Chemical and Physical Transformations of Aluminosilicate Clay

Aug 29, 2014 - (20, 21) During atmospheric transport, however, mineral dust can undergo physical and chemical processing, which can affect its cloud n...
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Chemical and Physical Transformations of Aluminosilicate Clay Minerals Due to Acid Treatment and Consequences for Heterogeneous Ice Nucleation Sarah K. Sihvonen,† Gregory P. Schill,‡ Nicholas A. Lyktey,†,§ Daniel P. Veghte,† Margaret A. Tolbert,‡ and Miriam Arak Freedman*,† †

Department of Chemistry, The Pennsylvania State University, 104 Chemistry Building, University Park, Pennsylvania 16802, United States ‡ CIRES and Department of Chemistry and Biochemistry, University of Colorado at Boulder, Boulder, Colorado 80309, United States S Supporting Information *

ABSTRACT: Mineral dust aerosol is one of the largest contributors to global ice nuclei, but physical and chemical processing of dust during atmospheric transport can alter its ice nucleation activity. In particular, several recent studies have noted that sulfuric and nitric acids inhibit heterogeneous ice nucleation in the regime below liquid water saturation in aluminosilicate clay minerals. We have exposed kaolinite, KGa-1b and KGa-2, and montmorillonite, STx-1b and SWy-2, to aqueous sulfuric and nitric acid to determine the physical and chemical changes that are responsible for the observed deactivation. To characterize the changes to the samples upon acid treatment, we use X-ray diffraction, transmission electron microscopy, and inductively coupled plasma−atomic emission spectroscopy. We find that the reaction of kaolinite and montmorillonite with aqueous sulfuric acid results in the formation of hydrated aluminum sulfate. In addition, sulfuric and nitric acids induce large structural changes in montmorillonite. We additionally report the supersaturation with respect to ice required for the onset of ice nucleation for these acid-treated species. On the basis of lattice spacing arguments, we explain how the chemical and physical changes observed upon acid treatment could lead to the observed reduction in ice nucleation activity.

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freezing, and condensation freezing.10 Several studies have shown evidence for heterogeneous nucleation in cirrus clouds.11−15 The largest component of cirrus ice residuals is mineral dust,15 which is formed through wind action over arid land. The major sources of mineral dust are the Sahara and Gobi deserts, though dust can be up to 50% anthropogenic in origin.16 Mineral dust is composed primarily of aluminosilicate clay minerals, with additional contributions from quartz, carbonates, oxides, evaporates, mica, and feldspars.17 Aluminosilicate clay minerals compose 85% of Asian dust, as measured by X-ray diffraction, and 50−64 wt % of Saharan dust.18,19 In the troposphere, aluminosilicate clay minerals are one of the most ice active types of mineral dust.20,21 During atmospheric transport, however, mineral dust can undergo physical and chemical processing, which can affect its cloud nucleation activity. Multiple recent laboratory and chamber studies have shown that exposure of aluminosilicates and Arizona test dust to sulfuric and nitric acid inhibits heterogeneous ice nucleation in

INTRODUCTION Aerosol−cloud interactions are the largest uncertainty in our understanding of the climate system.1 Aerosol particles act as seeds for cloud condensation and ice nuclei and influence cloud radiative properties and lifetime. Among different cloud types, the climate is especially sensitive to the microphysical properties of cirrus clouds.2 These ice clouds continually cover approximately 30% of the globe and persist in the tropical upper troposphere.3 Because few particles nucleate ice in these clouds, they transmit a larger portion of incoming solar radiation than liquid and mixed-phase clouds while still absorbing outgoing terrestrial radiation.4−6 As a result, optically thin cirrus clouds have a net warming effect on the climate system as measured at the top of atmosphere.7 In addition, these clouds dehydrate air in the upper troposphere, limiting water vapor transport to the stratosphere, and thereby affecting stratospheric ozone concentrations.8,9 Ice nucleates in clouds due to homogeneous and heterogeneous mechanisms. Homogeneous nucleation, in which aqueous droplets freeze to form ice, only occurs below approximately −40 °C.4 In heterogeneous nucleation, a solid particle acts a catalyst for the ice formation process. Four hypothetical modes for heterogeneous nucleation have been proposed: deposition mode, contact nucleation, immersion © XXXX American Chemical Society

