Biotransformation of lanthanum by Aspergillus niger

Abstract

Lanthanum is an important rare earth element and has many applications in modern electronics and catalyst manufacturing. However, there exist several obstacles in the recovery and cycling of this element due to a low average grade in exploitable deposits and low recovery rates by energy-intensive extraction procedures. In this work, a novel method to transform and recover La has been proposed using the geoactive properties of Aspergillus niger. La-containing crystals were formed and collected after A. niger was grown on Czapek-Dox agar medium amended with LaCl₃. Energy-dispersive X-ray analysis (EDXA) showed the crystals contained C, O, and La; scanning electron microscopy revealed that the crystals were of a tabular structure with terraced surfaces. X-ray diffraction identified the mineral phase of the sample as La₂(C₂O₄)₃·10H₂O. Thermogravimetric analysis transformed the oxalate crystals into La₂O₃ with the kinetics of thermal decomposition corresponding well with theoretical calculations. Geochemical modelling further confirmed that the crystals were lanthanum decahydrate and identified optimal conditions for their precipitation. To quantify crystal production, biomass-free fungal culture supernatants were used to precipitate La. The results showed that the precipitated lanthanum decahydrate achieved optimal yields when the concentration of La was above 15 mM and that 100% La was removed from the system at 5 mM La. Our findings provide a new aspect in the biotransformation and biorecovery of rare earth elements from solution using biomass-free fungal culture systems.

Keywords: Aspergillus niger; Biorecovery; Biotransformation; Lanthanum; Rare earth elements.

Fig. 1

Colony expansion rate of A. niger on solid Czapek-Dox media containing 0 mM (empty circle), 1 mM (filled circle), 5 mM (empty square) and 10 mM (filled square) LaCl₃ over 6-day incubation at 25 °C in the dark. Data are averages of at least three replicates with error bars (shown only when greater than symbol dimensions) showing the standard error of the mean

Fig. 2

Formation of crystals in solid Czapek-Dox medium after 2 weeks incubation of A. niger at 25 °C in the dark. Plates were supplemented with a, d 1 mM, b, e 5 mM and c, f 10 mM LaCl₃. (d–f, scale bars = 100 μm). Typical images are shown from several similar examinations

Fig. 3

Scanning electron microscopy of a, b mycogenic crystals purified from 1 mM La-containing Czapek-Dox medium after 14-day incubation at 25 °C in the dark with A. niger and c, d crystals from chemical reaction of 25 mM oxalic acid and 1 mM lanthanum chloride. (a–c, scale bars = 50 μm; d, scale bar = 5 μm). Typical images are shown from several similar examinations

Fig. 4

Energy-dispersive X-ray analysis of (a) mycogenic crystals formed in solid Czapek-Dox medium containing 1 mM LaCl₃ after incubation with A. niger for 14 days at 25 °C in the dark; EDXA of crystals precipitated by reactions of (b) 1 mM and (c) 40 mM LaCl₃ with biomass-free culture supernatant of NO₃⁻-containing AP1 medium that was incubated with A. niger for 7 days at 25 °C in the dark. Typical spectra are shown from several similar determinations

Fig. 5

X-ray diffraction of (a) mycogenic crystals obtained from solid Czapek-Dox medium containing 1 mM LaCl₃ and incubated with A. niger for 14 days at 25 °C in the dark; (b) XRD of the above-mentioned sample after thermogravimetric treatment at 1000 °C until constant weight. The standard patterns shown below the XRD patterns are (a) lanthanum oxalate decahydrate (JCPDS card no. 20-549) and (b) lanthanum oxide (JCPDS card no. 05-602). Typical patterns are shown from several similar determinations

Fig. 6

Thermogravimetric analysis of mycogenic crystals obtained from solid Czapek-Dox medium amended with 1 mM LaCl₃ and incubated with A. niger for 14 days at 25 °C in the dark. DTG denotes differential thermal gravimetry, which is expressed as percentage of weight loss per minute. A typical pattern is shown from several similar determinations, all of which gave similar results

Fig. 7

a Precipitation yield and c La removal rates for reactions of 1 ml LaCl₃ and 9 ml biomass-free supernatant of NO₃⁻-AP1 medium which was incubated with A. niger for 7 days at 25 °C in the dark. b Precipitation yield and d La removal rates for chemical reactions of 9 ml 25 mM oxalic acid with 1 ml LaCl₃. The amount of La removed after liquid reactions (empty circle); dry wt of precipitate (filled circle); removal rate of La (empty square); pH after reaction (filled square). All data are given as means of at least three independent replicates. Standard deviation is represented by error bars which are not shown when smaller than the symbols

Fig. 8

Scanning electron microscopy of crystal samples from reactions of a 1, b 7, c 30, and d 50 mM LaCl₃ with biomass-free culture supernatant of NO₃⁻-AP1 medium grown with A. niger for 7 days at 25 °C in the dark. (a–d, scale bars = 5 μm). Typical images are shown from several similar examinations

Fig. 9

X-ray diffraction of crystals from reaction of 20 mM LaCl₃ with biomass-free culture supernatant of NO₃⁻-AP1 medium after incubation with A. niger for 7 days at 25 °C in the dark. Standard patterns of La₂(C₂O₄)₃·10H₂O and La₇P₃O₁₈ are also shown. A typical result is shown from one of several similar determinations, all of which gave similar results

Fig. 10

a Speciation diagram of pH versus log [(C₂O₄)²⁻] in the presence of 1 mM LaCl₃ and b pH vs log [La³⁺] in the presence of 25 mM oxalic acid. All simulated reactions were set at 25 °C under 1.013 bars atmosphere pressure, and the activities of all chemicals contained in the media were calculated using Geochemist’s Workbench in accordance with their actual concentrations