Sonic Boom for Solar Waste: Organic Acids Unlock Critical Metals from Old Panels

The solar power boom has an awkward secret: every photovoltaic panel installed today will one day become waste. According to the International Renewable Energy Agency, between 1.7 and 8 million tonnes of panels will need decommissioning by 2030, and as installed capacity climbs toward 4,500 gigawatts by mid-century, that figure could balloon to 60–78 million tonnes. A new study published in Clean Technologies and Environmental Policy offers a greener way to mine that looming mountain of trash, showing that a combination of ultrasound and relatively benign acid solutions can selectively pull valuable and technology-critical elements out of crushed solar panels—without relying on the corrosive industrial chemicals that dominate current recycling practice.

The research, led by George Yandem, Katarzyna Grygoyć and Magdalena Jabłońska-Czapla of the Polish Academy of Sciences together with Joanna Willner and Tomasz Matuła of the Silesian University of Technology, tackles a question that has received surprisingly little systematic attention: how the way you pre-treat a dead panel interacts with the chemistry of the leaching solution to determine which metals dissolve, and how much. Instead of testing one process in isolation, the team ran a full factorial comparison of three pre-treatment routes against four chemically distinct leaching agents, all under identical ultrasonic conditions, using a decommissioned polycrystalline silicon module originally manufactured by ALGATEC Solar AG in Germany.

The three pre-treatments represented the main strategies competing in the recycling world today. Mechanical milling simply ground the module into particles sieved below 0.5 millimetres. Pyrolysis heated the material to 500 degrees Celsius for two hours, burning away the plastic encapsulants that glue the sandwich of glass, silicon and metal together. Acetone washing softened the ethylene–vinyl acetate encapsulant chemically, allowing the glass to be scraped away and the dissolved polymer to be filtered off under vacuum. Each route was then paired with citric acid, oxalic acid, the chelating agent EDTA, or concentrated nitric acid, and leached in an ultrasonic bath at 40 kilohertz and ambient temperature, with a solid-to-liquid ratio of one gram to twenty millilitres over treatment times from ten to sixty minutes.

Before any leaching began, the team characterised exactly what the waste contained and where those elements ended up after grinding. Using microwave digestion and inductively coupled plasma mass spectrometry, they found the material dominated by copper, tin, lead, antimony, chromium, silver and nickel, with lower but environmentally significant concentrations of cobalt, gallium, germanium, indium, molybdenum and tellurium. Crucially, sieving revealed that most of these elements concentrate in the finest dust. Particles smaller than 0.05 millimetres made up only 13.4 to 14.4 percent of the total mass, yet carried the highest concentrations of most elements. That means a recycler could process just the fine fraction, cutting reagent demand, treatment cost and pollutant load dramatically—an insight with immediate practical value, since industrial-scale crushing can push residues toward concentrations comparable to metal ores.

The headline results concern which combination extracts which element. Pyrolysis followed by nitric acid delivered the strongest overall performance, dissolving up to 300 milligrams per litre of silver, 888 milligrams per litre of copper, 256 milligrams per litre of lead and 109 milligrams per litre of tin. Thermodynamic analysis explains why: tin dissolution in nitric acid carries an equilibrium constant of roughly 10 to the power 37, and lead is barely less favourable, so once the polymer barriers are burned away the oxidation reactions proceed almost irreversibly. Silver, by contrast, is the most noble metal in the set, with a small cell potential of just 0.156 volts, yet the ultrasonically assisted nitric acid route still outperformed earlier studies by a factor of nearly six in leached silver concentration—a difference the authors attribute to the sonication and the fine pre-milling.

The greener acids proved unexpectedly powerful for the rarer, geopolitically sensitive elements. Oxalic acid, especially after pyrolysis or acetone treatment, was the best route for antimony and indium, reaching 5.07 and 8.11 milligrams per kilogram of indium respectively—close to the total indium content of the finest digestible fraction. Citric acid excelled at tellurium and germanium, while EDTA with simple milling pulled out cobalt, and acetone treatment followed by oxalic or citric acid preferentially recovered indium, tin and tellurium. The chemistry behind this selectivity is ligand-driven: citrate ions bind metals through tridentate carboxylate and hydroxyl groups, oxalate forms highly stable bis- and tris-oxalate complexes with indium(III) and gallium(III), and antimony(III) oxalate complexes carry a stability constant that makes dissolution spontaneous. In effect, the organic acids act as molecular tweezers, plucking specific ions from the particle matrix while leaving others behind.

