JEQ Journal of Natural Resources and Life Sciences Education
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Related articles in JEQ
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Citing Articles
Right arrow Citing Articles via ISI Web of Science (7)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Wang, Q.
Right arrow Articles by Lemley, A. T.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Wang, Q.
Right arrow Articles by Lemley, A. T.
Agricola
Right arrow Articles by Wang, Q.
Right arrow Articles by Lemley, A. T.
Related Collections
Right arrow Humic Substances
Right arrow Remediation
Right arrow Organic Compounds
Right arrow Other Models
Right arrow Agricultural Pesticides
Published in J. Environ. Qual. 33:2343-2352 (2004).
© ASA, CSSA, SSSA
677 S. Segoe Rd., Madison, WI 53711 USA

TECHNICAL REPORTS

Waste Management

Kinetic Effect of Humic Acid on Alachlor Degradation by Anodic Fenton Treatment

Qiquan Wang and Ann T. Lemley*

Graduate Field of Environmental Toxicology, TXA, MVR Hall, Cornell University, NY 14853-4401

* Corresponding author (ATL2{at}cornell.edu)

Received for publication March 26, 2004.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Contamination of water often results from the heavy use of agricultural chemicals, and the disposal of aqueous pesticide waste is a concern. Anodic Fenton treatment (AFT) has been shown to be a successful remediation method for pesticides in solution, but the effect of soil on the degradation kinetics of pesticides using this method has not been determined. The purpose of this study was to study the effect of humic acid, as a soil surrogate, on the degradation kinetics of alachlor [2-chloro-N-(2,6-diethylphenyl-N-(methoxymethyl) acetamide], a heavily used herbicide that has been studied in pure aqueous solution by AFT. The AFT consists of a controlled constant delivery of Fenton reagents, using an electrochemical half-cell to deliver ferrous iron. Alachlor was quickly degraded by AFT, and the kinetics were found to obey the previously developed AFT model well. Degradation of alachlor by AFT in humic acid slurry showed that when the amount of humic acid was increased, alachlor degradation was significantly slowed down and the degradation kinetics were shifted from the AFT model to a first-order model. Further experimentation indicated that humic acid not only competes with alachlor for hydroxyl radicals, reducing the degradation rate of the target compound, but also buffers the slurry at near neutral pH, blocking regeneration of ferrous ion from ferric ion and subsequently shifting the kinetics to first order. Degradation of several other pesticides in humic acid slurry also followed first-order kinetics. These results imply that higher concentrations of Fenton reagents will be required for soil remediation.

Abbreviations: AFT, anodic Fenton treatment • CFT, classic Fenton treatment • HPLC, high performance liquid chromatography


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
WITH THE HEAVY APPLICATION of pesticides in agriculture, generation of contaminated water is not always avoidable. Disposal of unwanted pesticides and rinse water from pesticide containers and application equipment have been of increasing concern in the United States (Felsot, 1996; Waxman, 1998, p. 349–371; Felsot et al., 2003). Efficient, fast, low-cost, and easily operated technology is needed for farmers and commercial applicators to treat this small-scale but highly concentrated pesticide wastewater on-site to reduce its pollution to the environment, especially when the wastewater contains toxic or recalcitrant pesticides. Among the kinds of proposed technologies, Fenton treatment, which is based on the Fenton reaction (Eq. [1]), has become an attractive alternative:

[1]

The main advantages of Fenton treatment are the simplicity of the treatment system and the high oxidation ability of hydroxyl radicals and their wide spectrum of target compounds (Walling, 1975; Walling and Johnson, 1975). Degradation of pesticides and other organic contaminants by Fenton treatment has been widely studied (Arnold et al., 1995; Barbeni et al., 1987; Sedlak and Andren, 1991; Tang and Huang, 1996). However, the direct use of the Fenton reagent [e.g., classic Fenton treatment (CFT)] to treat contaminants in wastewater has two disadvantages (Saltmiras and Lemley, 2000). One is that the treatment effluent is acidic and needs to be neutralized before drainage or further treatment. The other is that ferrous salts are very hygroscopic and readily oxidized and are thus difficult to handle in large amounts. Furthermore, it has been noted that treatment efficiency corresponding to single-dose delivery of Fenton reagent is generally lower than that of a step dose or constant dose with the same total amount of Fenton reagent (Li et al., 1997; Bier et al., 1999; Wang and Lemley, 2002a). To avoid these two problems and increase the treatment efficiency, Pignatello and colleagues (Sun and Pignatello, 1992, 1993; Huston and Pignatello, 1996, 1999) modified the Fenton reagent by using Fe(III) chelates instead of Fe(II) salts to treat pesticides in aqueous solutions at circumneutral pH and introduced photo-irradiation to enhance the treatment. However, the degradation rates of pesticides using Fe(III) chelates are much slower than ferrous salts and the contribution of photo assistance is quite limited compared with that of the traditional Fenton process (Chiron et al., 2000).

