Palladium

Typical wastewaters

• road runoff and stormwater (automobile catalytic converters) Pd(0) and PdO particles on road dust and runoff solids, with a small fraction solubilised as Pd(II) during transport 5.4 to 61.2 ng/g in Perth runoff basin sediment
• municipal sewage labile palladium passing sewage works with limited retention, following the traffic metals Cu, Zn and Pb into urban rivers trend only; no concentration in the abstract read

1 · Identity

Symbol, number

Pd, 46

Oxidation states in water

0 as metal particles shed by converters; +2 as chloro and hydroxo complexes once solubilised

Note

The element entry covers the metal, PdCl₂ and the catalysts. The water chemistry is that of a particle that slowly leaches, and the abstracts read give no dissolved speciation.

2 · Occurrence in water

Natural sources

Negligible; placer and sulfide ores (element entry).

Anthropogenic sources

Automobile three-way catalysts, the main source; catalyst and jewellery workshops, pharmaceutical hydrogenation catalysts leaving with waste (element entry); dental alloys. Road dust in Toronto held a geomean 152 µg/kg Pd in the inhalable fraction against 55 Pt and 21 Rh; Canadian road dust averaged 73 ng/g Pd, 23 Pt, 7.1 Rh.

3 · Speciation

Emitted as metal and oxide particles on the catalyst washcoat; in drainage systems a small part of the palladium in road dust is solubilised under natural conditions, which shifts the Pt to Pd ratio between road dust and basin sediment. Dissolved palladium is expected as Pd(II) chloro and hydroxo complexes but no speciation measurement was read.

Solubility

Low but the highest of the three converter metals.

Hydrolysis

Not read.

Complexation

Chloride and organic ligands expected; not read.

Precipitates

Not relevant.

4 · Role in treatment

5 · Removal and control

6 · Analytics

Sampling pitfalls

Spectral interferences on palladium masses (yttrium, rubidium, strontium, copper, zinc argides and oxides) must be monitored; the isopod study measured Hf, Cu, Y, Rb, Sr and Pb for corrections. The 2025 Canadian study warns that literature reports of high PGE in biota may be analytical artefacts.

7 · Regulatory limits

Limits change, and many are set locally. Treat these as the published values to start from, not as your compliance target: check the standard in force at your site and the numbers written into your own permit.

8 · Health and environmental effects

Toxicity

No human health guideline anywhere read. Palladium in inhalable road dust is only 3.4 percent bioaccessible in simulated lung fluid, against 16 percent for platinum and 14 percent for rhodium.

Bioaccumulation

Asellus aquaticus from an urban river held 155.4 plus or minus 73.4 ng/g Pd dry weight (Pt 38.0, Rh 17.9); 24 h bioaccumulation factors from standard solutions 150 for Pd, 85 for Pt, 7 for Rh, time dependent and higher for materials of higher PGE content. Chaoborus, by contrast, assimilated only 19 percent of dietary Pd and excreted it fast (efflux 0.69 per day), so trophic transfer is low.

Ecotoxicity

Not read.

Flags

• No dissolved palladium concentration in water was read as a number; the Scheldt abstract gives trends only and the Canadian water samples were below detection.
• Bioaccumulation evidence conflicts between the 2001 isopod field data and the 2025 predator-prey experiments; both are quoted.

Gaps

• No dissolved Pd concentration for river water, road runoff, sewage or seawater was read as a number.
• No speciation or solubility data were read.
• No removal efficiency for Pd in wastewater treatment was read; the Scheldt abstract says retention is limited.
• No ecotoxicity data were read.
• No GCC standard was read.

Sources

Identity

Name and symbol

Palladium, Pd

Atomic number

46 protons

Position

group 10 · period 5 · d-block · transition metal

CAS number

7440-05-3

Atomic structure

Atomic mass

106.42 u

Electron configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 4d¹⁰
[Kr] 4d¹⁰

Electrons per shell

2, 8, 18, 18

Valence electrons

28 ns and (n-1)d

Physical properties

State at room temperature

Solid

Melting point

1,828.05 K (1,554.9 °C)

Boiling point

3,236 K (2,962.85 °C)

Density

12 g/cm3

Appearance

silvery white

Thermal conductivity

71.8 W/(m·K)

Electrical resistivity

105.4 nΩ·m at 20 °C

Electrical conductivity

9.49 MS/m

Crystal structure

face-centered cubic

Molar heat capacity

25.98 J/(mol·K)

Chemical properties

Oxidation states

+3, +2

Electronegativity

2.2 (Pauling Scale)

Ionisation energy

8.337 eV
1st 804.4, 2nd 1,870, 3rd 3,177 kJ/mol

Electron affinity

0.557 eV

Atomic radius

empirical 139, covalent 139, van der Waals 202 pm

Ionic radius

Pd⁺ 59 (2-coordinate); Pd²⁺ 86; Pd³⁺ 76; Pd⁴⁺ 62 pm

Reactivity

The least dense and lowest-melting platinum-group metal, chemically inert in air but the most easily dissolved of the group: nitric acid and aqua regia attack it, and it absorbs up to 900 times its own volume of hydrogen at room temperature.

