Effects of Energy Sources on Rare Earth Elements Bioleaching from Phosphogypsum by Acidithiobacillus ferrooxidans

Abstract

Effects of different energy sources (ferrous iron, elemental sulfur and their mixture) on the bioleaching efficiency of rare earth elements (REEs) from phosphogypsum (PG) by using Acidithiobacillus ferrooxidans CL (A. ferrooxidans) were investigated in this paper. Potassium and ammonium jarosite was detected in the residues of bioleaching experiments using the ferrous iron and the mixture as the energy source. XRD patterns and SEM images showed that jarosite intercepted the microbial attachment to the mineral surfaces because it was formed as a passivation layer. During the bioleaching process using ferrous iron as the energy source, redoxolysis reaction of PG by ferric iron produced in the oxidation of ferrous iron doesn’t perform. In the bioleached residue of using elemental sulfur as the sole energy source, jarosite was not detected and the pH value was 1.25, much lower than that of using the ferrous iron. REEs in PG exist in the form of phosphate and the dissolution of REEs from PG is described as the acidolysis process. The maximum total REEs extraction was found in the test using the elemental sulfur due to the lowest pH value and no formation of jarosite.

Introduction

Rare earth elements (REEs) possess unique physicochemical properties essential for many modern technologies. Phosphogypsum (PG), a major by-product of phosphoric acid production, contains 0.01–0.4% REEs, and with 200–300 million tons of PG produced annually, up to 100,000 tons of REEs are lost each year—comparable to current global REE oxide production. This has motivated growing interest in recovering REEs from PG.

Bioleaching, particularly using the chemoautotrophic bacterium Acidithiobacillus ferrooxidans, is considered a promising method for extracting REEs from low-grade waste materials due to its high selectivity and efficiency. A. ferrooxidans can oxidize ferrous iron, elemental sulfur, or metal sulfides, producing ferric iron and sulfuric acid that enhance mineral dissolution. Earlier work showed that REE extraction from PG using A. ferrooxidans with ferrous sulfate is driven largely by proton generation during jarosite formation. However, the effectiveness of elemental sulfur as an energy source for REE bioleaching had not been evaluated.

Previous studies report varying effects of ferrous iron and sulfur—used alone or in combination—on bioleaching efficiency depending on the target mineral and metal. Therefore, this study investigates the influence of different energy sources (ferrous iron, elemental sulfur, and their mixture) on REE bioleaching performance and mechanisms for PG.

Phosphogypsum from Hubei, China, consisting mainly of gypsum and quartz, contained 58.8 mg/kg total REEs, primarily La, Ce, Nd, and Y. Bioleaching experiments were conducted for 30 days using A. ferrooxidans under three conditions: ferrous iron alone (test 1), elemental sulfur alone (test 2), and a mixed energy source (test 3), with abiotic controls included.

Results showed that REE extraction efficiencies were significantly higher in biotic systems than in controls. Among the energy sources, elemental sulfur (test 2) achieved the highest extraction of La, Nd, and Y (≈31–37%). Ce extraction was highest with the mixed energy source (test 3). Ferrous iron alone (test 1) gave the lowest REE recovery, likely due to its relatively higher pH (1.76), which reduces leaching efficiency. These trends differ from bioleaching behaviors observed in other minerals such as realgar or vanadium-bearing shale, indicating that energy-source effects are REE- and substrate-dependent.

Conclusion

In this paper, the performance and mechanism of REEs bioleaching from PG by A. ferrooxidans using different energy sources such as ferrous iron, elemental sulfur and their mixture. REEs in PG exist in the form of phosphate and the dissolution of REEs from PG can be described by acidolysis process. In the first three days of test, ferrous irons as the energy source were all oxidized to ferric irons and the ferric irons react with the potassium and ammonium cations in the solution to form jarosite. During the formation of jarosite, lots of protons are produced and the pH value decreases, however, jarosite decreases the bioleaching efficiency by forming passivation layer on the surfaces of mineral particles. During the test of using elemental sulfur as the sole energy source, jarosite was not formed and the pH value of leachate decreased to 1.25 by the sulfuric acid produced in microbial metabolism, which promoted the acidolysis of PG to extract the maximum REEs.

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Copyright

Copyright © 2025 Kichang Pak, Chol Hong, Chol Jin Jon. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.