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Water adsorption isotherms on fly ash from several sources Juan Gabriel Navea, Emily Richmond, Talia Stortini, and Jillian Greenspan Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02028 • Publication Date (Web): 07 Sep 2017 Downloaded from http://pubs.acs.org on September 11, 2017

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Water adsorption isotherms on fly ash from several sources Juan G. Navea a,*, Emily Richmond,a Talia Stortini,a and Jillian Greenspan,a a

Chemistry Department, Skidmore College, Saratoga Springs, NY, 12866-1632

* To whom correspondence should be addressed. Email: [email protected] (J. Navea)

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Abstract In this study, horizontal attenuated total reflection (HATR) Fourier-transformed infrared (FT-IR) spectroscopy was combined with quartz crystal microbalance (QCM) gravimetry to investigate the adsorption isotherms of water on fly ash, a byproduct of coal combustion in power plants. Because of composition variability with source region, water uptake was studied at room temperature as a function of relative humidity (RH) on fly ash from several regions: United States, India, Netherlands, and Germany. The FT-IR spectra show water features growth as a function of RH, with water absorbing on the particle surface in both an ordered (ice-like) and a disordered (liquid-like) structure.

The QCM data was modeled using the Brunauer,

Emmett, and Teller (BET) adsorption isotherm model. The BET model was found to describe the data well over the entire range of RH, showing that water uptake on fly ash takes place mostly on the surface of the particle, even for poorly combusted samples. In addition, the source region and power-plant efficiency play important roles in the water uptake and ice nucleation (IN) ability of fly ash. The difference in the observed water uptake and IN behavior between the four samples and mullite (3Al2O3·2SiO2), the aluminosilicate main component of fly ash, is attributed to differences in composition and the density of OH binding sites on the surface of each sample. A discussion is presented on the RH required to reach monolayer coverage on each sample as well as a comparison between surface sites of fly ash samples and enthalpies of adsorption of water between the samples and mullite.


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1.

Introduction The surface of atmospheric particles has the potential to open pathways for heterogeneous

reactions, leach metals into atmospheric water, and has an overall impact in air quality and visibility.1-6 It has been shown that water adsorption on the surface of atmospheric aerosols can have a dual impact on these atmospheric processes:7,8 First, adsorbed water competes for adsorption sites on the particle surface, decreasing the coverage and heterogeneous reactions of trace atmospheric gases on particle surfaces.9-11 On the other hand, adsorbed water can also enhance the uptake and reactivity of substrates with coadsorbed water participating in heterogeneous reactions or increasing the uptake of water-soluble atmospheric compounds.12-14 Such relative humidity (RH) effects are driven by the capacity of the particles to take up atmospheric water, making the study of water adsorption crucial for a better understanding of their impact in climate and the environment. Yet, our understanding of water uptake on anthropogenic particles, such as fly ash, remains significantly low compared to mineral dust. Fly ash particles are byproducts of coal combustion in power plants with an estimated global production of over 300 Tg per year.15 As the worldwide demand for energy increases, the production of fly ash may likely increase as well. Despite efforts to prevent its outflow, fly ash particles are continuously observed even in isolated regions of the atmosphere, suggesting longrange transport and relatively large atmospheric lifetimes.16-18 The focus of this study is to investigate the adsorption of water on fly ash particles and established its potential variability with respect to source region. Combustion particles, such as fly ash, are known to have an available surface for the uptake of water.19 Because of similarities with mineral dust, fly ash particles might contribute to the cloud condensation nuclei (CCN) and ice nuclei (IN) budget in the troposphere.µIn fact, 3 ACS Paragon Plus Environment

