Palladium is a shiny, silver-white metal that belongs to the platinum group elements.
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It is widely used in the automotive industry, especially in catalytic converters for car exhaust systems. Additionally, palladium serves as a catalyst in various chemical reactions within the chemical industry. It is also popular in jewellery making and has several medical applications. One of palladium’s unique properties is its remarkable ability to absorb large amounts of hydrogen, much like a sponge. Furthermore, being a precious metal, palladium is chemically very stable.
How could I come in contact with it?
The largest source of palladium in the environment primarily comes from catalytic converters in vehicles. As hot exhaust gases pass through these converters, some palladium is released into the gas stream and enters the environment with the exhaust. As a result, elevated concentrations of palladium can be detected on busy roads.
Additionally, jewellery manufacturers and dentists use palladium, often as an alternative to or together with platinum, in jewellery and dental implants that are in close contact with the body.
How dangerous is the material for humans and the environment?
Palladium is a precious metal that does not dissolve when it comes into contact with bodily fluids, such as sweat and blood. For this reason, experts consider it safe to use in jewelry and implants. However, palladium is often found in high concentrations along busy roads because it is released from catalytic converters in cars. This can be harmful to plants and certain animal species. A decline in combustion engines could help reduce this issue.
Conclusion
Palladium is harmless to humans when used in jewellery or as an implant material. However, microorganisms and plants on roads could be damaged by car exhaust emissions.
By the way…
In 1989, two researchers generated significant hype by claiming to have discovered cold fusion using a palladium catalyst. They believed they had demonstrated that energy could be produced by fusing hydrogen atoms at room temperature, which would have potentially solved all the Earth’s energy problems. In nature, the nuclear fusion process occurs inside the sun at temperatures of several million degrees. Human-built fusion reactors, currently being tested worldwide, aim to replicate these solar conditions. Unfortunately, the researchers’ measurements could not be confirmed and were likely the result of measurement errors.
Palladium is a shiny, silvery-white transition metal considered the lightest of the platinum group elements (ruthenium, rhodium, palladium, osmium, iridium, and platinum).
It was discovered in 1802 by British chemist William Hyde Wollaston and was named after the asteroid Pallas and the Greek goddess Athena. With an average concentration of only 0.015 ppm, palladium is one of the rarest metals on Earth. Due to its properties, it can be used in many technologies and applications.
Palladium is primarily used in the automotive industry. It is utilised either on its own or in combination with platinum and/or rhodium in three-way catalytic converters. As a catalyst, it converts harmful exhaust gases, such as unburned hydrocarbons, carbon monoxide (CO) and nitrogen oxides (NOx), into less harmful substances. In 2023, this sector was responsible for about 80% of global palladium consumption .
Palladium can absorb large amounts of hydrogen (H₂) or deuterium (D₂) and release them again, but not other gases. At ambient temperature and pressure, palladium can store up to 900 times its own volume of hydrogen. The adsorbed hydrogen is released again and this process can be repeated indefinitely. Due to this property, it is also metaphorically called as a ‘metallic sponge’. Palladium is therefore often used as a gas separation membrane in the form of a thin metallic layer on a porous carrier material to separate hydrogen from gas mixtures or to ‘store’ it ..
Palladium remains an important metal catalyst in the chemical industry because it facilitates hydrogenation and dehydration reactions. In the food industry, it is used for fat hardening or fat hydrogenation. This process converts unsaturated fatty acids and oils into margarine, which is used as a substitute for butter .
In 2010, Richard F. Heck, Ei-ichi Negishi and Akira Suzuki were awarded the Nobel Prize in Chemistry for their pioneering research into palladium-catalysed cross-coupling reactions in organic synthesis. These reactions combines two different chemical fragments, simplifying the production of complex molecules, especially in the synthesis of natural products and drugs. A practical example is discodermolide, which was originally extracted from a sea sponge (Discodermia dissoluta). This substance shows potential efficacy against cancer cells . Thanks to palladium-catalysed cross-coupling reactions, this molecule can now be produced in large quantities in the laboratory. This advancement enables researchers to test it extensively and develop it further as a potential cancer drug without having to rely on sea sponges.
Furthermore, the industry uses palladium as a catalyst to produce other drugs, such as the painkiller Naproxen.
The metal dissolves in aqua regia. Palladium is used as a metal complex in the Wacker-Höchst process, which is one of the oldest industrial methods for producing acetaldehyde.
Palladium is a precious metal and has high chemical stability. As a solid, it only reacts with oxygen when heated to 350 °C under extreme chemical conditions, which makes it a popular alternative to traditional metals in the jewellery industry. Palladium is a much harder metal than gold and unlike silver, it does not tarnish over time. It has similar properties to platinum but is lighter. This character makes palladium a popular alternative to platinum in hypoallergenic jewellery and for making white gold jewellery.
