GSOT P R É C I S
METALS & AQUATIC Contamination Workshop T BY
ERNEST
MERIAN
he ACS, EPA, and Delaware Workshop on metal speciation was held in conjunction with the 23rd Annual International Symposium on Environmental Analytical Chemistry at Jekyll Island, GA, in June 1993. Herbert Allen (University of Delaware) and colleagues organized the workshop, which provided an overview of present knowledge about metal speciation, local differences in speciation, and regulatory approaches for managing pending problems. Several analytical techniques, biomonitoring methods, and modeling were also discussed.
Overviews, inorganic interactions André Tessier (University of Quebec) reported on inflow, outflow, and internal processes of lakes in Quebec and Ontario. In particular, he discussed the binding of trace elements in oxic lake sediments. Some dissolved trace metals (Zn, Cd, Ni) show large concentration gradients at the sediment—water interface, which indicates that sediments of acid lakes act as a sink for these metal species. Local diffusion may lead to deviations in accumulation. Moreover, Tessier said that the "normal" concentration gradients of Zn, Cd, and Ni may change, particularly in anoxic sediment zones, because of accumulation as insoluble metal sulfides. Précis articles are reports of meetings of unusual significance, international or national developments of environmental importance, significant public policy developments, and related items. 144 A
Environ. Sci. Technol., Vol. 28, No. 3, 1994
0013-936X/94/0927-144A$04.50/0 © 1994 American Chemical Society
Little dissolved Zn was found in interstitial water, but m u c h more was present in the water above the sediment—water interphase. From field measurements a n d calculations using an equilibrium model, Tessier estimated zinc concentration. The binding intensity constants for sorption are in some way correlated to hydrolysis. Unlike Zn, Cd, and Ni, Cu forms complexes that make concentration estimations more difficult. Similar absorption behavior was seen for As and Fe. Low concentrations of some dissolved trace elements in the anoxic zones of sediments appear to be controlled by the solubility of the corresponding sulfides. Calculations indicate that the trace element concentrations (with the exception of iron and m a n g a n e s e ) in the lake w a t e r o v e r l y i n g oxic s e d i m e n t s are h i g h l y u n s a t u r a t e d (i.e., d i s solved completely) with respect to their carbonate or oxyhydroxide solid phases. The partitioning of trace element concentrations b e t w e e n c o m p o n e n t s of oxic lake s e d i m e n t s a n d t h e overlying water can be described by sorption processes using surface complexation concepts. Leo Azarraga (EPA, A t h e n s , GA) a p p l i e d l a n t h a n i d e i o n p r o b e s p e c t r o s c o p y (LIPS) in conjunction with inductively coupled plasma emission spectroscopy (ICP-ES) and atomic absorption spectroscopy (AAS) to investigate the complex metalbinding properties of aquifer materials. Such combinations are p a r t i c u l a r l y powerful, for ins t a n c e , w h e n t r a n s i t i o n s of Eu(III) p r o b e i o n s are u s e d . However, water association m a y affect s p e c t r a l l i n e s i n LIPS. LIPS requires that the dispersed material be completely suspended; ICP-ES a n d AAS require that the dispersed material be c o m p l e t e l y s e p a r a t e d from water or the dispersion media. Moreover, LIPS can distinguish between weak and strong binding sites. The author analyzed Florida aquifer materials, a mixture that contains about 90% sand, some clay, and organic substances. Si, Na, Mg, Fe, Ca, and Al were determined in the supernatant (in some cases competition with Ca and Cu was observed). George Luther (University of Delaware) discussed sampling methods for trace metals, processes that dissolve or p r e c i p i t a t e m e t a l s , a n d metal speciation in iron-rich pore-
water. Because metal concentration in porewaters is governed by precipitation, coprecipitation into and adsorption onto solid phases, and organic c o m p l e x a t i o n , efficient traps for o x i d a t i o n a n d r e d u c t i o n are i m p o r t a n t . Available oxidants i n c l u d e 0 2 , iron(III) (oxy)hydroxides, Mn(III, IV) oxides, and sulfate. In s u l f i d i c e n v i r o n m e n t s , F e S phases are among the first formed sulfide solid phases. The formation
