Properties and Applications

Nickel is a silver-colored metal that is referred to as a heavy metal due to its high density. It is more resistant to corrosion than other metals and is therefore used in many alloys to improve this property. It is also used extensively in technology as a surface coating; nickel is normally applied by electroplating. Nickel-cobalt-manganese-lithium mixed oxides are used in batteries (accumulators, rechargeable batteries).
Finely dispersed nickel can be self-igniting (pyrophoric), which is why it must be stored as nanoparticles under inert gas.

Nickel in everyday applications

Nickel is found in the form of alloys in coins (a "nickel"), eyeglasses and fashion jewelry (e.g. formerly: nickel glasses or wristwatches, today reduced due to allergy risk), magnets (alnico), guitar strings, microphone capsules, fittings coatings, for the green coloring of glass, and in ceramic pigments.

In the light of the energy transition, the most discussed application of nickel at the moment is probably in the cathode materials of rechargeable batteries. In current research work, nickel and other metals are hidden in abbreviations such as NMC 6:2:2, in the example it is a mixed oxide of lithium, nickel, manganese and cobalt, the numbers indicate the quantity ratios of the metals (without lithium), i.e. NMC 622 means that there are 2 atoms of manganese and 2 atoms of cobalt for every 6 atoms of nickel. Common ratios are, e.g:

• NMC111: LiNi_1/3Mn_1/3Co_1/3O₂
• NMC622: LiNi_0.6Mn_0.2Co_0.2O₂
• NMC811: LiNi_0.8Mn_0.1Co_0.1O₂

Class 1 nickel, which has a purity of more than 99%, is usually used for battery materials . More recent developments have centered on higher nickel contents, e.g. NMC811 in the cathodes, in order to reduce dependence on cobalt.

Nickel is mainly used to improve the quality of steel. V2A stainless steel, which is widely used around the world, contains 8% nickel. V4A steel with a nickel content of 11% is better known in everyday life under the brand names Cromargan or Nirosta. In cutlery labelled with the trade name "nickel silver", up to a quarter of the weight is nickel. It is used here to maintain the luster of the new cutlery over a long period of time because the nickel prevents surface corrosion (tarnishing). Surgery instruments can also be made of nickel-alloyed steel. Nickel plating is used to protect everyday objects made of steel and other metals from corrosion. This is done by electroplating, an electrochemical process.

Constantan, a copper-nickel alloy, is used for electrical devices that require particularly precise resistances that also need to remain constant over a wide temperature range.

Nickel in the industry

Steels containing nickel (see above) and nickel-plated appliances, fittings and utensils are particularly common in laboratories and in the chemical industry (nickel spatulas). The increased corrosion resistance of nickel is utilized here. Monel metal is another nickel alloy made from nickel, copper and iron, whose special resistance to fluorine gas is used in the manufacture of gas cylinders.
Nickel-based superalloys are used for highly stressed machine parts, which can also withstand the loads in aircraft turbines and gas turbines.
Nickel is used in a large number of catalytic processes and is one of the most important catalyst materials in the chemical industry . Raney nickel is a nickel-aluminum alloy that is used for the catalytic hydrogenation of organic compounds with hydrogen. A practical application of Raney nickel and other nickel-based catalytic processes, which also reaches the household dining table, is the hydrogenation of unsaturated fatty acids in vegetable oils during margarine production, known as fat hardening. This is necessary because although vegetable oils are produced relatively cheaply and with a low CO₂ footprint, they are not suitable as a butter substitute due to their liquid form. The challenge in fat hardening is to produce as few trans fatty acids as possible, which are produced as a by-product. They are considered to be a contributory cause of cardiovascular disease - in contrast to the cis fatty acids that occur naturally in vegetable fats.
Nickel compounds, often together with other components, are the focus of research work on hydrogen storage materials, including metal hydrides (for example as a nano-catalyst with zinc boranate (Zn(BH₄)₂) or magnesium hydride (MgH₂) or as a nickel-containing hydrogen compound nickel aluminum hydride (NiAlH₄) .

