Thulium

Typical wastewaters

• rare earth separation and processing (saponification wastewater) Tm³⁺ carried with the whole lanthanide row in ammonia rich separation plant wastewater (ammonia nitrogen 300 to 5,000 mg/L in Chinese saponification wastewater, 2005 estimate) no thulium specific concentration read
• coal mine drainage (acid) Tm³⁺ and the TmSO₄⁺ ion pair dissolved at low pH in sulfate rich drainage; total rare earths averaged 282 µg/L over 141 Appalachian sites, per-element values not read co-precipitates into the iron, aluminium and manganese hydroxide sludge when the drainage is neutralised

1 · Identity

Symbol, number

Tm, 69

Oxidation states in water

+3 only (Tm³⁺)

Note

The metal's reactions are in the book entry; in water thulium shares the trivalent carbonate and phosphate chemistry of the lanthanide row.

2 · Occurrence in water

Natural sources

Weathering of monazite and xenotime; heavy lanthanide, carbonate complexed in neutral to alkaline water; the lowest concentration of the row in the western Pacific profile (0.12 to 1.02 pmol/L).

Anthropogenic sources

None read. Rare earth separation wastewater carries the whole row.

3 · Speciation

Trivalent Tm³⁺ throughout the natural pH and Eh range. Below about pH 6, and in sulfate rich water such as mine drainage, the free ion and the TmSO₄⁺ ion pair dominate (sulfate complexes above 90 percent at pH 3.1 to 3.4 in the EPA groundwater study). From neutral to alkaline pH the carbonate complexes TmCO₃⁺ and Tm(CO₃)₂⁻ take over, and they bind the heavy lanthanides more strongly than the light ones (at pH 5.1 to 6.1 the carbonate share rose from 5.0 percent for La to 19.2 percent for Lu). Phosphate, carbonate and fluoride solids and sorption to iron and manganese oxides cap the dissolved concentration at ng/L in oxic neutral water, and the light lanthanides ride partly on colloids.

Solubility

Controlled by the phosphate, carbonate and fluoride solids and by sorption; the RIVM report puts total dissolved lanthanum and cerium in Dutch surface water at about 1 µg/L or less and the free ion fraction at 0.3 fM to 9 pM. No solubility product for a thulium solid was read this session.

Hydrolysis

Hydrolysis of Tm³⁺ is minor in natural water; the hydroxide Tm(OH)₃ forms only at high pH, so lime or caustic neutralisation of acid mine water strips the element with the iron and aluminium hydroxides rather than as its own hydroxide (no hydrolysis constant read this session).

Complexation

Sulfate at low pH, carbonate at neutral to alkaline pH (the sources read); humic substances compete for the element in organic rich water and are out-competed by carbonate in alkaline water. No stability constants were read this session.

Precipitates

TmPO₄ (hydrated phosphate), Tm₂(CO₃)₃, TmF₃ near fluoride rich discharges, Tm(OH)₃ at high pH; co-precipitated on iron, aluminium and manganese hydroxides.

4 · Role in treatment

Not relevant or not given for this element.

5 · Removal and control

6 · Analytics

Sampling pitfalls

Filtration defines the result: 0.2 or 0.45 µm filtrates still carry colloid bound lanthanides, and only ultrafiltration (10 kDa) separates the truly dissolved pool, which matters most for the light lanthanides. Acidify after filtration. Report the shale-normalised pattern so that anthropogenic anomalies (gadolinium, lanthanum, samarium) are visible. In the Rhine work Tm was interpolated from Er and Yb for the sum of REE because of its low concentration and interference; measure it directly only with adequate preconcentration.

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; no acute or chronic human toxicity data for this element were read this session.

Bioaccumulation

Field bioconcentration factors in Rhine estuary amphipods fall from the light to the heavy lanthanides (La 28,840 to Lu 4,786; Tm 6,310); carp muscle takes up little (BCF 0.22 to 1.10 for Ce, La, Nd, Pr, Sm) while internal organs reach 634 to 978. Anthropogenic La and Sm in the Rhine are taken into mussel shells, anthropogenic Gd is not, so speciation decides bioavailability.

Ecotoxicity

No MPC was derived for thulium: the RIVM report set risk limits only for Y, La, Ce, Pr, Nd, Sm, Gd and Dy. The one thulium number it carries is a field bioconcentration factor in amphipods of 6,310 (porewater basis).

Flags

• The RIVM MPCs, where they exist, are environmental risk limits from a 2000 report (LC₅₀ divided by 1000, background at the detection limit), not permit limits.
• The seawater range is one western Pacific station.

Gaps

• WHO GDWQ and EU DWD 2020/2184 Annex I were not read this session (eur-lex returned the articles without the annexes), so their absence of a rare earth parameter is not asserted here; only the US EPA table was read.
• No GCC discharge standard was read; no GCC row is written.
• World average river water concentrations (Gaillardet et al. 2003) were not reachable; river figures come from the Rhine and Dutch waters only.
• No source read this session gives municipal wastewater concentrations for this element beyond the gadolinium literature.
• ICP-MS oxide interference corrections between lanthanides are standard practice but no method text describing them was read, so none is written.
• No stability constants or solubility products for this element's carbonate, phosphate or hydroxide were read; the speciation is qualitative, taken from the EPA groundwater study and the RIVM report.
• No thulium specific river, groundwater or wastewater concentration was read, and no toxicity test.

