A Urine/Cr(VI) Fuel Cell: Electrical Power from Processing Heavy Metal and Human Excretion

Hexavalent chromium (CrO₄^2-, Cr₂O₇^2-, CrO₃, CrF₆, etc.) is seriously hazardous substance inducing various point mutations in DNA and oxidative changes in proteins due to its strong oxidational properties^[1]. Notably, Cr(VI) is strongly oxidative which can oxidize most organics such as urine in wastewater. Thus, it is possible to develop a fuel cell by using hexavalent chromium and urine as fuels according to the following reaction mechanism:

Anode reaction:     CO(NH₂)₂+6OH^-→N₂+CO₂+5H₂O+6e     E⁰= -0.746 V vs. SHE

Cathode reaction:  Cr₂O₇^2-+14H⁺+6e→2Cr³⁺+7H₂O      E⁰= +1.33 V vs. SHE

Overall reaction:    CO(NH₂)₂+Cr₂O₇^2-+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||KCl_aq||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^[4]) was achieved with an OCV of 1.3 V (twice of that in microbial fuel cells^[4]) in AEM||KCl_aq||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.

References:

1. A. D. Dayan, A. J. Paine, Hum. Exp. Toxicol. 2001, 20, 439-451.

2. W. Xu, H. Zhang, G. Li & Z. Wu, Scientific Reports, 2014, 4: 5863.

3. B. Yu, H. Zhang, W. Xu, G. Li & Z. Wu, Scientific Reports, 2014, 4: 5860..

4. L. P. Huang, X. L. Chai, S. A. Cheng, G. H. Chen, Chem. Eng. J. 2011, 166, 652-661.