Effect of reactor configuration on the kinetics and nitrogen byproduct selectivity of urea electrolysis using a boron doped diamond electrode

Water Research 168 (2020) 115130

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Effect of reactor configuration on the kinetics and nitrogen byproduct selectivity of urea electrolysis using a boron doped diamond electrode Andrew Schranck, Kyle Doudrick* Department of Civil and Environmental Engineering and Earth Sciences, University of Notre Dame, Notre Dame, IN, 46556, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 1 July 2019 Received in revised form 20 September 2019 Accepted 24 September 2019 Available online 27 September 2019

Electrochemical systems have emerged as an advantageous approach for decentralized management of source-separated urine with the possibility of recovering or removing nutrients and generating energy. In this study, the kinetics and byproduct selectivity of the electrolytic removal of urea were investigated using a boron doped diamond working electrode under varied operational conditions with a primary focus on comparing undivided and divided reactors. The urea removal rate in the undivided and divided reactors was similar, but the divided reactor had an increased required cell voltage needed to maintain the equivalent current density. The current efficiency was similar for 0.1, 0.25, and 0.5 A (33.3, 83.3, 167 mA/cm2), suggesting no interference from competing reactions at higher potentials. In a divided reactor, increasing the anolyte pH reduced the urea removal rate presumably from hydroxyl radical scavenging by hydroxide. Further, for all divided reactor experiments, the final pH was less than 1, suggesting that the transport of protons across the ion exchange membrane to the cathode was slower than the oxidation reactions producing protons. The nitrogen byproduct selectivity was markedly different in the undivided and divided reactors. In both reactors, nitrate (NO
3 ) formed as the main byproduct at the anode, but in the undivided reactor it was reduced at the stainless steel cathode to ammonia. In the presence of 1 M chloride, the urea removal kinetics improved from the generation of reactive chlorine species, and the byproduct selectivity was shifted away from NO
3 to presumably chloramines and N2. Overall, these results indicate that the electrochemical reactor configuration should be carefully considered depending on the desired outcome of treating source-separated urine (e.g., nitrogen recovery, H2 generation). © 2019 Elsevier Ltd. All rights reserved.

Keywords: Urea Urine Electrolysis Boron doped diamond Undivided reactor Nitrogen byproducts

1. Introduction Nitrogenous compounds in municipal wastewater increase the required time, space, and energy for treatment. Urine accounts for about 80% of influent municipal wastewater nitrogen (N) but just 1% of the volume, contributing significantly to eutrophication of natural waterways (Larsen et al., 2009; Maurer et al., 2006). This positions decentralized, urine source-separation as a practical solution for reducing N treatment costs (Ledezma et al., 2015; Lienert and Larsen, 2010). Urea (CO(NH2)2) is the primary N compound in fresh urine, and the development of sustainable technologies to decompose urea will be key for effective source-separation.

* Corresponding author. Department of Civil and Environmental Engineering and Earth Sciences, University of Notre Dame, 156 Fitzpatrick Hall, Notre Dame, IN, 46556, USA. E-mail address: [email protected] (K. Doudrick). https://doi.org/10.1016/j.watres.2019.115130 0043-1354/© 2019 Elsevier Ltd. All rights reserved.

Electrochemical systems have emerged as a treatment technology for source-separated urine, presenting a potentially lower cost alternative to conventional denitrification methods that are limited by biological process kinetics (Yan et al., 2012a). Other treatment technologies such as anaerobic ammonia oxidation (anammox) and precipitation also target the most common forms
of N in municipal plant influent (i.e., NH3, NO
2 , and NO3 ) with the goal of removing N by producing N2 gas in anammox and recovering N in a useful precipitate such as struvite (NH4MgPO4) (Maurer et al., 2006). While anammox is an energy efficient method for N removal, it requires a greater P:N:C balance than present in urine, and though recovery of N for agricultural use is widely investigated, struvite production requires chemical addition and increased infrastructure (Maurer et al., 2006). Though fresh urine has approximately 20 g/L urea, the urease enzyme present in urine naturally facilitates complete hydrolysis of urea to ammonia (NH3) within one to a few days (Kim et al., 2011; Ray et al., 2018; Udert et al., 2003). This raises the pH of fresh urine

