Article pubs.acs.org/est
Microwave-Induced Degradation of Atrazine Sorbed in Mineral Micropores Erdan Hu, Hefa Cheng,* and Yuanan Hu State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China S Supporting Information *
ABSTRACT: The herbicide atrazine is a common pollutant in reservoirs and other sources of drinking water worldwide. The adsorption of atrazine from water onto zeolites CBV-720 and 4A, mesoporous silica MCM-41, quartz sand, and diatomite, and its microwave-induced degradation when sorbed on these minerals, were studied. Dealuminated HY zeolite CBV-720 exhibited the highest atrazine sorption capacity among the mineral sorbents because of its high micropore volume, suitable pore sizes, and surface hydrophobicity. Atrazine sorbed on the minerals degraded under microwave irradiation due to interfacial selective heating by the microwave, while atrazine in aqueous solution and associated with PTFE powder was not affected. Atrazine degraded rapidly in the micropores of CBV-720 under microwave irradiation and its degradation intermediates also decomposed with further irradiation, suggesting atrazine could be fully mineralized. Two new degradation intermediates of atrazine, 3,5-diamino-1,2,4-triazole and guanidine, were first identified in this study. The evolution of degradation intermediates and changes in infrared spectra of CBV-720 after microwave irradiation consistently indicate the creation of microscale hot spots in the micropores and the degradation of atrazine following a pyrolysis mechanism. These results indicate that microporous mineral sorption coupled with microwave-induced degradation could serve as an efficient treatment technology for removing atrazine from drinking water.
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reaching the μg/L level.7 Atrazine contamination of soil, surface water, and groundwater is also prevalent in China. For example, atrazine levels in the Liao River, which is located in a major corn production zone, were up to 1.6 μg/L in late spring.8 Because of concerns about the ubiquitous and unpreventable surface water and groundwater contamination caused by atrazine, it has been banned in the European Union (EU) in 2004,3 while most countries, including U.S. and China, continue to use it on large scales. Atrazine is a known endocrine disruptor that interferes with reproduction and development, and it may cause cancer.3 It has been classified as a possible human carcinogen by the U.S. Environmental Protection Agency (USEPA). The EU has set pesticide standards for drinking waters at a maximum permissible concentration for a particular pesticide at 0.1 μg/ L and the sum of all pesticides (including their degradation products after drinking water treatment) at 0.5 μg/L.9 The maximum contaminant level (MCL) of atrazine set by the USEPA is 3 μg/L, while its drinking water guideline value set by the World Health Organization (WHO) is 2 μg/L. In China,
INTRODUCTION Atrazine, or 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine, is a widely used herbicide to stop pre- and postemergence broadleaf and grassy weeds in major crops, such as corn, grain sorghum, and sugar cane.1 It is one of the most effective, affordable, and trusted agricultural herbicides. The global consumption of atrazine in the late 1980s was between 70 000 and 90 000 tonnes/year.2 It was estimated that 27 000−36 000 tonnes of atrazine was used annually in the United States (U.S.) in the early 2000s, with 85% of it being applied on corn fields.3 Approximately 2800 tonnes of atrazine was applied in China in 2000, and its use grew at an average annual rate of 20%.4 Atrazine is also one of the most controversial herbicides in the world because of its high potential to contaminate surface water and groundwater. The chlorine, methylthioether, and Nalkyl substituents on the s-triazine ring all hinder microbial metabolism, thus atrazine degrades more slowly compared to most other herbicides used today.5 With a moderate water solubility (33 mg/L at pH 7 and 22 °C) and relatively low Kd (0.20−12.6) and Koc (40−394), atrazine is rather mobile and can be carried with runoff to surface water, percolate to groundwater, or be retained in the soil once applied to the field.6 Because of its widespread use, atrazine is one of the most commonly detected herbicides in surface and groundwater in Europe and North America with concentrations frequently © 2012 American Chemical Society
Received: Revised: Accepted: Published: 5067
