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Melamine-based organoclay to sequester atrazine
Chemosphere 64 (2006) 704–710 www.elsevier.com/locate/chemosphere
Melamine-based organoclay to sequester atrazine Susan L. Neitsch a, Kevin J. McInnes a,*, Scott A. Senseman a, G. Norman White a, Eric E. Simanek b a
Department of Soil and Crop Sciences, Heep Center, 370 Olsen Boulevard, Texas A&M University, College Station, TX 77843, United States b Department of Chemistry, Texas A&M University, College Station, TX 77843, United States Received 7 July 2005; received in revised form 3 November 2005; accepted 4 November 2005 Available online 5 January 2006
Abstract Sequestration of aqueous atrazine by organoclays prepared from the surfactant 6-piperazin-1-yl-N,N 0 -bis-(1,1,3,3-tetramethyl-butyl)(1,3,5)triazine-2,4-diamine and Gonzales bentonite was assessed using 14C-labeled atrazine. Organoclays with varying ratios of surfactant to clay were evaluated with respect to their ability to sequester atrazine from an aqueous solution. Organoclays containing 100– 200 g kg1 surfactant on a total weight basis provided the most efficient adsorption of atrazine, with apparent KOC values exceeding 5000 l kg1 at these loading fractions. Less than 12% of sequestered atrazine was released during four sequential daylong washings, supporting our expectation that the majority of the reaction of atrazine with the surfactant lead to irreversible chemical bond formation through nucleophilic aromatic substitution. 2005 Elsevier Ltd. All rights reserved. Keywords: Organoclay; Atrazine; Herbicide; Pesticide; Adsorption; Sequestration; Smectite; Bentonite
1. Introduction Triazine and acetanilide herbicides are widely used in American agriculture. Collectively, triazine and acetanilide herbicides possess low to moderate water solubility and soil sorptivity, and relatively long persistence in soil and water (Wauchope et al., 1992). As a consequence of these characteristics, triazine and acetanilide herbicides possess a moderate to high potential for offsite transport with storm water runoff and percolating water (Goss, 1992). Atrazine, a widely used triazine herbicide, has contaminated surface and ground water resources in regions where usage is widespread (Larson et al., 1997). Given the necessary efforts associated with remediation of contaminated waters, means of improving the efficiency of treatment would be desirable. The most common technique for removal of atrazine from water involves use of an activated carbon (activated
*
Corresponding author. Tel.: +1 979 845 5986; fax: +1 979 845 0456. E-mail address: [email protected] (K.J. McInnes).
0045-6535/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2005.11.033
charcoal) adsorption system. Unfortunately, activated carbon lacks selectivity for atrazine and adsorbs innocuous organic compounds. These organic compounds, such as soluble natural organic matter, compete with atrazine for adsorption sites causing inefficient sequestration or even displacement of adsorbed atrazine. As a consequence, it is desirable to investigate complimentary supplements to activated carbon such as inorganically intercalated clays (Abate and Masini, 2005) and novel regenerable organics (Brown et al., 2004). To remediate petroleum contaminated water, Alther (2002) used a granular organoclay as a pretreatment to activated carbon treatment. Treatment combining one-half equivalent of each the organoclay and the activated carbon was found to be appreciably more efficient than a full equivalent of either treatment alone. Similarly, it might be possible to enhance atrazine removal from water using a combination of an atrazine-sequestering organoclay and activated carbon. In the course of characterizing the properties of newly synthesized dendrimers based on melamine (2,4,6-triamino-1,3,5-triazine), researchers discovered that some of
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the dendrimer systems effectively sequestered triazine herbicides from solution (Zhang and Simanek, 2000; Zhang et al., 2001). This observation and the economical nature of producing these dendrimers were new discoveries. Analysis of the atrazine adsorption mechanism showed that monochlorotriazines, including atrazine and its major metabolites, are susceptible to nucleophilic aromatic substitution. In screening potential nucleophiles, constrained secondary amines were found to be the most reactive group. Subsequent research (Acosta, 2003) identified a promising surfactant [6-piperazin-1-yl-N,N 0 -bis-(1,1,3,3tetramethyl-butyl)-(1,3,5)triazine-2,4-diamine] that could be attached to smectite clay. In removal of triazine contaminated water, a sequestration strategy utilizing adsorption through nucleophilic aromatic substitution would provide a distinct advantage over that of activated carbon because it is selective for reactive molecules and the efficacy does not rely on an equilibrium position, but instead, irreversible chemical bond formation. When constrained secondary amines are attached to solid supports, it is possible to remove triazine herbicides from solution. Acosta et al. (2004) found that a polystyrene resin modified with secondary amines was very efficient at removing atrazine, desethylatrazine, desiopropylatrazine, and cyanazine from solution. The sequestration potential of the modified polystyrene resin was comparable to that of fresh activated carbon. Data indicated a covalent sequestration mechanism, and unlike with activated carbon, the presence of natural organic materials in water did not restrict removal of triazine contaminants. In addition, adsorption of triazines to the modified resin showed that the sequestration was relatively unaffected by pH and ionic strength. Use of inexpensive smectite clays as a solid support for similar nucleophilic surfactants has not been investigated. The objective of the research reported herein was to create organoclays with 6-piperazin-1-yl-N,N 0 -bis-(1,1,3,3tetramethyl-butyl)-(1,3,5)triazine-2,4-diamine and to investigate the ability of these organoclays to sequester atrazine from water. The studies characterized the apparent adsorption of atrazine on and desorption from organoclays created with the surfactant and a bentonite clay, and quantified the impact of the ratio of surfactant to clay in the organoclay. As a basis for comparison of the sequestration capacity for atrazine, experiments to measure apparent adsorption and desorption were performed on soil from the upper 10 cm of a Houston Black clay (fine, smectitic, thermic Udic Haplustert) pedon.
a chemical purity of 98% was purchased from Chem Service (West Chester, PA). 14C-ring-labeled atrazine with a chemical purity of 96.7%, a radioactive purity of 98.7%, and a specific activity of 340 TBq mol1 was obtained from Syngenta Crop Protection (Greensboro, NC). Atrazine was first dissolved in methanol then diluted with 0.01 M aqueous CaCl2 solution to produce desired concentrations of 0.01 mg l1 to 30 mg l1. The aqueous solutions contained 1% methanol. The atrazine solutions were created to provide 170 kBq l1 of working solution. The only exception was that the most dilute solution (0.01 mg l1) provided only 17 kBq l1.
2. Materials and methods
2.3. Organoclay
2.1. Atrazine
Surfactant–clay composites (organoclays) were created by sorbing the surfactant to the Ca-saturated smectite clay. Organoclays were prepared containing 10, 20, 30, 40, 50, 75, 100, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450 and 500 g kg1 surfactant on a total weight basis (i.e., 500 g kg1 consists of equal mass surfactant and clay).