Another approach to overcome these disadvantages of CFT is anodic Fenton treatment (AFT), in which the treatment system is divided into two half-cells connected by a salt bridge (Saltmiras and Lemley, 2000, 2001, 2002). Pesticide wastewater and NaCl solution are placed in anodic and cathodic half-cells, respectively. Hydrogen peroxide solution and ferrous ion are constantly delivered into the anodic half-cell by a pump and electrolysis, respectively. The electrode reactions for the two half-cells are shown below:

[2]

[3]

Acidity in the anodic solution is generally caused by hydrolysis of the ferric ion generated from the Fenton reaction. The effluent pH can be partially neutralized by combining solutions from the two half-cells at the end of the treatment, and a higher treatment efficiency is observed. A kinetic model was developed based on degradation of 2,4-D [(2,4-dichlorophenoxy)acetic acid] to describe the concentration change of the target contaminant during the AFT and to optimize the operating conditions of AFT (Wang and Lemley, 2001). The salt bridge was later replaced by an ion exchange membrane (Wang and Lemley, 2002a), taking AFT technology a major step closer to practical application. The AFT kinetic model was found to fit the degradation kinetics of several other pesticides by both salt bridge AFT and membrane AFT (Wang and Lemley, 2002a, 2002b, 2003a, 2003b). Recent work showed that metribuzin (4-amino-6-tert-butyl-4,5-dihydro-3-methylthio-1,2,4-triazin-5-one) and several other triazinone and triazine herbicides can be weakly chelated by ferric ion during AFT and are partially unavailable to hydroxyl radicals. Thus, the degradation kinetics of these herbicides do not obey the AFT model. A revised kinetic model was developed based on both the original AFT model and the consideration of a weak interaction between ferric ion and these herbicides (Wang et al., 2004).

Fenton technology has been applied not only to wastewater treatment, but also to the remediation of contaminated soil. Watts and colleagues (Watts et al., 1990; Tyre et al., 1991) first introduced the use of Fenton reagent in remediation of contaminated soil. It was found that the optimal pH for Fenton treatment in soil is 2 to 3 and the treatment efficiency is highest when no external ferrous ion is added to the soil, suggesting that soil iron minerals and added hydrogen peroxide in an acidic environment provide a system in which a Fenton-like reaction degrades contaminants in the soil. In other studies, dissolved natural organic matter was found to significantly impede degradation of organic contaminants by Fenton treatment (Lindsey and Tarr, 2000a, 2000b). Soil remediation by a Fenton-like reaction using different ferric salts and chelates and natural iron minerals and the effect of contaminant hydrophobicity on remediation were recently investigated (Pignatello and Baehr, 1994; Watts et al., 1999; Quan et al., 2003). Compared with oxidation by an aqueous Fenton reagent, the degradation of contaminants by a Fenton-like process in soil is much slower even if the soil pH has been adjusted to 2 to 3. Soil organic matter content was found to affect the remediation rate of contaminants in the soil (Tyre et al., 1991; Huling et al., 2001; Kanel et al., 2003), but almost all of the investigations were conducted at pH = 3, adjusted with H2SO4. The effect of organic matter on the Fenton degradation kinetics of contaminants in soil slurries without pH adjustment has not yet been well documented.

Since some soil is often mingled with pesticide wastewater from application sites, the effect of soil on the degradation kinetics of pesticides or other contaminants in wastewater by Fenton treatment needs to be better understood. Since AFT is a controlled Fenton process, it can be used to quantitatively investigate the effect of the operating conditions on the degradation kinetics of the target compound (Wang and Lemley, 2001, 2002b) and can also be an effective way to investigate the effect of soil or soil component(s) on the Fenton process, thus providing further understanding of soil remediation by Fenton or Fenton-like treatments.

In the present study, alachlor, a widely used herbicide and a frequently detected contaminant in water (USEPA, 2002; Kolpin et al., 1996), was chosen as the target compound for degradation by AFT. To simplify the system, humic acid, one of the most active ingredients of soil organic matter in adsorbing pesticides in the soil (Senesi, 1992), was used as a surrogate of soil organic matter. The degradation kinetics of alachlor by AFT with and without the presence of humic acid were investigated. The kinetic effect of humic acid on the degradation of alachlor is discussed and the causes of the kinetics shift are elucidated. Degradation kinetics of several other pesticides by AFT in humic acid slurry were also investigated.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Chemicals, Humic Acid, and Membrane
Alachlor (99.5%), metolachlor [2-chloro-6'-ethyl-N-(2-methoxy-1-methylethyl) aceto-o-toluidide] (97.8%), metribuzin (99.5%), 2,4-D (98%), and carbaryl (1-naphthyl methylcarbamate) (99%) were purchased from Chem Service (West Chester, PA). Hydrogen peroxide (30%, analytical grade), acetonitrile (high performance liquid chromatography [HPLC] grade), water (HPLC grade), sodium phosphate monobasic (AR), and potassium permanganate (AR) were purchased from Mallinckrodt (Hazelwood, MO). Hydrochloric acid (GR) and sulfuric acid (GR) were purchased from EM Science (Gibbstown, NJ). Sodium phosphate dibasic (certified), sodium chloride (certified), ferric chloride (certified), and ferrous sulfate (certified) were purchased from Fisher Scientific (Hampton, NH).

Humic acid (ash = approximately 20%) was purchased from Fluka (Buchs, Switzerland). It was used after grinding without further purification or analysis. The anion exchange membrane (ESC-7001) with an electrical resistance of 8 {Omega} cm–2 in 1 M NaCl at 25°C was purchased from Electrosynthesis (Lancaster, NY).

