Cadmium environmental aspects
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INTERNATIONAL PROGRAMME ON CHEMICAL SAFETY
ENVIRONMENTAL HEALTH CRITERIA 135
CADMIUM - ENVIRONMENTAL ASPECTS
This report contains the collective views of an international group of experts and does not necessarily represent the decisions or the stated policy of the United Nations Environment Programme, the International Labour Organisation, or the World Health Organization.
Published under the joint sponsorship of the United Nations Environment Programme, the International Labour Organisation, and the World Health Organization First draft prepared by Dr S. Dobson,
Institute of Terrestrial Ecology, United Kingdom World Health Orgnization
Geneva, 1992
The International Programme on Chemical Safety (IPCS) is a joint venture of the United Nations Environment Programme, the International Labour Organisation, and the World Health
Organization. The main objective of the IPCS is to carry out and disseminate evaluations of the effects of chemicals on human health and the quality of the environment. Supporting activities include the development of epidemiological, experimental laboratory, and risk-assessment methods that could produce internationally
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coordination of laboratory testing and epidemiological studies, and promotion of research on the mechanisms of the biological action of chemicals.
WHO Library Cataloguing in Publication Data Cadmium : environmental aspects.
(Environmental health criteria ; 135)
1.Cadmium - toxicity 2.Environmental exposure I.Series
ISBN 92 4 157135 7 (NLM Classification: QV 290) ISSN 0250-863X
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CONTENTS
ENVIRONMENTAL HEALTH CRITERIA FOR CADMIUM - ENVIRONMENTAL ASPECTS 1. SUMMARY
2. IDENTITY, PHYSICAL AND CHEMICAL PROPERTIES, AND ANALYTICAL METHODS
2.1. Physical and chemical properties 2.2. Analytical procedures
2.2.1. Sampling and preparation
2.2.2. Quantitative instrumental methods
3. NATURAL OCCURRENCE AND SOURCES OF ENVIRONMENTAL CONTAMINATION 3.1. Natural occurrence
3.2. Industrial uses
3.3. Sources of environmental cadmium 3.3.1. Sources of atmospheric cadmium 3.3.2. Sources of aquatic cadmium 3.3.3. Sources of terrestrial cadmium 3.4. Environmental transport and distribution
3.4.1. Atmospheric deposition
3.4.2. Transport from water to soil 3.5. Concentrations in various biota
3.5.1. Concentrations in fish 3.5.2. Concentrations in sea-birds
3.5.3. Concentrations in sea mammals 3.6. Concentrations adjacent to highways 3.7. Concentrations from industrial sources 4. KINETICS AND METABOLISM
4.1. Uptake
4.1.1. Uptake from water by aquatic organisms 4.1.1.1 Microorganisms
4.1.1.2 Aquatic molluscs
4.1.1.3 Other aquatic invertebrates 4.1.1.4 Fish
4.1.1.5 Model aquatic ecosystems 4.1.1.6 Uptake from aquatic sediment
4.1.1.7 Uptake from food relative to uptake from water
4.1.2. Uptake by terrestrial organisms 4.1.2.1 Uptake into plants
4.1.2.2 Terrestrial invertebrates 4.1.2.3 Birds
4.2. Distribution
4.2.1. Aquatic organisms 4.2.2. Terrestrial organisms
4.2.2.1 Terrestrial plants
4.2.2.2 Terrestrial invertebrates 4.3. Elimination
4.4. Bioaccumulation and biomagnification 5. TOXICITY TO MICROORGANISMS
5.1. Aquatic microorganisms
5.1.1. Freshwater microorganisms
5.1.2. Estuarine and marine microorganisms 5.2. Soil and litter microorganisms
6. TOXICITY TO AQUATIC ORGANISMS 6.1. Toxicity to aquatic plants
6.2. Toxicity to aquatic invertebrates 6.2.1. Acute and short-term toxicity
6.2.1.1 Effects of temperature and salinity on acute toxicity
6.2.1.2 Effect of water hardness
6.2.1.3 Effect of organic materials and sediment 6.2.1.4 Lifestage sensitivity
6.2.1.5 Other factors affecting acute and short-term toxicity
6.2.2. Long-term toxicity 6.2.3. Reproductive effects
6.2.4. Physiological and biochemical effects 6.2.5. Behavioural effects
6.2.6. Interactions with other chemicals 6.2.7. Tolerance
6.2.8. Model ecosystems 6.3. Toxicity to fish
6.3.1. Acute and short-term toxicity
6.3.2. Reproductive effects and effects on early life stages
6.3.3. Metabolic, biochemical and physiological effects 6.3.4. Structural effects and malformations
6.3.5. Behavioural effects
6.3.6. Interactions with other chemicals 6.4. Toxicity to amphibia
7. TOXICITY TO TERRESTRIAL ORGANISMS 7.1. Toxicity to terrestrial plants
7.1.1. Toxicity to plants grown hydroponically 7.1.2. Toxicity to plants grown in soil
7.1.3. In vitro physiological studies 7.2. Toxicity to terrestrial invertebrates 7.3. Toxicity to birds
7.3.1. Acute and short-term toxicity 7.3.2. Reproductive effects
7.3.3. Physiological effects 7.3.4. Behavioural effects 7.4. Toxicity to wild small mammals 8. EFFECTS IN THE FIELD
8.1. Tolerance
8.2. Effects close to industrial sources and highways 8.3. Effects on fish
8.4. Effects on sea-birds 9. EVALUATION
9.1. General considerations 9.2. The aquatic environment 9.3. The terrestrial environment
10. RECOMMENDATIONS FOR PROTECTING THE ENVIRONMENT 11. FURTHER RESEARCH
REFERENCES APPENDIX 1 APPENDIX 2 APPENDIX 3 APPENDIX 4 APPENDIX 5 RESUME RESUMEN
WHO TASK GROUP ON ENVIRONMENTAL HEALTH CRITERIA FOR CADMIUM - ENVIRONMENTAL ASPECTS
Members
Dr L.A. Albert, Consultores Ambientales Asociados, S.C., Xalapa, Veracruz, Mexico
Dr J.K. Atherton, Toxic Substances Division, Directorate for Air, Climate and Toxic Substances, Department of the Environment, London, United Kingdom
Dr R.W. Elias, Trace Metal Biogeochemistry, Environmental Criteria and Assessment Office, US Environmental Protection Agency, Research
Triangle Park, North Carolina, USA
Dr A.H. El-Sebae, Faculty of Agriculture, Alexandria University, Alexandria, Egypt
Dr R. Koch, Bayer AG, Leverkusen, Germany
Professor Y. Kodama, Department of Environmental Health, University of Occupational and Environmental Health, Japan School of Medicine, Yahata Nishi-ku, Kitakyushu City, Japan
Dr P. Pärt, Department of Zoophysiology, Uppsala University, Uppsala, Sweden
Dr J.H.M. Temmink, Department of Toxicology, Agricultural University, Wageningen, The Netherlands ( Chairman)
Secretariat
Dr S. Dobson, Institute of Terrestrial Ecology, Monks Wood
Experimental Station, Abbots Ripton, Huntingdon, Cambridgeshire, United Kingdom ( Rapporteur)
Dr M. Gilbert, International Programme on Chemical Safety, World Health Organization, Geneva, Switzerland ( Secretary)
Mr P.D. Howe, Institute of Terrestrial Ecology, Monks Wood
Experimental Station, Abbots Ripton, Huntingdon, Cambridgeshire, United Kingdom
NOTE TO READERS OF THE CRITERIA DOCUMENTS
Every effort has been made to present information in the criteria documents as accurately as possible without unduly delaying their publication. In the interest of all users of the Environmental Health Criteria documents, readers are kindly requested to communicate any errors that may have occurred to the Director of the International Programme on Chemical Safety, World Health Organization, Geneva, Switzerland, in order that they may be included in corrigenda.
