Ecotoxicity of Nickel-Containing Substances
The Basic Science Papers: ENV-2
This is one of a series of papers on the basic science issues associated with the production, use and
final disposal of nickel and nickel-containing materials. Other papers in the series deal with
carcinogenicity and nickel allergic contact dermatitis.
These papers reflect the current understanding of the science associated with health and environmental
issues.
The essential contributions of B.R. Conard, Ph.D., and of the Nickel Producers Environmental Research
Association (NiPERA) are gratefully acknowledged.
1. EXECUTIVE SUMMARY
1.1 General Comments
1.2 Ni Bioavailability is the Key to Ecotoxicity
1.3 Invalid Criteria for Judging Ecotoxicity
1.4 Natural Occurrence and Essentiality
1.5 Aquatic Ecotoxicity
1.6 Terrestrial Ecotoxicity
2. ECOTOXICOLOGY
2.1 Acute and Chronic Responses
2.2 Dose-Response
2.3 Factors Affecting the Fate of Nickel in the Environment
2.4 Proper Ecotoxic Criteria
2.4.1 Nickel is a Naturally Occurring Element
2.4.2 Essentiality
2.4.3 Homeostasis
2.4.4 Bioavailability
2.5 Improper Ecotoxic Criteria
2.5.1 Persistence
2.5.2 Bioaccumulation
2.5.3 Biomagnification
3. AQUATIC TOXICITY
3.1 Transformation Protocol
3.1.1 Transformation of Nickel Metal (Acute Toxicity)
3.1.2 The Critical Surface Area Approach
3.1.3 Transformation Testing of Nickel Alloys
3.1.4 Transformation of High Temperature NiO
3.2 Current Status of the Draft Transformation Protocol
3.3 Bioassays for Nickel Aquatic Toxicity
3.3.1 Acute Nickel Toxicity
3.3.2 Chronic Nickel Toxicity
4. TERRESTRIAL ECOTOXICITY
4.1 Terrestrial Toxicity Testing
4.1.1 Acute Testing
4.1.2 Chronic Testing
4.2 Terrestrial Ecotoxicity Classification
4.3 "Nickel" Terrestrial Toxicity
4.3.1 Nickel Speciation in Soils
4.3.2 Toxicity to Earthworms
4.3.3 Toxicity to Plants
- Ecotoxicity is complex. There are large varieties of aquatic and terrestrial media, forms of life and
toxic endpoints to be considered. While there is no single ecotoxic concentration for any substance,
sensitive and/or relevant organisms can be tested and results used to classify any substance into one of four
broad classes: "not toxic", "harmful", "toxic" and "very toxic" to the environment.
- Acute (relatively high exposures over a short duration leading to sudden and often severe toxicity) and
chronic (low doses over long duration with delayed toxicity onset and with long toxic response continuance)
ecotoxic endpoints are valid concerns for metal-containing substances in general. Where standard toxicity
testing has been done, or other relevant information exists, then new testing may not be required. When
conflicting, equivocal, or no information exists for a specific substance, however, ecotoxicity testing
should be carried out to allow more accurate hazard identification and ecotoxicity classification.
- Ecological field data, including historical information, should be integrated with relevant and well- conducted laboratory experiments for both hazard identification and risk assessment. Where conflicting evidence between field and laboratory exists, preference should be given to the field data because it corresponds to the real environment where multiple factors interplay and where organisms actually live.
1.2 Ni BIOAVAILABILITY IS THE KEY TO ECOTOXICITY
- Bioavailability of nickel, as the hydrated divalent ion Ni , is the parameter that is correlated with
toxicological effects of nickel-containing substances. Bioavailability is influenced and controlled by many
physical and chemical factors, both biotic and abiotic. Examples of such parameters are: the specific
substance under consideration; its particle size and surface area; and the media variables such as pH, redox
potential, hardness of water, organic and inorganic ligands, particulate matter, etc. All such factors, when
testing for ecotoxicity, must be chosen to correspond to those that predominantly occur in the natural
environmental. The experimental difficulties encountered in controlling relevant parameters cause results to
have high variability.
- The potential for a portion of unbioavailable nickel to mobilize or re-mobilize to the bioavailable
divalent ion at some later time or under a severe change in environmental conditions is a subject for risk
assessment, not a subject for hazard identification and ecotoxic classification.
- Highly bioavailable soluble nickel salts can be added to aquatic test media for determining their own
ecotoxicity, but they are not able to represent the ecotoxicity of sparingly soluble nickel compounds,
elemental nickel, or nickel alloys because dramatic differences in solubilities and/or corrosion affect
nickel bioavailability.
- A transformation test (a draft Protocol exists) can be used to measure bioavailable nickel arising from sparingly soluble nickel-containing substances. The steady state bioavailability of Ni is dependent on the surface area loading (i.e., m2/L) of the substance. The ecotoxicity classification can be derived by comparing the Ni concentration from the transformation test with a relevant L(E)C50 value from a soluble nickel salt, but this comparison has to be done using data from aquatic media of similar composition (pH, hardness, dissolved organic matter, etc.).
1.3 INVALID CRITERIA FOR JUDGING ECOTOXICITY
- Persistence, as used and measured for organic substances, is not a valid criterion upon which to judge
ecotoxicity of nickel and sparingly soluble nickel compounds, since it is the persistence of the bioavailable
Ni , not the parent nickel compound, that determines toxicity. Accordingly, established biodegradation tests
to measure persistence (developed for organic compounds) are not suitable for nickel or sparingly soluble
nickel compounds.
- Bioaccumulation is not a valid criterion for judging the ecotoxicity of nickel substances because nickel
is an essential element for many organisms and these organisms would suffer if they did not have the ability
to accumulate and utilize nickel. Additionally, as a naturally occurring element, many organisms have
mechanisms for detoxifying Ni through sequestration, thereby accumulating Ni in a non-toxic form.
- Biomagnification is not a valid criterion for nickel because nickel does not biomagnify.
1.4 NATURAL OCCURENCE AND ESSENTIALITY
- Nickel and most nickel compounds occur naturally. Life has evolved in their presence. Many organisms require nickel as an essential element for survival. Many organisms have developed mechanisms (homeostasis) to control and regulate their nickel body burdens.
- Aquatic toxicity is primarily evaluated using fresh water organisms (fish, daphnia, and algae). A number
of standardized ecotoxicity tests (e.g., OECD) exist, but care must be taken to control all relevant
parameters when testing nickel-containing substances. Acclimatization of organisms to the testing medium
prior to testing is important to optimize the test's validity.
- Massive nickel metal is not ecotoxic because it cannot supply enough bioavailable Ni . Even considering
the creation of small nickel particles (1 mm3) via normal manufacturing operations would still
result in massive nickel being classified as non-ecotoxic to any known organism for any toxic endpoint.
- Carbonyl nickel powders should be classified for ecotoxicity according to their specific surface
areas:
Not toxic <1.7 m2/gram Harmful 1.7-1.7 Toxic 17.0-170 Very Toxic >170
- 304L stainless steel and high temperature NiO have been tested using a draft Protocol for transformation
and were found to be not toxic in aquatic media because of their inability to provide Ni .
