Lanthanum arsenate is an inorganic compound of lanthanum and the arsenate ion, with the chemical formula LaAsO4. It is a sparingly soluble crystalline solid and adopts the monoclinic monazite structure type. The ideal composition LaAsO4 also occurs naturally as the rare mineral gasparite-(La).
Preparation Lanthanum arsenate can be prepared hydrothermally from lanthanum nitrate and arsenic acid. A modern synthesis used equimolar 0.3 M solutions of La(III) and As(V), reacted at 160 °C for 24 hours:
La(NO3)3 + H3AsO4 → LaAsO4 + 3 HNO3
Single crystals have also been obtained from alkali- and rare-earth-metal chloride fluxes. In one synthesis, unintended ingress of air oxidized arsenic to As(V), giving colorless LaAsO4 crystals as a by-product.
Properties Crystal structure Lanthanum arsenate crystallizes in the monoclinic crystal system, space group P21/n (No. 14), with the monazite structure type.
Single-crystal diffraction gives the lattice parameters a = 6.7615 Å, b = 7.2103 Å, c = 7.0056 Å, β = 104.507° with four formula units per unit cell.
The structure contains isolated tetrahedra and irregular La–O coordination polyhedra. Nine oxygen atoms from seven arsenate tetrahedra lie in the primary coordination sphere of La3+, with a more distant tenth oxygen contact also sometimes included.
As–O bond lengths lie at about 1.67–1.69 Å, while the La–O distances in the principal coordination sphere are approximately 2.48–2.91 Å.
LaAsO4 represents the large-cation, monazite-type end of the rare-earth arsenate structural series; the smaller rare-earth arsenates instead adopt the tetragonal xenotime structure.
Solubility and stability Lanthanum arsenate is only slightly soluble in water and is particularly stable under mildly acidic to neutral conditions.
Hydrothermally synthesized LaAsO4 released less than 1 mg/L arsenic during a 24-hour leaching test at pH 5. Longer equilibrium experiments at 22 °C gave dissolved arsenic concentrations of about 4 mg/L at pH 5 and 13 mg/L at pH 7.
The dissolution was found to be incongruent, rather than a simple stoichiometric dissolution of LaAsO4.
Natural occurrence The ideal composition LaAsO4 occurs naturally as gasparite-(La), a member of the monazite group approved as a mineral species in 2018.
Gasparite-(La) is monoclinic, space group P21/n, and is structurally equivalent to synthetic LaAsO4. Natural samples are not compositionally pure: La is partly replaced by Ce, Nd and other cations, while As may be partly replaced by P or V.
Arsenic immobilization The low solubility of LaAsO4 has led to investigation of lanthanum compounds for removing arsenate from contaminated water. Lanthanum hydroxide, carbonate and related materials can remove dissolved As(V), with formation of poorly soluble lanthanum arsenate contributing under acidic conditions.
Precipitation-based processes using lanthanum chloride have also been studied over approximately pH 3–10 for arsenic stabilization.
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