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a limit of 3 μg/L was set for source water in 2002, followed by a standard of 3 μg/L for drinking water in 2006. Conventional drinking water treatment systems are ineffective at removing atrazine from the water. Various treatment technologies, such as activated carbon adsorption, advanced oxidation processes (AOPs), biodegradation, and zerovalent metal reduction, have been developed to remove atrazine from water.10−16 Recently, microwave energy has been applied to assist photocatalytic and photochemical reactions for decomposing atrazine and has produced some remarkable results, although complete mineralization of atrazine cannot be achieved by these treatments.17−19 Because the microwave energy is insufficient to disrupt the bonds of common organic molecules, ultraviolet/visible radiation is responsible for the photochemical changes, while microwave irradiation is believed to affect the course of the subsequent reactions.20 Activated carbon adsorption, which can be rather expensive, is currently considered the most suitable technology for removing atrazine in drinking water treatment, whereas biodegradation is commonly used for treatment of atrazine in wastewater and soils.21−23 In light of the widespread occurrence of atrazine pollution in surface and groundwater, and increasing regulatory requirements and public health concerns, there is a significant need to develop effective and inexpensive treatment technologies for removing atrazine from drinking water supplies. Microwave irradiation, which can significantly increase the intrinsic rates of chemical reactions and selectively drive reactions, is a well established technique in organic synthesis.24 Cleaner and more efficient chemical reactions with higher yields can be achieved by microwave technology compared to conventional heating methods. In materials containing polar molecules having an electrical dipole moment, microwave heating occurs with the alignment and reorientation of the molecules in the applied microwave field.25 Dipolar polarization (heat is generated in polar molecules) and conduction (heat is generated through resistance to an electric current) are the two major mechanisms for microwave dielectric heating.26 The enhancement of reaction rates by microwaves has been attributed to the heating effect and the lowering of the overall activation energy for a kinetic process.27 Although microwave irradiation might be able to excite specific molecules or functional groups within the molecules in a homogeneous system, the oscillations produced by the radiation in these target molecules would be instantaneously transferred by collisions with the adjacent molecules, precluding the formation of “molecular hot spots”. In contrast, steady state hot spots might be present in solid phases due to the much higher heat transfer resistances. Furthermore, build-up of charge in interfaces (or interphases) between components in heterogeneous systems is known as interfacial space charge or Maxwell− Wagner polarization.28 With the interfaces between materials having different dielectric properties providing loci for microwave interactions, molecules adsorbed on solid surfaces are especially activated by microwave irradiation, which can excite the valence band electrons of a group of specific atoms. As a result, reactions occurring at surfaces and interfaces are particularly susceptible to microwave influences.27 A few studies have reported removal and destruction of organic pollutants from aqueous and solid phases employing microwave irradiation, sometimes in combination with catalysts.29−33 In particular, much effort has been devoted to the regeneration of exhausted carbon sorbents using microwave irradiation as an alternative to the conventional regeneration
with vacuum, hot air, or steam.34−37 Microwave irradiation can be effective at decomposing organic substances sorbed on granular activated carbon, which is an excellent dielectric material to absorb and convert microwave energy into thermal energy, although significant improvement in the technique is required for real world applications.36,37 In this study we investigated the removal of atrazine from aqueous solution using a dealuminated HY zeolite and its degradation in the zeolite micropores under microwave irradiation. The combination of molecular-dimension pores and the relatively hydrophobic surfaces of the zeolite make it highly efficient at sorbing atrazine. The HY zeolite framework is transparent to microwave,38 while atrazine molecules sorbed in the hydrophobic spaces of zeolite micropores are subjected to interfacial selective heating under microwave irradiation, leading to rapid decomposition via pyrolysis. To our knowledge, this is the first report on the degradation of an organic pollutant by microwave irradiation after adsorbing it from aqueous solution into the micropores of a microwavetransparent mineral. This process can potentially be optimized for removal and destruction of atrazine and other nitrogencontaining herbicides from aqueous streams.