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components common to both fly ash and mineral dust, have been shown to adsorb atmospheric water and behave as both CCN and IN particles.20-22 In addition, mineral dust/fly ash mixtures have been proposed as important IN particles.16 Yet, estimates of the CCN and IN activity of fly ash have not been assessed independently. The very limited knowledge on fly ash particle interaction with atmospheric water make the estimations of their role in cloud adjustments challenging.19 This limited knowledge has led to studies where the complex interactions between fly ash and atmospheric water are treated as either mineral dust or mixed emissions that including black carbon.23-25 However, any study attempting to assess the chemistry of fly ash in the atmosphere needs to evaluate the water uptake on fly ash particles itself rather than as mixtures with mineral dust or soot.26 Along with our inability to distinguish between the effects of mineral dust and those of fly ash particles in the atmosphere, the variable composition of fly ash brings an additional limiting factor in assessing it effects on water uptake and cloud dynamics. This composition variability is linked to coal mineralogy, combustion parameters, and the efficiency of the collection system in the power plant flue-gas stack. Similar to mineral dust, the source-dependent composition of fly ash makes their comparative studies paramount in understanding their impact on cloud adjustments.2,4 For instance, recent studies in our laboratory suggest that mineralogy variations due to source region impacts the iron mobility and atmospheric processing of fly ash.4 Particles produced in different coal-fired power plants will have distinctive physicochemical properties, with potentially different CCN and IN behavior. Yet, to date, little attention has been given to fly ash uptake of water, and no direct or comparative studies on fly ash water uptake have been performed.


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In this paper we investigate water uptake on fly ashes from four different source regions: United States, India, Netherlands and Germany. All samples were collected in the flue-gas stack before atmospheric process have taken place, so samples are representative of recently emitted fly ash particles. The significant difference in location ensures a different coal and thermal power-plant boiler, providing a comparative element to this study. All samples were systematically characterized prior to water uptake quantification and analysis in a twodimensional experimental apparatus. 2.

Experimental Section

2.1

Materials: Four fly ash F (FA) samples were obtained directly from coal-fired power

plants located in different regions: United States of America (USFA) from the Midwest region, Indian fly ash (INFA) from Northeastern India, and two samples from Europe: Netherlands fly ash (NLFA) and German fly ash (DEFA). Both European ashes were obtained from commercially available standards of fly ash from the European Commission, BCR®-176R and BCR®-038R for NLFA and DEFA, respectively. All reagents employed for sample characterization and water adsorption isotherm experiments were analytical grade. All fly ash samples were used without further purification. 2.2

Water adsorption: Experiments of water adsorption on fly ash particles were performed

in a two-dimensional system coupled to a stepwise relative humidity (RH) generator. As described in Figure 1, the first section of the experimental apparatus consists on a commercial horizontal attenuated total reflection Fourier transformed infrared spectrophotometer (HATRFTIR) (Pike Technologies) in a flow cell. The second section consists on a commercially available quartz crystal microbalance (QCM200, Stanford Research Systems). An air dryer 5 ACS Paragon Plus Environment

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(Balston 75-60) is used as the air source for the stepwise RH generator, as well as to purge the spectrophotometer compartment and the quartz crystal microbalance (QCM) enclosure.

A

B

C

Figure 1: Schematic diagram of the HATR-FTIR and QCM flow and humidification system. The experimental apparatus is divided in three segments: (A) represents the stepwise relative humidity (RH) generator; (B) is the HATR flow cell inside a dry spectrophotometer compartment; (C) is the QCM flow chamber in a dry enclosure.