* * *
A detailed data profile and a legal file can be obtained from the International Register of Potentially Toxic Chemicals, Palais des Nations, 1211 Geneva 10, Switzerland (Telephone No. 7988400 or 7985850).
ENVIRONMENTAL HEALTH CRITERIA FOR CADMIUM - ENVIRONMENTAL ASPECTS A WHO Task Group on Environmental Health Criteria for Cadmium - Environmental Aspects met at the Institute of Terrestrial Ecology (ITE), Monks Wood, United Kingdom, from 13 to 17 May 1991. Dr M.
Roberts, Director, ITE, welcomed the participants on behalf of the host institution and Dr M. Gilbert opened the meeting on behalf of the three cooperating organizations of the IPCS (UNEP/ILO/WHO). The Task Group reviewed and revised the draft criteria document and made an evaluation of the risks for the environment from exposure to cadmium.
The first draft of this document was prepared by Dr S. Dobson (ITE). Dr M. Gilbert and Dr P.G. Jenkins, both members of the IPCS Central Unit, were responsible for the technical development and editing, respectively.
The efforts of all who helped in the preparation and finalization
of the document are gratefully acknowledged.
ABBREVIATIONS
ALAD delta-aminolevulinic acid dehydratase DPTA diaminopropanoltetraacetic acid
EDTA ethylenediaminetetraacetic acid EEC European Economic Community
EIFAC European Inland Fisheries Advisory Commission of FAO FAO Food and Agriculture Organization of the United Nations GESAMP Group of Experts on the Scientific Aspects of Marine Pollution
MATC maximum acceptable toxicant concentration NOEL no-observed-effect level
NTA nitrilotriacetic acid NTEL no-toxic-effect level
1. SUMMARY
Cadmium (atomic number 48; relative atomic mass 112.40) is a metallic element belonging, together with zinc and mercury, to group IIb of the periodic table. Some cadmium salts, such as the sulfide, carbonate, and oxide, are practically insoluble in water; these can be converted to water-soluble salts in nature. The sulfate, nitrate, and halides are soluble in water. The speciation of cadmium in the
environment is of importance in evaluating the potential hazard.
The average cadmium content of sea water is about 0.1 µg/litre or less. River water contains dissolved cadmium at concentrations of between < 1 and 13.5 ng/litre. In remote, uninhabited areas, cadmium concentrations in air are usually less than 1 ng/m3. In areas not known to be polluted, the median cadmium concentration in soil has been reported to be in the range of 0.2 to 0.4 mg/kg. However, much higher values, up to 160 mg/kg soil, are occasionally found.
Environmental factors affect the uptake and, therefore, the toxic impact of cadmium on aquatic organisms. Increasing temperature
increases the uptake and toxic impact, whereas increasing salinity or water hardness decreases them. Freshwater organisms are affected by cadmium at lower concentrations than marine organisms. The organic content of the water generally decreases the uptake and toxic effect by binding cadmium and reducing its availability to organisms.
However, there is evidence that some organic matter may have the opposite effect.
Cadmium is readily accumulated by many organisms, particularly by microorganisms and molluscs where the bioconcentration factors are in the order of thousands. Soil invertebrates also concentrate cadmium markedly. Most organisms show low to moderate concentration factors of less than 100. Cadmium is bound to proteins in many tissues. Specific heavy-metal-binding proteins (metallothioneins) have been isolated from cadmium-exposed organisms. The concentration of cadmium is greatest in the kidney, gills, and liver (or their equivalents).
Elimination of the metal from organisms probably occurs principally
via the kidney, although significant amounts can be eliminated via the shed exoskeleton in crustaceans. In plants, cadmium is concentrated primarily in the roots and to a lesser extent in the leaves.
Cadmium is toxic to a wide range of microorganisms. However, the presence of sediment, high concentrations of dissolved salts or
organic matter all reduces the toxic impact. The main effect is on growth and replication. The most affected of soil microorganisms are fungi, some species being eliminated after exposure to cadmium in soil. There is selection for resistant strains after low exposure to the metal in soil.
The acute toxicity of cadmium to aquatic organisms is variable, even between closely related species, and is related to the free ionic concentration of the metal. Cadmium interacts with the calcium
metabolism of animals. In fish it causes hypocalcaemia, probably by inhibiting calcium uptake from the water. However, high calcium concentrations in the water protect fish from cadmium uptake by competing at uptake sites. Zinc increases the toxicity of cadmium to aquatic invertebrates. Sublethal effects have been reported on the growth and reproduction of aquatic invertebrates; there are structural effects on invertebrate gills. There is evidence of the selection of resistant strains of aquatic invertebrates after exposure to cadmium in the field. The toxicity is variable in fish, salmonids being particularly susceptible to cadmium. Sub-lethal effects in fish, notably malformation of the spine, have been reported. The most
susceptible life-stages are the embryo and early larva, while eggs are the least susceptible. There is no consistent interaction between cadmium and zinc in fish. Cadmium is toxic to some amphibian larvae, although some protection is afforded by sediment in the test vessel.
Cadmium affects the growth of plants in experimental studies, although no field effects have been reported. The metal is taken up into plants more readily from nutrient solutions than from soil;
effects have been mainly shown in studies involving culture in nutrient solutions. Stomatal opening, transpiration, and
photosynthesis have been reported to be affected by cadmium in nutrient solutions.
Terrestrial invertebrates are relatively insensitive to the toxic effects of cadmium, probably due to effective sequestration mechanisms in specific organs.
Terrestrial snails are affected sublethally by cadmium; the main effect is on food consumption and dormancy, but only at very high dose levels. Birds are not lethally affected by the metal even at high dosage, although kidney damage occurs.
Cadmium has been reported in field studies to be responsible for changes in species composition in populations of microorganisms and some aquatic invertebrates. Leaf litter decomposition is greatly reduced by heavy metal pollution, and cadmium has been identified as the most potent causative agent for this effect.
