Article pubs.acs.org/JPCC
Cite This: J. Phys. Chem. C 2018, 122, 18690−18704
Influence of Environmentally Relevant Physicochemical Conditions on a Highly Efficient Inorganic Ice Nucleating Particle Mainak Ganguly,† Simon Dib,‡ Uday Kurien,† Rodrigo Benjamin Rangel-Alvarado,‡ Yoichi Miyahara,§ and Parisa A. Ariya*,†,‡ †
Department of Atmospheric and Oceanic Sciences, McGill University, Montreal, Quebec H3A 0B9, Canada Department of Chemistry, McGill University, Montreal, Quebec H3A 0B8, Canada § Department of Physics, McGill University, Montreal, Quebec H3A 2T8, Canada J. Phys. Chem. C 2018.122:18690-18704. Downloaded from pubs.acs.org by UNIV PIERRE ET MARIE CURIE on 08/21/18. For personal use only.
‡
S Supporting Information *
ABSTRACT: Ice nucleation microphysical processes are identified to be of high importance in forecasting the magnitude of the Earth’s climate change. The environmental conditions often influence the ice nucleation processes in the Earth’s atmosphere. We herein study the impact of various environmental conditions on FeHg (maghemite−Hg2Cl2 composite), a highly efficient ice nucleating particle with similar freezing point to the best inorganic ice nuclei, AgI. FeHg is formed from FeCl2·4H2O and HgCl2, which are observed in the environment, in contrast to AgI, which is rarely found. The ice nucleation efficacy remained unchanged for FeHg under ambient conditions for a long duration. To mimic the atmosphere, we performed a series of experiments using a suite of complementary techniques, at various levels of radiation intensity, temperature, and pH for FeHg. Experiments were also performed in the presence of atmospheric pollutants, such as ozone (in the presence or absence of light), as well as nine emerging metal oxides and NO2. The emerging metal oxides at various pH levels produced significant effects on the ice nucleation ability of FeHg. Elevated temperatures changed the maghemite of FeHg to βFe2O3, whereas other studied environmentally relevant physicochemical conditions could not alter the maghemite phase. We describe potential reaction mechanisms using our observations. To evaluate the effect of surface alterations, the passivation of clay materials, namely, kaolin and montmorillonite, was also performed on FeHg. The observed alteration in crystal structure, as found in X-ray diffraction, was attributed to the change of the extent of lattice mismatch, resulting in a significant variation of ice nucleation ability. Our insights of the ice nucleating particles may help understand real atmospheric ice nucleation processes, in association with the establishment of more in-depth understanding of ice nucleation mechanism in the Earth’s atmosphere.
1. INTRODUCTION
although the results are expected to depend heavily on the location of the field study. Heterogeneous ice nucleating particles3−5 mainly consist of mineral dust, fly ash, sulfates, and organic and metallic compounds. Since the last decade, there has been a substantial amount of work6−8 devoted to learning the radiative effects of atmospheric mineral dust, in particular the capacity of mineral dust to absorb, scatter, and diffuse solar and terrestrial radiation. Dust particles are composed of a combination of several chemical species. Iron oxides are one of the most ubiquitous members of mineral dusts.9,10 Within dust particles, iron oxides are mostly accompanied by clays as aggregates.11 Arias et al.12 have demonstrated the consequences of interactions between iron and humic acids on the flocculation and aggregation of quartz and kaolin.
Airborne particles, or aerosols, and their physicochemical interactions with clouds, including nucleation microphysics, represent the largest research uncertainty in climate change.1 Aerosols, particularly nanoparticles (diameters < 100 nm), were also identified by the World Health Organization2 as a top priority and as a major cause for premature death of millions of humans every year. In clouds, the formation of ice at temperatures around or above −38 °C takes place through heterogeneous ice nucleation, introduced by ice nucleating particles.3−5 In the absence of such heterogeneous ice nucleating particles, water drops inside clouds would stay in the supercooled liquid phase until the temperature of spontaneous freezing initiated by homogeneous ice nucleation was reached (−38 °C). Many particles serve as ice nucleating particles in the atmosphere, which include metallic particles, mineral dust, soot, and biological particles. DeMott et al.3 have reported the contributions of different chemicals on ice nucleating particles, © 2018 American Chemical Society
Received: April 14, 2018 Revised: July 13, 2018 Published: August 1, 2018 18690
DOI: 10.1021/acs.jpcc.8b03551 J. Phys. Chem. C 2018, 122, 18690−18704
Article
The Journal of Physical Chemistry C
lattice structure and the composition of the ice nucleating particles significantly. Consequently, this study includes complementary synthesis, characterization, and microphysics studies using several spectroscopic techniques, which are herein presented. We explore the factors responsible for forming highly efficient ice nucleating particles and their potential impacts on the environment.
