Ruthenium

Typical wastewaters

• nuclear fuel reprocessing liquid effluent (nitric acid, La Hague and Sellafield) ruthenium-106 as nitrosyl Ru(III) complexes [RuNO(NO₃)x(NO₂)y(OH)z(H₂O)₅-x-y-z]^(3-x-y-z), 30 to 50 percent insoluble 70 to 80 percent of the beta-gamma activity of La Hague liquid discharges in 1983 figures; sediment Kd only 20 to 50
• alkaline and diluted nuclear waste (Savannah River, Oak Ridge waste pits) ruthenate RuO₄²⁻ and perruthenate RuO₄⁻ at high pH; about 60 percent anionic nitrosyl species in diluted waste, the rest neutral
• reactor cooling effluent (Hanford) ruthenium-103, 66 percent particulate, 16 percent cationic, 13 percent anionic and 5 percent nonionic; 91 percent anionic in riverbank springs

1 · Identity

Symbol, number

Ru, 44

Oxidation states in water

+3 and +4 hydrolysed species Ru(OH)n^(4-n)+ in natural water; nitrosyl Ru(III) complexes RuNO³⁺ with nitrate, nitrite, hydroxide and water ligands in nitric acid effluent; +6 and +7 ruthenate and perruthenate in alkaline waste; +8 as volatile RuO₄ in accidents

Note

The element entry covers the metal, RuO₂ and RuO₄. In water the chemistry is complex and ligand driven; the PNNL review is the reference used here.

2 · Occurrence in water

Natural sources

Platinum group sulfide ores and ophiolite chromitites (element entry); seawater 1.3 ppt in one Indian Ocean measurement, typical ocean 5 x 10⁻14 M (0.005 ppt) and rivers 4.7 x 10⁻9 M (0.48 ppb) as summarised in the review.

Anthropogenic sources

Ru-103 and Ru-106 from fuel reprocessing liquid and gaseous effluents (La Hague, Sellafield, Marcoule, Oak Ridge, Hanford tank leaks), reactor accidents (Chernobyl, the 2017 European Ru-106 release), weapons fallout; stable ruthenium from chlor-alkali anode coatings, chip resistors and catalysts (element entry).

3 · Speciation

In seawater ruthenium is expected as pH dependent hydrolysed species Ru(OH)n^(4-n)+; under strongly oxidising conditions perruthenate RuO₄⁻ can occur, and ruthenate and perruthenate dominate alkaline reprocessing waste, disproportionating to RuO₄ and hydrous RuO₂ on acidification or dilution. In nitric acid reprocessing effluent 30 to 50 percent of the ruthenium is insoluble and the rest is a family of nitrosyl complexes [RuNO(NO₃)x(NO₂)y(OH)z(H₂O)₅-x-y-z]^(3-x-y-z), which are the non-reacting species that let Ru-106 travel from La Hague to the North Sea and Barents Sea. Reactor effluent Ru-103 at Hanford was 66 percent particulate, 16 percent cationic, 13 percent anionic and 5 percent nonionic, but 91 percent anionic in riverbank springs.

Solubility

Ligand and oxidation state dependent; hydrous RuO₂ is insoluble, nitrosyl complexes are soluble and mobile.

Hydrolysis

Ru(III) and Ru(IV) hydrolyse strongly at natural pH.

Complexation

Nitrosyl, nitrate, nitrite, chloride (polychloro complexes in acid); the review notes that complexation constants for environmentally relevant ligands are incomplete.

Precipitates

RuO₂.nH₂O, ruthenium sulfides.

4 · Role in treatment

5 · Removal and control

6 · Analytics

Sampling pitfalls

Speciation decides everything: the La Hague study fractionated effluent mixed with seawater by ultracentrifugation and hollow fibre ultrafiltration with on line ion exchange over 62 hours because particulate, cationic, anionic and neutral fractions behave differently. RuO₄ is volatile and toxic; oxidising sample treatment can lose ruthenium as the tetroxide.

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

Radiological for the fission isotopes; chemically, gaseous RuO₄ is toxic with an ozone-like smell, and hydrous RuO₂ is retained in the deep lung after inhalation. Gastrointestinal absorption of ingested ruthenium is low (about 0.01 for chloro complexes of Ru(III) and Ru(IV), 0.03 for nitrosyl Ru(III), Yamagata 1969 as cited).

Bioaccumulation

Ru-106 Kd in soils: 6.6 x 10^3 L/kg inorganic soils, 55 sand, 990 loam, 400 clay (medians), all lower than Cs-137, so Ru-106 is the more mobile; it binds preferentially to organic matter. Brown algae and estuarine saltmarsh sediments in tidally inundated zones accumulate Ru-106 (Ravenglass).

Ecotoxicity

Not read.

Flags

• The review is a PNNL manuscript with visible editorial comments; the ocean and river ruthenium figures it summarises differ by orders of magnitude between sources and are quoted as given.
• The discharge shares (70 to 80 percent at La Hague, 10 percent at Sellafield) are 1983 figures and do not describe current discharges.
• No Bq/L concentration of Ru-106 in seawater or effluent was read.

Gaps

• No measured Ru-106 activity concentration in seawater, river water or effluent was read.
• No stable ruthenium data for groundwater, municipal or industrial wastewater were read.
• No complexation constants; the review itself states they are incomplete.
• No standard method number for Ru-106 in water was read.
• No GCC standard was read.
• No balanced equation is written: the sources read describe ruthenium hydrolysis and nitrosyl complexes in words only, without stoichiometry.

