Carbonaceous particle hydration

Atmospheric Environment 33 (1999) 2679—2687

Carbonaceous particle hydration A.R. Chughtai, G.R. Williams, M.M.O. Atteya, N.J. Miller, D.M. Smith* Department of Chemistry and Biochemistry University of Denver, Denver, Colorado 80208, USA Received 24 September 1997; received in revised form 9 July 1998

Abstract Microgravimetric measurements of the hydration of several different black carbons or soots and a series of commercial carbon blacks have been carried out, over a relative humidity range of 20—85%, in an extension of earlier work with the model n-hexane soot. All adsorption isotherms are of type III and were analyzed by the use of the Dubinin—Radushkevich (DR) equation which, although applicable over a limited range of intermediate relative humidity values, allows identification of chemisorption limit and onset of multilayer formation. While surface area determines the maximum adsorption possible for a given type, surface functionalities are determinative at lower humidity and are characteristic of the soot-producing fuel. Aging of carbon particles and oxygen chemisorption as well as O physisorption strongly  influence the extent of hydration for those soots studied, such as JP-8 aviation and diesel fuels. Infrared spectra confirm the surface oxidation of JP-8 soot by its reaction with O , a reaction of probable atmospheric importance, as underlying  its increased hydration.  1999 Elsevier Science Ltd. All rights reserved. Keywords: Carbonaceous particles; Soot; Water adsorption

1. Introduction Evidence has been accumulating that the global mobilization of black carbon from natural and anthropogenic sources is responsible for significant chemical and physical effects in our atmosphere. These effects include participation in tropospheric chemistry, light scattering/ absorption (thus affecting earth’s radiation balance), and human pulmonary health (as a constituent of respirable particles). While its quantitative impacts have not been fully delineated, world-wide anthropogenic emissions of black carbon recently have been estimated in the range of 12—24 Tg yr\ (Penner et al., 1993). The principal single source of anthropogenic black carbon in the atmosphere is biomass burning in connection with agriculture (Seiler

*Corresponding author. Tel.: #1 303 871 2938; fax: #1 303 871 2932; e-mail: [email protected].

and Crutzen, 1980), although other sources, such as diesel (Cass et al., 1982) and wood burning (Wolff et al., 1982), may dominate regionally or locally. The extensive distribution of black carbon particles throughout the earth’s atmosphere has been demonstrated by a variety of measurements (e.g. Smith et al., 1973; Rosen et al., 1981; Hansen and Rosen, 1984). The interaction of black carbon particles with an aqueous phase influences their role in numerous atmospheric reactions. The importance of water concentration in the wet oxidation of SO (Brodzinsky et al., 1980; Harrison  and Pio, 1983; Smith et al., 1989), and in the photodegradation of particle-associated polyaromatic hydrocarbons (McDow et al., 1995), are but two examples. The partitioning of chemical species, black carbon and sulfur (Hallberg et al., 1994), for example, between cloud droplets and interstitial air lies towards the more ‘‘soluble’’ particles forming droplets with the less soluble ones remaining in the interstitial air. Our own work (Chughtai

1352-2310/99/$ - see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 9 8 ) 0 0 3 2 9 - X

