Impact of Surface Chemistry on Microwave-Induced Degradation of


Dec 6, 2012 - Surface chemistry determines the interactions of sorbate and solvent molecules with the pore wall surfaces of microporous minerals, and ...
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Impact of Surface Chemistry on Microwave-Induced Degradation of Atrazine in Mineral Micropores Erdan Hu†,‡ and Hefa Cheng†,* †

State Key Laboratory of Organic Geochemistry Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: Surface chemistry determines the interactions of sorbate and solvent molecules with the pore wall surfaces of microporous minerals, and affects the transmission and absorption of microwave radiation for a given solvent−sorbate−sorbent system. The sorption and microwave-induced degradation of atrazine in the micropores of nine Y zeolites with different densities (0.16−2.62 site/nm2) and types (Mg2+, Ca2+, H+, Na+, and NH4+) of surface cations were studied. The influence of the content of cosorbed water in the mineral micropores on atrazine degradation rate was also examined. The results indicate the presence of surface cations at around 0.23 site/nm2 on the pore wall surface was optimal for atrazine degradation, probably due to formation of insufficient number of “hot spots” with too few cations but excessive competition for microwave energy with too many hydrated cations. Atrazine degraded faster in the presence of cations with lower hydration free energies, which could be attributed to less microwave energy consumption to desorb the bounded water molecules. Reducing the content of coadsorbed water in the micropores also increased atrazine degration rate because of less competition for microwave energy from water. Such mechanistic understanding can guide the design and selection of microporous minerals in the practical application of microwaveinduced degradation.



INTRODUCTION

is often used to evaluate the ability of materials to convert microwave into thermal energy.1,4 The heating effect arising from the interaction of the electric field component of high frequency electromagnetic waves with charged particles in materials has long been known.5 Microwave radiation is often used as an alternative to conventional thermal heating because of the faster heating rate, selective heating, better process control, and no direct contact of heated materials.1,3,6 In addition, microwave radiation can also have “microwave specific effect” by accelerating reaction rates, improving yields, and selectively activating or suppressing reaction pathways compared to the use of conventional heating.4,6−10 Microwave radiation has found uses in the environmental field as well: it has been applied in

Microwaves refer to the electromagnetic waves with wavelengths ranging from 0.001 to 1 m, or equivalently, with frequencies between 0.3 and 300 GHz. Electromagnetic waves interact with an object by reflection, absorption, and transmission, which may occur simultaneously in varying ratios depending on its dielectric properties.1,2 Interactions of materials with microwave occur at the molecular level, with the external electromagnetic field inducing reorganization of the electrical charges, including migration of free or bound charges and rotation of electric dipole.3 Complex permittivity (ε*), which contains a real part (ε′) and an imaginary part (ε″), is used to characterize the response of a given material to alternating electric fields. The relative dielectric constant, ε′, describes the ability of a dielectric material to store energy under the influence of an external electric field, whereas the loss factor, ε″, represents the dissipation of the absorbed energy as heat.4 The loss tangent (tanδ) calculated from the ratio of ε″/ε′ © 2012 American Chemical Society

Received: Revised: Accepted: Published: 533

August 28, 2012 December 2, 2012 December 5, 2012 December 6, 2012 dx.doi.org/10.1021/es303486j | Environ. Sci. Technol. 2013, 47, 533−541

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Table 1. Selected Physical and Chemical Properties of the Dealuminated Analogues of Y Zeolite Used in This Study sorbent

CBV-100

CBV-300

CBV-400

CBV-712

CBV-720

CBV-760

CBV-780

Si/Al mole ratioa bulk density, g/cm3 theoretical micropore volume, cm3/g nominal cation forma [Na2O]a, wt % unit cell sizea, nm BET surface areaa, m2/g monovalent cation densityb, sites/nm2

2.55 1.01 0.48 sodium 13 2.465 900 2.12c

2.55 1.01 0.48 ammonium 2.8 2.468 925 2.07c

2.55 1.01 0.48 hydrogen 2.8 2.450 730 2.62c

6 1.01 0.48 ammonium 0.05 2.435 730 1.11

15 1.01 0.48 hydrogen 0.03 2.428 780 0.42

30 1.01 0.48 hydrogen 0.03 2.424 720 0.23

40 1.01 0.48 hydrogen 0.03 2.424 780 0.16

a

From product data sheets of Zeolyst (Valley Forge, PA). bThe isomorphous positioning of aluminum in tetrahedral coordination of zeolite results in a net negative charge of the framework, which is counterbalanced by cations residing in the cages and channels. The density of surface monovalent cation is estimated from the Si/Al mole ratio and the general molecular formula of faujasite [(Mg, Na2, Ca)3.5 [Al7Si17O48]. 32(H2O)] assuming monovalent cations only; cTheoretically, CBV-100, CBV-300, and CBV-400 should have the same monovalent cation density as they have the same framework structure and Si/Al ratio. The calculated values are different because of the differences in their BET surface areas.