2. IDENTITY, PHYSICAL AND CHEMICAL PROPERTIES, AND ANALYTICAL METHODS 2.1 Physical and chemical properties
Cadmium (atomic number 48; relative atomic mass 112.40) is a metallic element belonging, together with zinc and mercury, to group IIb in the periodic table. It is rarely found in a pure state. It is present in various types of rocks and soils and in water, as well as in coal and petroleum. Among these natural sources, zinc, lead, and
copper ore are the main sources of cadmium.
Cadmium can form a number of salts. Its mobility in the
environment and effects on the ecosystem depend to a great extent on the nature of these salts. Since there is no evidence that
organocadmium compounds, where the metal is covalently bound to carbon, occur in nature, only inorganic cadmium salts will be discussed. Cadmium may occur bound to proteins and other organic
molecules and form salts with organic acids, but in these forms, it is regarded as inorganic.
Cadmium has a relatively high vapour pressure. The vapour is oxidized quickly to produce cadmium oxide in the air. When reactive gases or vapour, such as carbon dioxide, water vapour, sulfur dioxide, sulfur trioxide or hydrogen chloride, are present, the vapour reacts to produce cadmium carbonate, hydroxide, sulfite, sulfate or chloride, respectively. These salts may be formed in stacks and emitted to the environment.
Some of the cadmium salts, such as the sulfide, carbonate or oxide, are practically insoluble in water. However, these can be converted to water-soluble salts in nature under the influence of oxygen and acids; the sulfate, nitrate, and halogenates are soluble in water. The physical and chemical properties of cadmium and its salts are summarized in Table 1. Equilibrium data for complexes of group IIB cations, comparing cadmium with zinc and mercury, can be found in Table 2. A diagrammatic representation of the capacity of soil types for metals is given in Fig. 1.
The speciation of cadmium in soil water (Fig. 2) and surface water (Fig. 3) is important for the evaluation of its potential hazard.
Most of the cadmium found in mammals, birds, and fish is probably bound to protein molecules.
Table 1. Physical and chemical properties of cadmium and its salts
Cadmium Cadmium Cadmium Cadmium Cad chloride acetate oxide hyd CAS number 7440-43-9 10108-64-2 543-90-8 1306-19-0 Empirical formula Cd CdCl2 C4H6CdO4 CdO Cd(O Relative atomic or
molecular mass 112.41 183.32 230.50 128.40 146 Relative density 8.642 4.047 2.341 6.95 4.7 Melting point (°C) 320.9 568 256 < 1426 300 (de Boiling point (°C) 765 960 decomposes 900-1000
(decomposes) Water solubility insoluble 1400 very soluble insoluble 0.0 (g/litre) (20 °C) (26
Table 2. Equilibrium data for complexes of group IIB cations a System Metal log K1 DELTA H1 DELTA S1
(kJ mol-1) (J K-1 mol-1)
zinc 5.0 b 0 b 105 M2+-OH- cadmium 3.9 b 0 79 mercury 10.6 b - - zinc 0.8 7.5 42 M2+-F- cadmium 0.6 4.2 25 mercury 1.0 c 4.2 c 33 c
zinc - 0.2 5.4 16 M2+-Cl- cadmium 1.5 - 0.4 29 mercury 7.1 - 24.3 54 zinc - 0.6 1.7 - 4 M2+-Br- cadmium 1.7 - 4.2 21 mercury 9.4 - 40.1 46 zinc - 1.5 - - M2+-I- cadmium 2.1 - 9.2 8 mercury 12.9 c - 75.3 c - 8 c zinc 5.3 - - M2+-CN- cadmium 5.6 - 30.5 b 13 b mercury 18.0 c - 96 b 0 b zinc 0.7 d - 5.9 d - 4 d M2+-SCN- cadmium 1.3 d - 9.6 d - 8d mercury 9.1 d - 49.7 d 8 zinc 1.9 - - M2+-S2O32- e cadmium 4.7 - 6.3 d 67 d mercury 29.9 d - - zinc 2.4 f - 10.9 f 8 f M2+-NH3 cadmium 2.7 f - 14.6 f 4 f mercury 8.8 f - - zinc 4.8 c - 11.3 g 59 g
2+ - cadmium 4.1 d - 8.8 b 50 g (glycinate)- mercury 10.3 c - - zinc 16.4 - 20.5 247 M2+-(EDTA)4- cadmium 16.4 - 38.1 184 mercury 21.5 - 79.0 146
a From: Aylett (1979). Data, which refer to first stepwise stability constant, [ML]/[M][L], unless otherwise stated, are from Sillen (1964) and Smith & Martell (1974, 1975, 1976); see also Christensen et al. (1975). All values refer to measurements in water at 25 °C;
the ionic strength is 3 mol/litre unless otherwise stated.
b ionic strength 0
c ionic strength 0.5 mol/litre
d ionic strength 1.0 mol/litre
e Data refer to overall stability constant, ß2 = [ML2]/[M][L]2
f ionic strength 2.0 mol/litre
g ionic strength 0.1 mol/litre
2.2 Analytical procedures
The following is an outline of the analytical procedure for
cadmium; further information is given in Environmental Health Criteria 134: Cadmium (WHO, 1992).
2.2.1 Sampling and preparation
Only a few nanograms, or even less, of cadmium may be present in collected samples of air or water, whereas hundreds of micrograms may be present in small samples of kidney, sewage sludge, and plastics.
Different techniques are, therefore, required for the collection, preparation, and analysis of the samples.
In general, the techniques available for measuring cadmium in the environment and biological materials cannot differentiate between cadmium species. With special separation techniques,
cadmium-containing proteins can be isolated and identified. In most studies, the concentration or amount of cadmium in water, air, soil, plants, and other environmental or biological material is determined as the element.
Standard trace element methods can generally be used for the collection of samples (LaFleur, 1976; Behne, 1980). During the
handling and storage of samples, particularly liquid samples, special care must be taken to avoid contamination; coloured materials in containers, especially plastics and rubber, should be avoided. Glass and transparent, cadmium-free polyethylene, polypropylene or teflon containers are usually considered suitable for storing samples. All
containers and glassware should be precleaned in dilute nitric acid and deionised water. In order to avoid possible adsorption of cadmium onto the container wall, water samples or standards with low cadmium concentrations should not be stored for long periods of time.
To prepare samples for analysis, inorganic solid samples (such as soil or dust samples) are usually dissolved in an acid, e.g., nitric acid. Organic samples need to be subjected to wet ashing (digested) or dry ashing. When the cadmium concentration is low, special treatment is sometimes needed. The procedures for separating cadmium from
interfering compounds and concentrating the samples are very important steps in obtaining accurate results.
2.2.2 Quantitative instrumental methods
The most commonly used methods, at present, are atomic absorption spectrometry, electrochemical methods, neutron activation analysis, atomic emission spectrometry, atomic fluorescence spectrometry and proton-induced X-ray emissions (PIXE) analysis. Analytical methods for cadmium have been reviewed by Friberg et al. (1986). Detection limits of some of the methods are given in Table 3.
