In the present study, the feasibility to use phosphate solubilizing bacteria (PSB) to develop a biological leaching process of rare earth elements (REE) from monazite-bearing ore was determined. To predict the REE leaching capacity of bacteria, the phosphate solubilizing abilities of 10 species of PSB were determined by halo zone formation on Reyes minimal agar media supplemented with bromo cresol green together with a phosphate solubilization test in Reyes minimal liquid media as the screening

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... studies. Calcium phosphate was used as a model mineral phosphate. Among the test PSB strains, Pseudomonas fluorescens, P. putida, P. rhizosphaerae, Mesorhizobium ciceri, Bacillus megaterium, and Acetobacter aceti formed halo zones, with the zone of A. aceti being the widest. In the phosphate solubilization test in liquid media, Azospirillum lipoferum, P. rhizosphaerae, B. megaterium, and A. aceti caused the leaching of 6.4%, 6.9%, 7.5%, and 32.5% of calcium, respectively. When PSB were used to leach REE from monazite-bearing ore, ~5.7 mg/L of cerium (0.13% of leaching efficiency) and ~2.8 mg/L of lanthanum (0.11%) were leached by A. aceti, and Azospirillum brasilense, A. lipoferum, P. rhizosphaerae and M. ciceri leached 0.5-1 mg/L of both cerium and lanthanum (0.005%-0.01%), as measured by concentrations in the leaching liquor. These results indicate that determination of halo zone formation was found as a useful method to select OPEN ACCESS Minerals 2015, 5 190 high-capacity bacteria in REE leaching. However, as the leaching efficiency determined in our experiments was low, even in the presence of A. aceti, further studies are now underway to enhance leaching efficiency by selecting other microorganisms based on halo zone formation. Keywords: bioleaching; monazite; phosphate solubilizing bacteria; rare earth element Introduction Rare earth elements (REE) have been increasingly used in the fields of optics, permanent magnetism, electronics, superconductor technology, hydrogen storage, medicine, nuclear technology, secondary battery technology, and catalysis [1-3]. The minerals monazite (a phosphate mineral) and bastnasite (a fluorocarbonate mineral) are the main sources of REE in nature. Generally, monazite contains ~70% rare earth metal oxide, with the rare earth fraction comprising 20%-30% Ce2O3, 10%-40% La2O3, and substantial amounts of neodymium, praseodymium, and samarium. The thorium content is in the range of 4%-12% [2,3]. Caustic soda decomposition and concentrated sulfuric acid digestion have been widely used to decompose monazite for many decades [2, 4] . Due to its high chemical and thermal stability, monazite is very difficult to decompose; therefore, it is essential to eliminate the phosphate present in the ore by chemically attacking the mineral with sulfuric acid or sodium hydroxide at high temperature, so as to enhance the capacity to dissolve the REE. The sulfuric acid process results in a loss of phosphate as H3PO4, corrosion of the processing facilities, toxic gas and wastewater generation, as well as yielding impure products; the process is therefore no longer in commercial use. Caustic soda decomposition has some advantages in terms of the recovery of unreacted alkali and phosphorous, low energy consumption, and simplicity; however, the process also has limitations, such as the need for high-grade ore sources [5, 6] . Biohydrometallurgical technology is an attractive alternative emerging green technology for the recovery of metals due to its environmental friendly, simple, and economic processing. However, very few works have been published on the biological recovery of rare earth metals, in particular, from monazite. Recently thorium, uranium, and REE extraction by microorganisms from monazite concentrate was reported [7, 8] . The authors used Aspergillus ficuum, organic acid producing fungi, and Pseudomonas aeruginosa, organic acid/siderophore producing bacteria. They found that those microorganisms produced citric, oxalic, or 2-ketogluconic acid and dissolved 55% and 47% of REE from monazite by A. ficuum and P. aerunoginosa, respectively. Another study on REE leaching from phosphate minerals apatite and monazite by organic acids such as citric, oxalic, phthalic, and salicylic was also published, even though chemical organic acids were used in this study, which were not biologically produced [9] . Organic acid producing microorganisms secrete organic acids such as malic, gluconic, or oxalic acids [10, 11] , and the mechanism of metal dissolution by the microorganisms is both of acidolysis (protons dissociated from the organic acids) and complexolysis (metal-complexing anions from the acids) [12] .