strongly with the organic sorbate in absorbing the microwave energy (at 2.450 GHz) and affect its degradation. Therefore, the surface chemistry of microporous minerals can determine the interactions of sorbate and solvent molecules with the pore wall surfaces, and control degradation of the organic sorbate under microwave irradiation as well. The present study was conducted to explore the impact of mineral surface chemistry, more specifically, the type and density of surface cations on the pore wall surfaces, on sorption and microwave-induced degradation of atrazine in mineral micropores. A total of 9 dealuminated Y zeolites, 5 with the same Si/Al ratio (2.55) but different cations (with hydration free energy decreasing in the order of Mg2+, Ca2+, H+, Na+, and NH4+), while the rest having Si/Al ratios varying between 6 and 40, were studied as model microporous minerals. Differential scanning calorimetric (DSC) analysis was performed to study their thermal dehydration behaviors, while coaxial transmission line technique was used to characterize their dielectric properties. Sorption of atrazine from aqueous solution on these zeolites, and degradation of the atrazine sorbed in their micropores (after being separated from the aqueous solution) under microwave irradiation were investigated. The influence of the content of cosorbed water in the mineral micropores on microwave-induced degradation of the sorbed atrazine was studied on a hydrophilic zeolite (Si/Al = 2.55) and a hydrophobic one (Si/Al = 15). The degradation rate of atrazine sorbed in the micropores of four HY zeolites with different cation densities in the presence of water and a solvent that absorbs microwave only weakly (dichloromethane), was also compared. The results provide a mechanistic understanding on the microwave-induced degradation of atrazine sorbed in mineral micropores and the important roles played by surface cations and water molecules in the process.

decontamination of soils, sludge, and wastewater, and regeneration of activated carbon, with the significant benefit of drastic reduction in the processing time.6,11−14 Recently, we reported the first study on degradation of an organic pollutant by microwave irradiation after adsorbing it from aqueous solution into the micropores (<2 nm) of a microwave-transparent mineral.15 Because of its high micropore volume, suitable pore sizes, and surface hydrophobicity, a dealuminated HY zeolite (CBV-720) showed much higher atrazine sorption capacity than the nonporous, mesoporous (2−50 nm), and macroporous (>50 nm) minerals evaluated, while the atrazine sorbed in the mineral micropores could degrade rapidly under microwave irradiation. The degradation of atrazine followed pseudo zero-order kinetics and atrazine could be fully mineralized under continuous microwave irradiation via a series of dechlorination-hydroxylation, Ndealkylation, dechlorination, dealkylation, and ring cleavage reactions.15 The energy of microwave radiation (0.98 J/mol at 2.450 GHz) is very low and far less than the bond energies of common chemical bonds in organic molecules (for example, the dissociation energy for C−C bond is 348−356 kJ/mol).16 Therefore, microwave radiation cannot be directly responsible for the degradation of atrazine sorbed in mineral micropores. Instead, interfacial selective heating of microwave creates microscale “hot spots” near the pore wall surfaces, which causes degradation of the sorbed atrazine molecules via pyrolysis.10,15 In our experimental study, the microporous minerals were irradiated with microwave after adsorption of atrazine from aqueous solution. Microwave energy penetrates the pore wall (with some or little dissipation depending on the dielectric properties of the minerals) and is transferred to the pore wall surfaces, and the atrazine and water molecules sorbed in the micropores by interaction of the electromagnetic field. The microwave absorption in such solvent−sorbate−sorbent systems is very complex as several forms of energy absorption and energy transfer can be involved. Besides the interactions of microwave with the solid matrix (i.e., the mineral sorbent), the organic sorbate can have dielectric properties that are substantially different from the sorbent and modify the dielectric properties at the surface where they adsorb, thus change the system’s energy absorption.17 In addition, water, which has a dielectric constant of 78.5 at 25 °C, is also sorbed in the micropores through coordination to surface cations and hydrogen bonding with coordinated water molecules and surface silanol groups.18,19 Water is expected to compete