- Aquatic ecotoxicity testing has shown that NiSO4*6H2O and NiCl2*6H2O
fall into the "harmful" classification.
- The most sensitive aquatic plant is green algae. Algae EC50 values for Ni are quite variable and there exist significant questions regarding usefulness of algae data in predicting toxicity for nickel-containing substances.
- The terrestrial environment is more complex than the aqueous environment because of the presence of
complex solid phases that can adsorb nickel and significantly reduce bioavailability. Therefore, total Ni
analysis of a soil should not be used to predict ecotoxicity because it is not a valid surrogate for
bioavailable Ni .
- The most important soil parameter affecting nickel toxicity is pH. Soils that express nickel toxicity at
low pH can be easily and effectively remediated (by limestone addition) to a state where nickel toxicity is
no longer expressed.
- Terrestrial nickel ecotoxicity testing is usually done with soil invertebrates (e.g., earthworms) and
sensitive plants (e.g., oats). No nickel compound is ecotoxic to earthworms under environmentally relevant
conditions.
- Sparingly soluble nickel-containing substances, the element and all nickel alloys are classified as "not
ecotoxic" to even the most sensitive plant life.
- Soluble nickel compounds may be "harmful" or "toxic" to sensitive plants when pH levels of soil are low (pH<5.5), but these compounds are "not toxic" for naturally existing soils at pH>5.5.
Ecotoxicity is a shortened way of saying ecological toxicity. Both terms refer to the methodologies, models and the empirical results pertaining to harmful effects caused by natural or man-made substances to individual organisms, populations of organisms, communities or ecosystems in the natural environment.(1) Ecotoxicology is a branch of toxicology and therefore many of the principles of toxicology apply to ecotoxicology. There are many excellent texts (Casarett & Doull's Toxicology(2), for example) that can be referred to for the basics of general toxicology.
One of the challenges ecotoxicology faces is the large number of organisms that could be affected by environmental release of a chemical. Not only are there a large number of organisms, there are a variety of toxic endpoints that might be of interest. Mortality is only one such endpoint. Others include decreased growth, mobility effects, reproductive effects, etc. In order to reduce the testing required, scientists have chosen reasonably sensitive species in each environmental compartment as representatives of broad classes of organisms and standardized tests have been developed for these organisms. This part of ecotoxicology is still evolving and test conditions will likely be altered over time to improve reliability. Furthermore, the application of ecotoxicity results to risk assessments is undergoing rapid development.
Ecotoxicological observations are conducted both at the laboratory scale and in the field and a critical aim of scientists is to integrate these observations. Laboratory testing often is able to define the impact of a toxicant on individual organisms under conditions that may be far removed from the real environment. Such tests may cause unrecognized stress on the organism resulting in a toxic response that may be independent of exposure to a test substance. It is critical, using chemistry and biology, to understand the complex set of parameters that operate outside and within a healthy organism, and to use both laboratory and field data in assessing toxic responses for risk assessments.
2.1 ACUTE AND CHRONIC RESPONSES
The two basic types of toxicity are acute and chronic. Acute responses have a sudden onset after or during relatively high exposure that is often of short duration (typically 4-7 days). The endpoint can be lethal or non-lethal. A chronic response, involving endpoints that are realized over periods of several weeks to years, is caused by relatively low exposures that occur over a long time. A chronic toxic response is usually characterized by slow toxic progress and long continuance.
Often an acute to chronic ratio (ACR) is given for a particular organism and toxicant. This ratio is unitless provided the acute and chronic results are expressed in the same units (e.g., mg/L for aquatic systems). The ACR has been used in applications where chronic results must be derived from acute testing only. This approximation has provided a way to estimate chronic values because many measured ACRs cluster around 10. The broad assumption being made is that this general factor applies to any organism and toxicant of interest.
ACRs determined for soluble nickel (nickel chloride) for a single organism, Daphnia magna (an aquatic invertebrate), range from 14 to 122 with an average of 51.(3) For fathead minnows the ACRs for soluble nickel range from 24 to 53, and for Mysid shrimp a single value has been measured as 5.5. The average ACR for soluble nickel for all these organisms is about 30. However, the complexity of biological factors that control both acute and chronic responses, and the enormous variety of organism-specific chemistry, suggest that use of this ACR should be done with the full knowledge that large uncertainties exist for the chronic toxicities that may be derived therefrom.
It should also be mentioned that the fairly large range in ACRs for a single species of organism is due to the variations in both acute and chronic measurements and that such variations are due, in part, to slightly different water compositions. The importance of defining the characteristics of the water for testing acute or chronic endpoints cannot be overemphasized and is discussed under the heading "Factors Affecting the Fate of Nickel in the Environment."
A fundamental tenet of toxicology is that the studied response increases as the dose increases. A normal dose-response relationship for non-essential substances is shown below.
 Figure 1: Typical Dose-Response Curve |
This relationship implies that there is a dose below which the response does not occur (the threshold). There are also doses causing a 100% response. Between these extremes there is usually an S-shaped dose-response curve.
Under the best circumstances, the entire shape of the dose-response curve is determined. It is then possible to define the dose that results in a particular percentage response. The 50% response is commonly used. If the response is mortality, this is referred to as the LD50, which means the Lethal Dose for 50% of the animals to die. For a non-lethal response this is termed the ED50 or the Effective Dose to cause the endpoint in 50% of the animals. Other percentages can also be used, e.g., LD10 or ED25.
It is important to note the difference between "dose" and "exposure". Dose is the amount that is known to enter the organism or to interact with a membrane of an organism (e.g. a fish gill) for a given exposure. The dose is specifically associated with the toxic response. Exposure, on the other hand, is the amount or concentration of an agent in the ambient environment in which the organism resides. Simply being in the environment does not necessarily mean that the agent is absorbed by the organism at a dose and for a duration of time sufficient to reach a target site and exert a toxic effect. In the environment it is often difficult to know the dose being received and, therefore, it is common to specify the concentration in the water or soil to which an organism lives or is added to for testing. The resulting response curves are similar to that given above, but are called concentration-response curves and the "D" is substituted by a "C" (meaning "concentration") and the Lethal and Effective Concentrations are given as LC50 or EC50 values, respectively.
2.3 FACTORS AFFECTING THE FATE OF NICKEL IN THE ENVIRONMENT
The fate of nickel-containing substances in the environment depends on their sources, the processes that control chemical reactions ("transforming" nickel from one substance to another), the physical processes that control substance mobility and transportation, and biological processes that move chemical entities across membranes and influence distribution and metabolism within an organism. All of these processes are complex and interrelated. Some of the most critical aspects can be summarized as follows.
- The specific chemical entity, from either natural or anthropogenic sources, that enters an ecosystem is
of great importance. For example, refractory high temperature nickel oxide (NiO), if emitted from a nickel
smelter stack and deposited into water or soil, will be unreactive even at a very low pH of the receiving
medium. Such NiO would sink into sediment or remain unchanged in soil for a very long time (years). Nickel
from this form of NiO will not be available for biological uptake. Even if taken into a terrestrial
organism's gut, the NiO would not release Ni to be absorbed into the organism because of the insolubility of
the NiO even in high concentration acids. On the other hand, nickel sulphate hexahydrate, emitted from the
same smelter stack and falling into the same aqueous or terrestrial area, would initially be highly
solubilized in the lake water and in the soil pore water. However, bioavailability quickly changes as the
environment interacts with the divalent nickel ions resulting in them being bound to inorganic and organic
ligands that are not bioavailable.