Table 3. Analytical procedures a
Method Detection limit Matrix Atomic absorption 1 to 5 mg/litre water spectrometry
0.1 mg/kg biological samples electrothermal a few pg
atomization
Electrochemical method (potentiometric stripping
analysis) 0.1 mg/litre urine Neutron activation 0.1 to 1 mg/litre biological analysis samples/fluids X-ray atomic 17 mg/kg biological samples fluorescence
a From: Friberg et al. (1986)
3. NATURAL OCCURRENCE AND SOURCES OF ENVIRONMENTAL CONTAMINATION 3.1 Natural occurrence
A comparison of natural and anthropogenic sources of trace metals is given in the Appendix 1.
Cadmium is widely distributed in the earth's crust at an average concentration of about 0.1 mg/kg and is commonly found in association with zinc. However, higher levels are present in sedimentary rocks:
marine phosphates often contain about 15 mg/kg (GESAMP, 1984).
Weathering and erosion result in the transport by rivers of large quantities of cadmium to the world's oceans and this represents a major flux of the global cadmium cycle; an annual gross input of 15 000 tonnes has been estimated (GESAMP, 1987).
In background areas away from ore bodies, surface soil
concentrations of cadmium typically range between 0.1 and 0.4 mg/kg
(Page et al., 1981). The median cadmium concentration in non-volcanic soil ranges from 0.01 to 1 mg/kg, but in volcanic soil levels of up to 4.5 mg/kg have been found (Korte, 1983).
Volcanic activity is a major natural source of atmospheric cadmium release. The global annual flux from this source has been estimated to be 100-500 tonnes (Nriagu, 1979). Deep sea volcanism is also a source of environmental cadmium release, but the role of this process in the global cadmium cycle remains to be quantified.
The average cadmium content of sea water is about 0.1 µg/litre or less (Korte, 1983), while river water (Mississippi, Yangtze, Amazon, and Orinoco sampled between 1976 and 1982) contains dissolved cadmium at concentrations of < 1.1-13.5 ng/litre (Shiller & Boyle, 1987).
Cadmium levels of up to 5 mg/kg have been reported in river and lake sediments and from 0.03 to 1 mg/kg in marine sediments (Korte,1983).
Current measurements of dissolved cadmium in surface waters of the open oceans give values of < 5 ng/litre. The vertical
distribution of dissolved cadmium in ocean waters is characterized by a surface depletion and deep water enrichment, which corresponds to the pattern of nutrient concentrations in these areas (Boyle et al., 1976). This distribution is considered to result from the absorption of cadmium by phytoplankton in surface waters and its transport to the depths, incorporation to biological debris, and subsequent release. In contrast, cadmium is enriched in the surface waters of areas of
upwelling and this also leads to elevated levels in plankton
unconnected with human activity (Martin & Broenkow, 1975; Boyle et al., 1976). Oceanic sediments underlying these areas of high
productivity can contain markedly elevated cadmium levels as a result of inputs associated with biological debris (Simpson, 1981).
In remote, uninhabited areas, cadmium concentrations in air are usually less than 1 ng/m3 (Korte,1983).
3.2 Industrial uses
The principal applications of cadmium fall into five categories:
protective plating on steel; stabilizers for PVC; pigments in plastics and glass; electrode material in nickel-cadmium batteries; and as a component of various alloys (Wilson, 1988).
The relative importance of the major applications has changed considerably over the last 25 years. The use of cadmium for
electroplating represented in 1960 over half the cadmium consumed worldwide, but in 1985 its share was less than 25% (Wilson, 1988).
This decline is usually linked to the introduction of stringent effluent limits from plating works and, more recently, to the
introduction of general restrictions on cadmium consumption in certain countries. In contrast, the use of cadmium in batteries has shown considerable growth in recent years from only 8% of the total market in 1970 to 37% by 1985. The use of cadmium in batteries is
particularly important in Japan and represented over 75% of the total consumption in 1985 (Wilson, 1988).
Pigments and stabilizers accounted for 22% and 12% of the total world consumption in 1985. The share of the market by cadmium pigments remained relatively stable between 1970 and 1985 but the use of the metal in stabilizers during this period showed a considerable decline, largely as a result of economic factors. The use of cadmium as a
constituent of alloys is relatively small and has also declined in importance in recent years, accounting for about 4% of total cadmium use in 1985 (Wilson, 1988).
3.3 Sources of environmental cadmium
3.3.1 Sources of atmospheric cadmium
Estimates of cadmium emissions to the atmosphere from human and natural sources have been carried out at the worldwide, regional, and national level; examples of such inventories are shown in Table 4.
The median global total emission of the metal from human sources in 1983 was 7570 tonnes (Nriagu & Pacyna, 1988) and represented about half the total quantity of cadmium produced in the same year. In both the European Economic Community (EEC) and on a worldwide scale
(Nriagu, 1989), about 10-15% of total airborne cadmium emissions arise from natural processes, the major source being volcanic action.
Municipal refuse contains cadmium derived from discarded
nickel-cadmium batteries and plastics containing cadmium pigments and stabilizers. The incineration of refuse is a major source of
atmospheric cadmium release at country, regional, and worldwide level (Table 4).
Steel production can also be considered as a waste-related
source, as large quantities of cadmium-plated steel scrap are recycled by this industry. As a result, steel production is responsible for considerable emissions of atmospheric cadmium.
3.3.2 Sources of aquatic cadmium
Non-ferrous metal mines represent a major source of cadmium release to the aquatic environment. Contamination can arise from mine drainage water, waste water from the processing of ores, overflow from the tailings pond, and rainwater run-off from the general mine area.
The release of these effluents to local watercourses can lead to extensive contamination downstream of the mining operation. Mines disused for many years can still be responsible for the continuing contamination of adjacent watercourses (Johnson & Eaton, 1980).
At the global level, the smelting of non-ferrous metal ores has been estimated to be the largest human source of cadmium release to the aquatic environment (Nriagu & Pacyna, 1988). Discharges to fresh and coastal waters arise from liquid effluents produced by air
pollution control (gas scrubbing) together with the site drainage waters.
Table 4. Estimates of atmospheric cadmium emissions (tonnes/year) on a na
Source United EEC b Worldwide c Kingdom a
Natural sources ND 20 150-2600 d Non-ferrous metal
production
mining ND ND 0.6-3 zinc and cadmium 20 920-4600 copper 3.7 6 1700-3400 lead 7 39-195 Secondary production ND 2.3-3.6
Production of cadmium-containing
substances ND 3 ND Iron and steel production 2.3 34 28-284 Fossil fuel combustion
coal 1.9 6 176-882 oil 0.5 41-246 Refuse incineration 5 31 56-1400 Sewage sludge incineration 0.2 2 3-36 Table 4 (contd).