EXPERIMENTAL SECTION A series of dealuminated Y zeolites, which have a well-defined micropore structure but varying surface cation densities and types, were used in this study. The key properties of these zeolites, which were commercially supplied as powder of micrometric size without any binder, are summarized in Table 1. Ca/CBV-100 and Mg/CBV-100 were also prepared from CBV-100 by ion exchange. Ca2+ was exchanged into the framework cages and channels of CBV-100 by equilibrating 5 g of dry zeolite in 100 mL of 1.0 mol/L CaCl2 solution for 48 h (under constant stirring). The treated zeolite was separated by filtration followed by rinsing with triple-distilled water to 534

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Figure 1. Atrazine sorption on the dealuminated analogues of Y zeolite with different cations and Si/Al ratios at 25 °C and DSC thermographs of the wet zeolites: (a) influence of surface cation, where all the zeolites have the same Si/Al ratio of 2.55; and (b) influence of framework Si/Al ratio, where the Si/Al ratios are 2.55, 6, 15, and 40 for CBV-400, CBV-712, CBV-720, and CBV-780, respectively; and (c) DSC curves of the wet Y zeolites in the temperature range of 30 to 300 °C (at a heating rate of 5 °C/min). Means of triplicated experiments are plotted on the Freundlich isotherms in (a) and (b), and their fitting parameters are summarized in SI Tables S3 and S4.

remove the excess Ca2+, and then calcined at 120 °C for 12 h and 380 °C for 12 h, respectively. Calcium-exchanged Y zeolite (Ca/CBV-100) was obtained by repeating this process twice. Magnesium-exchanged zeolite (Mg/CBV-100) was prepared similarly using CBV-100 and 1.0 mol/L MgCl2 solution. Dry sorbents were prepared by calcining the zeolites at 380 °C for 12 h to remove the nonstructural water, while wet sorbents were prepared by equilibrating the zeolites with water vapor at room temperature for over 3 months. DSC analysis was carried out using a STA 409 PC Luxx simultaneous thermal analyzer (Netzsch-Geratebau, Wittelsbacherstrasse, Germany). Powders of wet zeolites were loaded into alumina pans and heated under a flow of dry nitrogen (60 mL/min) to 300 °C at a rate of 5 °C/min, and then to 500 °C at a rate of 10 °C/min. The heat flow between the pan loaded with zeolite sample and an empty alumina sample pan, which was used as reference, was recorded. Dielectric properties of the zeolites were measured by the coaxial transmission line technique,20,21 using an AV3618 network analyzer (CETC, Bengbu, China) that is responsible for the generation and reception of a signal at 2.450 GHz. A coaxial cell of 3 mm inner diameter and 7 mm outer diameter (80 mm long) was connected to the computer-controlled network analyzer through two standard transmission lines with impedance of 50 Ω. Toroidal-shaped composite samples were prepared by mixing the zeolites with polyethylene wax at a weight ratio of 1:2, and then pressing the homogenized mixtures into cylindrical-shaped molds. Because the zeolites had extremely low losses at 2.450 GHz, the sample thicknesses

were kept at about 4 mm to obtain suitable signal-to-noise ratios. The samples were further machined to fit a hollow cylinder and then inserted into the coaxial line. The values of relative dielectric constant (ε′) and loss factor (ε″) were calculated from the measured phase and amplitude of the reflected microwave signal from the composite samples using customized software. The system was carefully calibrated prior to each set of measurements. Atrazine (98.4%) and its degradation intermediates (95.0− 99.5%) were the same as those used in our previous work.15 Sorption isotherms of atrazine on the dealuminated analogues of Y zeolite were obtained with batch experiments. Accurately weighted dry sorbents (approximately 20 mg) were added into brown glass bottles containing 100 mL solutions of varying atrazine concentrations. The bottles were then sealed and agitated continuously (at 120 rpm) in a constant-temperature shaker at 25 °C for 24 h, which was sufficient for sorption to reach equilibrium.15 After equilibration, the supernatant was withdrawn from the bottles, filtered through 0.22 μm PTFE membrane filters, then analyzed for atrazine concentration. Data of atrazine sorption on the zeolites were fitted with the Freundlich isotherm model. For analytical reasons (higher atrazine mass loadings and controllable water contents), atrazine-enriched sorbents were prepared by equilibrating 2.5 g of zeolites with 200 mL of 40.7 μmol/L atrazine solution for 24 h, followed by separation of the sorbents by vacuum filtration and freeze-drying at −50 °C for 12 h. The water contents of the sorbents prepared were then determined from the weight losses after calcination at 380 °C 535

