Tannic acid coated single-wall carbon nanotube membranes for the recovery of Au from trace-level solutions

The efficient recovery of gold from industrial sewage is important for saving precious metals and remains a big challenge. We report the extraction of gold ions from a trace-level aqueous solution using a tannic acid (TA) coated single-wall carbon nanotube (SWCNT) film. The TA has many redox ligands that efficiently adsorb Au(III) from the solution and reduce them to Au particles. The interwoven SWCNTs not only act as a framework to improve the mechanical stability of the hybrid membrane, but also provide abundant paths for H₂O transport, and facilitate the full exposure of the TA. As a result, the hybrid membrane has an excellent ability to capture gold ions from solution with a high flux of 157 L/(m²·h·bar), and an ultra-high adsorption capacity of 2095 mg/g from solutions with an extremely low gold concentration of 20 ppm. The adsorbed gold ions are reduced to Au particles, which can be easily collected by oxidation. The recovered Au nanoparticles on the TA–SWCNT hybrid film had a remarkable surface-enhanced Raman scattering effect that enabled the sensitive detection of rhodamine 6G.