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The FTIR measurements were carried out in order to compare the molecular structures and signatures of water adsorption on each fly ash surface. Fly ash films were evenly deposited on the 7.3×0.7 cm area HATR crystal from a well-mixed suspension of the sample in 18 MΩ purified water and then dried overnight under dry air. The sample was then placed on the flow cell, which was made to enclose the HATR crystal and allow for a continuous flow of dry (RH < 2%) or humidified air by coupling it to a flow system as shown in Figure 1. The HATR flow cell was placed in the purged internal compartment of an FTIR spectrophotometer (Nicolet 6700). The flow system allows for the control of relative humidity, which is generated by bubbling dry air through 18 MΩ purified water in the humidification chamber. Dry air was humidified by passing air through a coalescing filter to remove organic trace compounds and particles. The humidified air was allowed to equilibrate in a 300 mL mixing chamber with a hygrometer to measure the RH before flowing it through the HATR crystal containing the sample of dry fly ash. Infrared measurements of water adsorption on the fly ash films were collected from 900 to 4000 cm-1 at 4 cm-1 resolution by averaging 100 scans. All water adsorption HATR-FTIR scans were referenced to the background of the dry fly ash sample. Because the multicomponent nature of fly ash, quantification via infrared spectroscopy is difficult as the necessary assumptions of uniformity in the adsorption sites and low intermolecular interactions are not appropriate. The variety of adsorption sites and possible interactions between them affect the absorption cross-sections of adsorbed water. Thus, the water uptake on fly ash was accurately measured using a gravimetric method via QCM.27,28 The QCM has a 1-inch diameter Au/Cr polished quartz crystal enclosed in a flow cell.29 The QCM uses the piezoelectric properties of quartz to measure the changes in frequency of the crystal. When the mass loading is less than 2% of the unloaded crystal frequency, multilayers of fly ash deposited 7 ACS Paragon Plus Environment

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on the crystal are considered an extension of the quartz crystal surface, and the change in frequency can be directly correlated to the mass of sample on the crystal by the Sauerbrey equation:30 ∆f = −C ∆m

(1)

where ∆f is the change in frequency of the crystal (Hz), ∆m is the change in mass (µg/cm2), and C is the quartz sensitivity factor, 56.6 Hz µg-1 cm-2 for the 5 MHz At-cut quartz crystal described in this work. Therefore, the change in frequency measured by the QCM can be directly related to the mass of water on the fly ash samples deposited on the crystal. In a typical experiment, before placing the sample on the QCM crystal, a baseline frequency was recorded with a confidence of ±0.1 Hz, keeping the air flow constant to 4 scfh. After the baseline was established, the flow cell was removed to apply the fly ash sample on the crystal to a mass loading less than 2% of the unloaded frequency of the crystal. Fly ash coatings were applied by suspending fly ash in 18 MΩ purified water and sprayed onto the QCM crystal using a glass atomizer. The resulting uniform thin layer of fly ash covering the active surface area of the QCM crystal (0.4 cm2) was then enclosed in the flow cell and dried overnight under dry air. The change in frequency of the QCM was then used to calculate the loading of fly ash. Fly ash samples on the QCM ranged from 2 to 20 µg per crystal area. After overnight drying, the frequency of the QCM crystal was zeroed so the changes in frequency only reflect changes in the sample as humidify air was introduced to the flow cell. Using a similar protocol than that for the HATR flow cell, humidified air was changed stepwise and allowed in the flow cell where the changes in the frequency were used to calculate the mass of water adsorbed on fly ash. Air humidity was only changed after the frequency remained 8 ACS Paragon Plus Environment

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constant, indicating equilibrium between surface and airflow. Typically, the sample was allowed to stabilize for 20 to 60 minutes before changing RH, and multiple experiments were performed on each fly ash sample. 2.3

Characterization of fly ash samples: The characterization of three of the samples used in

this work, including the elemental composition of USFA, INFA, and NLFA, has been reported recently in a previous work (see support material for elemental composition of DEFA).4 Briefly, Figure 2 shows characteristic scanning electron microscopy (SEM) images of the four fly ash samples reported in this work, acquired with a JEOL 6480LV. These micrographs show that USFA, INFA and DEFA contain predominantly spherical particles, a characteristic morphology resulting from the highly efficient combustion process taking place in power plants. Yet, NLFA shows a substantial amount of amorphous rock-like particles, indicating an incomplete combustion.4 Particle size measurements from SEM data show a mean particle diameter trend of NLFA>INFA>DEFA>USFA. In addition, surface areas for all fly ash samples were determined using an eleven-point N2-BET adsorption isotherm (Quantachrome Nova 1200). The samples were evacuated overnight prior to the surface area measurement, which simulates the protocol used for the water uptake experiments, with water uptake measurements collected after the sample was dried overnight (vide infra). A summary of particle size and surface area is shown in Table 1.


