A critical review of bioleaching of rare earth elements : The mechanisms and effect of process parameters

Rare earth elements (REEs) are important high-tech materials with widespread industrial applications and a high risk of supply disruption. Current physico-chemical methods for REE recovery are energy intensive, complex, and costly. Thus, it is crucial to undertake measures to secure future REE demand and protect the environment as well. Bioleaching as an alternative technology for the recovery and recycling of REEs from a variety of primary and secondary sources has been increasingly explored. The present review provides an overview on REE content of various primary and secondary sources, applied bioleaching microorganisms and methods, reactor setups, as well as a detailed description of known REE leaching mechanisms. The effects of different process parameters including temperature, pH, medium composition, pulp density, and particle size on efficiency of REE leaching are also evaluated in detail.acceptedVersionPeer reviewe

Low residual dissolved phosphate in spent medium bioleaching enables rapid and enhanced solubilization of rare earth elements from end-of-life NiMH batteries

Successful heterotrophic bioleaching with high metal yields requires an efficient leaching agent production and minimization of secondary reactions such as precipitation of leached metals with growth medium components. In this study, the role of the secondary reactions on bioleaching of spent nickel-metal-hydride batteries was investigated. Substitution of K2HPO4 by yeast extract (YE) reduced precipitation of both base metals and rare earth elements (REEs). REEs were proportionally more affected by precipitation than base metals. Optimizing the ratio of YE to glucose in the growth medium resulted in glucose to gluconic acid conversion yield of 90% by Gluconobacter oxydans. In one-day leaching with YE medium, 28.8% Mn, 52.8% Fe, 22.9% Co, 12.0% Ni, and 19.5% of total REEs were extracted. The leach liquor obtained with the YE medium resulted in leaching of 1.5 and 11.0 times more of total base metals and REEs, respectively, than with phosphate medium. Experimental results were consistent with geochemical modeling results corroborating the benefit of low phosphate concentrations in leaching systems at neutral to moderately acidic pH. In summary, substitution of K2HPO4 with YE in gluconic acid production phase with G. oxydans reduced subsequent base metal and especially REE precipitation during leaching and, thus, enhanced the overall metal extraction.publishedVersionPeer reviewe

Leaching of rare earth elements and base metals from spent NiMH batteries using gluconate and its potential bio-oxidation products

Gluconate is known to mediate metal leaching. However, during bioleaching by e.g., Gluconobacter oxydans, gluconate can be oxidized to 2-ketogluconate and 5-ketogluconate. The impact of bio-oxidation of gluconate on metal leaching has not been investigated. Therefore, the aim of this study was to investigate leaching of rare earth elements (REEs) and base metals from spent nickel-metal-hydride (NiMH) batteries using gluconate, 2-ketogluconate and 5-ketogluconate. Batch leaching assays were conducted under controlled and uncontrolled pH conditions for 14 days using 60 mM of either the individual leaching agents or their various combinations. At target pH of 6.0 ± 0.1 and 9.0 ± 0.1 and without pH control, complexolysis was the dominating leaching mechanism and higher REE leaching efficiency was obtained with gluconate, while 5-ketogluconate enabled more efficient base metal leaching. At target pH of 3.0 ± 0.1, acidolysis dominated, and the base metal and REE leaching yields with all the tested leaching agents were higher than under the other studied pH conditions. The highest base metal and REE leaching yields (%) were obtained using gluconate at target pH of 3.0 ± 0.1 being 100.0 Mn, 90.3 Fe, 89.5 Co, 58.5 Ni, 24.0 Cu, 29.3 Zn and 56.1 total REEs. The obtained results are useful in optimization of heterotrophic bioleaching.publishedVersionPeer reviewe

Bio-oil stability through stepwise pyrolysis of groundnut shells : Role of chemical composition, alkali and alkaline earth metals, and storage conditions

Bio-oil obtained from crop residues is unstable because of multiple reactive oxygenated compounds and alkali and alkaline earth metals (AAEMs), which hinder its use as a chemical feedstock. In the current study, stepwise pyrolysis of groundnut shells was performed in three-step (160/200–320/340−600 °C), two-step (320/340−600 °C), and continuous (600 °C) heating regimes to obtain a more stable bio-oil. The stability of bio-oils was compared in terms of changes to their pH, water content, AAEM concentration, and chemical composition over a fixed period. During three-step pyrolysis, 94.85–97.38 % of chemicals (anhydrosugars, organic acids, aldehydes, and ketones) were concentrated in step 2. In contrast, for two-step pyrolysis, 93.14–94.78 % were concentrated in step 1. The AAEMs transferred from groundnut shells to bio-oil were 7.04–9.63 % for three-step, 6.68–11.26 % for two-step, and 11.87 % for continuous pyrolysis. An accelerated aging test (80 °C for 24 h) showed that two-step pyrolysis at 340 °C and 600 °C, produced the most stable bio-oil despite a higher concentration of AAEMs. Further improvement in this bio-oil's stability was explored by altering storage temperature and adding solvents to the bio-oil. The decrease in concentrations of anhydrosugars and phenolics was <1% after four weeks of storage at 4 °C with the addition of methanol. The obtained results contribute to implementing stepwise pyrolysis units for crop residue management.publishedVersionPeer reviewe

