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Ind. Eng. Chem. Res. 2005, 44, 1634-1639
Piperidine-Functionalized Supports Sequester Atrazine from Solution Emily Hollink, Shane E. Tichy, and Eric E. Simanek* Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255
To achieve irreversible (covalent) capture of monochlorotriazine herbicides such as atrazine, three commercially available poly(styrene) resins (Janda Jels) that vary in size were modified with reactive isonipecotic acid groups. The materials were characterized using the Kaiser test, infrared (IR) spectroscopy, and titration experiments that indicated the relative number of accessible nucleophiles. Sequestration of atrazine from aqueous solutions using these polymers was monitored using liquid chromatography-mass spectrometry. Large (50-100 mesh) and medium (100-200 mesh) resins remove all detectable traces of atrazine from water at similar rates, whereas the smallest resin (200-400 mesh) removes the herbicide at a slower rate. Generally, the sequestration of atrazine by reactive, nucleophilic resins seems to be a suitable strategy for additional inquiry, particularly as an alternative to the current method of wastewater treatmentsactivated charcoal. Introduction Atrazine is one of the most commonly used herbicides in North America for the control of broadleaf weeds.1 Accordingly, atrazine and its metabolites represent a significant source of contamination in water in agricultural areas.1,2 Evidence of the carcinogenic effect to humans upon chronic exposure to triazine herbicides and their metabolites is mounting.3 Several methods that remove triazine herbicides from various media have been investigated, including adsorption onto solid supports such as clay minerals,4 and linear,5 cross-linked,6 or molecularly imprinted polymers.7 In practice, activated charcoal is used to remove triazine herbicides from drinking water. Although efficient and rapid in clean water, this method is less efficient for environmental applications because of the competition between atrazine and innocuous organic material which significantly reduces the lifetime of the charcoal beds.8 We are examining an approach based on the reactivity of monochlorotriazine herbicides resulting in covalent sequestration of atrazine rather than equilibrium adsorption/desorption processes. This strategy could ultimately be implemented with existing technologies, including modifying textiles similar to those already used for erosion control9 or functionalizing flocculants that could be reused in a subsequent herbicide application.10 We have recently demonstrated that commercially available poly(styrene) resins modified with piperidine or piperazine groups are effective solid supports for the covalent sequestration of atrazine.11 Using these resins, we showed that the cyclic secondary amine piperidine reacted to form a covalent bond between the resin and the triazine herbicide (see eq 1). This reactivity is consistent with solution-phase reactivity of monochlorotriazines. However, these studies were limited in scope, because only one piperidine-containing resin was available. This report describes the chemical modification of a series of commercially available cross-linked poly* To whom correspondence should be addressed. E-mail:
[email protected].
(styrene) polymers of different mesh sizesslarge (50100 mesh, denoted as L), medium (100-200 mesh, denoted as M), and small (200-400 mesh, denoted as S)susing a standard peptide coupling protocol12 to generate BOC-protected piperidine groups that provide secondary amines upon deprotection with trifluoroacetic acid (Scheme 1). To verify the reproducibility of the resins to sequester atrazine from solution, one synthetic strategy was repeated (designated by the superscripts 1 or 2) and then compared to the products of an additional synthetic strategy (designated by the superscript 3). The resins have been characterized using infrared (IR) spectroscopy, and titrations performed in water have determined the accessible amount of nucleophile. In addition, the kinetics of the atrazine uptake has been monitored using liquid chromatography-mass spectroscopy (LC-MS). Experimental Section Reagents. ACS-grade solvents were used for all the synthetic preparations. Distilled water was obtained inhouse. Cross-linked poly(styrene) resins13 modified with p-aminomethyl groups (Janda Jels), trifluoroacectic acid (TFA), and diisopropylethylamine (DIPEA) were purchased from Aldrich Chemical Co., isonipecotic acid from Acros Organics, di-tert-butyl dicarbonate fom Fluka, benzotriazole-1-yl-oxy-tris-pyrrolidino-phosphonium hexafluorophosphate (PyBOP) from NovaBiochem, and 1-hydroxybenzotriazole monohydrate (HOBt‚H2O) from
10.1021/ie0494566 CCC: $30.25 © 2005 American Chemical Society Published on Web 02/12/2005
Ind. Eng. Chem. Res., Vol. 44, No. 6, 2005 1635 Scheme 1. Nomenclature and Preparation of the Materials for Studya
a The letters L, M, and S refer to the size of the resin. The critical functional group is identified as NH2, BOC, or NH; BOCINP is 1-BOC-isonipecotic acid. The number (n ) 1-3) refers to the batch number of an otherwise identical product. Batches 1 and 2 were prepared using a two-step procedure, whereas batch 3 was prepared using a one-pot procedure; the ability of the separate batches to sequester atrazine was assessed separately to ensure reproducible results. TFA is trifluoroacetic acid.