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Figure 2: Representative micrographs of fly ash samples. (A) United States (USFA); (B) Northeast India (INFA); (C) Netherlands (NLFA); (D) Germany (DEFA). Table 1: Specific surface area and particle size of fly ash samples BET surface area, SBET (m2g-1)

Mean diameter, ̅ (µm)

USFA

1.8±0.1

1.59±0.05

INFA

0.98±0.03

2.07±0.04

NLFA

2.8±0.1

4.6±0.21

DEFA 3.5±0.1 1.73±0.04 1 Value obtained from spherical particles only Infrared spectroscopy was used to investigate the composition and chemical properties of all ash samples. Characteristic HATR-FTIR spectra of the dried fly ash samples are shown in Figure 3. A detailed assignment of the vibrational absorption bands on the fly ash samples has recently been published by our group.4 In general, the most intense vibrational features in all fly ash samples are observed from 900 and 1700 cm-1, assigned to vibrational absorptions bands due 10 ACS Paragon Plus Environment

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to lattice stretching motions of Si-O.29,31 The frequency differences between fly ash samples observed in this region are attributed to the various forms of the lattice structure of quartz and mullite, inferring surface OH terminals on the fly ash particles. The strongest distinction between absorbance features in the fly ash samples can be observed in Figure 3b: a relatively strong absorption band centered at 1411 cm-1 attributed to bulk carbonates, and is only observed in NLFA spectra.32,33 In addition, a small but observable band at 1628 cm-1 is also present and attributed to the stretching vibration of –COO– of bicarbonates.33,34

Figure 3: Infrared spectra of fly ash samples from several source regions: United States (USFA); Northeast India (INFA); Netherlands (NLFA); Germany (DEFA). The arrow under the NLFA spectrum indicate the band at 1628 cm-1.


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Figure 4 shows the X-ray diffraction (XRD) patterns of the four fly ash samples examined in this work. XRD was obtained for all fly ash samples and standards using a Phillips PW-1840 X-ray diffraction instrument equipped with a Cu Kα source to determine structural and composition differences between each sample. The main components of the crystal phase composition are mullite (3Al2O3·2SiO2) and quartz (SiO2), with lower fractions of hematite (Fe2O3), and magnetite (Fe3O4).35,36 In addition, XRD confirms the FTIR detection of carbonates in the form of calcite or lime. Carbonate is a trace component in all samples except NLFA, where an incomplete combustion leads to a higher proportion of carbonates. The XRD spectra also shows a broadening peak between 15 and 35 degrees, especially noticeable in INFA and DEFA but evident in all samples, indicating the presence of an amorphous phase of silica.


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Figure 4: X-ray diffraction characterization of fly ash samples. (a) United States (USFA); (b) Northeast India (INFA); (c) Netherlands (NLFA); (d) Germany (DEFA). Legend: Q-quartz; MMullite; H-Hematite/Magnetite; F-Feldspar; C-Calcite. The quartz and mullite content in all fly ash samples, indicated by the combined FTIR and XRD analysis, indicate that all fly ash samples have surface site terminals Al-OH and SiOH, which can serve as water binding sites for adsorption processes.37-39 The formation of mullite, fly ash main component in all samples, is consistent with the combustion process of aluminosilicate frameworks contained in coal. During the combustion process, clay components of coal are transformed in a first step into meta-clay, a pseudo-amorphous aluminosilicate structure,40,41

followed by a second combustion step to form mullite.42 Metakaolin

(Al2O3·2SiO2) and metaillite (KAl2AlSi3O11), from the initial combustion of kaolinite and illite, respectively, continue to combust above 1700 K as described by reactions (2) and (3):40 3 Al O ∙ 2SiO 3Al O ∙ 2SiO s + 4SiO s