Kinetic analysis added a further layer of control. The elements fell neatly into three time groups: silver, arsenic, cobalt, gallium, germanium, molybdenum and nickel peaked within ten minutes; copper, indium, manganese, lead and tin needed thirty; and chromium, antimony, tellurium, thallium and zinc required the full hour. This pattern reflects where each element sits in the material—surface-accessible phases dissolve quickly, while elements buried deeper in the matrix or held by slower ligand-exchange kinetics take longer. Shrinking-core models, the classical framework for hydrometallurgical dissolution, only fit gallium and germanium, indicating that those two elements leach from deep within the particles under chemical reaction control, with germanium showing R-squared values above 83 percent for EDTA and nitric acid. For everything else, multiple mechanisms operate simultaneously, a reminder that ultrasonic cavitation scrambles the tidy assumptions of conventional leaching theory.

Ultrasound itself deserves attention. Acoustic cavitation—the collapse of microscopic bubbles generated by sound waves—strips blockages from mineral surfaces and shatters particles, expanding the reactive area. Previous work has shown ultrasound can shrink waste particles from 30 micrometres to 1 micrometre and cut chemical activation energy from 34.68 to 6.21 kilojoules per mole, producing dramatic jumps in recovery. Here, the ultrasonic bath allowed leaching at ambient temperature in as little as ten minutes, avoiding the heated, hours-long acid treatments typical of the literature. The method’s principal component analysis further confirmed that pre-treatment dominates the overall variance, with pyrolysis samples clustering apart from milled and acetone-washed material, and low-melting-point elements such as cadmium, gallium, vanadium, zinc and thallium responding distinctly to thermal processing.

The industrial context is moving fast. Recent reporting from the International Energy Agency’s Photovoltaic Power Systems Programme shows commercial recyclers such as SOLARCYCLE, Reiling and SPR already running hybrid lines that combine automated deframing, shredding, mechanical sorting and chemical leaching, with capacities between 20,000 and 63,000 tonnes per year. Downstream, dissolved metals are typically converted into sellable products through clarification, selective separation, concentration and final recovery by precipitation, solvent extraction, ion exchange, cementation or electrolysis. The challenge this study addresses is that these hydrometallurgical stages traditionally depend on nitric, sulfuric and hydrofluoric acids—highly corrosive and toxic reagents whose handling and disposal carry their own environmental burden, threatening the very sustainability credentials that solar recycling is meant to deliver.

The authors are candid about the limits of scaling sound waves. Cavitation intensity becomes spatially uneven in large reactors, potentially producing inconsistent leaching, and the energy cost of prolonged ultrasonication must be weighed against the savings from milder reagents. They suggest that combining ultrasound with conventional agitation, rather than relying on it exclusively, may be the pragmatic path, and they call for pilot-scale trials with techno-economic analysis and life-cycle assessment. Still, the core message is striking: end-of-life solar panels are not merely a disposal problem but a concentrated, pre-sorted ore body, and with the right marriage of mechanical, thermal or solvent pre-treatment and green complexing acids—activated by nothing more exotic than sound—a recycler can dial in exactly which critical element to extract. As millions of tonnes of panels head toward retirement, that tunability may prove as valuable as the metals themselves.

Subject of Research: Green sonochemical recovery of technology-critical elements from end-of-life photovoltaic modules using aqueous organic-acid leaching solutions

Article Title: Green sonochemical recovery of technology-critical elements from the end-of-life photovoltaic module using aqueous organic-acid solutions

Article References: Green sonochemical recovery of technology-critical elements from the end-of-life photovoltaic module using aqueous organic-acid solutions. (n.d.). https://doi.org/10.1007/s10098-026-03606-5

Image Credits: AI Generated

DOI: 10.1007/s10098-026-03606-5

Keywords: photovoltaic recycling, sonochemical leaching, technology-critical elements, organic acids, urban mining, silver recovery, indium, tellurium, pyrolysis pretreatment, green chemistry, e-waste, circular economy