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from about 6.5 to over 9, partially converting ammonium to ammonia (pKa ¼ 9.24), which can lead to air quality hazards (Hernandez et al., 2014). N2 on the other hand is a benign, convenient product for point of use treatment in urban settings (GarciaSegura et al., 2018). Further, complete oxidation of urea in electrochemical treatment provides opportunities to recover energy in the form of hydrogen gas (H2) that may be used for fuel cell technologies (Urbanczyk et al., 2016; Xu et al., 2016a). Inorganic electrodes are easily mass produced and they are more stable over a wide range of conditions, whereas microbialbased electrochemical systems involve complex bacterial cultivation, long start-up times, and stringent working conditions (Logan et al., 2006). With recent advances in electrode synthesis and characterization methods, the performance of inorganic electrodes has improved and been applied for a broad set of environmental applications (Bezerra et al., 1997; Boggs et al., 2009; Chen et al., 2016; Cho and Hoffmann, 2017; Climent et al., 1999, 2000; Ding et al., 2016; Guo et al., 2015; King and Botte, 2011; Lan and Tao, 2011; Lan et al., 2010; Miller et al., 2012; Urbanczyk et al., 2017; Wang and Botte, 2014; Wang et al., 2012a, 2012b, 2013; Wu et al., 2014a, 2014b; Xu et al., 2014, 2016b; Ye et al., 2015; Yu et al., 2014). Noble metals (e.g., Pt, Ru, Ir) initially received most of the attention (Amstutz et al., 2012; Bezerra et al., 1997; Boggs et al., 2009; Chen et al., 2016; Climent et al., 1997, 1999, 2000; King and Botte, 2011; Miller et al., 2012; Wright et al., 1986; Yao et al., 1973, 1974), but their high cost, low-abundance, and inability to fully oxidize organics limits their scalability (Chaplin, 2014; Comninellis, 1994; Panizza and Cerisola, 2009). Nickel, which is also the active metal in the urease enzyme, has emerged as an alternative to noble metals due to the catalyst regeneration mechanism of nickel in the presence of OH
and the modest applied electrode potentials required for urea oxidation (0.40e0.65 V vs. Hg/HgO) (Boggs et al., 2009; Guo et al., 2015; Miller et al., 2012; Wang and Botte, 2014; Wang et al., 2017; Wu et al., 2014a; Xu et al., 2014). While promising, nickel suffers from alkaline pH requirements (Ding et al., 2016; Xu et al., 2014, 2016a; Yan et al., 2012b) and fouling by urine compounds (Schranck et al., 2018). Boron doped diamond (BDD) has been used frequently in electrochemical advanced oxidation processes for a wide range of wastewater contaminants (Chaplin, 2014; Panizza and Cerisola, 2005, 2009). BDD is an inactive electrode as an inactive electrode (i.e., does not change oxidation state) produces highly oxidative hydroxyl radicals (HO) at approximately 2.38 V (vs. SHE) (Bard et al., 1985; Chaplin, 2014; Dbira et al., 2015a, 2016; Hernandez et al., 2014; Kapalka et al., 2009, Kapalka et al., 2010b; Perez et al., 2012a; PerezGonzalez et al., 2012; Radjenovic and Sedlak, 2015). It has a higher overpotential for the oxygen evolution reaction compared to other electrodes (e.g., Ni, Pt), which provides a wider electrode potential window for oxidation of contaminants (Chaplin, 2014). Because BDD is an inactive electrode, it will be less susceptible to electrode deactivation by urine compounds. The electrochemical treatment of hydrolyzed urea (i.e., ammonium) has been studied in some detail (Amstutz et al., 2012; Christiaens et al., 2017; Kapalka et al., 2010a, Kapalka et al., 2010c; Luther et al., 2015; Zamora et al., 2017; Zollig et al., 2015a, 2015b, 2017), but limited attention has been given to the treatment of fresh urine containing concentrated urea. Due to the storage and transport time associated with wastewater as it flows from the toilet to the treatment plant effluent and the concomitant hydrolysis of urea to ammonia, kinetic studies are critical for developing technologies to address the issue of wastewater N. Additionally, the production of undesirable treatment byproducts, including N and Cl species, compromises the viability of many electrochemical systems (Garcia-Segura et al., 2018; Perez et al., 2012b; Zollig et al., 2015b).

Select studies have investigated kinetics and N selectivity for urea and ammonia electrooxidation for BDD in undivided reactors (Dbira et al., 2015a); however, divided reactors have been sparsely reported on. Separating the anode and cathode will be critical if considering H2 production from urine. Further, it may play a role in the N byproduct selectivity depending on the cathode material selected. For example, many active cathodes can reduce nitrate to various nitrogen byproducts (Garcia-Segura et al., 2018). A comprehensive evaluation of BDD in a divided reactor will provide insight into the reaction mechanisms of the anode and cathode chambers, ultimately determining wastewater treatment and H2 production opportunities and obstacles for source-separated urine treatment. In this study, the kinetics and byproduct selectivity of the electrolytic removal of urea were investigated using a BDD electrode under varying conditions. Specifically, comparing an undivided and divided reactor, this study evaluated the effect of (i) the anolyte and catholyte pH, (ii) varying current and voltage, (iii) the presence of reactive chlorine on the urea removal kinetics and the nitrogen byproduct selectivity. The outcomes provide insight into urea electrochemical treatment processes that will improve reactor design and approaches to wastewater treatment and resource recovery.

