Metal-Eating Microbe Meets Old Car Catalysts in Quest for Greener Platinum Recovery

Inside every catalytic converter that rolls off a production line sits a small fortune in platinum group metals, the platinum, palladium and rhodium that scrub toxic exhaust gases into something far less harmful. When those converters reach the end of their road life, they become one of the most valuable waste streams on the planet, typically holding between one and fifteen grams of recoverable precious metal per unit. Yet getting those metals back out has always demanded a brutal trade-off: furnaces running above 1200 degrees Celsius, or baths of concentrated nitric, hydrochloric and sulphuric acids that spew hazardous effluent and greenhouse gases into the environment. Now a team at the University of Nottingham has tested a radically gentler idea, letting a famously tough metal-resistant bacterium tackle untreated catalyst powder directly inside a microbial fuel cell, and the results reveal both a tantalising opportunity and a hard physical ceiling on what biology alone can achieve.

The stakes could hardly be higher. Primary platinum group metal production is geographically precarious, with South Africa’s Bushveld Igneous Complex supplying roughly 88.7 percent of the world’s platinum group metals and Russia’s palladium-rich Norilsk-Talnakh region alone accounting for about 43.8 percent of global palladium output. Life cycle assessments place the carbon footprint of producing a single kilogram of refined metal at between 28 and 42 tonnes of carbon dioxide equivalent, while the classic aqua regia leaching route releases toxic nitrogen oxide and chlorine-bearing gases. As millions of converters are scrapped each year, spent automotive catalyst has become the obvious secondary resource, but the industry has lacked a clean, low-energy route that works on the real, unprocessed solid material rather than pre-dissolved metal salts.

The Nottingham team, led by Christine Paul with Frankie Rawson, Katalin Kovács and Helena I. Gomes, turned to Cupriavidus metallidurans CH34, a bacterium with almost legendary resistance to toxic metals. Its defensive arsenal sits on two megaplasmids carrying metal-specific gene clusters including cop, czc and cnr, allowing it to shrug off millimolar concentrations of nickel, zinc, copper and cadmium. Crucially, the organism is also electrogenic: it forms biofilms on graphite electrodes and generates current densities of 15 to 150 milliamperes per square metre. Previous bioelectrochemical studies of precious metals had only ever worked with dissolved metal ions in model solutions, or with real catalyst material that had first been chemically leached. This new work, published in Cleaner Engineering and Technology, is the first to put an electroactive culture in direct contact with untreated spent catalyst solids.

The experimental design was elegantly simple. Dual-chamber H-type microbial fuel cells were built from borosilicate glass, with carbon felt electrodes separated by a perfluorinated sulfonic acid membrane and a ferricyanide catholyte chosen specifically so that the cathode would never limit the measured response. The anode chambers received untreated spent catalyst powder at loadings of 10,000, 50,000 and 100,000 parts per million, corresponding to pulp densities of 1, 5 and 10 percent, alongside catalyst-free controls. Eight reactor types, each run in biological triplicate, allowed the team to separate purely chemical effects from genuinely biological ones. After 168 hours of operation, inductively coupled plasma mass spectrometry tracked exactly where the platinum, palladium and rhodium had ended up across the liquid, biomass, electrode and residual solid fractions.

The first surprise came from the biomass data. At the lowest loading of 10,000 parts per million, the bacteria actually grew better than in the catalyst-free control, reaching 221.4 micrograms per millilitre of planktonic protein at 96 hours compared with 201.3 in the control, suggesting the moderate metal challenge stimulated rather than suppressed the culture. But at 50,000 and 100,000 parts per million, growth collapsed to just 60 to 75 percent below the control level. Scanning electron microscopy told part of the story: at the higher loadings, aggregated catalyst particles blanketed the carbon felt electrodes, masking the underlying fibre structure and leaving little visible biological material. The electrode-attached population fared better than the free-floating cells, but the overall picture was one of progressive physical and chemical suffocation.

That suffocation had two distinct sources. The first was acidity. Even in completely abiotic reactors, adding catalyst drove the anolyte pH down in a dose-dependent fashion, from 5.95 in the catalyst-free control to 4.87, 4.06 and 3.93 at the three loadings respectively. This chemical acid load occurred entirely without microbial help and, crucially, preceded the decline in bacterial growth, establishing that acidification was a cause rather than a consequence of the biological collapse. The second source was electrical. Open-circuit voltage peaked at an impressive 0.442 volts at 10,000 parts per million, actually exceeding the catalyst-free control, but fell to around 0.12 to 0.15 volts at higher loadings. Internal resistance told the same story, with activation resistance jumping from 142 ohms in the clean biotic reactor to 3688 ohms at low loading and over 7000 ohms at the highest concentrations, while power density plummeted by more than two orders of magnitude across the range.