(2)

2 KAl AlSi O s 3Al O ∙ 2SiO s + 4SiO s + K O s

(3)

∆

∆

These meta-clays might be responsible for the feldspar signatures in the XRD spectra of NLFA. In addition, XRD data shows that the main trace element, iron, is present in fly ash as hematite and magnetite, which is consistent with the combustion of coal component pyrite, (Fe"⁄# S). The two step combustion mechanism of pyrite leads to the formation of magnetite first, and upon further combustion, it forms hematite:43 4 22 1 4 Fe"⁄# S s + O g Fe O# s + SO g ∆ 3 5 15 5 1 3 Fe O# s + O g Fe O s ∆ 2 4

(4)

(5)


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Because iron oxide species are positively charged and show surface OH sites at near-neutral pH,44 they have also been shown to have relevant surface and aqueous chemistry.4,45 Yet, water uptake on iron oxides have been found to be lower than that on aluminum oxides, silicon oxides, or mullite, so a larger content of iron oxide on the surface of fly ash might reduce the more effective binding sites, decreasing water uptake on the sample.21 On the other hand, iron oxides 3,46 ( have been shown to bind species such as SO( # and CO , which leads to higher water uptake.

This is particularly important for NLFA, as carbonates are present already in the bulk of the sample as a result of an incomplete combustion. In addition, XRD data shows the presence of carbonates in INFA and DEFA as well, as trace surface components since these samples are the result of a more through combustion process. There is no clear evidence from either the XRD or FTIR data that indicates the presence of carbonates on USFA. In general, carbonate and carbonate-containing minerals such calcite and lime can form hydrogen bonding with surface water and OH group in neighboring mullite and iron oxides, making them important components for the water adsorption capacity of fly ash.12,32,33,46 3.

Results and Discussions Representative HATR-FTIR spectra of water adsorption on fly ash samples from United

States, Northeast India, Netherlands, and Germany are shown in Figure 5. Infrared spectra were collected at specific RH values after a passivation time when infrared absorption intensity was constant. Upon exposure to humidified air, all four fly ash films show a significant increase in absorption intensity in the 1600 to 1700 cm-1 and 3000 to 3600 cm-1 regions. These bands are assigned to the bending (δOH) and stretching (νOH) modes of water respectively.20 Similar absorption bands have been observed for water uptake on clay minerals, suggesting similarities between mineral dust and fly ash.29,47 In all samples, the O-H stretching mode of water is 14 ACS Paragon Plus Environment

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observed in two forms: an absorption band centered around 3400 cm-1, which corresponds to the OH stretching mode of liquid water, and a second band, centered around 3240 cm-1, assigned to a more structured arrangement of adsorbed water, similar to that of the structure of ice.20 In addition, small absorbance features are observed to grow with increase in RH around 2100 cm-1 for Northeast India, Netherlands, and Germany ashes, which are characteristic bands for both liquid water and different phases of ice.20,48 The spectra of the dry fly ash samples, particularly samples from India and the Netherlands, show a structural hydroxyl group vibrational band centered around 3673 cm-1.4 The growth of this structural OH stretching mode is observed in all the samples as RH increases. However, the growth is more noticeable in the fly ash sample from the Netherlands, suggesting the structural OH bonds could be relatively dominant in these ash particles.