Received: May 16, 2014 Revised: August 29, 2014

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the regime below liquid water saturation.22−27 Several different treatments have been used including exposure to sulfuric acid vapor, nitric acid vapor, sulfuric acid vapor with subsequent heating, and aqueous acid. In addition, several different ice growth chambers have been used including the aerosol interaction and dynamics in the atmosphere (AIDA) chamber, the Leipzig aerosol cloud interaction simulator (LACIS), and optical microscopes with environmental chambers. While evidence of the deactivation of ice nucleation has been consistently reported in the water subsaturated regime, the physical and chemical reasons behind this deactivation have only been hypothesized. For example, Tobo et al. state that the reduction of ice nucleation activity in kaolinite treated with sulfuric acid could be due to the formation of aluminum sulfate, which degrades the kaolinite surface.27 Reaction products or sulfate could cover active sites,22,24−26,28 or active sites for ice nucleation could be modified during the reactions.25,28 In this paper, we have used X-ray diffraction (XRD), transmission electron microscopy (TEM), and inductively coupled plasma-atomic emission spectroscopy (ICP-AES) to characterize the chemical and physical changes that the aluminosilicate clay minerals, kaolinite and montmorillonite, undergo when exposed to aqueous sulfuric and nitric acid. We have additionally performed experiments to detect the onset of heterogeneous ice nucleation for treated and untreated samples. Kaolinite is a nonswelling aluminosilicate clay that is commonly found in samples of mineral dust.20 Montmorillonite was used for comparison because it is an aluminosilicate clay that swells upon exposure to water. We use aqueous acids to mimic cloud processing of mineral dust.29 XRD provides a fingerprint of each mineral that is sensitive to physical and chemical changes upon reaction. Understanding structural changes may be especially important for deposition mode nucleation, in which water vapor adsorbs on a heterogeneous surface, forms a critical nucleus, and grows ice. For deposition mode nucleation, lattice match between the surface lattice and the growing ice lattice may be especially important for promoting nucleation. Poor lattice match may inhibit ice nucleation by causing strain in the initial molecular layers of ice that form on the nucleus,30 hindering the formation of the ice crystal that indicates the onset of ice nucleation. As a result, the particle may not nucleate until higher supersaturations of water vapor with respect to ice are reached. At these higher supersaturations, other modes of nucleation may dominate. Other types of nucleation have additional considerations besides lattice match: in immersion/condensation freezing, the change in the water structure surrounding the heterogeneous nucleus from liquid to ice is important; in contact nucleation, the nature of the collision may affect the nucleation. Through our study, we are able to argue that the observed deactivation in ice nucleation activity in the liquid water subsaturated regime is due to changes in lattice match between the reacted surface and ice due to product formation and structural changes in the minerals.

KGa-2, has more substitutions in the aluminosilicate sheets than the low defect form, KGa-1b. Major mineral impurities in source clays are detailed in Chipera and Bish.31 Approximately 0.4 g of each mineral was exposed to 15 mL of variable concentrations of sulfuric acid (EMD, ACS Reagent grade) and nitric acid (EMD, ACS Reagent grade) for 1.5 h. High concentrations (1.0 M, which is equivalent to 9.8 × 10−2 wt %) of sulfuric and nitric acids were used to produce sufficient physical and chemical changes to observe with X-ray diffraction (XRD), which can detect compounds that compose 0.5 to 5 wt % of a sample. Lower concentrations of acid down to 0.010 M (9.8 × 10−4 wt %) were also used. To determine the pH of condensed-phase aqueous sulfuric acid in the upper troposphere, we have used the extended aerosol inorganics model (E-AIM).32−34 At 240 K, water vapor is saturated with respect to ice at a relative humidity (RH) of 72.4%.35 Between a relative humidity of 70% RH and 99.9% RH, the concentration of sulfuric acid in solution corresponds to a pH of less than 0 to approximately 1.5, which is a similar range to the sulfuric acid concentrations chosen for our experiments.32−34 The treated minerals were centrifuged and the supernatant was collected for inductively coupled plasma-atomic emission spectroscopy (ICP-AES) analysis, from which we measure the concentration of dissolved cations. Two drying procedures were used and compared. In the first, the mineral was dried in the oven at 110 °C for 3 h, and in the second, it was dried under vacuum at room temperature for 3 days. Using X-ray diffraction (XRD), transmission electron microscopy (TEM), and diffuse reflectance infrared Fourier transform spectroscopy, we have confirmed that this heating temperature causes no physical changes to the mineral other than dehydration of montmorillonite. After the sample was dry, it was gently ground in a mortar and pestle and analyzed using XRD and TEM. For the XRD experiments, we use an X-ray diffractometer with Cu Kα radiation with a current of 40 mA and a voltage of 45 kV. Bulk powder samples are front loaded in a zero background silicon holder. Goniometer angles are set from 5 to 70° 2θ with a 0.0263° 2θ step size and a scan step time of 96.4 s. A beam knife is used to reduce low angle scattering. For samples where multiple concentrations of acid were used, we added a nickel standard (approximately 7 wt %; Alfa Aesar, 325 mesh, 99.8%) in order to calibrate the peak positions in the diffractograms. The XRD diffractograms were analyzed using Jade XRD libraries. Rietveld analysis was not performed due to the fact that the samples were not isotropically oriented in the sample holder. Samples for TEM were prepared using aerosol that is dry generated from the untreated and treated minerals, which was impacted on continuous carbon coated copper grids (SPI Supplies). Control experiments, which followed the above procedure using water instead of acid, were also performed to determine what effects were from acid treatment and which were from hydration and drying. Ice Nucleation Experiments. The experimental apparatus and techniques used for ice nucleation experiments have been described previously in detail.36,37 Thus, we only offer a brief description here. A Nicolet Almega XR Raman spectrometer coupled to an Olympus BX-51 microscope (10×, 20×, 50×, and 100× capabilities) has been outfitted with a Linkam THMS600 environmental cell. The temperature of a cold-stage inside the cell is governed by a Linkam TMS94 temperature controller, which has a temperature accuracy of 0.1 K. Water vapor inside the cell is controlled by mixing flows of dry and humidified N2. Water partial pressure (PH2O) inside the cell is

