Abstract
Monazite is a naturally occurring lanthanide (Ln) phosphate mineral [Ln x (PO4 ) y ] and is the main industrial source of the rare earth elements (REE), cerium and lanthanum. Endeavours to ensure the security of supply of elements critical to modern technologies view bioprocessing as a promising alternative or adjunct to new methods of element recovery. However, relatively little is known about microbial interactions with REE. Fungi are important geoactive agents in the terrestrial environment and well known for properties of mineral transformations, particularly phosphate solubilization. Accordingly, this research examined the capability of a ubiquitous geoactive soil fungus, Aspergillus niger, to affect the mobility of REE in monazite and identify possible mechanisms for biorecovery. It was found that A. niger could grow in the presence of monazite and mediated the formation of secondary Ce and La-containing biominerals with distinct morphologies including thin sheets, orthorhombic tablets, acicular needles, and rosette aggregates which were identified as cerium oxalate decahydrate (Ce2 (C2 O4 )3 ·10H2 O) and lanthanum oxalate decahydrate (La2 (C2 O4 )3 ·10H2 O). In order to identify a means for biorecovery of REE via oxalate precipitation the bioleaching and bioprecipitation potential of biomass-free spent culture supernatants was investigated. Although such indirect bioleaching of REE was low from the monazite with maximal lanthanide release reaching >40 mg L-1 , leached REE were efficiently precipitated as Ce and La oxalates of high purity, and did not contain Nd, Pr and Ba, present in the original monazite. Geochemical modelling of the speciation of oxalates and phosphates in the reaction system confirmed that pure Ln oxalates can be formed under a wide range of chemical conditions. These findings provide fundamental knowledge about the interactions with and biotransformation of REE present in a natural mineral resource and indicate the potential of oxalate bioprecipitation as a means for efficient biorecovery of REE from solution.
Highlights
Monazite is a group of monoclinic phosphates mainly comprising rare earth elements (REE), e.g. cerium, lanthanum and neodymium, and has long been regarded as a strategic resource coveted by the World’s great powers such as China, Europe, USA and Japan because of its critical involvement in high-technology sectors including consumer electronics, clean energy, hybrid electric vehicles and weapons systems (Humphries, 2012; Massari and Ruberti, 2013; Goodenough et al, 2016)
Such indirect bioleaching of REE was low from the monazite with maximal lanthanide release reaching >40 mg L−1, leached REE were efficiently precipitated as Ce and La oxalates of high purity, and did not contain Nd, Pr and Ba, present in the original monazite
These findings provide fundamental knowledge about the interactions with and biotransformation of REE present in a natural mineral resource and indicate the potential of oxalate bioprecipitation as a means for efficient biorecovery of REE from solution
Summary
Monazite is a group of monoclinic phosphates mainly comprising rare earth elements (REE), e.g. cerium, lanthanum and neodymium, and has long been regarded as a strategic resource coveted by the World’s great powers such as China, Europe, USA and Japan because of its critical involvement in high-technology sectors including consumer electronics, clean energy, hybrid electric vehicles and weapons systems (Humphries, 2012; Massari and Ruberti, 2013; Goodenough et al, 2016). More than 70% of the global supply of monazite is restricted to only a few rich mine deposits and the content of Ce in commercial monazite concentrate ranges from 42.7% (Guangdong, China) to 51.0% (Mount Weld, Australia) while that of La ranges from 17.5% (Green Cove Springs, USA) to 26.0% (Mount Weld, Australia) of the total REE present (Massari and Ruberti, 2013; Moss et al, 2013; Kumari et al, 2015). Gadd for recovering REE from concentrate include direct leaching using strong acids, e.g. sulphuric, nitric, and hydrochloric acid or strong alkalis, e.g. sodium hydroxide (Panda et al, 2014; Kumari et al, 2015): LnPO4ðsÞ + 3H + ðaqÞ ! Ln3 + ðaqÞ + H3PO4ðaqÞ ð1Þ
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