Trace Metal Accumulation in Surface Microlayers

J. Great Lakes Res. 8(2): 282-287 Internat. Assoc. Great Lakes Res., 1982

TRACE METAL ACCUMULATION IN SURFACE MICROLAYERS

David E. Armstrong Water Chemistry Program and Civil and Environmental Engineering University of Wisconsin, Madison, Wisconsin 53706 Alan W. Elzerman Environmental Systems Engineering Clemson University Clemson, South Carolina 29631

ABSTRACT. The accumulation of trace metals in surface microlayers is controlled by severalfactors, including microlayer composition, the physical and chemical form of the metal, the source of the metal, the processes controlling metal transport to the microlayer, the longevity ofthe microlayer, and the residence time of the metal in the microlayer. Recent investigations provide insight into trace metal-surface microlayer interactions. Evidencefor the importance ofvariousfactors and mechanisms in controlling accumulation is reviewed, and gaps in this information are discussed.

INTRODUCTION Trace metals such as Cu, Zn, Pb, and Cd accumulate to high concentrations in surface microlayers of marine and fresh water systems relative to the concentrations in the bulk water. Although the quantities of trace metals present in surface microlayers are insignificant relative to the amounts present within a few cm of the underlying water, the high concentrations are of interest in relation to possible biological and ecological significance. Furthermore, interactions within the microlayer may alter the chemical properties of trace metals and influence their geochemical and biochemical cycles. The purpose of this paper is to reveiw some of the evidence concerning the factors controlling trace metal accumulation in surface microlayers. The factors considered include the properties and composition of the microlayer, the form of the trace metal, the source of the trace metal, and processes controlling the transport of metals to and through the microlayer. Enrichment in the surface microlayer is frequently represented as the fractionation ratio (FR) or the surface excess (SE) defined as follows:

FR = [X]SM

[X]BW

(1)

where [X] is the concentration of element X, SM is the surface microlayer, and BW is the bulk water or underlying surface water. SE = d([X]SM - [X]BW)

(2)

where d is the thickness of the surface microlayer sample. When the screen sampling technique is used, d is about 300 J,Lm while the actual thickness of the microlayer is about 0.03 to 0.03 J,Lm (Elzerman and Armstrong 1979). Consequently, actual SM concentrations and FR values are underestimated by about 10+3 to 10+4 when the screen sampler is used. Data on FR and SE values from various sources are cited in this paper, usually as mean values for the observations made. The reader should recognize that trace metal concentrations and associated SE and FR values tend to vary over a relatively wide range, and this range is not reflected in the mean.

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TRACE METAL ACCUMULATION IN SURFACE MICROLAYERS SURFACE MICROLAYER COMPOSITION The accumulation of trace metals at the air-water interface has been related to the presence of surface active organic matter or "slicks" (Elzerman 1981, Elzerman and Armstrong 1979, Langevin 1978, Piotrowicz et al. 1972). For example, a greater frequency and extent of surface enrichment has been observed when surface slicks are present (Table 1). Piotrowicz et al. (1972) used visual evidence (capillary wave damping) to determine the presence of surface slicks, while Elzerman and Armstrong (1979) also used film pressure measurements. A film pressure> 1 dyne cm- I was taken as evidence of surface accumulated material. The occurrence of some surface enrichment when surface slicks were not evident is attributed either to the presence of surfactants of insufficient surface activity to cause capillary wave damping or to surface accumulated particulate matter. TABLE 1. Relationship between surface conditions and surface enrichment of trace metals. Surface Condition

Surface Obs. Enrichment

FR

SE J.Lg m- 2

REF

% No slick No slick [but surface accumulated matter present] Slick

7 12

60 6

1.35 1.43

0.14

(A) (B)

7

97

2.9

0.63

(A)

46 20

94 47

3.7 3.4

0.41

(A) (B)

(A) Data are averages for Zn, Cd, Pb, and Cu from Elzerman and Armstrong (1979). (B) Data are averages for Cu, Fe, Ni, and Pb from Piotrowicz et al. (1972).