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EXPERIMENTAL METHODS Mineral Preparation and Instrumental Analysis. Four clay samples were obtained from the Source Clays Repository of the Clay Mineral Society (West Lafayette, IN): low-defect kaolinite, Washington County, GA (KGa-1b); high-defect kaolinite, Warren County, GA (KGa-2); Ca-rich montmorillonite, Gonzales County, TX (STx-1b); Na-rich montmorillonite, Crook County, WY (SWy-2). The high defect kaolinite, B

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measured by a Buck Research CR-A1 chilled mirror dew point hygrometer, which has an accuracy of 0.15 K. The equilibrium vapor pressure over ice (VPice) was derived from the temperature reading by employing the parametrization of Murphy and Koop.35 We note that the Raman spectrometer was not used in this study because the minerals fluoresce, which masks chemical signatures from the samples. For the ice nucleation experiments, both untreated and treated clay mineral particles were generated by directing a flow of dry nitrogen over a powder of dry clay minerals, which was agitated by a stir bar; this method of dry particle generation is similar to the one utilized by Veghte and Freedman.38 The particle flow was directed at a fused silica disk that was treated with commercially available Rain-X to make a hydrophobic surface. Larger particles were removed from the disk by an additional burst of dry nitrogen. The resulting particles were clay aggregates with lateral diameters ranging from ∼1.8 to 20 μm, with the median lateral diameter being ∼2.5 μm. A typical ice nucleation experiment begins by placing a particle-laden fused silica disk into the environmental cell. The particles sit at 298 K and ∼0% RH for at least 10 min to ensure that they are nominally dry. The ice saturation ratio (Sice = PH2O/VPice) inside the cell is controlled by maintaining a constant dew point and steadily decreasing the temperature. Specifically, after a stable dew point is achieved, the temperature is decreased at a rate of 10 K min−1 until Sice was ∼0.9. The temperature is then decreased at a rate of 0.1 K min−1 until ice is observed visually. At the temperatures explored in this study, this corresponds to rate of Sice change of ∼0.01 min−1. After visual confirmation of ice, the humidified flow of nitrogen is turned off to sublime the ice away to reveal the ice nucleus. The size of the resulting ice nucleus and its position on the fused-silica disk are recorded. The sizes of the nucleating particle in this study ranged from ∼1.8 to 16 μm. For all ice nucleation experiments, at least three experiments were conducted on at least two separate disks. Errors are reported as one standard deviation of the mean.

Figure 1. XRD diffractograms of kaolinite (KGa-1b) untreated and treated with 1.0 M nitric acid and 1.0 M sulfuric acid. Mineral samples that were treated with sulfuric acid were either dried in a vacuum or in an oven, as described in the text. The dotted lines mark the (001) and (002) peaks. The peak in the inset and the arrows indicate the peaks corresponding to alunogen (hydrated aluminum sulfate; Al2(SO4)3· 17H2O) that are visible in the XRD diffractograms. The quartz (Q) and anatase impurities in this mineral are indicated. The quartz signal is enhanced in the nitric acid treated sample, perhaps due to preferential orientation in that sample.

sample, indicating that the bulk structure of kaolinite has not changed during acid treatment. Treatment of KGa-1b and KGa2 with nitric acid does not result in the formation of new peaks or changes in existing peaks in the XRD diffractogram. Using transmission electron microscopy (TEM) and inductively coupled plasma−atomic emission spectroscopy (ICP-AES), we obtain additional information about the effects of acid treatment on the minerals (Figures 2, 3, S2, and S3). In Figures 2 and S2, we show TEM images of untreated and treated minerals. KGa-1b and KGa-2 particles appear as stacked polygonal plates. The edges of these sheets become roughened after exposure to sulfuric acid, in agreement with the literature.39 In contrast, treatment with nitric acid does not change the structure of the particles. Exposure of KGa-1b and KGa-2 to water, sulfuric, and nitric acid results in the dissolution of ions from the clays (Figures 3 and S3). Based on ICP-AES, water primarily dissolves silicon and sodium from the clays. Nitric acid results in the dissolution of higher concentrations of silicon and sodium and additional types of ions including aluminum and calcium. Sulfuric acid results in dissolution of the same ions as nitric acid, but in the same or greater quantities (Figures 3 and S3). Because edge sites are the least stable sites in the lattice due to lower coordination, we expect that the dissolution of silicon and aluminum occur in greater quantities from the edge of the minerals for both sulfuric acid and nitric acid treatment. Because sulfuric acid dissolves more ions than nitric acid, we only observe a change in the edges of the minerals with TEM using sulfuric acid (Figures 2 and S2). The new peaks in the XRD diffractograms of sulfuric acid treated and oven-dried KGa-1b and KGa-2 could be due to a chemical change of this mineral (Figures 1 and S1). If the new peaks are due to a chemical transformation of kaolinite, we would expect that the new compound would form from sulfuric acid reacting with one of the ions that is dissolved into the solution to form a salt. Of the possible choices, the Jade XRD library shows that the best fit for these peaks is alunogen,