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EXPERIMENTAL SECTION Table 1 summarizes the key properties of the sorbents used in this study. Two microporous zeolites with different pore structures and size distributions (CBV-720 and 4A), a widely studied templated mesoporous silica (MCM-41), a nonporous silica sand powder (Min-U-Sil), and a macroporous biomineral (diatomite) were studied as model mineral sorbents. Powder of PTFE, which is nonporous and inert, was also selected to compare with the dielectric mineral surfaces in microwaveinduced degradation experiments. To remove the nonstructural water of the mineral sorbents, CBV-720, 4A, and MCM-41 were calcined at 380 °C for 12 h, whereas silica sand and diatomite were calcined at 200 °C for 5 h before use. The PTFE powder was used as received. Standards of atrazine (98.4%), and its degradation intermediates, hydroxyatrazine (99.0%), deethylatrazine (98.0%), N-ethyl-ammeline (99.5%), ammeline (98.0%), and 3,5-diamino-1,2,4-triazole (98.0%), were obtained from Dr. Ehrenstorfer (Augsburg, Germany); deisopropylatrazine (98.0%) and N-isopyl-ammeline (95.0%) were obtained from AccuStandard (New Haven, CT); guanidine hydrochloride (98.0%) was obtained from Aladdin Reagent (Shanghai, China). HPLC-grade methanol and other solvents were supplied by CNW Technologies (Dusseldorf, Germany). The standard mixtures for instrument calibration were prepared from the single-compound solutions in methanol. Laboratory triple-distilled water was used in preparation of all aqueous solutions. All samples were stored at 4 °C in the dark and analyzed within 2 weeks (aqueous-based samples were analyzed within 2 days). Sorption isotherms of atrazine on the mineral sorbents were obtained with batch experiments. To 30 mL solutions of varying atrazine concentrations in 100-mL brown glass bottles, 200 mg of sorbents were added. Because of the much higher sorption capacities of CBV-720 and MCM-41, their masses were reduced to 20 mg while the solution volume was increased to 100 mL. After sealing, the bottles were shaken continuously (at 120 rpm) in a constant-temperature shaker at 25 °C for 24 h. Preliminary testing showed that 24 h was sufficient for sorption to reach equilibrium in all the systems studied. 5068
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Supernatants were then withdrawn from the bottles using glass syringes and filtered with 0.22-μm PTFE membrane filters. Atrazine-laden sorbents were obtained after separating the aqueous solutions by vacuum filtration and oven drying at 60 °C for 1 h. The quantitative recovery of atrazine and its degradation intermediates was key for assessing the degradation kinetics. Our preliminary testing shows that microwave-assisted extraction with 20 mL of methanol at 80 °C for 15 min yielded the highest recovery efficiency (77.1%) for atrazine, while those of the atrazine degradation intermediates varied from 22.3% to 44.2% (Supporting Information). Microwave irradiation of the atrazine-laden sorbents was carried out using the OMNI vessels in a MARS system (CEM, U.S.) with the microwave power kept constant. Readings from the fiber-optic probe in the control reference vessel indicated that the temperature in the bulk of sorbents never rose above 134 °C during microwave irradiation. After cooling to room temperature, the irradiated sorbents were quantitatively transferred to the Greenchem extraction vessels. Atrazine and its degradation intermediates were then extracted from the sorbents by microwave-assisted extraction. Unirradiated control samples were also prepared in the degradation experiments. The identification and quantification of atrazine and its degradation intermediates were accomplished using ultraperformance liquid chromatography/tandem mass spectrometry (UPLC/MS/MS) on a Xevo-TQ triple quadruple mass spectrometer (Waters, U.S.) operated in the positive electrospray ionization mode (ESI+). Aliquots of 5 μL were injected onto an Acquity BEH C18 analytical column (50 mm ×2.1 mm, 1.7 μm) maintained at 40 °C using isocratic methanol/ water (80:20, v/v) at 0.3 mL/min, and the total run time was 3.0 min. Detection was carried out in multiple reaction monitoring (MRM) mode, and the tune page parameters and conditions (Table S1, Supporting Information) for each of the MRM transitions were optimized by infusing neat standard solutions of the individual analytes (1 mg/L) into the mass spectrometer at 10 μL/min. Quantification of all target analytes was based on external calibration using the mixed standard solutions prepared. To verify the mineralization of atrazine, the total C and N contents of CBV-720 samples laden with two degradation intermediates of atrazine after microwave irradiation were also measured (Supporting Information). Infrared spectra of CBV-720 and atrazine-laden CBV-720 powders before and after microwave irradiation were compared to study changes in surface chemistry. The measurements were performed on a Prestige-21 Fourier transform infrared (FTIR) spectrometer (Shimadzu, Japan) in transmission mode. The atrazine-laden CBV-720 powder (9 mg), as well as those after microwave irradiation, was mixed with KBr (90 mg) and compressed into 12-mm pellets. Infrared spectra were obtained for the wavenumber range of 4000 to 400 cm−1, and a linear baseline correction was applied using 4000, 2000, and 860 cm−1 as zero absorbance points. Data of atrazine sorption on the mineral sorbents were fitted with the Freundlich isotherm model, which could describe its sorption behavior reasonably well (Supporting Information). The masses of atrazine and its degradation intermediates were adjusted with their extraction efficiencies in estimating the degradation rates in microwave-induced degradation. The mass balance was within 7% of the original mass of atrazine during the first 6 min of microwave irradiation, but deteriorated significantly after that primarily because guanidine was much
Value reported by supplier. bCalculated from static water adsorption data reported by the supplier. cBET surface area determined in this study. dDetermined in this study on an ASAP 2020 apparatus (Micromeritics, U.S.) using the Horvath−Kawazoe method. eN/A − Not applicable.