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capacity increased in the order of CBV-400, CBV-712, CBV720, and CBV-780, following the clear trend of increasing Si/Al ratio (i.e., decreasing surface cation density). Dealumination lowers the negative charge carried by the AlO4 tetrahedra in the zeolite framework,22,23 and thus reduces the density of surface cations and weakens the electrostatic field in the micropores. As a result, the water affinity of the micropores decreases as the framework Si/Al ratio increases, allowing greater organic sorption in the hydrophobic pore spaces through outcompeting water.18,19 Overall, the Y zeolites with higher Si/ Al ratios (i.e., lower density of surface cations) serve as better sorbents for up-taking of atrazine from aqueous solution. The presence of water molecules in the micropores of zeolites affects their sorption and catalytic properties through hindering adsorption of other molecules, and thermogravimetric analysis has been commonly used to identify the different types of water sorbed in zeolites and other microporous minerals.24−26 Figure 1c shows the DSC thermographs of the wet Y zeolites. A major endotherm was observed for all the zeolites in the 30−300 °C region, and the endotherms of the hydrophilic zeolites Mg/CBV-100, Ca/ CBV-100, CBV-400, CBV-100, CBV-300, and CBV-712 were broader than those of the hydrophobic ones (CBV-720, CBV760, and CBV-780). These zeolites had been equilibrated with saturated water vapor for over 3 months, thus their micropores were fully filled by water.18,27 Water molecules existed in the zeolite micropores in several forms: coordinated water that formed coordinated covalent bonds with surface cations, “zeolitic water” that was hydrogen bonded with either silanol groups or coordinated water molecules, and “loosely bound” water filling the hydrophobic pore spaces via capillary condensation.18,24,25,28−30 Loosely bound water is relative easy to remove upon heating, while zeolitic water, coordinated water, and the surface hydroxyl groups are lost gradually as microporous minerals are heated to several hundred degrees Celsius.18,31,32 The broad endotherms are attributed to the removal of loosely bound and zeolitic water, which makes up the majority of the heat required to remove water from the zeolites. For the HY zeolites, the endotherm peak temperature was positively correlated with the surface cation density, whereas the content of loosely bound water was negatively correlated with the surface cation density (SI Figure S1a). Zeolites of lower Si/Al ratios (and correspondingly higher surface cation densities) have stronger electrostatic interactions among cations, water molecules, and frameworks.33 Thus their micropores have greater overall water affinity, resulting in less sorption of atrazine (Figure 1b). The water molecules sorbed in the micropores of the more hydrophobic zeolites exhibited narrower endotherms that peaked at lower temperatures because their overall interactions with the hydrophilic sites (surface cations and silanol groups) in the micropores were weaker with the presence of fewer surface cations. The interaction of water molecules with different surface cation varies, thus the water affinity of zeolite micropores also depends on the type of surface cations. The hydration capacity of a cation is determined by its ionic radius and ion electric charge. The hydration free energies for Mg2+, Ca2+, Na+, H+, and NH4+ are −447.2, −369.9, −96.3, −252.6 to −262.5, and −77 to −80.5 kcal/mol, respectively.34−36 For zeolites with the same Si/Al ratio (2.55), the endotherm peak temperature and the content of loosely bound water in their micropores were also strongly correlated with the surface cation density (SI Figure S1b). Small and high charge cations Ca2+ and Mg2+ are

for 18 h to remove the remaining nonstructural water. Sorbents with higher water contents were also obtained by reducing the freeze-drying time to 3 h. Water contents were expressed as the number of water molecules per unit cell (u.c.) of the zeolites. To evaluate the effect of solvent (water vs dichloromethane) on the rate of microwave-induced degradation, atrazine-enriched sorbents were prepared by adding 200 μL of 186 μmol/L atrazine solution (with water or dichloromethane as the solvent) to 0.2 g of dry zeolites in 100 mL PTFE vessels. The vessels were then sealed and stored at 25 °C for 12h to equilibrate. Microwave irradiation of the atrazine-enriched sorbents and microwave-assisted extraction of atrazine and its degradation intermediates were carried out following the procedures developed in our previous study.15 In brief, the atrazineenriched zeolites were irradiated by microwave at fixed power levels, then the atrazine remaining and degradation intermediates formed in the micropores were extracted with methanol using microwave-assisted extraction (no degradation occurred during the extraction step). Atrazine degradation followed pseudozero order kinetics,15 and the degradation rate was obtained by fitting of the masses of atrazine degraded after adjusting for the extraction efficiencies summarized in Table S1 of Supporting Information (SI). Concentrations of atrazine and its degradation intermediates in aqueous and methanol solutions were analyzed on a liquid chromatograph-triple quadrupole tandem mass spectrometer (HPLC-MS/MS) (Agilent Technologies, Palo Alto, CA) with positive electrospray ionization (ESI+). Analytes were separated on an Acquity BEH C18 analytical column (50 × 2.1 mm) with 1.7 μm particle size (Waters, Milford, MA.) and detected by MS/MS in the multiple reaction monitoring (MRM) mode (SI Table S2). Optimization of the instrument conditions and calibration were performed the same way we did previously on a different instrument.15 The mass balance discrepancy was within 7% of the original mass in the first several minutes of microwave irradiation, but deteriorated significantly later due to the unaccounted-for small degradation products.15