2. Experimental 2.1. Electrochemical setup Electrochemical experiments were conducted in an undivided or divided reactor. Complete details and images of the electrochemical reactor setups used for this study can be found in the Supplementary Information (SI; Fig. S1). The undivided reactor experiments were conducted in a 40 mL chamber. The divided reactor experiments were conducted in an H-cell consisting of a 40 mL anodic chamber and a 20 mL cathodic chamber. The chambers were separated by a Nafion 117 proton exchange membrane (PEM; Fuel Cells Etc., TGPH030-4005). A detailed description of the preparation of the PEM is described in the SI. Voltage was controlled using an HY3003D Dr. Meter DC Power Supply for the two-electrode experiments and a Biologic SP200 potentiostat (BioLogic USA) with EC Lab software for the three-electrode experiments. A commercial solid polycrystalline, free-standing boron doped diamond electrode (BDD; EP Poly, 15.0 (W) x 15.0 (H) x 0.45 (D) mm, Element Six Technologies, 145-500-0469) was used as the working electrode. Stainless-steel mesh (SS, Alfa Aesar, 46715) was used for the counter electrode. A saturated calomel electrode (SCE, VWR, 89501-040) was used as a reference electrode for the threeelectrode experiments. Prior to the electrolysis experiments, the BDD electrode was cleaned using bath sonication (40 kW, Branson, M3800), first in an aqueous solution of 10% Alconox (10 min) and then in H2O (10 min). SS counter electrodes were bath sonicated in separate solutions of ACS-grade acetone (VWR BDH, 1101) (10 min), 1 N hydrochloric acid (HCl; Millipore Sigma, HX0603-3) (10 min), and H2O (10 min), with H2O rinses between each step. The submerged geometric area of the BDD and SS electrodes was approximately 3 cm2 and 4 cm2, respectively. The distance between the working and counter electrodes for undivided and divided experiments was approximately 25 cm and 75 cm, respectively. The SCE was always placed approximately 15 mm from the working electrode. The working electrode solution was continuously stirred with a magnetic bar during experiments to reduce mass transfer limitations.

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2.2. Electrochemical experiments Electrochemical experiments were conducted using various solutions, including urea (CO(NH2)2; Amresco, 0568), urea in 1 N sodium perchlorate (NaClO4; Millipore Sigma, SX0694), synthetic urine, and dilute synthetic urine in 1 N NaClO4. Starting anolyte concentrations contained 117 mg-N L
1 in all experiments based on 250 mg L
1 urea. All aqueous samples were prepared with ultrapure H2O (18.2 MU-cm, Barnstead GenPure Pro). Synthetic urine was prepared as previously reported (Schranck et al., 2018; Wenzler-Roettele et al., 2006) and details are provided in the SI (Table S1); however, NH4Cl was not used when measuring total ammonia nitrogen (TAN) in the byproduct selectivity experiments. Synthetic urine was composed of the following chemicals: potassium hydroxide (100 mM, Sigma 306568), urea (330 mM), sodium chloride (154 mM, Fisher Scientific, S271-3) ammonium chloride (56 mM, BDH 9208), sodium sulfite (24 mM, Spectrum Chemical 51475), disodium phosphate (18 mM, Fisher Scientific S374-500), monopotassium phosphate (18 mM, Amresco 0781), creatinine (18 mM TCI C0398), gelatin (1 g/L Amresco 9764), and Difco nutrient broth (1.6  10
4 g/L, BD 234000). Dilute synthetic urine was prepared by diluting the synthetic urine 80 times to achieve 117 mg-N L
1 urea for equal comparison across experiments. A two-electrode configuration was used for chronopotentiometry (CP) experiments that were used to analyze the urea removal kinetics. CP establishes a constant current (I, A) with respect to the electrode area, while permitting E to fluctuate as needed to maintain a constant flow of current and allow for measurement of kinetics (i.e., galvanostatic electrolysis under specified current densities). CP results are not potential dependent, and thus only two electrodes (i.e., anode and cathode) were required. The reported voltages for CP experiments are cell voltages. A three-electrode configuration was used for chronoamperometry (CA) potentiostatic experiments that were used to analyze the byproduct selectivity for urea oxidation under constant voltage conditions. The constant voltage (E, V) established at the working electrode in CA establishes a voltage vs. a reference electrode for targeted redox phenomena, allowing current to fluctuate accordingly based on the thermodynamic conditions of the reactor. CA requires a reference electrode connected to the working electrode to maintain a constant working electrode potential. CP and CA experiments were conducted using various electrolyte salts as noted throughout the text, including: 1 N NaClO4, potassium hydroxide (KOH; Sigma-Aldrich, 306568), sulfuric acid (H2SO4; Millipore Sigma, SX1244-75), and sodium chloride (NaCl; Fisher Scientific, S271-3). Nitrogen sources used in the experimental anolytes included urea, sodium nitrate (NaNO3; Sigma Aldrich, 55506) and ammonium chloride (NH4Cl; BDH, 9208). 2.3. Chemical analysis Temperature and pH (HACH sensION Gel Filled Combination pH Electrode, VWR, 97021-580) and conductivity (HACH Conductivity Probe, VWR, 82026-966) of the reactor solution were measured with calibrated probes before and after electrolysis to characterize changes across the operational parameters and reactor configurations tested. During CP and CA experiments, extractions were taken at various timepoints from the chamber containing the working electrode. All samples were diluted ten times prior to analysis. Unless otherwise noted, dilutions were prepared inside a 10 mL syringe and then filtered through a 0.45-mm filter (EZFlow® Membrane Disc Filter, Nylon, Foxx Life Sciences, VWR, 76018-842). To minimize volume extraction from 40 mL experimental volume, nominal extraction volumes were selected according to the experimental method CP or CA) and target compounds being

