advantage to limestone, since preparation cost is the area in which the new sorbents compare least favorably. In other areas, for example, regeneration and sulfur recovery, one would expect a significant economic advantage for the new materials. Hence, the required sorbent lifetimes given above are quite conservative, and shorter lifetimes should result if all costs are taken into account.
Acknowledgment
Conclusions
Calcium aluminate cement and calcium and barium titanate have shown surprising potential as SO2 sorbents in tests conducted in a laboratory TGA and in a fluidized bed pilot plant. CAC can be made into pellets which, in a fluidized bed, are much more resistant to attrition than the conventional sorbent limestone. Although CAC is a more active sorbent than Grove limestone, its activity can be considerably improved by increasing the porosity of pellets made from the cement, or by using promoters. Fly ash was found to sharply increase the activity of CAC, perhaps by catalyzing the oxidation of SO2 to SO3. Possibly, the fly ash that is present in a fluidized bed coal combustor may also affect the absorption of SO2 by limestone. Calcium and barium titanate were unique among all sorbents tested because they did not deactivate when cycled between sulfating and regenerating conditions. With other sorbents, the rate of sulfation became slower with cycling and the fraction of sorbent utilized declined with each succeeding sulfation. Physical characterization of sulfated and regenerated samples did not reveal why the titanates maintain their activity. The explanation, if it can be found, may shed light on the mechanism for deactivation of other sorbents as well. Based on a preliminary estimate of sorbent preparation costs and performance data now available, there is no reason to eliminate either CAC or the titanates as prospects for commercial use. However, a much more thorough cost analysis is needed to accurately predict the real commercial potential of these materials. A modest amount of additional experimental work to further define the technical merits of these materials also appears justified. Future experiments should be conducted in a fluidized bed arranged so that the sorbent can be cycled between sulfating and regenerating conditions, and so that measurements can be made of SO2 retention by the sorbent, SO2 levels produced during regeneration, and attrition rates.
For their many useful suggestions, we wish to thank Andrej Macek and George Weth of the U.S. Department of Energy, Pic Turner of the U.S. Environmental Protection Agency, Henry Phillips of Fluidized Combustion Co., Irving Johnson of Argonne National Laboratory, Joseph Yerushalmi of the City University of New York, Thomas Wheelock of Iowa State University, and Rene Bertrand and Ronald Hoke of Exxon. The assistance of T. C. Gaydos and D. T. Ferrughelli with the experimental arrangements is also gratefully acknowledged. Finally, we wish to thank the many people from industry who supplied raw materials and made suggestions for preparing sorbents. L i t e r a t u r e Cited (1) Hoke, R. C., Bertrand, R. R., Nutkis, M. S., Ruth, L. A., Gregory, M. W., Magee, E. M., Loughnane, M. D., Madon, R. J.,Garabrant, A. R., Ernst, M., “Miniplant Studies of Pressurized Fluidized-Bed Coal Combustion: 3rd Annual Reaort”. EPA-600/7-78-069. Ami1 1978, pp 46-51. (2) Lowell, P. S., Parsons, T. B., “Identification of Regenerable Metal Oxide SO? Sorbents for Fluidized-Bed Coal Combustion”. EPA 650/2-75-665, July 1975. (31 Cusumano, J. A,. Levy. R. B.. “Evaluation of Reactive Solids for SO;! Removal During Fluidized-Bed Coal Combustion”, final report, E P R I TPS76-603, Oct 1975. 14) Newbv. R. A,. Keairns. D. L.. “Alternatives to Calcium-Based SO7 ‘Sorbents’ for Fluidized Bed Combustion: Conceptual Evaluation’? EPA-600/7-78-005.J a n 1978. (5) Snyder, R. B., Wilson, W. I., Johnson, I., Jonke, A. A.,“Synthetic Sorbents for Removal of Sulfur Dioxide in Fluidized Bed Coal Combustors”, ANL/CEN/FE-77-1, June 1977. (6) Pearce, T. A,, Conner, J. C., “Sulfur Dioxide Removal from Fluidized Bed Combustors”, report prepared by Dow Chemical Co. (Texas Division) under ANL Contract No. 31-109-38-3268, Aug 1975-0ct 1976. (7) Ruth, L. A,, Squires, A. M., Graff, R. A,, Enuiron. Sci. Technol., 6, 1009-14 (1972). (8) Hoke, R. C., Bertrand, R. R., Nutkis, M. S., Kinzler, D. D., Ruth, L. A,, Gregory, M. W., “Studies of the Pressurized Fluidized-Bed Coal Combustion Process”, Annual Report, EPA-600/7-76-011, pp 80-13 1. (9) Hoke, R. C., Ruth, L. A., Shaw, H., Combustion, 46,6-12 (Jan 1975). 110) Voorhoeve. R. J. H.. Johnson. D. W.. Jr.. Remeika. J. P.. Gallagher, P. K., Science, 195,827-33 (1977). (11) Yang, R. T., Shen, M., Steinberg, M., Enuiron. Sci. Technol., 12,915-8 (1978).