tion in some marshes and sulfide oxidation intermediates must be considered. For instance, u n d e r some conditions aqueous Fe(III) concentrations vary significantly. When plants grow in early summer, less oxygen e n t e r s t h e s e d i m e n t s , l e a d i n g to anaerobic oxidation and production of aqueous Fe(III) in marshes. In nonsulfidic zones, trace metal concentrations in porewaters can increase because of dissolution of pyrite and Fe and Mn oxide phases. That dissolution of the mineral phases may be both abiotic and biotic, but does not have to be reductive. Trace metal levels may increase and be stabilized by organic complexation. John Morse (Texas A&M University) discussed the interactions of trace metals with sulfide minerals in anoxic sediments. Trace metals that are mobilized during early diagenesis by p r o c e s s e s such as the reduction of iron and manganese oxides, and oxidation of organic matter, can be coprecipitated with and adsorbed on authigenic sulfide minerals in anoxic s e d i m e n t s . These sulfide minerals are generally classified as acid volatile sulfides (and may include the metastable minerals amorphous-FeS, mackinawite and greigite, and thermodynamically stable pyrite). Sediment pH is usually 6.9-7.5, but in anoxic sediments pH and r e d o x p o t e n t i a l s are l o w e r e d . Thus reduction a n d binding to new ligands are observed. Morse distinguished between suboxic zones (upper sediments), acid volatile sulfide zones, and pyritic zones (lower sediments). W h e n initially a n o x i c sediments are exposed to oxic seawater, a major portion (20% to over 90%) of the pyrite-bound metals can be released in a day or less. This does not have a major effect on Mn, Zn, Ni, and Pb, w h i c h have little tendency to pyritize. However, Hg, As, and Cu have an inc r e a s e d t e n d e n c y to p y r i t i z e a n d therefore are released and often exceed the concentration of their potentially bioavailable fraction. After oxidation metals may again be released (e.g., in seawater oxidation there is a fast and a slow reaction). Morse concluded that pyritization-depyritization of trace metals is an important process in controlling bioavailability.
When initially anoxic sediments are exposed to oxic seawater, a major portion of the pyrite-bound metals can be released in a day or less. of FeS solid phases may control the concentration of many other metals in p o r e w a t e r . T h e a u t h o r d i s t i n guished between the FeS 2 oxidation zone and the FeS 2 recycling (sulfide) zone in porewaters; relatively small changes in the m i n e r a l s e d i m e n t s may lead to tremendous changes in p o r e w a t e r s . For i n s t a n c e , these changes have been linked to the iron and manganese oxide phases, bacterial decomposition, chemical reduction, and chemical dissolution (e.g., of pyrites). In addition, seasonal sulfate reduc-
Sources of sediment contamination Herbert W i n d o m (Skidaway Institute of O c e a n o g r a p h y , S a v a n n a h ,
Environ. Sci. Technol., Vol. 28, No. 3, 1994
145 A
GA) stated that the proper analysis of sediment contamination requires appropriate quality assurance a n d quality control. He suggested as a first step total digestion and analysis of sediments. But if levels of contamination are biologically significant, something must be k n o w n about the sources of the contamination: do the sediments contain just natural concentrations? It is thus necessary to differentiate a n t h r o p o g e n i c c o m p o n e n t s in s e d i m e n t s from n a t u r a l o n e s . For example, acid volatile sulfides can be used to estimate biological exposure. In a comparison of analytical measurements from 60 laboratories (fractionation, including decomposition, extraction, and consideration of porosities a n d grain size fractions and " n o r m a l i z a t i o n " ) , W i n d o m r e p o r t e d that Cd deviations are often greater than those of Zn and Cu. Richard Thomas (Waterloo Centre for Groundwater Research, Ontario) r e p o r t e d on a s t u d y of p o l l u t i o n sources in the Great Lakes. Among the findings were that the southern lakes contain more Ca; Al is correlated to clay contents; redox potentials are reduced in some locations of the Georgian Bay of Lake Huron and in Lake Erie; Cu concentrations are high in areas of Lake Superior and Lake Michigan because of mining a n d deforestation; Hg concentrations are highest in the Niagara River, Lake Erie, and Lake Ontario locations; and Ni a n d Co concentrations are highest in Lake Superior. The a u t h o r c o n s i d e r e d seasonal variations in rivers and airborne inp u t . A n n u a l total metal loads increased from west to east in Lake Erie and Lake Ontario, w h i c h is reflected in t h e m a s s b a l a n c e s . In some cases, the movement of metal species did not follow the flows of the main water bodies (because of such factors as additional airborne input, local variations in sediment bonding, and biotic and abiotic transport). However, for Lake Ontario, it could be confirmed that the major input comes from the Niagara River. Bioavailability of metals Bjorn Sundby (University of Quebec) discussed the effects of bioturbation on trace metals in sediments of benthic ecosystems. In these systems, some metal species are transported d o w n , and others u p . Thus, there is a need to describe interactions between benthic organisms and the chemistry of trace metals. 146 A