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analyzed (urea, NO
2 , NO3 , TAN); a full description of the sample preparation methods are specified in the SI. Urea concentrations were measured using an assay adapted from previous reports (Jung et al., 1975; Zawada et al., 2009) using a black, clear bottom, 96-well plate (Corning, 3603) and UV-VIS spectrophotometer (Biotek, Synergy H1) with primaquine as the primary colorimetric reagent in the assay); a full description of the urea assay analysis is specified in the SI. The minimum detection limit (MDL) of the urea assay was determined to be 2.5 mg mg-N L
1 urea take from nine replicates using a 95% confidence t-test. Due to the sample dilution, this corresponds to an effective MDL of 25 mg mg-N L
1 urea. All urea concentrations below this MDL were set equal to the MDL. After urea measurement, 100 mL of 5 M KOH was added as an ion strength adjuster, converting NHþ 4 to NH3, and this remaining sample was used to determine TAN using an ion selective electrode (ISE;
Thermo Scientific, 9512BNWP). NO
2 and NO3 concentrations were quantified using ion chromatography [IC; Thermo Dionex, ICS-5000 with AS23 (analytical) and AG23 (guard) columns]. Statistical analysis was done using a two-tailed paired two-sample mean ttest. Exact P values are reported, and significance was determined herein using an a ¼ 0.05.

3. Results and discussion 3.1. Urea removal kinetics 3.1.1. Effect of initial anolyte and catholyte pH Fig. 1A shows the effect of initial anolyte pH on urea removal for the undivided reactor. The unadjusted pH of the anolytes was 1.31 (1 N H2SO4), 6.45 (1 N NaClO4), and 13.94 (1 N KOH). The observed average percent removals of urea after 80 min were 73 ± 11%, 71 ± 3.0%, and 12 ± 3.0%, respectively. There was no significant difference between the acidic and circumneutral anolyte conditions (P ¼ 0.20). The difference in conductivity values for the supporting electrolytes [e.g., 1 N H2SO4 ¼ 211 mS/cm; 1 N KOH ¼ 178 mS/cm (Rumble, 2019)] is unlikely to have a marked effect on the urea removal kinetics because of the high supporting electrolyte concentrations used (1 N). BDD is an inactive electrode with no unique electrochemical signature for urea (Fig. S2), which supports HO production as the dominant electrochemical oxidation pathway of urea. The low urea removal observed for alkaline conditions can be attributed to the reaction between and hydroxide (OH
), which can recombine with HO to form H2O (Hayashi et al., 2016). At alkaline pH, the abundance of OH
retarded the oxidation of urea by HO. The average required cell voltage negatively correlated (R2 ¼ 0.99) with pH (Fig. S3 and Table S3), with voltages decreasing from 6.3 V for pH 1.31e3.8 V for pH 13.94. This is presumably due to a combination of conductivity differences between electrolytes and/or a Nernst relationship, where the redox reactions will have a lower required potential with increasing higher pH. Though not directly observed here, alkaline conditions can also change the surface morphology of BDD, producing inhibitory conditions for organic oxidation, and ultimately degrading the BDD surface (DeClements and Swain, 1997; Gonzalez-Gonzalez et al., 2010; Griesbach et al., 2005). For remaining experiments, unless otherwise stated, NaClO4 was used as the conductive supporting anolyte to reduce resistance caused by large electrode spacing. In realistic applications, due to its toxicity, its use would be eliminated using advanced reactor design. In a divided reactor, the cathode pH is important because it can alter the flow of protons across the PEM and change the anolyte pH, thus affecting the urea oxidation rate and required cell potentials. Fig. 1B shows the effect of catholyte pH (0.58, 6.12, 13.98) on the urea removal kinetics at the anode for a divided reactor. All