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RESULTS AND DISCUSSION Throughout the following section, data from the low-defect kaolinite sample, KGa-1b, and the Ca-rich montmorillonite sample, STx-1b, are shown in the text of the manuscript, and data from the high-defect kaolinite sample, KGa-2, and the Narich montmorillonite sample, SWy-2, are shown in the Supporting Information. In addition, control experiments for both montmorillonite samples are included in the Supporting Information. Kaolinite. The bulk structure of kaolinite, as observed with X-ray diffraction (XRD), did not change upon heating, exposure to water, or during acid treatment (Figures 1 and S1, Supporting Information). The diffraction peaks of KGa-1b and KGa-2 remained unchanged after drying the powders in an oven or under vacuum, indicating that no changes to the structure of kaolinite occurred from this treatment. We note that drying under vacuum conditions is the more atmospherically relevant process because no heat is applied. In addition, exposure to water prior to drying the powders also had no effect on the X-ray diffraction peaks. Upon exposure to 1.0 M sulfuric acid, the samples of KGa-1b and KGa-2 have new peaks at 6.5° and 24.2° 2θ. As described below, these new peaks are due to a product that forms on the surface of the kaolinite. Other peaks have the same positions as in the untreated C

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vacuum-dried sample was not dried as aggressively as the ovendried sample. As a result, not as much water was driven off of the aluminum sulfate, allowing it to remain more amorphous in structure, which decreases its intensity in the XRD diffractogram. The fact that alunogen is observed in the XRD diffractogram even though it is highly soluble suggests that the reaction products that we observe form during the drying process. After the sample is centrifuged, the supernatant, which may contain soluble reaction products, is removed for analysis and the mineral is dried. The majority of soluble reaction products will be removed with the supernatant. During drying, however, the sulfuric acid can continue to react with the mineral, and product compounds are not removed. As a result, soluble species remain on the mineral surface. Aluminosilicate clay minerals consist of sheets of tetrahedrally bonded silicon and octahedrally bonded aluminum, with kaolinite having one sheet of silicon for every sheet of aluminum. Substitution of aluminum for silicon in the tetrahedral layers and divalent cations for aluminum in the octahedral sheets of clay minerals results in a net negative charge on the basal sheets. To balance this charge, cations (e.g., Na+, K+, Ca2+) adsorb on the basal planes of the mineral and OH occasionally replaces O in the aluminosilicate lattice.42 Because kaolinite is a nonswelling clay mineral, aqueous solutions access the outer surface of the mineral and pores, rather than the interior. As a result, the reaction products are likely located at the surface of the mineral. Dissolution of cations in kaolinite in aqueous solution starts from locations with incomplete bonding, and as a result, is likely to originate from the edge sites on the mineral.42 Silica and alumina are exposed to the surface along basal planes and edge sites. Acid treatment results in the protonation of Si−O−Al and Si−O−Si bonds, with preference for the former.43,44 Dissolution rates of kaolinite are dependent on pH and temperature, and have been found to decrease over time.45,46 At steady state, aluminum and silicon are released stoichiometrically at all pH values.46 The literature of clay mineral dissolution is consistent with the TEM images in which the edges of the mineral are roughened due to exposure to sulfuric acid, the aluminum sulfate and otherwise unchanged kaolinite structure seen from the XRD diffractograms, and the dissolved ions observed with ICP-AES. For our kaolinite samples, we observe more removal of silicon when sulfuric acid is used, and the same amounts of silicon and aluminum released with nitric acid within the error of our measurement (Figures 3 and S3). We note that our reaction is not performed under steady state conditions. We have additionally performed experiments to determine how acid treatment affects the onset of ice nucleation of KGa1b (Figure 4). Consistent with previous studies, kaolinite nucleates ice in the liquid water subsaturated regime at low supersaturations (Sice < 1.10).22 Nitric acid-treated mineral requires higher supersaturations to nucleate ice, meaning that nitric acid impairs the ice nucleation activity in the regime below liquid water saturation. Sullivan et al. have also found that nitric acid impairs the ice nucleation of Arizona test dust in this regime.24 We observe that sulfuric acid further impairs the ice nucleation activity of these minerals, in agreement with the literature.22,23,25,26,28,47 As seen in Figure 4, we do not observe as large a degree of deactivation as Chernoff and Bertram most likely due to differences in sample origin and preparation.26 In particular, their samples are treated in a more concentrated acidic solution for a longer period of time before the solution is

Figure 2. Transmission electron microscopy (TEM) images of (a) kaolinite (KGa-1b), (b) sulfuric acid-treated, oven-dried KGa-1b, (c) sulfuric acid-treated, vacuum-dried KGa-1b, and (d) nitric acid-treated, oven-dried KGa-1b. The acid concentrations were 1.0 M. The insets show the edges of the mineral. The smaller boxes on the full image of the particle indicate where the insets are taken.

