Marine Chemistry in the Coastal Environment - American Chemical

21 Release of Heavy Metals from Sediments: Preliminary Comparison of Laboratory and Field Studies Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 31, 2018 | https://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0018.ch021

KERILYNC.BURROWS*and MATTHEW H. HULBERT† Center for Marine and Environmental Studies and Department of Chemistry, Lehigh University, Bethlehem, Penn. 18015 The work reported in this paper is the initial portion of a study intended to clarify the rates and mechanisms of heavy metal uptake and release by sediments. Particular reference is made to estuarine sediments and the metals zinc, cadmium, and lead. The significance of these metals is, of course, their detrimental effects on a variety of animals, including man, and plants via interference with certain enzymes. At present the biologically critical levels of various chemical species of the heavy metals are imperfectly known, and these for only a few biological species. Evidence for synergistic and antagonistic effects has been reported, but data is so limited as to be merely tantalizing. Estuarine sediments are of particular interest since the estuaries serve as breeding grounds of many economically important species, and, in the anoxic state, the sediments appear to serve as traps or sinks for heavy metals. (6,7) Anoxic sediments are to be expected in areas where large quantities of organic wastes are discharged into overlying water; heavy metals accompany the organic wastes in industrialized, urban areas. Below Philadelphia such sediment must be dredged to maintain navigation. In the past this sediment has been used to "reclaim" salt marshes, but with the growing recognition of the values of unreclaimed marshland and the increasing political muscle of conservation interests, new methods of disposal have become mandatory. A particularly apt disposal technique is the construction of artificial wildlife habitats including structures designed to form fresh water ponds. But what happens when oxygenated water percolates through these anoxic sediments? Would reversal of the metal-sequestering reactions lead to undesirable water quality in the surroundings? Experimental Reagents. Stock solutions (1 mg/ml) of zinc, cadmium, copper, and lead were prepared by dissolving 100 mg high purity metal (Alfa-Ventron) in a few milliliters Ultra-Pure nitric acid *Present address: The Wetlands Institute, Box 91, Stone Harbor, N.J. 08247 tPresent address: Department of Chemistry, Connecticut College, New London, Conn. 06320 382

Church; Marine Chemistry in the Coastal Environment ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

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(65%, Alfa-Ventron) and diluting to 100 ml with d i s t i l l e d water. These solutions were stored i n polypropylene bottles. Working solutions were prepared daily by serial dilutions of the stock solutions with pH 1.3 HN0_. pH 1.3 n i t r i c acid, which was also used for rinsing glassware, was prepared by adding 750 y l Ultra-Pure HN0 to 1 i dist i l l e d water, and further purified by electrolysis at -1.5 V (vs. SCE) for approximately 24 hours. One M sodium citrate solution was prepared from USP tri-sodium citrate and purified by electrolysis. Hydrofluoric acid (48%, Fisher Reagent Grade) was r e d i s t i l l e d using a sub-boiling Teflon s t i l l (4). A l l other chemicals were reagent grade and used without further p u r i f i c a tion.

Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 31, 2018 | https://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0018.ch021

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Sediments. Unpolluted samples were collected manually from the upper 50 cm of bottom sediments i n approximately 0.5 meter of water at low tide i n Jenkins Sound, Stone Harbor, New Jersey. The Sound has a s a l i n i t y of 25 to 30% and i s located about one mile from both major roads and major pleasure boat t r a f f i c . Samples were transported i n covered plastic buckets, under several inches of overlying water. Portions not immediately processed were frozen at -20°C i n polyethylene bags. Polluted samples were collected i n the Schuylkill River i n south Philadelphia, just below the Gulf refinery and Naval Yard, on 3 November 1974 by the University of Delaware's R/V Ariadne, using a modified Foster Anchor Dredge. The upper 10 cm of the bottom sediments were taken, under 1.5 meters of water. Samples were stored i n plastic bags at 0°C for about 16 hours, then frozen to -20°C. o

