Development of efficient betaine-based natural deep eutectic solvents for metal leaching from waste printed circuit boards.

The rapid growth of electronic waste has intensified the need for sustainable approaches for metal recovery from waste printed circuit boards (WPCBs). In this study, betaine-based natural deep eutectic solvents (NADES) were developed as lixiviants for metal extraction from WPCBs. Betaine (BET) was used as the hydrogen bond acceptor, while ethane-1,2-diol (ETD), propane-1,2-diol (PPD), and butane-1,2-diol (BTD) served as hydrogen bond donors (HBDs) to investigate the influence of HBD structure on the physicochemical properties and leaching performance of the resulting NADES. Fourier transform infrared spectroscopy (FTIR) indicated intermolecular interactions between BET and the diol-based HBDs, while thermogravimetric analysis/derivative thermogravimetry (TGA/DTG) showed pronounced mass-loss-rate maxima above 240 °C for all three NADES. Rheological measurements revealed Newtonian flow behaviour, with viscosity increasing with increasing diol chain length; BET:ETD exhibited the lowest viscosity (50.1 mPa·s) at 20 °C. Under the experimental conditions of 75 °C, 6 h, 1.0 M H2O2, 500 rpm, and a solid-to-liquid ratio of 1:20, BET:ETD achieved leaching efficiencies of 99.3% for Cu, 99.5% for Pb, 97.8% for Fe, 95.4% for Al, 90.5% for Sn, and 73.9% for Ti. Kinetic analysis based on the shrinking core model and activation energy evaluation indicated a mixed-control mechanism involving interfacial reaction and diffusion processes. SEM, XRD, and FTIR analyses further indicated substantial dissolution of metallic phases while the polymeric and glass-fibre matrix was largely preserved. Overall, the results demonstrate that betaine-based NADES, most especially BET:ETD, offer tunable physicochemical properties and effective metal-leaching performance for WPCB recycling.