Figure 3. Inductively coupled plasma−atomic emission spectroscopy (ICP-AES) of kaolinite (KGa-1b) that is treated with water, 1.0 M sulfuric acid, and 1.0 M nitric acid.

hydrated aluminum sulfate (Al2(SO4)3·17H2O).40 Higher intensity peaks of kaolinite mask the other peaks of alunogen. While bulk minerals are treated with aqueous acids in this study, we do not know if the alunogen completely coats the particle surfaces. Both alunogen and aluminum hydroxide were observed in XRD diffractograms of kaolinite treated with sulfuric acid under harsher conditions.41 We do not observe the aluminum hydroxide perhaps due to the shorter exposure time and lower temperature of our experiment. Other compounds and other hydration states of aluminum sulfate may form in amounts of insufficient concentration to be observed with XRD. In addition, the peak due to alunogen is less intense in the vacuum-dried, acid-treated sample. We hypothesize that the D

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Table 1. Lattice Parameters for the Compounds Investigated in This Study lattice parameters (Å)

a

compound

structure

a

b

c

ice (Ih)a kaoliniteb montmorillonitec acid-treated montmorillonited Al2(SO4)3·17H2Oe

hexagonal triclinic hexagonal monoclinic

4.498 5.154 5.169 5.3348

4.498 8.942 5.169 9.2505

7.338 7.401 15.02 12.4370

triclinic

7.4200

26.9250

6.0520

Reference 67. e Reference 40.

b

Reference 68.

c

Reference 69.

d

Reference 70.

an active site, which may only be present on one of the three faces.48 The identity of these active sites is unknown for the compounds used in this study. As a result, the sets of lattice parameters ab, ac, and bc should be compared to these sets for ice. The ab, ac, or bc face of ice could form on any of the faces of the mineral. Note that we list literature values for the lattice spacings. Because the peak positions in an XRD diffractogram determine the lattice spacings, two samples with the same peak positions will have the same lattice spacings. While kaolinite does not appear to have lattice spacings close to ice, the hexagonal arrangement of hydroxyl groups on the basal plane of kaolinite has been hypothesized to lead to kaolinite being a good ice nucleus.49 During sulfuric acid exposure, the bulk lattice arrangement of kaolinite does not change, but the surface dissolves and alunogen is formed and is present at the surface of kaolinite. Because the XRD diffractograms of untreated and acid-treated kaolinite are identical with the exception of the peaks due to alunogen in the acid-treated sample, the lattice spacings of untreated and acid-treated kaolinite are the same. Alunogen has a more dissimilar lattice spacing compared to ice than kaolinite, and is presumably also worse than the ice-like arrangement of hydroxyl groups on the kaolinite surface. Our results therefore suggest that covering the kaolinite with a less good ice nucleus may be responsible for the higher supersaturations required for the onset of ice nucleation when the mineral is treated with sulfuric acid. Interestingly, the onset of ice nucleation also occurs at higher supersaturations for nitric acid, which resulted in no change to the physical or chemical structure of kaolinite except for the dissolved ions observed in ICP-AES. We hypothesize that in addition to possible changes to the surface of the mineral due to dissolution, treatment with nitric acid results in an amorphous product or a crystalline product with a concentration that is below our detection limit, such as an aluminum nitrate salt, which is not observed in XRD and is a less efficient nucleus for ice formation than kaolinite. Montmorillonite. The results for the montmorillonite samples, STx-1b and SWy-2, are more complex than the results for kaolinite due to the fact that montmorillonite, a type of smectite clay, swells with exposure to water (Figures 5 and S4− S6). Using TEM, we observe that the edges of the sulfuric acid treated montmorillonite look less thin and flakey than the original mineral, though these changes are less distinct than we found from kaolinite, perhaps because both the edges and the interstitial spacings between the basal planes are accessible to the acid (Figures 6 and S7). We note that quartz is a component of STx-1b and SWy-2, which is visible in the TEM images and the XRD diffractograms (Figures 5-6, S4−6). From ICP-AES, we observe that an order of magnitude more cations dissolve from montmorillonite than kaolinite (Figures 7 and

Figure 4. Supersaturation with respect to ice that is required for the onset of heterogeneous ice nucleation on kaolinite (KGa-1b), kaolinite treated with 1.0 M sulfuric acid and then dried in a vacuum, kaolinite treated with 1.0 M sulfuric acid and then oven-dried, kaolinite treated with 1.0 M nitric acid, and hydrated aluminum sulfate. The data are compared to the results of Chernoff and Bertram.26 The solid line corresponds to water saturation and the dashed line is the homogeneous freezing line for sulfuric acid droplets assuming a homogeneous nucleation rate of J = 5 × 109 cm−3 s−1 as designated in Koop et al.66