- Physical factors, such as particle size and surface area, are also important parameters in determining
how much and at what rate a substance reacts with an environmental medium.
- The physico-chemical properties of the receiving environment control the ultimate fate of nickel and
control the rates at which nickel ions move through and react with other components in the environment. Some
of the most important parameters are:
- pH
- redox potential
- hardness (Ca content)
- alkalinity
- organic ligands
- inorganic ligands (both complexing and precipitating agents)
- ionic strength
- other cations (that compete for binding sites)
- temperature
- solid organic matter
- inorganic solid matter
- cation exchange capacity
A critical discussion of these factors and others has been given by Parametrix(4) and Nieboer et al.(5)
2.4.1 NICKEL IS A NATURALLY OCCURRING ELEMENT
The fact that nickel is a natural element in the earth's makeup must be a factor in assessing its ability to harm the environment. Substances synthesized and released by humans represent a far more serious challenge for ecological systems than do natural substances simply because synthetic substances are new additions to the environment and organisms may not have developed coping mechanisms for them. Natural substances, such as nickel and most of its compounds, on the other hand, have existed on the earth for billions of years. Life has evolved among such minerals. For many organisms nickel has become incorporated into molecules that carry out life's processes, and organisms consequently need nickel to survive (See "Essentiality", below).
The earth's crust contains 0.008% (80 parts per million) nickel.(6) Due to natural geochemical activity and weathering, nickel occurs naturally in all soils, sediments and waters. The average farming soil in the United States contains 30 ppm Ni; in Ontario the average level in soil is 43 ppm Ni.(7)
It is important to note that simply being natural does not necessarily protect the environment from high additional fluxes of such substances from human activities. The ability of organisms to withstand some increase and the threshold point where toxic effects occur are the subjects of ecotoxicology.
The essential function of nickel in most plants and some microorganisms is well-known. Nickel is used by these organisms, for example, in enzymes such as urease and hydrogenase, without which plants either die or are retarded in their growth and yield. (8)
Animal studies have shown certain organ abnormalities in animals on nickel-deficient diets. Such animals often show general ill health, depressed growth rates, abnormal haemoglobin and red blood cell counts, and abnormal liver and kidney function. Nickel also appears to be related to the necessary calcium uptake by mammals. Given these observations, many biologists consider this to be a probable essential function for nickel in humans; in fact, some vitamin capsules intended for human consumption have included nickel sulphate among their constituents. One of the consequences of essentiality is that the nature of the dose-response curve is dramatically altered as shown in figure 2.
 Figure 2: Comparison of Non-Essential and Essential Metal responses [from Casarett & Doull's Toxicology]2 |
Thus, at low doses adverse effects may occur due to nickel deficiency. This means that there is a window of "no adverse effects" for optimal health. This window may be different for each animal or plant species.
Because organisms have incorporated many metals in their biological functioning, organisms have developed a variety of highly effective mechanisms by which they regulate their body burden of essential metals. These regulatory processes are collectively referred to as homeostasis. By this method organisms often are able to collect and store an excess of an essential metal without adverse effects. Indeed, evolutionary pressures seem to have favoured organisms that have developed this ability because they are less prone to experiencing a deficiency. It follows that a high body burden of nickel in an organism is not automatically indicative of harm being done, or even potentially being done. In fact, it is just the reverse: without the high body burden an organism may be in potential danger of suffering from deficiency.
Nickel can do no harm (or benefit) to an organism unless the organism absorbs the divalent nickel ion into its body or the nickel ion is bound strongly enough to a membrane (fish gills) so that the membrane cannot function properly. The concept of bioavailability of metals was recently discussed by a number of international workshops. The consensus report of one such workshop stated that the "Effects of metals on ecosystems are related to metal bioavailability and not to the total metal concentration" [p.14].(9) Only a small number of soluble ions containing nickel are small enough and/or have the necessary reactivity to be transported through membranes or adsorb strongly to membranes. Most environmental chemists and biologists consider the divalent nickel ion (double positive charge and hydrated) as the single most important species of nickel able to be transported through or interact with external membranes. Such ions are termed "bioavailable".(10) Other forms of nickel are not bioavailable because they are too large or unreactive to be of consequence via an animal or plant membrane.
It follows logically from this recognition that the total concentration of nickel in an environmental setting (aquatic or terrestrial) is not correlated with nickel's biological effects. Only the bioavailable portion of the nickel is relevant for ecotoxicity. While a condition might be constructed in a laboratory where nickel in a given medium is 100% bioavailable, it is virtually impossible to have such a situation under any naturally occurring conditions. In the natural environment there exists a complex soup of reactants which are able to bind nickel into complex ions, precipitate nickel into insoluble compounds, adsorb nickel strongly to their surfaces, all of which remove the bioavailable nickel divalent ion from the medium by converting the nickel into another form which is not bioavailable. (See "Factors Affecting the Fate of Nickel in the Environment" on page 8.)
2.5 IMPROPER ECOTOXIC CRITERIA
Recent activities in regulatory jurisdictions in Europe, Canada and the United States increasingly have stated their reliance on a set of criteria for assessing the dangers (hazard identification) of all classes of substances. These criteria are: Persistence, Bioaccumulation and Toxicity (PBT). While the PBT criteria were developed for synthetic organic substances and have been used with success in that area, these criteria are not appropriate for metals in general, and nickel specifically, for the reasons given below.
According to the U.S. EPA, a chemical's persistence refers to the length of time the chemical can exist in the environment before being destroyed (i.e., "transformed") by natural processes.(11) Since nickel is an element and has no environmentally- natural radioactive isotopes, it may be said to persist forever. Indeed, the U.S. EPA has concluded that metals are obviously persistent in the environment in some form. There is no argument with this conclusion. However, this conclusion is irrelevant to toxicology because only the bioavailable forms of nickel are toxicologically important.
It is not the persistence of the element that matters, but rather it is the persistence of bioavailable forms that is important. As discussed in the section "Bioavailability", the natural environment contains many reactants that transform the bioavailable divalent nickel ion into non-bioavailable forms.(12,13) As such, hydrated divalent nickel ions are not persistent. This transformation of bioavailable nickel into non-bioavailable forms (by natural processes) is precisely what the EPA requires in its own definition of persistence.
One common measure of persistence is the ready biodegradation test. The concept of biodegradation was developed for organic compounds and tests were devised for evaluating an organic compound's persistence. As was concluded by a 1995 Canada/EU Technical Workshop,(14) by the OECD's Advisory Group on Harmonization of Classification and Labelling(15) in 1998, and by the NAFTA CEC Task Force on Criteria on the Sound Management of Chemicals(16) in 1999, the concept of biodegradation and its concomitant tests are not applicable for inorganic substances.
This term refers to the amount of a substance taken up by an organism through all routes of exposure (water, diet, inhalation, epidermal). The bioaccumulation factor (BAF) is the ratio of the steady-state tissue concentration and the steady-state environmental concentration (assuming uptake is from food and water).* This term was developed for organic compounds where the concern was potential chronic toxicity and biomagnification. It was legitimately asserted that synthetic organic molecules that accumulate in an organism are far more likely to cause a long-term problem than a substance that does not accumulate.