structure supplier
FAU Zeolyst (Valley Forge, PA)
2−4 nominal pore size of 0.4 nm LTA Jiuzhou Chemicals (Shanghai, China) particle size (μm) pore size
hydrophilic, Na+
hydrophobic, H+ (0.42 sites/ nm2) 1−2 0.74−1.2 nm
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a
N/A Xingying Industry Development Inc. (Zhuhai, China) amorphous silica Kermel Chemicals (Tianjin, China) tectosilicate U.S. Silica (Berkeley Springs, WV)
median particle size of 2 N/Ae median particle size of 35 median pore diameter of 400 nm
both hydrophobic and lipophobic hydrophilic (primarily amorphous silica, with minor fractions of with 2−4% alumina and iron oxide)
hydrophilic (99.5% quartz, 0.2% alumina, 0.3% others) median diameter of 8 N/A
moderately hydrophobic, purely siliceous molecular sieve 0.4−1 2−5 nma; median pore width of 4.78 nmd templated mesoporous silica Jcnano Technology (Nanjing, China)
diatomite 1.60c Min-U-Sil 0.8a MCM-41 847c 4A 856b CBV-720 780a
property specific surface area (m2/g) hydrophobicity and surface cation type
Table 1. Summary of Selected Physical and Chemical Properties of the Sorbents Used in This Study
PTFE powder 10a
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sorption capacity compared to the other mineral sorbents. MCM-41 is composed of amorphous silica wall but possesses a long-range ordered framework with uniform mesopores. The atrazine sorption capacity of MCM-41 was lower than that of CBV-720, but much higher than that of the other minerals, which could be explained by its relatively large pore sizes (2−5 nm) and the lack of significant enhancement in adsorption potentials within the mesopores. Besides the pore sizes, hydrophobicity of the pore wall surfaces also plays a key role in controlling the sorption capacity of microporous minerals, as was demonstrated in earlier studies.39,44 The framework of CBV-720 (Si/Al = 15) is relatively hydrophobic as the density of surface negative charge (carried by the AlO4 tetrahedra in the framework) has been lowered through dealumination.45,46 To adsorb in the hydrophobic pore spaces within the framework of CBV-720 from aqueous solution, atrazine molecules had to out-compete the vastly abundant water molecules. Atrazine adsorbs on silica-rich mineral surfaces through hydrogen bonding of the amine moieties and the silanol groups, but water can successfully compete against atrazine for the surface silanol groups.47 Water molecules are less strongly bound and form less ordered structure on surfaces with low surface charge density, which allows sorption of sparingly water-soluble organic compounds, such as atrazine.39,48 Previous sorption studies have shown that the neutral siloxane surfaces in clay minerals were the key domain for the adsorption of hydrophobic organic compounds.48−50 Laird proposed that the alkyl-side chains of atrazine molecules out-compete water molecules for retention on hydrophobic siloxane surfaces, while the lone pair of electrons on the ring N atoms interact with water molecules solvating exchangeable cations associated with smectite surfaces,49 which was confirmed by molecular dynamic simulations.50 With removal of surface charge and crosscondensation of neighboring silanol groups (forming siloxane bonds) through dealumination,45 the micropores of CBV-720 are expected to have much higher affinity toward atrazine compared to those of natural zeolites in the presence of water. In fact, our previous study has shown that the water affinity of Y zeolites decreased with dealumination, and organic compound (trichloroethylene) could increasingly compete with water for adsorption in their micropores.39 In the micropores of CBV720, atrazine molecules were believed to sorb predominantly in the hydrophobic spaces, with possible hydrogen bonding formation between the lone pair of electrons on N atoms (of the ring and amino groups) and water molecules, which sorbed in the hydrophilic pore spaces through coordination to surface cations and hydrogen bonding to surface hydroxyl groups.39,44 Similarly, the mesopores of MCM-41 exhibited a relatively high atrazine sorption capacity because the silica molecular sieve possessed moderately hydrophobic pore wall surfaces. On the other hand, Min-U-Sil, diatomite, and 4A all had primarily hydrophilic surfaces, and their low atrazine sorption capacities resulted partially from the strong competition from water. 2. Microwave-Induced Atrazine Degradation. Figure 2a shows the degradation of atrazine sorbed on or associated with the mineral sorbents and PTFE powder after being irradiated with microwave at 800 W for 2 and 4 min. Degradation of atrazine occurred on all the mineral sorbents but not on PTFE powder. With a very low dielectric constant of 2.1, PTFE powder does not adsorb atrazine and is microwave-transparent. Atrazine remained in the droplets of water associated with the PTFE powder and did not experience any surface-enhancement
more difficult to quantify with elevated background noises (probably due to the interference from small degradation products). The degradation kinetics of atrazine and its degradation intermediates in the micropores of CBV-720 were also modeled. The rate constants were estimated through nonlinear least-squares fitting of the experimental data by solving the ordinary differential equations derived from the corresponding rate equations simultaneously.
