Effectiveness of metal–EDTA-polluted water treatment and metal recovery at drop-casted films of TiO2 under UV-LED irradiation: A photoelectrochemical study

Abstract

EDTA is an important metal sequestering agent in industrial processes and household products, but also an emerging pollutant and a challenge for wastewater treatment due to its high resistance to biodegradation. Conventional strategies for recovering metals from the metal chelates require a previous oxidation step. Herein, P25 TiO2 photocatalyst was applied to assemble nanoparticulate films on gold (model conducting substrate) with highly reproducible photocatalytic activity toward EDTA and Cu(II)-EDTA (model chelate) UVA-photooxidation. Films were obtained from P25 suspensions in acidic medium by a quick cold drop-casting method without thermal annealing. The involved TiO2 agglomerates were characterized by Dynamic Light Scattering (DLS), Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), voltammetric DC techniques and zero-current chronopotentiometry. Photocatalytic treatment of Cu(II)-EDTA solutions and electrochemical essays were accomplished in a thin layer-type cell coupled with a 365 nm UV-LED. Synthetic samples of the chelate at stoichiometric ratio or with EDTA in excess were exposed to the irradiated TiO2 film to promote the photooxidation of the ligand, and recovery of copper mainly by direct (photo)electrodeposition on gold. Three potential programs were tested during UV-irradiation: open circuit potential (OCP), +0.5 V and –0.3 V vs Ag/AgCl. Overall best results were obtained at –0.3 V, showing that it is more effective to favor in-situ metal reduction rather than collecting photogenerated electrons via +0.5 V biasing to diminish electron/hole (exciton) recombination. Nevertheless, operation without external biasing, i.e., at OCP, is nearly as effective since the substrate is spontaneously driven to circa –0.7 V by photogenerated electrons plus electrons injected into photoholes by EDTA, as it oxidizes. The Cu(II)-EDTA treatment can be abbreviated by coupling both redox processes at the same photoelectrode, thus, avoiding the transport of Cu(II) from the photoanode to the cathode, conceivably with risk of rechelation, like in conventional photoelectrochemical cells. Similar results were obtained with the TiO2 drop-casted on other conducting substrates including carbon, a relevant finding for a forthcoming scale-up of the treatment process.