Kitchen Chemistry Meets Solar Waste: Citric Acid Strips Silver and Aluminum from Recycled Solar Panels

Every solar panel installed today is a future recycling problem. Crystalline silicon modules dominate the global photovoltaic market, and with typical lifespans of 25 to 30 years, the first great wave of end-of-life panels is now approaching. Estimates suggest that 60 to 78 million tons of photovoltaic waste could accumulate worldwide by 2050. A team of Italian researchers has now tested a surprisingly gentle solution to one of the trickiest steps in recycling these modules: instead of attacking the silicon cells with corrosive sodium hydroxide and nitric acid, they used citric acid—the same food-grade compound found in lemons—assisted by ultrasound, to strip away the silver and aluminum contacts that contaminate recovered silicon.

The study, published in Clean Technologies and Environmental Policy by researchers from ENEA, the Italian National Agency for New Technologies, Energy and Sustainable Economic Development, together with the recycling company 9-Tech and the University of Padova, compared two purification strategies head to head. The conventional route relies on sequential leaching: a hot alkaline bath of sodium hydroxide dissolves the aluminum back-contact, converting it into sodium aluminate, followed by nitric acid treatment that oxidizes the silver grid lines into soluble silver nitrate. The alternative, patented by 9-Tech, immerses the cells in a 25 percent aqueous citric acid solution and then applies high-power ultrasound at 40 kilohertz, letting acoustic cavitation do the heavy lifting.

The results reveal a clear trade-off between purity and process safety. The traditional sodium hydroxide and nitric acid sequence removed 96 percent of the aluminum and essentially all of the silver, yielding a silicon powder of 92.6 percent purity with just 0.3 percent residual aluminum. The citric acid route was less aggressive, removing 69 percent of aluminum and 83 percent of silver, and producing a powder of 89.4 percent silicon purity with 2.3 percent aluminum and 0.2 percent silver. Yet the greener method has a decisive structural advantage: it works in a single step, uses a non-toxic and biodegradable reagent, and—crucially—recovers the leached silver and aluminum as solid metallic particles rather than dissolved ions, eliminating the need for downstream reduction or electrodeposition stages.

The mechanism behind the ultrasound-assisted process is a synergy of chemistry and mechanics. On its own, citric acid proved nearly powerless: blank experiments without ultrasound removed less than 5 percent of the silver and aluminum. Citric acid is a weak organic acid and an effective polydentate chelating agent, but it lacks the oxidizing strength to dissolve silver completely, and silver oxidation by dissolved oxygen proceeds too slowly. What the acid does accomplish is the chemical etching of less-noble adhesion oxides, such as lead oxide and zinc oxide, within the metal pastes. This weakens the bond between the metallic contacts and the silicon substrate. The ultrasonic bath then generates microscopic cavitation bubbles whose collapse produces high-energy micro-jets and localized shockwaves that mechanically shear the loosened silver tracks and sintered aluminum layers away from the silicon surface in solid metallic form.

The recovered cells came from a genuine industrial pipeline rather than laboratory simulants. In 2023, four batches of photovoltaic panels totaling 47 units were processed at a 30-kilogram-per-hour pilot plant in Porto Marghera, near Venice, operated by 9-Tech under authorization from the Veneto region. The process, known as 9-Tech technology, begins with manual removal of the aluminum frame and junction box, slices the panels into strips, and thermally treats them in a continuous furnace equipped with a heat exchanger and fume abatement system to burn off the ethylene-vinyl acetate encapsulant and polymeric backsheet. Mechanical separation then isolates three fractions: glass, copper ribbons, and silicon cells, the latter recovered as foil pieces between 1 and 20 square centimeters. Twenty-five kilograms of these cells were shipped to the University of Padova for the purification trials under the PARSIVAL project, supported by EIT Raw Materials.

After leaching, both purified samples were ground into fine powders using a high-energy ball mill at 800 revolutions per minute for 80 minutes. Scanning electron microscopy with energy-dispersive X-ray spectroscopy revealed that the chemically treated cells fractured far more readily than untreated controls: 83 percent of grains from the sodium hydroxide and nitric acid route and 75 percent from the citric acid route fell within the 0 to 3 micrometer range, compared with only 54 percent for the reference powder, whose particles reached up to 14 micrometers. The explanation lies in the aluminum coating itself. The porous aluminum back-contact acts as a cushion that absorbs impact energy during milling, and its ductility combined with a passivating oxide layer resists fracture, producing coarser, less homogeneous powder. Removing or thinning this coating dramatically improves milling efficiency.

