthoquinone. The oxidation of Triton X-100 mainly involves the breakdown of the EO chain and oxidation of the methyl group at the end of the surfactant molecules (the o carbon) to a hydroxyl group, and finally to a carboxyl group, which is know as ‘‘o-oxidation’’. Based on the degradation pathway, Triton X-100 could be fully deethoxylated to octyl phenol, which is potentially more toxic and persistent than the original surfactant, a typical drawback of the chemical oxidation process. To solve this problem, further treatment is required to detoxify the degradation products.

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4 1 (200 7) 279 4 – 280 2

O O

C

O N CH3 H

OH

O carbaryl (m/z=201)

1-naphthol (m/z=144)

O CH2CH2O

C8H17

C8H17

O

CH2CH2O H n-m

C8H17

1,4--naphthoquinone (m/z=158)

n

H Triton X-100

O CH2CH2O

(APEOs)

COOH n-1

(CAPEOs)

+ H OCH2CH2

OH m

OCH2CH2OH

C8H17

(PEG)

+ OCH2COOH n-2 (MCPEG)

H OCH2CH2

H2CCOOH

OCH2CH2

m'

OCH2COOH + C8H17

(DCPEG)

O

CH2CH2O H n'

(APEOs)

Fig. 6 – Proposed pathways of carbaryl and Triton X-100 degradation (a) carbaryl (Wang and Lemley, 2002) and (b) Triton X-100 (n, m, n0 , and m0 represent EO numbers) (Franska et al., 2003).

4.

Conclusions

(1) The presence of nonionic surfactants (Triton X) in carbaryl wastewater retarded the pesticide degradation during the AFT process, which can be attributed to the consumption of hydroxyl radicals by TX and the formation of the carbaryl?TX?Fe3+ complex. The modified AFT kinetic model was found to fit experimental data very well due to the formation of the weak complex between pesticide, nonionic surfactant and ferric ion. (2) The degradation rate of carbaryl decreases with increasing concentration of Triton X-100. Below the CMC, both the consumption of hydroxyl radicals by TX and the formation of a carbaryl?TX?Fe3+ complex influence the carbaryl degradation process. Above the CMC, the formation of the weak complex (carbaryl?TX?Fe3+) and micelle becomes a more dominant factor to reduce the availability of carbaryl to hydroxyl radical attack, and consequently slow the carbaryl degradation. (3) The length of the EO chain in TX is another important factor determining the carbaryl degradation rate. The longer the chain, the slower the degradation rate. The

carbaryl degradation rate in the presence of various TX surfactants were found to be in the order of Triton X454X-1004X-405, which is most likely caused by the consumption of dOH by surfactants. (4) No evidence was found that the carbaryl degradation pathway was affected by the presence of Triton X-100. Carbaryl is oxidized by breaking off the carbamate branch to form 1-naphthol, and/or further oxidized to 1,4naphthoquinone. The oxidation of Triton X-100 mainly involves the breakdown of EO chains and o-oxidation of the terminal methyl group.

Acknowledgments The authors thank Mr. Robert Sherwood and Dr. Sheng Zhang in the Biotechnology Center at Cornell University by helping with the LC–ESI–MS measurements. This research was supported in part by the Cornell University Agricultural Experiment Station federal formula funds, Project no. NYC329806 (W-1045), received from Cooperative State Research, Education, and Extension Service, US Department of Agriculture. Any opinions, findings, conclusions, or recommenda-

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tions expressed in this publication are those of the author(s) and do not necessarily reflect the view of the US Department of Agriculture. R E F E R E N C E S

Ahmad, R., Kookana, R.S., Alston, A.M., 2004. Surfactant-enhanced release of carbaryl and ethion from two long-term contaminated soils. J. Environ. Sci. Health, B B39, 565–576. Ba´n, T., Papp, E., Incrze´dy, J., 1992. Reversed-phase high-performance liquid chromatography of anionic and ethoxylated non-ionic surfactants and pesticides in liquid pesticide formulations. J. Chromatogr. 593, 227–232. ˜ ez, O., 2005. Conditions for Barrios, N., Sivov, P., D’Andrea, D., Nu´N selective photocataclytic degradation of naphthalene in triton X-100 water solutions. Int. J. Chem. Kinet. 37, 414–419. Bianco Prevot, A., Pramauro, E., de la Guardia, M., 1999. Photocatalytic degradation of carbaryl in aqueous TiO2 suspensions containing surfactants. Chemosphere 39, 493–502. Brambilla, A., Bolzacchini, E., Orlandi, M., Polesello, S., Rindone, B., 1997. Reactivity of two models of non-ionic surfactants with ozone. Water Res. 31, 1839–1846. Brand, N., Mailhot, G., Sarakha, M., Bolte, M., 2000. Primary mechanism in the degradation of 4-octylphenol photoinduced by Fe(III) in water-acetonitrile solution. J. Photochem. Photobiol., A 135, 221–228. Calvosa, L., Monteverdi, A., Rindone, B., Riva, G., 1991. Ozone oxidation of compounds resistant to biological degradation. Water Res. 25, 985–993. Cho, Y., Park, H., Choi, W., 2004. Novel complexation between ferric ions and nonionic surfactants (Brij) and its visible light activity for CCl4 degradation in aqueous micellar solutions. J. Photochem. Photobiol., A 165, 43–50. Cross, B., Scher, H.B., 1987. Pesticide Formulations: Innovations and Developments. American Chemical Society, Washington, DC. Cserha´ti, T., Forga´cs, E., 1997. Separation and quantitative determination of non-ionic surfactants used as pesticide additives. J. Chromatogr., A 774, 265–279. Ding, W.-H., Tzing, S.-H., 1998. Analysis of nonylphenol polyethoxylates and their degradation products in river water and sewage effluent by gas chromatography-ion trap (tandem) mass sepctrometry with electron impact and chemical ionization. J. Chromatogr., A 824, 79–90. Felsot, A.S., Racke, K.D., Hamilton, D.J., 2003. Disposal and degradation of pesticide waste. Rev. Environ. Contam. Toxicol. 177, 123–200. Foy, C.L., 1993. Progress and developments in adjuvant use since 1989 in the USA. Pestic. Sci. 38, 65–76. Franska, M., Franski, R., Szymanski, A., Lukaszewski, Z., 2003. A central fission pathway in alkylphenol ethoxylate biodegradation. Water Res. 37, 1005–1014. Friedman, C.L., Lemely, A.T., Hay, A., 2006. Degradation of chloroacetanilide herbicides by anodic Fenton treatment. J. Agric. Food Chem. 54, 2640–2651. Haas, S., Ha¨sslin, H.-W., Schlatter, C., 2001. Influence of polymeric surfactants on pesticidal suspension concentrates: dispersing ability, milling efficiency and stabilization power. Colloids Surf., A 183-185, 785–793. Huckaba, C.E., Keyes, F.G., 1948. The accuracy of estimation of hydrogen peroxide by potassium permanganate titration. J. Am. Chem. Soc. 70, 1640. Johnson, S., Dureja, P., 2002. Effect of surfactants on persistence of azadirachtin-A (neem based pesticide). J. Environ. Sci. Health, B 37, 75–80.

