Fungi Ferment a Greener Route to Critical Metals from Electronic Waste

The mountains of discarded smartphones, LEDs and circuit boards piling up around the world contain a hidden treasure of critical metals, including gallium and rare earth elements, that modern electronics cannot function without. Recovering those metals has traditionally meant dousing e-waste in harsh petrochemical acids such as hydrochloric and sulfuric acid. Now, researchers in Germany have engineered a cleaner supply chain from the ground up: a fungal fermentation process that churns out oxalic acid, a biological leaching agent capable of selectively plucking valuable metals from electronic scrap. The study, published in Applied Microbiology and Biotechnology, transforms a century-old observation about mold metabolism into a scalable industrial blueprint.

Oxalic acid is hardly a novelty. The simple dicarboxylic compound has served industries from pharmaceuticals to textiles for generations, and it was first synthesized in 1776 by the Swedish chemist Carl Wilhelm Scheele through the oxidation of sugar with nitric acid. Today it is produced almost exclusively through petrochemical routes that involve volatile intermediates and substantial carbon dioxide emissions. That matters for metal recycling, because oxalic acid’s green credentials collapse when its entire life cycle is considered. A recent assessment found that, weighed against its toxicity and recovery performance, conventionally produced oxalic acid failed to qualify as a green chemical. The paradox is what motivated Aylin Nur Erkmen of FH Münster University of Applied Sciences and her colleagues to ask whether the molecule could instead be brewed by living cells.

Filamentous fungi had long been suspected of doing exactly that. As early as 1893, the microbiologist Carl Wehmer identified Aspergillus niger as an oxalic acid accumulator, distinguishing it from Penicillium species that follow different metabolic fates. Decades later, researchers mapped the underlying biochemistry to a cytoplasmic, tricarboxylic-acid-independent pathway driven by the enzyme oxaloacetate hydrolase, which cleaves oxaloacetate into oxalate. Crucially, that enzyme only operates efficiently when the surrounding pH remains above 4, a detail that turns out to be the linchpin of the entire process. Industrial citric acid fermentation has historically treated oxalic acid as an unwanted by-product to be suppressed; the German team inverted that logic entirely.

The researchers began with a head-to-head comparison of two Aspergillus niger strains in small shake-flask cultures. Over a 240-hour fermentation, the ATCC 1015 strain secreted a mean peak of 71.0 millimolar oxalic acid, clearly outpacing the CECT 2807 strain, which managed only 37.9 millimolar under the same conditions. The difference tracks closely with how fast each strain acidified its environment: ATCC 1015 drove the pH down to about 3.8 within 72 hours, while its competitor hovered near 4.5. Because oxaloacetate hydrolase is suppressed at low pH, the team then intervened, periodically raising the culture pH back toward neutral. The effect was dramatic. Cultures with intermittent pH adjustment on days one, four and seven reached a final titer of 90.4 millimolar, roughly seven times the yield of unregulated cultures by the 168-hour mark. Keeping the medium above the critical pH 4 threshold, the authors conclude, is not merely helpful but essential.

Carbon source selection proved equally consequential. When the fungus was grown on glucose, sucrose or lactose, glucose produced the highest titer, 136.9 millimolar, with a product yield on substrate of about 0.4 grams per gram, outperforming sucrose by a factor of 1.6 and lactose by nearly five. The explanation lies in metabolic plumbing: glucose enters glycolysis directly, channeling carbon efficiently toward oxaloacetate, the immediate precursor of oxalate. More complex sugars like sucrose and lactose require enzymatic breakdown steps first, diverting flux toward other metabolites. Prior work has suggested that glucose can theoretically support yields approaching 75 percent of the maximum, making the monosaccharide the obvious choice for scale-up.

The decisive test came in a 10-liter stirred-tank bioreactor, where the team compared conventional batch fermentation against a fed-batch strategy with pulsed glucose feeding every 72 hours. Batch operation, seeded with a high initial glucose load of 150 grams per liter, struggled through a prolonged lag phase; substantial acid secretion began only after roughly 192 hours, and the culture only hit its stride once residual glucose dipped below about 20 grams per liter, a sign of carbon catabolite repression lifting. Fed-batch operation avoided that pitfall by keeping glucose below the inhibitory threshold throughout, producing a cyclical, sustained surge of acid after each pulse. The result was a peak oxalic acid titer of 260.1 millimolar over 14 days, with a volumetric production rate of 0.061 grams per liter per hour, more than 1.6 times the batch rate.