Adsorption of Pesticides on Humic Acid
A preliminary study showed that the adsorption equilibrium of alachlor and other investigated pesticides on humic acid in slurry can be reached after 16 h of moderate magnetic stirring. More than 95% adsorption takes place after 4 h of stirring (data not shown). Adsorption isotherms of pesticides on humic acid were obtained using the slurry-type method (Wang et al., 1999). A series of pesticide concentrations, ranging from 10 to 200 µM, was prepared. Fifty milliliters of pesticide solution with 0.02 M NaCl and 0.125 g of humic acid were added into a 250-mL triangle flask and stirred for 4 h at 25 ± 1°C. The supernatant was removed for analysis by HPLC after centrifugation at 5000 rpm for 10 min. The difference between the original and the equilibrium concentrations was used to calculate the amount of adsorbed pesticide on humic acid.

Anodic Fenton Treatment
Humic acid–pesticide slurry for membrane AFT treatment was prepared by adding 0.50 g (if not specified) ground humic acid into 200 mL pesticide solution containing 0.02 mol L–1 NaCl and 200 µmol L–1 alachlor or other pesticide and then stirring for 4 h at 25 ± 1°C. Humic acid extract solution was obtained by filtering 2.5 g L–1 humic acid slurry after 4 h of magnetic stirring using Whatman (Maidstone, UK) no. 1 filter papers. Humic acid extract–alachlor solution was prepared by dissolving alachlor and NaCl into humic acid extract solution. The final concentrations of alachlor and NaCl were 200 µmol L–1 and 0.02 mol L–1, respectively.

Treatment experiments were performed in an H-shaped glass apparatus, shown in our previous work (Wang and Lemley, 2002a). Two 300-mL beakers served as anodic and cathodic half-cells and were separated by an anion exchange membrane. An iron plate (2 cm wide x 10 cm long x 0.2 cm thick) and a graphite rod (1-cm i.d. x 10-cm length) were used as anode and cathode, respectively. Pesticide aqueous solution, humic acid extract–pesticide solution, or humic acid–pesticide slurry was added into the anodic half-cell and 0.08 mol L–1 NaCl solution was added into the cathodic half-cell. Hydrogen peroxide solution (0.0311 M if not specified) was added into the anodic half-cell at 0.50 mL min–1 by a peristaltic pump (Fisher Scientific), and ferrous ion was delivered into the anodic half-cell by electrolysis using a dc power supply (B&K Precision Corporation, Yorba Linda, CA) at 0.050 A (if not specified). The delivery molar ratio of H2O2 to Fe2+ was 10:1. Treatment temperature was controlled at 25 ± 0.1°C by a Model K20 water circulator (HAAKE Instruments, Paramus, NJ). At different treatment times, 1.00 mL of solution–slurry was taken out for pesticide concentration analysis.

The alachlor AFT degradation experiments at different reagent delivery rates were conducted at electrolysis currents of 0.010, 0.020, 0.030, 0.050, 0.070, 0.090, and 0.120 A. Different concentrations of H2O2 were used to keep the delivery ratio of H2O2 to Fe2+ at 10:1.

In those treatments of humic acid extract–alachlor with initial pH adjusted, 37% HCl was added before AFT treatment. When investigating the effect of the possible interaction of ferric ion with humic acid, solid ferric chloride was weighed and dissolved into humic acid extract–alachlor solution before AFT treatment. When investigating degradation of alachlor in pH buffer solution by AFT, 200 µmol L–1 alachlor solution was prepared in 1.6 mmol L–1 (each) NaH2PO4 and Na2HPO4 solutions with 0.02 mol L–1 NaCl. All treatments were performed in triplicate.

Classic Fenton Treatment
A given amount of ferrous sulfate and hydrogen peroxide were added simultaneously into the prepared humic acid–alachlor slurry with magnetic stirring. At different times, 1.00 mL of slurry was taken out for alachlor concentration analysis.

Concentration Analysis
For treatment of alachlor solution or humic acid extract–alachlor solution, samples from anodic half-cells were added into 2-mL gas chromatography (GC) vials with 0.10 mL methanol (for quenching subsequently generated hydroxyl radicals). Samples were then analyzed for alachlor by an HP 1090 HPLC (Hewlett-Packard, Palo Alto, CA) equipped with a diode array detector.

For treatment of humic acid–pesticide slurries, samples were mixed with 2.00 mL of methanol in 10-mL serum vials. Sample vials were vigorously vortexed for 5 min after being sealed using aluminum caps with Teflon-surfaced liners. After the slurry settled, a portion of the supernatant from each serum vial was transferred into a GC vial for HPLC analysis. The preliminary study showed that the extraction efficiency of this method for each investigated pesticide is greater than 95% (data not shown).

A C18 PRISM reverse-phase (RP) column (5-µm particle size x 250-mm length x 4.6-mm-i.d.) was used for HPLC separation. For analysis of alachlor and metolachlor, the detector wavelength was set at 230 ± 15 nm with 450 ± 80 nm as reference; the mobile phase was composed of H2O (pH adjusted to 3 with H3PO4) and CH3CN at 22:78 (v/v). Their retention times were 5.13 and 5.36 min, respectively. For analysis of 2,4-D and carbaryl, the detector wavelength was set at 280 ± 20 nm; the mobile phase was composed of H2O and CH3CN at 38:62 and 50:50, respectively. Their retention times were 4.56 and 6.73 min, respectively. For metribuzin analysis, detector wavelength was 225 ± 20 nm; the mobile phase was 55% H2O and 45% CH3CN. The retention time was 7.03 min.