Received for revieus October 30, 1978. Accepted February 12, 1979. This Lcork was supported by Grant No. AER75-16194 from the R A N N Program of the National Science Foundation.
Particulate Zinc, Cadmium, Lead, and Copper in the Surface Microlayer of Southern Lake Michigan Alan W. Elzerman” Department of Environmental Systems Engineering, Clemson University, Clemson, S.C. 29631
David E. Armstrong and Anders W. Andren Water Chemistry Program, University of Wisconsin-Madison,
The surface microlayer (SM) of natural waters has received increased attention in relation to fluxes and accumulation of certain organic and inorganic materials at the air/water interface (1-3). Trace metals have been of interest due to their tendency for wide distribution and possible synergistic and toxic effects. Results from previous investigations indicate that particulate matter is important to surface accumulation of trace metals ( 4 , 5 ) . A t the same time, the significance of 720
Environmental Science & Technology
Madison, Wis. 53706
inputs of atmospheric particulate matter to natural waters has been recognized ( 2 ) ,especially for waters such as southern Lake Michigan which are located near industrial areas (6, 7 ) . Also, bubble flotation of trace metals followed by ejection into the atmosphere upon bubble breaking has been established as a mechanism affecting trace metal distribution (2,8). Despite recent advances, our present knowledge of surface enrichment mechanisms and the sources and nature of par0013-936X/79/0913-0720$01.00/0 @ 1979 American Chemical Society
Particulate matter of diverse composition and morphology commonly occurred in higher concentrations in screen-collected surface microlayer (SM) samples than in the bulk water of southern Lake Michigan. Filtration experiments (0.4 pm Nuclepore) indicated that enrichment of Zn, Cd, Pb, and Cu in the S M occurred substantially in the particulate phase, especially when surface enrichments were large. Surface en-
richment of total particulate matter and Zn, Cd, Pb, and Cu was related to the presence of surface accumulated material and film pressure. Chemical analysis indicated that a significant portion of the particulate Zn, Cd, Pb, and Cu in the SM may be atmospherically transported anthropogenic particulate matter.
ticulate matter in the SM is incomplete. Information on freshwater systems has been particularly sparse. Also, the importance of SM processes to the fluxes and interactions of particulate matter at the aidwater interface has received little attention, especially concerning the influx of atmospheric particulate matter. Since trace constituents may leach from or be adsorbed by particulate matter, and the biological, chemical, and physical interactions as well as the residence time of particulate matter in the SM are a function of the source and form of the particulate matter, investigation of particulate matter in the SM may contribute to an understanding of the distribution and fate of trace materials in aquatic systems. Surface accumulation of Zn, Cd, Pb, and Cu is common in freshwater lakes.,including Lake Michigan (9).Local sources, weather conditio'ns, and the presence of surface accumulated material (SAM) that decreases the surface tension of the water surface have been found to affect the degree of surface enrichment (9). In. the investigation reported here, filtration techniques were used to investigate the enrichment of particulate matter in the SM of southern Lake Michigan relative to the bulk water in order to evaluate the contribution of particulate matter to the surface enrichment of Zn, Cd, Pb, and Cu. Additionally, the relationship of SAM to surface enrichment of particulate matter was studied. Information on the significance of atmospheric sources of particulate matter to the SM was also obtained by scanning electron microscopy combined with X-ray fluorescence microprobe (SEM-XRF) analysis.