The herbicide atrazine (C8H14ClN5) (Fig. 1) is a white powder or crystalline solid with a water solubility of 28 and 33 mg l1 at 20 and 25 C, respectively (Montgomery, 1993; Hornsby et al., 1996). Analytical grade atrazine with
clay surfactant H N
H N
N N
H N
N
N N
N H
N N N H
Cl N N H
N
N
+
N N
sequestered atrazine N H
+
N N
H N
N
N H
HCl
atrazine
Fig. 1. Potential covalent bonding of atrazine with 6-piperazin-1-yl-N,N 0 bis-(1,1,3,3-tetramethyl-butyl)-(1,3,5)triazine-2,4-diamine organoclay.
2.2. Surfactant and clay The surfactant [6-piperazin-1-yl-N,N 0 -bis-(1,1,3,3tetramethyl-butyl)-1,3,5-triazine-2,4-diamine; C23 H46 Nþ 7] (Fig. 1) used in the experiments was prepared at Texas A&M University. A summary of the surfactant synthesis was provided by Acosta (2003). The clay was Gonzales bentonite (GB) obtained from Southern Clay Products (Gonzales, TX). The clay was pretreated following the procedure detailed in Deng (2001) to remove particles >2 lm diameter and to saturate the clay’s cation exchange sites with Ca. Deng (2001) confirmed that the mineral composition of the Gonzales clay was smectite using X-ray diffraction and Fourier-transform infrared spectroscopy. Cation exchange capacity for the smectite was 81 cmolc kg1, specific area was 790 m2 g1, charge density was 1.0 lmolc m2, and the pH was 6.4 (Deng, 2001).
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For each composition, 500 mg of clay was suspended in 20 ml of distilled water in a 50-ml polypropylene conical centrifuge tube. The required amount of surfactant was dissolved in 5 ml of tetrahydrofuran and added to the clay suspension by pipette. The suspensions were then shaken for 24 h at 25 C. The resulting organoclays were centrifuged at 15 000 · g for 10 min and the supernatant was decanted. The organoclays were washed three times with distilled water to remove tetrahydrofuran. Washing was done by adding 15 ml distilled water to the organoclay sediment, shaking until all the sediment was suspended, centrifuging at 15 000 · g for 10 min, and then decanting the supernatant. The organoclays were then freeze-dried and ground to a powder. The amount of surfactant sorbed to the clay was verified with a Carlo Erba NA-1500 combustion unit (CE Elantech, Lakewood, NJ) by analyzing for organic C and N contents.
counting of 1 ml of the supernatant mixed with 10 ml of Ecolite liquid scintillation cocktail (ICN Biomedicals, Costa Mesa, CA) using a Beckman LS6500 multipurpose scintillation counter (Beckman Instruments, Inc., Fullerton, CA). The counting time was set to obtain a maximum 2% standard deviation for the disintegration mean. Centrifuge tubes containing no solid material were prepared in triplicate and processed alongside the sample centrifuge tubes. The concentration bound to the solid was calculated as the difference between the concentrations in solution of centrifuge tubes containing no solid material and of the samples with the solid. Background radioactivity was measured by processing centrifuge tubes with solid material to which 50 ll methanol was added in place of the atrazine solution.
2.4. Soil sample
Temporal sorption studies were carried out with 0, 20, 50, 125, 250 and 450 g kg1 organoclays and Houston Black soil at a solution concentration of 2 mg l1 atrazine. The amount of atrazine sequestered from solution was measured at 0, 4, 8, 24, 48, 72, 96, and 120 h. The sequestration percentages were calculated as
The soil sample was collected from the top 10 cm of a Houston Black clay (HBC) pedon located on the Blackland Research Center, Texas Agricultural Experiment Station. The sample site (31 3 0 11.600 N, 97 21 0 0.900 W) was located at the base of a 1–3% slope. The soil sample was air dried and ground to pass a 2-mm sieve, then pulverized with a laboratory ring pulverizer (Armstrong Inc., Chicago, IL). Particle size analysis of unpulverized clay using the hydrometer method (Gee and Or, 2002) established a size distribution of 13% sand (2000–50 lm), 34% silt (50– 2 lm), and 53% clay (<2 lm). A pulverized sample had 1.5% organic carbon and 31.4% CaCO3 equivalent (29.9% calcite and 1.4% dolomite) as determined by the Soil Characterization Laboratory, Soil & Crop Sciences Dept., Texas A&M University. Total carbon was determined using combustion (Nelson and Sommers, 1982). Calcium carbonate equivalent was measured using HCl treatment and a Chittick apparatus (Drenianis, 1962). Organic carbon was calculated as the difference between total carbon and CaCO3 equivalent. 2.5. Experimental protocols All sorption and desorption experiments were carried out in 35-ml glass centrifuge tubes with Teflon-lined caps at 25 C. To minimize variation in atrazine adsorption by the clay component of the organoclay, samples were weighed so that all vials contained 15 mg of Gonzales bentonite clay. Triplicate 15 mg clay-equivalent samples were weighed into centrifuge tubes and equilibrated for 24 h in 5 ml of 0.01 M CaCl2 solution. After the 24-h equilibration period, 50 ll of a desired atrazine solution was added to each centrifuge tube by pipette. The tubes were shaken for a length of time that varied depending on the specific experiment then centrifuged for 20 min at 622 · g and the supernatant decanted. The concentration of atrazine in the decanted liquid was measured by liquid scintillation
2.6. Sequestration kinetics
% sequestered ¼ 100
Ci Ce Ci
ð1Þ
where Ci is the initial concentration in solution (mg l1) and Ce is the concentration in solution at the end of the reaction period (mg l1). The concentration of atrazine sorbed mx to the organoclay or soil was calculated as x vsol ¼ ðC i C e Þ m msub
ð2Þ
where x is the mass of atrazine sorbed (mg), m is the mass of organoclay or soil (kg), vsol is the total solution volume (l), and msub is the total mass (kg) of substrate exposed to the solution. Reaction was considered complete when the difference in the amount of atrazine sorbed from solution between the current and subsequent 24 h time step was less than or equal to 5% (Moreau and Mouvet, 1997). 2.7. Sorption isotherms Apparent sorption isotherms were determined for all organoclays and Houston Black clay using solution concentrations selected to represent concentrations that might be found in runoff (0.01 mg l1) up to the saturation value (30 mg l1), namely, 0.01, 0.1, 1, 2, 5, 10, 20 and 30 mg l1 atrazine. Apparent partition coefficients Kd were calculated as the slope of a linear regression through the origin of mx against Ce. Organic-carbon-adjusted partition coefficients Koc were calculated as the ratio of Kd to the mass fraction (kg kg1) organic carbon OC. Freundlich partition coefficients Kf and 1n were calculated from log-linearized adsorption data.