RESULTS AND DISCUSSION 1. Impact of Surface Chemistry on Atrazine Sorption in Mineral Micropores. The effect of surface chemistry on atrazine sorption was studied using a series of dealuminated Y zeolites, which have the same pore structure with nominal pore opening of 0.74 nm. Figure 1a shows the sorption isotherms of atrazine on a series of Y zeolites with the same Si/Al ratio of 2.55 but different surface cations. The exchangeable cations in the micropores of CBV-100, CBV-300, CBV-400, Ca/CBV100, and Mg/CBV-100 are Na+, NH4+, H+, Ca2+, and Mg2+, respectively. With pKa of 1.68, atrazine was expected to be sorbed predominantly as neutral molecules in the hydrophobic micropore spaces of the zeolites when the pH value of the solution was higher than the atrazine pKa.15,18 Atrazine sorption behaviors on the zeolites with different surface cations were almost identical, indicating that these cations made small difference on the hydrophobicity of the zeolite micropores. Figure 1b shows the sorption isotherms of atrazine on a series of Y zeolites with Si/Al ratios of 2.55 (CBV-400), 6 (CBV712), 15 (CBV-720), and 40 (CBV-780). The exchangeable cations are H+ for CBV-400, CBV-720, and CBV-780, and NH4+ for CBV-712. Atrazine sorption on all four zeolites exhibited the Freundlich-type isotherms, and the sorption 536

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ing negative lattice charges.38 Dealumination (i.e., removal of Al(III) from the anionic aluminosilicate frameworks) reduces the electrostatic fields in zeolites, which explains the decrease in the relative dielectric constants of Y zeolites as their Si/Al ratio increases. The loss tangent (tanδ) describes the ability of a material to extract energy from an applied microwave field. Materials with tanδ values greater than 0.1 can be effectively heated by microwave while the rest cannot undergo significant microwave heating.39 All the dealuminated Y zeolites tested in this study exhibited low losses, with tanδ values no larger than 0.03. The tanδ values of CBV-100, Ca/CBV-100, and Mg/CBV-100 are relatively higher than the other zeolites because of the greater mobility of Na+, Ca2+, and Mg2+ in the zeolite cages and channels compared to H+ and NH4+.37 Consequently, they are expected to give relatively stronger microwave heating compared to the zeolites containing nonmetallic cations. Nonetheless, the dielectric properties of all the dealuminated Y zeolites studied here indicate that none of their frameworks can absorb significant amount of microwave energy. Figure 2 shows the changes of temperature and tanδ of “wet” CBV-720 under 800 W microwave irradiation. Although the

more energetically hydrated, and hold water molecules in charged clusters more strongly compared to the monovalent cations. As a result, thermal desorption of water sorbed on Ca/ CBV-100 and Mg/CBV-100 was relatively difficult and consumed more energy than CBV-100, which contains Na+. The thermal desorption behaviors of water sorbed on the two nonmetallic cation zeolites, CBV-300 and CBV-400, were also different. With its significantly smaller ionic radius, H+ has a much stronger hydration capacity, thus it is more difficult to remove water from the micropores of HY zeolite CBV-400 compared to the NH4+-containing CBV-300. Compared to the effect of surface cation density, the type of surface cations has much less influence on the content of loosely bound water (and the endotherm peak temperature), which explains the small difference in the atrazine sorption isotherms among the Y zeolites with Si/Al ratio of 2.55 but different cations (Figure 1a) 2. Dielectric Properties of Dealuminated Y Zeolites and Implications for Microwave Heating. For a given material, the magnitude of reflection, absorption, and transmission of microwave radiation is determined by its dielectric properties. Table 2 summarizes the dielectric properties of the Table 2. Summary of Dielectric Properties of the Dealuminated Y Zeolites Used in This Study Measured by the Coaxial Transmission Line Technique sorbent