dried in droplets containing the minerals, which could result in more alteration to the minerals due to acid treatment and a thicker coating of product compounds.26 Vacuum drying and oven drying produce the same results within the uncertainty of the measurement except at the highest temperatures. Experiments were also performed to compare the ice nucleation activity of hydrated aluminum sulfate to kaolinite. The form of hydrated aluminum sulfate that was obtained commercially was Al2(SO4)3·16H2O. At the lowest temperature of 235 K, hydrated aluminum sulfate nucleates ice at the same supersaturation as the sulfuric acid-treated KGa-1b. At 240 and 245 K, hydrated aluminum sulfate deliquesces. As the supersaturation is further increased, heterogeneous freezing could occur on a KGa-1b particle. The results at the lowest temperature suggest that nucleation on hydrated aluminum sulfate rather than kaolinite may be responsible for the observed deactivation. At the warmer temperatures, the soluble coating may cause heterogeneous nucleation to occur by immersion/condensation freezing rather than deposition mode nucleation. A similar hypothesis is used to explain the ice nucleation behavior of kaolinite coated with ammonium sulfate in Eastwood et al.22 We can roughly compare the ice nucleation activity of different crystalline surfaces by considering their lattice spacings. As a first approximation, compounds with lattice spacings similar to ice are better heterogeneous nuclei for ice formation. We have listed lattice spacings in Table 1. The first two numbers (ab) for each clay mineral correspond to the basal plane. The pairs of numbers ac and bc, correspond to the lattice parameters for the mineral edges. For the reaction product, the pairs of numbers ab, ac, and bc describe the lattice spacings on each face of the salt crystal. Ice can form on the basal plane and/or the edges of the mineral or on any of the faces of the salt. Some compounds will have a preferred site of nucleation, E

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Figure 6. Transmission electron microscopy (TEM) images of (a) montmorillonite (STx-1b), (b) sulfuric acid-treated, oven-dried STx1b, (c) sulfuric acid-treated, vacuum-dried STx-1b, and (d) nitric acidtreated, oven-dried STx-1b. The acid concentrations were 1.0 M.

Figure 5. XRD diffractograms of montmorillonite (STx-1b) exposed to varying concentrations of (a) nitric acid and (b) sulfuric acid. The acid-treated samples were all oven-dried. The dotted lines mark the (001), (003), and (005) peaks. A nickel standard is used to align the spectra. The labels “O” and “Q” correspond to opal-CT and quartz impurities. The arrows in part b indicate the peak positions for alunogen (Al2(SO4)3·17H2O).

S8), which is expected because montmorillonite has a cation exchange capacity that is an order of magnitude larger than that of kaolinite.42 Exchangeable cations primarily originate from external and interstitial basal planes with more limited contribution from edge sites.42 The swelling of montmorillonite facilitates the dissolution of cations into solution. During acid treatment, we observe that the calcium-rich montmorillonite (STx-1b) primarily releases calcium, while aluminum, sodium, silicon, and magnesium were dissolved to a lesser extent. The Na-rich montmorillonite (SWy-2) releases sodium and calcium in similar amounts followed by magnesium. Magnesium is often substituted for aluminum in the octahedral sheets of montmorillonite.42 During the initial steps of dissolution, cations are rapidly released from smectites, a class of minerals that includes montmorillonite.50−52 At steady state, the dissolution of aluminum and silicon are stoichiometric.50,51,53,54 Similar to kaolinite, the dissolution is expected to originate from the edges of the mineral.51,55 We note that STx-1b releases more aluminum than silicon and SWy-2 releases more silicon than aluminum during dissolution in acidic solution. In comparison, literature studies of smectites and chlorite have shown that during the initial steps of dissolution, silicon is often released in greater concentrations than aluminum.50,54,56−58

Figure 7. Inductively coupled plasma−atomic emission spectroscopy (ICP-AES) of montomorillonite (STx-1b) that is treated with water, 1.0 M nitric acid, and 1.0 M sulfuric acid.

Unlike kaolinite, montmorillonite undergoes structural changes with changes to hydration state with no acid added (Figures S4 and S5). Similar results are obtained when samples are dried in a vacuum or in an oven. For STx-1b, we observe that the (001), (003), and (005) peaks move to larger values of 2θ. Because the (00l) direction is along the axis that contains the interlayer spacing and XRD is an inverse spectroscopy, the change in peak position indicates that the interlayer spacing is smaller in the treated sample. We expect that we are drying these samples to a greater extent than the starting sample, resulting in the decreased interlayer spacing. The shifts in our XRD diffractograms are consistent with what we would expect from humidifying and drying the sample,59 though, in our experiment, cations are also dissolved during the wetting process. In addition, these control experiments can change the F

