Lanthanum

Typical wastewaters

• fluid catalytic cracking catalyst manufacture (Rhine, river-km 447.4) dissolved La³⁺ and its carbonate complexes at up to 49 mg/kg in the effluent (total REE 52 mg/kg) raised dissolved La in the Rhine from 1.91 to 338 ng/kg; about 1.5 t a year to the North Sea
• phosphate fertiliser manufacture (phosphogypsum slurry) La³⁺ precipitating as LaF₃ near the outfall and as phosphate further away 93 t of La emitted to the Rhine estuary in 1994 by the larger of two plants
• mine drainage (coal) La³⁺ and the LaSO₄⁺ ion pair in acid sulfate water; total REE averaged 282 µg/L over 141 sites leaves solution with the Fe, Al and Mn hydroxides on neutralisation

1 · Identity

Symbol, number

La, 57

Oxidation states in water

+3 only (La³⁺); no redox chemistry in water

Note

The metal's reaction with water is in the book entry; in natural water lanthanum is a large, hard trivalent cation that pairs with carbonate, phosphate and fluoride.

2 · Occurrence in water

Natural sources

Weathering of monazite and bastnaesite and of the accessory phosphates in ordinary rock; released most where water is acid. Dutch groundwater carried up to 2 µg/L at pH at or above 6.2 and up to 105 µg/L below pH 6.2 (RIVM, citing Stuyfzand 1991).

Anthropogenic sources

Effluent of a fluid catalytic cracking catalyst plant on the Rhine at river-km 447.4 (dissolved La up to 49 mg/kg in the effluent, total REE 52 mg/kg), which raised dissolved La from 1.91 ng/kg upstream at Mannheim to 338 ng/kg at Mainz and still 46.6 ng/kg (94 percent anthropogenic) 400 km downstream at the Dutch border, about 1.5 t of anthropogenic La a year to the North Sea; phosphogypsum slurry from phosphate fertiliser plants in the Rhine estuary (93 t of La emitted to water in 1994 by the larger of two plants, the REE precipitating as fluorides near the outfall and as phosphates further away); lanthanum-modified bentonite applied to about 200 lakes and reservoirs; rare earth processing wastewater.

3 · Speciation

Trivalent La³⁺ 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 LaSO₄⁺ 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 LaCO₃⁺ and La(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. For lanthanum specifically, the RIVM report names lanthanum fluoride, carbonate and phosphate as the solubility controlling solids, the carbonate mattering most at high pH, and puts the free La³⁺ fraction at 0.3 fM to 9 pM.

Solubility

Lanthanum fluoride, carbonate and phosphate control solubility (carbonate most at high pH); total dissolved La in Dutch surface water about 1 µg/L or less; free ion 0.3 fM to 9 pM (RIVM citing Maas and Botterweg 1993). REE phosphate solubility products can be as low as 10⁻25.

Hydrolysis

Hydrolysis of La³⁺ is minor in natural water; the hydroxide La(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

LaPO₄.nH₂O (rhabdophane, confirmed by NMR and EXAFS in treated lake sediments, with monazite LaPO₄ in the oldest), LaF₃ near fluoride rich discharges, La₂(CO₃)₃ at high pH, La(OH)₃ only under lime.

4 · Role in treatment

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. A positive La anomaly (La normalised to shale against its neighbours) of more than 1.5 was the thesis's conservative threshold for calling La anthropogenic.

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. Kulaksiz and Bau note that the catalyst plant effluent La exceeds concentrations with reported ecotoxicological effects; Herrmann et al. (2016) found no regulatory thresholds for REE anywhere and made preliminary water and sediment quality suggestions (values not read). Anthropogenic La is bioavailable to freshwater mussels.

Bioaccumulation

Field bioconcentration factors in Rhine estuary amphipods fall from the light to the heavy lanthanides (La 28,840 to Lu 4,786; La 28,840); 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

RIVM (2000) environmental risk limits, not discharge limits: MPC 10.1 µg/L in fresh surface water and 1.01 µg/L in salt water (negligible concentrations 0.18 and 0.02 µg/L), derived as MPA 10 µg/L from the lowest NOEC divided by 10 (fresh); lowest LC₅₀ divided by 1000 (salt) plus a background set at the detection limit (0.08 µg/L). Acute data behind it: Daphnia magna 48 h EC₅₀ 24 mg/L, zebrafish 96 h LC₅₀ 23 mg/L, alga 72 h EC₅₀ 1.3 mg/L; chronic: Daphnia magna 21 d NOEC 0.1 mg/L, carp 21 d NOEC 0.26 mg/L, alga 72 h NOEC 1.4 mg/L. Field bioconcentration factor in amphipods (porewater basis) 28,840. Lanthanum concentrations during lanthanum-modified bentonite applications are generally below acute thresholds except in low alkalinity waters (Copetti review).