ε′

ε″

tanδ

CBV-100 Ca/CBV-100 Mg/CBV-100 CBV-300 CBV-400 CBV-720 CBV-760 CBV-780

5.65 5.11 4.65 4.89 4.50 4.50 3.87 3.96

0.11 0.11 0.08 0.03 −0.04 −0.03 −0.04 −0.05

0.02 0.02 0.02 0.01 −0.01 −0.01 −0.01 −0.01

a

The microwave complex permittivities of the dealuminated Y zeolites were calculated from the effective complex permittivities (εeff) measured for the composites of zeolite and polyethylene wax based on the volumetric mixing law: εeff = VZεZ + VPεP, where VZ and VP are the volume fractions and εZ and εP are the permittivities of zeolite and polyethylene wax, respectively.45 For the polyethylene wax used in this study, the values of ε′ and ε″ are 2.2 and 0, respectively. bThe ε′ and ε″ values reported are the means determined from quintuplicated measurements. cNegative tanδ value indicates the sample is lossless. Conventional cavity perturbation theory, which allows simple calculations and measurements of dielectric constant and loss tangent, assumes the quality of the cavity would be decreased because of the dielectric loss of the sample in the metal cavity.46 However, the quality factor of the cavity will actually increase if the sample is lossless, which can make the measured loss tangent go to zero and even negative.47

Figure 2. Changes of temperature and loss tangent (tanδ) of a “wet” CBV-720 sample containing water at 14 H2O/u.c. and atrazine at 194.9 μmol/g during the course of 800 W microwave irradiation. The effective complex permittivities (εeff) measured for the composites of zeolite (containing water) and polyethylene wax were used directly for calculating tanδ values (based on quintuplicated measurements, with negative values indicating lossless specimens) here. The volumetric mixing law was not applicable because the composites contained water, which has a significantly higher complex permittivity than the zeolite and polyethylene wax. For comparison, the temperature profile of dry CBV-720 (without any atrazine or water sorbed) was also plotted.

zeolite framework was essentially transparent to microwave, it could be gradually heated under continuous irradiation. Compared to the dry zeolite, the heating rate was reduced by the presence of water sorbed in the micropores, which served as a significant microwave-absorbing phase (SI Figure S2). The polar atrazine molecules sorbed in the micropores also absorbed some of the microwave energy. The tanδ value of “wet” CBV-720 decreased sharply in the first 3 min and then more slowly during microwave irradiation, indicating it became essentially lossless after removal of the sorbed water. The microwave heating, particularly formation of microscale “hot spots” within the micropores, was expected to cause gradual removal of nonstructural water from the zeolite (SI Figure S3), leaving behind the relatively microwave-transparent mineral phase.2,3 As the zeolite got increasingly dry, it became more permeable for the microwave. Overall, the essentially micro-

dealuminated Y zeolites characterized in this study. The cations in the zeolite cages and channels have greater mobility than the framework atoms, thus generate significant instantaneous dipole moments that interact efficiently with external electromagnetic fields.37 The low relative dielectric constants (around 4.5) of the dealuminated Y zeolites are indicative of poor mobility of the ionic charges within their frameworks. This is as expected as siliceous zeolites are insulators with very low dielectric constants.38 Large electrostatic fields can be generated within the zeolite frameworks only when cations within the cages and channels are located next to the tetrahedra containing A1(III) instead of Si(IV), but they are short-ranged and the positive cation charges are compensated for by the correspond537

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Figure 3. Impact of surface chemistry on degradation of atrazine sorbed in the zeolite micropores under 700 W microwave irradiation: (a) degradation rate of atrazine sorbed on the dealuminated analogues of HY zeolite as a function of surface cation density; (b) schematic illustration of the influence of surface cation density on atrazine degradation, where microwave energy cannot act efficiently on the atrazine molecules sorbed in the micropores when too many hydrated surface cations are present, whereas only insufficient number of “hot spots” can form in the largely hydrophobic micropores having sparse surface cations; and (c) degradation of atrazine sorbed on the dealuminated analogues of Y zeolite (Si/Al ratio of 2.55) with different cations: the cations are Na+, NH4+, H+, Ca2+, and Mg2+ for CBV-100, CBV-300, CBV-400, Ca/CBV-100, and Mg/CBV-100, respectively. Error bars represent standard deviations based on triplicated experiments. Water contents of the zeolites prior to microwave irradiation were 18 H2O/u.c. for CBV-300, 16 H2O/u.c. for CBV-400 and CBV-100, 14 H2O/u.c. for Ca/CBV-100, Mg/CBV-100, CBV-720, and CBV-760, and 12 H2O/u.c. for CBV-780, respectively.