Chem-Impex International; all reagents and solvents were used without purification. The reagent 1-BOCisonipecotic acid was prepared according to a literature method, and the spectroscopic features of the product were in agreement with reported values.14 Preparation of the Resins. The general protocol for modification of the resins is described. The number (n ) 1-3) refers to the batch number of an otherwise identical product. These three batches were treated as separate resin products for the purposes of assessing their ability to sequester atrazine to determine if the results were dependent on the resin lot. Synthesis of BOC-Protected Resins (L1-BOC, M1BOC, S1-BOC, L2-BOC, M2-BOC, S2-BOC). A measured amount of the Janda Jel (L-NH2, M-NH2, and S-NH2, 335 mg, ca. 0.335 mmol amine) was placed into a syringe reactor that was equipped with a frit, and CH2Cl2 (10 mL) was drawn into the syringe. The slurry was shaken in a wrist-action shaker for 1 h at room temperature (RT) to pre-swell the resin for the reaction. The solvent was removed, and a clear, freshly prepared solution of 1-BOC-isonipecotic acid (384 mg, 1.67 mmol), PyBOP (854 mg, 1.64 mmol), HOBt‚H2O (226 mg, 1.67 mmol), and DIPEA (0.58 mL, 3.35 mmol) in CH2Cl2 (10 mL) was drawn into the syringe. The slurry was shaken in a wrist-action shaker for 15 h at RT, after which time the solution was removed. Complete reaction was assumed on the basis of a negative Kaiser test result.15 The resin was thoroughly washed (three times, 10 mL each: CH2Cl2, MeOH, dimethylformamide (DMF), MeOH, CH2Cl2) and was isolated at this stage, followed by drying in vacuo.
Deprotection of the Resins. A measured amount of the BOC-protected resin (453 mg, ca. 0.453 mmol) was placed into a syringe reactor that was equipped with a frit, and CH2Cl2 (10 mL) was withdrawn into the syringe. The slurry was shaken in a wrist-action shaker for 1 h at RT to pre-swell the resin for the reaction. A solution of 50:50 v/v TFA:CH2Cl2 (10 mL total volume) was drawn into the syringe, and the slurry was shaken in a wrist-action shaker for 30 min. The solution was removed, and the resin was thoroughly washed (three times, 10 mL each: CH2Cl2, MeOH, DMF, MeOH, CH2Cl2) before drying in vacuo for at least 12 h. In the case of the precursor resin with a mesh size of 200-400, different lots were provided from Aldrich Chemical Co., and these were subjected to separate modification to assess this influence on atrazine sequestration. One-Pot Strategy for the Synthesis of L3-NH, M3NH, and S3-NH. Resins were prepared using the previously described method without isolation of the BOC-protected intermediate. Instead, deprotection by TFA was achieved in situ from the amide-coupled product, followed by exhaustive washing and preswelling of the resin with CH2Cl2. Titrations. Titration experiments were performed in triplicate using resins that were prepared using a onepot method (batch 3); these data were confirmed by performing a single titration with each of the batches (1, 2) prepared using the two-step protocol. A representative procedure is described. All amine-functionalized resins were pretreated16 before titration by washing with 0.1 M NaOH (aq), then stirring in a fresh solution of 0.1 M NaOH (aq) for 12 h. The resin was washed with distilled water until the effluent had a neutral pH and then was dried in vacuo for 48 h. Following transfer to a dry flask (∼0.5 g, accurately weighed; theoretical loading of nucleophile was 1 m(eq)/g of resin), standard aqueous HCl (∼0.1 M, 15 mL) and 0.030 M of NaCl (10 mL) were added. The slurry was stirred at RT for 30 min, after which time an indicator (phenolphthalein) was added, and the slurry was back-titrated with standard aqueous NaOH (∼0.1 M). The same values were determined if a potentiometric titration was performed instead of the use of indicator solution. Standards. Atrazine was prepared by a modification of a literature method,17 