Capacitive-faradaic fuel cells (CFFCs) for selective separation of copper(II) ions from water and wastewater

Abstract

• The mechanism of Cu 2+ ions removal from water by the CFFCs was studied. • Cu 2+ are catalytically reduced to Cu 0 by H 2 on the Pt within the separation step. • The Cu 0 is oxidized by oxygen within the regeneration step. • The electrosorption mechanism is insignificant in Cu 2+ separation by the CFFCs. • Cu 2+ ions are selectively separated from Ni 2+ , Cd 2+ , Fe 3+ , Ca 2+ , Mg 2+ and Zn 2+ . The capacitive-faradaic fuel cells (CFFCs), previously proposed for separation of anions and cations from water and wastewater, were investigated for Cu 2+ ions removal. It is shown in this study that the electrosorption of copper ions plays an insignificant role in the process and the major mechanisms that govern the H 2 -induced Cu 2+ separation and the O 2 -induced regeneration of the CFFCs are ( i ) catalytic reduction of Cu 2+ ions into the elemental copper (Cu 0 ), and ( ii ) oxidation of Cu 0 into Cu 2+ ions, respectively. The process was investigated in pure CuCl 2 solutions at different pH values with an activated granular charcoal on which different loadings of Pt catalyst were embedded (0, 0.1, 0.25, 0.5, 1.0 and 2.5 %wt.). Higher adsorption density of Cu was obtained at higher Pt loading (i.e., 30 mgCu/gCFFCs in 0.1%Pt-CFFCs and 75 mgCu/gCFFCs in 2.5%Pt-CFFCs), but the Pt-utilization efficiency decreased from 30 mgCu/mgPt in 0.1%Pt-CFFCs to 3 mgCu/mgPt in 2.5%Pt-CFFCs. Better Cu 2+ removal was achieved at higher pH due to lower reversible potential of the hydrogen oxidation reaction. Excellent selectivity of the process for Cu 2+ ions removal was obtained in experiments conducted with aqueous solution that comprised Cu 2+ , Ni 2+ , Cd 2+ , Fe 3+ , Ca 2+ , Mg 2+ and Zn 2+ ions (100 mg/l each). Further studies are required to develop the CFFCs with ultralow Pt loading or/and to replace Pt with non-noble metal catalysts.