Recycling of critical metals by granular biomass
The increasing world population, the striving for a high standard of living, the glory of high-tech products and the emerging low-carbon energy technologies, have all together resulted in an ever-increasing demand for specialty metals. Unfortunately, the supply of these precious resources is unsteady, since the reserves are unequally distributed over the world and are often susceptible to geopolitical tensions. Besides, their primary mining is environmentally taxing. Manufacturing industries worldwide are highly dependent on the import of these crucial building blocks for modern day technologies, and want to secure the supply of these raw materials, which are therefore declared critical raw materials. These comprise two important groups of metals; the platinum group metals (PGMs), known for their excellent catalytic activities, and rare earth elements (REEs), essential in batteries and permanent magnets for their superconductivity and magnetism. As one of the main measures to be taken, metal recovery is essential to reduce the sensitive dependence on virgin materials, especially for resource-limited countries. Conventional metallurgical technologies are available for the recovery of valuable metals from highly concentrated waste streams, while economically viable recovery technologies for low concentrated streams are lacking.
Currently, few technologies exist for the treatment of diluted streams, underlining the need for the development of effective, low-cost strategies for metal recovery. Biometallurgical technologies, based on microbe-metal interactions, are low in energy and chemicals consumption, unlike many conventional metallurgical technologies. They can serve as sustainable and environmentally friendly alternatives for metal recovery from low concentrated streams, such as secondary sources, residues, process and waste streams. Therefore, this work aimed at the investigation and development of biometallurgical technologies for PGM and REE recovery from liquid and solid waste streams and studied the metal recovery potential of different microbial strategies. Out of these metal groups, platinum (PGM), neodymium and lanthanum (REEs) are focused on.
In a first research line, the biological recovery of platinum from aqueous streams was studied. Due to the growing importance of platinum, many Pt loaded waste streams exist, which might cause the accumulation of this precious metal in the environment. The biological recovery of Abstract a selection of common Pt-complexes was studied to assess the fate of Pt-complexes once they have entered a traditional wastewater treatment plant or the environment. Five axenic microbial cultures, which might reside in such systems, were used; a G- Shewanella, Cupriavidus, Geobacter and Pseudomonas species, and a G+ Bacillus species. This study links the relationship between the Pt-speciation, which is highly dependent on the pH and chloride concentration of the target stream, and the microbial recovery and the effect on the microbial viability. Full recovery was achieved at pH 2 and in the presence of H2-gas as electron donor for the platinum chloro-complexes Pt(II)Cl42- and Pt(IV)Cl62- (which can result from leaching or metal refinery processes) and the chemotherapy complex cisplatin. A second commonly applied chemotherapy complex carboplatin was hardly recovered (max. 25%), while no recovery was observed in the case of the Pt-tetraamine complex Pt[NH3]4Cl2. A decrease in intact bacterial cells was seen during platinum reduction, and indicated Cupriavidus metallidurans to be the most resistant species.
Herewith, a proof of concept was given for the biological recovery of different platinum complexes under circumneutral conditions. These tests are representative for the conditions of U+2018friendlyU+2019 matrices such as hospital and municipal wastewaters. Yet, many industrial streams are characterized by more harsh conditions such as extreme pH values and the distinct presence of salts or contaminants. The well-studied microbial species such as Shewanella or Geobacter are generally not able to withstand such challenging conditions. Halophilic species to the contrary originate from hypersaline extreme environments and could possibly cope with these conditions. This inspired us to explore the use of halophilic mixed cultures, enriched from Artemia cyst samples originating from a salt lake. Cultures were enriched in different salt matrices (20 U+2013 210 g L-1 salts; sea salt mixture and NH4Cl) and at low to neutral pH (pH 3 U+2013 7). High throughput sequencing revealed the main taxonomic families present in these halophilic enrichments; Halomonadaceae, Bacillaceae, and Idiomarinaceae. The halophilic cultures recovered 98% Pt(II)Cl42- and 97% Pt(IV)Cl62- at pH 2 and 24.1 g L-1 salts. The platinum was reduced and precipitated both intra- and extracellularly. This demonstrated the ability of mixed halophilic cultures to recover platinum from process streams and to withstand such challenging conditions.
Next, closer-to-application conditions are developed to test the robustness and application potential of the halophilic enrichments. The most appropriate halophilic mixed culture was selected for use in real industrial streams, characterized by more complex matrix conditions compared to synthetic lab streams. The halophiles demonstrated their suitability for acidic and saline conditions as they were able to recover 79 U+2013 99% Pt(II)Cl42- at 20 U+2013 80 g L-1 salts and pH 2.3. Halophilic cells, with preprecipitated Pt particles on their cell wall, were even able to recover the Pt-tetraamine complex from a real process stream (46 U+2013 95% recovery). High recoveries were maintained during sequential batch treatments. These experiments showed the recovery from a refinery process stream and the transformation of soluble Pt from diluted streams into Pt rich biomass, which can easily be separated.
A second research line focused on the recovery of REEs (especially neodymium and lanthanum as a proxy for the light REEs) from solid streams such as the mineral ore monazite. Since conventional extraction techniques are chemical and energy intensive and produce toxic waste streams, the development of an environmentally friendly REE extraction and recovery technology based on bioleaching and membrane electrolysis was aimed at. The effective leaching from pretreated (roasted) monazite by citric acid was demonstrated in sequential batch experiments; up to 392 mg L-1 Nd and 281 mg L-1 La were leached. The electrochemical system concomitantly extracted and precipitated Nd as Nd-hydroxides in the catholyte (up to 880 mg L-1 Nd), while thorium and citrate and unwanted counterions such as phosphate were separated into the anolyte. Citrate could be recovered from this anolyte to treat the next batch of monazite. This study shows a promising REE recovery technology that could be suitable for both primary and secondary resources, such as mine tailings and metallurgical slags.
Finally, potential interesting streams can be selected. Since the matrix conditions of the waste streams determine the metal recovery technology (pH, salts, metal speciation, dilution), a strategy can be developed for selecting the most appropriate technology. Depending on the industrial operation and requirements, waste streams can be treated in batch or continuously, in sequential batch reactors or membrane bioreactors. Through settling or filtration, the Pt loaded biomass can be separated from the treated stream. To implement maximal metal recovery in a circular economy, a metallurgical toolbox can be designed, consisting of a series of metallurgical technologies to pretreat the stream, extract the metals and valorize the residual matrix. Finally, an economic evaluation can be made, including an assessment of the intrinsic value of the stream and an estimation of the capital and operation expenses of the implemented technology.