Source United EEC b Worldwide c Kingdom a
Phosphate fertilizer manufacture ND ND 68-274 Cement manufacture 1 ND 8.9-534 Wood combustion ND ND 60-180 TOTAL EMISSIONS 14 130 3350-14 640
a From: Hutton & Symon (1986); data apply to 1982-1983
b From: Hutton (1983); data apply to 1979-1980 (the EEC consisted, at that time, of Belgium, Denmark, Federal Republic of Germany, Italy, Luxembourg, The Netherlands, Republic of Ireland, and the United Kingdom)
c From: Nriagu & Pacyna (1988); data apply to 1983
d From: Nriagu (1979) ND Not determined
The manufacture of phosphate fertilizer results in a
redistribution of the cadmium present in the rock phosphates between the phosphoric acid product and gypsum waste. In many cases, the gypsum is disposed of by dumping in coastal waters, which leads to considerable cadmium inputs. Some countries, however, recover the gypsum for use as a construction material and thus have negligible cadmium discharges (Hutton, 1982).
The atmospheric fallout of cadmium to fresh and marine waters represents a major input of cadmium at the global level (Nriagu &
Pacyna, 1988). A GESAMP study of the Mediterranean Sea indicated that this source is comparable in magnitude to the total river inputs of cadmium to the region (GESAMP, 1985). Similarly, large cadmium inputs to the North Sea (110-430 tonnes/year) have also been estimated, based on the extrapolation from measurements of cadmium deposition along the coast (van Alst et al., 1983a,b). However, another approach based on model simulation yielded a modest annual cadmium input of 14 tonnes (Krell & Roeckner, 1988).
Acidification of soils and lakes may result in enhanced
mobilization of cadmium from soils and sediments and lead to increased levels in surface and ground waters (WHO Working Group, 1986).
3.3.3 Sources of terrestrial cadmium
Solid wastes are disposed of in landfill sites, resulting in
large cadmium inputs at the national and regional levels when expressed as total tonnage (Hutton, 1982; Hutton & Symon, 1986).
Sources include the ashes from fossil fuel combustion, waste from cement manufacture, and the disposal of municipal refuse and sewage sludge.
Of greater potential environmental significance are the solid wastes from both non-ferrous metal production and the manufacture of cadmium-containing articles, as well as the ash residues from refuse incineration. These three waste materials are characterized by
elevated cadmium levels and as such require disposal to controlled sites to prevent the contamination of the ground water.
The agricultural application of phosphate fertilizers represents a direct input of cadmium to arable soils. The cadmium content of phosphate fertilizers varies widely and depends on the origin of the rock phosphate. It has been estimated that fertilizers of West African origin contain 160-255 g cadmium/tonne of phosphorus pentoxide, while those derived from the southeastern USA contain 35 g/tonne (Hutton, 1982).
The annual rate of cadmium input to arable land from phosphate fertilizers has been estimated at 5 g/ha for the countries of the EEC (Hutton 1982). This only represents about 1% of the surface soil cadmium burden. Despite the relatively small size of this input, long-term continuous application of phosphate fertilizers has been shown to cause increased soil cadmium concentrations (Williams &
David, 1973, 1976; Andersson & Hahlin, 1981).
The application of municipal sewage sludge to agricultural soils as a fertilizer can also be a significant source of cadmium; a value of 80 g/ha has been estimated for the United Kingdom (Hutton & Symon, 1986). On a national or regional basis, however, these inputs are much smaller than those from either phosphate fertilizers or atmospheric deposition (see section 3.4).
Polluted soils can contain cadmium levels of up to 57 mg/kg (dry weight) resulting from sludge applied to soil and up to 160 mg/kg in the vicinity of metal-processing industry (Fleischer et al., 1974).
The highest cadmium levels reported appear to be from ancient mining areas with levels of up to 468 mg/kg.
3.4 Environmental transport and distribution
3.4.1 Atmospheric deposition
Cadmium is removed from the atmosphere by dry deposition and by precipitation. In rural areas of Scandinavia, annual deposition rates of 0.4-0.9 g/ha have been measured (Laamanen, 1972; Andersson, 1977).
Similarly, in a rural region of Tennessee, USA, a deposition rate of 0.9 g/ha was observed (Lindberg et al., 1982). Hutton (1982) suggested that 3 g/ha per year was a representative value for the atmospheric deposition of cadmium to agricultural soils in rural areas of the EEC.
The corresponding input for these areas from the application of phosphate fertilizers is 5 g/ha per year (see section 3.3).
Many industrial sources of cadmium possess tall stacks which bring about the wide dispersion and dilution of particulate emissions.
Nevertheless, cadmium deposition rates around smelter facilities are often markedly elevated nearest the source and generally decrease rapidly with distance (Hirata, 1981). Soil cadmium concentrations in excess of 100 mg/kg are commonly encountered close to long established
smelters (Buchauer, 1972).
Crop plants growing near to atmospheric sources of cadmium may contain elevated cadmium levels (Carvalho et al., 1986). However, it is not always possible to distinguish whether the cadmium is derived directly from surface deposition or originates from root uptake, since soil levels in such areas are generally higher than normal.
3.4.2 Transport from water to soil
Rivers contaminated with cadmium can contaminate surrounding land, either through irrigation for agricultural purposes, by the dumping of dredged sediments, or through flooding (Forstner, 1980;
Sangster et al., 1984). For example, agricultural land adjacent to the Neckar River, Germany, received dredged sediments to improve the soil, a practice that produced soil cadmium concentrations in excess of 70 mg/kg (Forstner, 1980).
Much of the cadmium entering fresh waters from industrial sources is rapidly adsorbed by particulate matter, where it may settle out or remain suspended, depending on local conditions. This can result in low concentrations of dissolved cadmium even in rivers that receive and transport large quantities of the metal (Yamagata & Shigematsu, 1970)
3.5 Concentrations in various biota
Table 5 indicates the levels of cadmium found in various biota (Eisler, 1985).
Eisler (1985) concluded that there are at least six trends evident from the abundant residue data available for cadmium.
* Marine organisms generally contain higher cadmium residues than their freshwater and terrestrial counterparts.
* Cadmium tends to concentrate in the viscera of vertebrates, especially the liver and kidneys.
* Cadmium concentrations are generally higher in older organisms.
* Higher cadmium residues are generally associated with industrial and urban sources, although this does not apply to sea birds and sea mammals.
* Cadmium residues in plants are normally less than 1 mg/kg.
However, plants growing in soil amended with cadmium (e.g., from sewage sludge) may contain significantly higher levels.
* The species analysed, season of collection, ambient cadmium levels, and the sex of the organism probably all affect the residue level.
Table 5. Concentrations of cadmium in biota
Organisms Parts of the Cadmium concentration organisms (mg/kg dry weight)
Marine organisms
Algae < 1 to 16 Molluscs soft parts up to 425 kidney up to 547
liver up to 782 digestive gland up to 1163 Crustaceans whole body < 0.4-6.2 Annelids whole body 0.1-3.6 Fish whole body up to 5.2 Birds kidney up to 231 Mammals kidney up to 300
Freshwater organisms
Plants whole plant 0.5-1.8 roots up to 6.7 Molluscs soft parts; fresh weight 0.2-1.4 Annelids whole body; fresh weight 0.5-3.2 Fish whole body; fresh weight 0.01-1.04 Table 5 (contd).
