Team:Calgary/Bioleaching Wetlab
OVERVIEW
DISSOLVING METAL WITH CELLS
The goal of the bioleaching workflow is first to demonstrate our ability to leach neodymium from NdFeB magnets using biologically generated acids, and to measure its acid production and percent efficiency of metal extraction. Next, we wanted to demonstrate the compatibility of bioleaching with hard drive shredding dust as feedstock, another major industrial source of REEs. Finally, we hope to compare the practical application and effectiveness of two bioleaching organisms, Acidothiobacillus thiooxidans and Gluconobacter oxydans, under different conditions. These two organisms have been commonly used in past REE bioleaching studies due to their strong potential in metal dissolving, making them standard models in the field. Together these experiments will help future Neocycle users to make informed decisions about incorporating bioleaching into their systems as an eco-friendly and cost-effective solution for solubilizing REEs.
OUR ORGANISMS
A. thiooxidans is a sulfur-reducing, acidophilic bacteria which generates large quantities of sulfuric acid as part of its metabolism [1]. Past research has demonstrated its high capacity to dissolve REEs, despite its slow growth [1]. G. oxydans is an aerobic bacteria which generates organic acids including gluconic and citric acid [2]. It has in the past shown reduced total efficiency at REE leaching compared to A. thiooxidans. However, it is a faster grower and leaches more quickly, and its biolixiviant is less hazardous due to being less strongly acidic [2]. We therefore sought to compare both for their use in solubilizing REEs from e-waste. However, although we were able to demonstrate the successful use of G.oxydans, Our plans were unfortunately limited by shipping blockages which delayed G. oxydans culture delivery by three months and prevented A. thiooxidans from arriving.
Experiments
QUANTIFYING OUR DATA
First and foremost of our priorities is to determine the rate of bioleaching from NdFeB magnets using our cells, and the total percentage of REEs they are able to mobilize from the metallic matrix. In order to do this, we needed a quick, accurate, and reliable method to quantify REEs in solution. As undergraduate students, we don’t have reliable, consistent access to analytical equipment such as an ICP-OES. In the future, we hope that this challenge can be helped for other teams by the use of our measurement system.
In the meantime, to supplement our ICP-OES access, we initially utilized a colorimetric assay called the Arsenazo III assay. This assay can be used under a spectrometer to assess the total REE concentration of a solution. We validated the efficacy of the Arsenazo assay under our conditions by generating standard curves for its use with Nd solutions in pure water, Glycine/Acetate buffer, and several growth medias. We found that only those medias which did not contain calcium could be accurately assayed using Arsenazo, as calcium interfered with the results. However, after experimenting with inoculating G. oxydans into a variety of medias, we found that the calcium-containing PKM media produced significantly stronger growth. As a result, it became necessary to obtain ICP-OES access as our sole analysis for future tests, and plan our experiments carefully in order to make efficient use of our very limited access to the machine.
EVALUATING BIOLEACHING'S POTENTIAL
We set up rounds of bioleaching experiments, which used G. oxydans cultures in PKM media to dissolve samples of NdFeB magnets ranging at concentrations from 10 to 50 g/L of magnet in solution. These magnets were obtained by separation from hard drives, demagnetizing them under heat, and removing the nickel coating. In addition, we also tested the ability to solubilize Nd from the residual metal dust left over from shredding hard drives during the recycling. This shredding dust was obtained from eCycle Solutions e-waste processing in Airdrie, Alberta, and was considered a likely source of neodymium due to its observed magnetic behaviour and its hard drive content. We took samples from these experiments after 4 and 7 days to measure their pH, OD, and Nd content under ICP-OES.
To use the ICP-OES, we created standards using pure Nd to calibrate the machine. We next dissolved samples of our magnet stock in the extremely strong acid aqua regia: a volatile combination of nitric acid and HCl, used in past literature to solubilize the full total amount of Nd from NdFeB. [3] At the same time, we ran experimental samples that had been incubated with varying concentrations of metal feedstock for 4 or 7 days. These experiments could then be used to compare the extraction of Nd from NdFeB in our bioleaching experiments against the total Nd content extracted by aqua regia.
In comparing two different organisms, experimental design must be carefully controlled in order to ensure a fair comparison that accurately reflects the strengths of both species. The two species prefer different media compositions, and therefore cannot be compared under identical conditions. Instead, a thorough literature review was conducted in order to find what would be the optimal conditions for each species as described by previous studies. By ensuring that each is kept in a media, temperature, and aeration optimal for its species, the two can be compared at their strongest, allowing for meaningful conclusions to be drawn about their varying pros and cons in practical implementation. In the future, when our A. thiooxidans cultures become accessible, we plan to put this planned experimental workflow into action.
Results
Measurement of the NdFeB and shredding dust bioleaching after 4 and 7 days incubation showed that we were able to successfully leach Nd from all samples. It was found that the samples with the lowest amount of metal feedstock addition had the highest percent yield efficiency of total Nd present. The highest percent efficiency was found in 10 g/L NdFeB magnet after 7 days incubation, which had 82% of total Nd content extracted. However, although percent efficiency decreased with additional feedstock content, total Nd recovery increased, with a maximum of 7.65 g/L extracted after 7 days from a 50 g/L magnet solution. The total Nd content of shredding dust was not known; however, up to 9.6% of the mass of the shredding dust was extracted as Nd after 7 days at 25 g/L.
Figure 1. Results of bioleaching with Gluconobacter oxydans after 4 and 7 days at various concentrations, with NdFeB magnet and hard drive shredding dust as feedstock.
Additionally, G.oxydans was found to produce prodigious amounts of acid. Samples of PKM media with precipitated buffer removed that were inoculated with G. oxydans were found to drop to a pH of 2 after 4 days and 1 after 7 days, while uninoculated samples remained at a pH of 7. Samples inoculated with metal dropped only to a pH of 4 after 4 days and remained there, showing that feedstock presence somewhat reduces net acid production.
Conclusions
Taken together, these results indicate that G. oxydans has strong potential for bioleaching neodymium from NdFeB magnets and hard drive shredding dust, with best results in magnets. This validates our choice of NdFeB magnets as a primary target for future Neocycle implementation. It also shows that G. oxydans possesses a better extraction efficiency when used with lower concentrations of metal feedstock, possibly due to toxic inhibitory effects on cell function, or increasing mass volume to surface area ratios of feedstock. Further experiments should test bioleaching under these conditions with a greater variety of e-waste in order to further inform our choice of potential feedstocks for implementation. More experimentation with feedstock at varying concentrations and degrees of coarseness could also help to maximize the efficiency of feedstock use. Overall, bioleaching is validated as a solution with strong potential to be used to solubilize e-waste as part of recycling through an environmentally sustainable Neocycle system.
REFERENCES
Marra A, Cesaro A, Rene ER, Belgiorno V, Lens PNL. Bioleaching of metals from WEEE shredding dust. Journal of Environmental Management. 2018;210. doi:10.1016/j.jenvman.2017.12.066
Kwok R. Inner Workings: How bacteria could help recycle electronic waste. Proceedings of the National Academy of Sciences. 2019 [accessed 2021 Oct 15];116(3):711–713. https://www.pnas.org/content/116/3/711. doi:10.1073/PNAS.1820329116
Gergoric M, Ravaux C, Steenari BM, Espegren F, Retegan T. Leaching and Recovery of Rare-Earth Elements from Neodymium Magnet Waste Using Organic Acids. Metals. 2018; 8(9):721. https://doi.org/10.3390/met8090721
