Environ. Sci. Technol. 2002, 36, 1405-1413
Fossil Fuel and Wood Combustion As Recorded by Carbon Particles in Lake Erie Sediments 1850-1998 A N D R E W C . K R A L O V E C , †,‡ E R I K R . C H R I S T E N S E N , * ,†,§ A N D RYAN P. VAN CAMP† Department of Civil Engineering and Mechanics, Department of Chemistry, and Center for Great Lakes Studies, University of WisconsinsMilwaukee, Milwaukee, Wisconsin 53201
Carbon particle analysis was performed on a dated sediment core from Lake Erie in order to explore the inputs of pollution from incomplete combustion of coal, oil, and wood. Carbon particles were isolated from the sediment by chemical digestion, and elemental carbon content was determined by CHN analysis. The type of carbon particle (from burning coal, oil, and wood) and particle size and relative abundance were determined using scanning electron microscopy on 100 particles from each core section. The elemental carbon content in the Lake Erie core ranges from 2.5 to 7.4 mg of carbon/g of sediment (18501998), and the maximum carbon content in the sediment occurs in the late 1960s to early 1970s. It is shown that particle mass is a better predictor than particle number of historical energy consumption records. This is especially clear for wood where variable particle volumes play a significant role in determining the record of elemental carbon mass from wood burning. Lake Erie core’s content of total carbon and carbon particle type is in agreement with U.S. energy consumption records, except that a wood maximum occurs during 1905-1917, about 36 yr after the U.S. consumption maximum from 1870 to 1880.
Earlier work in this area considered numbers of different particles (coal, oil, or wood) rather than their mass (2-4). In addition, the dating resolution was rather limited, with only five (2) or four (3) core sections since around 1850. However, it was demonstrated that numbers of spheroidal carbon particles or total charcoal mass exhibited a general correlation with bituminous coal consumption or overall energy consumption with a peak in the carbon record around 19601970 (4-6). It was also shown that there was a general correlation between numbers of spheroidal carbon particles (6) or total charcoal mass (5) and the metals Zn, Cr, Cd, Pb, and Cu. Carbon particles are transmitted from their source, atmospherically, until they settle on the earth. After settling on lakes or rivers, they travel through the water column and become part of the sedimentary record. If the sediment remains relatively undisturbed, carbon particles can become locked in the sediment. The age of the sediment and the types and amounts of carbon particles can then be used to create a time record of carbon pollution for the area from where the sediment was taken. Relative numbers of types of carbon particles in sediment cores were used to confirm PAH apportionment of sources (7) and to identify coal gasification as a source of PAHs in the environment (8). This was done for the time period 1900-1995 (7) or 1896-1990 (8) using percent carbon particles by type as a measure of energy input. In addition, the examination of the amounts, types, and dimensions of the carbon particles can indicate the effectiveness of pollution control devices used by facilities that burn coal, oil, and wood (2). The objectives of this work are to use scanning electron microscopy (SEM) to identify types of carbon particles in a dated sediment core from Lake Erie spanning the last 150 yr and to determine a source apportionment based on the carbon particles; to obtain measurements of their size and shape factor; and to explore any possible correlations between numbers or mass of carbon particles and historical energy consumption data.
Introduction
Materials and Methods
Adverse effects of fossil fuel combustion were first identified during the 1700s when an increase in the incidence of cancer was detected in chimney sweeps in England. The cancers were caused by polycyclic aromatic hydrocarbons (PAHs) in soot from chimneys (1). While complete combustion of fuels such as coal, oil, and wood produces carbon dioxide and water, complete combustion is difficult to obtain and is rarely achieved. During incomplete combustion of coal, oil, and wood, several harmful byproducts can be formed including PAHs and particulate matter, including carbon particles. Carbon particles have a unique morphology that is determined by the type of fuel that is burned (2). By examining these byproducts, one can determine their sources and processes in which they are generated. Our interest in carbon particles is primarily motivated by a desire to explore and refine correlations with historical energy consumption data. These relationships are useful as time markers in sediment dating, as an aid in PAH source apportionment, and in studies of correlations with other pollutants such as metals.