Hydrogen peroxide concentration was determined by titration using standard potassium permanganate solution.

Anodic Fenton Treatment Kinetic Model
The development of the AFT kinetic model was published previously (Wang and Lemley, 2001). To better understand the discussion in this study, a brief introduction of the AFT kinetic model is presented here. During AFT treatment, the ferrous ion is delivered into the system at a rate defined as v0 (µmol L–1 min–1). It is rapidly consumed by reaction with hydrogen peroxide and slowly regenerated from the reaction of ferric ion with hydrogen peroxide and other species. It is assumed that the concentration of ferrous ion is constant:

[4]
where [Fe2+] is the instantaneous concentration of ferrous ion (µmol L–1) and {pi} is the average life of ferrous ion in the reaction system (min).

Hydrogen peroxide, which is continuously added into the AFT system at a constant rate, is controlled to be in excess of ferrous ion; thus hydrogen peroxide can be gradually accumulated. It is then assumed that the hydrogen peroxide concentration [H2O2] (µmol L–1) increases linearly with treatment time and can be described as:

[5]
where {omega} is a constant related to the delivery ratio of hydrogen peroxide to ferrous ion and to the consumption ratio of hydrogen peroxide, and t is treatment time (min).

Since the Fenton reaction obeys second-order kinetics, the generation rate of the hydroxyl radical can be expressed as:

[6]
where k1 is the rate constant of the Fenton reaction (µmol L–1 min–1).

The reaction kinetics between the hydroxyl radical and the target compound are also second order. The degradation rate of the target compound can be written as:

[7]
where [D] and [·OH] are the concentrations of the target compound and hydroxyl radical (µmol L–1), respectively, and k is the reaction rate constant for this reaction [(µmol L–1)–1 min–1].

Since hydroxyl radicals are very reactive and short-lived, it can be assumed that the instantaneous concentration of hydroxyl radical is proportional to its generation rate; Consequently, Eq. [7] can be written as:

[8]
where {lambda} is the average life of the hydroxyl radical (min). After integration of Eq. [8], the AFT kinetic model is obtained:

[9]
where K = kk1 [(µmol L–1)–2 min–2] and [D]t and [D]0 are the target compound concentrations (µmol L–1) at t and 0 min.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Degradation of Alachlor by Anodic Fenton Treatment in Aqueous Solution without Humic Acid
Alachlor in pure aqueous solution can be promptly degraded by AFT (Fig. 1) . Even at an electrolysis current as low as 0.010 A, equaling a ferrous ion delivery rate of 15.6 µmol L–1 min–1, about 50% of a 200 µmol L–1 alachlor solution is degraded after 8 min of treatment. No significant degradation was found when alachlor was treated only by electrolysis or hydrogen peroxide (data not shown), confirming that the removal of alachlor is not caused by electrolysis or by hydrogen peroxide oxidation. Alachlor degradation is significantly enhanced by an increased delivery rate of Fenton reagent, and all alachlor in the treated solution is degraded within 6 min of treatment at 0.070 A. Meanwhile, degradation kinetics at each delivery rate obey the AFT model well. Fitting results of experimental data by the AFT kinetic model are listed in Table 1. The rate parameter, K{lambda}{pi}{omega}v20, increases with the Fenton reagent delivery rate, accelerating the degradation.



View larger version (20K):
[in this window]
[in a new window]
  		Fig. 1. Degradation of alachlor in pure aqueous solution by membrane anodic Fenton treatment (AFT) at different delivery rates of Fenton reagent. Points are experimental data and lines are fitting results using the AFT kinetic model.

 

View this table:
[in this window]
[in a new window]
  		Table 1. Values of rate parameter, K{lambda}{pi}{omega}v20, obtained from fitting of experimental data by the anodic Fenton treatment (AFT) model and the calculated half-life, t1/2, of alachlor at different delivery rates of Fenton reagent.

 
When t = t1/2, Eq. [9] can be written as:

[10]

Then, the half-life of alachlor can be calculated from the following equation:

[11]

The calculated values of alachlor half-lives from Eq. [9] are also listed in Table 1. If the amount of Fenton reagent needed to degrade a certain amount of alachlor (i.e., the treatment efficiency) is constant with the change of Fenton reagent delivery rate, the value of v0t1/2 should be the same for all these treatments. Equation [11] can be then written as:

[12]

A regression equation between v0t1/2 and v0 has been obtained:

[13]

It can be seen that v0t1/2 is not a constant with Fenton reagent delivery rate, but increases with an increase in delivery rate, signifying that at a higher delivery rate, more Fenton reagent is consumed to degrade the same amount of alachlor, and the treatment efficiency decreases. When the delivery rate is increased but the concentration of the target compound remains the same, a greater percentage of hydroxyl radicals may be self-quenched or react with Fenton reagent (Neyens and Baeyens, 2003). This is a reasonable explanation of why the treatment efficiency decreases with increasing delivery rate. Conversely, a treatment solution with a high concentration of the target compound would increase the probability that hydroxyl radicals will react with this compound and reduce the percentage of hydroxyl radicals that are self-quenched or react with the Fenton reagent.