Natural Occurence of Nickel

Nickel ores, i.e. mineral deposits of nickel that are industrially exploited, can be found in Indonesia (33.6% of global production in 2019), the Philippines (12.7%), New Caledonia (8.3%), Canada (7.4%), Australia (6.3%) and other countries . It is mined there, often in open-cast mines, which causes massive damage to the landscape. If nickel is extracted by underground mining, the impact on the landscape is less, but the overburden (i.e. the rock that is produced when the nickel ore is excavated, but which itself contains no or too little nickel to be processed further) must be accommodated.
The most important minerals with a nickel content worth mining are pentlandite (also known as nickel magnetic pyrites) and various silicate minerals with different nickel contents such as garnierite, népouite, nontronite and nickel-bearing goethite. In addition to nickel, these minerals also contain other elements such as iron .
In addition to the deposits of nickel that are used to produce the metal, nickel is also present in traces in plants and microorganisms. Here nickel plays an important role in enzymes (e.g. in ureases, Co-F430). In mammals, however, no evidence of nickel-containing enzymes has been found to date .

Production of Nickel

Nickel metal is obtained from ores. In addition to nickel, these often contain iron and copper. During nickel extraction, iron is removed as an unwanted component in the form of iron oxide or iron silicate slag using metallurgical processes. This produces a copper and nickel containing intermediate. In contrast to the formerly removed unwanted iron, copper is not thrown away due to its relatively high value. Instead, the two metals are separated from each other in a further high-temperature process and processed further, producing crude nickel. It is used directly in steel production. The crude nickel is then refined by electrolysis into pure nickel with a standard purity of > 99.8%, which is already suitable for many applications, e.g. batteries. If even purer nickel is required, the pure nickel can be refined to ultra-pure nickel using the Mond process. It then consists of more than 99.99% nickel.

To produce nickel nanoparticles, nickel compounds of oxidation state +2 are usually reduced in solution to nickel metal in a carefully controlled process. To prevent the nickel nanoparticles from forming larger aggregates again immediately after their formation, a surfactant, i.e. a type of soap, is added, which prevents the nanoparticles from forming aggregates by attaching to one another .

Production

Nickel is mined both by open pit and deep mining. The main producing countries are Indonesia, the Philippines and Russia (see also https://de.statista.com/statistik/daten/studie/37062/umfrage/produktion-von-nickel-weltweit-nach-laendern/). The global demand for nickel is constantly increasing, e.g., for stainless steel and in cathodes of lithium-ion batteries. In addition to traditional extraction from onshore ores, future plans therefore include extraction from manganese nodules, which often contain copper, cobalt, gold and silver in addition to manganese and nickel. Manganese nodules are located in various areas on the seabed and their extraction would destroy a sensitive and largely unexplored ecosystem.

Rocks from nickel ore. @Adwo-stock.adobe.com

Mining on land is associated with environmental impacts due to land consumption, especially in open pit mining. Compared with other metals, nickel mining produces little overburden because some of the slag can be used as a raw material in road construction. This is positive because no land is required for the storage of the overburden.

Resource consumption during processing

For nickel production, natural resources are consumed through the loss of forest areas and the associated biodiversity. Depending on the source mineral, energy consumption during smelting can be very high.

Safety

Nickel can cause contact allergy in humans, which is why its use in jewelry, toys, cutlery and pots etc. has been reduced by law. Inhalation of dust containing nickel with nickel particles smaller than 1 mm can trigger lung carcinomas in humans, which is particularly relevant for occupational safety. Nickel compounds and nickel salts sometimes have a higher toxicity than pure nickel. For aquatic organisms such as water fleas and fish, nickel in fine powder form is also toxic, while it is an essential element for plants.

Emissions

Nickel mining produces slurry, which is often retained by technically inadequate dams ("slurry ponds"). There have been many accidents here in the past due to leakage. Deep mining produces mine water, which can contain acids and heavy metals. For every ton of nickel mined, 10-50 tons of slag are produced during smelting, some of which is used as raw material in road construction. In the past, nickel smelting released large quantities of SO₂, which contributed to the so-called "acid rain". Today, SO₂ is no longer released due to filter systems. In the vicinity of some mining sites, dust containing heavy metals affects the vegetation.

Depending on the starting material, 13-45 kg of CO₂ are emitted per kg of nickel product, 68-87% of which is emitted during smelting, the remaining 13-32% for mining, processing and transport.