Sources

Identity

Name and symbol

Ruthenium, Ru

Atomic number

44 protons

Position

group 8 · period 5 · d-block · transition metal

CAS number

7440-18-8

Atomic structure

Atomic mass

101.07 u

Electron configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s¹ 4d⁷
[Kr] 5s¹⁴d⁷

Electrons per shell

2, 8, 18, 15, 1

Valence electrons

8 ns and (n-1)d

Physical properties

State at room temperature

Solid

Melting point

2,607 K (2,333.85 °C)

Boiling point

4,423 K (4,149.85 °C)

Density

12.1 g/cm3

Appearance

silvery white metallic

Thermal conductivity

117 W/(m·K)

Electrical resistivity

71 nΩ·m at 0 °C

Electrical conductivity

14.08 MS/m

Crystal structure

hexagonal close packed

Molar heat capacity

24.06 J/(mol·K)

Chemical properties

Oxidation states

+3

Electronegativity

2.2 (Pauling Scale)

Ionisation energy

7.361 eV
1st 710.2, 2nd 1,620, 3rd 2,747 kJ/mol

Electron affinity

1.05 eV

Atomic radius

empirical 146, covalent 146, van der Waals 207 pm

Ionic radius

Ru³⁺ 68; Ru⁴⁺ 62; Ru⁵⁺ 57; Ru⁷⁺ 38 (4-coordinate); Ru⁸⁺ 36 (4-coordinate) pm

Reactivity

A hard platinum-group metal that is inert at room temperature: it does not tarnish, resists all common acids including aqua regia, and is attacked only by strong oxidants, the halogens on heating, and fused alkalis.

with water

Does not react with water.

with oxygen, air

Does not tarnish at room temperature; on heating to about 800 C it oxidises to the dioxide: $\ce{Ru + O2 -> RuO2}$.

with acids

Not attacked by acids or by aqua regia alone; it dissolves in fused alkalis to ruthenates and is oxidised by sodium hypochlorite at room temperature.

with halogens

Attacked by the halogens at elevated temperature; chlorine gives the trichloride: $\ce{2 Ru + 3 Cl2 -> 2 RuCl3}$, and fluorine the hexafluoride RuF6.

Typical compounds

• RuO₂ ruthenium dioxide stable oxide; photocatalyst on CdS for splitting H2S
• RuO₄ ruthenium tetroxide volatile, toxic, explosive strong oxidant
• RuCl₃ ruthenium trichloride brown solid, the usual synthetic starting material
• K₂RuO₄ potassium ruthenate Ru(VI) product of fused alkali
• RuF₆ ruthenium hexafluoride highest halide, hydrolyses violently

Occurrence, production and use

Crustal abundance

1×10-3 milligrams per kilogram

Oceanic abundance

7×10-7 milligrams per liter

Occurrence and sources

A member of the platinum group, ruthenium occurs native with other members of the group in ores found in the Ural mountains and in North and South America. It is also found along with other platinum metals in small but commercial quantities in pentlandite in the Sudbury, Ontario nickel-mining region, and in the pyroxinite deposits of South Africa.

• with other platinum-group metals in pentlandite and pyroxenite; occasionally native Bushveld Complex (South Africa), Norilsk (Russia), Sudbury (Canada), Great Dyke (Zimbabwe), Stillwater (Montana)
• crustal and oceanic abundance 0.000037 ppm (BGS group figure for all PGMs via RSC); 0.001 mg/kg crust and 7 x 10^-7 mg/L seawater (PubChem)

Extraction, production

By-product of nickel and platinum refining

recovered from the wastes of nickel refining; no separation chemistry is stated by either source

Uses

Ruthenium is primarily used as an alloying agent. Adding 0.1% ruthenium to titanium makes titanium 100 times more resistant to corrosion. Small amounts of ruthenium are added to platinum and palladium to strengthen them. These alloys are used in jewelry and in electrical contacts that must resist wear.

• Electronics: thick-film chip resistors and electrical contacts; most ruthenium goes here; hard-disk and multilayer capacitor applications shared with other PGMs
• Chemicals: ruthenium oxide coating on dimensionally stable anodes for chlorine production; ruthenium catalysts for ammonia synthesis and for acetic acid production
• Jewellery and alloys: hardener for platinum and palladium; wear-resistant contact alloys; platinum-ruthenium jewellery alloys
• Energy: ruthenium dye complexes in dye-sensitised solar cells
• Mining: co-product of platinum-group and nickel-copper sulfide mining

Safety, toxicity

GHS classification, signal word Danger

• H228 Flammable solid Flammable solids
• H413 May cause long lasting harmful effects to aquatic life to the aquatic environment, long-term hazard

Discovery and name

Discovered by

Karl Ernst Claus

Discovered

1844

First isolated

not in sources

Named by

not in sources

Origin of the name

from Latin Ruthenia for Russia

Ruthenium is a hard, white metal and has four crystal modifications. It does not tarnish at room temperatures, but oxidizes explosively. It is attacked by halogens, hydroxides, etc. Ruthenium can be plated by electrodeposition or by thermal decomposition methods. The metal is one of the most effective hardeners for platinum and palladium, and is alloyed with these metals to make electrical contacts for severe wear resistance. A ruthenium-molybdenum alloy is said to be superconductive at 10.6 K. The corrosion resistance of titanium is improved a hundredfold by addition of 0.1% ruthenium. It is a versatile catalyst. Hydrogen sulfide can be split catalytically by light using an aqueous suspension of CdS particles loaded with ruthenium dioxide. It is thought this may have application to removal of H2S from oil refining and other industrial processes. Compounds in at least eight oxidation states have been found, but of these, the +2, +3, and +4 states are the most common. Ruthenium tetroxide, like osmium tetroxide, is highly toxic. In addition, it may explode. Ruthenium compounds show a marked resemblance to those of cadmium.