2.450 GHz and can be poorly heated by microwave.40 Meanwhile, atrazine and water sorbed in their micropores can absorb microwave energy, with water having much stronger microwave absorption than atrazine. The polar silanol groups (Si−OH) on the pore wall surfaces can absorb microwave energy, while the hydrated surface cations in the zeolite cages, which have properties similar to the hydrated ions in salt solutions, also act as effective microwave absorbers.41 With the rate of microwave energy absorption much greater than that of heat transfer from the surface,17 microscale “hot spots” can form surrounding the hydrated surface cations, the surface silanol groups, the sorbed atrazine molecules, and the loosely bound water molecules in the micropores. Atrazine undergoes significant thermal decomposition at above 400 °C,42 whereas infrared spectra of atrazine-enriched CBV-720 clearly indicate the condensation of silanol groups to form siloxane bonds after microwave irradiation,15 which occurs at temperatures above 500 °C.43 These observations, together with the ring-cleavage products observed, consistently support formation of “hot

wave-transparent frameworks of the Y zeolites allow the microwave energy to interact predominantly with the atrazine and water molecules sorbed in their micropores without causing excessive heating of the microporous sorbents. 3. Impact of Surface Chemistry on MicrowaveInduced Degradation of Atrazine in Mineral Micropores. Figure 3a shows the degradation rate of atrazine sorbed on HY zeolites under 700 W microwave irradiation as a function of the surface cation density. The degradation rate increased as the monovalent surface cation density of the zeolite decreased from 2.62 sites/nm2 (CBV-400) to 0.42 site/nm2 (CBV-720), and then to 0.23 site/nm2 (CBV-760). However, a further decrease in the cation density to 0.16 site/nm2 (CBV-780) resulted in significant reduction of the atrazine degradation rate. Absorption of microwave radiation by the surfaces of the mineral sorbents depends on the surface chemistry such as hydroxylation and the permittivity of the sorbed species. The ε″ and tanδ values of the four HY zeolites (Table 2) indicate that their frameworks are essentially transparent to microwave at 538

dx.doi.org/10.1021/es303486j | Environ. Sci. Technol. 2013, 47, 533−541

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spots” with elevated temperatures in the micropores and degradation of the sorbed atrazine via pyrolysis.15 In the microwave-induced degradation, the hydrated surface cations are believed to play dual roles as sinks of microwave energy competing with atrazine molecules and as reactive “hot spots” (after absorbing enough microwave energy) promoting atrazine degradation. The presence of greater amount of hydrated surface cations resulted in much less microwave energy available to the atrazine sorbed in the micropores of CBV-400 compared to CBV-720, CBV-760, and CBV-780. Atrazine degraded more slowly in the micropores of CBV-780 compared to CBV-720 and CBV-760, probably because only insufficient number of “hot spots” could form surrounding the fewer hydrated surface cations. These results suggest the existence of an optimal density of surface cations (around 0.23 site/nm2) in the micropores that allows formation of enough “hot spots” without diverting too much microwave energy from the sorbed atrazine molecules (Figure 3b). Figure 3c shows the degradation of atrazine sorbed on the dealuminated analogues of Y zeolite with different cations under 700 W microwave irradiation. All the zeolites have the same Si/Al ratio of 2.55, while the cations are Na+, NH4+, H+, Ca2+, and Mg2+ for CBV-100, CBV-300, CBV-400, Ca/CBV100, and Mg/CBV-100, respectively. The rate of atrazine degradation increased in the order of Ca/CBV-100, Mg/CBV100, CBV-400, CBV-100, and CBV-300. It appears that the degradation rate generally follows the order of decreasing hydration free energy of the cation (with the exception of Ca/ CBV-100 and Mg/CBV-100). Clusters of water molecules were held more strongly by the surface cation with greater hydration free energy, and consequently more microwave energy was consumed to desorb them, resulting in slower atrazine degradation. Overall, the cation type affected the degradation rate of atrazine sorbed in micropores, although the influence was not as significant as that of cation density (i.e., framework Si/Al ratio) of the zeolite. 4. Influence of Water Content on Microwave-Induced Degradation of Atrazine in Mineral Micropores. Because water in the micropores competes with the sorbed atrazine for microwave energy, water content is expected to significantly influence the rate of atrazine degradation. Figure 4 contrasts the microwave-induced degradation of atrazine sorbed in the micropores of a hydrophilic zeolite (CBV-100) and a hydrophobic one (CBV-720) with comparable water contents. As expected, higher water contents led to slower degradation of atrazine sorbed on both zeolites, although the effect was more pronounced on the hydrophobic zeolite CBV-720. Due to the presence of more hydrated surface cations, the sorbed atrazine degraded much slower in the micropores of CBV-100 (Na+, at 2.12 sites/nm2) than in those of CBV-720 (H+, at 0.42 site/ nm2) at comparable water contents. Increasing the water content had a smaller impact on atrazine reactivity in the micropores of CBV-100 because the abundant hydrated surface cations diverted most of the microwave energy from acting on the sorbed atrazine molecules. The competition of water for microwave energy was further corroborated by comparing the degradation rates of atrazine sorbed in the micropores of Y zeolites using water and dichloromethane as the solvents (Table 3). Within the zeolite micropores, atrazine degraded 1.9−3.7 times faster in the dichloromethane system than the corresponding aqueous system. With a dielectric constant of 8.93, dichloromethane has relatively weak microwave absorption. In the absence of