thawing, and drying for 3 days in a desiccator. This measurement was used as an approximation of the concentration of total particulate matter. Trace Metal Concentrations. Zn, Cd, Pb, and Cu concentrations were obtained for filtered and unfiltered samples by differential pulse anodic stripping voltammetry (DPASV) following UV irradiation to destroy organic matter (9).Analytical precision for the DPASV method, based on averaging the results of triplicate analyses, was about f 5 % for each metal. Total metal concentrations were obtained from the unfiltered samples. Dissolved metal concentrations were operationally defined as the concentrations in the filtered samples, and particulate metal concentrations as the difference between total and dissolved metal concentrations. SEM-XRF. Sections (about 25 mm2) of separate filters were mounted on carbon stubs and vacuum coated with carbon. A JEOL Model 50A SEM with a tungsten filament and equipped with a Princeton Gamma-Tech GE (Li) detector for energy-dispersive XRF microprobe analysis and a Northern/Tracor Model NS-880 computer data system was used. Definitions. Fractionation ratio (FR) = X S / X B .Surface where d = thickness of SM Samexcess (SE) = d(X,- XB), pled by screen (0.3 mm), XS= concentration in screen sample, and X B = concentration in bulk water sample. The measured FR is dependent on the thickness sampled by the screen relative to the thickness of the enriched layer, and the actual FR may be up to lo4 or more greater than the measured FR (9, 24).
Experimental
Sampling Procedures. Samples were collected during the summer of 1975 in the southern basin of Lake Michigan near Milwaukee, Chicago, Gary, Michigan City, Benton Harbor, Grand Haven, and the southern central basin. All samples were collected from an inflatable launch away from the main vessel. Bulk water samples were collected by opening an acid-leached polyethylene bottle about 30 cm below the surface, avoiding contamination from the launch. Surface microlayer samples were collected by touching an all-plastic screen to the surface and draining the sample into an acidleached polyethylene bottle as described by Garrett ( I O ) . The screen collected a sample from a layer about 0.3 mm thick. Individual screen samples were combined until a sample of 1 to 2 L was obtained at each site. In situ film pressure measurements were made using the oil drop method of Adam (11). The minimum measurable film pressure was about 1dyn/cm. Although SAM frequently may be present at lower film pressures (9, I 2 ) , a film pressure of 1 dyn/cm or greater was used here to confirm the presence of SAM. Filtration. Samples were filtered through 47 mm diameter Nuclepore filters (0.4 pm) as soon as possible (usually within a few hours of collection). An all-plastic, acid-leached, and covered apparatus was used for vacuum filtration. Filters were immediately frozen, and portions of the filtrate and unfiltered water were placed in separate acid-leached polyethylene bottles, acidified with concentrated H N 0 3 (3 mL/L), and frozen. Particulate Matter. Particulate matter was operationally defined as the weight gain by the filter after filtration, freezing,
Results Particulate Matter. Large quantities of particulate matter of diverse nature were common in the SM, especially when SAM was present. Particulate matter present on apparently SAM-free surfaces also may be important to surface enrichment of trace metals (9). Total particulate matter concentrations in southern Lake Michigan bulk water were typically in the range of 1to 4 mg/L for lake samples and 4 to 15 mg/L for samples in and near rivers. Total particulate matter measurements for screen samples collected indicate that the S M was frequently enriched in particulate matter relative to the bulk water, especially when detectable SAM was present (Table I). Surface excesses for particulate matter were commonly greater than 1 mg/m2 and values as high as 9 mg/m2 were found near the highly industrialized southern tip of the lake. Particles larger than about 0.5 mm diameter were rarely observed on the filters, probably because the number concentration of large particles was low and because the screen sampling technique and the filtration procedure discriminated against larger particles. Within the group of samples with detectable SAM, the degree of surface enrichment of total particulate matter appeared to be related to film pressure. Correlation coefficients between film pressure and the particulate matter fractionation ratio and between film pressure and the particulate matter surface excess were +0.5 and +0.8, respectively. Similar relationships between film pressure and FR and S E values for total Zn, Cd, Pb, and Cu have been observed ( 9 ) , and are probably related to increases in film pressure and surface excess values resulting from lateral compression of surface microlayers. Volume 13, Number 6, June 1979
721
Table I. Surface Enrichment of Total Suspended Particulate Matter in Lake Michigan Samples
parameter
% of samples with surface
33
enrichment av fractionation ratioC av surface excess, mg/m2
-0.12 (0.4)"
0.8 (0.5)"
86 4.3 (2.4)d 3.5 (2.7)"
*Film pressure 1 dynlcm. Fourteen observations. CFractionation ratio = Concentration in screen Sampielconcentration in bulk water sample. dThe Standard deviation of the observations is presented in parentheses.