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BDD½HO   þ R / BDD½  þ CO2 þ H2 O þ H þ þ e


Fig. 1. Urea removal kinetics as a function of Q0 for (A) an undivided reactor with varying anolyte pH and (B) a divided reactor with varying catholyte pH. The different pH was obtained using 1 N supporting electrolytes, including H2SO4 (acidic), NaClO4 (circumneutral), and KOH (alkaline). Current was maintained at 0.1 A (33.3 mA/cm2) for 140 min using CP. The volume of the undivided reactor and the anolyte chamber in the divided reactor was 40 mL, and the volume of the catholyte chamber in the divided reactor was 20 mL. The initial concentration of urea was 117 mg-N L
1. The error bars represent one standard deviation of the urea concentration for triplicate sampling for each time point. All urea values below the MDL are marked with an asterisk.

catholyte pHs tested resulted in similar urea removal kinetics (P ¼ 1.0) and a decrease in the anolyte pH. The initial anolyte pH for the three conditions was an average of 6.7 and it decreased to an average of 0.65 after 140 min. The pH of the catholyte increased less than 0.5 over 140 min for the acidic (0.58) and alkaline (13.98) reactors, but for the circumneutral condition (6.12), the pH increased to 12.95. The negligible change in the catholyte pH for the acidic and alkaline conditions is attributed to the buffer intensity afforded by water at the extreme pH (i.e., pH < 3 or pH > 11). In contrast, the pH of the electrolyte in the undivided experiments (i.e., Fig. 1A) did not change significantly (<5%) (Table S3). The pH change observed in the divided reactor is attributed to Hþ reaction and transport kinetics. For BDD, the oxidation of H2O forms HO (Equation (1)), which remains physisorbed at a BDD[ ] site (Chaplin, 2014). The weakly bound HO (i.e., BDD[HO]) can then react with H2O (Equation (2)) or an organic species, R (Equation (3)). All reactions produce Hþ at the anode and Hþ can be reduced at the cathode. The surplus of Hþ produced at the anode should migrate through the PEM to the cathode to maintain a steady-state pH. But, because of the drastic pH changes observed in the anode and cathode chambers, this suggests that the kinetics of Hþ production (anode) and consumption (cathode) were much faster than transport of Hþ through the PEM (Comninellis, 1994; Panizza and Cerisola, 2009).

BDD½  þ H2 O / BDD½HO   þ Hþ þ e


(1)

BDD½HO   þ H2 O / BDD½  þ O2 þ 3H þ þ 3e


(2)

(3)

3.1.2. Comparison of undivided and divided reactors for urea and synthetic urine Separating the electrodes into two chambers can prevent the back reaction of byproducts in solution or at the electrode surfaces, but this may have unwanted consequences on the kinetics. The kinetics of urea removal were compared for an undivided and divided reactor for urea and dilute synthetic urine solutions (Fig. 2). Under all conditions, urea was removed to the MDL within 140 min. There was no significant difference observed between the two solutions for each reactor (Undivided: P ¼ 0.31; Divided: P ¼ 1.0), nor did dividing the reactor have any significant impact on the urea removal kinetics of either solution (Urea: P ¼ 1.0; Synthetic Urine: P ¼ 0.36). The temporary stabilization of the rate around Q0 ¼ 0.5 observed for synthetic urine solutions in both reactors is presumably due HO competition by intermediates (e.g., ammonium). Other constituents in urine could also inhibit the urea oxidation kinetics, whether due to side and competing reactions or electrode interference and degradation (Griesbach et al., 2005; Schranck et al., 2018). However, this was not observed for either reactor configuration and voltammetry characterization supported this result (i.e., Fig. S2). Compared to the undivided reactor, maintaining 0.1 A in the divided reactor required an approximately two-fold increase of the required cell voltage from 5.1 to 11 V. This increase is due to an increase in the total internal resistance, R, which is the sum of nonohmic and ohmic resistances (ElMekawy et al., 2013). Generally, non-ohmic resistances are caused by activation (i.e., charge transfer) and concentration (i.e., mass transport) losses, and ohmic resistances are caused by poor electronic contacts, ion migration within the electrolyte, and ion conductivity of the separation membrane (Fan et al., 2008; Sleutels et al., 2009). Using Ohm’s law (I ¼ V R
1), R was calculated for the undivided and divided reactors as 51 and 110 U, respectively. Using current interrupt (Logan et al.,