little direct evidence exists on properties contolling enrichment, other than film pressure. FORM OF TRACE METAL Trace metal enrichment frequently occurs in both the dissolved and particulate phases of surface accumulated material (Piotrowicz et al. 1972, Duce et al. 1972, Elzerman 1981). Dissolved phase enrichment is assumed to result primarily from trace metal complexation by dissolved organic material in the surface microlayer (Hunter and Liss 1981). Perhaps the most direct evidence for complexation is based on the extractability of trace metals by an organic solvent immiscible with water. For example, Piotrowicz et al. (1972) and Owen and Meyers (1978) measured trace metal concentrations and enrichment in the CHC13_ extractable fraction (Table 2). Enrichment in the CHC13_ extract could result from entrainment of particulate matter in the emulsion formed during extraction, but indicates the presence of a non-ionic, presumably organically-complexed form. As the surface microlayer may contain glycoproteins, proteoglycans (Baier et al. 1974), and other ionic or high molecular weight materials not readily extracted by CHC13_, the CH3C13_-extractable metal fraction is likely an underestimate of the amount retained in the microlayer by complexation with dissolved organic material. TABLE 2. Trace metal enrichment in the "dissolved organic" fraction of surface microlayer samples. Metal

Obs.

FR

eu

5 7 5 7 5 7 7 7

10.1 4.7 3.4 1.8 4.6 1.0 1.8 1.15

Ni

Even though trace metal accumulation has been related to the presence of surface accumulated organic matter, relatively little is known about the relationship between chemical composition of the microlayer and trace metal accumulation. Fatty acids, esters, and alcohols and proteinaceous materials have been identified in surface microlayers (Baier et al. 1974) and could provide sites for trace metal complexation. Adsorption of trace metals on surface-accumulated particulate material likely occurs. Incorporation of metal-containing particles into the surface microlayer has also been demonstrated (Elzerman et al. 1979). Although evidence exists for enrichment in both the dissolved and particulate phases of surface-accumulated material,

Pb Zn Fe Mn

Location and Reference (A)

(B) (A)

(B) (A)

(B) (B) (B)

(A) Data from Piotrowicz et al. (1972) for Narragansett Bay samples with visible slick present. (B) Data from Owen and Meyers (1978) for Lake Michigan-St. Joseph River mouth samples.

Complexation of trace metals by dissolved organic material in the surface microlayer is likely influenced by several factors, including the nature and concentration of functional groups such as -COOH, -OH, and -NH2 in the dissolved organic

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material, the concentration of competing metal ions (e.g., Ca++) and inorganic ligands such as Cland C03= (Hunter and Liss 1981), and the rate of transport to the microlayer relative to residence time in the microlayer. Possibly, some of the enrichment of dissolved trace metals occurs through partial dissolution of particulate matter within the microlayer. Enrichment of particulate trace metals may be more important than dissolved trace metal enrichment in the surface microlayer (Elzerman 1981). For Zn, Pb, and Cu in Lake Michigan surface microlayer samples, a higher frequency of enrichment occurred in the particulate than the dissolved fraction, and the particulate surface excess accounted for more than 50% of the total surface excess (Elzerman et al. 1979). In Narragansett Bay samples, Piotrowicz et al. (1972) observed about the same frequency of enrichment in the particualte and CHCI3_-extractable fractions. However, the degree of surface enrichment (FR) tended to be higher in the particulate fraction. Examples of average surface enrichment values observed in the particulate fraction in three separate investigations are given in Table 3. The differences for a given metal reflect a combination of differences in surface microlayer characteristics and sampling techniques as well as differences in location. The high FR values for Cu, Ni, and Fe in the Narragansett Bay samples are strongly influenced by one sample described as a "heavy" slick. Piotrowicz et al. (1972) and Elzerman et al. (1979) used the screen sampling technique, while Mackin TABLE 3. Trace metal enrichment in the particulate fraction of surface microlayer samples. Metal Cu Ni Pb Cd Zn Fe (A)

Obs.