Surface enrichment of total particulate matter and total Zn, Cd, Ph, and Cu also seemed to he related. When one or two divergent values were omitted, correlation coefficients for trace metal and total particulate matter fractionation ratios and for trace metal and total particulate matter surface excess values were between +0.5 and +0.9 for Zn, Cd, Ph, and Cu. More than 25% of the trace metal fractionation ratio values were within 10% of the total particulate matter fractionation ratio in the same sample. Surface Enrichment of Particulate Zn,Cd, Pb, and Cu. Results of particulate and dissolved Zn, Cd, Pb, and Cu measurements are presented in Table 11. Although the frequency of surface enrichment in the dissolved phase was similar to the frequency of enrichment in the particulate phase (about 90% of all samples collected), the extent of particulate phase surface enrichment was usually greater than dissolved phase surface enrichment (Table 11). SEM-XRF. Scanning electron microscopy combined with X-ray fluorescence microprobe analysis (SEM-XRF) provided a means for investigating the chemical and physical nature of submicroscopic particulate matter. Because the particulate matter was diverse in morphology and composition, the XRF data were interpreted only semiquantitatively. On the basis of visual interpretation, SM samples with SAM present appeared to contain a greater diversity and quantity of particulate matter than either hulk water samples or screen samples from apparently SAM-free surfaces. Fly ash-like spheres (13) and other materials of apparent anthropogenic origin were observed in SM samples from all locations. Examples of a fly ash-like particle and other particulate matter from the same sample are shown in Figure 1.
I
2
3
4
5
6
7
8
9
KeV
Figure 1. A scanning electron micrograph of surface microlayer particulate matter and an X-ray fluorescence spectrum of a fly ash-like sphere. The SEM micrograph is of particulate matter on a Nuclepore filter from a sample collected near Gary, Ind. The XRF spectrum was taken on the small sphere just to the left of the arrow. The bar below the micrograph represents a length of about 10 wm The spherical shape of the particle suggests a high-temperature source, and the composition from the XRF spectrum is similar to spheres found in fly ash ( 1 4 ) . Similar spheres ohserved in SM samples ranged from I0 wm in size. The greatest number of particles was Idynlcm pWameIW
av FRa dissolveda particulate av SEe(~glm2) dissolvede particulate av particulate SE as a % of total SE"
1.6 (0.7-3. 1) 7.4 (1.1-32)
1.2 (-1.0-3.3) 4.8 (0.06-16) 68 (18-100) 75
2.6 (0.5-5.0)
1.6 (0.9-3.: 11 (1.6-31
0.04 (-0.012-0.027) 0.05 (-0.15-0.12)
0.15 (-0.12~-o.., 99) 2.6 (0.96-1 9)
50 (0-97)
92 (67-10( 1)
71 (0-100)
100
75
1.8
(0.5-5.5)
samples with particulate SE > dissolved SE as % of 50 total of samples * Based on 14 ObServationS for Zn. 12 for Cd, 17 for Pb, and 9 lor Cu. Based on 1 1 ooservar~ons,or 310 excluded from average. "Average Of calcuiations lor individual samples of % = [(total SE samples is presented in parentheses.