Sundby distinguishes between various layers in bottom sediments of lakes and seas, for example, oxygen reduction in the u p p e r layers a n d nitrogen r e d u c t i o n s o m e w h a t below. This influences Mn cycling [ M n 0 2 to Mn(II)]. I n t e r p r e t a t i o n s are complex because there are noncontinuous processes from the surface water to the bottom water zone of precipitation to the zone of dissolution and to the historical layer. O r g a n i s m s can s p e e d u p t h e s e processes. Organisms can even increase metal concentrations within the zones w h e r e metals dissolve, but it is not clear w h y they do that, because it is not advantageous for them. Samuel N. Luoma (U.S. Geological Survey, Menlo Park, CA) examined the bioavailability of metals in bivalves. T h e b i o a c c u m u l a t i o n of Cd in bivalves living in Pacific versus Atlantic waters a p p e a r s to be different. Luoma d e t e r m i n e d concentrations of Cd in soft tissues of m u s s e l s in c o a s t s a n d b a y s a n d compared it to water values. The filter feeder Potamocarbula amurensis retains about 8 0 % of Cd, the deposit feeder Macona balthica m u c h less. A kinetic bioaccumulation model predicts that assimilation is possible if mussels take u p c a d m i u m via bacteria or d i a t o m a . But the efficiency is increased in the cases of a s s i m i l a t i o n from s o m e t y p e s of p h y t o p l a n k t o n or from iron oxide surfaces. However, the observed differences of bioaccumulation between the two clams does not exist, if they get only dissolved Cd. Sediment management T h o m a s A r m i t a g e (EPA, Washington, DC) reported on EPA's Contaminated Sediment Strategy, w h i c h w a s d e v e l o p e d from EPA studies of the extent and severity of s e d i m e n t c o n t a m i n a t i o n at s i t e s t h r o u g h o u t t h e U n i t e d States, as w e l l as from t h e 1989 N a t i o n a l Academy of Sciences report on the extent of contaminants and potentials for far-reaching effects. Unfortunately, information on sources is not yet complete. Bioaccumulation in fish tissue has been studied for possible h u m a n health risk, including case studies of cancer risk. Goals of the n e w strategy developed between 1989 and 1993 are prevention of future contamination of sediments and management of existing contaminations (remediation may be needed, w h e n natural processes are not sufficient).
Environ. Sci. Technol., Vol. 28, No. 3, 1994
An office of wetlands, oceans, and watersheds within EPA has been established. The office's strategy has involved assessment, research, prevention, remediation, and management of dredged materials. Bioassays are standardized and validated. A manual for dredged material disposal has been developed. Robert M. Engler (Waterways Exp e r i m e n t Station, Vicksburg, MS) asked, "Are contaminated sediments a problem and h o w are they managed?" The U.S. Army Corps of Engineers is responsible for maintaining 25,000 miles of navigable waterways and 500 ports and harbors of the United States. About 500 million cubic yards of dredged materials must be r e m o v e d , transported, a n d placed a n n u a l l y . The material contains mainly sand, silt, and clay, but also rock, gravel, org a n i c matter, d e b r i s , a n d a w i d e range of c o n t a m i n a n t s . Unacceptable c o n t a m i n a t e d s e d i m e n t s acc o u n t for less t h a n 1 0 % . For the protection of waters at the end of the stream, the Long-term Effects of Dredging O p e r a t i o n program was developed. Ocean d u m p i n g of dredged material has b e e n b a n n e d s i n c e 1988. About 340 million cubic yards must therefore be placed inland. About half goes into rivers (e.g., the Mississippi). Cleanup outside navigation c h a n n e l s is also important. There are natural loads and real contaminations of metals such as Cd a n d Zn. As m u c h as 12 million cubic yards of dredgings require special handling. Subject to funding aproval from EPA, the w o r k s h o p will return to Jekyll Island in 1995 in conjunction w i t h the S y m p o s i u m on Environmental Analytical Chemistry.
Ernest Merian received a degree in chemical engineering from the Swiss Federal Institute of Technology (Zurich). He worked at Sandoz Inc. (Basel) from 1947 to 1973; there his research interests included dyestuffs and organic chemicals. He was secretary of the Swiss Association for Environmental Research and of the International Association of Environmental Analytical Chemistry until 1990. Among his hobbies is paper history, and he was president (now honorary) of the Basel Papermill, a wellknown museum and producer of paper and prints made with historic methods.