Iron oxides are well reported for their affinity toward mercury and the removal of this highly toxicant global metal13 from water and air matrices.14 Elhouderi et al.15 have synthesized Ag/Fe3O4 composites for mercury removal from groundwater. On the contrary, Diagboya et al.16 have synthesized covalently bonded graphene oxide−iron magnetic nanoparticles for mercury removal. Furthermore, Cao et al.17 have optimized the removal of mercury at trace levels from water by Fe3O4, supported by reduced graphene oxide with the aid of an artificial neural network and genetic algorithm. Therefore, iron and iron oxides have significant affinity toward Hg. It is noteworthy that mercury does not form an amalgam with iron. However, iron oxides often absorb Hg, not only in the solution phase, but also in the vapor phase.18 Moreover, the observation of Wiatrowski et al.19 has indicated the solidphase interaction of iron oxide and Hg(II). They have demonstrated the surface-catalyzed reduction of Hg(II) to Hg(0), kinetically favored in magnetite-containing soils and sediments. Hu et al.20 have used iron oxides under atmospheric conditions as a means to recycle Hg from used mercurycontaining fluorescent lamps, which have been found to be efficient. Cloud formation is a function of atmospheric dynamics and microphysical processes in the Earth’s atmosphere. As such, some environmental conditions can affect aerosol ice nucleation microphysical processes. In clouds, there are specific environmental situations, such as light intensity and chemistry, to affect bacterial ice nucleation activity, as reported by Attard et al.21 Clouds of oceanic origin possess less acidic pH (pH 5.0−6.5) than do clouds of the north, which have been influenced by anthropogenic activities (pH 3.0−4.5).22 The role of heating and filtering on the ice nucleation of biogenic ice nucleating particles has been reported by Wilson et al.23 In highly polluted urban areas, elevated levels of NO2 and O3 have been found in the atmosphere. The influence of mineral dust on nitrate, sulfate, and ozone in transpacific Asian pollution plumes has been investigated by Fairlie et al.24 Many studies25,26 have illustrated the impacts of O3, NO2, and so on on mineral dust. Subsequently, ice nucleating particles of inorganic origin get affected due to environmental alterations. To better understand their impact on the environment, many researchers have been involved in producing mineral and anthropogenic dust of iron oxides in the laboratory.27−29 Studies reveal that numerous forms of iron oxides are able to react with atmospheric components.30 However, such mineral dusts are found to be poor ice nucleating particles compared to the most efficient ice nucleating particles (e.g., Gram-negative bacteria and selected biomaterials, with a mean freezing temperature (MFT) of −1.8 °C).31 The effect of copollutants, like toxic Hg on mineral dust (in the context of nucleation ability), is also a matter of interest to explore the interaction of toxic materials with mineral dust.32 Recently, there has been a report33 of a highly efficient inorganic ice nucleating particle from our laboratory, FeHg, which is close in effectiveness to the most efficient biomolecules. In this manuscript, we explore the effect of various environmental conditions, such as temperature, radiation, pH, and copollutants, on an efficient ice nucleating particle, FeHg, to explore whether the environmental effects are significant. Indeed, as surface characteristics are key factors in ice nucleation, we evaluated the effects of the association of clay materials (kaolin and montmorillonite) on FeHg. The various environmental conditions are expected to influence the
2. EXPERIMENTAL SECTION In this section, we describe the methods for synthesis, as well as various analytical techniques for chemical and physical analyses. In addition, the materials and supplies required are discussed. 2.1. Synthesis. 