722
Environmental Science & Technology
L",
,10r
GO, 1 0 TOT m, and
1.5 (0.9-2.4) 2.6 (0.5-5.4)
4 lor Cu.
0.25
(-n ns-n-.--, ~fii \
1.0 (-0.3-2.1)
' values 01be ana
- dissolved SE)/total SEI 100. e The range of values for the observed
Fix",-
_.
Ir
_.
__..__......i.
P...
collected in SM 1 km off Kenosha, Wis.
Figure 2. Micrographs of selected surface microlayer particulate matter. Upper left, width of fieid 45 pm. XRF of rough sphere in middle essentially showed only Fe. Sample taken 5 km off Gary, Ind. Upper right, width of field is 900 pm. XRF spectrum of complete area showed large DeakS for Si and AI. smaller DeakS for K. Ca. and Fe. and trace Deaks bf S and Ti. Small spheres and large sphere to right-center had XRF spectra similar to Figure 1. Sample taken 1.5 km off Michigan City, Ind., downwind of power plant. Lower left, width of fieid is 90 pm. XRF of whole particle or small parts showed Similar composition of primarily Ca and Fe with smaller peaks for Mg, AI, Si, and Mn. Sample taken 3 k m off Calumet Harbor. Lower right, width of field is 30 urn. XRF of e ntire particle showed large Si arid S peaks with lesser peaks for K. C:a, AI, Fe, and V. Same sample23s lower left Dicture
... .
foam lines (Y, 15) likely accounts tor the greater abundance of fly ash-like particles in the foam-line sample. Figure 3 shows the XRF spectra and major element peak identifications for a roughly spherical 1.5pm diameter particle collected from a SM 1 km off Kenosha, Wis. Similar XRF spectra, with the addition of Br, were obtained for some particles filtered from the exhaust of an automobile burning leaded gas. The Br peaks were not noticeable in the automobile exhaust particles after 2 L of lake water was drawn through the filter. Elements generally observed in highest abundance in the XRF spectra were Si, Al, Ca, K, and Fe. Lesser amounts or fewer cases of Mg, S, Ti, Mn, Cr, V, C1, P, Pb, and other elements were observed. Cu and Zn peaks were observed in many XRF spectra, but the construction of the SEM sample holder sometimes produced nonsample Cu and Zn signals. Diatom frustrules (showing Si as the only major element in their XRF spectra) and other remains of organisms were common in the SM samples. Particles similar in morphology and composition to clay materials (16)were also common. Examination of atmospheric particulate matter collected on filters during the sampling cruises confirmed the presence in the air of fly asblike particles and other particulate matter of apparent anthropogenic origin similar to the particles observed in the SM samples. The similarities between air and SM particulate matter suggested that the atmosphere is an important source for particulate matter in the SM. Discussion Interpretation of Particulate T r a c e Metal Data. Determination of particulate Zn, Cd, Pb, and Cu at low concentrations in lake waters poses several analytical problems. v:l*"-A:"..
I Il~'alll"II
:..^"^^^^^ +l.^ LLLb.ITauGu lllr " p p " L b u L . , b J
e..-
,.,."tn.":""i:~" I V L C"I.YOLII,.I.OLY,",I.
n , "c-
casionally, samples showed dissolved concentrations greater than total concentrations and were omitted. Determination of particulate concentrations by difference lacks the precision
of a direct technique, and the efficiency of the screen in collecting particulate matter is uncertain ( I 7,181. Furthermore, during filtration, colloidal material (1 dyn/cm (50(2-25) 1800)b
590 (30-1900)
(50-640)
bulk water
40
30
150
5
190
a Calculated as [total metal - dissolved metal &g/L)]/particuiate matter (g/L). Range of values.
Table IV. Average Metal Ratios In Particulate Matter sample
Zn/Pb
Cd/Pb
aerosol a
0.55
0.009
60
SM with film pressure >1 dyn/cm
1.2
0.018
65
bulk water
3.8
0.12
30
a
Table V. Estimated Atmospheric Contributions to Particulate Matter in Example Surface Microlayer Samples a
ZnlCd
sample locallon
samples with film pressure >1 dyn/cm mid-lake near Chicago near Gary range of 11 samples samples with film pressure < 1 dyn/cm near-shore, away from rivers near Chicago near Gary
Oh of total part. matter derived from almos
3 20 20 1-20
0.4 0.9
6
aSee text for method of calculation.