Social Aspects

At some nickel mining sites, there are land use conflicts with the indigenous population. According to the Worldwide Governance Indicator, 76% of nickel comes from countries with medium to weak governance.

Value creation is good for most mining countries, as not only mining but also processing takes place in one country. This keeps profits in the country and generates revenue in the form of taxes.

For responsible metal production, due diligence in supply chains is becoming increasingly important in the EU. This also includes the documentation or certification of social standards and occupational health and safety.

Disposal and recycling

Stainless steel is a durable product and can be recycled. The end-of-life recycling rate for nickel is 68%. Around 15% of the recycled nickel is used in carbon steels with no functional use (so-called downcycling), while a further 17% ends up in landfills with nickel-containing products.

Overall, the recycling rate for nickel is higher than for most other metals, with around 57% of the nickel mined since the 18th century still in use. The European Battery Regulation sets a recycling rate of 90% for nickel in 2027.

How can Nickel be made more sustainable?

A further increase in the recycling rate from batteries and accumulators as foreseen in the European Battery Regulation would contribute to a reduction in the mining of nickel, and hence reduce the associated environmental impacts.

Nickel is a very versatile element that is used in various chemical forms (metal, alloy, oxide and other compounds) in different products. Due to its widespread use, exposure can occur mainly in the workplace, but also through skin contact with consumers.

Everyday Contact

It has been known for a hundred years that nickel leads to changes in the skin on intensive contact . For this reason, its effects have been investigated in various forms and compounds in the workplace and among users over the past decades. It has been known since then that nickel in metallic alloys can trigger contact allergies (contact dermatitis) on physical contact. Nickel is the most common of over 2,000 contact allergens and the prevalence of nickel allergy is between 8 and 19% within the European population .

For this reason, the EU has set very low limits for the use of nickel wherever nickel could be released as an ion (e.g. cutlery, piercing jewelry, spectacle frames, buttons and zippers, toys) [Ahlstrom et al., 2019], which, however, do not seem low enough for very sensitive people . Nevertheless, there are always new products and developments that provide a new source of exposure, such as cell phones and their metallic edges or buttons or euro coins . However, tobacco products and e-cigarettes are also a source of nickel exposure.

Innovative products such as batteries or fuel cells, on the other hand, are hardly a source for consumers to come into contact with nickel.

Situation at the Workplace

Rolls of sheet metal in a warehouse for aluminium material @ amorn-stock.adobe.com

There have been many studies on nickel and its compounds over the past decades. Without repeating this multitude of study results, only a few representative studies will be quoted here. These studies make it clear that unprotected work in dust-laden workplaces where nickel is processed can lead to various biological effects. Probably the most important effect is the direct triggering of inflammatory processes in the lungs. These are based on oxidative stress, but also include interaction with DNA and DNA repair enzymes . This can cause severe organ damage and even tumor formation. As soon as particles consisting of nickel or its compounds dissolve in the lungs, nickel ions can easily enter the body and thus also the bloodstream and reach internal organs.

Two studies from India recorded the connection between nickel or chromium exposure and the semen quality of male workers or possible DNA damage in white blood cells . The effect on DNA is intensified by the fact that the DNA repair mechanisms of the cells are also inhibited by nickel ions and thus damage persists longer . A retrospective study of retired workers at a nickel refinery showed that workers exposed to nickel had an increased rate of chromosomal defects . Studies on the genotoxicity and carcinogenic effects of nickel, including Ni-particles of different sizes, have been well summarized in various reviews . Based on many studies, the World Health Organization (WHO) has classified nickel and its compounds as carcinogenic since 1973. Currently (2023), nickel in its compounds (e.g. NiO etc.) is classified in Group 1 (CLP – Carc 1A: carcinogenic) in the EU and in Group 1 (IARC Group 1 "carcinogenic to humans") by the WHO [WHO, 2012]. This effect is mainly associated with uptake via the lungs, whereas oral uptake is not associated with a tumor-causing effect. As a metallic particle, nickel is classified as Carc 2 (suspected carcinogenicity to humans) in the EU and as IARC Group 2B (possibly carcinogenic) by the WHO.