Flags

• Dutch groundwater and drinking water figures are 1991 to 1994 data compiled by RIVM in 2000.
• The Rhine figures are single sampling campaigns with a point source; they are not a river background.
• The RIVM MPCs are environmental risk limits from a 2000 report, mostly LC₅₀ divided by 1000, with background set at the detection limit; they are not permit limits.
• The lanthanum-modified bentonite figures are maxima across 16 lakes; the modelled La³⁺ values depend on the speciation code used.
• The Spears 2013 title carries a product name; it is not written here.
• The 5 percent lanthanum content of the modified bentonite comes from a search summary of the Copetti review, not from the abstract read.

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.
• Lanthanum carbonate as an oral phosphate binder (a pharmaceutical) was not read; its excretion route into sewage is not described.
• No removal percentage for lanthanum in activated sludge was read.
• Herrmann et al. 2016 proposed water and sediment quality criteria for lanthanum; the values are in the full text, which was not read.

Sources

Identity

Name and symbol

Lanthanum, La

Atomic number

57 protons

Position

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

CAS number

7439-91-0

Atomic structure

Atomic mass

138.905 u

Electron configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 5d¹
[Xe] 6s²⁵d¹

Electrons per shell

2, 8, 18, 18, 9, 2

Valence electrons

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

Physical properties

State at room temperature

Solid

Melting point

1,191 K (917.85 °C)

Boiling point

3,737 K (3,463.85 °C)

Density

6.15 g/cm3

Appearance

silvery white

Thermal conductivity

13.4 W/(m·K)

Electrical resistivity

α, poly: 615 nΩ·m

Electrical conductivity

1.63 MS/m

Crystal structure

double hexagonal close packed

Molar heat capacity

27.11 J/(mol·K)

Chemical properties

Oxidation states

+3

Electronegativity

1.1 (Pauling Scale)

Ionisation energy

5.577 eV
1st 538.1, 2nd 1,067, 3rd 1,850.3 kJ/mol

Electron affinity

0.5 eV

Atomic radius

empirical 207, covalent 207, van der Waals 240 pm

Ionic radius

La³⁺ 103 pm

Reactivity

One of the most reactive of the rare-earth metals, soft enough to cut with a knife: it tarnishes within hours in air, the oxide spalls off so a small piece corrodes completely in a year, and it reacts directly with water, the halogens and most non-metals.

with water

Attacked slowly by cold water and much faster by hot water, giving lanthanum hydroxide and hydrogen: $\ce{2 La + 6 H2O -> 2 La(OH)3 + 3 H2}$.

with oxygen, air

Oxidises rapidly in air, turning dark within hours, and burns readily to the strongly basic sesquioxide: $\ce{4 La + 3 O2 -> 2 La2O3}$.

with acids

Dissolves readily in dilute acids to colourless La3+ salts and hydrogen: $\ce{2 La + 3 H2SO4 -> 2 La^3+ + 3 SO4^2- + 3 H2}$; hydrofluoric acid forms a protective LaF3 layer instead.

with halogens

Reacts with the halogens even at room temperature to the trihalides: $\ce{2 La + 3 Cl2 -> 2 LaCl3}$.

Typical compounds

• La₂O₃ lanthanum(III) oxide optical glass for camera and telescope lenses
• LaF₃ lanthanum fluoride reduced with calcium for the metal; fluoride electrodes
• LaCl₃ lanthanum chloride typical trihalide, deliquescent
• La(OH)₃ lanthanum hydroxide product of the water reaction
• LaNi₅ lanthanum nickel hydrogen-storage alloy in NiMH batteries
• La₂(CO₃)₃ lanthanum carbonate phosphate binder drug for kidney disease

Occurrence, production and use

Crustal abundance

3.9×101 milligrams per kilogram

Oceanic abundance

3.4×10-6 milligrams per liter

Occurrence and sources

Lanthanum is found in rare-earth minerals such as cerite, monazite, allanite, and bastnasite. Monazite and bastnasite are principal ores in which lanthanum occurs in percentages up to 25 percent and 38 percent respectively. Misch metal, used in making lighter flints, contains about 25 percent lanthanum.

The availability of lanthanum and other rare earths has improved greatly in recent years. The metal can be produced by reducing the anhydrous fluoride with calcium.