Thanks to its chemical stability, it has a wide range of applications. In medicine, for example, it is used as an alloy additive in dental alloys and surgical instruments. It also plays an important role in electrical sensors due to its adsorption capacity and is utilised in coins, fuel cell electrodes and membrane catalysts.
Occurrence and Production
Palladium is rarely found in its metallic form but is more common in ore deposits. It often occurs with other platinum group metals or in copper and nickel ore deposits. The metal is usually obtained as a by-product of metal mining such as platinum or nickel because of their commercial interest.
Catalyst material @ Mariia Human – stock.adobe.com
Major palladium producers include Russia, South Africa, Canada, the United States and Zimbabwe. Finland was the largest producer of primary palladium within the European Union in 2022.
The refining of palladium (and platinum group elements in general) is a complex separation process. As a result, palladium recycling is becoming more and more important and accounts for an increasing proportion of total palladium supply. The majority of the recycled material originates from used automotive catalytic converters, old jewellery and electronic devices.
While palladium recycling is on the rise, the supply still cannot satisfy the global demand.
In March 2024, the platinum group elements were added as the 27th critical material to the list of 34 strategic raw materials (SRM) under the European Critical Raw Materials Act.
Regulation (EU) 2024/1252 of the European Parliament and of the Council of 11 April 2024 establishing a framework for ensuring a secure and sustainable supply of critical raw materials and amending Regulations http://data.europa.eu/eli/reg/2024/1252/2024-05-03
Palladium belongs to the platinum group elements (PGE). Like all representatives of this group, it has a very wide range of applications. The most important source of release is the car exhaust catalytic converter, from which a minimal amount of palladium (in addition to platinum and/or rhodium) is emitted and can be found in particulate matter. It is also found in jewellery or dentures, for example. Thus, direct contact with human tissue is possible.
Everyday contact
Palladium is used in various chemical forms in a wide range of products. As a metal or in alloys, it is used for jewellery, dentures and in other medical products. Scientists are researching it in organic complexes as a potential drug. Automotive catalytic converters are the main source of human contact with palladium, but also platinum and rhodium. This releases tiny amounts of the precious metals into the air. They combine with fine dust, which can be inhaled. A study from 2022 investigated the concentrations of platinum and palladium in urban dust fractions in Moscow . The values for palladium are between 155 and 456 ng/g dust. The worldwide concentrations for palladium quoted in this paper vary from 0.6 ng/g to 516 ng/g. The largest amount for both platinum and palladium is bound to microparticles.
Situation at the Workplace
In the workplace, palladium plays a role in ore extraction, in dental practices and in the chemical industry. A distinction must be made between metallic palladium and palladium salts. Metallic palladium is inert and is considered less critical. However, palladium salts, such as Pd(II) chloride (PdCl2), can have a skin-sensitising effect. For this reason, the German MAK Commission generally classifies palladium(II) compounds as ‘sensitising for the skin’. However, in Germany and many other European countries, no limit values have been set for palladium and its compounds in the workplace (MAK and BAT values list 2024). In Finland, on the other hand, there is an occupational exposure limit of 500 µg/m3 for insoluble Pd and 1.5 µg/m3 for soluble Pd .
In a factory that manufactures and recycles automotive catalytic converters, workers were examined to determine whether their exposure to PGE was higher than that of control subjects. It was found that all four precious metals in this group (iridium, platinum, palladium and rhodium) were slightly elevated in the blood of the test subjects (0.1 to 2.94 µg/l blood). In the control group, all values were below the detection limit. A follow-up study showed that exposure to PGE salts in the workplace also leads to an increased manifestation of allergic symptoms, particularly in the lungs, skin and eyes. An important observation also dates from the time of these studies: sensitisation by palladium salts is almost always accompanied by a nickel allergy .
Products and Customer
Palladium is not a significant risk for the general population, even if it is released in measurable quantities by the exhaust catalytic converter. The exposure is still so low that adverse reactions can be ruled out. The situation is different in the workplace, where exposure has been detected and increased skin sensitisation is triggered. The different reactivity in the population and at the workplace may well be related to the chemical form, as the metallic palladium (catalyst) has no effect, but the palladium salts (workplace) have a sensitising/allergising effect on the skin and in the lungs.
Palladium is generally harmless. However, in certain forms, such as salts, it can trigger allergies – especially in people who come into contact with it a lot, for example through jewellery or dentures. Allergic reactions are particularly common when there is simultaneous contact with nickel.
The absorption of palladium in the body is strongly dependent on its chemical form or oxidation status. As a metallic palladium particle, it is hardly absorbed at all. As a palladium salt, e.g. PdCl2, it does enter the body via the lungs or the gastrointestinal tract.
Uptake via the Lung
Palladium is mainly released from car exhaust catalytic converters. It binds to fine dust particles in the air and can be inhaled together with them. A study shows that human exposure in blood and urine is in the order of ng/l. However, toxicological effects only occur from µg/kg or mg/kg (i.e. three to six orders of magnitude above the exposure values) . Even if an increase in car exhaust catalytic converters is predicted, this will be offset by the increase in electrically powered cars and there will be no critical additional exposure to palladium for humans and the environment.