Products and Consumer

Nickel acts in humans mainly in its ionic form. It must therefore be released from its metallic form from alloys or coatings and other applications. This usually occurs on the skin via sweat, e.g. from jewelry, piercings, coins and cell phone surfaces, other nickel-containing products or from inhaled dust particles in the lungs . The known toxic effect of nickel as a contact allergen and the carcinogenic reaction in the event of long-term exposure via the lungs have consequently led to strict limit values and safety measures in the workplace, but also for consumers.

The release of nickel nanoparticles into the environment occurs at various stages of their lifecycle, including manufacturing, usage, disposal, and accidental spills. Here, aquatic organisms are particularly at risk, as nickel is introduced into aquatic ecosystems by leaching. Furthermore, nickel nanoparticles can interact with other pollutants present in the environment.

Nickel mining, Philippines ©MARYGRACE - stock.adobe.com

Release of nickel

Nickel is released into the environment through various man-made sources, such as ore mining, combustion processes and disposal of nickel-containing products. This occurs in the form of various nickel compounds, including sulphides, oxides, silicates, soluble compounds, and, to a lesser extent, the element nickel itself. Nickel and nickel compounds can be found in the ambient air, drinking water, food, and tobacco products .
In respect to soil, mining is a major source of anthropogenic nickel contamination due to the release of nickel and other metals during the extraction process. Contaminated soil can be washed out by rainwater or surface water and enter the aquatic ecosystem .

During the use phase, nickel can be released from various application (read more at “Material Information”) into the air due to wear effects. Since the disposal is sometimes not handled properly, nickel can also be released in the soil through leaching from landfills. Ultimately, nickel enters the aquatic environment through rain or groundwater .
Inefficient vehicle engines, owing to their lubricants, also emit nickel during transportation. Additionally, nickel plays a role in several applications, including oil refining, cryogenic containers, pollution control equipment, and plumbing materials.
There is a potential for nickel to seep into drinking water as pipes and other materials corrode, although these leaks are typically minor. Other sources of nickel exposure include nickel alloys and items plated with nickel, such as steel, coins, and jewellery. Residual traces of nickel may also be found in soap, oils, and fats .

Released amount of nickel

Nickel is predominantly released into the atmosphere through human activities, with approximately 1.4–1.8 times more emissions generated by humans than by nature. In the 1980s, fossil fuel combustion accounted for 62% of anthropogenic atmospheric nickel emissions, totalling 570,000 tons worldwide in 1999. Of this, electric utilities contributed 326 tons, municipal incineration 12%, nickel-metal refining 17%, steel production 3%, and other nickel-containing alloys 2%.
In water, aside from industrial processes like mining and smelting, nickel can be found in industrial wastewaters, effluents, domestic wastewaters, and landfill leachate. Seawater contains approximately 0.5–2 mg/L of nickel, while rivers hold about 0.3 mg/L of nickel species. The recommended acceptable limit for nickel in drinking water, according to the European parliament, is 20 µg/L (DIRECTIVE (EU) 2020/2184).
Groundwater with a pH below 6.2 can have dissolved nickel concentrations as high as 0,98 g/L. Urban storm runoff water samples have shown nickel levels ranging from <1–87 g/L.
Nickel levels in soil primarily originate from anthropogenic sources like metal manufacturing waste, commercial waste, fallout, sludge, coal fly ash, coal bottom ash, mining, and smelting. In 2002, for example, it was estimated that 5530 metric tonnes of nickel and 14,800 metric tonnes of nickel compounds were released into the environment from US manufacturing and processing facilities, representing about 82% and 87% of the estimated total nickel released into the environment. Although the issue related to nickel pollution is most pronounced in urban areas, it also impacts agricultural soils due to reduced soil liming and the effects of acid rain from industrialization .

Nickel and nickel compounds are ubiquitous in the environment, and human activities result in their release into all environmental compartments.

Primary sources of nickel intake are food and, at a low level, air and drinking water. In general, however, nickel can also be released from products, e.g. from nickel-containing steels in cooking pots or from buttons, jewelry and toys. Therefore, the skin, the gastrointestinal tract and, in the workplace due to nickel-containing dusts, the lungs are the potential routes of absorption.

