Rhodium is a rare metal that does not rust.
Rhodium is a rare metal that does not rust.
Rhodium is a rare, shiny silver-white transition metal known for its exceptional corrosion resistance. It belongs to the platinum group elements (ruthenium, rhodium, palladium, osmium, iridium and platinum). It was discovered in 1803 by British chemist William Hyde Wollaston, who named it after the Greek word ‘rhodon’ (rose) due to the pink crystals observed during its isolation.
Rhodium is one of the hardest and rarest elements on Earth. It is durable, has high reflectivity and catalytic activity. These properties contribute to its popularity in various industrial applications. In addition to its use in alloys in catalytic converters, rhodium also finds application in a wide range of technologies .
The automotive industry uses rhodium in three-way catalytic converters. Here, it converts harmful nitrogen oxides into less harmful substances: nitrogen (N₂) and carbon dioxide (CO₂). In addition to rhodium, three-way catalytic converters often contain platinum and palladium. These are responsible for converting hydrocarbons and carbon monoxide (CO) into water and carbon dioxide (CO2) .
The chemical industry uses rhodium catalysts to produce certain basic chemicals. One example is the Ostwald process for producing nitric acid.
Rhodium is also very popular in the jewellery industry. It is extremely resistant to corrosion and oxidation, even at high temperatures. That is why it is used to refine jewellery, such as white gold or silver. This process is called ‘rhodium plating’. It ensures a bright, shiny and abrasion-resistant surface that keeps the jewellery beautiful for longer. Rhodium is very skin-friendly in jewellery (e.g. in nickel-free jewellery).
Additionally, it is utilized in the electronics industry to coat electrical contacts, preventing corrosion and enhancing conductivity. Researchers also use thin layers of rhodium for highly reflective mirrors in devices such as X-ray imaging equipment.
Rhodium is also used in nuclear reactor detectors to measure local power or neutron flux within specific reactor areas.
Rhodium also plays a specialized role in the hydrogen economy. In certain electrolysis processes, such as PEM electrolysis, it is used as a coating or alloy component to improve the efficiency of hydrogen evolution (HER – hydrogen evolution reaction). In fuel cells, rhodium alloys help prevent catalyst poisoning by carbon monoxide, extending the catalyst’s service life . Due to its high cost and limited supply, rhodium is reserved for demanding, high-performance applications.
Rhodium has no known biological function and may be harmful to health in some forms.
Rhodium is among the rarest metals on earth, with an average concentration of 0.0002 ppm. It occurs in pure form as a separate mineral and is often found alongside platinum group elements or gold.
Extracting rhodium is complex because platinum group elements are chemically similar and unreactive, making separation both difficult and costly.
As a result, recycling rhodium is increasingly important. Most recovered rhodium comes from used catalytic converters, electronic waste, and jewelry. Major producers include South Africa, Russia, and Zimbabwe, which introduces geopolitical risks .
In March 2024, platinum group elements were listed as the 27th critical material on the list of 34 strategic raw materials (SRM) under the European Critical Raw Materials Act.
Further information:
Rhodium belongs to the platinum group elements (PGE) and has versatile properties. The most important source of release is the automotive exhaust catalyst, from which a minimal amount of rhodium (alongside platinum and/or palladium) is released and ends up in particulate matter. Like platinum, palladium, or ruthenium, rhodium could be used in complex compounds as a medicine, e.g., to treat cancer.
There are two main sources of exposure to rhodium (Rh) in everyday life: firstly, rhodium-plated jewelry that comes into direct contact with the skin, and secondly, car exhaust catalytic converters, which can release minimal amounts of the catalytic platinum group elements (PGE) platinum, palladium, and rhodium. When bound to fine dust in the air, these elements can also enter the lungs through respiration. A study shows that German cities can contain up to 1 µg of platinum but only 0.1 µg of rhodium per g of dust .
Although platinum accounts for the largest proportion of PGE in road dust, the amounts of rhodium (and also palladium) excreted in human urine are higher than those of platinum, indicating better bioavailability of rhodium (and also palladium) .
In certain specialized industries (e.g., in the production of automotive catalytic converters, electroplating, or jewelry processing), workers may be exposed to rhodium, e.g., during the rhodium plating of surfaces. There is also potential for direct contact with rhodium in specialized laboratories, electronics manufacturing (e.g., rhodium-coated contacts), and the recycling industry. The main routes of exposure are inhalation of vapors, dust, or aerosols and direct contact with the skin.