Some regulatory agencies have taken the view that one common "safe" BAF value exists for all substances. Any BAF higher than the "safe" BAF, they assert, is indicative of potential toxic responses. The nickel industry does not agree. As pointed out under the "Essentiality" discussion, no single BAF value accounts for the necessity that organisms have to obtain and retain (see "Homeostasis") essential metals such as nickel. In addition, the bioaccumulation of nickel occurs by different mechanisms than the bioaccumulation of organic substances(17) and therefore it would not be expected that a safe BAF level would be the same for nickel as for organic substances.
The test developed to measure the ability of a substance to bioaccumulate, namely, the octanol-water partition, Kow, is not valid for nickel species in virtually all cases. The Kow for this use is based on the preferential partitioning of lipophilic organic compounds into the octanol phase because this partitioning can be correlated with the attraction for such compounds to the fatty tissue (lipid) of organisms. Virtually all soluble nickel species do not partition according to lipophilicity and therefore the Kow for nickel-containing substances is meaningless regarding ecotoxicity predictions.
*Bioconcentration is a sister term (with its bioconcentration factor, BCF) when only water is the source of exposure.
Biomagnification is the process by which tissue concentrations of a bioaccumulated substance increase as a substance passes up the food chain through at least two trophic levels. Biomagnification is a concern for synthetic organics and for some metals, such as methyl mercury, but nickel is not observed to biomagnify.
The fundamentals of aquatic toxicology closely follow the general comments made in the preceding sections. An excellent reference text is Fundamentals of Aquatic Toxicology.(18)
A purpose of aquatic toxicity testing is to identify the concentration at which a substance exerts a harmful effect on aquatic organisms. These tests, called "bioassays", are conducted in parallel with one series of tests using the substance of interest while a parallel test is a "control" in which the organisms have no exposure to the test substance. Only if the control population has no adverse effect at a particular exposure, and the test organism experiences toxicity at the same exposure, can the results from the exposed organisms be considered valid. There are different testing procedures for fresh-water and marine waters. Most of the detailed procedures have been specified for fresh-water since the composition of fresh-water is more highly variable than that of sea water and the testing in fresh-waters must control or record more variables than tests using marine water. Testing procedures have been specified as "standards" by a number of agencies including the ASTM, the EPA (U.S.), Environment Canada and the OECD. In the international arena, the OECD tests are commonly carried out.
The OECD tests for fresh-water are summarized as follows:
| Acute | OECD 203 | fish* | 96 h LC50 |
| OECD 202 | daphnia** | 48 h EC50 - immobilization | |
| OECD 201 | algal | 72-96 h EC50 - growth inhibition | |
| Chronic | OECD 210 | fish (early life) | 7-200 day NOEC - hatching success; growth |
| OECD 211 | daphnia | 3 broods NOEC - number of offspring |
*Fish species commonly used are: rainbow trout (Oncorhynchus mykiss) and fathead minnows (Pinephales promelas).
**Ddaphnia species commonly used is Daphnia magna.
For chronic aquatic toxicity, instead of testing a range of concentrations to give an entire concentration-response curve, the results are often reported as the "No Observed Effects Concentration" (NOEC).
It must be emphasized that the composition of water used for either fresh or marine water testing is highly significant in determining the results. This is because of the large number of parameters (see discussion in "Factors Affecting Nickel in the Environment") that interact to control the bioavailability of the metal ion, allowing it to enter the organism or be adsorbed onto external membranes (gills). It is essential that all relevant parameters of water quality be specified when reporting bioassay results, even if such parameters were not "controlled".
It is also recognized that organisms may acclimate to modest changes in their environment. For this reason it is important to have a pretest acclimation period for organisms within the test medium prior to the addition of the test substance. In addition, it is helpful to know the environmental conditions from which the organisms were obtained to determine the degree of acclimation necessary. Ideally, testing should be done within the standardized protocol at conditions similar to those the organism was originally accustomed. Water pH is particularly important: for standard tests of fish and daphnids, acclimation at test pHs of 6-8.5 is done; for algae the test pH is less broad at 7.5-8.0.
As testing is expensive to conduct, researchers are also developing models that aim to predict toxicity. These models are based on the recognition that the total metal in water is not 100% bioavailable. Indeed, a large variety of metal species are present in a real water with only a small fraction being bioavailable. The models use fundamental information about equilibrium thermodynamics of each species, together with kinetic formation and dissociation reactions, to calculate the relevant concentration of the bioavailable species. These data are then compared with the results from standard ecotoxicity tests to determine expected toxicity under a specified condition.
Free-Ion Activity Model: This model was initially proposed by Morel(19) and has been recently summarized by Campbell.(20) The model calculates the chemical activity (or concentration) of the divalent metal ion which is the bioavailable form that can be correlated with toxicity.
The Gill-Surface-Interaction Model: Pagenkopf (21) developed this model to account for the competition of binding sites between the free metal ion of interest and the other major cations such as H , Ca , and Mg . Some difficulties with the formulation of this model caused it to be generally ignored, but it laid a foundation for subsequent models.
The Biotic-Ligand Model: The work of Playle(22) has rekindled interest in considering the interacting biological membrane as a component in the competition for binding metals such as Cu , Ag and Co . Preliminary data suggests that this model is applicable also to nickel.
The correct procedures to be used to test toxicity for environmental organisms are not straightforward and considerable debate among scientists is still ongoing. Some of the concerns are:
- What compositions of standard test media (aquatic, sediment, soil) best represent the range of conditions
found in natural environment?
- How should testing design account for the essentiality of metals such as nickel?
- How should tests account for the ability of organisms to acclimatize to varying environments?
- What organisms best represent the range of life in nature? There exists an assertion by some regulatory agencies that the most soluble form of a metal should be used to predict the ecotoxicity of all other forms of the metal, including the element itself. The nickel industry strongly rejects such an assertion due to the differing chemical and physical properties of various metal-containing species and compounds. Expert workshops have agreed(14,23) and, as a result, the OECD formed a Metals Working Group (representatives from government and industry) to develop testing methods for metals and sparingly soluble inorganic compounds.
At the heart of the work of the OECD Metals Working Group was the fact that metals and sparingly soluble compounds do not supply bioavailable metal ions in the amounts or at the rates as do highly soluble metal compounds. The approach of the OECD Group was to use a dissolution test (termed "transformation" test because a non-bioavailable compound is being transformed into a bioavailable form) to determine the steady state solubilities of metals* and sparingly soluble compounds.
If a compound solubilizes to release the bioavailable metal ion at a concentration that is greater than the EC50 of a soluble form of the same metal, then its toxicity can be established based on the loading used. If, on the other hand, an insoluble compound is incapable of providing the EC50 concentration within a reasonable time (7 days for acute and 28 days for chronic) at 100 mg/L loading, then it would be classified as not ecotoxic. Testing of a range of insoluble metal compounds revealed that the transformation reaction was dependent on the total number of milligrams (particle size-specific) of the compound per litre. This is not surprising given that most dissolution and/or corrosion reactions are dependent on the surface area of the solid phase. This means that 1 mg/L of solid at a certain particle size may reach a different steady state condition than 3 mg/L of the same solid having the same particle size. The number of mg/L of a solid is called its "loading", and this is an important parameter for metals and sparingly soluble compounds.
