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

Abstract

Hexavalent chromium (CrO 4 2- , Cr 2 O 7 2- , CrO 3 , CrF 6 , 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(NH 2 ) 2 +6OH - →N 2 +CO 2 +5H 2 O+6e E 0 = -0.746 V vs. SHE Cathode reaction: Cr 2 O 7 2- +14H + +6e→2Cr 3+ +7H 2 O E 0 = +1.33 V vs. SHE Overall reaction: CO(NH 2 ) 2 +Cr 2 O 7 2- +8H + →N 2 +CO 2 +2Cr 3+ +6H 2 O E 0 = +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 H 2 SO 4 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.

Key findings

• ▸The fuel cell's anode reaction involves the oxidation of urea, releasing electrons and producing water, with an estimated potential of -0.746 V vs. SHE.
• ▸The cathode reaction involves the reduction of chromate, releasing electrons and producing chromium(III) ions, with an estimated potential of +1.33 V vs. SHE.
• ▸The overall reaction combines the anode and cathode reactions, generating electricity through the oxidation of urea and reduction of chromate.

Keywords

Identifiers

DOI

10.1149/ma2015-02/37/1366

Journal

ECS Meeting Abstracts

Year

2015