Uptake via the Lung

Nickel-containing dusts play an important role, especially in the workplace, when considering the potential effect of nickel and its compounds in the lungs. Due to the classification of nickel in its compounds as clearly carcinogenic when ingested via the lungs (IARC Group 1 "carcinogenic in humans") and metallic nickel particles in IARC Group 2B ("possibly carcinogenic"), the limit values at the workplace are also correspondingly low. Personal protective measures are prescribed by law.

The limit values in Europe and abroad are slightly different from each other [ECHA, 2018] with Germany has the lowest value of 6 µg/m³ for metallic nickel in the A-dust fraction (for dust fractions, see also Granular biopersistent dust particles). However, acute exposure to nickel can also lead directly to death, as a well-documented case in South Africa showed . In this case, a male employee was engaged in vaporizing turbine blades with nickel. The 38-year-old was neither well instructed nor did he use sufficient protective measures and had taken off his breathing mask several times during the 90-minute process. He consulted a doctor the following day because of breathing problems, was admitted to hospital 4 days later and nevertheless died of "acute respiratory distress syndrome (ARDS)" 13 days after exposure. In his urine, 780 µg/l nickel could be detected and also in the lungs, many nickel oxide nanoparticles, which were used during vaporization, were found in macrophages.

Tobacco products can contain relatively high amounts of nickel of 1.7 to 4.2 µg/g. This means that the lungs of smokers are additionally exposed to nickel through the condensate from tobacco smoking . However, both studies come to slightly different conclusions with regard to uptake into the human body. While the Norwegian study found no difference in the amount of nickel in the blood and urine between smokers and non-smokers in the occupationally exposed group, but nevertheless found an increased risk of cancer in the smoking employees , the second study describes a clear difference in the nickel content in the urine. While no difference could be detected in the blood between smokers and non-smokers, the nickel level in the urine of smokers was up to twice as high as that of non-smokers . In general, however, these studies showed an increased risk of lung disease in all cases of double exposure to nickel and tobacco smoke.

Uptake via the Skin

Skin rash as a reaction to nickel jewellery  ©HASPhotos-stock.adobe.com

Metallic nickel plays an important role on the skin and is one of the most common contact allergens we know of. The most common sources of nickel exposure are jewelry (e.g. costume jewelry, especially ear studs and piercings), but also zippers, buttons, tattoos (nickel-containing tattoos are not permitted in Germany but still many tattoo inks contain nickel ), toys, cell phones and coins. Nickel is released from the alloys through sweat on the skin and can penetrate deep into the skin layers in its ionic form and thus reach living immune cells. There it unfolds its allergizing effect as a so-called "hapten". Ear studs or piercings should be viewed particularly critically here, as contact with blood is possible. Blood can dissolve nickel ions faster than sweat. For this reason, the limit values for nickel release from piercings and ear studs are significantly lower (0.2 µg Ni/cm²/week) than those for other consumer products or cell phones (0.5 µg Ni/cm²/week) . However, these limits can be considerably exceeded, as studies on euro coins (>100 µg Ni/cm²/week), cell phones (20 µg Ni/cm²/week) and metal construction kits have shown

Uptake via the Gastro-Intestinal Tract

Nickel is usually absorbed via the gastrointestinal tract through food or drinking water. However, the levels of nickel in food and drinking water in Europe are so low that this route is unlikely to pose a risk to humans. An amount of significantly more than 100 mg/kg body weight per day would be acutely toxic. However, the sensitizing effect of nickel plays a role in chronic intake. The European Food Safety Authority (EFSA) therefore set an acceptable daily intake (TDI) of max. 13 µg/kg body weight and day for adults and 11 µg/kg body weight for adolescents . The nickel absorbed into the body is excreted via the urine. A reference value of 3 µg nickel/l urine has been described in several studies for uncontaminated humans, so that a higher excretion can be assumed to be an indication of a particular exposure.

Nickel usually enters the human body via food, but the level of exposure in industrialized countries is very low and poses no risk. However, smoking can significantly increase the intake and workplaces are also contaminated, so that the limit values for nickel and its compounds have been set very low due to the carcinogenic effect of nickel

Nickel, in its ionic form, is a well-known toxicant and classified as hazardous for the aquatic environment. The toxicity of nickel nanoparticles and nickel compounds may differ from that of the ionic form or bulk nickel due to differences in particle size, surface reactivity, and potential for cellular uptake.