Organisms Parts of the Cadmium concentration organisms (mg/kg dry weight)
Terrestrial organisms
Plants whole plant up to 27.1 grain up to 257 Annelids whole body 3-12.6 Birds whole body; fresh weight < 0.05-0.24 kidney; fresh weight up to 7.4 Mammals kidney up to 8.1
3.5.1 Concentrations in fish
May & McKinney (1981) monitored freshwater fish from the USA in 1976 and 1977 and found cadmium concentrations ranging from 0.01 to 1.04 mg/kg (wet weight), the mean being 0.085 mg/kg. This represented a significant decline from the mean 1972 concentration of 0.112 mg/kg.
The authors pointed out that this decline parallels a decline in cadmium metal production and consumption over the same period.
Hardisty et al. (1974a) sampled flounder ( Platichthyes flesus) from the Severn estuary, United Kingdom, and found mean cadmium concentrations of 3.4-7.3 mg/kg (dry weight). No overall correlation between cadmium concentration and length or age was observed, although the largest (27-29 cm) and the oldest („ 5 years) fish gave the
highest mean concentrations. Hardisty et al. (1974b) found a positive correlation between the cadmium content of a variety of fish species and the crustacea content of their diet. Lovett et al. (1972) sampled fish from New York State, USA, and reported mean cadmium
concentrations of < 10-142.7 µg/kg (fresh weight). There was no
relationship between total residues and size, sex or age of lake trout ( Salvelinus namaycush).
3.5.2 Concentrations in sea-birds
Cadmium has been found in a wide variety of birds, and
particularly high levels have been reported in pelagic sea-birds. Much of the cadmium occurs in the kidney and liver, and relatively little is transferred to the eggs. A review of the uptake of cadmium and of the factors that affect it can be found in Scheuhammer (1987).
Interestingly, the concentrations of cadmium in sea-birds are often higher in areas with little or no contamination from industrial sources (Bull et al., 1977; Hutton, 1981; Osborn & Nicholson, 1984).
3.5.3 Concentrations in sea mammals
High levels of cadmium have been reported in sea mammals from areas around the world, which they are assumed to take up from their diet of fish. Roberts et al. (1976) showed that kidney levels of cadmium in the common seal off the United Kingdom coast were age
related. Drescher et al. (1977) showed a similar relationship in seals off the German coast and Hamanaka et al. (1982) in stellar sea lions off the coast of Japan. Similar trends in dolphins and porpoises have been reported (Falconer et al., 1983; Honda & Tatsukawa, 1983; Honda et al., 1986). Muir et al. (1988) sampled white-beaked dolphins ( Lagenorhynchus albirostris) and pilot whales ( Globicephala
melaena) from the coast of Newfoundland, Canada, and reported mean cadmium levels in kidney (dry weight) of 13.6 mg/kg and 108 mg/kg, respectively. Cadmium concentrations were age related in pilot whales.
The lower levels found in dolphins were probably related both to species differences and to the fact that they were all young animals.
3.6 Concentrations adjacent to highways
Muskett & Jones (1980) monitored levels of cadmium adjacent to a heavily used road. The concentrations in air were highest at a
distance from the road of 0-10 m, and a similar pattern was found in soil. Cadmium levels in earthworms sampled at known distances from a highway revealed levels of 12.6 mg/kg (dry weight) within 3 m falling to 7.1 mg/kg approximately 50 m from the highway. The level in
earthworms from control sites was 3 mg/kg (Gish & Christensen, 1973).
The land snail Cepaea hortensis accumulates cadmium from roadside verges (Williamson, 1980). The highest concentration of cadmium was found in the digestive gland (40.3 mg/kg dry weight) and kidney (12.8 mg/kg dry weight). There was little metal in the head and foot, which make up most of the body tissue. The author showed that age accounted for 80% of the total variance of soft tissue body burdens. The cadmium body burdens were found to be effectively immobile, accumulating
progressively with age.
3.7 Concentrations from industrial sources
Burkitt et al. (1972) analysed the cadmium content of ryegrass at various distances from a zinc smelter and found 50, 10.8, and 1.8 mg/kg dry weight at distances of 0.3, 1.9, and 11.3 km, respectively, from the smelter.
Teraoka (1989) found that cadmium levels in rice roots were significantly higher in industrial urban and roadside areas of Japan compared to sparsely populated areas. The mean level in industrial areas was 10 mg/kg (dry weight).
Beyer et al. (1985) monitored biota from the vicinity of two zinc smelters in eastern Pennsylvania, USA. Cadmium concentrations were highest in carrion insects (25 mg/kg dry weight), followed by fungi (9.8 mg/kg), leaves (8.1 mg/kg), shrews (7.3 mg/kg), moths (4.9
mg/kg), mice (2.6 mg/kg), songbirds (2.5 mg/kg), and berries (1.2 mg/kg).
Van Hook (1974) sampled soil and earthworms from soil that had not been disturbed for 30 years and reported mean cadmium levels in the soils and earthworms of 0.35 and 5.7 mg/kg dry weight,
respectively. Ma et al. (1983) analysed soil and earthworms ( Lumbricus rubellus) at varying distances from a zinc-smelting plant. Cadmium concentrations ranged from 0.1 to 5.7 mg/kg for the soil and 20 to 202 mg/kg for the worms, and there was a correlation between decreasing distance from the smelter and increasing cadmium levels. Pietz et al. (1984) sampled soil and earthworms ( Aporrectodea
tuberculata) and ( Lumbricus terrestris) from mine soil and non-mine soil, either amended or not with sewage sludge. Soil and worms from mine soil gave residues of 0.6 and 3.8 mg/kg dry weight, respectively, in non-amended soil and 2 and 22 mg/kg in sludge-amended soil. Residues in soil and worms from non-mined soil were 1 and 12 mg/kg for non-amended and 3.5 and 36 mg/kg for sludge-amended soil, respectively. The much lower capacity of worms from areas already contaminated with cadmium to take up the metal suggests some selection for varieties that control metal uptake. Morgan & Morgan (1988)
sampled earthworms ( Lumbricus rubellus and Dendrodrilus rubidus) from one uncontaminated site and fifteen metal-contaminated sites (in the vicinity of disused non-ferrous metalliferous mines) in the United Kingdom. Cadmium concentrations in the worms ranged from 8 mg/kg (dry weight) to 1786 mg/kg; they were generally higher than soil levels, and the total soil cadmium explained 82% to 86% of the variability in earthworm cadmium concentrations. The authors found some evidence that cadmium accumulation was suppressed in extremely organic soils.
Martin et al. (1980) reported cadmium levels in a variety of invertebrates sampled from sites contaminated by airborne cadmium. The woodlouse was shown to accumulate cadmium principally in the
hepatopancreas.
Van Straalen & van Wensem (1986) analysed 13 species of
arthropods from an area polluted by zinc factory emissions. They found no effect of body size or trophic level on the cadmium content of the arthropods.