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RESULTS AND DISCUSSION 1. Atrazine Sorption on Mineral Sorbents. Figure 1 shows the sorption isotherms of atrazine on CBV-720, 4A,
Figure 1. Freundlich isotherms of atrazine sorption on Min-U-Sil, diatomite, 4A, CBV-720, and MCM-41 at 25 °C. The data points represent the mean values determined from triplicated experiments, while the lines represent Freundlich isotherm fits (the fitting parameters are summarized in Table S2, Supporting Information).
MCM-41, Min-U-Sil, and diatomite. Although atrazine sorption on all five mineral sorbents exhibited the Freundlich-type isotherms, there were significant differences in their sorption capacities. Min-U-Sil is a nonporous grounded quartz sand with very low microporosity.39 It exhibited low atrazine sorption because of its low surface area. Diatomite (median pore diameter of 400 nm) is a silica-based macroporous mineral, and its internal surface is not expected to be different from the external one due to the very large sizes of its pores. This explains the comparable atazine sorption on diatomite and on Min-U-Sil. The surface areas of microporous zeolites CBV-720 and 4A are located predominantly within the micropores, which cannot be accessed by sorbate molecules larger than their pore openings. The critical molecular diameter of atrazine is 0.54 nm,40 while the nominal pore openings of CBV-720 and 4A are 0.74 and 0.4 nm, respectively. As a result, atrazine molecules could access the internal surface of CBV-720, but not that of 4A. That is why atrazine sorption on zeolite 4A was similar to that on Min-U-Sil and diatomite. Pore size of the sorbent plays an important role in sorption, besides dedicating the accessibility of internal surface. Molecules occluded inside porous solids experience the interactions with all surrounding pore wall surfaces, and the dispersion force fields acted by various parts of the surface overlap each other when the pore dimension is small enough.41,42 Appreciable enhancement of adsorption potentials can occur in micropores of six molecular diameters or even larger.41,43 The enhancement of adsorption fields in the micropores of CBV-720, which can be accessed by atrazine molecules, partially explains its uniquely high atrazine 5070
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degradation proceeded approximately linearly with time and the rate of degradation was highly dependent on the microwave power. These results clearly indicate that supply of microwave energy to the atrazine−mineral interface played an important role in controlling the degradation rate. In contrast, the initial mass loading of atrazine on CBV-720 had little effect on its degradation rate (Supporting Information). 3. Atrazine Degradation Mechanism. A total of eight degradation intermediates of atrazine with molecular weights above 50 Da were identified and quantified by UPLC/MS/MS: hydroxyatrazine, deethylatrazine, deisopropylatrazine, N-isopropyl-ammeline, N-ethyl-ammeline, ammeline, 3,5-diamino1,2,4-triazole, and guanidine. To our knowledge, 3,5-diamino1,2,4-triazole and guanidine are two new degradation intermediates of atrazine that have not been reported in previous studies on atrazine degradation. Figure 3a and b shows the degradation of atrazine in the micropores of CBV-720 and the evolution of the degradation intermediates during microwave irradiation at 800 W. Atrazine degraded quickly under microwave irradiation, following an apparent zero-order kinetics. This indicates that the microwave-induced atrazine degradation might be surface-controlled (i.e., limited by the availability of surface sites where hot spots could form) under a given microwave energy input. Hydroxyatrazine, deethylatrazine, deisopropylatrazine, N-isopropyl-ammeline, and N-ethylammeline were detected at relatively low levels, suggesting they degraded easily in the micropores under microwave irradiation. In contrast, ammeline, 3,5-diamino-1,2,4-triazole, and guanidine appeared to be the major degradation intermediates because they were relatively more stable. The formation of 3,5-diamino1,2,4-triazole and guanidine clearly indicates the cleavage of the triazine ring in the microwave-induced degradation. Figure 3c shows that C/N ratios of CBV-720 laden with guanidine and