2.1.1. Synthesis of KaFe/BHFe/MtFe. KaFe is kaolin-passivated iron oxide. A 30 mL solution of 2.67 g of FeCl2·4H2O was prepared in a 4:1 ethanol/water mixture. Then, 1.5 g of kaolin was added to the solution, creating a muddy beige mixture. This suspension was sonicated for 2 h to homogenize Fe2+ in kaolin. Meanwhile, 100 mL of a 1 M NaBH4 solution was made by dissolving solid NaBH4 in icecold milli-Q water. After sonication, the cold NaBH4 solution was added slowly into the kaolin-iron chloride suspension. An instant color change was observed from beige to black. After adding all of the NaBH4 solution, the reaction mixture was shaken for another 10 min and left for 24 h. After 24 h, the supernatant was decanted, leaving the solid inside the container with a strong magnet. The black solid was washed thoroughly with milli-Q water and then with ethanol. The ironkaolin composite was then placed in a vacuum oven at 50 °C to dry fully. The dry solid was crushed with a mortar and a pestle to acquire the KaFe. Likewise, BHFe was synthesized according to a similar protocol as KaFe, without kaolin. MtFe was synthesized according to a similar protocol as KaFe, with montmorillonite instead of kaolin. 2.1.2. Synthesis of HgKaFe/HgBHFe/HgMtFe. First, 0.0025 g of KaFe was sonicated with 10 mL of a 3.69 × 10−3 M aqueous HgCl2 solution for a few seconds to form a HgKaFe solution. A HgBHFe solution and a HgMtFe solution were obtained according to similar protocols by using BHFe and MtFe, respectively, instead of KaFe. 2.2. Analytical Techniques for Physical and Chemical Characterization. 2.2.1. Droplet-Freezing Assays. Heterogeneous ice nucleation is more common in the environment in comparison to homogeneous nucleation. There are several modes or pathways to achieve heterogeneous ice nucleation, namely, (a) deposition nucleation, (b) immersion freezing, (c) condensation freezing, and (d) contact freezing.32 In this paper, we employed the immersion freezing mode via droplet-freezing experiments. Droplet-freezing experiments are the most useful methods to understand the ice nucleation efficiency of a heterogeneous ice nucleus via immersion mode.33−35 For each sample, droplets of 10 μL were positioned on a cooling plate (in-house-made copper plate) with a cooling rate of 1 °C/min, and the freezing temperatures for each droplet were recorded. A thin homogeneous layer of petroleum jelly (Vaseline) was spread on the copper plate before the placing of the droplets to minimize the interactions between the copper plate and the droplets and maintain hydrophobicity. At a certain temperature (T), the cumulative number of ice nucleation active sites per unit volume of water (K) can be expressed by the following equation 18691
DOI: 10.1021/acs.jpcc.8b03551 J. Phys. Chem. C 2018, 122, 18690−18704
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The Journal of Physical Chemistry C K (T ) = −
2.2.4. Exposure of NO2 Gas. We treated NO2 gas (at 5 KPa) to HgBHFe, HgKaFe, and HgMtFe solutions as well as vacuum-dried solids from a NO2 cylinder (TC-3ALM-139/ Dot 3AL). 2.2.5. X-ray Diffraction (XRD) Spectroscopy. X-ray diffraction (XRD) was recorded with a Bruker D8 Discovery X-ray Diffractometer (VANTEC Detector Cu-Source λ = 1.5418 Å). XRD patterns were recorded for 3 ≤ 2θ ≤ 80° with increments of 0.005°. Samples of different HgKaFe solutions were vacuum dried at room temperature at 20 psi before taking the measurements. Software of “DIFFRAC.EVA V4.2.1” combined with the latest database “PDF-4+2018” from International Center For Diffraction Data was used for data processing. The background of XRD pattern was first subtracted. Subsequently, phase identification and quantitative analysis were performed by searching and matching the literature peaks. It is to be noted that we did not perform Rietveld refinement.41 Fitting to a library gives a qualitative composition assignment and approximate quantitative compositional information. 2.2.6. Atomic Force Microscopy (AFM). AFM images were taken using a Bruker Multimode AFM. The cantilevers were OPUS 160AC-NA models, with the resonance frequency and spring constants being 300 kHz and 26 N/m, respectively. The substrates used were pieces of a commercial silicon wafer. Droplets (10 μL) of KaFe, BHFe, MtFe, HgKaFe, HgBHFe, and HgMtKe were placed on a silicon wafer and dried in air. 2.3. Materials and Supplies. All of the reagents were of analytical reagent grade, and Milli-Q water was employed throughout the experiment. Kaolin, montmorillonite, all metal oxides, iron(II) chloride, and mercury(II) chloride were purchased from Sigma-Aldrich. Sodium borohydride was obtained from Alfa Aesar, whereas hydrochloric acid and sodium hydroxide were obtained from Fisher Scientific. Then, 15-watt UV A, visible, and UV B were obtained from Hitachi, Philips, and Sankyo Denki, respectively. All glassware was cleaned with freshly prepared aqua regia, then subsequently rinsed with copious amounts of distilled water, and dried well before use. All of the reagents were used without any further purification.
