Photoelectrochemical cell for simultaneous electricity generation and heavy metals recovery from wastewater

The feasibility of simultaneous recovery of heavy metals from wastewater (e.g., acid mining and electroplating) and production of electricity is demonstrated in a novel photoelectrochemical cell (PEC). The photoanode of the cell bears a nanoparticulate titania (TiO₂) film capped with the block copolymer [poly(ethylene glycol)-b-poly(propylene glycol)-b-poly(ethylene glycol)] hole scavenger, which consumed photogenerated holes, while the photogenerated electrons transferred to a copper cathode reducing dissolved metal ions and produced electricity. Dissolved silver Ag⁺, copper Cu²⁺, hexavalent chromium as dichromate Cr₂O₇^2- and lead Pb²⁺ ions in a mixture (0.2mM each) were removed at different rates, according to their reduction potentials. Reduced Ag⁺, Cu²⁺ and Pb²⁺ ions produced metal deposits on the cathode electrode which were mechanically recovered, while Cr₂O₇^2- reduced to the less toxic Cr³⁺ in solution. The cell produced a current density J_sc of 0.23mA/cm², an open circuit voltage V_oc of 0.63V and a maximum power density of 0.084mW/cm². A satisfactory performance of this PEC for the treatment of lead-acid battery wastewater was observed. The cathodic reduction of heavy metals was limited by the rate of electron-hole generation at the photoanode. The PEC performance decreased by 30% after 9 consecutive runs, caused by the photoanode progressive degradation.