Holmium

Typical wastewaters

• rare earth separation and processing (saponification wastewater) Ho³⁺ 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 holmium specific concentration read
• coal mine drainage (acid) Ho³⁺ and the HoSO₄⁺ 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

Ho, 67

Oxidation states in water

+3 only (Ho³⁺)

Note

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

2 · Occurrence in water

Natural sources

Weathering of monazite, bastnaesite and xenotime; heavy lanthanide, carbonate complexed in neutral to alkaline water.

Anthropogenic sources

None read. Ho-166 is used in medicine (not read this session); rare earth separation wastewater carries the whole row.

3 · Speciation

Trivalent Ho³⁺ 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 HoSO₄⁺ 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 HoCO₃⁺ and Ho(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 holmium solid was read this session.

Hydrolysis

Hydrolysis of Ho³⁺ is minor in natural water; the hydroxide Ho(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

HoPO₄ (hydrated phosphate), Ho₂(CO₃)₃, HoF₃ near fluoride rich discharges, Ho(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.

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; Ho 8,511); 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 holmium: the RIVM report set risk limits only for Y, La, Ce, Pr, Nd, Sm, Gd and Dy. The one holmium number it carries is a field bioconcentration factor in amphipods of 8,511 (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 holmium specific river, groundwater or wastewater concentration was read, and no toxicity test.

Sources

Identity

Name and symbol

Holmium, Ho

Atomic number

67 protons

Position

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

CAS number

7440-60-0

Atomic structure

Atomic mass

164.93 u

Electron configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹¹
[Xe] 6s²⁴f¹¹

Electrons per shell

2, 8, 18, 29, 8, 2

Valence electrons

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

Physical properties

State at room temperature

Solid

Melting point

1,747 K (1,473.85 °C)

Boiling point

2,973 K (2,699.85 °C)

Density

8.8 g/cm3

Appearance

silvery white

Thermal conductivity

16.2 W/(m·K)

Electrical resistivity

poly: 814 nΩ·m

Electrical conductivity

1.23 MS/m

Crystal structure

hexagonal close packed

Molar heat capacity

27.15 J/(mol·K)

Chemical properties

Oxidation states

+3

Electronegativity

1.23 (Pauling Scale)

Ionisation energy

6.022 eV
1st 581, 2nd 1,140, 3rd 2,204 kJ/mol

Electron affinity

not in sources

Atomic radius

empirical 192, covalent 192, van der Waals 216 pm

Ionic radius

Ho³⁺ 90 pm

Reactivity

A late lanthanide (4f11 6s2, electronegativity 1.23) that is trivalent in essentially all its compounds; the metal is fairly corrosion resistant in dry air but oxidises quickly in moist air or when heated and reacts with water and acids.

with water

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

with oxygen, air

Stable in dry air at room temperature but tarnishes in moist air to a yellowish oxide film and burns readily on heating: $\ce{4 Ho + 3 O2 -> 2 Ho2O3}$, the pale yellow sesquioxide.

with acids

Dissolves readily in dilute sulfuric acid to yellow Ho3+ solutions with evolution of hydrogen: $\ce{2 Ho + 3 H2SO4 -> Ho2(SO4)3 + 3 H2}$, the sulfate.

with halogens

Reacts with all the stable halogens to the trihalides: $\ce{2 Ho + 3 Cl2 -> 2 HoCl3}$, yellow, while the fluoride is pink.

Typical compounds

• Ho₂O₃ holmium(III) oxide holmia, yellow oxide that colours glass and cubic zirconia
• HoCl₃ holmium(III) chloride yellow, soluble trihalide
• HoF₃ holmium(III) fluoride pink trihalide, reduced to make the metal
• Ho(OH)₃ holmium(III) hydroxide product of the metal with water
• Ho₂(SO₄)₃ holmium(III) sulfate yellow salt from sulfuric acid

Occurrence, production and use

Crustal abundance

1.3 milligrams per kilogram

Oceanic abundance

2.2×10-7 milligrams per liter

Occurrence and sources

• monazite and bastnaesite a minor component; recovered from ores processed for yttrium

Extraction, production

not in sources

Uses

not in sources

Safety, toxicity

GHS classification, signal word Danger

• H228 Flammable solid Flammable solids

Discovery and name

Discovered by

Per Theodor Cleve, Jacques-Louis Soret and Marc Delafontaine

Discovered

1878

First isolated

Heinrich Bommer

Named by

not in sources

Origin of the name

from Holmia, the Latin name for the city of Stockholm, home town of its discoverer

Pure holmium has a metallic to bright silver luster. It is relatively soft and malleable and is stable in dry air at room temperature but rapidly oxidizes in moist air and at elevated temperatures. The metal has unusual magnetic properties. Few uses have yet been found for the element. The element, as with other rare earths, seems to have a low acute toxic rating.