Environ. Sci. Technol. 1994, 28, 1487-1490
New Particle-Labeling Technique for Use in Biological and Physical Sediment Transport Studies Ilhan Olmez' and Francis X. Pink
Nuclear Reactor Laboratory, Environmental Research & Radiochemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Robert A. Wheatcroft Applied Ocean Physics & Engineering Department, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543
A new particle-labeling technique is presented that can be used for biological and physical sediment transport studies. The technique improves upon previous methodologies by using natural sediment from the area of interest as tracers. Sediment particles are labeled with noble metals, such as gold or silver, that are present in only trace amounts in most naturalsettings. The elements are introduced in a liquid phase to the sediment surfaces and are then diffused at a high temperature into the crystalline lattice. Acid etching subsequently removes all unreacted, including surface-adsorbed, metal. The metals can be sensitively detected at low levels (ng/g) using instrumental neutron activation analysis (INAA). Other analytical techniques [e.g., inductively coupled plasma atomic emission spectroscopy (ICPAES), inductively coupled plasma mass spectrometry (ICPMS), and atomic absorption (AA)] would be suitable for this analysis following sediment dissolution and concentration of the dopants. Scanning electron microscopy and energy dispersive X-ray microprobe analysis have been used to examine particles before and after labeling. Neither detectable morphological changes (e.g., size, shape, surface roughness) nor nondiffused surface metals were found following a final etch to remove unreacted dopants. Only minor amounts of particle agglomeration were observed in one sample of the gold-labeled sediments. Sediments labeled by this technique have been used in a study of seasonal and particle-size-dependent bioturbation at a subtidal site in Massachusetts Bay. Application of this technique to the study of other physical transport properties is suggested. Introduction In a marine environment, the transport and dispersal of nonreactive substances is relatively straightforward, as they follow the mass movement of water under the particular conditions of a given region. However, a large number of substances, including many pollutants, are reactive and have an affinity for particle surfaces. Consequently, they are removed from the water and are concentrated on particles, their transport and dispersal being governed by the movement of the sediment through both physical and biological processes. The kinematics of sediment transport are much more complex than the mass movement of water. Dynamically based theories of particle and pollutant dispersal are insufficient and often in need of empirical information, which may only be attained by particle-tracer methods. Extant particle tracers can be differentiated on the basis of whether they are naturally or artificially present in the environment, or whether they have been deliberately introduced. In the first instance, the most commonly used 0013-936X/94/0928-1487$04.50/0
0 1994 American Chemical Society
particle tracers are natural (e.g., Pb-210, Th-234, Be-7) or bomb-produced radionuclides (e.g., Cs-137). These radionuclides have been used successfully in studies of sedimentation ( I , 2) and biological particle mixing ( 1 , 3 , 4 ) . Deliberate tracers of particle transport include various radionuclides Sc-46 (5), CO-58 (61, CS-137 67); fluorescent dye-coated particles or luminophores (8-10); and exotic particles such as fluorescent pigments (111, microtaggants (12),glass beads (131,or rare earth element oxides mixed with natural sediments (14). While these tracers have proven useful, each has its drawbacks. Sources for many naturally occurring radionuclides are widely distributed in the atmosphere or dissolved in seawater; thus, these radionuclides cannot be easily used to study point-source dispersal. Further, these radionuclides cannot be manipulated: the specific component (e.g., particle size distribution, composition, etc.) of the sediment that is tagged may not be compatible with the purpose of the study. Bomb-produced or cosmogenic radionuclides have the additional shortcomingof extremely complex and often indeterminate input functions. Deliberate tracers can be manipulated, but they may cause other difficulties. Researchers who propose to use radionuclide-labeled particles in environmental studies are often faced with immovable regulatory agencies or skeptical funding agents. Lumniphores are difficult to enumerate, requiring tedious manual counting under a microscope. Another potential problem with coated, luminous particles is that energetic, grain-to-grain collisions may dislodge significant amounts of the fluorescent paint. A major problem with exotic particles is that they differ in critical ways (morphology,surface chemistry, size distribution, etc.) from ordinary ambient particles. While this might not be critical for studies of physical sediment transport, it is potentially important for biological studies of particle mixing. Also, quantitative analysis with adequate sensitivity is difficult to attain with exotic particles (13). An ideal tracer for studies of biological and physical sediment transport would overcome the shortcomings listed above and possess the following attributes: (1) detectable at extremely low concentrations, that is, able to undergo extensive dilution; (2) noninterfering with the transport kinetics of the sediments; (3) permanently attached to the particles; and (4) environmentally benign. We have developed such a tracer, suitable for studies of both biological and physical sediment transport. Our technique is based on thermal diffusion of noble metals into the crystalline matrix of ambient sediment grains and subsequent analysis by INAA. The only possible alteration in the physical characteristics of the natural sediments is the density, which depends upon the amount of noble metals diffused and the density of the sediments Environ. Sci. Technol., Vol. 28, No. 8, 1994
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w Dopant solution
natural sediment
i High-temp diffusion
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