FR

5

II

9

2.6 2.7 12.7 7.8 10.0 2.6 7.4 8.2 3.5

10 5 5 17 12 14 5 10

Location and Reference (A)

(B) (C) (A) (A)

(B) (B) (B) (A) (C)

Data from Piotrowicz et al. (1972) for Narragansett Bay samples with visible slick present. (B) Data from Elzerman et al. (1979) for Lake Michigan samples with film pressure I dyne em-I. (C) Data from Mackin et al. (1980) for Lake Michigan samples.

et al. (1980) used the glass plate technique. Furthermore, Mackin et al. did not report the microlayer characteristics at their sampling sites. Nevertheless, the data in Table 3 clearly illustrate the importance of particulate matter in trace metal surface enrichment. Data on wind-generated lake foam samples provide dramatic evidence of the enrichment of both dissolved and particulate trace and transition metals in surface accumulated material (Eisenreich et al. 1978). Enrichment in the foam is much greater than in surface microlayer samples (Table 4). This reflects the dilution by bulk lake water inherent in surface microlayer samples. The data are averages of 30 observations for each metal. Fe and Cu tended to be enriched mainly in the dissolved phase, while Pb enrichment occurred mostly in the particulate phase. Zn and Cd showed appreciable enrichment in both phases. These differences among the metals suggest differences in sources and/ or foams or in mechanisms of interaction within the surface microlayer. TABLE 4. Trace metal enrichment in the "dissolved" and ''Particulate'' fractions of destabilized lake foams. Enrichment (FR) Metal

Dissolved

Particulate

Zn Cd Pb Cu Fe Na

409 301 199 453 963

236 800 1580 80 230

1.6

o

Data from Eisenreich et al. (1978).

Comparison with Na used as a conservative tracer indicates that little "inorganic" phase enrichment occurred in the foam and that dissolved phase enrichment was due to complexation with organic matter. Gel permeation chromatography confirmed the association of dissolved Fe with high molecular weight organic matter (Eisenreich et al. 1978). Some of the particulate matter may have been entrained during foam generation. However, the differences in FR values for particulate metals indicate that either selective accumulation of certain particulate phases from the lake water occurs, or that the accumulated particulate matter is possibly from another source, presumably atmospheric.

TRACE METAL ACCUMULATION IN SURFACE MICROLAYERS TRACE METAL SOURCE Sampling location can have a large influence on trace metal concentrations and enrichment in the surface microlayer, implying that local conditions and sources play an important role in accumulation. Piotrowicz et al. (1972) observed the highest trace metal concentrations in New York Bight samples collected at near-shore sites. Similarly, Elzerman and Armstrong (1979) observed a tendency for increasing surface enrichment (FR) and surface excess (SE) in comparing mid-lake to near-shore or mixing zone sampling sites (Table 5). The higher trace metal concentrations in near-shore samples are not entirely reflected in FR and SE values because bulk water concentration enters into the calculation of both parameters. The relatively low values in river and harbor samples may partially reflect relatively high bulk water concentrations (Mackin et al. 1980, Elzerman 1981).

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are relatively high. The observed concentrations of Zn, Pb, and Cu were appreciably higher in surface microlayer particulate matter than in particulate matter from the bulk lake water, soils, sediments, or plankton, indicating the surface microlayer particulate matter was partly of atmospheric origin (Table 6). Assuming all of the surface microlayer particulate Pb was atmospheric, the contribution of atmospheric particulate matter to surface microlayer particulate matter was estimated, based on a model Chicago area aerosol (Gatz 1975). These calculations indicated that up to 20% of the surface microlayer particulate matter was atmospheric and that relatively high proportions of the particulate Zn, Cd, and Cu were of atmospheric origin. While particulate matter containing high trace element concentrations could be selectively enriched from the bulk particulate matter in the underlying water, it seems more likely that atmospheric particulate matter is a significant source of particulate trace metals in Lake Michigan surface microlayers.

TABLE 5. Relation of trace metal surface enrichment to sample location in Lake Michigan. Location

N

FR

SE JJ.g m- 2

Rivers and harbors Mixing zones Nearshore Midlake

19 14 8 5

2.5 4.4 3.9 2.4

1.5 1.4 1.0 0.56

Data are averages for Zn, Cd, Pb, and Cu from Elzerman and Armstrong (1979).