S.L. Neitsch et al. / Chemosphere 64 (2006) 704–710
Desorption studies were carried out with 0, 10, 50, 200, 350 and 500 g kg1 organoclays and Houston Black clay samples. Two studies were conducted. In the first study, the samples were reacted with atrazine solution containing 10 mg l1 14C-labeled atrazine solution. Liquid scintillation analysis was used to determine the amount of atrazine desorbing from the substrates. In the second study, the samples were reacted with 10 mg l1 non-labeled atrazine solution. HPLC analysis was used to qualitatively determine the material that desorbed from the substrate—atrazine molecules, atrazine degradation products, or larger molecules derived from the surfactant-atrazine reaction product. For both studies, samples were reacted with atrazine solution for 3 d then centrifuged for 20 min at 622 · g and the supernatant decanted for analysis. Five milliliters of 0.01 M CaCl2 solution was then added to the centrifuge tubes, the tubes were shaken for 24 h, centrifuged for 20 min at 622 · g, and the supernatant decanted for analysis. This washing process was performed four times for each sample. In the first study, the concentration of atrazine in the supernatant was measured by liquid scintillation counting. The percentage of pesticide released for each step was calculated as x mx t m t1 % released ¼ 100 ð3Þ x m t1
x
1 where m t is the concentration (mg kg ) remaining bound x to the solid at time t, and m t1 is the concentration remaining bound to the solid at the previous step. Apparent Kd were calculated for from the data for each washing. Apparent Kf and 1n were calculated from data for all washings. The type of material released was qualitatively and quantitatively analyzed using high-performance liquid chromatography (HPLC). Analytes were separated using a Waters RP8 Symmetry Shield C8 column with a Waters HPLC chromatograph equipped with a photodiode array detector (PDA) (Waters Inc., Milford, MA) set at the 225-nm wavelength. The injection volume was 20 ll and the flow rate was 0.3 ml min1. The mobile phase consisted of acetonitrile–water–70 mM ammonium acetate buffer (10:75:15 volume basis) for the initial 5 min followed by acetonitrile–water–70 mM ammonium acetate buffer (80:5:15) for 20 min.
values calculated assuming 100% adsorption of applied surfactant to the clay (Fig. 2). Measured organic carbon content differed from expected values at low loading fractions (Fig. 2). The additional organic carbon was found to be caused by the retention of tetrahydrofuran, the solvent used to dissolve the surfactant. Sorption of tetrahydrofuran decreased as the amount of surfactant increased, indicating preferential sorption of the surfactant to the clay. In light of these unexpected findings, tests were conducted to assess the impact of tetrahydrofuran on atrazine sorption by the organoclays. These tests showed no variation in atrazine adsorption on the organoclay due to differences in the amount of tetrahydrofuran retained and further consideration of the effects of tetrahydrofuran were dismissed. X-ray diffraction analysis of the samples showed no significant change in basal spacing of the organoclays relative to that of the pure clay. While some of the surfactant and tetrahydrofuran, or parts of these compounds, probably entered the interlayer space, not all the loaded surfactant could have entered the clay’s interlayer space without altering the basal spacing. We expect that at high loading fractions the surfactant built upon itself, coating the clay particles. 3.2. Sequestration The quantity of atrazine sequestered from solution increased as the amount of surfactant present in the organoclay increased (Fig. 3). Rates of sequestration decreased with time (Fig. 3) suggesting progressive reaction with less accessible sites (Moreau and Mouvet, 1997). For all organoclays, there was generally <5% change in the amount of 350
300
250
Measured (g kg-1)
2.8. Desorption
707
200
150 organic C
100
organic N 1:1 line
3. Results 3.1. Organoclay characterization
50
0 0
The Gonzalez bentonite clay contained no nitrogen and very little organic C (0.42% on a total weight basis). The surfactant molecules contain 23.3% N and 65.7% C. Measured nitrogen contents of the organoclays agreed with
50
100
150
200
250
300
350
Expected (g kg-1) Fig. 2. Expected and measured organic C and N concentrations in manufactured organoclays. Concentrations are ratio of element mass to organoclay mass.
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S.L. Neitsch et al. / Chemosphere 64 (2006) 704–710 Table 1 Partition coefficients
HBC 0 gk g-1 (GB) 20 g kg-1 50 g kg-1 125 g kg-1 250 g kg-1 450 g kg-1
80
Adsorbent
60
40
20
0
0
24
48
72
96
120
Time (h) Fig. 3. Atrazine adsorption kinetics graphed as percent of atrazine removed from a 2 mg l1 solution by Houston Black clay (HBC), Gonzales bentonite (GB), and five organoclays with differing surfactant loading. Loading fraction is the ratio of surfactant mass to organoclay mass-a loading fraction of 450 g kg1 is an organoclay with 0.9 kg of surfactant to 1.0 kg of clay. Values whose error bars overlap are not significantly different at the 0.05 probability level.
atrazine sequestered over a 24-h period after 72 h of reaction. Of the total amount of atrazine sequestered during the first 72 h of reaction, approximately 70% was removed from solution within the first 24 h. While we (below) and Acosta et al. (2004) found the reaction of atrazine to the melamine-based surfactants mostly to be irreversible, a linear partitioning model described the atrazine sorption by the organoclays (Table 1). R2 values for the linear partitioning model were above 0.9 for all but the least loaded organoclay, the base Gonzales bentonite, and the Houston Black clay soil. The partition coefficients increased as the fraction of surfactant increased (Table 1). Kd values were similar to those reported for smectite loaded with Rhodamine-B (Borisover et al., 2001) or dimethyl distearyl ammonium chloride (Bottero et al., 1994). The difference in adsorption capability between the Houston Black clay and the organoclays became more apparent when the partition coefficients were adjusted for organic carbon content (Koc). The most efficient organoclays were loaded between 100 and 200 g kg1 (Fig. 4) and produced Koc > 5000 l kg1. 3.3. Desorption Release of atrazine from the 0, 10, 50, 200, 350 and 500 g kg1 organoclays and the Houston Black clay sample were measured with four sequential daylong washings. Total amounts of atrazine released from the solids for the 4-day period ranged from a high of 12% of initially sorbed atrazine for the 10, 50, and 200 g kg1 organoclays to 2% of initially sorbed atrazine for the Houston Black clay (Fig. 5). Percent of initially sorbed atrazine released during the four washings averaged 10% for the five organoclays
Kd (l kg1)
r2
Houston 1.37 (1.03–1.72)b Black clay Gonzales 9.50 (6.73–12.3) bentonite Organoclay loadingc (g kg1) 10 24.9 (21.9–27.9) 20 38.3 (35.5–41.1) 30 74.8 (69.1–80.5) 40 108 (103–113) 50 155 (148–163) 75 267 (239–295) 100 379 (357–400) 125 445 (420–471) 150 509 (471–547) 175 519 (471–566) 200 688 (633–743) 225 673 (591–755) 250 661 (631–690) 300 675 (619–731) 350 861 (778–944) 400 856 (803–908) 450 540 (508–573) 500 1050 (938–1160)
Kfa (mg11/n l1/n kg1)
1/na
0.796
1.12
0.979
0.885
4.42
1.101
0.969 0.972 0.970 0.988 0.988 0.946 0.983 0.983 0.971 0.956 0.967 0.926 0.989 0.964 0.953 0.980 0.981 0.943
28.1 42.8 72.3 119 174 291 411 484 559 576 813 779 699 864 1162 1180 809 1519
0.949 0.934 0.993 0.971 0.967 0.984 0.976 0.959 0.949 0.950 0.947 0.934 0.914 0.874 0.841 0.840 0.803 0.853
a
From linear regression of log-transformed data. Numbers in parenthesis are the limits of the confidence interval (CI) about the mean value. Means whose CI overlap are not significant at the 0.05 probability level. c Ratio of surfactant mass to organoclay mass. b
6000
5000
4000
KOC
Sequestration (% of applied)
100
3000
2000
1000
0
0
100
200
300
400
500
Loading Fraction (g kg-1) Fig. 4. Koc as function of targeted loading of surfactant on the organoclays. Loading fraction is the ratio of surfactant mass to organoclay mass. Koc values whose error bars overlap are not significantly different at the 0.05 probability level.