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Figure 5: Representative IR spectra of fly ash samples ratioed to the dry fly ash sample as a function of increasing RH. (A) United States (USFA); (B) Northeast India (INFA); (C) Netherlands (NLFA); (D) Germany (DEFA). RH values included on the top of each panel. 16 ACS Paragon Plus Environment

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All the O-H stretching regions of adsorbed water grow as RH increases (see support material). Based on their apparent height as observed in Figure 5, the absorption band centered around 3400 cm-1 becomes the dominant O-H stretching band as relative humidity increases. At higher RH, the water molecules start to interact and form “liquid-like” water on the surface of the particles, resulting in a relatively higher growth of the bands centered around 3400 cm-1. For RH above 45%, there is a more rapid growth in absorbance of both stretching and bending features, supporting the argument of a higher proportion of “liquid-like” water due to clustering. The “ice like” adsorbed water on the surface of fly ash samples have been observed on other aerosol samples such as mineral dust and carbonaceous combustion aerosol.16,19,49 The “ice-like” observations show that aluminosilicate frameworks in the fly ash crystalline structure, mostly present as mullite, have a partial ice-nucleation (IN) ability that can lead to mixed-phase water nucleation on the surface of the particles.19 Ultimately, the presence of ice-like adsorbed water suggests that fly ash could be a relevant surface for ice nucleation in the atmosphere. However, IN on fly ash samples varied with sample source, probably due to mineralogy and combustion efficiency of the emitting power plant. Figures 5B and 5D also show a small loss in absorption around 1685 cm-1 in the adsorbed water spectrum. This may be associated with the loss of carbonate/bicarbonate in the framework of aluminosilicate.33,46,50 The presence of bicarbonate, coexisting with carbonate on the surface, may increase the hydrophilic nature of fly ash,12 where carbonate/bicarbonate can be solubilized at the interface and removed from the aluminosilicate framework of fly ash in the flow of humid air.32,51 Recent X-ray photoelectron spectroscopy (XPS) studies have established the presence of HCO3- on surface carbonates.51-53 While fly ash major components are mostly insoluble in water under our experimental conditions, surface carbonate and formation of surface bicarbonate can 17 ACS Paragon Plus Environment

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lead to more water uptake from the atmosphere as these species increase the charge density on the surface.52-54 The negative absorption band near the bending region of water was only observed for surface carbonate/bicarbonate, and was absent in NLFA where bulk carbonate species are the result of incomplete combustion and are not associated to the aluminosilicate framework of fly ash, which are more suitable for detection using HATR-FTIR. Although recent studies have quantified water adsorption using infrared absorption bands,47,55,56 the multicomponent complexity of the samples render impossible the assumptions required for water quantification using vibrational absorption bands. As mentioned above, we have quantified water uptake via QCM, which has been proven a more accurate method for quantification of adsorbed water in complex interfaces.29,37 Representative QCM measurements for the water uptake on the four fly ash samples are shown in Figure 6. Changes in the QCM crystal frequency (∆f) were recorded as the RH in the system was increased and subsequently decreased for adsorption/desorption investigation. Relative humidity was only changed after the system was in equilibrium, evidenced by a constant value of QCM frequency. Stepwise changes in RH are controlled by adjustable valves, resulting in different RH steps for each experiment. Figure 6 shows that there is hysteresis in the water uptake in two of the four fly ash samples examined: the USFA and DEFA frequency does not return to its original value on the timescale of the measurements (usually 12 hours). On the other hand, INFA does not show hysteresis as it dries out to the original frequency, with a small difference likely due to slight drifts in the in the frequency of the QCM crystal (80%) surface water behaves as bulk water. Altogether, USFA requires higher pressures of water and lower energies to reach surface coverage; all other samples examined in this work reach surface coverage at lower water partial pressures with higher enthalpies of adsorption. Water adsorption on fly ash takes place mostly through hydrogen bonding with OH terminal sites on the aluminosilicate network. For all the isotherms investigated, the enthalpies of adsorption of water on fly ash, as reported in Table 2, are close to the energy of liquefaction of bulk water, indicating a low density of OH binding sites on the surfaces.22,60-62 Yet, the more exothermic adsorption processes on fly ash suggest a higher density of water adsorption sites. The proximity between OH terminals increases with its surface site density, making it possible for one water molecule to interact with OH-pair terminals, and as a consequence less partial pressure of water is needed to occupy all the sites and form a ML at lower partial pressures of water. In fact, Fubini et al. suggested that water adsorption on SiO2 polymorphs, a good model system for fly ash, takes place on Si-OH pairs when ∆B@C 4 ° < −44 kJ mol( .63 Thus, the 24 ACS Paragon Plus Environment