PGE can enter the lungs from road dust. Studies show that ionic components of platinum, palladium or rhodium can be dissolved out of the dust particles . PGE must be absorbed into the cells for this purpose. The PGE are only dissolved in significant proportions when they come into contact with lysosomal fluid in the cells.
Uptake via the skin
Absorption of palladium nanoparticles via the skin is rather unlikely. Nevertheless, the skin is an important organ when considering palladium. It has long been known that palladium can trigger a contact allergy in both its metallic (weak) and ionic (strong) forms . It is very noticeable that palladium allergy is associated with nickel allergy in very many cases (94% of cases). An increased susceptibility can be observed particularly in dermatitis patients or patients with dentures.
Uptake via the gastrointestinal Tract
A scientific review article from the 1990s already pointed out the difference between metallic palladium and palladium salts (ionic form of palladium, Pd ions) . The summarised studies focus on palladium in dental prostheses. The authors came to the conclusion that only the Pd ions have an allergising effect, while the metallic palladium does not affect the internal organs (is hardly released) and if it does, it is excreted again very quickly.
A more recent animal study from 2024 describes a comparison of different palladium nanoparticles. Scientists administered particles of different sizes via the gastrointestinal tract: 20 nm, 80 nm and 20-100 nm long rods. In the study, the animals were treated daily for 28 days with a dose of 1 to 10 µg/kg via gavage. The 20 nm particles triggered an immune response. However, this was weaker with the 80 nm particles and the nanorods (20 – 100 nm). If the self-produced particles in this study were not contaminated with Pd ions, this means that some of the nanoparticles may well enter the body and have a sensitising effect by releasing Pd ions into the lungs. It remains to be seen whether these results can be confirmed by further studies .
Uptake via medical applications
Researchers are currently investigating palladium as a possible anti-tumour drug . It is comparable to ruthenium or platinum, both of which also have an anti-tumour effect in complex compounds. However, to date only drugs containing platinum have been authorised for treatment. Palladium therefore only plays a role as a metal in alloys that are used as dental prostheses, for example. Intake via medical applications is so low that there is no risk.
Metallic palladium poses no risk to humans. The ionic form (palladium salts) can trigger allergic reactions. The intake of particulate matter from car exhausts in cities is probably the largest source of palladium and will also decrease with the reduction of combustion cars.
Palladium is considered to be well tolerated by the body and is not considered toxic. In dissolved form, however, it can affect the immune system – which can lead to skin irritation or allergies in some people.
Distribution and Effects in the Body
Metallic palladium is hardly absorbed by the body and therefore has little effect in this form. For this reason, many studies are investigating the effect of palladium by administering it directly into the bloodstream or via the gastrointestinal tract. In animal studies with female rats, repeated injection of palladium nanoparticles into the blood led to an increase in follicle-stimulating hormone (FSH) and also influenced certain inflammatory mediators .
In both studies, the palladium dose was administered three times at 30-day intervals. The animals were examined after 90 days. The administered doses of at least 0.36 to 36 micrograms per kilogramme of body weight were significantly higher than what humans are exposed to in heavily polluted workplaces – in some cases up to 500 times higher. In a short-term study with just one injection and a blood test after two weeks, many inflammatory mediators were elevated. However, this effect only occurred in the animals that had received the highest dose .
Uptake and Effects in Cells
Very high concentrations of palladium cause human and animal cells to become less viable. This is due to the ‘particle effect’. However, many cell types, but also laboratory animals, react much more sensitively to palladium salts or very small palladium particles with changes in the immune system (see study examples in this overview: ).
In one of the examples from this review article, cells from healthy women without known allergies (non-atopic) were analysed. It was shown that both palladium nanoparticles and palladium salts can alter inflammatory mediators . The reaction was triggered when the cells came into direct contact with palladium – especially in combination with certain bacterial components (lipopolysaccharide).
In more realistic experiments with palladium nanoparticles (5-10 nm), the viability of the cells was not impaired. The cells reacted with a typical response to foreign particles (see above) .
Cell experiments also show the same picture as in humans: Cells from women with a known palladium allergy react much more strongly to palladium salts than cells from healthy women. More inflammatory mediators are released.
In contrast, metallic palladium nanoparticles often show no effect . The results reflect other studies. These particles only achieved an effect at high doses (>10µg/ml). The dissolved ions from palladium salts, on the other hand, had an effect even at significantly lower concentrations – up to 100 times lower.
A direct comparison of various platinum compounds with palladium complexes showed that the platinum complexes had a significant genotoxic effect in human lymphocytes. In contrast, the palladium compounds only had a cytotoxic effect (reduction in viability) at very high concentrations, but no genotoxic effect (DNA damage) .
Metallic palladium has hardly any effect on the human body. However, in the form of salt (ionic form) it can trigger allergies. People with a palladium allergy often also have a nickel allergy.
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