Impacts of Phosphorous Source on Organic Acid Production and Heterotrophic Bioleaching of Rare Earth Elements and Base Metals from Spent Nickel-Metal-Hydride Batteries

This study investigated heterotrophic bioleaching of rare earth elements (REEs) and base metals from spent nickel-metal-hydride (NiMH) batteries. Furthermore, the impacts of phosphorous source [Ca3(PO4)2, KH2PO4 and K2HPO4] and its concentration on organic acid production by Gluconobacter oxydans and Streptomyces pilosus were evaluated. Phosphorous source affected microbial acid production and metal leaching. Among the studied phosphorous sources, use of K2HPO4 resulted in highest organic acid production by both bacteria. Increasing K2HPO4 concentration from 2.7 to 27 mM enhanced pyruvic acid production by S. pilosus from 2.2 to 10.7 mM. However, no metal was leached from the spent NiMH batteries with S. pilosus using either one-step, two-step or spent-medium bioleaching. With G. oxydans, highest gluconic acid concentration of 45.0 mM was produced at the lowest K2HPO4 concentration of 2.7 mM. When using two-step bioleaching with G. oxydans, higher leaching efficiencies were obtained for base metals (88.0% vs. 68.0% Fe, 41.5% vs. 35.5% Co, 18.5% vs 16.5% Ni), while more REEs were leached using spent-medium bioleaching (9.0% vs. 6.0% total REEs). With both bioleaching methods, base metals leaching was faster than that of REEs. Surplus of phosphorous should be avoided in bioleaching cultures as precipitation especially with REEs is possible. Graphic Abstract: [Figure not available: see fulltext.]publishedVersionPeer reviewe

Removal and recovery of metal ions from acidic multi-metal mine water using waste digested activated sludge as biosorbent

Highly acidic and toxic metal ion containing acid rock drainage (ARD) can cause severe environmental problems and endanger aquatic life. However, due to the high metal ion containing ARD is an auspicious source of metals for recovery and reuse. The feasibility of using waste digested activated sludge (WDAS) as a biosorbent for selective metal recovery from a highly complex mine water characterized by low pH (2.6), high sulfate concentration (80.8 g L−1) and a multitude of metals (Co, Cr, Cu, Fe, Mn, Ni, Th, U, V, Y, and Zn) at concentrations from few mg L−1 to several g L−1 was investigated. The effect of pH (2.0–10.0) and WDAS concentration (3.8–22.2 g L−1) on metal ion removal and the effect of several recovery solutions and their concentrations on metal recovery were explored in batch experiments. Metal removal was influenced by pH and WDAS concentration. A strong selectivity was observed in the recovery step employing 1 M Na2CO3 solution. The combination of a one-step removal and a two-step recovery approach enabled recovery of U (108.0 ± 6.6%), Cu (39.2 ± 2.5%), and Th (53.7 ± 7.7%) due to formation of strong carbonate complexes increasing the purity of U and Cu up to tenfold compared to the mine water. The results revealed that careful adaption of pH, WDAS concentration, and number of steps of the process according to the combination of metals present in solution is of great importance.publishedVersionPeer reviewe

Primary and secondary data on dementia care as an example of regional health planning

Health service planning that takes into account as far as possible the regional needs and regional discrepancies is a controversial health issue in Germany. In a pilot scheme, we tested a planning process for regional healthcare services, based on the example of dementia care. The aim of this article is to present the strengths and limitations of this planning process. We developed an indicator set for dementia care based on routine regional data obtained from two German statutory health insurance companies. Additionally, primary data based on a questionnaire sent to all GPs in the area were evaluated. These data were expanded through the addition of official socio-demographic population data. Procedures and evaluation strategies, discussion of the results and the derivation of planning measures followed, in close agreement with a group of local experts. Few epidemiological data on regional variations in health care planning are publicly available. Secondary data from statutory health insurance companies can be assessed to support the estimation of regional health care needs, but interpretation is difficult. The use of surveys to collect primary data, and the assessment of results by the local health board may facilitate interpretation and may contribute towards more valid statements regarding regional health planning. Despite the limited availability of data and the considerable efforts involved in data analysis, the project demonstrates how needs-based health service planning can be carried out in a small region, taking into account the increasing demands of the local health care providers and the special local features