A very soluble compound completely dissolves rapidly and will continue to dissolve up to its solubility limit. The amount of the soluble compound put into the water (the loading) is directly related, by the ratio of formula weight of the compound to the atomic weight of the ion, to the amount of bioavailable ion released. The classification of toxicity for soluble compounds is generally done according to the following:
| Compound loading for L(E)C50, in mg/L | Aquatic Acute Toxicity Class |
| < 1 | Very Toxic |
| 1-10 | Toxic |
| 10-100 | Harmful |
| >100 | Not Toxic |
This kind of classification system makes sense. A very small amount of a substance that causes a certain end-point is more dangerous than a large amount of another substance that causes the same endpoint. In other words, much more care has to be taken to monitor releases of a more potent hazard.
The same system can be used for the results from the transformation test on sparingly soluble metal compounds. In this case the loadings are those that result in the EC50 concentration as determined using a soluble form of the metal. For example, if 200 mg/L of a metal sulphide in the transformation test generates the EC50 that was measured using the soluble metal chloride salt, then the metal sulphide is classified as "not toxic". However, if 15 mg/L of a metal carbonate were to generate the same EC50, then the metal carbonate would be classified as "harmful".
3.1.1 TRANSFORMATION OF NICKEL METAL (ACUTE TOXICITY)
Testing of nickel metal has been carried out by CANMET.(24) For these tests they used standard aqueous media defined for toxicity testing. These media were OECD 201 (for testing algae) and OECD 203 (for testing daphnia) at pH 8; both media were used without EDTA or micronutrients because the addition of complexing agents, such as EDTA, were not recommended by a workshop of experts. Micronutrients were not needed because the transformation test did not involve the presence of organisms.
The metallic nickel tested was Inco 123 powder (50% - 10 mm) and Cerac N-1021 particles (screened to 710-850 mm size). Both of these products are produced by decomposition of nickel tetracarbonyl gas. The steady state soluble concentrations CNi (in mg/L) at 7 days for both these products were well-correlated with the measured surface area, A(in mm2/L), according to
| log CNi,201 = - 5.12 0.99 log A | r2 = 0.98 |
| log CNi,203 = - 4.76 0.89 log A | r2 = 0.95 |
Nickel powders made by hydrogen reduction of ammonia solutions were also tested, but showed different coefficients than those for the carbonyl-produced powders. Even though good relationships can be empirically obtained for nickel at small particle sizes, different methods of preparation of nickel result in different relationships.
3.1.2 THE CRITICAL SURFACE AREA APPROACH
The critical surface area loading (CSAL) is defined(25) as the surface area of the solid per litre of aqueous median that, after a given period of time, will generate the L(E)C50 of the metallic bioavailable ion as determined in separate ecotoxicity tests using one of the metal's soluble salts. It is necessary that the critical surface area testing be done using virtually an identical medium (same pH, hardness, etc.) to the medium that was used for the L(E)C50 determination.
By testing different amounts of mass per litre, one can obtain results for a variety of surface area loadings (for a given particle size distribution). This relationship for carbonyl nickel powders was given above. One can then look up an L(E)C50 for a soluble nickel salt. An example is the value of EC50 = 1.102 mg/L for Daphnia magna which is the most sensitive aquatic animal according to the U.S. EPA.(3) The log of this LC50 concentration for nickel sulphate correlates with a critical surface area loading for carbonyl nickel powder of 169,800 mm2/L (this is the surface area loading that produces the EC50 concentration). Any other carbonyl powder (with its measured surface area) can have its critical loading calculated, and its ecotoxicity classification assigned by where the critical loading falls within the <1, 1-10, 10-100 and >100 mg/L categories discussed previously.
Applying this to a variety of powders shows that a carbonyl powder with 70 m2/g surface area would be judged acutely "toxic" to Daphnia magna, while a coarser powder having 0.65 m2/g is not toxic. Going to another L(E)C50 changes the toxicity because a different critical surface area loading exists. For example, for duckweed, a 96-hour growth inhibition EC50 has been reported by Wang(26) as 0.33 mg Ni/L. The CSAL therefore is 50,070 mm2/L for duckweed and a powder having 0.65 m2/g would be "harmful" and the 70 m2/g powder would be "very toxic". This demonstrates that ecotoxicity classification is very dependent on the surface area (size) of nickel powder and the organism one is using for the measure of toxicity.
This work also points out that, even for the most sensitive organism (the lowest value of EC50), a specific surface area of < 0.5 m2/g nickel powder would always be non-ecotoxic. Massive forms of nickel have very low specific surface areas. For example, a 1" square of nickel, 1/4 inch thick, has 0.00005 m2/g. Even the typical turnings and drilling chips from working such massive forms would only have 0.02 m2/g and are not toxic in aquatic media.
3.1.3 TRANSFORMATION TESTING OF NICKEL ALLOYS
Alloys are not simple mixtures because the constituents of alloys cannot be separated by mechanical means. Alloys are substances having unique chemical/physical properties. For example, the properties of stainless steels, containing major constituents of iron, nickel and chromium, are substantially different than the properties of their respective pure constituents. In other words, it is impossible to predict the corrosion of a stainless steel simply by adding the corrosion of its pure components. Likewise, it is invalid to predict the ecotoxicity of stainless steel from the ecotoxicity of its constituents.
As a substance, an alloy, or a closely related (chemically and physically) family of alloys, should be independently classified for ecotoxicity. Preliminary testwork carried out by CANMET(27) indicates that the draft Transformation Protocol is applicable to alloys.
In the case of a specific stainless steel, namely Type 304L (in wt.% : 69 Fe, 19 Cr, 11 Ni) at 100 µm particle size, no observable amount of soluble nickel ions were released into an OECD 203 medium at pH 7.8-8.0 over 7 days of agitation (maximum amount analyzed was 0.003 mg/L at a detection limit of 0.001 mg/L). Therefore, Type 304L stainless steel, even in fine powder form, is not ecotoxic.
3.1.4 TRANSFORMATION OF HIGH TEMPERATURE NiO
CANMET also used a high temperature (h.t.) -nickel oxide, Cerac N-1110 (37-44 µm and 88-105 µm size ranges), to test its solubility in OECD 201 and OECC 203 media. The concentration of nickel in solution after 7 days is typically used for assessing acute toxicity and that for 28 days is used for chronic toxicity. The maximum concentration of nickel in either medium over 28 days was 0.005 mg Ni/L and the average was 0.002 mg Ni/L. These concentrations were so consistently low in all tests that CANMET concluded that "[h.t.] NiO does not transform to the bioavailable form in OECD 203 or OECD 201 to any significant extent above background." This work implies that h.t. NiO is not ecotoxic in either acute or chronic time frames.
Transformations of other sparingly soluble nickel compounds are currently being tested.