and stock solutions were prepared by weighing at least 10 mg of atrazine into a 500 mL volumetric flask. The atrazine was dissolved in a minimum amount of ACS-grade methanol ( L > S, and are entirely consistent with the rate data.19 Because not all of the atrazine was removed from the 100 ppb solution over a 24 h period of time, the atrazine uptake was monitored over a period of two weeks to determine if all detectable atrazine could be removed (Figure 4). The same sets of sequestration experiments were performed using the precursor amine and BOCprotected resins to exclude their utility for this application. For two of the three piperidine-functionalized resins, all detectable atrazine was removed from solution over a two week period. Interestingly, the smallestsized resin beads (S-NH) had the slowest uptake of atrazine and removed 84% of the atrazine from solution
Figure 5. Linear regression to obtain pseudo-first-order rate constants for atrazine uptake by (a) S-NH and (b) M-NH and L-NH (where ai is the initial concentration of atrazine and p is the concentration of the covalent product, calculated from the difference between the measured concentration of atrazine in solution pre-sequestration and post-sequestration).
over the same time period. We speculate that the smallest resin beads probably did not swell as well as the larger beads in aqueous solution, which could account for the slower observed rate. Several additional control experiments were performed to rule out leaching of material from the resins for the duration of the sequestration conditions studied. The piperidine-substituted resin was shaken in distilled water for a period of 2 weeks, and LC-MS indicated that nothing was being leached from the resins that could be detected using this method. To further support that the resins were not leaching organic residues under these conditions while the atrazine sequestration was performed, an atrazine solution that was shaken with the piperidine-substituted resins for a period of two weeks was also subjected to LC-MS. Similarly, nothing was detected in solution using this technique. Kinetic Analysis. The pseudo-first-order rate constants for atrazine sequestration were obtained for the resins L-NH, M-NH, and S-NH (see Figure 5 and Table 3).20 The equation ai - p ) aie-kt (where ai is the initial concentration of atrazine, p the concentration of the covalent product, calculated from the difference between the measured concentration of atrazine in solution preand post-sequestration, and k the pseudo-first-order rate constant) indicates that the concentration of the reactant should decrease exponentially with time if the reaction is first-order. Analysis of these data suggests that the uptake of atrazine by the resins L-NH, M-NH,
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Table 3. Pseudo-First-Order Rate Constants for the Uptake of Atrazine by Piperidine-Functionalized Resins
a
resin
pseudo-first-order rate constanta (s-1)
L M S
8.4 × 10-6 (0.3) 9.28 × 10-6 (0.02) 1.55 × 10-6 (0.01)
Relative errors are provided in parentheses.
and S-NH can be described by a pseudo-first-order rate of reaction. Although the rate constants are on the order of 10-6 s-1, these are significantly faster than the pseudo-first-order rate constant for the minimum rate of hydrolysis (1.24 × 10-11 s-1).21 Consistent with previous studies, this rate of atrazine uptake is slower than the rate at which activated charcoal removes natural organic matter.11 However, this could prove advantageous, should these materials be applied as erosion control textiles,9b because the benefits of the herbicide would not perturbed but the textiles could remove the excess herbicide before it reaches the groundwater. Control experiments performed with LNH2, M-NH2, and S-NH2 and with L-BOC, M-BOC, and S-BOC indicated that only minimal amounts of atrazine were removed from solution over a two-week time frame (