with water

Does not react with water.

with oxygen, air

Does not tarnish in air at room temperature; heated in air to about 800 C it forms a layer of palladium(II) oxide: $\ce{2 Pd + O2 -> 2 PdO}$.

with acids

Dissolves slowly in concentrated nitric acid and in hot concentrated sulfuric acid, readily in aqua regia at room temperature, and when finely divided in hydrochloric acid: $\ce{3 Pd + 12 HCl + 2 HNO3 -> 3 H2PdCl4 + 2 NO + 4 H2O}$.

with halogens

Combines with chlorine on heating to palladium(II) chloride: $\ce{Pd + Cl2 -> PdCl2}$; fluorine gives PdF2 and higher fluorides.

Typical compounds

• PdCl₂ palladium(II) chloride principal starting material for palladium compounds and catalysts
• PdO palladium(II) oxide oxide layer formed above 800 C
• Pd(O₂CCH₃)₂ palladium(II) acetate versatile coupling-reaction catalyst
• H₂PdCl₄ tetrachloropalladic acid product of dissolving the metal in aqua regia
• PdH₀.7 palladium hydride non-stoichiometric hydride from absorbed hydrogen

Occurrence, production and use

Crustal abundance

1.5×10-2 milligrams per kilogram

Oceanic abundance

Not Applicable

Occurrence and sources

Discovered in 1803 by Wollaston, Palladium is found with platinum and other metals of the platinum group in placer deposits of Russia, South America, North America, Ethiopia, and Australia. It is also found associated with the nickel-copper deposits of South Africa and Ontario. Palladium's separation from the platinum metals depends upon the type of ore in which it is found.

• braggite and other PGM sulfides in layered intrusions Norilsk (Russia), Bushveld (South Africa), Great Dyke (Zimbabwe), Sudbury (Canada), Stillwater (Montana)
• native palladium and palladium-gold alloy Brazil
• crustal abundance 0.000037 ppm (BGS group figure for all PGMs via RSC); 0.015 mg/kg crust (PubChem)

Extraction, production

Mined PGM concentrate and by-product of nickel, copper and zinc refining

world mine production about 190,000 kg in 2024; about 120,000 kg of palladium plus platinum recovered globally from scrap in 2024; no refining chemistry stated

Uses

Palladium is used to make springs for watches, surgical instruments, electrical contacts and dental fillings and crowns. Finely divided palladium acts as a catalyst and is used in hydrogenation and dehydrogenation processes. Palladium at room temperature can absorb up to 900 times its own volume of hydrogen. Hydrogen will easily pass through heated palladium, a property that allows for the easy purification of hydrogen. Palladium alloys are used to make jewelry and, when alloyed with gold, forms a material known as white gold.

Palladium dichloride (PdCl2), a palladium compound, can absorb large amounts of carbon monoxide (CO) gas and is used in carbon monoxide detectors.

Finely divided palladium is a good catalyst and is used for hydrogenation and dehydrogenation reactions. It is alloyed and used in jewelry trades.

White gold is an alloy of gold decolorized by the addition of palladium. Like gold, palladium can be beaten into leaf as thin as 1/250,000 in. The metal is used in dentistry, watch making, and in making surgical instruments and electrical contacts.

• Automotive catalysts: catalytic converters, the leading domestic use of PGMs; palladium substitutes for platinum in most gasoline-engine converters
• Pharmaceuticals and fine chemicals: palladium hydrogenation and dehydrogenation catalysts in multipurpose batch synthesis; spent catalyst and metal residues leave with the waste
• Chemicals and refining: catalysts for bulk-chemical production and petroleum refining; palladium membranes for hydrogen purification
• Electronics: multilayer ceramic capacitors in phones and laptops; hybridised integrated circuits
• Jewellery and dentistry: white gold alloying, jewellery, dental crowns and fillings, investment
• Mining: PGM mining in Russia, South Africa, Zimbabwe, Canada, United States

Safety, toxicity

GHS classification, signal word Danger

• H228 Flammable solid Flammable solids
• H315 Causes skin irritation Skin corrosion/irritation
• H319 Causes serious eye irritation Serious eye damage/eye irritation
• H335 May cause respiratory irritation Specific target organ toxicity, single exposure; Respiratory tract irritation
• H413 May cause long lasting harmful effects to aquatic life to the aquatic environment, long-term hazard

Discovery and name

Discovered by

William Hyde Wollaston

Discovered

1802

First isolated

not in sources

Named by

not in sources

Origin of the name

after asteroid Pallas, itself named after Pallas Athena

The element is a silvery-white metal, it does not tarnish in air, and it is the least dense and lowest melting of the platinum group of metals. When annealed, it is soft and ductile; cold-working greatly increases its strength and hardness. Palladium is attacked by nitric and sulfuric acid.

At room temperatures, the metal has the unusual property of absorbing up to 900 times its own volume of hydrogen, possibly forming Pd2H. It is not yet clear if this is a true compound. Hydrogen readily diffuses through heated palladium, providing a means of purifying the gas.