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adsorption sites of INFA, NLFA, and DEFA likely involve two OH terminals binding one water molecule, resulting in a more negative average ∆B@C 4 ° and lower RH to reach ML coverage. On the other hand, USFA may have a slightly lower OH density than that in all other fly ash samples examined in this work, leading to higher RH values needed to reach the ML formation, where some water molecule will bind to a single OH surface terminal. Figure 7 also suggests that water uptake on fly ash is similar to that on mineral dust, a similarity that has also been observed on studies of the atmospheric fate of fly ash.2,4,64 In addition, like mineral dust, fly ash can play a significant role as IN. In fact, formation of ice below water saturation (RH = 100%) has been observed on mullite, where ice nucleation was more effective than alumina or iron oxide alone.21 Overall, all fly ash particles examined are effective surfaces for the heterogeneous formation of ice under tropospheric conditions. Yet, there are some apparent differences between the samples: Table 2 shows a higher amount of adsorbed water at ML formation on INFA and NLFA than the other samples. The difference in water uptake between the four fly ashes studied here may relate to fundamental properties and composition, primarily traces of soluble material, such as carbonates, that can enhance water uptake. Thus, the relatively large fraction of carbonates on NLFA can account for the relatively higher amount of surface water. In addition, particle size and OH surface density play an important role in binding water to the surface of ash and the heterogeneous formation of ice, which may account for the large uptake at low RH of both INFA and NLFA, the two samples with larger mean particle diameter (Table 1). Studies of ice nucleation on mullite show that larger particles are more effective as IN and can form surface ice at lower relative humidity, in agreement with the relatively high RH required for ML formation and low surface water


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observed on USFA.21 In general, and in good agreement with observations by Archuleta et al., as fly ash particle diameter increases, the enthalpy of adsorption becomes more exothermic.21 Metal cation hydration seems to play a minor role in water uptake. Previous XRF and AAS studies on the fly ash particles presented in this work show that iron oxide is the main trace component in the samples, in agreement with XRD analysis as presented in Figure 3.4 The total iron content in the samples was found to be 38 ± 2, 25 ± 3 and 9.4 ± 0.8, and 33.8 ± 0.7 mg g-1 for USFA, INFA, NLFA, and DEFA, respectively.4 Well-combusted fly ash contains a higher fraction of Fe3+, while incomplete combustion, such as NLFA, contain a relatively larger fraction of Fe2+; because the hydration enthalpy of Fe3+ is larger than that of Fe2+, trace iron in wellcombusted ashes can drive water uptake.65 Yet, the ashes with higher content of iron show lower water adsorbed on the particle surface. While hydration of trace metal cation is likely to take place,29 water uptake is driven mostly by surface binding and not necessarily by hydration of iron cations. In addition, the samples with the higher fraction of iron, USFA and DEFA show the lowest water uptake. On the other hand, recent dissolution experiments suggest that iron oxide in INFA is not available on the surface of the particles, which might indicate that Fe2O3 in INFA might not be as significant.4 While Fe2O3 also nucleates water and is an IN, it is not as effective at water uptake and IN as aluminosilicate.21 Therefore, samples with less iron oxide available on the surface, such as INFA and NLFA, leave mullite and other aluminosilicates to drive the water uptake. Indeed, Table 2 shows that pure mullite forms a monolayer with more water mass than fly ash samples. For the experiments described here, water uptake can proceed via two possible mechanisms: (1) water vapor is condensed on the surface of fly ash particles and ice is formed directly on the surface via deposition nucleation; alternatively (2) adsorbed liquid-like water can 26 ACS Paragon Plus Environment