Fig. 2. Urea removal kinetics as a function of Q0 in (A) an undivided and (B) a divided reactor. The anode chamber (V ¼ 40 mL) contained either urea or dilute synthetic urine in both reactor configurations and with a 1 N NaClO4 supporting electrolyte. Divided experiments contained 1 N H2SO4 as the catholyte (V ¼ 20 mL). Current was maintained at 0.1 A (33.3 mA/cm2) for 140 min using CP. The starting concentration of urea was 117 mg-N L
1. The error bars represent one standard deviation of the urea concentration for triplicate sampling for each time point. All urea values below the MDL are marked with an asterisk.

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2006), an ohmic resistance of 14.3 and 15.2 U was measured for the undivided and divided reactors, respectively. Assuming the ohmic contact resistance was similar in the divided and undivided reactors, the minor increase (i.e., 0.9 U) in ohmic resistance when dividing the reactor can be attributed to the increased electrode distance and addition of the PEM (Cho and Hoffmann, 2014, 2017; Garcia-Segura et al., 2018; Hu et al., 2008; Logan et al., 2006; Rozendal et al., 2007). The increase in ohmic resistance was minor, thus most of the increased resistance for the divided cell was due to non-ohmic resistances, which were presumably caused by concentration polarization losses from the pH imbalance between the anode and cathode chambers (ElMekawy et al., 2013). This imbalance occurs because the Hþ diffusion through the PEM is slower than the Hþ reaction at the electrodes (Daud et al., 2015), causing the pH of anode and cathode cells to decrease and increase, respectively, as observed in this study. In practical applications, ohmic resistances can be reduced by using a reactor that has a negligible electrode spacing (Chouler et al., 2016; Logan et al., 2006; Moon et al., 2015; Mousset et al., 2019) and using thin, conductive membranes (Daud et al., 2015). Reducing non-ohmic resistances is more challenging, but in the scenario presented herein, the resistances imparted by the pH imbalance can be mitigated by using a more proton conductive exchange membrane. 3.1.3. Effect of varied current in a divided reactor Fig. 3A shows the impact of varying applied current on urea removal as a function of time in the divided reactor. The 0.1, 0.25, and 0.5 A (33.3, 83.3, 167 mA/cm2) applied currents corresponded to cell voltages of 8.90, 15.8, and 21.9 V. Increasing the current density resulted in a linearly proportional increase of the urea conversion (R2 ¼ 0.99, not shown). Current efficiency (CE) was calculated assuming an 8-electron oxidation to nitrate (based on selectivity data in Section 3.2). The current efficiencies at 0.1, 0.25, and 0.5 A for approximately 80% urea conversion were 39%, 39%, and 42%, respectively. The current efficiency decreased with passing time (e.g., for 0.1 A: at t ¼ 20 min, CE ¼ 94%, and at t ¼ 60 min, CE ¼ 50%), indicative of a pseudo first-order reaction (Table S3). The urea conversion at each Q0 was similar for all current densities, indicating relatively no change in the current efficiency with increasing current and voltage. This suggests that side reactions and mass transfer limitations were not impacting urea conversion (Bard and Faulkner, 2001). This data further promotes the stability of BDD for urea removal at high potentials, and predictable nature of urea removal between 0.1 and 0.5 A (33.3e167 mA/cm2) for this divided reactor.