Release of nickel in environmental organisms

Nickel is an essential micronutrient for animals and plants, playing a vital role in cellular redox reactions and overall development. However, when the concentration of nickel exceeds permissible limits, it disrupts various cellular components, potentially leading to damage. Most nickel compounds, except nickel carbonyl, are relatively non-toxic when ingested due to poor gastrointestinal absorption. Nickel's bioavailability is influenced by food consumption and is excreted rapidly in urine, following a non-dose-dependent pattern. Studies in hens show that nickel exposure reduces levels of magnesium, manganese, and zinc in various tissues, with nickel and zinc exhibiting antagonistic effects in animals.
Regarding the effects of nickel and nickeloxide nanoparticles towards environmental organisms, ingestion and uptake by different environmental organisms (animals like zebrafish, amphipods or water flea as well as plants like barley, cabbage or alga) have been observed. Organisms can accumulate nickel nanoparticles in their tissues, leading to concerns about potential accumulation and therefore higher concentration of nickel in animals and plants.

Toxicity of nickel in environmental organisms

Nickel and nickel oxide nanoparticles were studied regarding their effects in several animal and plant species, including worms, insects, amphipods, sea urchins, fish, green algae, cabbage and yeast. Several life stages were considered. The most common mechanisms that were observed are genotoxic action as well as the induction of oxidative stress, leading to systemic effects such as organ damage and reduced growth in animals like zebrafish, amphipods or fish and inhibition of photosynthesis in plants like duckweed or barley.
Overall, nickel ions are thought to be responsible for the toxic effects. Whether there are also specific effects of nickel nanomaterials is discussed somewhat controversially. Some studies report on a specific, mostly higher toxicity of nickel nanomaterials . However, a recent review on nickel nanoparticle toxicity in aquatic organisms reveals inadequate data quality compared to studies on nickel ions. Some common findings about nanomaterial toxicity don't apply to nickel nanoparticles. Nickel and nickel oxide nanoparticles generally show similar toxicity despite different compositions, and toxicity doesn't increase with smaller particle size. Nickel nanoparticles isn't more toxic than nickel ions on a mass-concentration basis. However, toxicity mechanisms of nickel nanoparticles align with oxidative stress seen in other metal-based nanomaterials.
For some groups of organisms, however, nanoparticle specific effects have been observed. For example, zebrafish embryos showed higher toxicity to dendritic nickel nanoparticles compared to other sizes. Amphipods exposed to nickel oxide nanoparticles accumulated higher levels of nickel in their tissues, suggesting potential risks for higher trophic levels. Copepods were more sensitive to dissolved nickel ions in comparison to nickel nanoparticles, and the presence of surfactants influenced the toxicity of nickel nanoparticles to water flea. Plant roots have been demonstrated to take up nanoparticles which entered the soil.

The studies also highlight that factors, such as exposure duration, long-term retention of nanoparticles in organisms, and specific nanoparticle shapes play significant roles in determining the overall toxicity. The toxicity of nickel and nickel oxide nanoparticles may lead to cellular oxidative stress, alterations in antioxidant enzyme activities, and damage to various organs and tissues. Also, effects such as altered behaviour, reduced growth, and reproductive effects have been observed .

Assessment of the effects of nickel in environmental organisms

The studies on the toxicity of nickel and its compounds, as well as nickel and nickel oxide nanoparticles have revealed varying levels of toxicity and potential hazards to different organisms. Concerning nickel nanoparticles, studies emphasize the importance of considering specific physicochemical characteristics of nanoparticles in toxicity assessments. Nickel in its various forms of appearance is ubiquitously distributed in the environment. In most places, the concentrations are in ranges that are well tolerated by animals and plants. However, in specific areas such as mining sites or landfills, high nickel concentrations damaging organism occur. The sensitivity of different organisms towards nickel, however, differs.
In terms of relating the determined toxicity values to actual or modelled environmental concentrations, the EU has set up annual average concentrations for bioavailable nickel under the Water Framework Directive which is limited to 4µg/L (Environmental Quality Standards for Priority Substances listed by ECHA).