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et al., 1991) has elucidated the mechanisms by which hydrolysis of carbon surface carboxylics underlies an unusual ‘‘solubilization’’ of soot particles. A surprising variation in the water nucleation properties of soot particles from similar fuels (diesel and JP-8 aviation fuel) based upon their soluble mass fraction has been reported (Lammel and Novakov, 1995). Some soot particles emitted to the atmosphere are hydrophilic while others are hydrophobic; in the former case, the primary particles grow in size in an aqueous environment while in the latter case they do not. It was observations such as these that prompted our first effort to understand the mechanism(s) of soot particle hydration (Chughtai et al., 1996). In that work, microgravimetric measurements revealed soot aging, surface oxidation, physisorbed O , and the incorporation of  trace metals to increase particle hydration in the range of 0.33—0.52 relative humidity (o/o ). The application of  a modified Dubinin—Radushkevich isotherm (Dubinin, 1966; Kobayashi et al., 1993) enabled the determination of several important parameters such as limiting surface coverage and the limit of chemisorption to be estimated. Limitations of that work include a somewhat narrow o/o range (dictated by the original apparatus), and the  exclusive use of n-hexane soot (employed throughout our studies as a model for liquid fossil fuel combustion). Recent studies of the hygroscopic properties of diesel soot particles (Weingartner et al., 1997) show that aging and oxidation of these particles lead to a higher hygroscopicity. These results are consistent with ours from the hydration experiments using n-hexane soot (Chughtai et al., 1996). Hydration experiments with a commercial carbon black (Degussa’s FW2) in a low-pressure Knudsen cell reactor (Rogaski et al., 1997) showed no effect of NO  and O treatment of the carbon black on its water up take, results at variance with ours. The Knudsen cell experiments are not comparable to previous ones, however, because the carbon black is post-treated with NO  by the manufacturer, and the relative humidity in these experiments did not exceed 1%. The research reported in this article was undertaken to explore soot hydration over a broader range of relative humidity (20—85%), and to examine the hydration behavior of soots produced from a variety of fuels and combustion conditions. The choice of 14 different carbonaceous particles for this study reflects an effort to compare the hydration behavior of soots produced from a variety of common fuels with each other, with our n-hexane model soot, and with a typical commercial carbon black, several of which have been used by researchers to estimate soot’s atmospheric impact. Soots from diesel, plant material and coal, which represent principal sources of anthropogenic black carbon in the atmosphere, were of particular interest, as was the soot from JP-8 aviation fuel believed to react rapidly (Smith and Chughtai, 1996) with ozone in the lower stratosphere (Bekki, 1997; Lary et al., 1997;

Smith and Chughtai, 1997). Materials were selected to examine the effects of surface oxidation, surface area, and aging, as well as composition, on their hydration characteristics.

2. Experimental 2.1. Carbonaceous particles Eleven black carbons were prepared from the combustion of gaseous, liquid or solid fuels. All n-hexane soots were derived from the Chem AR grade reagent (Mallinckrodt, Paris, KY, USA), while JP-8 aviation fuel (Continental Oil Company, CONOCO) and diesel fuel (Total Petroleum) from Denver area commercial sources were used to generate those soots. Solid fuels included dried lodgepole pine needles and Utah hard coal (Trinidad Benham Corp., Denver, USA) from which soots were prepared through combustion. The sole soot of gaseous fuel origin was from acetylene (Matheson). All black carbons were prepared using the technique described in earlier publications (e.g. Akhter et al., 1991). In general, burning of the fuel under least turbulent conditions (oxidant O furnished solely by diffusion/convection  from surroundings) deposited soot on the inside surface of an inverted Pyrex funnel (13 cm;23 cm) from which it was scraped, collected in a glass vial, mixed with a mechanical shaker, and stored in a desiccator for use in experiments. Oxidized black carbons were prepared by exposure to gaseous NO /N O or 1.12% O in O by      procedures detailed in the first paper of this series (Chughtai et al., 1996). Three commercial carbon blacks of Cabot’s Black Pearls series (Cabot Corporation, Billerica, MA, USA) having greatly differing surface areas were used as obtained from the manufacturer: BP2000 (1500 m g\), BP1100 (240 m g\), and BP280 (42 m g\). These materials are prepared by the combustion of a mixture of gaseous and liquid hydrocarbons utilizing a rapid cooling process for collection of the solid. 2.2. Hydration measurements Isotherms were obtained, as mass increase versus both time and relative humidity (RH), o/o , from a high re solution data collection system interfaced with a Cahn RG electrobalance, modified to increase both accuracy and reproducibility ($1 lg and $0.1 lg, respectively) as described in the earlier paper (Chughtai et al., 1996). A diagram of the apparatus and description of the technique is found in that paper. The principal modification for this work was the replacement of the hydration chamber with one constructed of hermetically sealed Plexiglas (polymethylmethacrylate) panels. A water saturated

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N stream flowed through at various accurately estab lished flow rates and relative humidity was monitored with two Cole—Parmer Instrument Company hygrometers as a function of time. Experiments were generally carried out in duplicate either over the o/o range of  0.20—0.36 or of 0.36—0.85 at flow rates for which the establishment of equilibrium separately has been demonstrated. 2.3. Other details All infrared spectra were obtained from samples at the same concentration (w/w) in the KBr matrix and acquired with an extensively modified FT infrared spectrometer (BioRad FTS-14) (Smith and Chughtai, 1995). Surface area values for each carbon were obtained from low temperature N adsorption measurements (the BET  technique).