3.2 CURRENT STATUS OF THE DRAFT TRANSFORMATION PROTOCOL
The OECD Metals Working Group continues to refine the Transformation Protocol. Several issues remain contentious, as discussed below. The nickel industry's position on each of these is:
- The transformation results and their subsequent use in ecotoxicity classification must be able to
discriminate between toxic and non-toxic. The testing and classification should clearly not classify all
metal and all their compounds as toxic. The conditions of the transformation test and its application must be
specified in such a way as to reveal the truly hazardous substances. For example, experts generally agree
that iron and most iron compounds should not be classified as toxic (from an Fe perspective) to the
environment due to their natural ubiquity and necessity for life.
The lowest EC50 for Fe is for carp at 0.6 mg Fe /L. Transformation tests run on Fe powder using the draft Protocol showed the following:
Loading of
Fe PowderConc. in mg/l Highest Fe 7-day Fe 100mg/L 6.1 0.09 10 mg/L 1.4 0.46 1 mg/L 0.29 0.17 The application of the transformation test results involves comparing the concentrations obtained with the most sensitive and relevant EC50. In the Fe case, the EC50 is 0.6 mg/L. If one were to use the highest Fe conc. observed during the transformation of Fe powder, then Fe powder would classify as "toxic" because a 10 mg/L loading generated 1.4 mg Fe /L which is higher than the EC50. Using the transformation concentrations at 7 days would not result in classifying Fe powder, but the 10 mg/L loading result is quite close to the EC50.
- The pH of the transformation medium should reflect the pH values found in the natural environment and the
pH values used in the ecotoxity tests. Initially, the expert Working Group thought a pH range of 6-8 was
reasonable because this range not only encompasses most of the natural pH levels, but also covers the range
of pH used in ecotoxicity testing of organisms. There is a suggestion that the lower limit of this range
should be still lower, e.g., 5.5 or even 5.0. However, the majority of experts disagree with this lowering
and, indeed, are considering increasing the lower limit in order to make the test results and their
application more discriminatory (see above).
- The pH should be kept relatively constant during the transformation test. This can be most conveniently
done by using a chemical buffer consisting of CO2(g) and bicarbonate in solution. Realistic pHs
can be achieved by this buffer and introduction of uncommon water components can be avoided.
- The transformation test's resultant [M ] concentration in solution is to be compared to the L(E)C50 value for an appropriate and a sensitive organism. Most experts contend that this comparison is only valid if values from tests run at about the same pH are compared. It is well-known, for example, that lowering the pH increases metal "solubility" and that a transformation result at pH 6, giving a high M concentration, should not be compared with an L(E)C50 conducted at pH 8 where the M concentration is much lower.
3.3 BIOASSAYS FOR NICKEL AQUATIC TOXICITY *
There are hundreds of measured acute toxicity values for nickel. These results can generally be summarized in the following manner:
Rainbow trout: 96 hours LC50 values range from 7.8 to 35.5 mg Ni/L for highly soluble nickel compounds (chloride, sulphate and acetate salts). This range occurs due to differences in water hardness, with the higher LC50 values being correlated with higher hardness. Acute toxicity of Ni to fish is believed to be chiefly caused by iono-regulatory and osmo-regulatory failure, which disrupts Na and possibly Ca transport.
Fathead minnows: reported 96-hour LC50 values vary by over an order of magnitude, namely 3.1-89.8 mg Ni/L, and decreasing toxicity (increasing LC50) is correlated with increasing pH, alkalinity and hardness, in combination.
Daphnia magna: 48 hours, LC50 values range from 0.51 to 7.3 mg Ni/L with toxicity decreasing as a combination of pH, alkalinity and hardness (in combination) increase.
Algal species: Few data exist, but it is known that nickel toxicity increases with increasing pH. For a 50% reduction in growth, EC values range from 0.05 to 5 mg Ni/L.
The Janssen studies: In 1993 NiPERA(28) contracted Janssen Pharmaceutica to conduct ecotoxicity tests on a number of nickel-containing substances, namely, NiSO4*6H2O, NiCl2*6H2O, high temperature NiO, and Type 123 nickel powder.
The tests were conducted according to OECD protocols using zebra fish (Brachydanio rerio), water fleas (Daphnia magna) and unicellular green algae (Selenastrum capricoruium). The pH was maintained at 7.9±0.3, the hardness was 200-210 mg CaCO3/L. O2 saturation was 90-100% and the temperature of the test solution was 22°C. Under these conditions, the following was reported:
| Substance | Organism | Duration | mg Substance/L L(E)C50 | Acute Toxic Classification |
| NiSO4 | fish | 24 h | > 100 | not toxic |
| daphnia | 48 h | 9.5 | toxic | |
| algae | 72 h | 0.75 | very toxic | |
| NiCL2 | fish | 24 h | > 100 | not toxic |
| daphnia | 24 h | > 10 | harmful | |
| daphnia | 48 h | 6.7 | toxic | |
| algae | 72 h | 0.7 | very toxic | |
| Ni0 (high temperature) | fish | 24 h | > 100 | not toxic |
| daphnia | 48 h | > 100 | not toxic | |
| algae | 72 h | not measurable | not toxic | |
| Type 123
Ni powder |
fish | 24 h, 96 h | > 100 | not toxic |
| daphnia | 48 h | > 100 | not toxic | |
| algae | 72 h | 0.2 | very toxic |
Algae tests have a number of specific problems that can influence results and reproducibility.(29) One problem is that algae go through a series of physiological states in the batch cultures that are most commonly used (e.g., OECD). Considerable variability in the medium results from changes in algae metabolism. These changes, if not well controlled, can significantly affect the rate of growth that is commonly used to measure toxicity. Another problem is that the most common algae cultures used, based on their ease of culturing and lower testing costs, are rarely found in the natural environment. There is some question, therefore, as to their accuracy in predicting toxicity for natural species of algae. As a result of these problems, and others, EC50 results on the same algae culture, for the same test substance, can vary by several orders of magnitude. Such uncertainty should be taken into account during both hazard identification and ecological risk assessments.
Chronic toxicity measurements for Ni in aqueous solutions are sparse. Ni has been associated with: stress hormone production; modification of enzyme production; and alteration of immune, cardiac and respiratory functions. The mechanisms through which Ni might cause these effects are unknown.
Rainbow trout: show a NOEC of 0.06 to 0.24 mg Ni /L (nickel chloride) at a hardness of 50 mg CaCO3/L. The presence of Ca is known to decrease gill permeability.
Daphnia magna: show a decreasing toxicity with increasing hardness, ranging from NOEC = 0.01-0.22 mg Ni/L in the range of 50-200 mg CaCO3/L.
Most of the aquatic parameters that were identified as being important in controlling the bioavailability of metal ions are also important in terrestrial media.(30) The terrestrial media can be described as solid phases in which pore water (an aqueous system) exists. However, the chemistry of the pore water is not solely controlling the situation as it did for the aqueous medium. Certain solids play a critically important role in controlling metal bioavailability in soils. The phase heterogeneity of soils represents a significant challenge in trying to develop standard ecotoxicity tests, and for this reason the reliability of terrestrial tests have lagged behind the aquatic tests.
It is also clear that terrestrial organisms may not be biologically similar to aquatic organisms in the way they interact with the media in which they live. For example, the root zone of plants can be significantly altered by excretions put out by plants. The plants must do this in order to extract nutrients from the pore water and solids. Monitoring the composition of pore water in soil, difficult to do reliably under the best of conditions, is therefore not a reliable indication of what the root membrane encounters.