Evidence exists for both sub-surface and atmospheric contributions to the accumulation of trace metals in surface microlayers (Elzerman 1981). Enrichment of 210po in the sea-surface microlayer was attributed primarily to concentration from the bulk water while 210Pb appeared to have both bulk water and atmospheric sources (Bacon and Elzerman 1980). Based on factor analysis and longitudinal distribution, Mackin et al. (1980) concluded in situ processes controlled the enrichment of particulate Fe and Mn in Lake Michigan surface microlayers. However, the particualte trace metals in. ~ke Michigan surface microlayers apparently ongmate partly from atmospheric sources (Elzerman et al. 1979). Direct examination of surfaceaccumulated material using scanning electron microscopy and X-ray fluorescence showed the presence of fly-ash-like particles in Lake Michigan surface microlayer samples. Concentrations of trace elements in atmospheric particulate matter

TABLE 6. Calculated atmospheric contribution to suspended particulate matter and particulate trace metal content of Lake Michigan surface microlayers. Source

Zn

Cd

Pb

Cu

Concentration in particulate matter (JJ.g/ g) Surface microlayer Bulk water Model aerosolB

720 150 5000

11 5 100

590 40 11 ,000

190 30 1000

Calculated atmospheric contribution (%)

30-93

25-68

100

19-100

Results: SPMatmos in SM SPM

= 1-20%

= [Pb]aerosol

SPMtotal

SPMatmos in BW SPM

= <1%

AFrom Elzerman et al. (1979) BGatz (1975).

TRANSPORT PROCESSES Surface microlayers are transitory in nature. They are periodically disrupted by wave action, and components are continuously metabolized and regenerated by biological processes. Consequently, accumualtion of trace metals cannot be viewed as

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ARMSTRONG and ELZERMAN

an equilibrium process. More likely, the amount of accumulation occurring is controlled by the flux to the air-water interface and the "capture" efficiency in the microlayer (i.e., the residence time). The processes controlling transport to the surface microlayer include gravitational settling and impaction for air particulates and diffusion and advection (including bubble flotation) for materials in the underlying water (Elzerman 1981, Wallace and Duce 1975). The residence time of surface microlayers or surface-accumulated material provides some perspective on the possible role of these transport processes. Hoffman et ai. (1974) concluded the residence time of particulate Fe in surface microlayers in the Atlantic Ocean was only a few seconds under wind conditions of 5 to 10 m sec-I. However, it appears the residence time of particulate Pb in Lake Michigan surface microlayers is somewhat longer. Based on a Pb dry depostion rate of about 110 J..I.g m- 2 day-I (Schmidt 1977) and a particulate Pb surface excess of 2.6 J..I.g m- 2 (Elzerman et ai. 1979), the surface microlayer residence time of particulate Pb would be about 0.6 hour. This residence time seems possible for the relatively calm conditions under which surface microlayers are sampled. Of course, residence times may differ for other surface accumulated materials or meteorological conditions. Using Pb as an example, the approximate distance of transport necessary for accumulation from the underlying water can be calculated. Based on lake water Pb concentrations of about 1.3 J..I.g/ L and a total Pb surface excess of 2 J..I.g m-2 (Elzerman and Armstrong 1979), sufficient Pb would be present in the underlying 2 mm of lake water to account for the Pb accumulated in the surface microlayer. Values for other trace metals are similar. Consequently, even slow processes (diffusion) could contribute significantly to transport of materials to the surface microlayer. The similarity of scale between this calculated depth and the depths actually sampled by SM sampling devices also suggests the need for awareness of potential sampling artifacts. CONCLUSIONS In spite of recent information, factors controlling trace metal accumulation in the surface microlayer are not well characterized, particularly in a quantitative sense. Research on these factors is impeded by the transitory and varying nature of surface