ln(1 − ffrozen (T )) V
(1)
If the mass of particles of unit volume of water is estimated, the ice nucleation active site density per unit mass (nm) can also be presented according to the following equation nm(T ) = −[K (T )d ] /Cm
(2)
where Cm is the mass concentration of the particles in the initial suspension and d denotes the dilution ratio of the suspension relative to Cm. Background correction was performed by subtracting the ice nucleation data of pure milli-Q water. In other words, first frozen fractions were calculated and then nucleation sites per unit mass were presented in all of the freezing spectra to make comparison, taking care of mass variation for different samples. It is an improved approach for measuring immersion freezing, and many studies33,36,37 follow a similar protocol to show the ice nucleation ability. KaFe (0.0025 g in 10 mL of water), HgKaFe, HgBHFe, and HgMtFe, as described in Section 2.1.2, were used at different conditions for ice nucleation experiments. KaFe (0.0025 g in 10 mL of water), HgKaFe, HgBHFe, and HgMtFe were also vacuum dried at room temperature at different conditions for ice nucleation experiments (after adding 10 mL of water). Then, 0.0025 g of AgI and 0.01 g of HgCl2 were sonicated in 10 mL of milli-Q water at different conditions for ice nucleation experiments. To measure the stability of FeHg as ice nucleating particles, we preserved the samples in screw-capped test tubes on the laboratory benchtop with diffuse light at room temperature. However, we collected samples at every 5 day interval from the screw-capped test tubes and performed droplet-freezing experiment on an ice nucleator. 2.2.2. Evaluation of Particle Size Distribution and Contact Angle. The angle that is measured through the liquid, at a liquid−vapor interface, meeting a solid surface is known as contact angle.38 Researchers usually make thin films of their samples and place water droplets on them to measure contact angle.38 Contact angles, used in the context of the ice nucleation literature, do not correspond to a true contact angle, which is the property of a material.39 For some reason, they are used in the literature of this field to provide a sense of the activation energy of forming a critical nucleus. Geometric observations have shown that as the contact angle decreases, so does the spherical surface area. Therefore, the smaller the contact angle is, the lower the activation barrier is, and the faster the nucleation rate is. Savre and Ekman39 demonstrated the relation of nucleation ability of a given compound to the contact angle. The contact angles of BHFe, KaFe, MtFe, HgBHFe, HgKaFe, and HgMtFe were calculated (Table S1) as described in Marcolli et al.40 The procedure in details has been stated in the Supporting Information. 2.2.3. Production and Exposure of Ozone. O3 was generated via an ozone analyzer from O2 with a flow rate of ∼60 mL/min, and ∼46 000 ppm/mL was the concentration of O3 that was produced. The concentration of ozone (to obtain 100 ppb and 100 ppm) was estimated with a UV−vis spectrophotometer (Cary 50 Bio). O3 was bubbled slowly (one bubble/3 s) in a 10 mL solution of HgBHFe, HgKaFe, and HgMtFe in an open container. We also treated O3 gas to HgBHFe, HgKaFe, and HgMtFe, as vacuum-dried solids.