Overall, exposure is relatively low. However, as some case studies show, appropriate protective measures are still required in these fields of work. A female worker in the jewelry industry developed severe skin inflammation through contact with rhodium. A 27-year-old worker suffered mild asthma attacks and inflammation of the nasal mucosa at a rhodium surface coating plant. These are rare isolated cases. Nevertheless, sensitization to rhodium compounds cannot be ruled out, as is also the case with other PGE zotpressInText item="{4274171:XIITX6U4},{4274171:NRCGLCMM}"] (see platinum, palladium, and ruthenium).
There are no fixed occupational exposure limits for rhodium in Germany, Europe, or the USA. For some rhodium compounds, there are guidelines for possible skin contact, but for rhodium-containing dusts, only the general dust limit must be observed (see Granular biopersistent dust particles).
Private individuals rarely come into contact with rhodium. Rhodium is hardly toxic in metallic form. It is mainly used in products such as jewelry (especially silver and white gold jewelry, known as rhodium-plated jewelry) and car catalytic converters. Rhodium-plated jewelry is considered hypoallergenic and harmless. However, rhodium can cause allergic reactions in a few very sensitive individuals.
Environmental pollution caused by rhodium (e.g., from car catalytic converters) has been proven, but it is minimal and poses no health risk . Rhodium is not used in cosmetics or food. Overall, rhodium does not pose a relevant health risk to consumers.
Under normal conditions, consumers face a very low risk of exposure to rhodium. Rhodium-plated jewelry is generally safe. Environmental pollution from rhodium is low. However, people with known metal allergies should be aware of possible reactions to rhodium salts and, if necessary, avoid wearing rhodium-plated jewelry. The World Health Organization (WHO), the Agency for Toxic Substances and Disease Registry (ATSDR) in the USA, the European Chemicals Agency (ECHA), and other authorities do not currently classify rhodium as relevant for widespread consumer exposure.
Rhodium can be absorbed through both the lungs and the skin, particularly in its ionic form (as rhodium salts). However, the amounts are very small. Significant exposure only occurs in specific workplaces.
The uptake of rhodium (Rh) through the lungs is caused almost exclusively by car exhaust catalysts. A German study showed that people in cities had more rhodium in their urine than people in rural areas. The differences were very small. Although the amounts of rhodium absorbed are higher than those of platinum (Pt), they are well below levels of concern. Similar results had previously been found in tram drivers in Rome. They showed slightly elevated levels of platinum and rhodium in their urine compared to a control group. An Indian study also showed a correlation: traffic police officers in Hyderabad were tested for blood levels of platinum, palladium, and rhodium depending on their length of service . All three elements were elevated in the blood of the individuals examined compared to the unexposed control group. Further studies show that artificial lung fluid is capable of dissolving rhodium and other platinum group metals from road dust – which explains the exposure in the body .
The few studies that have investigated the absorption of rhodium through the skin have used a special in vitro system known as the Franz diffusion cell. In this system, real skin (e.g., pig skin or skin samples from volunteers) is clamped into a vessel in such a way that the “outside” and “inside” are spatially separated from each other. Although this experimental setup is prone to errors (e.g., due to leaks on the clamped sides of the skin sample), it can replace many animal experiments.
When skin in the Franz diffusion cell was exposed to soluble platinum or rhodium salts, significantly more platinum was transported through the skin or deposited in the skin than rhodium salts. However, after 24 hours, the amounts that had passed through the skin were still very small. The results were quite different when platinum and rhodium nanoparticles were examined. Although these particles were very small (diameter < 10 nm), no transport through healthy skin could be detected. Only when the skin was injured with a needle could the penetration of particles from “outside” to “inside” be measured in the Franz diffusion cell. In a third study, a soluble rhodium salt (rhodium trichloride, RhCl3) was applied to the skin at different pH values and its penetration through the skin was analysed. The study was able to demonstrate that acidic conditions (low pH) led to an increase in the amount of rhodium transported through the skin .