Degradation of Alachlor in Humic Acid Slurry
As shown in Fig. 2 , the alachlor degradation kinetics by AFT are significantly affected by the presence of humic acid. The more humic acid present in the slurry, the slower the degradation of alachlor. When the added amount of humic acid is 0.25 g L–1, the degradation of alachlor is just slightly slower than that in pure aqueous solution (Fig. 2a), and both degradation kinetics fit the AFT model well, with correlation coefficients greater than 0.999. However, when the added amount of humic acid reaches 1.00 g L–1, the degradation kinetics begin to deviate from the AFT model. Neither the AFT model nor a first-order model fit the experimental data (Fig. 2b). When the amount of humic acid is 1.50 g L–1 or higher, alachlor degradation obeys first-order kinetics (Fig. 2c). All correlation coefficients are greater than 0.996.



View larger version (17K):
[in this window]
[in a new window]
  		Fig. 2. Degradation of alachlor in slurry with different amounts of humic acid by anodic Fenton treatment (AFT) at 0.050 A. Points are experimental data. Lines are fitting results using (a) the AFT kinetic model, (b) both the AFT model and the first-order model, and (c) the first-order model.

 
It has been reported that soil organic matter can compete with target compounds for hydroxyl radicals, thus slowing down the degradation of target compounds (Watts et al., 1990; Kanel et al., 2003). This result is a possible explanation of why alachlor degradation by AFT becomes slower with increased humic acid in the slurry, assuming that the concentration of soluble humic acid is also increasing. But this fact cannot explain why the degradation kinetics follow different models with different amounts of humic acid. Based on our previous studies (Wang and Lemley, 2001, 2002b, 2003b), the degradation kinetics of the target compound should follow the same AFT model as in the pure solution no matter how many competitors coexist in the system. Alachlor adsorption on humic acid might be one of the reasons for the kinetics shift. This and other possibilities will be discussed later in this study.

Degradation of alachlor by AFT in humic acid slurry at different delivery rates of Fenton reagent was also investigated (Fig. 3) . The amount of added humic acid was controlled at 2.50 g L–1. Alachlor degradation is effectively enhanced with the increase of Fenton reagent delivery rate. All degradation kinetics are first order for treatments with iron delivery from 0.010 to 0.070 A. For treatments at 0.090 and 0.120 A, alachlor degradation kinetics can also be fitted by the first-order model if the last two points in each treatment are not included in the regression. All correlation coefficients are greater than 0.996 (data not shown). Correlation of calculated first-order rate constants with delivery rate of ferrous ion is shown in Fig. 4 . The first-order rate constant increases linearly with ferrous ion delivery rate from 15.6 to 140 (µmol L–1)–1 min–1 (e.g., from 0.010 to 0.090 A), suggesting that the acceleration of alachlor degradation rate by increasing Fenton reagent delivery rate has not caused a loss of treatment efficiency as occurred in the pure aqueous system. This phenomenon is most probably due to the fact that there are many more organic compounds, which are available to hydroxyl radicals, in the humic acid slurry than in pure aqueous solution. In the slurry system the self-quenching reaction of hydroxyl radicals and their reaction with Fenton reagent might be mitigated to some extent. Thus, within a certain range of Fenton reagent delivery rate, the percentage of quenched hydroxyl radicals is the same.



View larger version (25K):
[in this window]
[in a new window]
  		Fig. 3. Degradation of alachlor in 2.50 g L–1 humic acid slurry by anodic Fenton treatment (AFT) at different delivery rates of Fenton reagent. The H2O2 to Fe2+ ratio is kept at 10:1. Points are experimental data and lines are fitting results using the first-order model. The last two points are not included in model fitting for data from treatment at 0.090 or 0.120 A.

 


View larger version (13K):
[in this window]
[in a new window]
  		Fig. 4. Correlation between first-order rate constant of alachlor anodic Fenton treatment (AFT) degradation in 2.50 g L–1 humic acid slurry and delivery rate of ferrous ion.

 
However, the first-order rate constant at 0.120 A is significantly lower than it should be, indicating that treatment efficiency begins to decrease at this high rate of Fenton reagent delivery. The percentage of quenched hydroxyl radicals in humic acid slurry can be increased if the Fenton reagent delivery rate is very high. This is further confirmed by a comparison between alachlor degradation by AFT and by CFT (Fig. 5) . The CFT can be regarded as an AFT with extremely high delivery rate of Fenton reagent. Alachlor concentration in the slurry after AFT is significantly lower than after CFT with the same amount of Fenton reagent, indicating that a low delivery rate of Fenton reagent is more effective than a higher one. This might help explain why the treatment efficiency of PCP in soil without the addition of ferrous ion is higher than that with the addition, as was shown by Watts et al. (1990). When no external ferrous ion is added, limited soil iron minerals and low release rate of iron ions act as a low delivery rate of Fenton reagent. The difference in treatment efficiency between AFT and CFT, and between a high delivery rate and a low one, might be even greater if more humic acid is added to the slurry since more hydroxyl radicals in CFT or in a high delivery rate process can be quenched or react with coexisting organics before alachlor is desorbed.