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freeze on the surface of fly ash through condensation freezing.64 Studies on water nucleation on fly ash are necessary to better determine the mechanisms of ice nucleation on these particles. Yet, in this study we have provided the first direct evidence that differences in sample due to mineralogy, size, and combustion efficiency can impact water uptake ash surfaces. Overall, fly ash may contribute to the tropospheric budget of CCN and IN, which can contribute to our understanding of cloud adjustments due to anthropogenic aerosols. 4.

Conclusions We used a QCM coupled with FTIR experiments to investigate the water uptake and ice

formation below water saturation conditions on fly ash samples from several source regions. From this study, we have shown that fly ash composition and the combustion process that generates the particles influence its water uptake. Given the non-pours character of fly ash, BET adsorption isotherm is a suitable model for the study of water uptake on its surface. While it is assumed some homogeneity on the sites within a single fly ash sample, some differences in site density and binding are inferred between ashes. These differences may be due to fundamental particle properties that affect water uptake and catalysis of ice formation, such as particle composition, size, and adsorption site density.

All these properties can be linked to the

mineralogy of the combusted coal and the efficiency of the combustion process in the power plant that generated the fly ash. While mullite (3Al2O3·2SiO2) was consistantly the main component of fly ash, iron oxides were also found in the samples. In addition, incomplete combustion lead to the presence of feldspars and carbonates, as observed in NLFA. Some traces of carbonates in INFA and DEFA were also observed, which can lead to CCN enhancement on the particles as soluble materials increase the water uptake.


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In addition, several works on alumina and silica have shown that OH terminal binding is the main mechanism of water adsorption, and OH site density drives water nucleation on these surfaces.60-63 Puibasset and Pellenq suggested that low OH density on alumina surfaces, good proxies of fly ash, results in relatively high pressures of water for monolayer formation and enthalpies of adsorption close to -44 kJ mol-1, the energy of condensation of pure water.22 In similar studies on silica, Fubini et al. suggested that low densities of OH lead to single molecules per site, with enthalpies −∆B@C 4 ° ≤ 44 kJ mol-1. When two OH terminals bind to a single water molecule, the absolute value of energy increases above 47.5 kJ mol-1, reaching monolayer saturation at lower pressures.63 It is inferred that USFA binds water on single sites while the rest of the ashes examined in this work have OH pairs per molecule of water. The composition and structural differences between fly ash particles play a role in water uptake, and the ash ability to act as CCN and/or as an IN. Fly ash from the United States requires larger partial pressure of water to reach full surface coverage; therefore, U.S. fly ash particles will have fewer layers of water and subsequently have less liquid-like water binding. In general, as the fly ash particle diameter increases and the OH surface density decreases. Finally, the findings in this study support the observation that fly ash has similar nucleation abilities to that of mineral dust, with lower water uptake probably due to a composition made primarily by non-soluble materials and the absence of clay minerals.12,47 This information provides some insight on how anthropogenic particles can contribute to cloud adjustments, although it continues to be difficult to assess the effect of fly ash on cloud dynamics due to the lack of information on the relative abundance of fly ash particles in the atmosphere. In addition, it has been shown recently that atmospheric processing of fly ash can lead to dissolution of particles in atmospheric water, with possible enhancements in its CCN and IN 28 ACS Paragon Plus Environment

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ability as ionic species and soluble matter becomes part of the complex interface of fly ashwater.3,4 Acknowledgment. This work was supported by Skidmore College startup funds and the Schupf Scholar program. Authors also acknowledge Joseph Marto for important foundation experiments and Professor Michael E. Hagerman, from Union College, for XRD analysis.


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Water Adsorption Isotherms on Fly Ash from ... - ACS Publications

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