Sediment Digestion. Sediment samples were dissolved by an adaptation of the method used by Presley.(5) Prior to analysis, the samples were dried at 70°C, ground by hand, and washed by shaking for one minute with equal volumes of d i s t i l l e d water, centrifuged 1.5 minutes at 4100 rpm, and dried again. This was designed to remove excess salts from the sediments. Adsorbed fraction. Two grams of ground, dried sediment were placed i n a 15 ml polypropylene centrifuge tube with 10 ml d i s t i l l e d water, shaken by hand for five minutes, centrifuged at 4100 rpm for five minutes, f i l t e r e d into a 25 ml volumetric flask, and taken to volume with pH 1.3 HNO^. Reducible fraction. The residue remaining i n the centrifuge tube and on the f i l t e r paper was washed into a 125 ml flask with 50 ml of 0.25 M NH 0H.HC1 i n 25% acetic acid, and stirred magnetically for 4 hours at room temperature (23°C). The solution was transferred to the centrifuge tube, centrifuged at 4100 rpm for three minutes, and f i l t e r e d into a 250 ml Teflon beaker. The residue was washed by shaking for one minute with five 10 ml 2


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portions of d i s t i l l e d water, centrifuging at 4100 rpm for 30 seconds, and combining the supernatant with the f i l t r a t e . The f i l t r a t e was heated slowly to near dryness on a hot plate. Af ter cooling, one ml HNO and 5 ml HC1 were added. The solution was f i l t e r e d into a 25 ml volumetric flask and taken to volume with pH 1.3 HN0 .


3

Oxidizable fraction. The residue remaining i n the centri fuge tube and on the f i l t e r paper was washed back into the flask with 50 ml d i s t i l l e d water, heated to dryness, and cooled. Twenty-five ml 30% H ^ was added i n 10 ml portions, and the solution l e f t to stand overnight. On the following morning, i t was heated for 30 minutes, cooled, centrifuged at 4100 rpm for three minutes, f i l t e r e d into a Teflon beaker and prepared as for the reducible fraction. Residual fraction. Ten ml d i s t i l l e d water was used to wash the residue into the 25 ml Teflon cup of a Parr bomb, and the sample was heated to dryness. After cooling, 10 ml HF and 2.4 ml HNO were added, the solution heated i n the bomb for three hours at I25°C, cooled overnight, and taken to dryness. This process was repeated using another 10 ml of HF and 2.6 ml of HNO3. After cooling, 3 ml HC1 and approximately 15 ml pH 1.3 HNO^ were added and heated for ten minutes, then the solution was transferred to a 50 ml glass flask and boiled to dissolve the remaining solids. The solution was f i l t e r e d , when cool, into a 50 ml volumetric flask and taken to volume with pH 1.3 HNQ^. Total. One-half gram of ground, dried sediment was prepared as for the residual fraction. A l l samples were stored i n two-ounce polypropylene bottles which had been 1eached for a week prior to use with 6 M n i t r i c acid. Leaching Studies. Short term 1eaching studies were con ducted using washed and dried samples of clean and polluted sedi ments i n both d i s t i l l e d and saline waters under atmospheres of a i r and nitrogen. The caps of one l i t e r wide mouth polypropylene bottles were d r i l l e d for gas inlet and outlet tubes, and leached for a week with 6 M n i t r i c acid. Twenty-four hours before use, they were f i l l e d with the 1eaching 1iquid, which was either dis t i l l e d water or fresh sea water f i l t e r e d through a three micron f i l t e r (30%o s a l i n i t y ) . For the studies, this was discarded, replaced with 500 ml of fresh solution, and the bottles connected to a nitrogen tank or a i r pump and purged for one hour at a flow rate of 200 ml/min. Ten grams of polluted sediment or 20 grams of the clean were added, and the systems stirred magnetically for twenty-four hours. After settling for 45 minutes, a 50 ml por tion was pipetted into a two ounce polypropylene bottle and acid i f i e d with 20μ 1 HNOo.