• monazite, a rare earth phosphate (about 25 percent lanthanum) heavy-mineral-sand concentrates, stockpiled as a separated concentrate or an accessory mineral in the southeastern United States; the principal thorium mineral
• bastnaesite, a rare earth fluorocarbonate (about 38 percent lanthanum) mined as a primary product at Mountain Pass, California; the main Chinese deposits are not itemised by the source
• rare earth metals (mainly lanthanum, cerium, praseodymium, neodymium) carried with phosphate rock into wet-process phosphoric acid and discharged with phosphogypsum reported emission to water on disposal of phosphogypsum: 2,200 g and 360 g of rare earth metals per tonne P2O5 at two plants (HDH-1 and HDH-2 processes, 1996/97 data, plants since closed because of the discharge to sea); Table 5.8 of the LVIC-AAF BREF, PDF p253, printed p225; the ledger's chemical chapter holds this under the phosphoric-acid hub

Extraction, production

Separation of the rare earths from monazite or bastnaesite concentrates by ion exchange and solvent extraction

no balanced equation is printed by the sources; the oxide and chloride are the traded intermediates

Reduction of anhydrous lanthanum fluoride with calcium to the metal

$\ce{2 LaF3 + 3 Ca -> 2 La + 3 CaF2}$

the usual route to lanthanum metal; stoichiometry not printed by the source, so no equation is written

Uses

Lanthanum is one of the rare earth elements used to make carbon arc lights which are used in the motion picture industry for studio lighting and projector lights. Lanthanum also makes up about 25% of Misch metal, a material that is used to make flints for lighters. Lanthana (La2O3) is used to make the glass used in camera lenses and in other special glasses.

Rare-earth compounds containing lanthanum are extensively used in carbon lighting applications, especially by the motion picture industry for studio lighting and projection. This application consumes about 25 percent of the rare-earth compounds produced. La2O3 improves the alkali resistance of glass, and is used in making special optical glasses. Small amounts of lanthanum, as an additive, can be used to produce nodular cast iron.

There is current interest in hydrogen sponge alloys containing lanthanum. These alloys take up to 400 times their own volume of hydrogen gas, and the process is reversible. Every time they take up the gas, heat energy is released; therefore these alloys have possibilities in an energy conservation system.

• Petroleum refining catalysts: lanthanum salts in catalysts for petroleum refining catalysts were the estimated leading United States end use of rare earths in 2024 (usgs-mcs2025, PDF p148, printed p144)
• Batteries and hydrogen storage: lanthanum-nickel alloy anodes of nickel metal hydride batteries in hybrid cars; lanthanum-nickel alloy for storing hydrogen gas limited quantities of rare earths were recovered from batteries in the United States in 2024 (usgs-mcs2025)
• Glass and optics: lanthanum(III) oxide in special optical glass, raising the refractive index and alkali resistance for camera and telescope lenses ceramics and glass were among the United States end uses of rare earths in 2024 (usgs-mcs2025)
• Steel and metallurgy: mischmetal (about 20 percent lanthanum in the RSC account; 65 percent cerium and 35 percent lanthanum in the USGS price grade) for lighter flints and pyrophoric alloys; small additions to iron and steel to reduce brittleness and to tungsten for arc-welding electrodes mischmetal averaged about 5 dollars per kilogram in 2024 (usgs-mcs2025, PDF p148, printed p144)
• Rare earth mining and separation: mined as bastnaesite (a rare-earth fluorocarbonate) at Mountain Pass, California, and as monazite (a phosphate) in heavy-mineral-sand concentrates; separated from the other rare earths by ion exchange and solvent extraction world mine production of rare earths 390,000 t of rare-earth-oxide equivalent in 2024 (USGS estimate), of which China 270,000 t (production quota), the United States 45,000 t, Burma 31,000 t, Australia, Nigeria and Thailand 13,000 t each; reserves over 90 million t, China 44 million t, Brazil 21 million t (usgs-mcs2025, PDF p149, printed p145)

Safety, toxicity

Lanthanum and its compounds have a low to moderate acute toxicity rating; therefore, care should be taken in handling them.

GHS classification, signal word Danger

• H228 Flammable solid Flammable solids
• H260 In contact with water releases flammable gases which may ignite spontaneously Substances and mixtures which in contact with water, emit flammable gases

Discovery and name

Discovered by

Carl Gustaf Mosander

Discovered

1838

First isolated

not in sources

Named by

not in sources

Origin of the name

from the Greek λανθάνειν, "to lie hidden", for 'hiding' in cerite

Lanthanum is silvery white, malleable, ductile, and soft enough to be cut with a knife. It is one of the most reactive of the rare-earth metals. It oxidizes rapidly when exposed to air. Cold water attacks lanthanum slowly, while hot water attacks it much more rapidly.

The metal reacts directly with elemental carbon, nitrogen, boron, selenium, silicon, phosphorus, sulfur, and with halogens.

At 310°C, lanthanum changes from a hexagonal to a face-centered cubic structure, and at 865°C it again transforms into a body-centered cubic structure.