microlayers and problems in selective sampling of the surface microlayer. In view of the short residence time of surface microlayers, the kinetics of transport processes and interactions occurring within the microlayer are undoubtedly important. Becaue of the small amounts of trace metals present in the surface microlayer (even with enrichment factors of 104 ), trace metal accumulation may be relatively unimportant unless interactions unique to the surface microlayer are occurring. Cycling of materials in the surface microlayer is more likely to be significant. To establish the importance of trace metal accumulation, information is needed on the role of surface microlayers in the biological and geochemical cycles of trace metals. Potentially important interactions include solubilization of atmospheric particulate matter, packing trace elements into biogenic particles (e.g., fecal pellets), subsequent removal by sedimentation and/ or biomagnification, and ejection of enriched material into the atmosphere. Elucidation of variations in the sources and fates of particulate matter, in contrast to dissolved, in the surface microlayer is especially needed. REFERENCES Bacon, M. P., and Elzerman, A W. 1980. Enrichment of Po-210 and Pb-210 in the sea-surface microlayer. Nature 284:332-334. Baier, R. E., Goupil, D. W., Perlmutter, S., and King, R. 1974. Dominant chemical composition of sea-surface films, natural slicks, and foams. J. Rech. Atmos. 8:571-600. Duce, R. A, Quinn, J. G., Olney, C. E., Piotrowicz, S. R., Ray, B. J., and Wade, T. L. 1972. Enrichment of heavy metals and organic compounds in the surface microlayer of Narragansett Bay, Rhode Island. Science 176:161-163. Eisenreich, S. J., Elzerman, A. W., and Armstrong, D. E. 1978. Enrichment of micronutrients, heavy metals, and chlorinated hydrocarbons in windgenerated lake foam. Environ. Sci. Technol. 12:413417. Elzerman, A W. 1981. Mechanisms of enrichment at the air-water interface. In S. J. Eisenreich (ed.), Atmospheric Pollutants in Natural Waters. Ann Arbor Science Publishers. _ _ _ _ , and Armstrong, D. E. 1979. Enrichment of Zn, Cd, Pb, and Cu in the surface microlayer of Lakes Michigan, Ontario, and Mendota. Limnol. Oceanogr. 24:133-144. and Andren, A W. 1979. Particulate 'zinc, cadm'ium, lead, and copper in the surface microlayer of southern Lake Michigan. EnViron. Sci. Technoi. 13:720-725.

TRACE METAL ACCUMULATION IN SURFACE MICROLAYERS Gatz, D. F. 1975. Pollutant aerosol depositions into southern Lake Michigan. Water, Air, Soil Pollut. 5:239-251. Hoffman, G. I., Duce, R. A., Walsh, P. R., Hoffman, E. J., Ray, B. J., and Fasching, J. L. 1974. Residence time of some particulate trace metals in the oceanic surface microlayer: significance of atmospheric deposition. J. Rech. Atmos. 8:745-759. Hunter, K. A., and Liss, P. S. 1981. Principles and problems of modeling cation enrichment at natural air-water interfaces. In S. J. Eisenreich (ed.), Atmospheric Pollutants in Natural Waters. Ann Arbor Science Publishers. Langevin, S. A. 1978. Part I. Size distribution of trace metals in northeastern Minnesota aerosols, Part II. The role of metals in air-water interactions on Lake Superior. Unpublished M.Sc. dissertation, University of Minnesota, Minneapolis, Minnesota.

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Mackin, J. E., Owen, R. M., and Meyers, P. A. 1980. A factor analysis of elemental associations in the surface microlayer of Lake Michigan and its fluvial inputs. J. Geophys. Res., Abstract Paper 9C1607. Owen, R. M., and Meyers, P. A. 1978. Petroleum hydrocarbons and heavy metals in Great Lakes surface films. Michigan Sea Grant Technical Report No. 60, University of Michigan, Ann Arbor, Michigan. Piotrowicz, S. R., Ray, B. J., Hoffman, G. L., and Duce, R. A. 1972. Trace metal enrichment in sea-surface microlayers. J. Geophys. Res. 77:5243-5254. Schmidt, J. 1977. Selected metals in air particulates over Lake Michigan. Unpublished M.Sc. dissertation, University of Wisconsin, Madison, Wisconsin. Wallace, G. T., Jr., and Duce, R. A. 1975. Concentration of particulate trace metals and particulate organic carbon in marine surface waters by a bubble flotation mechanism. Marine Chem. 3:157-181.