Change of Forms of Cadmium in the Aquatic Environment R. S. S. Wu Department of Environmental Management, Victoria University of Technology, St. Albans, Victoria 3021, Australia H e a y metal.; ; ~ nimpi~rtant t pollutants commonl~found m industrial cmuent and thejr wxic eftects on the aquaticen\ironment have been well documented ( 1 4 ) .In the aquatic environment, heavy metals may exist in various forms. For examole. thev mav amear as free ions in different oxidation staies; be a"bso&ed:r adsorbed onto the surface of clay, iron, and manganese oxyhydroxides or organic matters, or bind to dissolved inorganic and organic ligands in solution and form stable lizand comolexes. Thev also mav be chelated. or complexed &h parti&late matter and sediment (5, 6).Humic substances, which are ubiquitous in the aquatic environment h pwticulatr.: and diiiolved urg:mic assoc~atedM ~ t sedment, matters! I 7.81,show a particular strong - affinitv to cht'late or complex metal ions (9-il). In teaching environmental chemistry, it is, therefore, important for students to understand the changes of metal forms in the aquatic environment, because the specific form in which a metal exists will greatly affect its availability and hence toxic effects on the environment. For example, heavy metals bound to sediment become unavailable to most of the aquatic organisms (12). On the other hand, the biological uptake of some metals (e.g., Hg and Cd) may increase significantly when metal ions are complexed or chelated by particulates (13). Thus. it is also imoortant for the student to understand that thk measurement of the total concentration of a metal in the aquatic environment provides little information about the potential of the metal to interact with the abiotic and biotic environment, and hence the environmental effects of the metal. This paper presents some simple experiments to study the change of various metal forms in the aquatic environment, and to demonstrate: (1) the decrease of free Cd2+in the presence of sea water, (2) the decrease of free Cd2+in the presence of EDTA, (3) the chelation and complexation of Cd2+on sediment can-
taining organic matters and humie substances, and (4) the absorptionladsorption of Cd2+onto incinerated sediment without organic matters. The concentration of Cd2+in the aqueous solution can be measured easilv bv a n ion analvzer with a cadmium electrode. In this eGpehment, a n 0riou ion analyzer (Model EA 940) with a cadmium electrode (Model 94-98)'was used. The detection limit and reproducibility of the Cd electrode used is 1 porn and + 4%. respectively. Other ion analyzer systems &h similar detection limits and reproducib:llity (for example, Cole-Parmer ion analyzer with Cd2+electrode G-27502 or TOA ion meters IM-5S with Cd2+electrode CD-125) will work equally well. Because electrode potential is affected by temperature changes, samples and solutions should be maintained a t + 1 "C. Principles of Measurements The sensing element of a Cd electrode is composed mainlv of cadmium sulfide. The electrode ootential across the sensing element is directly proportional to the level of free cadmium ions in solution. Because the electrode will only measure free cadmium ions in the solution, any change in the reading represents a decrease in free cad264
Journal of Chemical Education
mium ions due to adsorption, absorption, chelation, or complexation of the free cadmium ions. ~ r e s h w a t e r l m a r i n esediments collected from river bedlsea bed normally contain organic matters and humic substances, and are-suitable forkse in the experiment to illustrate the chelatinglcomplexing effects of Cd2+occurring in the natural aquatic environment. I n the present experiment, freshwater sediment was collected from a river near the campus of the Victoria University of Technology. (As a n alternative, garden soil also may be used to illustrate the same principle if freshwaterlmariue sediment is difficult to obtain). Dried sediment was obtained after drvinz the sediment in a n oven a t 100 "C for 48 h. Incinerated sediment without organic matter was obtained by incinerating the dried sediment in a muffle furnace a t 500 "C for 6 h. To allow direct comparison and provide the same surface area for adsorption and absorption for both sediment types, the particle size of both types of sediment must be standardized. This can be done bv sieving the dried and incinerated sediments. In this experiment, only those sediment particles that passed through a 500 pm mesh but were retained on a 250 pm mesh were used. In each of the following four experiments, 50 mL of 800 ppm (mgL) cadmium nitrate is delivered to a beaker and continuous mixing is provided, using a magnetic stirring bar. Initial concentration of free Cd ions is measured using a n Orion ion analyzer and a Cd electrode after a two-point calibration. Samples of distilled water are used a s controls. 1 mL of 5M NaN03 is added to the solution a s a n ionic strength adjustor in the measurement of Cd2+.
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Experiment 1: ComplexationlChelation of cd2' by Sea Water Sea water contains oarticulate matter and a varietv of ligands that would chelate or complex Cd2+.To illustrate the chelating/complexing effects of sea water, measure-
200
P
distilledwater
C
Seawater
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"-0
. 2
4
;
volume (mL) Figure 1. Effect of distilled water and sea water on free cadmium ion in the solution.