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3.2. Nitrogen byproduct selectivity 3.2.1. Effect of varied voltage Byproduct selectivity is critical for understanding the fate of N during urea removal, and it can be used to elucidate reaction mechanisms. Considering all other factors to be constant, the voltage must be increased to get increased reaction rates; however, changing voltage may alter the urea oxidation pathway and byproduct selectivity. Fig. 4 shows the impact of reactor configuration and applied working electrode voltage (3 and 4 V vs. SCE) on the removal of urea and subsequent formation of NO
3 and TAN. No urea removal was observed when 2 V was applied, presumably because the overpotential required to produce HO for BDD [2.62 V vs. SCE (Bard et al., 1985; Kapalka et al., 2009; Panizza and Cerisola, 2009)] was not reached. As expected, in both undivided and divided reactors, the urea removal at 60 min was greater for 4 V compared to 3 V, with respective removals of 92% and 42% for the undivided reactor (Figs. 4A) and 72% and 31% for the divided reactor (Fig. 4B and Table S3).
NO
3 , NO2 , and TAN were measured as aqueous byproducts of urea removal for undivided and divided reactors (Fig. 4CeD). NO
2 concentrations were typically low (not shown), with maximum values of 0.9 and 1.5 mg-N L
1 observed in both the undivided and divided reactors, respectively. In previous studies on urea or TAN
oxidation, much less NO
2 was formed than NO3 and oftentimes
NO
2 concentrations peaked as NO3 continued to increase (Dbira et al., 2015a, 2016; Lacasa et al., 2012; Li et al., 2015; Perez et al., 2012a, 2012b; Zollig et al., 2017). In the undivided reactor, NO
3 production peaked at approximately 55 mg-N L
1 around 60 min for 4 V and at 42 mg-N L
1 around 120 min for 3V. For both 3 and 4 V conditions, NO
3 was subsequently removed in conjunction with TAN formation. These results complement the anticipated conclusion that TAN is a byproduct of NO
3 reduction at the cathode (Garcia-Segura et al., 2018), and that voltage increases the rate of reaction without altering the product pathway. For 4 V, TAN reached a peak of 84 mg-N L
1 after 180 min before leveling and then it began to decrease. Peak by-product selectivities for TAN and NO
3 were approximately 65% and 20%, respectively. There are two hypothesized reactions that can account for the missing 15% N byproduct. In the undivided reactor, the final pH for the 4 V experiment was approximately 10.21, so NH3 would account for approximately 90% of TAN (i.e., pKa ¼ 9.25). NH3 can volatilize (Tarpeh et al., 2018) or it can be oxidized at BDD to N2 (Michels et al., 2010). Without the addition of a stripping gas, the NH3 mass volatilized would be low

Fig. 3. Urea removal kinetics in a divided reactor at various currents as a function of (A) time and (B) charge passed normalized to reactor volume and electrode surface area, Q0 . The current was held constant at 0.1, 0.25, and 0.5 A (33.3, 83.3, 167 mA/cm2) over 140 min. The anode chamber (V ¼ 40 mL) contained 117 mg-N L
1 urea and 1 N NaClO4, and the cathode chamber (V ¼ 20 mL) contained 1 N H2SO4. The error bars represent one standard deviation of the urea concentration for triplicate sampling for each time point. All urea values below the MDL are marked with an asterisk and negative values are plotted as 0.

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Fig. 4. Urea removal (A and B) and NO
3 and TAN formation (C and D) in undivided and divided reactors operated at 3 and 4 V (vs. SCE). The theoretical initial urea concentration was 117 mg-N L
1 prepared in 1 N NaClO4 supporting electrolyte. The volume of the undivided reactor and the anode chamber of the divided reactor was 40 mL. For the divided reactor, urea was only added to and N species tracked in the anode chamber, and the cathode chamber (V ¼ 20 mL) contained 1 N H2SO4. The voltage was maintained for 240 min using CA. The error bars represent one standard deviation of the urea concentration for duplicate sampling for each time point. All urea values below the MDL are marked with an asterisk and negative values are plotted as 0.

at the timescales used here. Thus, the remaining 15% N in this study was attributed to mostly N2 gas formation as it is the most thermodynamically stable N gas species (Garcia-Segura et al., 2018); however, other N-gases are also plausible for N organics (e.g., N2O, NO) (Garcia-Segura et al., 2017). In the divided reactor, the byproduct selectivity was markedly different. NO
3 formation continually increased throughout the experiment, reaching a maximum of 83 and 102 mg-N L
1 for 3 and 4 V, respectively, before the reaction period ceased. For both 3 and 4 V conditions, minor amounts of TAN (<15 mg-N L
1) were formed at approximately the same rate up to 60 min and then steadied. The final pH in anode chamber of the divided reactor was <1, so nearly 100% of the TAN would be NHþ 4 . For 3 and 4 V, a maximum byproduct selectivity of approximately 80% and 20% was observed þ
þ for NO
3 and NH4 , respectively. All NO3 or NH4 formed was not þ removed by electrooxidation, and the sum of NO
3 and NH4 accounted for approximately 100% of N, so no N2 was formed from urea oxidation. This agrees with previous studies that showed NH3 must be present for N2 formation (Michels et al., 2010). In both reactors, urea oxidation forms NO
3 . In the undivided reactor, NO
3 migrates to the cathode and is reduced to TAN (as mostly NH3), which is then either volatilized at the cathode or transports to the anode and becomes oxidized to N2. In the divided reactor, NO
3 cannot pass through the PEM to the cathode and thus accumulates in the anode chamber. Since urea and TAN have the same (-III) oxidation state, the observed formation of TAN in the divided reactor cannot be from a redox reaction, thus TAN is being produced by some other mechanism. Hydrolysis of urea to TAN has been observed chemically (Khan et al., 1996; Warner, 1942) and electrochemically in the presence of OH
(Lu and Botte, 2015, 2017). Presumably, some minor hydrolysis of urea occurred, which reached equilibrium after 60 min in the divided reactor. This reaction likely occurred in the undivided reactor as well but was not observed due to the formation of TAN from NO
3 reduction.