4.1 TERRESTRIAL TOXICITY TESTING
A number of criteria for terrestrial testing are emerging(31), including the following:
- Testing conditions may optimize metal bioavailability, but such conditions should be maintained within
the naturally occurring ranges of soil composition and chemistry.
- Soil processes are rarely in equilibrium. Therefore, while some thermodynamic modelling shows potential
for predicting speciation for aquatic media, the terrestrial media requires additional complex kinetic
information.(32) Terrestrial toxicity will therefore be empirically measured for quite a few years
to come.
- The bioavailabilities of metals in soils change with time. This is called "aging" and must be included in
an ecotoxicity testing procedure.
- The substance of interest should be added to the soil. Addition of only soluble forms fails to account for the properties of the substance itself and its intrinsic ability to release metal ions under the specific test conditions.
Acute testing in terrestrial systems is currently focused on plants and soil invertebrates. Plant tests include end-points such as seedling emergence, root elongation and vegetative growth.(33) The invertebrates used are commonly earthworms (Eicenia fetida).
The concept of tolerance has been introduced (see Fairbrother(34)). This is important because, for naturally-occurring substances, toxicity is a function of both sensitivity and tolerance. That is, toxic responses may begin to occur at the same level (same sensitivity), but the ability of organisms to tolerate increasing metal exposure may differ (different tolerance). The hazard should be greater for a substance that causes a large increase in toxic response for a relatively small increase in soil concentration.
Chronic testing in terrestrial systems is usually also done using Brassica and earthworms, but under a longer time frame. For Brassica the chronic test is 45 days with end-points of reproduction measured as number of flowers and seed production. For earthworms the test is extended to 8 weeks and a reproduction endpoint is also measured. For earthworm chronic testing it is important to supply food that has a low content of the substance being tested.
4.2 TERRESTRIAL ECOTOXICITY CLASSIFICATION
As with aquatic testing, it is desired to specify toxic thresholds in several categories. One scheme suggested(35) is:
| Threshold (ECx for substance, mg/kg soil) | ||
| Terrestrial Class | Plant | Invertebrate |
| Extremely Hazardous | < 10 | < 50 |
| Hazardous | 10 - 100 | 50 - 500 |
| Not Hazardous | > 100 | > 500 |
This scheme appears reasonable as long as it is recognized that the terrestrial medium used for testing must be within the range of naturally occurring soils and that the substance of interest is added as itself (and not in a soluble surrogate) and that aging of the soil plus substance is done prior to the test organisms being added.
4.3 "NICKEL" TERRESTRIAL TOXICITY
4.3.1 NICKEL SPECIATION IN SOILS
|
Figure 3: Sorption of Nickel on Hydrous Oxides of Iron and Manganese or Silica as a function of Solution
pHNickel speciation in soils and pore water is dependent on many parameters including the sources and mineralogy of particulates depositing onto soil, the age of the soil, historical land uses, the presence of naturally-occurring humic acid complexing ligands and the presence of silica and hydrous oxides of iron and manganese. Significant amounts of nickel, for example, can be strongly bound to the surfaces of hydrous oxides and silica, rendering it unbioavailable. This binding is very dependent on pH, as shown in figure 3 on page 23.
At low pH oxides of manganese adsorb nickel. As the pH increases, oxides of iron become important and are seen, with oxides of manganese, to be strong adsorbers at a neutral pH of 7. Silica is a weaker binder. It is clear from this diagram that the presence of these oxides can dramatically alter the concentration of the divalent nickel ion.
A site-specific example of this effect has been calculated by Theis and Richter and shown in figure 4 on page 24.
For the specific soil composition at the site, the calculations show that Ni is an important species at pH
5, but nickel bound to iron oxides increases dramatically from pH 5 to 6 to become the most abundant species,
followed at pH 6 to 7 by nickel being increasingly bound to manganese oxides.
|
For nickel acetate (when added to horse manure) the no adverse effect concentration for cocoon production was 200 mg Ni/kg soil. This study, however, used a bioavailable form of nickel without aging the soil prior to earthworm testing. In contrast, Chaney(38) observed a good earthworm population existing in serpentinic soils containing up to 4,000 mg/kg. Studies have shown(39) that earthworm tissue does not contain a significant level of nickel even when living within a soil containing high amounts of total nickel. In view of the field soil observations, the nickel industry does not believe any nickel compound or the metal should be classified as ecotoxic to earthworms.
One of the most important factors in nickel phytotoxicity is soil pH. For very acidic soils, particularly for those low in organic matter, nickel can cause toxicity in sensitive crops. However, remediation of toxicity for such soils can be readily done by addition of limestone to increase pH.(37) Oats and barley show high sensitivity to increasing nickel levels in soils. In oats, for example, laboratory sand cultures show visible toxicity as longitudinal banding of yellow and green tissue (described as chlorotic stripes) across young leaves.(40) This symptom has recently been explained by Chaney(41) as a nickel-induced iron deficiency. Grasses, particularly oat, have a unique mechanism by which nickel can inhibit iron uptake. If the laboratory sand soil medium is already deficient in iron, then additions of soluble nickel can create an iron deficiency in the plant. Chaney has shown that real soils usually have enough iron to avoid this toxic endpoint and he has shown that small additions of iron to a soil successfully removed the chlorotic banding observed at high nickel levels.
Furthermore, natural soils (unlike some laboratory synthetic soils) have an enormous capacity for removing bioavailable nickel ions by adsorbing them onto natural soil components (hydrous oxides of iron and manganese). Thus, nickel in a natural soil would be far less ecotoxic to oats, other plants, and organisms in general. Serpentinic soils that are naturally rich in nickel, for example, rarely demonstrate nickel phytotoxicity. Total nickel levels in soil, as in aquatic media, bear little relationship to plant toxicity since they do not correlate with the nickel that is bioavailable in the soil. Scientists have increasingly pointed out that phyto-available nickel (not total nickel) should be used for setting safe limits for nickel in soils.(42,43,44,45,46,47)
It is clear from the above that nickel compounds are easily rendered non-toxic to the terrestrial environment by increasing soil pH. Accordingly, while soluble nickel compounds may be tested as having a slight possibility of harm to sensitive organisms under laboratory conditions, these same compounds, when emitted to the natural environment, are usually rapidly rendered non-bioavailable and have no measurable effects. Insoluble or sparingly soluble nickel compounds, the element itself and nickel alloys are not ecotoxic to the terrestrial environment because they release negligible amounts of the bioavailable Ni under naturally-occurring conditions.