Roberts & Johnson (1978) sampled invertebrates and their diet from the area of an abandoned lead-zinc mine in the United Kingdom.
They found cadmium levels higher in herbivorous invertebrates than in the vegetation on which they fed (but not markedly so). There were much higher levels of cadmium in carnivorous invertebrates, suggesting that cadmium might have a capacity for accumulation in food chains.
In contrast to mercury levels, total cadmium body burdens were higher in sparrows ( Passer domesticus) caught in industrialised
areas of Poland than in those caught in agricultural regions (Pinowska et al., 1981). Pigeon brain, liver, and kidney sampled in rural,
suburban, and urban areas gave a good indication of the level of environmental pollution with cadmium (Hutton & Goodman, 1980).
Hunter & Johnson (1982) monitored small mammals near to an industrial works complex and found that cadmium accumulated
particularly in the liver and kidney. Cadmium levels in the liver ranged rom 1.5 to 280 mg/kg (dry weight) and in the kidney from 7.4 to 193 mg/kg. Small mammals from unpolluted sites contained liver levels ranging from 0.5 to 25 mg/kg and kidney levels of 1.5-26 mg/kg. The insectivorous common shrew ( Sorex areneus) was found to be a more prominent accumulator of cadmium than omnivorous and herbivorous small mammals, based on body burden to dietary metal concentration ratios.
Similar results were obtained by Andrews et al. (1984) who monitored cadmium levels in the herbivorous short-tailed field vole ( Microtus
agrestis) and the insectivorous common shrew ( S. araneus) from a revegetated metalliferous mine site. Mean cadmium concentrations were 1.84 mg/kg (dry weight) and 52.7 mg/kg for voles and shrews,
respectively, values that were significantly higher than those found in control sites.
4. KINETICS AND METABOLISM
Appraisal
In aquatic systems, cadmium is most commonly taken up by organisms directly from water, but may also be ingested with substantially contaminated food. The free metal ion, Cd2+, is the form most available to aquatic species. Uptake from water may be reduced by the concentration of calcium and magnesium salts (water hardness). Cadmium uptake from sea water may be greatly reduced by the formation of less available complexes with chloride. Organic complexes with cadmium can be classified in three groups: those that are unavailable (e.g., EDTA, NTA, DPTA), those that are available but less so than the free Cd2+ (e.g., fulvic acids of low relative
molecular mass), and those that form readily available hydrophobic complexes with cadmium (xanthates and dithiocarbamates).
Organisms in the freshwater environment are contaminated according to their ability to absorb or adsorb cadmium from the
water, rather than to their position in the food chain. Consequently, differences in cadmium concentration between species at the same trophic level are common and there is no evidence for
biomagnification. Conversely, marine organisms take up cadmium principally from food. The primary source of cadmium in terrestrial systems is the soil, and uptake follows the typical food chain
pathway, although deposition of cadmium on plant and animal surfaces can account for some additional contamination at each trophic level.
Variations in uptake and retention occur, and there is some evidence for biomagnification in carnivores. Organisms that feed on sediment or detritus may accumulate more cadmium than those in the grazing food chain. High levels of cadmium have been reported in sea mammals, pelagic sea-birds, and terrestrial invertebrates.
Within a variety of organisms, cadmium is distributed throughout most tissues, but tends to accumulate in the roots, gills, livers, kidneys, hepatopancreas, and exoskeleton. Cadmium in the cell is often bound to cytoplasmic proteins, a possible detoxifying
mechanism. Elimination probably occurs primarily via the kidney but also via moulting of the exoskeleton.
There is some evidence of an interaction between cadmium and other metals, especially calcium and zinc. Cadmium may replace
calcium on the calcium-specific protein calmodulin and is affected by other physiological processes that regulate the uptake of calcium. In certain circumstances, zinc increases cadmium retention in the liver and kidneys of aquatic vertebrates. In terrestrial systems, high soil zinc levels can reduce cadmium uptake appreciably.
Selection can lead to cadmium-tolerant populations in both the aquatic and terrestrial environments.
4.1 Uptake
4.1.1 Uptake from water by aquatic organisms
Several studies have shown that the free metal ion, Cd2+, is
the form of cadmium most available to aquatic organisms (Sunda et al., 1978; Borgmann, 1983; Part et al., 1985; Sprague, 1985).
Inorganic cadmium complexes appear not to be taken up, at least by fish (Part et al., 1985). This is particularly important in marine water where cadmium is mainly present in soluble chloride complexes (Zirino & Yamamoto 1972). It is most probable that chloride
complexation is responsible for the reduced cadmium accumulation and toxicity in a variety of organisms observed with increasing salinities (Coombs, 1979).
In the case of organic cadmium complexes, the chemical properties are of importance with respect to bioavailability. Three categories can be distinguished. The first comprises cadmium complexes with EDTA, NTA, and DPTA, which are unavailable to aquatic organisms (Sunda et al., 1978; Part & Wikmark, 1984). The second consists of complexes that to some extent contribute to the total metal uptake, i.e. uptake is higher than predicted from the actual Cd2+ activity, but the
complex is still less available than the free Cd2+ ion. This group includes fulvic acids of low relative molecular mass (Giesy et al., 1977; John et al., 1987), the amino acid histidine (Pecon & Powell, 1981), and carboxylic acids like citric acid (Guy & Ross Kean, 1980;
Part & Wikmark, 1984). The third category includes compounds such as xanthates and dithiocarbamates that form hydrophobic complexes with heavy metals. These hydrophobic complexes act as metal carriers across biological membranes and they lead to a greater uptake of cadmium in aquatic organisms than when the metal is present as the free ion (Poldoski, 1979; Block & Part, 1986; Gottofrey et al., 1988; Block, 1991). This latter observation is of particular environmental concern because xanthates are used in the mining industry in the enrichment of metals from sulfide ores by flotation. Xanthate concentrations of between 4 and 400 µg/litre have been measured in waters receiving effluent from metal refineries (enrichment plants) (Waltersson, 1984).
Another water quality parameter affecting cadmium uptake is the Ca2+ and Mg2+ concentration (hardness) of the water. Increasing
Ca2+ concentration reduces cadmium uptake through fish gills (Part et al., 1985; Wicklund, 1990), cadmium accumulation (Carroll et al., 1979), and cadmium toxicity for fish (Calamari et al., 1980). Two mechanisms can be distinguished for the Ca2+-mediated reduction in cadmium uptake. The first is an inhibitory effect on uptake into gill tissue, while the second is related to the adaptive response of the fish to increased Ca2+ concentrations (Calamari et al., 1980,
Wicklund 1990). Mg2+ also reduces cadmium uptake through fish gills but at 5 times higher concentrations than Ca2+ (Part et al., 1985).
Cadmium uptake in fish is not strongly pH dependent; uptake in rainbow trout gills was not affected over the pH range 5-7 (Part et al., 1985).