View larger version (14K):
[in this window]
[in a new window]
  		Fig. 5. Degradation of alachlor in 200 mL of 2.50 g L–1 humic acid slurry by anodic Fenton treatment (AFT) and classic Fenton treatment (CFT) with the same amount of ferrous ion and hydrogen peroxide. The H2O2 to Fe2+ ratio is kept at 10:1.

 
The equilibrium concentration of 200 µmol L–1 alachlor in 2.5 g L–1 humic acid slurry was determined to be 135.9 µmol L–1. This means that 64.1 µmol L–1 alachlor out of 200 µmol L–1 total alachlor concentration is adsorbed on humic acid. When alachlor in the slurry system is degraded by AFT, and the total concentration becomes lower than 64.1 µmol L–1, it can be assumed that some of the originally adsorbed alachlor has been degraded. As shown in Fig. 3, the degradation of alachlor at 0.050 and at 0.070 A obeys first-order kinetics well and the final alachlor concentration after 20 min of treatment is 48.9 and 29.9 µmol L–1, respectively. Both of these final concentrations are lower than 64.1 µmol L–1; hence some of the previously adsorbed alachlor has been desorbed and degraded. But it appears that not all of the adsorbed alachlor can be consistently desorbed and degraded. After 12 min of AFT at both 0.090 and 0.120 A, the concentration decreases more slowly than the kinetics would predict, based on the experimental points before 12 min when some of the previously adsorbed alachlor has been degraded and its degradation fits the predicted kinetics well. It is possible that strongly adsorbed alachlor may not desorb quickly enough to maintain the first-order degradation kinetics.

Alachlor Degradation Kinetics
As stated above, based on previous work the coexistence of organic compounds would not be expected to cause a shift of alachlor degradation kinetics from the AFT model to the first-order model. We believe that there are two possible explanations why alachlor degradation by AFT in humic acid slurry obeys first-order kinetics. One is the adsorption and slow release of adsorbed alachlor from humic acid. The other is the blocking of ferrous ion regeneration from ferric ion.

If the adsorption of alachlor on humic acid and its subsequent slow release from humic acid during AFT is the cause for this shift in kinetics, degradation kinetics of alachlor in a similar system without adsorption should follow the AFT model. The degradation kinetics of alachlor by AFT in a humic acid extract solution was investigated. It is assumed that there is no significant adsorption of alachlor in this solution. As shown in Fig. 6 , experimental data cannot be fitted by the AFT kinetic model. This result indicates that the alachlor degradation kinetics do not follow the AFT model even though no adsorption exists in the system, suggesting that the adsorption of alachlor on humic acid is not the cause of the first-order kinetics of alachlor degradation in humic acid slurry.



View larger version (17K):
[in this window]
[in a new window]
  		Fig. 6. Degradation kinetics of alachlor by anodic Fenton treatment (AFT) in humic acid extract solution with and without pre-addition of ferric ion. Points are experimental data and lines are fitting results using the AFT model.

 
If the regeneration of ferrous ion from ferric ion is blocked, the generation rate of hydroxyl radicals will depend totally on the delivery rate of ferrous ion, since this is the only source of ferrous ion and also the control step in the Fenton process. The hydrogen peroxide concentration and its delivery rate should have no effect on hydroxyl radical generation if they are in excess of ferrous ion. The following equation can be established:

[14]
where {theta}, a constant, is the ratio of ferrous ion reacted with hydrogen peroxide to the total ferrous ion delivered into the system. By substituting Eq. [14] into [8], the following is obtained:

[15]

After integration of both sides of Eq. [15], a pseudo first-order model is obtained:

[16]

Since this derived first-order kinetic model is consistent with the degradation kinetics of alachlor in humic acid slurry, it is probable that the blocking of ferrous ion regeneration from ferric ion is the cause of the kinetic shift from the AFT model to first-order kinetics. To confirm this conclusion, the basis for the blocking of ferrous ion regeneration from ferric ion must be elucidated.

It is known that humic acid has many hydroxyl, carbonyl, and carboxyl groups and different kinds of aromatic rings (Senesi, 1992). As an electron acceptor, ferric ion can possibly be chelated by the electron-donating functional groups in humic acid, making the ferric ion unavailable to hydrogen peroxide and other species, thus blocking the regeneration of ferrous ion from ferric ion. If this is the case, the degradation kinetics of alachlor by AFT in a slurry of Fe3+–saturated humic acid should obey the AFT model. To verify this, degradation of alachlor by AFT in humic acid extract solution with pre-addition of ferric ion was investigated. Ferric chloride solid was added and dissolved into humic acid extract–alachlor solution until the pH decreased from 6.90 to 5.90, to ensure that the added ferric ion was in slight excess of the chelating capacity of the soluble humic acid in the solution. As shown in Fig. 6, the degradation kinetics still cannot be fit by the AFT model, although they are slightly faster than those without the addition of ferric ion, showing that the possible interaction between ferric ion and humic acid does not exist, or exists but is not the cause of the blocking of ferrous ion regeneration from ferric ion.