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Long-term leaching studies were conducted on clean sediment exposed to the weather. A hole was d r i l l e d i n the bottom of a 20 gallon plastic trash barrel and a plastic drainage plug (11 cm inside diameter) was inserted. The cap of a one l i t e r narrow mouth polypropylene bottle was d r i l l e d to accommodate the plug, and held i n place by a rubber 0 ring. The cans were supported in a wooden frame. Seepage samples were collected i n polypropylene bottles screwed into the cap. A similar system was used for rain- and dust-fall collection. In early June, the barrel was f i l l e d with clean wet sediment. Three bottles of seepage were collected within the next two days; afterwards, the bottles were changed at irregular intervals, generally after rain f a l l s . Samples were preserved by the addition of 0.5 ml HNO^. A surface sample of weathered sediment was taken for analysis i n late January. Instrumentation. Samples were analyzed for zinc, cadmium, lead, and copper by d i f f e r e n t i a l pulse anodic stripping voltammetry (DPASV) using a hanging mercury drop electrode (HMDE). The analyses were performed with a Princeton Applied Research model 174 polarographic analyzer, and data recorded on a HewlettPackard 7001 A XY flatbed recorder. The analytical c e l l consisted of 50 ml capacity borosilicate glass bottom (PAR 9301) with p l a s t i c top and f i t t i n g s supplied by PAR. A metro-ohm mercury micro-feeder (model E-410) served as the HMDE; a standard colomel electrode (SCE) isolated by a pH 1.3 HN0 bridge and Pt wire were the reference and counter electrodes, respectively. The instrument was used i n the d i f f e r e n t i a l pulse mode, with a 1.5 V scan range, 25 mV modulation amplitude, and one second drop time; the recorder's X axis was calibrated at 100 mV/inch. A setting of three divisions of the micro-feeder provided a mercury drop with a surface area of 1.8 mm . One analytical cycle consisted of purging with nitrogen and s t i r r i n g at 15 rpm for one minute, plating and s t i r r i n g for one minute, plating for 30 seconds, and scanning. During the analysis the nitrogen flow was directed over the solution. 3

Analysis. Sediment fractions were diluted 1:5 with pH 1.3 HNCL before analysis. A ten ml aliquot of the solution to be analyzed was placed i n the c e l l and purged with nitrogen for 15 minutes. Sodium citrate solution (0.5 ml for rainwater samples, 1.0 ml for seepages, 2.0 ml for sediments and leachates) was added (2), and a preliminary analysis made. This was run from -1.4 V to +0.1 V, at 10 mV/sec from -1.4 V to -0.8 V, 5 mV/ sec from -0.8 V to -0.4 V, and 10 mV/sec from -0.4 V to +0.1 V, at a current range of 10 yA. This scan was used to estimate the peak potentials and current ranges needed for the individual metals. In general -1.4 V served as the i n i t i a l potential for Zn, -0.8 V for Cd, -0.7 V for Pb, and -0.4 V for Cu. Zinc and copper were scanned at 10 mV/sec and cadmium and lead at 5 mV/sec. Once the peak had been scanned, the drop was rapidly scanned to


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+0.1 V ; this portion was not recorded as i t s only purpose was to strip the remaining metals back into solution. Quantitation was accomplished by standard additions; two spikes of 20 to 100 yl and appropriate concentrations were added.


Results and Discussion Sediments used i n these studies were selected to represent a relatively unpolluted area and an urban area subjected to i n dustrial pollution. These w i l l be referred to i n tables and figures as "clean" and "dirty" respectively. Both sediments were dark, anoxic, and rich i n organic matter ; the unpolluted sample was characterized as a clayey s i l t , the polluted as s i l t . TABLE I Grain Size Analysis of Sediments Sample %Sand %Silt %Clay CLEAN 16,7 63.0 20.3 DIRTY 9.2 81.1 9.7 Short-term Studies. Short-term leaching experiments were designed to test the effects of oxygen and s a l i n i t y on heavy metal removed from unpolluted and polluted sediments (Figure 1 ) . With respect to zinc, the unpolluted sediments showed very l i t t l e difference with the various treatments, losing approximately 15 yg zinc per gram of dry sediment. Loss from the polluted sediments under salt water was somewhat less, around 5 yg/g. Under fresh water the polluted sediment lost 60 yg Zn/g. Presence or absence of oxygen had no appreciable effect. These variations imply that zinc i s bound i n different ways i n the different sediments. Cadmium studies show a tendency for the sediments to actually remove the metal from fresh water, amounting to an increase of 0.9 yg/g for the unpolluted sample and 1.7 yg/g for the polluted one. In salt water this appears to be affected by the oxygen content. The unpolluted sample releases 3 yg Cd/g i n the absence of oxygen ; release from the polluted sample, 2 yg/g, occurs i n the presence of oxygen. Here a synergistic effect may be taking place, with the organic content of the sea water being of greater importance than the s a l i n i t y . Under fresh water the unpolluted sample shows similar losses of lead, 1 yg/g, regardless of oxygen content. The polluted sample shows a release more than doubled i n the absence of oxygen, 14 yg Pb/g as opposed to 6. Meaningful data i n salt water i s d i f f i c u l t to obtain as a consequence of analytical error, but the lead i s below detection limits when the unpolluted sample is exposed to oxygen, and when the polluted sample i s not. This may also result from a synergistic effect. Sediment Analysis.