3. Results and discussion

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The lower o/o isotherms of Fig. 1B also are type III  and are clustered in three groups as to the rate of adsorption (mass increase per unit of o/o ):  E high-soots from diesel fuel, Utah hard coal and JP-8 aviation fuel; E intermediate-soots from n-hexane and pine needles; E low-BP2000 carbon black and acetylene soot. It is interesting to note that the highest surface area material, the BP 2000 carbon black, 1500 m g\ versus 65—90 m g\ for the five black carbons, undergoes the lowest rate of surface hydration initially but hydrates much more extensively than all others at high o/o .  Further, there are only small differences between the surface areas of the other six materials (all black carbons, soots): n-hexane (89 m g\), diesel (71 m g\), JP-8 (65 m g\), pine needles (80 m g\), coal (84 m g\), acetylene (78 m g\). The hydration behavior of these carbons, especially at lower o/o , cannot be related sim ply to surface area; there are differences in the surface structures of these particles, both physical and chemical, which also determine their uptake of water.

3.1. Adsorption isotherms 3.2. Analysis of adsorption data Fig. 1A illustrates the adsorption of water at 22°C by six different soots in the relative pressure (humidity) range o/o "0.33—0.83. Four of these, BP2000, n-hexane  soot, diesel soot, and pine needle soot, are type III (Brunauer et al., 1940) isotherms over this range of relative humidity. Only soots from JP-8 aviation fuel and Utah hard coal are not convex to the o/o axis by 83%  RH, although separate experiments have suggested that this behavior also is exhibited at higher o/o values by  both. Such type III behavior indicates that the adsorption of water by each of these soots is cooperative; that is, the more H O molecules are adsorbed, the greater the  attraction of the surface for the additional adsorbate molecules. Fig. 1B shows isotherms of the same water—carbon particle systems over the o/o range of 0.20—0.36, to  which have been added data for acetylene soot. Two sets of isotherms were obtained for each material as follows: (1) 20—36% RH; 74 ml min\ flow rate (2) 33—85% RH; 800 ml min\ flow rate Sample sizes generally were 20 mg for the 20—36% RH profile and 4 mg for the 33—85%RH profile. The 20—85% RH region was thus covered separately in two segments to ensure equilibrium data at both ends of the isotherm for each carbon. At the higher flow rate of 800 ml min\, necessary to achieve the higher RH values in a reasonable time frame, it is possible to create non-equilibrium conditions at low o/o where the rise is rapid; thus, data  were acquired over the region 20—36% RH at a slower flow rate and slower increase in o/o . 

No single equation will describe the entire adsorption isotherm of water on these carbonaceous particles, in this case from 20—85% relative humidity, because each represents at least three processes: (1) reactive adsorption or chemisorption beginning at low relative humidities, generally in the range up to o/o +0.25, which we term the chemisorption limit;  (2) quasi-reversible adsorption of water which may be facilitated by hydrogen bonding between surface sites and the adsorbate, generally in the o/o range from  0.25 to 0.5 or 0.6; (3) multi-layer adsorption which develops through the cooperative interaction between adsorbed and gas phase molecules as indicated by type III isotherms; in this case, the cooperative interaction is hydrogen bonding. For analysis of water—carbon isotherms in the first paper (Chughtai et al., 1996), we employed a modified form of the Dubinin—Radushkevich (DR) equation (Marsh and Siemieniewska, 1967; Kobayashi et al., 1993). That equation may be expressed as log a"log a !D(log o /o) (1)   where a is the mass of H O (micrograms in our usage)  adsorbed per gram of soot, a is the limiting adsorption  per gram of soot, and D is 9.03 ¹ (K/b) in which K is a constant related to the adsorption potential distribution and b is the affinity coefficient of the adsorbate. This equation generally applies up to o/o "0.55 (Dubinin, 

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Fig. 1. Water-various black carbons adsorption isotherms at 22°C. A: Relative humidity, 33—85%; B: Relative humidity, 20—36%.