In everyday life, platinum group elements (PGEs) play hardly any role in the gastrointestinal tract, as rhodium is not absorbed through food or drinking water. Nevertheless, a study investigated whether platinum, palladium, or rhodium particles released from automotive exhaust catalysts would be bioavailable at all if they entered the stomach and intestines . For this purpose, artificial stomach and intestinal fluid was used to investigate how much platinum, palladium, and rhodium could be dissolved from particulate matter. Palladium and rhodium were more soluble than platinum. Nevertheless, the available amount was highest for platinum because it occurs most frequently in catalytic converters. However, since very little is absorbed through the gastrointestinal tract, this route plays hardly any role in health .
Like other platinum group elements, rhodium could also play a role as a potential tumor drug. Scientists use rhodium in compounds that force their way into the DNA of tumor cells (technical term: “intercalation”). As so-called “metalloinsertors,” these compounds prevent tumor cells from dividing. Another application is the combination of rhodium citrate with so-called SPIONs, small magnetic iron nanoparticles. These can be directed to the tumor site from outside using a magnetic field, thus delivering the effective rhodium citrate to the tumor cells [16-18]. Both applications are still in the research stage and have not yet reached clinical application .
Rhodium intake is mainly seen in occupational settings (employees in transport or industry). For the general population, all findings to date show that rhodium is hardly absorbed and poses no health risk.
Metallic rhodium has virtually no harmful effects on the body. Large quantities of rhodium salt may have some effects on the body under certain circumstances. However, even road traffic dust and exhaust fumes do not contain quantities of this magnitude.
In a scientific study, researchers treated rats with a rhodium salt (rhodium-(III)-chloride) in their drinking water for two weeks. Dosages of 0.25 mg, 0.5 mg, and 1 mg of rhodium salt per liter led to mild kidney dysfunction. However, the effects varied greatly and have not yet been confirmed .
A clinical study reports on a 27-year-old male patient who was observed to have rhodium sensitization .. The man had worked for three years with electrolytic baths for plating, during which time he came into contact with platinum-, gold-, and mainly rhodium salts. According to this study, rhodium can, in very rare cases, have a sensitizing or allergenic effect on humans.
However, the body of research available for assessing the toxicity of rhodium in humans remains very limited. This is also due to the fact that this metal has a very low effect when it interacts with biological systems. For this reason, the International Agency for Research on Cancer (IARC) has not classified rhodium as a possible carcinogen. However, both the Health Council of the Netherlands in 2002 and the German MAK Commission (2023) point out that there are too few studies on rhodium salts and that these compounds cannot be evaluated with sufficient certainty. The Health Council of the Netherlands therefore called for classification in the former EU category 3B (possibly carcinogenic), which currently corresponds to category 2 following the redefinition of classifications in the EU in 2008 (EUR-Lex No. 1272/2008, Annex 1, 3.6). The MAK Commission has also classified rhodium and its compounds as possibly carcinogenic, which corresponds to KanzKat 3 in the special regulations in Germany. In all cases, however, the classification was made due to a lack of studies and not because of existing evidence. Therefore, these classifications are currently to be seen as a safety measure in accordance with the precautionary principle and not as a scientifically justified measure.
Most data on rhodium comes from experiments with cell cultures. Comparisons with other platinum group elements (PGEs) in particular allow some important conclusions to be drawn about rhodium.
Several studies have shown that rhodium is the element with the weakest biological effect (compared to platinum and palladium). In various cell tests, platinum was 30 times more effective than rhodium and palladium was 3 times more effective [20]. Another study using human lung cells confirmed the same order and the same differences in toxicity .
Rhodium nanoparticles (15 nm) have even been observed to reduce the formation of oxygen radicals, thereby inhibiting lipid peroxidation and protecting human intestinal cells (Caco-2) from the harmful effects of hydrogen peroxide (H2O2) without themselves causing any toxic effects .
However, the situation is different when soluble salts are examined. Rhodium salts inhibit the growth, division, and viability of various cultured cells .
When in contact with freshly isolated human lymphocytes, platinum, palladium, and rhodium salts can negatively affect cell growth and the formation of messenger molecules (cytokines) . Here, too, rhodium is the least effective and requires very high concentrations to have an effect.
Rhodium is very low in toxicity and, as a metallic nanoparticle, even has a protective effect. For this reason, there are no specific limit values for this element.