Samples of both sediments, as well as



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unpolluted sediment weathered for six months, were subjected to a four-stage analysis to determine metal distribution. The f i r s t stage, a brief washing with d i s t i l l e d water, was designed to remove labile adsorbed species. This was followed by treatment with hydroxylamine hydrochloride i n acetic acid to dissolve the reducible fraction, consisting of carbonates and iron-manganese oxides. Hydrogen peroxide was employed to dissolve oxidizable species such as sulfides and organic complexes. The final residual portion consists primarily of s i l i c a t e s ; under normal conditions the metals i n this phase are environmentally inert. The summations of the fractional metal analyses are i n generally good agreement with a total dissolution of the sample. Prior to weathering no one fraction of the unpolluted sample can be identified as metal-rich (Figure 2). Zinc i s concentrated in the reducible fraction; less than 1% occurs as oxidizable. Cadmium i s detectable only i n the oxidizable fraction; i t i s below the detection limit i n the total digestion. Roughly half the lead i s adsorbed, and an additional third i s associated with the reducible fraction. After weathering; v i r t u a l l y a l l the cadmium and lead are found i n the residual fraction; about 2% of the latter remains adsorbed. Approximately three-fourths of the zinc i s also residual, but 20% remains associated with the reducible fraction (note added i n proof - Ref · 8). The greatest contrast between the polluted and unpolluted sediment metal distributions i s found with cadmium. The oxidizable fraction of the former contains no detectable amount, over half i s associated with the reducible fraction. This portion also accounts for 40% of the zinc; a l l but 1% of the remainder i s divided evenly between the oxidizable and residual portions. Lead i s undetectable i n the oxidizable fraction ; i t is located primarily i n the residual component with less than 1% adsorbed. Seepage Studies. During the six-month period of natural weathering, seepage and r a i n f a l l were monitored for zinc, cadmium, and lead. The r a i n f a l l averaged 40 ppb, 8 ppb, and 20 ppb in the respective metals; the variation was generally random, with two exceptions, and a l l three metals followed similar patterns. Two marked increases i n metal content were noted, one of roughly ten-fold i n the sample taken over the Fourth of July and one of three-fold i n the sample taken over Labor Day. As these samples were collected i n a resort community, the increases probably result from the influx of land and marine vacationers. Metals i n the seepage from unpolluted sediment over the same period exhibit a gradual but definite increase (Figure 3). Zinc levels range from undetectable to 13 ppm, cadmium from 10 to 130 ppb, and lead from 10 to 90 ppb. Thus, r a i n f a l l appears to be a minor source of metal input to the seepage. Analysis of the sediment at this stage i s ambiguous, showing a slight decrease in zinc content, but increases i n both cadmium and lead. However,



BURROWS AND HULBERT

21.

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a distinct change i n metal distribution i s observed, as previously noted. Copper. Under the experimental conditions reported here, the determination of copper was d i f f i c u l t . At analyte levels below 0.5 ppm Cu, the copper peak could not be effectively detected on the shoulder of the mercury oxidation wave. In order to exhibit well-defined peaks, copper levels above 1 ppm were necessary. Such levels are not desirable for simultaneous determination of zinc, due to the formation of a zinc-copper intermetallic which decreases the zinc signal and enhances the copper response (1). Conclusions The three metals under consideration exhibit differing responses to short-term leachings. Zinc removal i s not i n fluenced by the presence of oxygen, but changes from fresh to saline water do appear to affect removal from the polluted sample. Cadmium responds to the presence or absence of oxygen i n both sediments, but only under salt water. A similar result for lead i s shown only by the polluted sample i n fresh water. Correlations between metal distribution i n the sediment fractions and metal removal have not become apparent. Changes i n sediment zinc content due to weathering and to short-term fresh water leaching are comparable, and the negative short-term result for cadmium may be paralleled by the slight i n crease i n cadmium content of the weathered sediment, but no correlation i s found for lead. However, the seepage shows that a l l three metals are, and continue to be, leached from the unpolluted sediment at appreciable levels. As may be anticipated for natural systems, none of the relatively simple laboratory techniques used seem sufficient to reflect the heavy metal release which actually occurs as sediments are weathered. Total sediment dissolution, i n addition to being time-consuming, provides no distinction between environmentally-active metals and those which may be classed as inert. Results of the various partial digestion procedures employed here are not easily correlated with metal removed. Simple shortterm leaching i s qualitative at best, and even here active elements may be undetected. The key to this question may l i e i n specific portions of the gross sediment fractions, notably hydrous iron and manganese oxides i n the reducible fraction and organics or sulfides i n the oxidizable fraction (3). More selective techniques than those currently employed are called for. Natural weathering of polluted sediment samples i s essential to determine the u t i l i t y of shortterm techniques, although i t appears reasonable to expect higher metal levels than those found i n the seepage from unpolluted sediments. Rates of such release may be expected to depend as