tested. The level of release implied that the majority of the bonds holding the atrazine to the surfactant were strong, possibly covalent (Fig. 1) as expected from previous studies (Acosta et al., 2004). Atrazine that was released from the organoclays did so primarily during the first
S.L. Neitsch et al. / Chemosphere 64 (2006) 704–710
Atrazine Released (% of sorbed)
25 HBC 0 g kg-1 (GB)
20
10 g kg-1 50 g kg-1 200 g kg-1
15
350 g kg-1 500 g kg-1
10
5
0 1st
2nd
3rd
4th
Washing Fig. 5. Desorption as percent of total adsorbed atrazine at the start of the particular washing for Houston Black clay (HBC), Gonzales bentonite (GB), and five organoclays with differing surfactant loading. Each point is the means of three replicates. Error bars represent 95% confidence intervals.
washing (first day). By the third and fourth steps, desorption of the remaining sorbed atrazine was less than 2.5% per washing. Apparent Kd values increased with each subsequent washing (data not shown). Apparent 1n values estimated from the desorption data were <0.04 for all loading fractions tested (data not shown) because of the high degree of retention. Characterization of desorption products by HPLC-PDA showed only atrazine molecules being released. Release of atrazine metabolites, surfactant molecules, or atrazine-surfactant reaction products during washing was negligible. 4. Discussion These studies showed that organoclays prepared with the surfactant 6-piperazin-1-yl-N,N 0 -bis-(1,1,3,3-tetramethyl-butyl)-(1,3,5)triazine-2,4-diamine are capable of sequestering considerable amounts of aqueous atrazine. Sorption of atrazine by the organoclays was described by linear partition isotherms, but it was not reversible. Statistically, the most efficient loading of the surfactant on the organoclays, with respect to amount of atrazine sequestered, were from 100 to 200 g kg1. As the 100 g kg1 loaded organoclay could be more widely dispersed in practice and clay is inexpensive, there appears to be no benefit in loading the surfactant-clay composite to greater values. The number of surfactant molecules to sequester one molecule of atrazine varied with the atrazine solution concentration. For the 100 g kg1 surfactant-loaded organoclay, the maximum atrazine sequestration was roughly 6 g atrazine per kg organoclay in equilibrium with a solution containing 9 mg l1 atrazine. This rate produced a sequestration of 60 mmol atrazine by 1 mol surfactant, and improving efficiency of the surfactant appears to be a suitable area for future research.
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Currently, the best available technology for removal of micropollutants such as atrazine from water is activated carbon (Li et al., 2002). Adsorption isotherms for atrazine on activated carbon are fit better with a Freundlich isotherm rather than the linear isotherm that fit sorption to our surfactant. Sorption of atrazine to other organoclays (Borisover et al., 2001) has also been reported to be nonlinear. Values of 1n for activated carbon reported in the literature range from 0.3 to 0.7 (Speth and Miltner, 1990; Adams and Watson, 1996; Knappe et al., 1998; Streat and Horner, 2000). Values of 1n for 6-piperazin-1-yl-N,N 0 -bis-(1,1,3,3-tetramethyl-butyl)-(1,3,5)triazine-2,4-diamine loaded clay decreased with increasing loading fraction, but were always >0.8 (Table 1). In addition to this difference, activated carbon lacks selectivity for micropollutants and adsorbs natural organic compounds (NOC) which are normally found in much greater concentrations than micropollutants (Streat and Horner, 2000; Li et al., 2002). As a consequence, sorption sites on activated carbon can saturate with NOC which limit sorption of targeted micropollutants. While we did not determine the selectivity of the organoclays for atrazine, we expect from the findings of Acosta et al. (2004) for similar nucleophilic surfactants that the binding sites on the organoclays studied here to favor triazines. Since adsorption of atrazine by the organoclays compared favorably with that reported for activated carbon without NOC, the organoclays may prove to be a suitable complimentary supplement to activated carbon for removal of atrazine from water. While the overall ability of the surfactant to sequester atrazine from solution was orders-of-magnitude greater than a natural soil, if the organoclay were unconfined in the natural environment and the surfactant were to degrade, the nature of the metabolites would need to be characterized. Due to the structural similarity between the surfactant and atrazine, there is a possibility that the surfactant may be susceptible to the same degradation mechanisms as atrazine.
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Bottero, J.Y., Khatib, K., Thomas, F., Jucker, K., Bersillon, J.L., Mallevialle, J., 1994. Adsorption of atrazine onto zeolites and organoclays, in the presence of background organics. Wat. Res. 28, 483–490. Brown, N.W., Roberts, E.P.L., Chasiotis, A., Cherdron, T., Sanghrajka, N., 2004. Atrazine removal using adsorption and electrochemical regeneration. Wat. Res. 38, 3067–3074. Deng, Y., 2001. Organic polymer–clay interactions and intercalation of kaolinite. Ph.D. diss. Texas A&M Univ., College Station, TX, United States. Drenianis, A., 1962. Quantitative gasometric determination of calcite and dolomite by using the Chittick apparatus. J. Sedimentary Petrol. 32, 520–529. Gee, G.W., Or, D., 2002. Particle-size analysis. In: Dane, J.H., Topp, G.C. (Eds.). Methods of Soil Analysis. Part 4: Physical Methods. SSSA Book Series No. 5. SSSA, Madison, WI, United States, pp. 255–293. Goss, D.W., 1992. Screening procedure for soil and pesticides relative to potential water quality impacts. Weed Technol. 6, 701–708. Hornsby, A.G., Wauchope, R.D., Herner, A.E., 1996. Pesticide Properties in the Environment. Springer-Verlag, New York. Knappe, D.R.U., Matsui, Y., Snoeyink, V.L., Roche, P., Prados, M.J., Bourbigot, M., 1998. Predicting the capacity of powdered activated carbon for trace organic compounds in natural waters. Environ. Sci. Technol. 32, 1694–1698. Larson, S.J., Capel, P.D., Majewski, M.S., 1997. Pesticides in surface waters: distribution, trends, and governing factorsPesticides in the Hydrologic System, vol. 3. Ann Arbor Press, Chelsea, MI, United States.