Lake Erie. At an elevation of 173 m above sea level, Lake Erie is the 11th largest lake in the world by surface area of 25 700 km2 (9). With an average depth of only about 19 m and a maximum of 64 m, Lake Erie is the shallowest Great Lake.
* Corresponding author phone: (414)229-4968; fax: (414)229-6958; e-mail:
[email protected]. † Department of Civil Engineering and Mechanics. ‡ Department of Chemistry. § Center for Great Lakes Studies. 10.1021/es011018s CCC: $22.00 Published on Web 02/21/2002
2002 American Chemical Society
Lake Erie includes three major basins: western, central, and eastern. The Sandusky Subbasin (Figure 1) is separated from the remainder of the Central Basin by the Pelee-Lorain Ridge in the east and is fed in the west by the South Passage from the Western Basin (10). Sampling of Cores. Lake Erie core samples were taken May 12, 1998. The core samples were all taken from the Sandusky Subbasin of the Central Basin of Lake Erie. The rationale for selecting these sites was that they would provide relatively unpolluted reference levels for a study of PAHs in the Black River sediments (11). This is because of the dominant counterclockwise gyre in the Western Basin (10). The sites were separated by approximately 10 km (Figure 1). One 80-95 cm long core was acquired from each site with a gravity corer of 3 in. (7.6 cm) diameter fitted with a polybutyrate tube. A hydraulic extruder was then used to remove the sediment in increments of 5 cm. Fifteen subsections were taken for a total of 75 cm of sediment. Approximately half of each section sample was placed in a plastic container for laboratory analysis of VOL. 36, NO. 7, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
1405
FIGURE 1. Map of Lake Erie sampling sites. porosity, loss on ignition, and dating studies, and the other amount was put into amber, glass jars with Teflon liners in the caps to be examined for PAH concentrations (11). The samples were all stored in a freezer at -18 °C for 1 week at the Ohio Department of Natural Resources in Sandusky, OH, and then transported frozen from Ohio to the Great Lakes Water Institute in Milwaukee where the plastic containers were stored at 4 °C and the jars were stored at -18 °C. 210Pb and 137Cs Dating. For 210Pb analysis (12), sediment previously dried and pulverized was stored in scintillation vials until needed. The 210Pb procedure includes digestion, filtering, volume reduction, pH adjustment, and plating on copper disks. Yield tracer for chemical separations and counting was 209Po (T1/2 ) 109 yr; NIST SRM 4326). The total 210Pb activity of each sample was determined by counting the R activity of 209Po and 210Po on copper disks. This was done using surface barrier R detectors (EG&G Ortec model 576 R detector) coupled to a multichannel analyzer with software (EG&G Ortec Maestro II). The supported 210Pb levels were estimated from the deep layers where the 210Pb activity is nearly constant. To determine 137Cs activities, we used previously dried and ground sediment. About 40 g of sediment was weighed and processed to form a cylindrical pellet (12). The pellets were oven-dried at 60 °C overnight. For counting 137Cs activity, a γ detector (EG&G Ortec high purity Ge detector model GEM09175-P-S) was utilized along with the Maestro II software. The pellets were inserted into small plastic bags, before being placed on the detector, and counted for a minimum of 80 000 s. The 137Cs γ radiation was detected at 661.65 keV. 133Ba (356.01 keV) and 22Na (1274.54 keV) standards were used to calibrate the counter. Mass sedimentation rates were found by linear regression of log excess 210Pb activity versus cumulative sediment mass. Time intervals for each core section were calculated by dividing the cumulative sediment mass by the mass sedimentation rate. Linear rates (cm/yr) were determined by dividing mass rates by the average bulk density or from the depth of onset or maximum 137Cs activity along with the corresponding dates (1954 and 1963, respectively). Depth of onset was determined as the point exhibiting 10% of the maximum 137Cs activity. Rates based on onset are maximum values. 1406
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 36, NO. 7, 2002