"
8
0
200
400
600
800
EDTA (W Figure 2. Effect of EDTA on free cadmium ion in the solution.
ments of the concentrations of free Cd2+are taken after each progressive addition of 0.5 mL of distilled waterlsea water and decrease i n Cd2+is plotted against the volume of distilled waterlsea water added and shown in Figure 1. The ligands found in natural sea waters contain a variety of functional groups that can donate the electron required to bond the ligand to a metal ion (14).Among the most common of these groups are carhoxylate, heterocyclic nitrogen, phenoxide, aliphatic and aromatic amino and phosphate: Cd2+alsok a y f o r m complexes with a wide variety of species that occur naturally in sea water including ammonia, bromide, chloride, a n d may be precipitated by sulfide, carbonate, and phosphate (11). Experiment 2: Chelation of cd2+by EDTA EDTAis a strong synthetic chelating agent used in large quantities i n the metal plating industry and industrial waste water treatment. Consequently, EDTA often is discharged with heavy metals in industrial efluent. To illustrate the chelating effects of EDTA on trace metals, measurements of free Cd ions are taken after each progressive addition of 50 pL of 10% EDTA and decrease in Cd2+levels i n the beakers i s plotted against the volume of EDTA added and shown i n Fieure 3. This prnpnrtion:il decrr:ise in free Cd ions clearly demonstrate? that EDTAchrlntci CdL- in t h ~solutiun . I I1 Experiment 3: ComplexingIChelating to Organic Matters in Dried Sediment Sediment in the natural environment contains a.rich organic matrix with humic substances. To illustrate the chelatinglcomplexing effects of sediment on trace metals, measurements of free Cd ions are taken after each addition of 0.5 g of dried sediment to the Cd solution in the beaker. The decrease in Cd2+levels i s plotted against the weight of dried sediment added and shown in Figure 3. Experiment 4: AbsorptionlAdsorption onto Incinerated Sediments The process of incineration removes all the organic matrix and humic substance from the sediment. To illustrate
0
2
i
6
sediment (g) Figure 3. Effectof dried and incinerated sediments on free cadmium ion in the solution. the ahsorption/adsorption effect of incinerated sediment, measurements of free Cd ions are taken after each progressive addition of 0.5 g of incinerated sediment to the Cd solution in the beaker. The decrease in Cd2+levels is plotted against the amount of incinerated sediment addedand shown in Figure 3. The decrease in free Cd ions unon addition of incinerated sediment represents physical absorptiodadsorption of the metals because no oreanic matter or humic substances are present. The difference i n the decrease of metal ions between Experiments 3 and 4 represents the amount of metals complexedichelated to the organic matrix of the sediment. The binding of metal ions by humic substances in the organic fraction is likely to be most important in contributine to the decrease i n free metal ions (i5).The binding may o& as a chelation between a carhoxyl group and a phenolic hydroxyl group, as a chelation between two carhoxyl groups, or as a complexation with a carhoxyl group (9). ~
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Acknowledgment I wish to thank Stephen Bigger for reading the draft of this manuscript. Literature Cited
Press: Landon, 1987. 4. Goldberg, E. The Health offhe Oceans. The UNESCO Ress, 1982; p 172. 5. Tessie~A ; Camphell. P G . C. Pnrtilioning o/l)nee Metals in Sediments. In: Metal Spmotion. Throry,AnolysisondApplrealion..J.R. Kramer J. R.:Allen. H. E.,Eds.: Lewis Publirhem: Michigan, 19RR: Chapter 9. 6. Samiullah, Y. Pmdidion ollha Enuironmenlol Fate olChemicnls. Elsewer Applied Suenee: London. 1990; p 285. 7. Sontheimel H.; Gimhel. R. Woser Abwasser 1971. 118. 165. 8. Saw R. A : Weber J.H. Enuiran. Sci. Technoi. 1982.16, 510A517A. 9. Lnxen.D. P H. Sci. Tot.Enuiron. 1983.30. 129-146. 10. Cabanisr. S. E.:Shuman M.S.; Collins, InComplaotion o/Tmce Meiolsm Natuml Wnlen;Kramer C. J. M.: Duinker.J. C. E d r ; JunkPublisher: the Hague, 1984;pp 165-179. 11. Msnahan. S. E. Enuimnmeninl Chemistry, 5th ed. Lewis Publisher: Michigan. 1991; p 583. 12. Forstner, U.Conlominoted Sdiments. Springer-Verlag: Berlin. 1989;p 157. 13. Phillips. D. J. H. Quonfildiu~Ayuntic Biolwicol Indimtom. Applied Science Publishcr: London, 1980; p488. 14. Malte1l.A. E. In 0,ponicCnmpoundsin AquotrcEnuimnmnis. Fausl S. D.; Hunter J. Y,Edr. Mareell Dekker: New York, 1971, pp 262-392. IS. Sposito, G.CRC C~iticolRevin Environ. Control 1986. 16. 19&229.
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