Li, Q.L., Snoeyink, V.L., Campos, C., Marinas, B.J., 2002. Displacement effect of NOM on atrazine adsorption by PACs with different pore size distributions. Environ. Sci. Technol. 36, 1510–1515. Montgomery, J.H., 1993. Agrochemicals Desk Reference: Environmental Data. Lewis Publishers, Chelsea, MI, United States. Moreau, C., Mouvet, C., 1997. Sorption and desorption of atrazine, deethylatrazine, and hydroxyatrazine by soil and aquifer solids. J. Environ. Qual. 26, 416–424. Nelson, D.W., Sommers, L.E., 1982. Total carbon, organic carbon and organic matter. In: Page, A.L. (Eds.), Method of Soil Analysis. Part II, second ed., Agronomy 9, 539–580. Speth, T.F., Miltner, R.J., 1990. Technical note—adsorption capacity of GAC for synthetic organics. J. Am. Water Works Assoc. 82, 72–75. Streat, M., Horner, D.J., 2000. Adsorption of highly soluble herbicides from water using activated carbon and hypercrosslinked polymers. Process. Saf. Environ. 78, 363–382. Wauchope, R.D., Butler, T.M., Hornsby, A.G., Augustijn-Beckers, P.W.M., Burt, J.P., 1992. The SCS/ARS/CES pesticide properties database for environmental decision-making. Rev. Environ. Contam. Toxicol. 123, 1–156. Zhang, W., Nowlan III, D.T., Thomson, L.M., Lackowski, W.M., Simanek, E.E., 2001. Orthogonal, convergent synthesis of dendrimers based on melamine with one or two unique surface sites for manipulation. J. Am. Chem. Soc. 123, 8914–8922. Zhang, W., Simanek, E.E., 2000. Dendrimers based on melamine: divergent and orthogonal, convergent synthesis of a G3 dendrimer. Org. Lett. 2, 843–845.
Melamine-based organoclay to sequester atrazine Susan L. Neitsch a, Kevin J. McInnes a,*, Scott A. Senseman a, G. Norman White a, Eric E. Simanek b a
Department of Soil and Crop Sciences, Heep Center, 370 Olsen Boulevard, Texas A&M University, College Station, TX 77843, United States b Department of Chemistry, Texas A&M University, College Station, TX 77843, United States Received 7 July 2005; received in revised form 3 November 2005; accepted 4 November 2005 Available online 5 January 2006
Abstract Sequestration of aqueous atrazine by organoclays prepared from the surfactant 6-piperazin-1-yl-N,N 0 -bis-(1,1,3,3-tetramethyl-butyl)(1,3,5)triazine-2,4-diamine and Gonzales bentonite was assessed using 14C-labeled atrazine. Organoclays with varying ratios of surfactant to clay were evaluated with respect to their ability to sequester atrazine from an aqueous solution. Organoclays containing 100– 200 g kg1 surfactant on a total weight basis provided the most efficient adsorption of atrazine, with apparent KOC values exceeding 5000 l kg1 at these loading fractions. Less than 12% of sequestered atrazine was released during four sequential daylong washings, supporting our expectation that the majority of the reaction of atrazine with the surfactant lead to irreversible chemical bond formation through nucleophilic aromatic substitution. 2005 Elsevier Ltd. All rights reserved. Keywords: Organoclay; Atrazine; Herbicide; Pesticide; Adsorption; Sequestration; Smectite; Bentonite
1. Introduction Triazine and acetanilide herbicides are widely used in American agriculture. Collectively, triazine and acetanilide herbicides possess low to moderate water solubility and soil sorptivity, and relatively long persistence in soil and water (Wauchope et al., 1992). As a consequence of these characteristics, triazine and acetanilide herbicides possess a moderate to high potential for offsite transport with storm water runoff and percolating water (Goss, 1992). Atrazine, a widely used triazine herbicide, has contaminated surface and ground water resources in regions where usage is widespread (Larson et al., 1997). Given the necessary efforts associated with remediation of contaminated waters, means of improving the efficiency of treatment would be desirable. The most common technique for removal of atrazine from water involves use of an activated carbon (activated
*
Corresponding author. Tel.: +1 979 845 5986; fax: +1 979 845 0456. E-mail address: [email protected] (K.J. McInnes).
0045-6535/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2005.11.033
charcoal) adsorption system. Unfortunately, activated carbon lacks selectivity for atrazine and adsorbs innocuous organic compounds. These organic compounds, such as soluble natural organic matter, compete with atrazine for adsorption sites causing inefficient sequestration or even displacement of adsorbed atrazine. As a consequence, it is desirable to investigate complimentary supplements to activated carbon such as inorganically intercalated clays (Abate and Masini, 2005) and novel regenerable organics (Brown et al., 2004). To remediate petroleum contaminated water, Alther (2002) used a granular organoclay as a pretreatment to activated carbon treatment. Treatment combining one-half equivalent of each the organoclay and the activated carbon was found to be appreciably more efficient than a full equivalent of either treatment alone. Similarly, it might be possible to enhance atrazine removal from water using a combination of an atrazine-sequestering organoclay and activated carbon. In the course of characterizing the properties of newly synthesized dendrimers based on melamine (2,4,6-triamino-1,3,5-triazine), researchers discovered that some of
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the dendrimer systems effectively sequestered triazine herbicides from solution (Zhang and Simanek, 2000; Zhang et al., 2001). This observation and the economical nature of producing these dendrimers were new discoveries. Analysis of the atrazine adsorption mechanism showed that monochlorotriazines, including atrazine and its major metabolites, are susceptible to nucleophilic aromatic substitution. In screening potential nucleophiles, constrained secondary amines were found to be the most reactive group. Subsequent research (Acosta, 2003) identified a promising surfactant [6-piperazin-1-yl-N,N 0 -bis-(1,1,3,3tetramethyl-butyl)-(1,3,5)triazine-2,4-diamine] that could be attached to smectite clay. In removal of triazine contaminated water, a sequestration strategy utilizing adsorption through nucleophilic aromatic substitution would provide a distinct advantage over that of activated carbon because it is selective for reactive molecules and the efficacy does not rely on an equilibrium position, but instead, irreversible chemical bond formation. When constrained secondary amines are attached to solid supports, it is possible to remove triazine herbicides from solution. Acosta et al. (2004) found that a polystyrene resin modified with secondary amines was very efficient at removing atrazine, desethylatrazine, desiopropylatrazine, and cyanazine from solution. The sequestration potential of the modified polystyrene resin was comparable to that of fresh activated carbon. Data indicated a covalent sequestration mechanism, and unlike with activated carbon, the presence of natural organic materials in water did not restrict removal of triazine contaminants. In addition, adsorption of triazines to the modified resin showed that the sequestration was relatively unaffected by pH and ionic strength. Use of inexpensive smectite clays as a solid support for similar nucleophilic surfactants has not been investigated. The objective of the research reported herein was to create organoclays with 6-piperazin-1-yl-N,N 0 -bis-(1,1,3,3tetramethyl-butyl)-(1,3,5)triazine-2,4-diamine and to investigate the ability of these organoclays to sequester atrazine from water. The studies characterized the apparent adsorption of atrazine on and desorption from organoclays created with the surfactant and a bentonite clay, and quantified the impact of the ratio of surfactant to clay in the organoclay. As a basis for comparison of the sequestration capacity for atrazine, experiments to measure apparent adsorption and desorption were performed on soil from the upper 10 cm of a Houston Black clay (fine, smectitic, thermic Udic Haplustert) pedon.