3. RESULTS AND DISCUSSION We performed complementary synthesis, chamber experiments, and microphysics studies. The Experimental Section explores all of the systematic steps, which led to the effect of atmospherically relevant physicochemical conditions on ice nucleation. These conditions can expand the scope of known ice nucleating materials, particularly knowing that there are emerging nanoparticles with more diverse emission in the atmosphere. 3.1. FeHg: A Purely Inorganic Highly Efficient Ice Nucleating Material. FeHg was discovered to be one of the most efficient inorganic ice nucleating particles (−4.7 to −6.6 °C) from our laboratory.33 We were specifically interested in understanding the effect of different atmospherically relevant physicochemical conditions on the ice nucleating properties of FeHg, as a model of other efficient ice nuclei. We used clay materials as a passivating agent to prevent unwanted aggregation, which enhanced the surface of the particles. The passivating agents used were kaolin and montmorillonite, which are very common clay materials. Because kaolin and montmorillonite are very common clay materials on Earth,42 their use as a scaffold is environmentally relevant to study. 18692
DOI: 10.1021/acs.jpcc.8b03551 J. Phys. Chem. C 2018, 122, 18690−18704
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The Journal of Physical Chemistry C
Figure 1. Active site density per unit mass against temperature of KaFe, BHFe, MtFe, HgKaFe, HgBHFe, and HgMtFe.
Figure 2. (A) Composition analysis from XRD of HgKaFe and (B) effect of aging of HgKaFe, HgBHFe, and HgMtFe.
iron chloride to Fe(0). Then, spontaneous aerial oxidation and subsequent treatment of HgCl2 converted Fe(0) to maghemite-C syn and Hg2Cl2. 3.2. Stability of FeHg as Ice Nucleating Particle. Highly efficient ice nucleating particles of FeHg were formed within 10 s of sonication of BHFe/KaFe/MtFe with HgCl2. To determine the ice nucleation efficiency of FeHg, its stability as an ice nucleating particle should be investigated. We performed ice nucleation experiments with HgBHFe, HgKaFe, and HgMtFe every 5 days for 60 days in closed screw-capped test tubes with diffuse light (Figure 2B). It is noteworthy that throughout the 60 day period, all of the compounds studied had very little change in ice nucleation efficiency or the MFT (−5.4 to −6 °C). Freezing spectra were found to be similar, and mean freezing temperatures were also similar, indicating the stability of the ice nucleating particles over aging. Therefore, FeHg was quite stable and had insignificant effects on the aging of long time. 3.3. Effect of Light Exposure. We performed experiments by irradiating the solution under UV A (315−400 nm), visible (∼400−700 nm), and UV B (280−315 nm) radiation. In earlier similar experiments, ice nucleation efficiency was adversely affected by exposed light.23,43 For example, Attard et al.19 and Anderson et al.37 exposed biological molecules to UV light and observed a reduction in the ice nucleation ability of biological molecules. Gobinathan et al.44 demonstrated that UV radiation exposure to inorganic materials (AgI) adversely affected their ice nucleation efficiency. Our samples of FeHg were exposed to three different energy radiations (UV A, vis, and UV B), individually. Water froze at much warmer temperatures after exposure to radiation. The samples were exposed to radiation for 0.5−3 h periods on FeHg (without heating the sample). The MFTs of HgKaFe were −5.4, −5.3, −5, and −5 °C after 0.5, 1, 2, and 3 h periods