Reference 31.
have found evidence for differences in either surface enrichment mechanisms or sources of trace metals for SM samples from areas with varying characteristics. Recreational boating on Lake Michigan, for example, could be a significant, but variable, source of SM materials. The Importance of Atmospheric Particulate Matter. Some insight into the source of SM particulate matter may be gained from its composition. Concentrations of Zn, Cd, Pb, and Cu in the particulate matter of SM and bulk water samples from southern Lake Michigan are compared in Table 111. The bulk water values agree favorably with the P b and Cu values reported by Leland and Shimp (27) and the Zn, Cd, Pb, and Cu values for the Grand River mouth reported by McMillan and Thomas (28). In some samples collected in or near harbors and river mouths, higher concentrations of trace metals in the suspended solids were observed, probably due to industrial sources. However, the concentrations of Zn, Cd, Pb, and Cu in the particulate matter from bulk water samples were generally consistent with concentrations reported for soils (29) and Lake Michigan sediments, suspended particulate matter, and plankton (27, 30). In contrast, on either an individual site or an average basis (Table 111),Zn, Cd, Pb, and Cu concentrations in SM particulate matter were significantly higher than concentrations found in bulk water particulate matter, soils, sediments, or plankton. The composition of atmospheric particulate matter, especially in an urban area, may be quite different from average soil and may be a source of particulate matter with high concentrations of trace metals. Gatz (31) adopted a model Chicago area atmospheric particulate aerosol containing about 5000 pg/g Zn, 100 pg/g Cd, 11 000 pg/g Pb, and 1000 pglg Cu. The southern basin of Lake Michigan is surrounded by urban and industrial areas, so the trace metal concentrations in Chicago area atmospheric particulate matter may serve as a reasonable first approximation for calculations. Based on the above trace metal concentrations, atmospherically derived particulate matter could comprise a small portion of the total particulate matter in the SM while accounting for a major portion of the particulate Zn, Cd, Pb, and Cu. A possible relationship between atmospheric particulate matter and SM particulate matter is shown by the element ratios presented in Table IV. Although less conclusive for Cu, the average concentration ratios in the SM particulate matter were intermediate between the bulk water and model aerosol ratios, consistent with the particulate matter in the SM being a mixture of bulk water and atmospheric particulate matter. 724
Environmental Science & Technology
Further insight may be gained by observing the more than two orders of magnitude greater P b concentration in the model atmospheric particulate matter compared to the bulk water particulate matter and using P b as a tracer of atmospheric particulate matter. As a first approximation, the atmosphere may be assumed to be the source of all particulate P b in the SM and no significant loss to or gain from solution occurs after impaction. With these assumptions, the fraction of the total particulate matter in the SM derived from the atmosphere is the ratio of the concentration of P b in the SM particulate matter to the concentration of P b in the aerosol (11000 pg/g in model aerosol). Caution in making interpretations from the available data must be exercised, but an indication of the importance of atmospheric inputs may be gained using this approach. Calculated estimates of the atmospheric contribution to the total particulate matter in the SM samples ranged from 1 to 2096, with an indication of higher fractions of atmospherically derived particulate matter in samples near industrialized areas, and with SAM present as compared to samples without SAM present (Table V). Similar calculations for the estimated atmospheric contribution to the particulate matter in the bulk water samples yielded results of less than 1%in all cases. Using the above calculated values, the model aerosol (31), and similar assumptions, an estimate of the atmospheric contribution to the Zn, Cd, and Cu in the SM may also be obtained. Calculations for lake SM samples with SAM present indicated that atmospheric particulate matter accounted for 30-93% of the particulate Zn, 25-68% of the particulate Cd, and 19-100% of the particulate Cu. Samples collected near the highly industrialized southern tip of the lake generally showed the highest atmospheric contributions. River and harbor SM samples tended to show lower percent atmospheric contributions than lake SM samples. This trend may be related to the observed lower fractionation ratios for total Zn, Cd, Pb, and Cu in river and harbor samples compared to lake samples, ( 9 ) , and the importance of other sources in the river and harbor areas. The effects of SAM on the fluxes and interactions of particulate matter in the SM are not well known, but the apparent abundance of atmospherically derived particulate matter in the SM when the film pressure was greater than 1dyn/cm may provide some insight. Mechanisms which could promote high concentrations of particulate matter in the SM might involve (a) a greater flux of particulate matter to the SM, (b) production of particulate matter in the SM, and (c) a longer residence time of particulate matter in the SM. Variations in conditions at the air/water interface may af-