Jonkers, N., Govers, H., Voogt, P.D., 2005. Adduct formation in LC–ESI–MS of nonylphenol ethoxylates: mass spectrometrical, theorectical and quantitiative analytical aspects. Anal. Chim. Acta 531, 217–218. Kibbey, T.C.G., Yavaraski, T.P., Hayes, K.F., 1996. High-performance liquid chromatographic analysis of polydisperse ethoxylated non-ionic surfactants in aqueous samples. J. Chromatogr., A 752, 155–165. Knepper, T.P., Petrovic, M., de Voogt, P., 2003. Occurrence of surfactants in surface waters and freshwater sedimients I. Alkylphenol ethoxylates and their degradation products. Comprehensive. Anal. Chem. 40, 675–693. Kong, L., Lemley, A.T., 2006. Modeling evaluation of carbaryl degradation in a continuously stirred tank reactor by anodic Fenton treatment. J. Agric. Food Chem. 54, 10061–10069. Krogh, K.A., Halling-Sørensen, B., Mogensen, B.B., Vejrup, K.V., 2003. Environmental Properties and effects of nonionic surfactant adjuvants in pesticides: a review. Chemosphere 50, 871–901. Mata-Sandoval, J.C., Karns, J., Torrents, A., 2001. Influence of Rhamnolipids and Triton X-100 on the biodegradation of three pesticides in aqueous phase and soil slurries. J. Agric. Food Chem. 49, 3296–3303. Muller, K., Bach, M., Hartmann, H., Spiteller, M., Frede, H.-G., 2002. Point-and nonpoint-source pesticide contamination in the Zwester Ohm Catchment, Germany. J. Environ. Qual. 31, 309–318. Pennell, K.D., Adinolfi, A.M., Abriola, L.M., Diallo, M.S., 1997. Solubilization of dodecane, tetracholorethylene, and 1,2dichlorobenzene in micellar solution of ethoxylated nonionic surfactants. Environ. Sci. Technol. 31, 1382–1389. Petrovic, M., Barcelo´, D., 2000. Determination of anionic and nonionic surfactants, their degradation products, and endocrine-disrupting compounds in sewage sludge by liquid chromatography/mass spectrometry. Anal. Chem. 72, 4560–4567. Petrovic, M., Barcelo´, D., 2001. Analysis of ethoxylated nonionic surfactants and their metabolites by liquid chromatorgraphy/ atmospheric pressure ionization mass spectrometry. J. Mass Spectrom. 36, 1173–1185. Sa´chez-Camazano, M., Arienzo, M., Sa´nchez-MartI´n, M.J., Crisanto, T., 1995. Effect of different surfactants on the mobility of selected non-ionic pesticides in soil. Chemosphere 31, 3793–3801. Saxe, J.K., Allen, H.E., Nicol, G.R., 2000. Fenton oxidation of polycyclic aromatic hydrocarbons after surfactant-enhanced soil washing. Environ. Eng. Sci. 17, 233–244. Wang, Q., Lemley, A.T., 2001. Kinetic model and optimization of 2,4-D degradation by anodic Fenton treatment. Environ. Sci. Technol. 35, 4509–4514. Wang, Q., Lemley, A.T., 2002. Oxidation of carbaryl in aqueous solution by membrane anodic Fenton treatment. J. Agric. Food Chem. 50, 2331–2337. Wang, Q., Lemley, A.T., 2003a. Competitive degradation and detoxification of carbamate inseticides by membrane anodic Fenton treatment. J. Agric. Food Chem. 51, 5382–5390. Wang, Q., Lemley, A.T., 2003b. Oxidative degradation and detoxification of aqueous carborfuran by membrane anodic Fenton treatment. J. Hazard. Mater. B 98, 241–255. Wang, Q., Scherer, E.M., Lemley, A.T., 2004. Metribuzin degradation by membrane anodic Fenton Treatment and its interaction with ferric ion. Environ. Sci. Technol. 38, 1221–1227. White, F.A., Wood, G.M., 1986. Mass Spectrometry, Applications in Science and Engineering. Wiley, New York. Zheng, Z., Obbard, J.P., 2002. Evaluation of an elevated non-ionic surfactant critical micelle concentration in a soil/aqueous system. Water Res. 36, 2667–2672.