Another possible cause of the blocking of ferrous ion regeneration is the maintenance of a high pH at which ferric ion can be precipitated and become unavailable to hydrogen peroxide and other species. Changes in hydrogen ion concentration in pure alachlor solution, humic acid extract–alachlor solution, and humic acid–alachlor slurry during the AFT are shown in Fig. 7 . In pure alachlor solution, [H+] increases rapidly and almost linearly with treatment time. After 20 min of AFT at 0.050 A, [H+] increases from 3.02 x 10–6 mol L–1 to as high as 1.38 x 10–3 mol L–1. However, in the humic acid–alachlor slurry [H+] increases very slowly over the same treatment time ending at 8.38 x 10–5 mol L–1 compared with the original [H+] at 1.26 x 10–6 mol L–1 (pH = 5.90). The increase of [H+] in humic acid extract–alachlor solution is slower than that in pure alachlor solution but faster than that in the slurry. Specifically, it increases almost as slowly as in the slurry within the first 4 min of AFT but gradually increases in rate more quickly than the slurry after 4 min and much more quickly after 10 min. These results indicate that humic acid is a strong pH buffer. Different amounts of humic acid in the solution–slurry resulted in different buffering capacities. Twenty minutes of AFT at 0.050 A still cannot overcome the pH buffering capacity of the humic acid slurry at 2.5 g L–1; thus, ferric ion is precipitated and the regeneration of ferrous ion is blocked. This helps explain why the degradation kinetics of alachlor in humic acid slurry obey first-order kinetics instead of the AFT model. Since less humic acid is contained in humic acid extract solution than in the slurry, the pH buffering capacity of humic acid is more easily overcome in this case. This is why [H+] increases quickly after the first 4 min and even faster after 10 min. It also explains why alachlor degradation in humic acid–extract solution is slow, and almost linear, in the beginning but becomes faster when a high concentration of H+ is built up and the regeneration of ferrous ion occurs.



View larger version (20K):
[in this window]
[in a new window]
  		Fig. 7. Changes of [H+] in pure aqueous alachlor solution, humic acid extract–alachlor solution, and humic acid–alachlor slurry during anodic Fenton treatment (AFT) at 0.050 A.

 
To confirm that it is the humic acid buffering capacity at neutral pH that results in first-order kinetics, the degradation of alachlor in a phosphate buffer solution by AFT was studied. The solution pH was decreased from 6.86 to 5.89 after 12 min of AFT at 0.050 A. As shown in Fig. 8 , alachlor degradation obeys first-order kinetics well with a correlation coefficient of 0.999. This result directly verifies the possibility of the first-order kinetics in a buffered neutral pH environment, but it does not confirm that the buffered neutral pH is the only cause.



View larger version (12K):
[in this window]
[in a new window]
  		Fig. 8. Degradation of alachlor by anodic Fenton treatment (AFT) at 0.050 A in phosphate buffer solution. Concentrations of NaH2PO4 and Na2HPO4 are each 1.6 mmol L–1. Points are experimental data and line is the fitting result using the first-order model.

 
To confirm that this buffering is the only cause of the first-order kinetics, alachlor degradation in a humic acid extract solution, in which the pH buffering ability is overcome, was investigated. Before AFT, humic acid extract–alachlor solution pH was adjusted to 3.05 using HCl. If the buffered, neutral pH is the only cause of the kinetics shift from the AFT model to the first-order model, alachlor degradation kinetics in this solution should obey the AFT model. As shown in Fig. 9 , alachlor degradation kinetics can be fitted by the AFT model well with a correlation coefficient of 0.996. This confirms that the buffered neutral pH in humic acid slurry is the only cause of the first-order kinetics of alachlor degradation by AFT. Experimental points at 1 and 2 min are slightly lower in alachlor concentration as compared with the fitting result, signifying a slightly faster degradation rate than predicted by the AFT model. This might be caused by the high initial [H+] from the pH adjustment, which creates an artificially optimal pH for the Fenton reaction from the beginning of the treatment. If there were a way to remove the pH buffering ability of humic acid without changing the initial pH, all experimental data could be fitted by the AFT model just as they are in the pure aqueous alachlor solution. As a comparison, degradation of alachlor in a pure solution by AFT at the same operating conditions is also shown in Fig. 9. Even though the initial pH of humic acid extract–alachlor solution was adjusted to 3.05, alachlor degradation is still slower than in the pure solution without the initial pH adjustment. This confirms even more directly than the data in Fig. 2 that humic acid competes with the target compound for hydroxyl radicals.



View larger version (20K):
[in this window]
[in a new window]
  		Fig. 9. Degradation kinetics of alachlor by anodic Fenton treatment (AFT) at 0.050 A in humic acid extract solution with initial pH adjusted to 3.05 and in pure solution without initial pH adjustment. Points are experimental data and lines are fitting results using the AFT model.

