A Urine/Cr(VI) Fuel Cell: Electrical Power from Processing Heavy Metal and Human Excretion
Anode reaction: CO(NH2)2+6OH-→N2+CO2+5H2O+6e E0= -0.746 V vs. SHE
Cathode reaction: Cr2O72-+14H++6e→2Cr3++7H2O E0= +1.33 V vs. SHE
Overall reaction: CO(NH2)2+Cr2O72-+8H+→N2+CO2+2Cr3++6H2O E0= +2.076 V
This work shows a strategy of reducing Cr(VI) by human urine with self-generation electricity via a urine/Cr(VI) fuel cell (UCrFC), in which urine functions as fuel and Cr(VI) severs as electrons acceptor. Urine is electro-oxidized on carbon supported nano-Ni catalyst at anode and Cr(VI) is electro-reduced at catalyst-free cathode in acid medium[2,3]. An ion selective separator, consisting of an anion exchange membrane (AEM) and a cation exchange membrane (CEM), AEM||KClaq||CEM separator, is introduced to improve the cell performance by hindering the crossover of dichromate and ammonium.
A maximum power density of 3400 mW m-2 (159 times to microbial fuel cells) was achieved with an OCV of 1.3 V (twice of that in microbial fuel cells) in AEM||KClaq||CEM-UCrFC, when using 50 mg L-1 Cr(VI) in 0.25 M H2SO4 as catholyte and neat urine as anolyte. The apparent first-order kinetic constants of Cr(VI) reduction were -0.224, -0.145 and -0.118 h-1, respectively, when initial concentration of Cr(VI) were 13, 22 and 50 mg L-1. Total organic carbon and total nitrogen removal from urine were observed in 71 h in the anode chamber, with removal efficiency of 79.2% and 78.4%, respectively. At the same time, ~93% of Cr(VI) was removed in the cathode chamber. A high cathodic coulombic efficiency of more than 98% was achieved. One liter of fresh urine could reduce about 28 g of Cr(VI) via UCrFC. This fuel cell provides an alternative technology for using waste (human urine) to treat another waste (hexavalent chromium), and is potentially applicable to other waste/waste system.
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