Recent data from fish gills indicate that, to some extent, Cd2+
shares uptake mechanisms with Ca2+; these two ions are about the
same size and also form complexes with the same kind of ligands. Thus Cd2+ can replace Ca2+ in the calcium-specific protein calmodulin
(Flik et al., 1987). In the gills, Cd2+ is assumed to enter the epithelial cells down its concentration and electrical gradient by facilitated diffusion through a calcium channel in the apical membrane (Verbost et al., 1989). Several lines of evidence support this
assumption. Firstly, increasing water Ca2+ concentrations reduce cadmium uptake. Secondly, cadmium in the water inhibits Ca2+ uptake in the gills (Verbost et al., 1987; Reid & McDonald, 1988). Thirdly, La3+, a calcium channel blocker in cell membranes, inhibits both
Ca2+ and Cd2+ uptake in the gills. Fourthly, the hypocalcaemic hormone stanniocalcin reduces both Ca2+ and Cd2+ uptake in the
gills (Verbost et al., 1989). Stanniocalcin has been shown to close the apical calcium channel in the gill epithelial cells thereby reducing Ca2+ uptake from the water (Lafeber et al., 1988). The hormone is secreted when the fish has a surplus of Ca2+, i.e.
hypercalcaemic. The two-fold effect of Ca2+ on cadmium uptake in fish discussed previously can be well explained by this model. A direct competition between Ca2+ and Cd2+ at the apical calcium channel reduces the uptake of cadmium into the cells, while the adaptive response in Ca2+-rich water probably involves an increased stanniocalcin level, which closes the apical calcium/cadmium channel.
The transport mechanism from the epithelial cells to the blood is unclear. Cadmium is not transported by the high affinity Ca-ATPase in the basolateral epithelial membrane which transports Ca2+ (Verbost et al., 1988). The possible involvement of the Na+/Ca2+ exchange mechanism, where Cd2+ replaces Ca2+, has recently been suggested as a translocation mechanism to the blood (personal communication to the IPCS by G. Flik).
Zinc also has been shown to reduce cadmium uptake through the gills (Wicklund, 1990). Like cadmium, zinc is assumed to enter the epithelial cell by facilitated diffusion (Spry & Wood, 1989) and, furthermore, Ca2+ acts antagonistically on zinc uptake.
Taken together, these data suggest that the apical epithelial membrane of fish gills contains an ion channel shared by cadmium and calcium, and probably also zinc. The movement of metals through this channel is controlled both by external factors such as the Ca2+
content of the water and internal factors such as hormones.
Increasing temperature increases the uptake of cadmium from water (Vernberg et al., 1974; Zaroogian & Cheer, 1976; Denton &
Burdon-Jones, 1981).
4.1.1.1 Microorganisms
In the alga Chlorella pyrenoidosa, uptake of cadmium was
completely blocked by 0.2 mg manganese/litre and inhibited by 2 to 5 mg iron/litre, but calcium, magnesium, molybdenum, copper, zinc, and cobalt had no effect on uptake (Hart & Scaife, 1977).
Cultures of Chlorella accumulate twice as much cadmium at pH 7.0 as at pH 8.0 when exposed to 0.5 mg cadmium/litre (Hart & Scaife, 1977).
4.1.1.2 Aquatic molluscs
Hardy et al. (1984) found greater uptake of cadmium from sea water into oysters given an uncontaminated phytoplankton food source than into those without food. The authors explain their findings on the basis that the presence of phytoplankton increases the flow of water through the oysters. Studies on oysters without a food source may thus underestimate cadmium uptake. Oysters fed phytoplankton containing cadmium retained only 0.59% of this cadmium; the majority of the cadmium in molluscs is taken up directly from the water. The oyster accumulates about twice as much cadmium in summer as in the winter. This is presumed to reflect the increased flow of water through the animal at higher temperatures (Zaroogian & Cheer, 1976).
Hardy et al. (1981) showed that clams ( Protothaca staminea) took up much less cadmium from water in the presence of sediment at
3.6 g/litre. The uptake was only 17% of that measured in sediment-free water.
Langston & Zhou (1987a,b) found no evidence of cadmium uptake into the bivalve Macoma balthica involving metallothionein or
metallothionein-like proteins. Accumulation in soft tissues was linear throughout a 29-day exposure period, whereas uptake onto the shell was characterized as saturation kinetics. In contrast, the gastropod
Littorina littorea did show induction of specific cadmium-binding proteins, which contributed to uptake and storage of cadmium.
Watling & Watling (1983) demonstrated uptake of cadmium in a dose-dependant manner into sandy beach gastropod molluscs in
laboratory experiments. Much of the cadmium (as chloride) accumulated in the gill. The rate of cadmium uptake was 0.01 mg/kg per day for
Donax serra and 0.16 mg/kg per day for the smaller Bullia
rhodostoma after exposure to cadmium at 20 µg/litre. The freshwater snail Physa integra took up more cadmium as exposure increased, concentrations ranging between 1 and 40 µg/litre. The highest concentration factors were found with the lowest exposure
concentration (Spehar et al., 1978a). Wier & Walter (1976) exposed the freshwater snail Physa gyrina to 1.3 mg cadmium/litre (as the
chloride) and found an average cadmium uptake rate of 0.55 mg/kg per hour over 24 h. Heavier snails took up less cadmium, after the same exposure, than lighter individuals.
4.1.1.3 Other aquatic invertebrates
Rainbow & White (1989) investigated uptake of cadmium and zinc in three marine crustaceans, Palaemon elegans (Decapoda),
Echinogammarus pirloti (Malacostraca), and Elminius modestus
(Cirripedia) at water concentrations of cadmium between 0.5 and 1000 µg/litre and zinc between 2.5 and 4000 µg/litre. All three crustaceans accumulated the non-essential cadmium at all dissolved cadmium
concentrations without regulation. Differences between species were interpreted by the authors in terms of differences in cuticle
permeability and way of life. All three species took up zinc more rapidly than cadmium; the ratios between molar uptake rates of zinc to cadmium were 11.4:1, 2.7:1, and 3.7:1 for the three species,
respectively, following an exposure to a molar ratio of 1.7:1.
4.1.1.4 Fish
Cadmium uptake in fish continues for some considerable time in fish exposed to the metal. The peak of tissue residues may not be reached for several weeks, particularly after exposure to low
concentrations of the metal (Cearley & Coleman, 1974; Benoit et al., 1976; Sullivan et al., 1978a).
Douben (1989a) exposed the stone loach Noemacheilus barbatulus to cadmium in water (as the sulfate) at a concentration of 1 mg/litre and monitored uptake and loss at different temperatures with fed and starved fish. The size of the fish affected both uptake and loss of cadmium, bioconcentration factors decreasing with size. Uptake of cadmium increased with temperature up to about 16 °C and decreased as the concentration of cadmium in the water increased. Feeding the fish increased the rate of uptake of cadmium from the water. The author concluded that metabolic rate was an important factor in the uptake of cadmium into the fish and in its subsequent loss.
4.1.1.5 Model aquatic ecosystems
Ferard et al. (1983) investigated the transfer of cadmium through