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treated with acid, only the (001) peak moves to larger values of 2θ. The (003) peak is reduced in intensity, disappears, or moves to smaller values of 2θ and the (005) peak moves to smaller values of 2θ (Figures 5 and S6). Because this result is different than that observed in the control experiments, we hypothesize that it is due to a chemical change in the montmorillonite lattice. We can obtain additional information about our observations by investigating the changes in the XRD diffractograms when the starting material (STx-1b) is treated with lower concentrations of acid (Figure 5). As the concentration of sulfuric acid is lowered from 1.0 to 0.010 M, the (001) peak shifts to smaller values of 2θ and becomes narrower and more intense, the (003) peak reappears, and the (005) peak shifts to higher values of 2θ. The alunogen peak is most pronounced for the 1.0 M treatment and decreases in intensity as the acid concentration is reduced. As the concentration of nitric acid is lowered from 1.0 to 0.010 M, the (001) and (003) peaks become more intense and narrower and the (005) peak shifts to higher values of 2θ. Through these experiments, the progression of the reaction with acid can be observed. The literature has shown that acidic protons can replace dissolved cations including aluminum in montmorillonite, forming a hydrogen clay (H-montmorillonite).60 The process of forming H-montmorillonite is due to ion exchange and protonation of surface, interlayer, and cavity spacings.61 This ion exchange process is fast compared with the hydrolysis of Al−O−Si bonds and dissolution.52 Unlike kaolinite, the surface does not become saturated with protons.56 Instead, Hmontmorillonite is unstable and rearranges to form Almontmorillonite, in which Al cations are interstitial ions in the silica-rich montmorillonite.61−63 To determine the lattice spacings of the acid-treated form of montmorillonite we observe in the XRD diffractograms, we use the Jade XRD library. Experiments to detect the onset of ice nucleation for treated and untreated montmorillonite STx-1b are shown in Figure 8. Untreated montmorillonite exhibits ice nucleation in the liquid water subsaturated regime at low supersaturations with respect to ice. Our results for untreated montmorillonite are in agreement with previous studies that show that supermicron montmorillonite nucleates below a Sice of 1.2 for the temperature range studied.22,26,64,65 In contrast to the results for kaolinite, nitric acid treatment does not affect the onset of ice nucleation for montmorillonite. Sulfuric acid treatment results in nucleation onsets that occur at significantly higher supersaturations, especially at higher temperatures. Our results do not show as large a deactivation as Chernoff and Bertram perhaps due to sample origin and preparation, as discussed for the kaolinite sample.26 At the lowest temperature, nucleation on aluminum sulfate or, at the higher temperatures, freezing of aqueous aluminum sulfate surrounding a montmorillonite particle could explain the onset supersaturation, similar to kaolinite. We can again approximately compare the ice nucleation activity of acid-treated montmorillonite with untreated montmorillonite using arguments based on lattice spacings (Table 1). Similar to kaolinite, the formation of alunogen could result in a full or partial coating of surface active sites for ice nucleation, which will reduce the ice nucleation activity of the montmorillonite. In addition, montmorillonite converts to a different form during the acid treatment. As for kaolinite, we use literature values for the lattice spacings, using compounds that have identical peak positions in the XRD diffractograms

shape of the peaks in the XRD diffractogram. For STx-1b, the (001) peak of the oven-dried sample is not as sharp as in the original sample. The broadness and sharpness of peaks in XRD is related to the degree of ordering within the sample as well as crystallite size. For SWy-2, similar results are observed for the (001) and (003) peaks. The (005) peak is much reduced in intensity, and in most diffractograms, there is no distinct peak. For both montmorillonite samples, just drying the samples generally produces less intense peaks, while wetting and then drying the montmorillonite samples results in sharper peaks or peaks of similar intensity. Montmorillonite undergoes significant chemical and physical changes when reacted with acids (Figures 5 and S6). Both STx1b and SWy-2 have new peaks after sulfuric acid treatment due to the product compound, alunogen (hydrated aluminum sulfate, Al2(SO4)3·17H2O). We observe these peaks in the oven-dried samples, but not consistently in the vacuum-dried samples. When one of the vacuum-dried montmorillonite samples was further dried by being stored in the lab for 11 months, we then observed alunogen in the XRD diffractogram. We expect that the product formed in the vacuum-dried sample retains more water and as a result, it is not sufficiently crystalline to be observed with the XRD. In a study in which montmorillonite was exposed to sulfuric acid at pHs = 0, 2, and 4 at 60 °C for 30 days, hydrated forms of calcium sulfate, iron sulfate, bisodium magnesium sulfate, potassium aluminum sulfate, and silica are seen in addition to alunogen.41 This result indicates that other salts may form in the montmorillonite sample, but not in large enough concentration to observe with XRD. In addition, we do not know what percentage of each montmorillonite particle is covered with reaction products. We also observe significant changes in the positions and shapes of the (00l) peaks. For both montmorillonite samples, the (001) peak moves to higher values of 2θ, indicating that the crystal is compressed along this axis. For the montmorillonite, STx-1b, acid-treatment followed by oven drying reduces the intensity and broadens the (001) peak. In contrast, for the Na-rich montmorillonite, SWy-2, treatment with acid narrows the (001) peak. These changes suggest that the STx-1b is becoming more disordered and the SWy-2 is becoming more ordered along the (00l) direction. For STx-1b, the (003) peak is not present in the 1.0 M sulfuric acid treated samples. In SWy-2, the peak that corresponds to the (003) peak in the original sample moves to lower values of 2θ in the vacuum-dried sample and is additionally much reduced in intensity in the oven-dried sample. For both montmorillonite samples, the peak that corresponds to the (005) peak in the untreated sample also moves to lower values of 2θ in the acidtreated samples. Nitric acid also affects the peaks in the XRD diffractogram of montmorillonite. For both montmorillonite samples, the (001) peak moves to larger values of 2θ, the (003) peak is reduced in intensity, and the (005) peak moves to smaller values of 2θ. This effect is more pronounced in STx-1b than SWy-2. Similar to the sulfuric acid, treatment with nitric acid broadens the STx1b (001) peak and narrows the SWy-2 (001) peak. The shifts in peak positions are much less pronounced for nitric acid treated samples than for sulfuric acid-treated minerals. In addition, there are no additional peaks present due to product formation. The control experiments show that contraction of the interstitial spacing of both montmorillonite samples results in (00l) peaks that move uniformly to larger values of 2θ (Figures S4 and S5). In contrast, when the montmorillonite samples are G