Sources

Identity

Name and symbol

Thulium, Tm

Atomic number

69 protons

Position

no group (f-block) · period 6 · f-block · lanthanide

CAS number

7440-30-4

Atomic structure

Atomic mass

168.934 u

Electron configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹³
[Xe] 6s²⁴f¹³

Electrons per shell

2, 8, 18, 31, 8, 2

Valence electrons

15 ns, (n-1)d and (n-2)f

Physical properties

State at room temperature

Solid

Melting point

1,818 K (1,544.85 °C)

Boiling point

2,223 K (1,949.85 °C)

Density

9.32 g/cm3

Appearance

silvery gray

Thermal conductivity

16.9 W/(m·K)

Electrical resistivity

poly: 676 nΩ·m

Electrical conductivity

1.48 MS/m

Crystal structure

hexagonal close packed

Molar heat capacity

27.03 J/(mol·K)

Chemical properties

Oxidation states

+3

Electronegativity

1.25 (Pauling Scale)

Ionisation energy

6.184 eV
1st 596.7, 2nd 1,160, 3rd 2,285 kJ/mol

Electron affinity

not in sources

Atomic radius

empirical 190, covalent 190, van der Waals 227 pm

Ionic radius

Tm²⁺ 103; Tm³⁺ 88 pm

Reactivity

A late lanthanide (4f13 6s2) whose solution chemistry is entirely Tm3+ (pale green), with a +2 state reachable only in solids because the 4f shell is nearly full; the metal tarnishes slowly in air and is attacked by water, acids and halogens.

with water

Reacts slowly with cold water and quickly with hot water to the hydroxide and hydrogen: $\ce{2 Tm + 6 H2O -> 2 Tm(OH)3 + 3 H2}$, like the other heavy lanthanides.

with oxygen, air

Tarnishes slowly in air and burns readily at about 150 C to the only known oxide: $\ce{4 Tm + 3 O2 -> 2 Tm2O3}$, thulia; the dust is a fire and explosion hazard.

with acids

Dissolves readily in dilute sulfuric acid to pale green Tm3+ solutions with evolution of hydrogen: $\ce{2 Tm + 3 H2SO4 -> Tm2(SO4)3 + 3 H2}$, and hydrochloric acid gives TmCl3, nitric acid Tm(NO3)3.

with halogens

Reacts with all the halogens, slowly at room temperature and vigorously above 200 C, to the trihalides: $\ce{2 Tm + 3 Cl2 -> 2 TmCl3}$, yellow, while the fluoride and bromide are white.

Typical compounds

• Tm₂O₃ thulium(III) oxide thulia, the only known oxide, pale green
• TmF₃ thulium(III) fluoride white trihalide, one of the Los Alamos-listed compounds
• TmCl₃ thulium(III) chloride yellow trihalide, crystallises as a green heptahydrate
• TmI₃ thulium(III) iodide yellow iodide listed by Los Alamos
• Tm(NO₃)₃ thulium(III) nitrate soluble salt from nitric acid

Occurrence, production and use

Crustal abundance

5.2×10-1 milligrams per kilogram

Oceanic abundance

1.7×10-7 milligrams per liter

Occurrence and sources

• monazite about 20 parts per million thulium; the principal source

Extraction, production

Reduction of anhydrous thulium fluoride with calcium, or of the oxide with lanthanum

$\ce{2 TmF3 + 3 Ca -> 2 Tm + 3 CaF2}$

no equation printed

Uses

Thulium is the least abundant of the naturally occurring rare earth elements. Metallic thulium is relatively expensive and has only recently become available. It currently has no commercial applications, although one of its isotopes, thulium-169, could be used as a radiation source for portable X-ray machines.

Thulium forms no commercially important compounds. Some of thulium's compounds include: thulium oxide (Tm2O3), thulium fluoride (TmF3) and thulium iodide (TmI3).

Because of the relatively high price of the metal, thulium has not yet found many practical applications. 169Tm bombarded in a nuclear reactor can be used as a radiation source in portable X-ray equipment. 171Tm is potentially useful as an energy source. Natural thulium also has possible use in ferrites (ceramic magnetic materials) used in microwave equipment, and can be used for doping fiber lasers. As with other lanthanides, thulium has a low-to-moderate acute toxic rating. It should be handled with care.

Safety, toxicity

GHS classification, signal word Danger

• H228 Flammable solid Flammable solids
• H319 Causes serious eye irritation Serious eye damage/eye irritation
• H335 May cause respiratory irritation Specific target organ toxicity, single exposure; Respiratory tract irritation

Discovery and name

Discovered by

Per Teodor Cleve

Discovered

1879

First isolated

Wilhelm Klemm and Heinrich Bommer

Named by

not in sources

Origin of the name

after Thule, a mythical region in Scandinavia

Thulium can be isolated by reduction of the oxide with lanthanum metal or by calcium reduction of a closed container. The element is silver-gray, soft, malleable, and ductile, and can be cut with a knife. Twenty five isotopes are known, with atomic masses ranging from 152 to 176. Natural thulium, which is 100% 169Tm, is stable.