Mass Transport Influence on Electrooxidation of Urea for Wastewater Remediation
Authors: Carpenter Kelly , Stuve Eric M. 
Published: February 2020

Introduction

Urea [CO(NH2)2] is one of the world’s most abundant waste products and is therefore commonly found in waste streams in varying concentrations. Human urine is 0.33 M urea while ruminant livestock urine has about half that concentration due to their herbivore diet [1]. Urea is also a versatile chemical that is industrially manufactured and commonly used. In effluent streams, urea hydrolyzes to form toxic ammonia (NH3) gas that can oxidize to form other pollutants like nitrates, nitrites, and nitrogen oxides [2]. Electrooxidation of urea for wastewater remediation is of interest whereby Eqn. (1) occurs at the anode of an electrochemical cell at a standard potential of E0 = –0.46 V vs. SHE at pH 14 [3]. Urea oxidation can be paired with concurrent reduction of water to produce H2
for energy storage, or reduction of O2 to directly produce energy in a direct urea fuel cell (DUFC) [4].

DUFCs and H2 production from urea both require high oxidation rates to make them economically viable. As a result, these studies typically use concentrations of 0.33 M urea to imitate human urine concentrations. Urea concentrations in wastewater will be diluted and therefore much lower, yet urea removal remains desirable for environmental reasons. At such low urea concentrations, transport to the electrode surface dominates the overall oxidation rate. The effects of low urea concentration on oxidation have yet to be fully studied, which is what this work aims to address.

In an alkaline aqueous environment, nickel provides an active surface for urea oxidation due to the Ni2+/Ni3+ reversible redox couple described by Eqn. (2) at a standard electrode potential of E0= 0.49 V vs. SHE at pH 14 [3]. Subsequent urea oxidation occurs on Ni3+ and regenerates a Ni2+ site [Eqn. (3)]. At sufficiently anodic potentials, Eqns. (2–3) will occur in tandem and oxidize urea. Experimentally, urea oxidation is observed on Ni at potentials that favor Ni3+ which suggests that Eqn. (3) dominates over Eqn. (1). According to Eqn. (2), an alkaline environment is required for urea oxidation on Ni. To increase reaction rate, studies commonly use 1–5 M KOH, which is too harsh for an effluent stream. This work aims to investigate oxidation kinetics from pH 7–14.

Materials and Methods

Voltammetric studies were performed using a Solartron 1287A electrochemical interface coupled with CorrWare/CorrView software for data collection and visualization. Electrochemical experimental methods include cyclic voltammetry (CV), linear sweep voltammetry (LSV), and chronoamperometry (CA) in a three-electrode cell utilizing a 3.0 mm diameter Ni disk working electrode (WE), Hg/HgO reference electrode (RE), and a platinum coil counter electrode (CE). Urea is the analyte in all experiments, varying in concentration from 1.0 mM to 1.0 M. Supporting electrolytes include KOH and KHCO3 in varying concentrations to maintain pH between 7 and 14. Concentration and temperature variation are used to investigate transport and kinetic mechanism control, respectively.

Results and Discussion

Results will be discussed including electrochemical surface area (ECSA) and electrode characterization and the concentration regimes for both urea and KOH that define transport vs. kinetic reaction mechanism control. Initial experiments confirm mass transfer limited reaction at 15 mM urea and 0.1 M KOH at room temperature as seen in Figure 1. Experiments focusing on the temperature and concentration dependence are in progress.

Significance

Waste stream remediation is a major application of urea oxidation that focuses on removal of urea over maximizing current density. Further, it requires lower urea concentration and more neutral pH than typically studied. We lay the groundwork for mechanistic understanding over a range pH and urea concentrations relevant to such motivations.

References

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Figure 1