cyanamide, respectively, decreased with continued microwave irradiation. In contrast to the ready degradation of cyanamide, guanidine was much more resistant to the microwave-induced degradation, which is consistent with the UPLC/MS/MS results, probably because of the resonance structures of the guanidinium cation ([CH6N3]+) (Supporting Information). Guanidine at the levels produced from atrazine degradation is not expected to pose any significant environmental concern (Supporting Information), while prolonged microwave irradiation should be able to completely destruct guanidine as it is known to decompose thermally at elevated temperatures (310 °C for guanidine hydrochloride). Together, these results indicate that atrazine could be fully mineralized in micropores of CBV-720 by the microwave treatment. Figure 4 depicts the proposed degradation pathways of atrazine sorbed in mineral micropores under microwave irradiation based on the occurrence of degradation intermediates. The C−Cl bond is relatively weak (331 kJ/mol) and Cl− is a good leaving group, making it susceptible to cleavage and nucleophilic substitution. That is why dechlorination− hydroxylation products appeared in the early stage of degradation. Meanwhile, N-dealkylation of secondary amines also occurred with the cleavage of the C−N bond.51 Substitution of the amine groups on the triazine ring by hydroxyl groups after dechlorination and dealkylation was commonly observed in atrazine degradation studies. For example, treatment of atrazine by AOPs typically yields a stable final product, cyanuric acid, without complete mineralization of atrazine.52,53 In microwave-induced degradation, ammline was observed to transform to a five-member-ring
Figure 2. Microwave-induced degradation of atrazine sorbed on or associated with the sorbents studied: (a) atrazine degradation on CBV720, 4A, MCM-41, diatomite, Min-U-Sil, and PTFE powder after microwave irradiation at 800 W for 2 and 4 min (error bars indicate 95% confidence intervals from triplicated experiments); (b) degradation of atrazine sorbed in the micropores of CBV-720 as a function of microwave power and irradiation time (the dashed lines represent linear fit to the degradation data).
effect. Consequently, no degradation occurred, as was observed for direct microwave irradiation of atrazine solution (data not shown). Atrazine degradation on diatomite, Min-U-Sil, and MCM-41 was comparable, probably because atrazine molecules adsorbed primarily on the relatively “flat” surfaces of these minerals (pores in diatomite and MCM-41 have rather large diameters compared to the size of atrazine molecules) and they were all made of essentially microwave-transparent SiO2. Zeolite 4A brought slightly greater degradation of the sorbed atrazine compared to diatomite, Min-U-Sil, and MCM-41, which might be caused by its strong microwave-absorption and subsequent pyrolysis of the sorbed atrazine.38 In contrast, atrazine degraded more rapidly under microwave irradiation on CBV-720, where the atrazine molecules were predominantly sorbed in the micropores. Together, these results suggest that micropore confinement played an important role in the microwave-induced degradation, which deserves further investigation. Given its uniquely high atrazine sorption capacity and the much faster atrazine degradation observed, we studied the microwave-induced degradation of atrazine sorbed in the micropores of CBV-720 in detail. As atrazine molecules were sorbed predominantly in the micropores, a major factor limiting the rate of degradation might be the microwave energy input. Figure 2b shows the effect of microwave power and irradiation time on degradation of atrazine sorbed on CBV-720. Atrazine 5071
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Figure 3. Degradation of atrazine in micropores of CBV-720 under 800 W microwave irradiation: (a) evolution of atrazine (mass loading: 84.65 nmol/g) and its degradation intermediates, along with the modeling results; (b) magnified view for evolution of the minor degradation intermediates; and (c) mineralization of two degradation intermediates (0.65 mmol/g guanidine and 0.55 mmol/g cyanamide, respectively) sorbed on CBV-720, as indicated by the C/N ratio, during the course of microwave irradiation (Supporting Information).