fect the rate or efficiency of capture of atmospheric particulate matter or bulk water particulate matter. Paterson and Spillane (32)report that SAM can reduce the amount of material ejected into the atmosphere by single bubbles, which might lead to accumulation in the SM, but Garrett (33)reports that SAM promotes ejection from bubbles in foams. Films of SAM have been observed to move from tributary sources out over open water, presumably causing horizontal surface fluxes (9). The effects of SAM on the solubility of atmospheric particulate matter and associated trace metals may also be significant but currently are not sufficiently understood. Similarly, the importance of encapsulated fly-ash spheres (13) and other particles which might have a density less than the lake water and the significance of the surface predominance of the trace metals in fly ash ( 3 4 ) to SM processes have not been evaluated. Residence times in the SM may be important in controlling the interactions of particulate matter in various chemical, biological, and physical processes. Hoffman et al. (26) estimated the residence time of particulate matter in the SM of the Atlantic Ocean to be only a few seconds and to be independent of atmospheric deposition rate. However, the presence of SAM may be related to particle residence time in the SM. For example, if the P b in an area of the Lake Michigan S M results from a direct deposition rate of 1.5 yg of P b ern+ yr-I ( 3 5 ) , the microlayer is 0.3 mm thick, and the P b concentration in the SM is 3 yg/L, the residence time would be about 30 min. Using this calculation technique, samples with SAM present, which showed greater surface enrichment, would also show longer particle residence times. Because the mass of particulate matter or of an element in the SM is small compared to the total mass in the lake water, the significance of the SM relates mainly to its effects on fluxes and interactions of material at the air/water interface. This investigation indicates the residence time of some particulate matter in the SM could be sufficiently long to allow significant interactions. Conclusions
Particulate matter of diverse nature commonly occurs in the SM of southern Lake Michigan in concentrations greater than in the bulk water. High concentrations of particulate matter in the SM seem to be related to the presence of SAM and to film pressure. Surface enrichment of Zn, Cd, Cu, and, especially, P b in the SM is highly variable but seems to occur substantially in the particulate phase. Although surface enrichment occurs in the less than 0.4-ym fraction, particulate phase surface enrichment tends to be greater and more variable and usually constitutes the major portion in samples exhibiting large surface enrichments. Surface enrichment of total suspended solids and that of total Zn, Cd, Pb, and Cu in the S M seem to ‘be related. A significant portion of the particulate Zn, Cd, Pb, and Cu in the southern Lake Michigan SM may be atmospherically transported anthropogenic particulate matter. Results suggest that SAM may influence the flux or residence time of particulate matter a t the air/water interface. Consequently, SM processes may be significant in air/water transfer of particulate materials and postdepositional chemical, physical, and biological interactions. Acknowledgment
We wish to thank Captain Dave Melvin of the RAI Aquarius and Paul Choitz for their assistance in collection and analysis of the samples.
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Receiced for reuieu’ October 10, 1978. Accepted February 1.5, 1979. This incestigation was supported in part by the National Oceanic and Atmospheric Administration Office of Sea Grant, C.S. D e partment of Commerce, through an institutional grant to the L’niuersity of Wisconsin. T h e support of the Department of Cic,il and Enuironmental Engineering and College of Engineering, L‘nicersity o f W i s c o n s i n ~ M a d i s o ni ,s acknowledged. Volume 13, Number 6, June 1979
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