Figure 4. Microwave-induced degradation of atrazine sorbed on the dealuminated analogues of Y zeolite with comparable water contents: (a) CBV-720 with water contents of 14 and 71 H2O/u.c.; and (b) CBV-100 with water contents of 16 and 78 H2O/u.c. Error bars represent standard deviations based on triplicated experiments.

Table 3. Comparison of the Degradation Rates of Atrazine Sorbed on Four Dealuminated Analogues of Y Zeolite under Microwave Irradiation at 800 W When Using Water and Dichloromethane As the Solvent (0.2 g of Dry Sorbent Was Equilibrated with 200 μL of 186 μmol/L Atrazine Solution in Water or Dichloromethane Prior to Microwave Irradiation) degradation rate (nmol/min) sorbent CBV-400 CBV-720 CBV-760 CBV-780

H2O 0.94 1.99 2.93 1.42

± ± ± ±

0.22 0.08 0.47 0.18

CH2Cl2 2.31 6.29 9.36 5.24

± ± ± ±

0.34 0.30 0.30 0.33

strongly microwave-absorbing water, the microwave energy could be absorbed more efficiently by the sorbed atrazine molecules and the surface silanol groups in the micropores, resulting in faster degradation rates. These results confirm that water sorbed in the mineral micropores competes strongly for microwave energy in microwave-induced degradation. Implications for Microwave-Induced Degradation. The density of surface cation affects the rate of microwave-induced degradation significantly, and a monovalent cation density around 0.23 site/nm2 appears to be optimal for degradation of atrazine sorbed in the micropores of Y zeolites. Zeolites with lower surface cation densities also have more hydrophobic frameworks, which make them highly efficient at up-taking atrazine from aqueous solution. Cations with lower hydration 539

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free energies favor faster atrazine degradation in the micropores, and the rate is expected to be much faster for the sorbed atrazine when the zeolite contains NH4+ as the exchangeable cation as compared to H+ and metal cations. Water is a strong microwave absorber (at 2.450 GHz) and its presence can significantly modify the bulk dielectric properties of the dealuminated Y zeolites. The rate of atrazine degradation decreases with increases in the initial water content in the micropores on both hydrophilic and hydrophobic zeolites. Even though water is removed from the micropores under microwave irradiation, the removal rate becomes increasingly slow as the water content decreases (SI Figure S3), and this process appears to have little effect on the overall degradation rate. Large amounts of energy may be required to significantly reduce the water contents of the wet zeolites, particularly for the more hydrophilic ones. Therefore, a trade-off has to be made between the energy saved from faster degradation rates at lower material water contents and the extra energy spent on water removal in practical applications. Microporous mineral sorption coupled with microwave-induced degradation can be potentially used for removing atrazine and other polar organic pollutants from aqueous solution. The microwave-induced degradation is estimated to require about 0.1 kWh of electricity per mole of atrazine degraded (using CBV-720 as the sorbent), in contrast to 0.46 kWh of electricity in photochemical treatment.44 Nonetheless, how to efficiently couple sorption with microwave-induced degradation and prevent microwave leakage from the cavity should be carefully considered in scaling up from laboratory experiments.



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ASSOCIATED CONTENT

S Supporting Information *

Additional information on the efficiencies of microwaveassisted extraction for recovering atrazine from the zeolites, MS/MS conditions for MRM analysis of atrazine and its degradation intermediates, the Freundlich isotherm fits of atrazine sorption on the Y zeolites, correlations of endotherm peak temperature and loosely bound water content of the zeolites with their surface cation density and cation hydration free energy, heating rates of dry and “wet” CBV-100 and CBV400, and the removal of water from “wet” CBV-720 during microwave irradiation. 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.



ACKNOWLEDGMENTS We gratefully acknowledge the anonymous reviewers for valuable comments and suggestions. This work was supported in parts by the Natural Science Foundation of China (Grant No. 41073079), Guangzhou Institute of Geochemistry (Grant No. GIGCX-11-03), and the Chinese Academy of Sciences (“One Hundred Talents” and “Interdisciplinary Collaboration Team” programs). This is contribution No. IS-1587 from GIGCAS. 540

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