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irreversible physical and chemical changes to active sites could result in the reported decrease.25,27,28

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CONCLUSIONS We have observed that kaolinite and montmorillonite that are treated with aqueous acids have an increase in the supersaturation required for the onset of heterogeneous ice nucleation in the regime below liquid water saturation, with the exception of nitric acid treatment of montmorillonite. The deactivation of heterogeneous ice nucleation of sulfuric acid treated minerals is accompanied by the production of measurable amounts of alunogen (hydrated aluminum sulfate). In addition, during acid treatment, the edges of kaolinite dissolve and montmorillonite undergoes additional physical and chemical changes in which it converts to a different form of montmorillonite. We have used these physical and chemical changes to explain the reduction in ice nucleation activity of these materials in terms of lattice match to ice. We find that both the formation of a reaction product in kaolinite and montmorillonite and structural changes to the minerals can explain the reduction in ice nucleation activity. Our results directly demonstrate a chemical basis for why a reduction in ice nucleation activity in the liquid water subsaturated regime is observed in acid-treated aluminosilicate clay minerals.

Figure 8. Supersaturation with respect to ice that is required for the onset of heterogeneous ice nucleation on montmorillonite (STx-1b), montmorillonite treated with 1.0 M sulfuric acid and then dried in a vacuum, montmorillonite treated with 1.0 M sulfuric acid and then oven-dried, montmorillonite treated with 1.0 M nitric acid, and hydrated aluminum sulfate. The data are compared to the results of Chernoff and Bertram (ref 26). The solid line corresponds to water saturation and the dashed line is the homogeneous freezing line for sulfuric acid droplets assuming a homogeneous nucleation rate of J = 5 × 109 cm−3 s−1 as designated in Koop et al. (ref 66).

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ASSOCIATED CONTENT

S Supporting Information *

Data for the high-defect kaolinite sample, KGa-2, the Na-rich montmorillonite sample, SWy-2, and control experiments for both montmorillonite samples (STx-1b and SWy-2) as described in the text. This material is available free of charge via the Internet at http://pubs.acs.org

and, therefore, identical lattice spacings as our experimental systems. The lattice spacings of these two types of montmorillonite differ, with the acid-treated sample having a poorer lattice match with ice. We therefore expect that montmorillonite does not nucleate as readily in the liquid water subsaturated regime after treatment with sulfuric acid due to the formation of the alunogen product and the conversion to a different form of montmorillonite. Comparison to Previous Studies on the Ice Nucleation of Coated Mineral Dust Aerosol. Several different hypotheses for the decrease in ice nucleation activity have been proposed in the literature. In the case of nitric acid exposure, deactivation of heterogeneous ice nucleation in the liquid water subsaturated regime has been suggested to be due to the formation of reaction products at the surface of the mineral, which cover active sites for ice nucleation.24 We expect that soluble salts form from the reaction of nitric acid with kaolinite and montmorillonite. Depending on their lattice spacings, the reaction products may cause the observed deactivation for kaolinite. In the case of sulfuric acid, the literature suggests that deactivation could be due to the adsorption of sulfate or reaction products on the surface of the mineral,22,25−28 physical and chemical changes to active sites for ice nucleation,25,27,28 or changes in how water interacts with the surface.47 Our XRD diffractograms show the formation of alunogen in the case of reactions with sulfuric acid, which has lattice spacings that are more dissimilar to ice than the original minerals. These results suggest that reaction products covering active sites on the mineral surface could result in a decrease in ice nucleation activity in the liquid water subsaturated regime. In addition, structural changes observed in TEM images of kaolinite treated with sulfuric acid and XRD diffractograms of montmorillonite with sulfuric and nitric acid support the hypothesis that

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AUTHOR INFORMATION

Corresponding Author

*(M.A.F.) E-mail: [email protected]. Present Address §

Department of Chemistry, Indiana University, 800 E. Kirkwood Ave., Bloomington, IN 47405

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank N. Wonderling and P. Heaney for helpful discussions regarding X-ray diffraction and clay minerals, K. T. Mueller and G. Kulkarni for reading drafts of this paper, and J. V. Badding for use of a centrifuge. We additionally thank V. J. Alstadt for the diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) data and K. T. Jansen for sample preparation. We acknowledge the Materials Characterization Lab run by the Materials Research Institute at Penn State for use of the Philips EM420T TEM, the PANanalytical Empyrean XRD, and the Bruker IFS 66/S DRIFTS instrument. We also acknowledge the Emission Spectroscopy Lab run by the Penn State Institutes of Energy and the Environment for use of the PerkinElmer Optima 5300 UV ICP-AES. The XRD, ICP-AES, and TEM work was supported by start-up funding from The Pennsylvania State University and the ice nucleation work was supported by NSF AGS 1048536. H

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