compound, 3,5-diamino-1,2,4-triazole, which is a clear indication of heterocyclic pyrolysis. Formation of a fivemember-ring from cleavage of a six-member ring during degradation is feasible but not very common. Under continuous microwave irradiation, the selective heating was believed to create steady state microscale hot spots at the organic−mineral interface in micropores,26 resulting in pyrolysis of the sorbed organic molecules. With continued input of microwave energy, 3,5-diamino-1,2,4-triazole underwent further ring cleavage and eventually degraded into small organic fragments, such as guanidine and cyanamide. Guanidine is a precursor compound used for production of s-triazines,54,55 but it has not been detected as a degradation product of atrazine previously. Guanidine and cyanamide are expected to be eventually degraded into ammonia and carbon dioxide in the microwave-induced degradation (Supporting Information). The estimated degradation rate constants for atrazine and its degradation intermediates in the micropores of CBV-720 under 800 W microwave irradiation are also shown in Figure 4. It should be noted that rates of chemical reactions are known to be dependent on temperature, while the temperatures at the reaction sites within the micropores (i.e., hot spots) are not
easily measurable. The degradation rate estimations were made by assuming that the temperature of the hot spots was uniform and constant during microwave irradiation. Atrazine degradation followed a pseudo-zero-order kinetics (kapp = 2.45 nmol/ min at 84.65 nmol/g atrazine mass loading), instead of a firstorder one commonly observed in AOPs (e.g., 12,13), and the degradation also proceeded much faster. This probably resulted from the intense microwave irradiation on atrazine (and water) molecules in the micropores, which was not shielded or absorbed by the zeolite framework. The degradation intermediates of atrazine degraded further under microwave irradiation, which could be described reasonably well with pseudo-first-order kinetics. 4. Insights from Infrared Spectroscopy. Figure 5 shows the FTIR spectra of CBV-720 and atrazine-laden CBV-720 before and after microwave irradiation. The strong and broad band centered at 3500 cm−1, a characteristic absorption band for Si−OH−Al hydroxyl groups located in the sodalite cages or hexagonal prisms of zeolite framework, was unaffected by sorption of atrazine. As water was present on CBV-720, the absorption bands for Si−OH−Al hydroxyl groups located in the supercages (3634 cm−1) and silanol groups terminating the 5072
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Figure 4. Proposed pathways and the estimated rate constants for microwave-induced degradation of atrazine sorbed in the micropores of CBV-720 (under 800 W microwave irradiation; atrazine mass loading: 84.65 nmol/g) based on the degradation intermediates detected.
zeolite lattice (3734 cm−1) could not be observed clearly. The strong bands centered at 1100 cm−1 were due to stretching of silica tetrahedra and aluminum tetrahedra, while the bands below 1000 cm−1 corresponded to the vibrations of zeolite framework.56 Comparison of the spectra shows that the broad and discrete bands between 2800 and 3000 cm−1, which are assigned to O−H and N−H stretching, changed with microwave irradiation. The band at 2975 cm−1 arose from O−H stretching of water in zeolite micropores, and its intensity decreased substantially with microwave irradiation, which is indicative of thermal desorption of water from micropores due to interfacial selective heating. Other bands in the region could be assigned to N−H stretching of atrazine and its degradation products, and they also varied with irradiation time. Significant changes also occurred on the band at 1395 cm−1, which is assigned to O−H bending vibration of silanol groups terminating the zeolite lattice. It shifted to 1385 cm−1 and became much stronger after microwave irradiation, which
indicates that microwave irradiation altered the surface silanol groups of CBV-720. Uytterhoeven et al. reported the following equilibrium in HY zeolite framework:57
where Ke is a constant related to temperature. The reaction is exothermic and the fraction of Bronsted acid (the left-hand) increases with temperatures, which explains the shift of the bending vibration of O−H at 1395 to 1385 cm−1 after microwave irradiation. Although the IR measurements were not made in situ during the microwave irradiation, with the rapid heating and fast cooling of microwave irradiation (instant on and off), the product state on the zeolite surface was expected to be “frozen” by the fast cooling once the microwave irradiation stopped. As a result, the IR spectra could indicate the thermal history of the zeolite surface under microwave 5073
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Figure 5. FTIR spectra of CBV-720 and atrazine-laden CBV-720 before and after microwave irradiation (800 W): (a) CBV-720 before irradiation, (b) CBV-720 after 12 min irradiation, (c) atrazine-laden CBV-720 before irradiation, (d) atrazine-laden CBV-720 after 6 min irradiation, and (e) atrazine-laden CBV-720 after 12 min irradiation.