1980), a range which makes it useful to identify the chemisorption limit, the onset of the multilayer adsorption region, and to calculate several adsorption parameters useful to interpretation. Fig. 2 illustrates the plots of log a versus log(o /o), which we term ‘‘DR’’ plots, for  the soots from n-hexane, diesel fuel, JP-8 aviation fuel,

pine needles, and the BP2000 carbon black for the 33—85% RH isotherms. Least squares treatment of the linear portion of these plots yields values for D, a , h ,   and h , where h and h are the particle surface cover 

age at limiting adsorption and the onset of multilayer formation, respectively. Also calculated from the DR

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Fig. 2. Dubinin—Radushkevich (DR) equation plots, log a versus [log(o /o)] for the isotherms of Fig. 1. A: Relative humidity, 33—85%;  B: Relative humidity, 20—36%.

plots are the specific adsorption and surface coverage of each carbon at o/o "0.83 (83%RH). From the DR  plots at lower RH, Fig. 2B, the surface coverages at the chemisorption limit (h ) also are calculated. These results  are presented in Table 1 for eight of the black carbon and carbon black samples for which isotherms were obtained. Several generalizations can be drawn from the data of Table 1. (1) The adsorption capacity of a carbon particle for water is a function of surface area for a particular type of carbon; e.g. the greatest adsorption at high RH is exhibited by the 1500 m g\ BP 2000, decreasing for BP 1100 and BP 280.

(2) The specific adsorption for these carbon particles at a relative humidity of 83%, grams of water adsorbed per gram of soot, suggest some characteristics of these surfaces. The diesel and pine needle soots adsorb about 9;10\ g H O m\ of soot surface and, in the process,  develop the equivalent of 2—3 surface layers of the adsorbate. The JP-8 soot adsorbs 4.0;10\ g H O m\ of  soot surface and develops about 1.5 monolayers while the coal soot adsorbs 2.6;10\ H O m\ of soot surface  and develops the equivalent of about a monolayer of adsorbate. (It should be noted that the BET surface area measurement uses N which has a cross-sectional area  about a third larger than the adsorbate water molecule.) The commercial carbon black materials accomodate

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Table 1 Adsorption parameters for H O on various carbon particles calculated from the DR equation and isotherms  Soot

Surface area (m g\)

h 

Multi layer, h



Chemisorption limit, h A

Specific adsorption, lg H O g\ soot;10\ 

o/o "0.83  h

n-Hexane Diesel Pine needles Coal JP-8 BP 280 BP 1100 BP 2000

89 71 80 84 65 42 240 1500

0.848 1.610 1.320 1.010 1.620 0.210 0.480 0.085

0.758 1.310 0.844 0.981 1.290 0.200 0.398 0.046

0.0680 0.1600 0.0590 0.1030 0.1760 0.0290 0.9450 0.0016

4.0 6.8 6.7 2.2 2.1 0.4 3.8 3.7

1.83 3.28 2.53 1.09 1.30 0.37 0.65 0.97

from 1—2;10\ g H O m\ of soot surface and develop  from one-third to a monolayer of adsorbate. These data are consistent with a variation in density of surface sites for initial adsorption of the H O molecule, on the basis  that the type III isotherms reflect cooperative adsorption. (3) The chemisorption limit, in terms of surface coverage, which is a function of the density of sites for irreversible H O adsorption, and thus should be related to  surface oxygen-containing functionalities, varies widely from 0.16% (BP 2000) to about 16—18% for diesel and JP-8 soots. In illustration of the processes underlying carbon particle hydration, Fig. 3A and B presents the lower pressure (20—36% RH) and full range (20—85%) isotherms, respectively, for the BP carbon black series. While their hydration at 85% RH correlates roughly with surface area, expansion of the 20—35% RH region shows clearly that chemisorption on the BP 1100 material (240 m g\) greatly exceeds that of BP 2000 (1500 m g\) in this region. Thus, one expects the BP 1100 to be more hydrophilic (Chughtai et al., 1991) than the BP 2000 at low relative humidities despite the factor of 6 difference in their surface areas. It is reasonable to assume that underlying this difference is a higher density of hydrolyzable functionalities on the BP 1100 surface. 3.3. The role of surface oxygen Table 2 presents data for the chemisorption limit, expressed as surface coverage (h ), of soots from diesel  and JP-8 aviation fuel. The increase in the h for the  ozonated particles is consistent with the oxidative formation of carboxylic groups on these surfaces, as observed in earlier work (Smith et al., 1996) with n-hexane soot. Fig. 4A shows the isotherms of JP-8 and ozonated JP-8 soots in the lower o/o region, from the DR plots of  which h is calculated, revealing the greater hydration of  the ozonated JP-8 soot in that region. The infrared spectra of JP-8 and ozonated JP-8, Fig. 4B, reveals the