a chemical purity of 98% was purchased from Chem Service (West Chester, PA). 14C-ring-labeled atrazine with a chemical purity of 96.7%, a radioactive purity of 98.7%, and a specific activity of 340 TBq mol1 was obtained from Syngenta Crop Protection (Greensboro, NC). Atrazine was first dissolved in methanol then diluted with 0.01 M aqueous CaCl2 solution to produce desired concentrations of 0.01 mg l1 to 30 mg l1. The aqueous solutions contained 1% methanol. The atrazine solutions were created to provide 170 kBq l1 of working solution. The only exception was that the most dilute solution (0.01 mg l1) provided only 17 kBq l1.
2. Materials and methods
2.3. Organoclay
2.1. Atrazine
Surfactant–clay composites (organoclays) were created by sorbing the surfactant to the Ca-saturated smectite clay. Organoclays were prepared containing 10, 20, 30, 40, 50, 75, 100, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450 and 500 g kg1 surfactant on a total weight basis (i.e., 500 g kg1 consists of equal mass surfactant and clay).
The herbicide atrazine (C8H14ClN5) (Fig. 1) is a white powder or crystalline solid with a water solubility of 28 and 33 mg l1 at 20 and 25 C, respectively (Montgomery, 1993; Hornsby et al., 1996). Analytical grade atrazine with
clay surfactant H N
H N
N N
H N
N
N N
N H
N N N H
Cl N N H
N
N
+
N N
sequestered atrazine N H
+
N N
H N
N
N H
HCl
atrazine
Fig. 1. Potential covalent bonding of atrazine with 6-piperazin-1-yl-N,N 0 bis-(1,1,3,3-tetramethyl-butyl)-(1,3,5)triazine-2,4-diamine organoclay.
2.2. Surfactant and clay The surfactant [6-piperazin-1-yl-N,N 0 -bis-(1,1,3,3tetramethyl-butyl)-1,3,5-triazine-2,4-diamine; C23 H46 Nþ 7] (Fig. 1) used in the experiments was prepared at Texas A&M University. A summary of the surfactant synthesis was provided by Acosta (2003). The clay was Gonzales bentonite (GB) obtained from Southern Clay Products (Gonzales, TX). The clay was pretreated following the procedure detailed in Deng (2001) to remove particles >2 lm diameter and to saturate the clay’s cation exchange sites with Ca. Deng (2001) confirmed that the mineral composition of the Gonzales clay was smectite using X-ray diffraction and Fourier-transform infrared spectroscopy. Cation exchange capacity for the smectite was 81 cmolc kg1, specific area was 790 m2 g1, charge density was 1.0 lmolc m2, and the pH was 6.4 (Deng, 2001).
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For each composition, 500 mg of clay was suspended in 20 ml of distilled water in a 50-ml polypropylene conical centrifuge tube. The required amount of surfactant was dissolved in 5 ml of tetrahydrofuran and added to the clay suspension by pipette. The suspensions were then shaken for 24 h at 25 C. The resulting organoclays were centrifuged at 15 000 · g for 10 min and the supernatant was decanted. The organoclays were washed three times with distilled water to remove tetrahydrofuran. Washing was done by adding 15 ml distilled water to the organoclay sediment, shaking until all the sediment was suspended, centrifuging at 15 000 · g for 10 min, and then decanting the supernatant. The organoclays were then freeze-dried and ground to a powder. The amount of surfactant sorbed to the clay was verified with a Carlo Erba NA-1500 combustion unit (CE Elantech, Lakewood, NJ) by analyzing for organic C and N contents.
counting of 1 ml of the supernatant mixed with 10 ml of Ecolite liquid scintillation cocktail (ICN Biomedicals, Costa Mesa, CA) using a Beckman LS6500 multipurpose scintillation counter (Beckman Instruments, Inc., Fullerton, CA). The counting time was set to obtain a maximum 2% standard deviation for the disintegration mean. Centrifuge tubes containing no solid material were prepared in triplicate and processed alongside the sample centrifuge tubes. The concentration bound to the solid was calculated as the difference between the concentrations in solution of centrifuge tubes containing no solid material and of the samples with the solid. Background radioactivity was measured by processing centrifuge tubes with solid material to which 50 ll methanol was added in place of the atrazine solution.
2.4. Soil sample
Temporal sorption studies were carried out with 0, 20, 50, 125, 250 and 450 g kg1 organoclays and Houston Black soil at a solution concentration of 2 mg l1 atrazine. The amount of atrazine sequestered from solution was measured at 0, 4, 8, 24, 48, 72, 96, and 120 h. The sequestration percentages were calculated as
The soil sample was collected from the top 10 cm of a Houston Black clay (HBC) pedon located on the Blackland Research Center, Texas Agricultural Experiment Station. The sample site (31 3 0 11.600 N, 97 21 0 0.900 W) was located at the base of a 1–3% slope. The soil sample was air dried and ground to pass a 2-mm sieve, then pulverized with a laboratory ring pulverizer (Armstrong Inc., Chicago, IL). Particle size analysis of unpulverized clay using the hydrometer method (Gee and Or, 2002) established a size distribution of 13% sand (2000–50 lm), 34% silt (50– 2 lm), and 53% clay (<2 lm). A pulverized sample had 1.5% organic carbon and 31.4% CaCO3 equivalent (29.9% calcite and 1.4% dolomite) as determined by the Soil Characterization Laboratory, Soil & Crop Sciences Dept., Texas A&M University. Total carbon was determined using combustion (Nelson and Sommers, 1982). Calcium carbonate equivalent was measured using HCl treatment and a Chittick apparatus (Drenianis, 1962). Organic carbon was calculated as the difference between total carbon and CaCO3 equivalent. 2.5. Experimental protocols All sorption and desorption experiments were carried out in 35-ml glass centrifuge tubes with Teflon-lined caps at 25 C. To minimize variation in atrazine adsorption by the clay component of the organoclay, samples were weighed so that all vials contained 15 mg of Gonzales bentonite clay. Triplicate 15 mg clay-equivalent samples were weighed into centrifuge tubes and equilibrated for 24 h in 5 ml of 0.01 M CaCl2 solution. After the 24-h equilibration period, 50 ll of a desired atrazine solution was added to each centrifuge tube by pipette. The tubes were shaken for a length of time that varied depending on the specific experiment then centrifuged for 20 min at 622 · g and the supernatant decanted. The concentration of atrazine in the decanted liquid was measured by liquid scintillation
2.6. Sequestration kinetics
% sequestered ¼ 100
Ci Ce Ci
ð1Þ
where Ci is the initial concentration in solution (mg l1) and Ce is the concentration in solution at the end of the reaction period (mg l1). The concentration of atrazine sorbed mx to the organoclay or soil was calculated as x vsol ¼ ðC i C e Þ m msub
ð2Þ
where x is the mass of atrazine sorbed (mg), m is the mass of organoclay or soil (kg), vsol is the total solution volume (l), and msub is the total mass (kg) of substrate exposed to the solution. Reaction was considered complete when the difference in the amount of atrazine sorbed from solution between the current and subsequent 24 h time step was less than or equal to 5% (Moreau and Mouvet, 1997). 2.7. Sorption isotherms Apparent sorption isotherms were determined for all organoclays and Houston Black clay using solution concentrations selected to represent concentrations that might be found in runoff (0.01 mg l1) up to the saturation value (30 mg l1), namely, 0.01, 0.1, 1, 2, 5, 10, 20 and 30 mg l1 atrazine. Apparent partition coefficients Kd were calculated as the slope of a linear regression through the origin of mx against Ce. Organic-carbon-adjusted partition coefficients Koc were calculated as the ratio of Kd to the mass fraction (kg kg1) organic carbon OC. Freundlich partition coefficients Kf and 1n were calculated from log-linearized adsorption data.