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well on the type of pollution*, industrial, essentially metalcontaminated sediments may d i f f e r substantially from urban sources containing significant amounts of organic matter. When several areas are being considered as sediment sources for creation of a r t i f i c i a l habitats, a six-month period of weathering i s impractical as a technique of sample preparation. Rapid, representative methods are essential, both for determination of sediment s u i t a b i l i t y and for estimation of the removal times involved. For example, a sediment with a small but constant metal loss may prove satisfactory i n an open, easily flushed situation, while the construction of a fresh water pond may lead to the accumulation of potentially harmful metal levels. This work has been confined to the effects of geochemical processes. In r e a l i t y biologically induced transformations, such as s o i l oxidation adjacent to plant roots, or active biological transport may prove to be of equal or greater importance in heavy metal mobilization. While the construction of phys i c a l l y suitable w i l d l i f e habitats poses no great d i f f i c u l t i e s , the question of chemical s u i t a b i l i t y i s much more subtle, complex, and v i t a l . Future work i n this laboratory i s directed toward reducing the time required to obtain an environmental1y meaningful sample from raw sediments, and toward improving both the speed and sens i t i v i t y of the analytical technique, particularly with respect to copper. The f i r s t objective involves investigations of both selective sediment digestion and other parameters of short-term leaching, notably organic content. Currently we are investigating the applicability of thin-film mercury electrodes and suppression of intermetallic interferences following the gallium technique proposed by Copeland et a l . (1). Acknow1edgement s. The authors wish to acknowledge the f i nancial support of this work by the Victoria Foundation, and express appreciation for the cooperation of the staff of the Wetlands Institute. Sampling assistance was generously provided by Fred Bopp of the University of Delaware and Mike Criss of the Wetlands Institute, and Joe Kelley of Lehigh University performed the grain size analysis. We are especially grateful to Mark Brindie of Lehigh for his invaluable aid during the early stages of the polarographic work. Abstract Sediments taken from polluted and unpolluted areas were exposed to short-term leaching i n the laboratory, and an unpolluted sample was exposed to natural weathering for six months. The sediments were subjected to fractional digestion. Zinc, cadmium, and lead levels i n leachates, seepage, and sediment were determined by differential pulse anodic stripping voltammetry. The metals continue to be leached during natural


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weathering, a process not adequately reflected by short-term studies, and become associated primarily with the residual sedi ment fraction. Implications of these results must be considered in the use of dredge spoils for a r t i f i c i a l habitat construction.


Literature

Cited

1.

Copeland, T. R., R. A. Osteryoung, and R. K. Skogerbee, Anal. Chem. (1974) 46, 2093.

2.

Florence, T. M., J. Electroanal.

3.

Jenne, E. A. in "Trace Inorganics in Water," Advances in Chemistry Series No. 73, p. 337, American Chemical Society, Washington, D. C., 1968.

4.

Mattinson, J. Μ., Anal. Chem. (1972) 44, 1715.

5.

Presley, B. J., California.

6.

Presley, B. J., Y. Kolodny, A. Nessenbaum, and R. J. Kaplan, Geochim. Cosmochim. Acta (1972) 36, 1073.

7.

Windom, H., J. Waterways, Harbors, Coastal Engineering Division, Amer. Soc. Civil Eng. (1972) 98, 475.

Chem. (1972) 35, 237.

PhD Thesis (1969), UCLA, Los Angeles,

8. Note: Added in proof: The hydroxyl amine HCl technique is not sufficiently rigerous to dissolve free iron oxides and hydroxides; this may account for the increased amount of heavy metals found in the residual (HF soluble) fraction of the weathered sample (E.A. Jenne, personal communication).


Marine Chemistry in the Coastal Environment - American Chemical

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