Cyclic voltammetry added a further layer of insight. The anodic peak current at 10,000 parts per million reached 688.89 microamperes, more than double the 327.65 microamperes of the catalyst-free control, and the Randles-Ševčík slope, a measure of diffusion-coupled electron transfer, was likewise highest at moderate loading. But the weaker linearity of that fit hinted at extra processes beyond simple diffusion control, and at 50,000 and 100,000 parts per million the peak currents fell to 278.25 and 96.24 microamperes respectively, with increasingly non-Nernstian behaviour indicating quasi-reversible to irreversible electron transfer governed by kinetic limitations. In plain terms, the electrochemical conversation between bacteria and electrode was sharpest at moderate catalyst loading and progressively garbled as particles piled up on the surface.

The metal redistribution results were the most striking of all. Rhodium proved to be the star performer: at 10,000 parts per million, the biotic reactors mobilised 17.93 percent of the rhodium input, compared with just 0.87 percent of platinum and 1.20 percent of palladium. Moreover, 10.3 percent of the rhodium ended up associated with the bacterial biomass, a fraction that was below detection in the abiotic controls, pointing to a genuinely selective biological interaction. The explanation likely lies in catalyst chemistry: road-aged three-way catalysts contain rhodium predominantly in oxidised form, whereas platinum sits mostly as metal. Dissolving metallic platinum group metals requires both a high oxidation potential and a complexing ligand such as chloride, neither of which was present in the anolyte, but rhodium that is already oxidised can bypass that demanding first step. The biological contribution, however, was confined to biosorption and accumulation on the biomass rather than true dissolution of the solid matrix, and the authors are careful to note that no oxidation-state analysis confirmed reduction.

Perhaps the most counterintuitive finding was that mobilisation per gram of catalyst was highest at the lowest loading, despite the more favourable pH there. Platinum recovery per unit mass fell from 2.01 hundredths of a milligram per gram at 10,000 parts per million to 0.332 hundredths at 100,000 parts per million, with palladium and rhodium showing the same pattern. This points to accessible surface area, not acidity, as the true bottleneck: at high solids loadings, particles aggregate, the reactive surface shrinks, and the sheer mass of solid available for re-adsorption pulls dissolved species back out of solution. The same pulp-density penalty has been observed in conventional hydrometallurgical processing, where palladium and platinum recovery drops from above 90 percent at 5 to 8 percent solids to below 70 percent at 20 percent.

The honest conclusion is that this system, as it stands, cannot yet compete with industrial recovery: even at the optimum loading, less than 18 percent of rhodium and under 2 percent of platinum and palladium were mobilised. But the study does something arguably more valuable than demonstrating a working process. It maps the operational window, showing that the transition between sustained and suppressed performance lies somewhere between 10,000 and 50,000 parts per million, and it identifies the two mechanisms, chemical acidification and particulate fouling of the electrode, that constrain it. The authors sketch a roadmap of possible fixes: pH control to decouple acidification from surface effects, pre-washing the catalyst to reduce the acid load at source, fed-batch dosing to keep instantaneous loading within the functional window, or a two-stage configuration that separates mild chemical solubilisation from the bioelectrochemical step that the biology performs best, namely selectively capturing mobilised rhodium. Achieving process-relevant yields, they conclude, will require intervention at the solid-liquid interface rather than tinkering with the microbe itself. For a field desperate to break its dependence on South African ore bodies and 1200-degree furnaces, that is a map worth having.

Subject of Research: Bioelectrochemical recovery of platinum group metals from untreated spent automotive catalyst using Cupriavidus metallidurans

Article Title: Bioelectrochemical treatment of untreated spent car catalyst: Effects of solids loading on platinum group metal redistribution and process limitations

Article References: Paul, C., Rawson, F., Kovács, K., & Gomes, H. I. (2026). Bioelectrochemical treatment of untreated spent car catalyst: Effects of solids loading on platinum group metal redistribution and process limitations. Cleaner Engineering and Technology, 34, Article 101322. https://doi.org/10.1016/j.clet.2026.101322

Image Credits: AI Generated

DOI: 10.1016/j.clet.2026.101322

Keywords: platinum group metals, spent car catalyst, microbial fuel cell, Cupriavidus metallidurans, rhodium recovery, bioelectrochemical systems, urban mining, circular economy, biomining, hydrometallurgy, electroactive bacteria, metal recycling