Spectroscopic and thermal analyses added further depth to the picture. Raman spectroscopy showed the characteristic silicon peak shifting from 518 wavenumbers in the control sample to 515 and 514 wavenumbers in the citric acid and conventional samples respectively, a signature of reduced particle size and increased lattice defects. X-ray diffraction with Rietveld refinement revealed compressive strain in the silicon lattice of the treated powders, with peak broadening and shrinking unit cell parameters indicating that the chemical treatments—particularly the more aggressive conventional route—introduce crystal defects. Thermogravimetric analysis detected the melting of the aluminum-silicon alloy formed during the original cell metallization at around 580 degrees Celsius, a fingerprint sensitive enough to reveal even minute aluminum traces in the conventionally purified sample.

One stubborn contaminant resisted both treatments: roughly 5 percent by weight of pyrolytic carbon, originating from incomplete thermal decomposition of the polymeric encapsulant during the initial furnace treatment. Because amorphous carbon is chemically inert, neither leaching route could digest it, and the researchers note that future process iterations may need a secondary calcination step in an oxygen-rich atmosphere or optimization of the furnace atmosphere itself. Interestingly, this residual carbon is not necessarily a liability. For metallurgical applications, silicon blended with carbon can promote the formation of silicon carbide, a valuable additive in steelmaking. And for lithium-ion battery anodes, pyrolytic carbon is deliberately added to improve electrical conductivity and buffer the mechanical stress that silicon undergoes during lithium insertion.

The economic case for the greener route is compelling. Silver accounts for roughly 0.053 percent of a photovoltaic module’s weight—about 6 grams in a typical 20-kilogram panel—and at a market price of around 2.1 euros per gram, the theoretical gross metal value exceeds 12 euros per module, a sum largely lost in conventional bulk shredding. While citric acid costs more per kilogram than industrial nitric acid, the method’s low corrosiveness eliminates the need for specialized reactors and vapor treatment, and because the metals are recovered in solid form, wastewater treatment is simplified and expensive electrochemical recovery steps are bypassed. The researchers also acknowledge a caveat from recent literature: organic leachants are not automatically free of environmental or economic drawbacks, and a genuine industrial trade-off between final purity, safety, cost, and footprint remains.

Perhaps the most striking context comes from comparing recycled silicon with virgin metallurgical-grade silicon. Industrial silicon production relies on open three-phase electric arc furnaces of 10 to 30 megawatts operating at temperatures up to 2400 kelvin, consuming roughly 11 to 13 kilowatt-hours of electricity per kilogram of silicon and emitting 4.7 to 5.0 tons of carbon dioxide per ton produced—figures increasingly incompatible with climate mitigation goals. Silicon recovered from end-of-life panels requires substantially less energy and avoids this carbon footprint entirely, and pilot-scale life cycle assessments have already demonstrated significant environmental benefits from a circular economy perspective. With silicon now classified as a critical and strategic raw material by the European Commission since 2023, turning retired solar panels into a domestic silicon supply is no longer just an environmental nicety—it is a matter of resource security. The citric acid route, still a preliminary proof of concept, may prove to be the gentle giant that makes this circular vision industrially real.

Subject of Research: Green chemical purification of silicon recovered from end-of-life photovoltaic panels using citric acid leaching

Article Title: Purification of recycled silicon from end-of-life photovoltaic panels: citric acid as a green alternative to traditional leaching

Article References: Taurisano, N., Burresi, E., Schioppa, M., Palmisano, M., Pesce, E., Capodieci, L., Valenzano, V., Miserocchi, F., Pitacco, E., Cerchier, P., & Protopapa, M. L. (2026). Purification of recycled silicon from end-of-life photovoltaic panels: citric acid as a green alternative to traditional leaching. Clean Technologies and Environmental Policy, 28(11), Article 273. https://doi.org/10.1007/s10098-026-03624-3

Image Credits: AI Generated

DOI: 10.1007/s10098-026-03624-3

Keywords: silicon recycling, photovoltaic waste, citric acid leaching, ultrasound-assisted extraction, silver recovery, aluminum removal, circular economy, solar panel recycling, green chemistry, metallurgical-grade silicon, lithium-ion battery anodes, hydrometallurgy