- R. Truhaut. Ecotoxicol. Environ. Safety, 1, 151-173 (1977)
- Casarett & Doull's Toxicology, 5th Edition, Ed. by C.D. Klassen. McGraw-Hill: New York (1966)
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Standards, Washington, D.C. (1986)
- Persistence, Bioaccumulation and Toxicity of Metals and Metal Compounds. Published by ICME: Ottawa
(1995)
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and biological monitoring of the elements. J. Environ. Monit. 1, 1-14 (1999)
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Workshop held under the auspices of the Canada/European Union Metals and Minerals Working Group, Brussels,
Belgium, December 11-13, 1995
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Compounds in the Aquatic Environment. ICME: Ottawa (1996)
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Metals and Metal Compounds. Canada/EU Metals and Minerals Working Group, Brussels, Belgium (1995)
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1998
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(1995)
- F.M.M. Morel. Principles of Aquatic Chemistry. Wiley & Sons: New York (1983)
- P.G.C. Campbell. Interactions between trace metals and aquatic organisms. A critique of the free ion
activity model. In Metal Speciation and Bioavailability in Aquatic Systems, Ed. by A. Tessier and D.R.
Turner. Wiley & Sons: New York (1995)
- G.K Pagenkopf. Env. Sci. and Technol., 17 (6), 342-347 (1983)
- R.C. Playle. Modelling metal interactions at fish gills. The Science of the Total Environ., 219, 147-163
(1998)
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Compounds and Minerals. Ottawa, September 5-8, 1995
- J. Skeaff and P. King. Canada Centre for Mineral and Energy Technology (CANMET), Natural Resources
Canada, Report MMSL 97-089, Ottawa
- Skeaff et al. A Critical Surface Area concept for acute hazard classification of relatively insoluble
metal-containing powders in aquatic environments. Environ. Toxicol. Chem. (in press) (1999)
- W. Wang. Environ. Toxicol. Chem. 6, (12), 961-967 (1987)
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for Metals and Sparingly Soluble Inorganic Compounds. CANMET Report MMSL99-025, Ottawa (1999)
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- H.G. Peterson and N. Nyholm. Algal Bioassays for Metal Toxicity Identification. Water Poll. Res. J.
Canada, 28(1), 129-153 (1995)
- P. Adamo, S. Dudka, M.J. Wilson and W.J. McHarly. Chemical and mineralogical forms of Cu and Ni in
contaminated soils from the Sudbury mining and smelting region. Environ. Pollut., 91, 11-19 (1995)
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terrestrial environment". Proceedings of the International Workshop, Madrid, 4-6 November, 1998
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H.E. Allen et al. Lewis Publishers: Boca Raton, FL, p.35-58 (1995)
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Tests. STM: Conshohochen, PA (1999)
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environment. In press, ICME, Ottawa (1999)
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Paper. ICME, Ottawa (1997)
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Envion. Sci. Technol., 13: 219-224 (1979)
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soils of the Sudbury area. In Proceedings of 8th Annual Meeting of Can. Land Reclamation Assoc., Guelph
(1983)
- R. L. Chaney. Personal communication (1998)
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(1952)
- R. L. Chaney. In press (1999)
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the uptake of nickel and cadmium from soils. Z. Pflanzenernahr. Bodenk., 152:85-91 (1989)
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chromium, copper, nickel and zinc in agricultural land application of municipal land sewage sludges. J.
Environ. Qual. 21:521-536 (1992)
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geogenic nickel contents (in German). Angew. Botanik, 64: 317-328 (1990)
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extractions. Water Air Soil Pollut. 57-58: 861-871 (1991)
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ACR: Acute to Chronic Ratio. The acute response concentration divided by the chronic response concentration (both in the same units).
Acute Response: A toxic response following a relatively high exposure. The response is characterized by sudden onset and can be lethal or non-lethal.
Aging: The processes operating over time that alter the bioavailable species in a soil.
Alkalinity: The measure of the acid-neutralizing capacity of an aqueous system, given in moles of protons (or equivalents)/litre.
ASTM: American Society of Testing Materials.
BAF: The Bio-Accumulation Factor is the steady state ratio of a substance's tissue concentration with the substance's environmental concentration.
Bioaccumulation: The steady state amount of a substance taken up and retained by an organism through all routes of exposure.
Bioassay: The series of tests on aquatic animals to determine the concentration of a substance at which toxicity occurs.
Bioavailability: The ability of a chemical entity to gain entry into an organism by means of being transported through a membrane or the ability of a chemical entity to adversely affect the performance of an external membrane (gill) by being strongly adsorbed to it. The divalent hydrated ion is the only confirmed bioavailable species of nickel.
Biomagnification: The process by which tissue concentrations of a bioaccumulated substance increase as the substance passes up the food chain through at least two trophic levels.
CANMET: The Canada Centre for Mineral and Energy Technology.
Chronic Response: A toxic response following relatively low exposures over an extended period of time. The response is characterized by slow toxic onset and usually long continuance.
Critical Surface Area Loading (CSAL): The surface area of solid particles per litre of solution that will generate the L(E)C50 of the bioavailable metal ion as determined separately from one of the metal's soluble salts.
E(C)Dx: The effective dose (or concentration in water) at which X % of the animals tested show the specific toxic endpoint being investigated.
Ecotoxicology: The study of health hazards pertaining to individual organisms, communities or populations of organisms existing within the aquatic and terrestrial environment.
EDTA: Ethylene Diamine Tetra-acetic Acid. This is a strong complexing agent for nickel.
EPA: U.S. Environmental Protection Agency.
Essentiality: The use of an element that an organism needs in order to live in a healthy state. Nickel is known to be essential in plants and some microorganisms; it has a probable essential function in mammals, including humans.
Hardness: In general, this is a measure of all metallic cations, except the alkali metals (e.g., Na , K ), present in solution. Operationally, it is the concentration of Ca plus Mg in solution, expressed frequently as mg CaCO3/L.
Homeostasis: A collection of processes that regulates the body burden of an essential element. Ionic Strength: A measure of the interionic effect resulting primarily from electrical attractions and repulsions between various ions in solution.
Kow: The octanol-water partition coefficient, used to determine the lipidphilicity of a substance.
L(C)Dx: The lethal dose (or concentration in water) at which X % of the animals tested show mortality.
Ligands: Chemical entities in solution that bind with metal ions to transform simple metal ions into complex ions or precipitates.
Loading: The weight of a substance put into a given volume of water.
LOAEL: The Lowest Observed Adverse Effect Level (concentration or dose).
h.t. NiO: The high temperature form of nickel oxide.
NiPERA: Nickel Producers Environmental Research Association.
NOAEL: The No Observed Adverse Effect Level (concentration or dose).
NOEC: No Observed Effects Concentration.
OECD: Organization for Economic Cooperation and Development.
Persistence: The ability of a chemical entity to exist in the environment over an extended period of time.
pH: The negative logarithm of the H concentration. Distilled water has a pH of 7.
Phytotoxicity: Toxicity to a plant species.
Risk Assessment: The process of quantifying the probability of a hazard being realized.
Solubility: The thermodynamic (equilibrium) (steady state) amount of a substance in solution that coexists with the solid (or liquid) phase of the substance.
Speciation: The occurrence of an element in separate, identifiable forms (i.e., chemical, physical or morphological state).
Terrestrial Toxicity: Harmful effects for animals living within the soil and for plants receiving nutrients from the soil.
Threshold: The dose at which a toxic response begins to occur.
Tolerance: The ability of an organism to withstand an increasing exposure without suffering adverse health effects.
Transformation: The reactions through which a non-bioavailable entity becomes bioavailable within a particular medium. This concept is important for metals and sparingly soluble inorganic substances which may slowly react with the medium to release a bioavailable ion.