Carbon Particle Analysis. The chemical separation used was a variation of the procedure described by Griffin and Goldberg (13). The main difference was that we used here smaller volumes of HCl and HF for digestion in order to minimize the volume of hazardous waste generated (14). The core sections were thoroughly mixed and dried in an oven at 100 °C for 36 h. After being dried, the samples were again thoroughly mixed, and approximately 10 g of sample was digested in 100 mL of 6 N HCl at 94 °C for 2 h. The initial acid digestion was done to remove metals and carbonates. Griffin and Goldberg (13) used 250 mL of 6 N HCl in this step. The samples were cooled, and the mixture was separated by centrifugation. The supernatant was decanted, and the residue was digested in 50 mL of 12 N HCl and 100 mL of 48% HF. The amount of HF in the original procedure was 300 mL of 10 N HF (13). The digestion mixture was mixed at room temperature for 1-2 weeks on an orbital shaker. The second acid digestion was used to remove silicate and more metals. The samples were then centrifuged, and the residue was washed with water until the wash water pH was approximately 7.0. The next step was oxidation by 100 mL of 6 N KOH and 800 mL of 35% hydrogen peroxide. The samples were then centrifuged and washed until the pH was approximately 7.0. The oxidation step removes organic matter by converting the organic material into water-soluble compounds. Digestion was carried out in 100 mL of 12 N HCl for 1 h at 95 °C. Here the original procedure called for 250 mL of 6 N HCl (13). After the acid digestion, the samples were centrifuged and washed until the pH was 7.0. The final acid digestion was performed to remove any remaining metals. The samples were then dried at 110 °C for 24 h. After being cooled, the samples were weighed and stored for CHN analysis. CHN Analysis. The percent carbon of each sample was determined by a CHN analyzer. The samples were analyzed in duplicate, and an average percent carbon was determined. The CHN instrument used was a Carlo Erba CHN model 1105 fitted with a thermal conductivity detector. The standard used was acetanilide, and all samples weighed approximately 0.5 mg. SEM Analysis. After the carbon particles were chemically separated and the percent of carbon in each sample was determined, the particles were examined by scanning electron
FIGURE 2.
137Cs
activity vs depth for Lake Erie cores.
microscopy (SEM) using a Top Con model ABT-32 electron microscope. The samples were examined by moving the sample in a side-to-side, top-to-bottom, zigzag pattern. A magnification window of 500× was used to examine the sample. Higher magnifications were then used to confirm the identity of each particle. The search criteria for the particles were that they should be identifiable (positive identification) and unobstructed (outer edges must be seen) so that they could be measured. The carbon particles were identified based on their morphologies. There are three main sources of carbon particles from the combustion of coal, oil, and wood. The combustion process produces carbon particles with characteristic morphologies. The shapes of the carbon particles can be categorized into three groups: (i) porous, spherical; (ii) elongated, prismatic; and (iii) irregular, spherical. The surface texture can be characterized as smooth, homogeneous, rough irregular with cells or pits, or having etched/ convoluted layer structures (1). Wood particles commonly have a length-to-width ratio greater than 3, usually contain a cellular structure, and are elongated in shape (1). The surface texture of wood particles is often rough with cells, but it can also be smooth (Figure 4).
Carbon particles produced from the combustion of oil have length-to-width ratios of approximately 1. This gives the particle a spherical shape. Oil particles have a convoluted or etched surface texture (Figure 5). Coal particles produce the largest variety of morphologies. They can be either spherical (or elliptical) in shape or form irregular fragments. The surface texture of coal particles also varies from smooth to highly porous. The irregular shape of coal particles yields a length-to-width ratio between 1 and 2 (Figure 6). Microscopic examination of the carbon particles was done to determine a source apportionment of sediment composition (SC) by particle type. The apportionment was based on the identity of 100 randomly selected particles from each core section. The sediment composition for a given particle type T (SCT) is expressed as the particle mass per unit mass of sediment (g of particles/g of sediment):
SCT )
NTVTF g of carbon (1) NCVCF + NOVOF + NWVWF g of sediment
where NT is the number of particles of type T per core section, NC is the number of coal particles per core section, NO is the VOL. 36, NO. 7, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
1407
FIGURE 3. Excess with arrows.