Yet the scale-up experiments also exposed two stubborn complications. First, the phosphate buffering used to protect against acid-induced product inhibition turned out to be a double-edged sword: excess phosphate fueled exuberant biomass growth at the direct expense of acidogenesis, delaying the onset of exponential oxalate production until roughly 312 hours, when the phosphate surplus had been depleted. The authors interpret this as a metabolic switch consistent with phosphate-starvation-driven acid synthesis and suggest that future work should pinpoint the precise phosphorus threshold that triggers the shift from growth to acid production. Second, operating the bioreactor at pH 6 to favor oxaloacetate hydrolase inadvertently created ideal conditions for glucose oxidase, an enzyme that converts glucose into gluconic acid. The culture accumulated 226.1 millimolar of gluconic acid alongside the oxalate, which does not reduce the final titer but complicates downstream separation of the two organic acids in an industrial setting.

These hurdles did not prevent the team from framing the process as a genuine industrial candidate, and the reasoning is strategic rather than purely about yield. The volumetric product formation rates reported here trail benchmarks from highly specialized fermentation systems, but those systems carry extreme complexity and high capital expenditure. The goal was instead a simplified, robust framework that can reliably supply enough biogenic leaching agent for real-world metal recovery operations without the operational fragility of a bespoke fermentation plant. The authors argue that a consistent supply of oxalic acid matters more for recycling economics than squeezing out the absolute maximum titer, and the fed-batch protocol they describe delivers that consistency at modest technical cost.

The destination for this biologically brewed acid is the gallium locked inside waste light-emitting diodes and other electronic components. Gallium is indispensable for LEDs, smartphones and tablets, and its long-term supply is increasingly viewed as precarious. Oxalic acid is unusually well suited to the task because it performs two jobs at once: it dissolves metal-bearing phases and forms robust octahedral coordination complexes with hard trivalent metals such as aluminum, iron and gallium, while selectively precipitating divalent ions, including rare earth elements. Earlier work by the same group showed that under optimized conditions, oxalic acid could recover up to 37 percent of gallium from waste minerals. Related research has achieved gallium recoveries exceeding 90 percent from waste surface-mounted LEDs at 90 degrees Celsius using 700 millimolar acid.

What makes the study resonate beyond the lab is the broader industrial and political context. The European Union has committed to climate neutrality by 2050 and is pushing to replace fossil feedstocks with sustainable alternatives for material production, while the global demand for oxalic acid stands at roughly 350,000 tons per year. Fermentation based on renewable sugars offers a route to satisfy that demand without petrochemical emissions, and it plugs directly into a circular economy vision in which yesterday’s electronics become tomorrow’s metal mines. Fungal leaching fluids may also carry synergistic secondary metabolites that enhance metal extraction beyond what pure acid alone can achieve, a phenomenon documented in earlier bioleaching studies with other microorganisms. There is still work to do: biological variability inherent to filamentous fungi, visible in the wide error bars across replicates, will demand larger reproducibility studies and possibly morphological stabilization strategies before industrial batches can be standardized. But the trajectory is clear. A mold that chemists once berated for contaminating citric acid fermentations is now being cultivated, carefully and deliberately, as the engine of a greener metallurgy, one whose raw material is glucose and whose product may help defuse the e-waste crisis one fermented liter at a time.

Subject of Research: Fermentation-based biogenic oxalic acid production for sustainable critical metal recovery from electronic waste

Article Title: Fermentation-based oxalic acid production aimed at sustainable critical metal recovery from electronic waste

Article References: Fermentation-based oxalic acid production aimed at sustainable critical metal recovery from electronic waste. (n.d.). https://doi.org/10.1007/s00253-026-14040-4

Image Credits: AI Generated

DOI: 10.1007/s00253-026-14040-4

Keywords: oxalic acid, Aspergillus niger, fermentation, electronic waste, critical metals, gallium, fed-batch, bioleaching, bioprocess engineering, circular economy, rare earth elements, pH control