In conclusion, the extensive body of research on the toxicity of nickel across various organisms and ecosystems highlights the complex and diverse nature of their effects. Nickel can induce oxidative stress, damage cells and tissues, and disrupt vital physiological processes in a range of species, from aquatic organisms like zebrafish, amphipods, and copepods to terrestrial organisms like plants, worms, and even yeast. The specific mechanisms of toxicity as well as sensitivity varies among organisms. Regarding the understanding of nickel and nickel oxide nanoparticles, results on specific effects are inconclusive, and needs detailed knowledge of properties, environmental behaviour, and composition.

Even at low doses, nickel has effects both on the skin and after absorption through the lungs or in the body. Because nickel in its compounds has been proven to be carcinogenic, the limit values are set very low, both for the workplace and for consumers.

Distribution and Effects in the Body

Metallic nickel or metallic nickel particles are less biologically available to the organism than ionic nickel compounds (e.g. nickel oxide NiO or nickel chloride NiCl₂). However, nickel only acts in the body when it is dissolved out of the material or molecule as an ion (Ni²⁺). This happens much more slowly or weak from metallic material than from its compounds in which the nickel is already ionically present, such as in nickel oxide (NiO). Nickel oxide is also very frequently used as nanoparticles and is therefore an important starting material for toxicological studies. If nickel is used as a salt (nickel chloride), it is easily soluble and is available as an ion. A comparative study on hamsters was able to show that nickel chloride applied to the skin enters the body where it is distributed and excreted in the urine . However, this occurs considerably more slowly compared to intramuscularly injected nickel chloride, which was largely excreted within 24 hours.

Although the basic statement on the uptake and distribution of nickel in the organism is consistent, this early study has a number of methodological shortcomings. Another exemplary study on NiO nanoparticles was conducted 20 years later and produced similar results in principle . The NiO particles orally administered to the rats were absorbed into the organism and distributed in the organs. As has already been shown for other nanoparticles, the nickel oxide particles accumulate primarily in the liver. However, other organs such as the kidney and spleen also showed significantly increased levels of NiO after 24 hours. An important finding is that at high concentrations, DNA damage in the liver and kidneys could already be detected after a short time. Similar effects caused by various nickel compounds (Ni₃S₂, NiO, NiCl₂ and NiSO₄) were found quite early on in workers in a nickel factory . At that time, the air at the workplace was contaminated with more than 1 mg/m³ and DNA strand breaks were increasingly detected. This is consistent with later studies in which an increased frequency of micronuclei (an effect that indicates DNA damage) and DNA strand breaks occurred in employees at highly polluted workplaces . These and many other studies on the effects of nickel oxides on animals and cells were summarized in a very good review article . After inhalation of NiO and Ni(OH)₂ particles, inflammatory processes and oxidative stress were always detected in rats and mice. Furthermore, systemic effects were subsequently observed in other organs such as liver, kidney, spleen and others. The same effects were observed after injection into the abdominal cavity or after oral intake, which means that nickel has similar effects when taken up via all routes. However, the accompanying genotoxic effects, which can also explain the carcinogenic effect, are always particularly serious.

Uptake and Effects in Cells

The uptake of nickel in its compounds was comprehensively described and summarized several years ago . This article lists many studies that have shown the uptake of different nickel compounds and particles, as well as the intracellular conversion of the particles into dissolved Ni ions in the lysosomes. The fact that both metallic and oxidic nickel can be dissolved in lysosomes was demonstrated by a research group in Sweden . Metallic nano- or microparticles as well as NiO nanoparticles dissolved in artificial lysosomal fluid, the metallic ones even to almost 100% after 24 hours. The dissolved nickel ions can then interact with different molecules in the cell and also with the DNA in the cell nucleus. An important reaction is the inhibition of the repair of DNA damage, which has been demonstrated in various cells . The good absorption into the organism and into the cells of various organs, the rapid dissolution of even poorly soluble particles in the lysosomes of the cells and the interaction with proteins (contact allergy) and DNA and its repair mechanisms (cancer induction) makes nickel a potential risk in the workplace and also for consumers who are exposed to sufficiently high concentrations of nickel.