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Desorption studies were carried out with 0, 10, 50, 200, 350 and 500 g kg1 organoclays and Houston Black clay samples. Two studies were conducted. In the first study, the samples were reacted with atrazine solution containing 10 mg l1 14C-labeled atrazine solution. Liquid scintillation analysis was used to determine the amount of atrazine desorbing from the substrates. In the second study, the samples were reacted with 10 mg l1 non-labeled atrazine solution. HPLC analysis was used to qualitatively determine the material that desorbed from the substrate—atrazine molecules, atrazine degradation products, or larger molecules derived from the surfactant-atrazine reaction product. For both studies, samples were reacted with atrazine solution for 3 d then centrifuged for 20 min at 622 · g and the supernatant decanted for analysis. Five milliliters of 0.01 M CaCl2 solution was then added to the centrifuge tubes, the tubes were shaken for 24 h, centrifuged for 20 min at 622 · g, and the supernatant decanted for analysis. This washing process was performed four times for each sample. In the first study, the concentration of atrazine in the supernatant was measured by liquid scintillation counting. The percentage of pesticide released for each step was calculated as x mx t m t1 % released ¼ 100 ð3Þ x m t1
x
1 where m t is the concentration (mg kg ) remaining bound x to the solid at time t, and m t1 is the concentration remaining bound to the solid at the previous step. Apparent Kd were calculated for from the data for each washing. Apparent Kf and 1n were calculated from data for all washings. The type of material released was qualitatively and quantitatively analyzed using high-performance liquid chromatography (HPLC). Analytes were separated using a Waters RP8 Symmetry Shield C8 column with a Waters HPLC chromatograph equipped with a photodiode array detector (PDA) (Waters Inc., Milford, MA) set at the 225-nm wavelength. The injection volume was 20 ll and the flow rate was 0.3 ml min1. The mobile phase consisted of acetonitrile–water–70 mM ammonium acetate buffer (10:75:15 volume basis) for the initial 5 min followed by acetonitrile–water–70 mM ammonium acetate buffer (80:5:15) for 20 min.
values calculated assuming 100% adsorption of applied surfactant to the clay (Fig. 2). Measured organic carbon content differed from expected values at low loading fractions (Fig. 2). The additional organic carbon was found to be caused by the retention of tetrahydrofuran, the solvent used to dissolve the surfactant. Sorption of tetrahydrofuran decreased as the amount of surfactant increased, indicating preferential sorption of the surfactant to the clay. In light of these unexpected findings, tests were conducted to assess the impact of tetrahydrofuran on atrazine sorption by the organoclays. These tests showed no variation in atrazine adsorption on the organoclay due to differences in the amount of tetrahydrofuran retained and further consideration of the effects of tetrahydrofuran were dismissed. X-ray diffraction analysis of the samples showed no significant change in basal spacing of the organoclays relative to that of the pure clay. While some of the surfactant and tetrahydrofuran, or parts of these compounds, probably entered the interlayer space, not all the loaded surfactant could have entered the clay’s interlayer space without altering the basal spacing. We expect that at high loading fractions the surfactant built upon itself, coating the clay particles. 3.2. Sequestration The quantity of atrazine sequestered from solution increased as the amount of surfactant present in the organoclay increased (Fig. 3). Rates of sequestration decreased with time (Fig. 3) suggesting progressive reaction with less accessible sites (Moreau and Mouvet, 1997). For all organoclays, there was generally <5% change in the amount of 350
300
250
Measured (g kg-1)
2.8. Desorption
707
200
150 organic C
100
organic N 1:1 line
3. Results 3.1. Organoclay characterization
50
0 0
The Gonzalez bentonite clay contained no nitrogen and very little organic C (0.42% on a total weight basis). The surfactant molecules contain 23.3% N and 65.7% C. Measured nitrogen contents of the organoclays agreed with
50
100
150
200
250
300
350
Expected (g kg-1) Fig. 2. Expected and measured organic C and N concentrations in manufactured organoclays. Concentrations are ratio of element mass to organoclay mass.
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S.L. Neitsch et al. / Chemosphere 64 (2006) 704–710 Table 1 Partition coefficients
HBC 0 gk g-1 (GB) 20 g kg-1 50 g kg-1 125 g kg-1 250 g kg-1 450 g kg-1
80
Adsorbent
60
40
20
0
0
24
48
72
96
120
Time (h) Fig. 3. Atrazine adsorption kinetics graphed as percent of atrazine removed from a 2 mg l1 solution by Houston Black clay (HBC), Gonzales bentonite (GB), and five organoclays with differing surfactant loading. Loading fraction is the ratio of surfactant mass to organoclay mass-a loading fraction of 450 g kg1 is an organoclay with 0.9 kg of surfactant to 1.0 kg of clay. Values whose error bars overlap are not significantly different at the 0.05 probability level.