210Pb
activity vs cumulative mass for Lake Erie cores. Negative excess
number of oil particles per core section, NW is the number of wood particles per core section, VT is the average particle volume (of particle type T) per core section (µm3), and F is the density of carbon particles. The volume of a coal or oil particle was approximated by the volume of a sphere, with a radius equal to half of the geometric average of the maximum and minimum diameters for each carbon particle. The volume of a wood particles was approximated by the volume of a cylinder, where the radius equaled half of the minimal particle diameter and the height equaled the maximum particle diameter. The maximum and minimum particle diameters were also used to determine a 1408
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 36, NO. 7, 2002
210Pb
activities, taken as zeros, are indicated
shape factor for each particle. The shape factor was defined as the maximum particle diameter divided by the minimum diameter for each particle.
Results and Discussion Sediment Dating. A summary of sedimentation rates for the Lake Erie cores is provided in Table 1. Values of porosity, loss on ignition, 210Pb activity, and 137Cs activity for each depth interval along with calculated cumulative mass, bulk density, excess 210Pb activity, uncertainties, and detector counts, are given in Van Camp (15).
FIGURE 4. Electron micrographs of carbon particles from the burning of wood. Magnification: (A) 2000× and (B) 400×.
FIGURE 5. Electron micrographs of carbon particles from the burning of oil. Magnification: (A) 1000×, (B) 5000×, (C) 1000×, and (D) 7500×. Cesium-137 activity versus depth and log excess 210Pb activity versus depth are shown graphically for cores LE98-1, LE98-2, and LE98-3 in Figures 2 and 3, respectively. Unsupported levels of 210Pb activity were found in layers 1-4 of LE98-1, in layers 1-6 of LE98-2, and in layers 1-8 of LE98-3. Supported levels of 1.84, 1.94, and 2.04 dpm/g were estimated for each core by averaging the 210Pb activities in the bottom layers. Peak 137Cs activities, corresponding to ∼1963, are observed in all three cores in layer 3. Activities tail to zero by layer 5 in all three cores. Although sample intervals were fairly thick (5 cm) so that details in 210Pb and 137Cs activities versus depth are somewhat limited, they provide roughly twice the time resolution of other related studies (2-4). Also,
while the slices are thick, they are not thick enough to significantly affect the 210Pb-derived sedimentation rates. For the purpose of comparing sedimentation rates based on 210Pb and 137Cs, we are primarily interested in the more recent sedimentation patterns corresponding to layers 1-4 in each core where 137Cs activity is observed. These layers were deposited during the period 1954-1998. A comparison of linear sedimentation rates v (cm/yr) using excess 210Pb, the 137Cs 1963 peak, and the 1954 onset of 137Cs activity is presented in Table 1. Rates based on 210Pb and 137Cs (0.290.59 cm/yr) are consistent and are also in agreement with the findings of Robbins et al. (16). VOL. 36, NO. 7, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
1409
FIGURE 6. Electron micrographs of carbon particles from the burning of coal. Magnification: (A) 2850×, (B) 8000×, (C) 1600×, and (D) 5000×.
TABLE 1. Sedimentation Rates for Lake Erie Cores Determined by 210Pb and 137Cs sample site
mass sediment. rate, r (g cm-2 yr-1)
LE98-1 0.1285 ( 0.0052 LE98-2 0.1753 ( 0.0079 LE98-3 0.1759 ( 0.0135
av bulk density, Gav (g/cm3)
sediment. rate, v (cm/yr) 137Cs 137Cs 210Pb peak onset
0.32 0.38 0.30
0.40 0.34 e0.45 0.46 0.29-0.43 e0.45 0.59 0.36-0.43 e0.57
Carbon Particles. The carbon particle source apportionment was based on the identity of 100 carbon particles per core section. The results of the particle identification and measurements for each of the 15 sections for core LE-2 are listed in Kralovec (14). Results of calculations of average shape factor and diameter for each particle type are given in Table 2. Oil particles are the most spherical, i.e., their shape factors are closest to 1. The coal particles are more elliptical in shape than the oil particles. The wood particle shape factors have the largest variance, and the particles have a definite elongated shape. The particle sizes for the Lake Erie core illustrates an increasing trend for all three particle types since about 1934. Note that no oil particles were found below layer 7 for which the number was 2 out of 100 particles; compared to the maximum of 20 out of 100 in layer 3. This fact supports the dating of layer 7 (1894-1916) in that the U.S. energy consumption records (14) indicate very low (