Nickel is a toxic metal that is carcinogenic in its compounds and causes contact allergies on the skin. For this reason, very low limit values have been set for nickel and its compounds in order to protect both workers and consumers.

Nickel is widely distributed throughout the environment. Natural nickel sources are released by geological processes (e.g. erosion, deposition or shifting of the earth's plates) into soils and sediments, while anthropogenic activities such as industrial emissions and combustion processes introduce nickel into the air and water. In the atmosphere, nickel exists primarily as particulate matter. In aquatic systems, the behavior and bioavailability of nickel are influenced by factors like pH, redox potential, and the presence of organic ligands, impacting its speciation and potential environmental impact.

Transport

Various geological processes continuously redistribute nickel among the land, water, and air. In the atmosphere, nickel is present at relatively low concentrations. Nickel released into the atmosphere primarily takes the form of particulates, with varying diameters depending on their source. Anthropogenic nickel particulates typically range in size from 0.1 mm to 2 mm, while those originating from natural sources have diameters ranging between 2 mm to 10 mm. Fine nickel particulates, measuring less than 10 mm, remain suspended in the atmosphere for a longer period, typically 5–8 days, and can be transported over greater distances compared to larger nickel particulates.
These nickel particulates disperse through the action of wind, and they settle from the atmosphere through gravitational settling (for particulates larger than 5 mm) or dry and wet depositions (for particulates smaller than 5 mm).
Notably, nickel particulates do not absorb infrared radiation, which means they are unlikely to contribute to global warming or stratospheric ozone depletion .

Transformation

In general, numerous metal oxide engineered nanoparticles exhibit rapid aggregation to the micron scale when introduced into seawater, leading to their sedimentation from the water column. As a consequence, nanomaterials have the potential to accumulate within sediments. Once deposited in sediments, they may interact with particulate organic matter, becoming sequestered, and subsequently undergo physical and biogenic processes such as bacterial decomposition or bioturbation . For nickel nanoparticles, sediment was identified as a significant sink, primarily due to their aggregation, binding, and deposition by mussels or oysters . Understanding the fate and transport of nickel nanoparticles is crucial, considering their potential accumulation in sediments and the potential for transfer into sediment organisms.
With respect to the atmosphere, our understanding of the chemical forms and the chemical and physical transformations that nickel undergoes remains limited. It is commonly assumed that nickel from anthropogenic sources, particularly those originating from combustion processes, is primarily in the form of nickel oxide. Conversely, windblown dust particles are likely to contain mineral sulfide species of nickel. Additionally, it is probable that nickel sulfate is formed in the atmosphere through the oxidation of nickel in the presence of sulfur dioxide.
Nickel in aquatic systems is mostly found as soluble salts, often linked to particulate matter, suspended solids, or organic material from biological sources. Its behaviour in water is determined by processes like adsorption, precipitation, and complexation, influenced by factors like redox potential, pH, ionic strength, metal and ligand concentrations, and solid surfaces.
Under anaerobic conditions, nickel levels are low due to the formation of nickel sulfide. In aerobic conditions and pH below 9, nickel forms soluble compounds, maintaining nickel ion concentrations above 60 mg L-1. In natural waters (pH 5-9), the dominant form is a hydrated nickel ions.
Some nickel compounds are highly soluble (e.g., nickel chloride and nickel sulfate), while others have lower solubility (e.g., nickel nitrate and nickel hydroxide). Nickel carbonate is the least soluble and hydrated nickel ions are the most common nickel compound in water.
Most of the released nickel accumulates in soil, strongly binding to various species like amorphous iron and manganese oxides, along with other minerals. Soil properties such as bulk density, pH, texture, organic matter, clay minerals, and hydroxides, as well as groundwater flow, influence nickel adsorption, making it site-specific.
In acidic soil, nickel is more mobile, whereas in alkaline soil, its adsorption is irreversible, limiting its availability and mobility .

Nickel is distributed widely in all environmental compartments. Depending on the prevalent conditions, nickel occurs in various forms, which govern the further fate. Further research is needed to comprehensively evaluate the ecological implications of nickel nanoparticles in particular, and their fate in aquatic ecosystems, taking into account different environmental media and organisms with varying ecological traits.