 
Humic acid not only competes with the target compound for hydroxyl radicals, but also buffers the slurry pH, thus blocking the regeneration of ferrous ion from ferric ion and shifting the degradation kinetics from the AFT model to the first-order model. To regenerate ferrous ion and thus achieve a high remediation rate and efficiency, the pH buffering ability of soil or its components must be overcome before soil remediation by Fenton or Fenton-like treatment. This may be the reason why almost all soil remediation studies were conducted at a pH of approximately 3, which is taken as the optimal pH for soil remediation using Fenton or Fenton-like treatment. However, this strong acidic adjustment can result in dramatic ecological impact in soil (Yeh et al., 2002). To avoid this problem, treating extraction solutions from soil with environmentally friendly solvents by Fenton or Fenton-like technologies (Li et al., 1997; Palma et al., 2003; Bogan et al., 2003) instead of directly treating contaminated soil might be a good alternative worth further study.

Degradation of Other Pesticides in Humic Acid Slurry
The AFT degradation kinetics of several other commonly used pesticides, including metolachlor, 2,4-D, carbaryl, and metribuzin, in humic acid slurry were also investigated. The humic acid content in the slurry is the same as in degradation experiments. The degradation rate follows the order: 2,4-D > carbaryl > alachlor {approx} metolachlor > metribuzin. Their degradation kinetics in humic acid slurry obey the first-order model well (Fig. 10) , with correlation coefficients greater than 0.992. Thus, the first-order degradation kinetics by AFT in humic acid is not a phenomenon unique to alachlor, but it may well be a phenomenon common to all organic contaminants in humic acid slurry.



View larger version (19K):
[in this window]
[in a new window]
  		Fig. 10. Degradation kinetics of alachlor, metolachlor, 2,4-D, carbaryl, and metribuzin in 2.50 g L–1 humic acid slurry by anodic Fenton treatment (AFT) at 0.050 A. Points are experimental data and lines are fitting results using the first-order kinetic model.

 
Adsorption isotherms of these pesticides on humic acid were also obtained, and all of them can be fitted by the Freundlich equation:

[17]
where Q is the adsorption amount (µmol g–1), Ce is the equilibrium concentration of the pesticide, and KF and 1/n are Freundlich coefficients. These values are shown for all the pesticides in Table 2. Their adsorption decreases in the following order: alachlor {approx} metolachlor > carbaryl > metribuzin > 2,4-D. It is probable that the more adsorptive the pesticide, the slower the degradation rate. However, no direct correlation was found between the order of adsorption and the order of degradation rates, suggesting that the degradation rate of these pesticides in humic acid slurry is not or not only affected by adsorption.


View this table:
[in this window]
[in a new window]
  		Table 2. Values of Freundlich coefficients, KF and 1/n, for the adsorption of different pesticides on humic acid.

 
Reactivity of pesticides toward hydroxyl radicals could be another important factor affecting the degradation rate of pesticides in humic acid slurry. As reported, the rate constant of alachlor with hydroxyl radicals is 7 x 109 (mol L–1)–1 s–1, which is greater than that of 2,4-D, 5 x 109 (mol L–1)–1 s–1 (Haag and Yao, 1992). But as shown above, the degradation of 2,4-D is faster than that of alachlor. Thus, this reactivity is not the only factor affecting the degradation rate. It appears possible that the degradation rate of a pesticide by the Fenton process in humic acid slurry is affected by both the adsorption of the pesticide on humic acid and the reactivity of the pesticide toward hydroxyl radicals, as well as other factors.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Alachlor can be quickly degraded in pure aqueous solution by AFT, and its degradation kinetics obey the AFT model well. The treatment efficiency decreases with an increasing delivery rate of Fenton reagent, signifying that there is also an increase of hydroxyl radicals that are quenched in side reactions. The presence of humic acid significantly slows alachlor degradation by AFT. In a slurry with a low amount of humic acid, alachlor degradation kinetics still obey the AFT model. But when humic acid content reaches 1.5 g L–1, alachlor degradation begins to obey first-order kinetics. Because of the coexistence of a high concentration of soluble humic acid, the percentage of quenched hydroxyl radicals in side reactions is mitigated to some extent, and the treatment efficiency remains constant with an increased delivery rate of Fenton reagent within a certain range. The shift in kinetics from the AFT model in pure solution to the first-order model in humic acid slurry was found to be caused by the pH buffering ability of humic acid, which blocks the regeneration of ferrous ion from ferric ion and makes the generation rate of ferrous ion during AFT depend totally on the delivery. Degradation of several other pesticides in humic acid slurry by AFT also obeys first-order kinetics. The degradation rate of pesticides in humic acid slurry is most likely affected by both adsorption to the slurry and reactivity of pesticides toward hydroxyl radicals.


   ACKNOWLEDGMENTS
 
This work was supported by the Cornell University Agricultural Experiment Station federal formula funds, Project 329423 (Regional Project W-045), received from the USDA Cooperative State Research, Education, and Extension Service (CSREES).


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 


Related articles in JEQ:

This Issue in Journal of Environmental Quality

JEQ 2004 33: 1947-1953. [Full Text]  




This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Related articles in JEQ
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Citing Articles
Right arrow Citing Articles via ISI Web of Science (7)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Wang, Q.
Right arrow Articles by Lemley, A. T.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Wang, Q.
Right arrow Articles by Lemley, A. T.
Agricola
Right arrow Articles by Wang, Q.
Right arrow Articles by Lemley, A. T.
Related Collections
Right arrow Humic Substances
Right arrow Remediation
Right arrow Organic Compounds
Right arrow Other Models
Right arrow Agricultural Pesticides

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE