The study tackles two problems at once. On one side is the mounting environmental burden of e-waste, whose mixture of ceramics, polymers, and metals makes conventional recycling difficult and often polluting. On the other is a growing global copper shortfall: demand for the metal is surging with digitalization, electrification, and the low-carbon energy transition, while ore grades decline and mining capacity strains to keep up. Copper has now been designated a strategic raw material under the European Union’s Critical Raw Materials Act, and it is officially recognized as a critical mineral by the United States, Canada, China, and Japan. Turning discarded circuit boards into a domestic source of copper, and then into value-added nanomaterials, is exactly the kind of urban mining the circular economy envisions.

Printed circuit boards are unusually rich targets. The metal fraction typically makes up 25 to 30 percent of a board’s total mass, and copper dominates that fraction, accounting for 60 to 70 percent of the metal content. The researchers obtained three types of waste boards from a Banja Luka recycling company: a main board from a vintage Canon photocopier-printer (PCB1), a Fujitsu Siemens desktop PC motherboard (PCB2), and an ECS P6VXA motherboard (PCB3), all mass-produced in the early-to-mid 2000s. The boards were ranked by their expected gold content, from low to high grade, but the team deliberately focused on copper, the most significant metal constituent by mass, rather than chasing the small quantities of precious metals that drive most e-waste economics.

The extraction chemistry itself was deliberately gentle. Instead of the corrosive mineral acids and strong oxidants common in hydrometallurgy, such as hydrochloric, sulfuric, and nitric acid often paired with hydrogen peroxide or chlorine, the team used three ammonium salts: ammonium chloride, ammonium citrate, and ammonium carbonate. When five grams of ground board material were immersed in these solutions at 30 degrees Celsius for 24 hours, most mixtures almost immediately turned dark blue. That color signaled success at the molecular level: copper had dissolved as the tetraamminecopper(II) complex ion, a stable coordination compound that selectively pulls copper into solution while leaving the refractory components of the board largely untouched.

Before any chemistry, however, the boards underwent a crucial mechanical transformation. Larger polymer components were removed by hand, and the boards were then crushed in a jaw crusher and ground for 90 minutes in a laboratory ball mill running in dry mode. Sieving through a 630-micrometer sieve split each board into two radically different fractions: a fine powder dominated by pulverized polymer and a coarse fraction containing macro pieces of metal. Fourier transform infrared spectroscopy confirmed that the fine fraction matched the spectrum of epoxy resin, the standard substrate material of circuit boards, validating the separation strategy. The insight was counterintuitive: by removing the polymer-rich fines, the copper-rich coarse fraction becomes far more accessible to the leaching solution, improving percolation and reagent efficiency.

The headline result came from the coarse fraction of PCB3, the highest-grade board, leached with ammonium citrate. The extract contained 29.93 weight percent copper relative to the starting waste mass, which corresponds to roughly 99.8 percent effective extraction when referenced to a copper-rich 30 weight percent feed. According to the authors, this sits at the top of all published data on ammoniacal e-waste leaching. Yet across all six board-fraction combinations, ammonium chloride produced the most reproducible copper feed values, while the citrate leachate ranged wildly from 0.09 to 29.93 weight percent. That reproducibility, combined with chemical logic, led the team to choose chloride extracts for the next stage. Citrate is itself a strong chelating ligand that competes with ammonia for copper and would leave organic residue in the final oxide, while the carbonate system tends to form basic copper carbonates rather than copper oxide on standing.

The nanoparticle synthesis exploited those copper-laden leachates directly, a shortcut that bypasses the energy-intensive purification stages of traditional metallurgy. The extract was diluted, and hexadecyltrimethylammonium bromide (CTAB) was added at twice its critical micellar concentration, a dose chosen to create a high density of micelles that act as nanoscale reactors while avoiding the formation of undesirable rod-shaped agglomerates. At 37 degrees Celsius, sodium hydroxide raised the pH to 12.5, precipitating intermediate copper hydroxide forms during a three-day ageing period. The solid was then filtered, dried at 110 degrees Celsius, and annealed at 800 degrees Celsius in air, converting it into copper oxide.

Characterization revealed both triumph and nuance. X-ray diffraction confirmed the monoclinic tenorite structure of CuO in all three products, but purity depended strongly on the starting board. Nanoparticles derived from the PCB1 extract were exceptionally clean: 98.1 percent tenorite, with only 1.4 percent metallic copper and 0.5 percent cuprite. In contrast, products from PCB2 and PCB3 were contaminated with calcium and magnesium phases, including calcite, portlandite, and periclase, which co-leached from inorganic fillers and flame retardants in the epoxy-resin substrates and competed with CuO for nucleation sites during precipitation. Scanning electron microscopy showed equally distinctive morphologies: PCB1-derived particles were uniform cylinders interlinked into porous networks, PCB2-derived particles were mostly spherical with sizes from about 30 to 500 nanometers, and PCB3-derived particles formed heterogeneous polyhedra with grooved surfaces. Overall particle sizes spanned roughly 30 to 900 nanometers, a diversity the authors suggest could be exploited to tailor surface-to-volume ratios for catalytic applications.

The analytical story also carried honest caveats. Energy-dispersive X-ray spectroscopy confirmed copper, oxygen, calcium, magnesium, and carbon in all products, with copper loading of 60.59 weight percent in the PCB1-derived material versus around 33 to 34 percent for the other two. Quantitative phase analysis used the reference intensity ratio method, which the authors note is quasi-quantitative and carries relatively high uncertainty because uncorrelated diffraction peaks can bias weight percentages upward. The team also emphasizes that future process optimization, through selective filtration, chelating agent adjustments, or pH-window control of calcium and magnesium ions, will be needed to suppress co-precipitation and push the lower-grade boards toward single-phase purity.

Even so, the broader implications are striking. The process uses non-aggressive, recyclable reagents, operates at modest temperatures, avoids the hazardous emissions and dust of pyrometallurgy, and produces not merely recovered copper but a catalytic nanomaterial in high demand for industrial organic synthesis. The work was supported by the EIT RawMaterials project RM SLO PLUS Hub, reflecting European interest in securing critical metal supplies from secondary sources. The researchers plan next to quantify the full gravimetric yield across the precipitation-ageing-calcination chain, refine the suppression of calcium and magnesium co-leaching, and run catalytic benchmarks to prove the nanoparticles’ practical worth. If those steps succeed, the mountains of retired circuit boards piling up worldwide could become feedstocks not for landfills, but for a cleaner, copper-secure industrial future.

Subject of Research: Green hydrometallurgical recovery of copper from waste printed circuit boards and synthesis of copper oxide nanoparticles

Article Title: Ammonia-salt-assisted green hydrometallurgy for CuO nanosynthesis from waste PCBs

Article References: Ammonia-salt-assisted green hydrometallurgy for CuO nanosynthesis from waste PCBs. (n.d.). https://doi.org/10.3389/fenvc.2026.1896574

Image Credits: AI Generated

DOI: 10.3389/fenvc.2026.1896574

Keywords: e-waste, printed circuit boards, copper recovery, hydrometallurgy, ammonium salts, copper oxide nanoparticles, CuO, urban mining, circular economy, critical raw materials, microemulsion synthesis, green chemistry