Fig. 3. Water-various black carbons adsorption isotherms for Black Pearl series (Cabot); BP2000 (1500 m g\); BP1100 (240 m g\); BP280 (42 m g\). A: Relative humidity, 20—36%; B: Relative humidity, 20—85%.

origin of this greater hydration. The reaction has produced a significant additional concentration of carboxylic species, interpreted as anhydride (Chughtai et al.,

A.R. Chughtai et al. / Atmospheric Environment 33 (1999) 2679—2687 Table 2 The chemisorption limit, expressed as surface coverage, of the hydration of soots prepared from the combustion of diesel and JP-8 aviation fuels Soot

Surface area, m g\

h A

Diesel Diesel, ozonated Diesel, aged JP-8 JP-8, ozonated

71 — 48 65 —

0.144 0.224 0.365 0.176 0.208

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1991), the hydrolysis of which increases the particle hygroscopicity. Adsorption isotherms (20—35% RH) of a series of diesel soots, freshly generated, aged, evacuated, ozonated, and nitrated, are presented in Fig. 5. From these plots, it is evident that: (1) both the rate and extent of hydration of diesel soots at 20—36% RH is in the order aged'ozonated'fresh, which will depend on the length of aging and ozonation; (2) the H O uptake of diesel soot at 83%RH exceeds  that of other liquid fuel soots, which generally have the

Fig. 4. Hydration of JP-8 aviation fuel soot and JP-8 soot reacted with ozone. A: Water-JP-8 soot adsorption isotherm for relative humidity 20—36%; B: FTIR spectra of JP-8 soot and the ozonated JP-8 soot (in KBr matrix).

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Fig. 5. Water—diesel soot isotherms at 22°C after various soot treatments.

same morphology and surface area. This effect must be due to either surface functionalities or pore size distribution. The sulfur content differentiates the diesel fuel from the others, and there is spectroscopic evidence for the formation of soot surface S—O species which may also serve as hydration sites. A separate study is underway. (3) evacuation of both aged and fresh diesel soots diminish the extent of hydration; (4) nitrated diesel soot initially hydrates more rapidly than fresh soot, but its hydration diminishes to a significantly lesser value at 27% RH. The effects of aging and ozonation on diesel and JP-8 soots are qualitatively the same as those earlier observed (Chughtai et al., 1996) with n-hexane soot. Further, evacuation of both the aged and fresh diesel soot lowering their hydration is consistent with the n-hexane experiments, which showed that exposure of the soot to both zero air and oxygen enhances hydration. On the other hand, nitration of the diesel soot yields lower hydration while water uptake was at its greatest following soot’s nitration in the n-hexane studies.

4. Conclusions Consistent with the results of an earlier study (Chughtai et al., 1996) on n-hexane soot, the hydration of

black carbons produced from a variety of fuel types generally is increased by aging, surface oxidation, and O physisorption. Measurements on a series of commer cial carbon blacks show that larger surface areas determine the adsorption capacity at high relative humidity (83%). In lower humidity environments, however, the extent of hydration is determined by surface functionalities which, in turn, are characteristic of the fuel type. Of particular note are experiments with JP-8 aviation fuel which reveal that the reaction of its soot with ozone (Smith et al., 1996) creates additional surface carboxylic groups that underlie the increased hydration of these particles. Experiments with diesel fuel specifically show the effect of aging, surface oxidation and adsorbed O molecules. At least for soot hydration, n-hexane ap pears to have been a fairly good model for liquid fuelproduced soots, and even for those from pine and coal. Commercial carbon blacks are not acceptable models.

Acknowledgements The authors are grateful to the National Science Foundation for support of this work through grants ATM 9200923 and ATM 9618495. Support by Research Corporation of GRW’s participation in this research is appreciated. Thanks also are extended to M.L. Rosenberger for preliminary hydration experiments.

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