Iron oxides were synthesized in the presence of kaolin (KaFe) and montmorillonite (MtFe), as well as in the absence of clay (BHFe). They were also suspended in an aqueous HgCl2 solution via sonication to obtain the highly efficient ice nucleating particle, FeHg. FeHg (without clay), kaolin-FeHg, and montmorillonite-FeHg are called HgBHFe, HgKaFe, and HgMtFe, respectively, in the rest of this manuscript. For the purpose of simplicity of comparison among various studied compounds, in this manuscript, ice nucleating particles with a mean freezing temperature (MFT) of >−6.5 °C are labeled as highly efficient ice nucleating particles. On the contrary, ice nucleating particles with an MFT of ≤−6.5 °C are herein considered as nonefficient or poor ice nucleating particles. Indeed, setting a temperature to delineate efficient and poor ice nucleating particles was chosen arbitrarily. In this study, HgBHFe, HgKaFe, and HgMtFe were found to be highly efficient ice nucleating particles and were found to have low contact angles throughout the freezing temperatures. On the contrary, BHFe, KaFe, and MtFe possessed poor ice nucleation ability and higher contact angles at the freezing temperatures (Table S1, Supporting Information). The mean freezing temperatures (MFTs) of HgBHFe, HgKaFe, and HgMtFe were −5.8 °C (−4.9 to −6.9 °C), −5.8 °C (−4.4 to −6.9 °C), and −5.7 °C (−4.6 to −6.6 °C), respectively. All of these species had very narrow freezing ranges. BHFe, KaFe, and MtFe demonstrated mean freezing temperatures (MFTs) of −17.4 °C (−11.3 to −22.9 °C), −16.7 °C (−8.8 to −20.2 °C), and −10.2 °C (−3.4 to −11.8 °C), respectively, with a wide range of freezing temperatures (Figure 1). From XRD analysis of HgKaFe (Figures 2A, S1, and S2, Supporting Information), we found that HgKaFe consisted of mullite (30.9%), nacrite-2M2 (20.4%), maghemite-C syn (11.9%), Hg2Cl2 (32.6%), and HgCl2 (4.2%). NaBH4 reduced 18693
DOI: 10.1021/acs.jpcc.8b03551 J. Phys. Chem. C 2018, 122, 18690−18704
Article
The Journal of Physical Chemistry C
Figure 3. (A) MFTs of HgKaFe, HgBHFe, and HgMtFe after the exposure of UV A, visible, and UV B light at different time intervals. (B) Comparison of MFTs of HgKaFe, HgBHFe, and HgMtFe after the exposure of UV A, visible, and UV B light for 3 h. (C) Active site density against the temperature of HgKaFe at different exposure times of UV A. (D) Composition analysis of HgKaFe after 3 h of UV A exposure.
3.4. Effect of the Exposure of Selected Atmospherically Key Gaseous Species. 3.4.1. O3 Exposure. 3.4.1.1. Exposure Experiments to Ozone Under Gas-Tight Conditions. Several naturally and anthropogenically produced volatile compounds, such as NOx (NO and NO2), and volatile organic compounds, and semivolatile compounds, which are emitted from both natural and human-made sources, upon photochemical reactions produce key atmospheric gases, namely, ozone, which is also a greenhouse gas.21 Furthermore, ozoneinitiated reactions are known to start a series of secondary reactions, which lead to reactive radicals, including OH and HO2, in the atmosphere.45 Salam et al.46 reported that ozoneaged montmorillonite (clay) mineral dust did not change its ice nucleation efficiency. Kaolinite particles, with a low exposure to ozone, exhibited improved ice nucleation activity (median freezing temperature of 1.5 °C warmer than untreated kaolinite). On the contrary, Arizona test dust particles displayed inhibited ice nucleation, requiring a median freezing temperature of 3 °C colder than untreated Arizona test dust particles after high ozone exposure.47 Seemingly, ozone-mediated oxidation cannot significantly modify the ice nucleation capability of soot particles and humic-like substances.48−50 Enhanced ozone concentration, due to enhanced anthropogenic activities, may or may not have significant effects on ice nucleation efficiency. It is important to note that the composition of dust is quite variable around the globe. Moreover, aerosols containing soot do not exhibit identical physical and chemical properties.51 Therefore, the direct comparison of values between different experiments should be considered with caution. Under our experimental conditions, HgBHFe, HgKaFe, and HgMtFe were vacuum dried and exposed to a high
of UV A light exposure, respectively. The MFTs of HgKaFe were −5.7, −5.4, −5.3, and −5 °C after 0.5, 1, 2, and 3 h periods of visible light exposure, respectively. Furthermore, the MFTs of HgKaFe were −5.7, −5.5, −5.3, and −5 °C after 0.5, 1, 2, and 3 h periods of UV B light exposure, respectively. HgBHFe and HgMtFe had MFTs at −4.8 and −4.9 °C, respectively, after a 3 h period of UV A exposure. Shorter pulses of UV (