atrazine sequestered over a 24-h period after 72 h of reaction. Of the total amount of atrazine sequestered during the first 72 h of reaction, approximately 70% was removed from solution within the first 24 h. While we (below) and Acosta et al. (2004) found the reaction of atrazine to the melamine-based surfactants mostly to be irreversible, a linear partitioning model described the atrazine sorption by the organoclays (Table 1). R2 values for the linear partitioning model were above 0.9 for all but the least loaded organoclay, the base Gonzales bentonite, and the Houston Black clay soil. The partition coefficients increased as the fraction of surfactant increased (Table 1). Kd values were similar to those reported for smectite loaded with Rhodamine-B (Borisover et al., 2001) or dimethyl distearyl ammonium chloride (Bottero et al., 1994). The difference in adsorption capability between the Houston Black clay and the organoclays became more apparent when the partition coefficients were adjusted for organic carbon content (Koc). The most efficient organoclays were loaded between 100 and 200 g kg1 (Fig. 4) and produced Koc > 5000 l kg1. 3.3. Desorption Release of atrazine from the 0, 10, 50, 200, 350 and 500 g kg1 organoclays and the Houston Black clay sample were measured with four sequential daylong washings. Total amounts of atrazine released from the solids for the 4-day period ranged from a high of 12% of initially sorbed atrazine for the 10, 50, and 200 g kg1 organoclays to 2% of initially sorbed atrazine for the Houston Black clay (Fig. 5). Percent of initially sorbed atrazine released during the four washings averaged 10% for the five organoclays
Kd (l kg1)
r2
Houston 1.37 (1.03–1.72)b Black clay Gonzales 9.50 (6.73–12.3) bentonite Organoclay loadingc (g kg1) 10 24.9 (21.9–27.9) 20 38.3 (35.5–41.1) 30 74.8 (69.1–80.5) 40 108 (103–113) 50 155 (148–163) 75 267 (239–295) 100 379 (357–400) 125 445 (420–471) 150 509 (471–547) 175 519 (471–566) 200 688 (633–743) 225 673 (591–755) 250 661 (631–690) 300 675 (619–731) 350 861 (778–944) 400 856 (803–908) 450 540 (508–573) 500 1050 (938–1160)
Kfa (mg11/n l1/n kg1)
1/na
0.796
1.12
0.979
0.885
4.42
1.101
0.969 0.972 0.970 0.988 0.988 0.946 0.983 0.983 0.971 0.956 0.967 0.926 0.989 0.964 0.953 0.980 0.981 0.943
28.1 42.8 72.3 119 174 291 411 484 559 576 813 779 699 864 1162 1180 809 1519
0.949 0.934 0.993 0.971 0.967 0.984 0.976 0.959 0.949 0.950 0.947 0.934 0.914 0.874 0.841 0.840 0.803 0.853
a
From linear regression of log-transformed data. Numbers in parenthesis are the limits of the confidence interval (CI) about the mean value. Means whose CI overlap are not significant at the 0.05 probability level. c Ratio of surfactant mass to organoclay mass. b
6000
5000
4000
KOC
Sequestration (% of applied)
100
3000
2000
1000
0
0
100
200
300
400
500
Loading Fraction (g kg-1) Fig. 4. Koc as function of targeted loading of surfactant on the organoclays. Loading fraction is the ratio of surfactant mass to organoclay mass. Koc values whose error bars overlap are not significantly different at the 0.05 probability level.
tested. The level of release implied that the majority of the bonds holding the atrazine to the surfactant were strong, possibly covalent (Fig. 1) as expected from previous studies (Acosta et al., 2004). Atrazine that was released from the organoclays did so primarily during the first
S.L. Neitsch et al. / Chemosphere 64 (2006) 704–710
Atrazine Released (% of sorbed)
25 HBC 0 g kg-1 (GB)
20
10 g kg-1 50 g kg-1 200 g kg-1
15
350 g kg-1 500 g kg-1
10
5
0 1st
2nd
3rd
4th
Washing Fig. 5. Desorption as percent of total adsorbed atrazine at the start of the particular washing for Houston Black clay (HBC), Gonzales bentonite (GB), and five organoclays with differing surfactant loading. Each point is the means of three replicates. Error bars represent 95% confidence intervals.
washing (first day). By the third and fourth steps, desorption of the remaining sorbed atrazine was less than 2.5% per washing. Apparent Kd values increased with each subsequent washing (data not shown). Apparent 1n values estimated from the desorption data were <0.04 for all loading fractions tested (data not shown) because of the high degree of retention. Characterization of desorption products by HPLC-PDA showed only atrazine molecules being released. Release of atrazine metabolites, surfactant molecules, or atrazine-surfactant reaction products during washing was negligible. 4. Discussion These studies showed that organoclays prepared with the surfactant 6-piperazin-1-yl-N,N 0 -bis-(1,1,3,3-tetramethyl-butyl)-(1,3,5)triazine-2,4-diamine are capable of sequestering considerable amounts of aqueous atrazine. Sorption of atrazine by the organoclays was described by linear partition isotherms, but it was not reversible. Statistically, the most efficient loading of the surfactant on the organoclays, with respect to amount of atrazine sequestered, were from 100 to 200 g kg1. As the 100 g kg1 loaded organoclay could be more widely dispersed in practice and clay is inexpensive, there appears to be no benefit in loading the surfactant-clay composite to greater values. The number of surfactant molecules to sequester one molecule of atrazine varied with the atrazine solution concentration. For the 100 g kg1 surfactant-loaded organoclay, the maximum atrazine sequestration was roughly 6 g atrazine per kg organoclay in equilibrium with a solution containing 9 mg l1 atrazine. This rate produced a sequestration of 60 mmol atrazine by 1 mol surfactant, and improving efficiency of the surfactant appears to be a suitable area for future research.
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Currently, the best available technology for removal of micropollutants such as atrazine from water is activated carbon (Li et al., 2002). Adsorption isotherms for atrazine on activated carbon are fit better with a Freundlich isotherm rather than the linear isotherm that fit sorption to our surfactant. Sorption of atrazine to other organoclays (Borisover et al., 2001) has also been reported to be nonlinear. Values of 1n for activated carbon reported in the literature range from 0.3 to 0.7 (Speth and Miltner, 1990; Adams and Watson, 1996; Knappe et al., 1998; Streat and Horner, 2000). Values of 1n for 6-piperazin-1-yl-N,N 0 -bis-(1,1,3,3-tetramethyl-butyl)-(1,3,5)triazine-2,4-diamine loaded clay decreased with increasing loading fraction, but were always >0.8 (Table 1). In addition to this difference, activated carbon lacks selectivity for micropollutants and adsorbs natural organic compounds (NOC) which are normally found in much greater concentrations than micropollutants (Streat and Horner, 2000; Li et al., 2002). As a consequence, sorption sites on activated carbon can saturate with NOC which limit sorption of targeted micropollutants. While we did not determine the selectivity of the organoclays for atrazine, we expect from the findings of Acosta et al. (2004) for similar nucleophilic surfactants that the binding sites on the organoclays studied here to favor triazines. Since adsorption of atrazine by the organoclays compared favorably with that reported for activated carbon without NOC, the organoclays may prove to be a suitable complimentary supplement to activated carbon for removal of atrazine from water. While the overall ability of the surfactant to sequester atrazine from solution was orders-of-magnitude greater than a natural soil, if the organoclay were unconfined in the natural environment and the surfactant were to degrade, the nature of the metabolites would need to be characterized. Due to the structural similarity between the surfactant and atrazine, there is a possibility that the surfactant may be susceptible to the same degradation mechanisms as atrazine.
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