it is expected that the dielectric molecules (atrazine and water) sorbed on the pore wall surfaces of the microwave-transparent CBV-720 would be rapidly heated by the microwave irradiation, forming microscale hot spots, which is supported by the changes in the surface chemistry of CBV-720 and the formation of pyrolysis products of the sorbed atrazine. Potential Application of Microwave-Induced Degradation. The microporous mineral sorption coupled with microwave-induced degradation studied here could potentially be used for treatment of atrazine in drinking water. Microporous minerals with hydrophobic pore wall surfaces, such as CBV720, can be used as an effective sorbent for taking up atrazine from aqueous streams. After saturation of the sorption capacity, the mineral sorbents can be regenerated by microwave irradiation on-site and used again. Microwave irradiation has been investigated as a potential means of regeneration for activated carbon.37 The rapid heating of the activated carbon by microwave energy could raise the temperature inside the carbon bed to above 1000 °C, leading to thermal desorption and oxidation of the sorbed pollutants. Despite the promising results, this technique suffers the distinct drawback of thermal instability, i.e., lack of uniformity in the magnitude and spatial
irradiation, despite the reversible thermal equilibrium at high temperatures. The changes in surface hydroxyl groups in the pore wall surfaces support our hypothesis that hot spots were created near the pore wall surfaces by selective heating of microwave. With atrazine sorbed in the micropores, the band strength at 1385 cm−1 was weaker compared to the case without atrazine sorption after 12 min of microwave irradiation, probably due to consumption of the microwave energy in heating the sorbed atrazine molecules. Although microscopic hot spots were not directly observed in this study, previous studies have clearly shown that microwave irradiation could cause rapid heating of dielectric materials. For example, Jou et al. observed the occurrence of sparks on the surfaces of granular activated carbon (2 g) under 800 W microwave irradiation after 30 s.58 Dawson et al. also recorded the random generation of plasma on fluidized carbon granules in the microwave cavity while the temperatures of the carbon granule bed measured in situ never rose above 160 °C.59 Pallavkar et al. found that the temperature of a bed of strongly microwave-absorbing SiC foam was raised to over 800 °C by microwave irradiation within 2−3 min, and higher microwave powers resulted in higher steady state temperatures.60 Similarly, 5074
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distribution of energy.37,61 Burning of the activated carbon can occur on some parts, while the other parts may not be regenerated at all. In contrast, the HY zeolite used in this study is microwave-transparent, and allows selective heating of the atrazine and water molecules sorbed in its micropores. Consequently, the microwave energy is efficiently used without heating the sorbents. Furthermore, the sorbed atrazine undergoes decomposition and eventually mineralization in the micropores, while pollutants sorbed on activated carbon undergo thermal desorption and oxidation, which may produce secondary pollutants under microwave irradiation. The microwave-induced degradation process is superior to photochemical and nonphotochemical oxidation in treatment of atrazine-polluted water. Microwave energy is used to cause degradation of atrazine sorbed in the micropores of the mineral sorbents, in contrast to direct applications of chemicals and/or energy to the vast volume of aqueous solution, and thus allows significant savings in energy and chemicals. Furthermore, degradation of atrazine is very fast under microwave irradiation and it can be fully mineralized, instead of being transformed to cyanuric acid. This process shows promise for removal of atrazine and other nitrogen-containing herbicides from drinking water.
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ASSOCIATED CONTENT
S Supporting Information *
Additional information on MS/MS conditions for MRM analysis of atrazine and its degradation intermediates, extraction of atrazine from the mineral sorbents by ultrasonic extraction, Soxhlet extraction, and microwave-assisted extraction, safety features of the microwave system, the recovery efficiencies for atrazine degradation intermediates by microwave-assisted extraction, the Freundlich isotherm fits of atrazine sorption on the mineral sorbents, influence of atrazine mass loading on degradation rate, microwave-induced degradation of guanidine and cyanamide, and the ecotoxicity of guanidine. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone: (+86) 20 8529-0175; fax: (+86) 20 8529-0706; e-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge the anonymous reviewers for their valuable comments and suggestions. This work was supported in part by Guangzhou Institute of Geochemistry (Grant GIGCX-11-03), the Natural Science Foundation of China (Grants 41073079 and 41121063), the SRF for ROCS, SEM, and the “One Hundred Talents” program of the Chinese Academy of Sciences. This is contribution IS-1479 from GIGCAS.
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