3.2.2. Effect of NaNO3 and NH4Cl on N selectivity during urea removal To confirm the reaction pathways observed in Fig. 4, additional

experiments were conducted in the undivided and divided reactors starting with either NO
3 or TAN (i.e., no urea). In the undivided
reactor starting with NO
3 , NO3 was reduced at the cathode to TAN with approximately 75% selectivity (Fig. 5A). No NO
3 removal was observed in the divided reactor (Fig. 5B), confirming NO
3 is reduced at the cathode in the undivided reactor. When starting with TAN, TAN decreased by 18% and 27% in the undivided and divided reactors, respectively, and insignificant NO
3 was formed (<5%) in both reactors (Fig. 5C and D). While this suggests TAN is slowly volatilized or oxidized to N-gases, the presence of Cl
in these experiments may cause misleading results, as significant TAN oxidation was not observed in the divided reactor experiments starting with urea (i.e., Fig. 4D). Cl
can oxidize on the BDD anode to reactive chlorine species (RCS), which then further oxidize urea,74 and this may change the urea removal kinetics and byproduct selectivity. This is further explored in the next section.

3.2.3. Effect of reactive chlorine species on byproduct selectivity Urine contains a significant amount of Cl
(~200 mM), which can be oxidized at the anode to RCS. Many studies have investigated RCS as an oxidation mediator but also as a producer of undesirable Cl disinfection byproducts (Bergmann et al., 2009; Cho and Hoffmann, 2014; Cho et al., 2014; Dbira et al., 2015b; GarciaSegura et al., 2018; Kapalka et al., 2010c; Lacasa et al., 2012; Landolt and Ibl, 1970; Mostafa et al., 2018; Perez et al., 2012b; Zollig et al., 2015b). Fig. 6 shows the NO
3 and TAN byproduct formation during urea oxidation in undivided and divided reactors containing dilute synthetic urine in either an unreactive electrolyte (NaClO4) or RCS electrolyte (NaCl). In the undivided reactor with NaCl, urea conversion at 60 min reached approximately 100%, compared to 40% for NaClO4. For the divided reactor with NaCl, urea conversion at 60 min reached approximately 65%, compared to 45% for NaClO4. The faster rate of urea removal with NaCl is attributed to the production of Cl2, which rapidly reacts in H2O to form the RCS hypochlorite (ClO
) and hypochlorous acid (HOCl). Experiments with NaCl also presented unique byproduct selectivity compared to those with NaClO4 (Fig. 6C and D). With NaCl, in both the undivided and divided reactors, NO
3 and TAN

A. Schranck, K. Doudrick / Water Research 168 (2020) 115130

7

Fig. 5. NO
3 and TAN removal and formation as a function of time for an undivided (A, C) and divided (B, D) reactor. The starting solutions were (A, B) NaNO3 or (C, D) NH4Cl, and NO
3 and TAN concentrations were measured. All solutions were prepared in a background electrolyte of 1 N NaClO4. The volume of the undivided reactor and the anode chamber of the divided reactor was 40 mL. For the divided reactor, the N species were only added and tracked in the anode chamber, and the cathode chamber contained 1 N H2SO4 (V ¼ 20 mL). Voltage was maintained at 3 V vs. SCE for 140 min using CA.

Fig. 6. Urea removal (A and B) and NO
3 and TAN formation (C and D) as a function of time in undivided and divided reactors. Experiments contained dilute synthetic urine and either a 1 N NaClO4 or 1 N NaCl background electrolyte in the reactor or anode chamber (40 mL). The cathode chamber of the divided experiments contained 1 N H2SO4 (20 mL). Voltage was maintained at 3 V (vs. SCE) for 140 min using CA. The error bars represent one standard deviation of the urea concentration for duplicate sampling for each time point. All urea values below the MDL are marked with an asterisk and negative values are plotted as 0. The dilute synthetic urine contained an additional 1.9 mM Cl prior to the addition of 1 N NaCl.

accounted for less than 5% of the byproducts in both the undivided and divided reactors. In contrast, with NaClO4, (NO
3 þ TAN) and NO
3 accounted for approximately 60e75% of the byproducts for the undivided and divided reactor, respectively. In the presence of NaCl, nearly 95% of the byproducts were not accounted for by NO
3 and TAN, suggesting formation of either N-gases (e.g., N2, N2O) or inorganic chloramines (e.g., NHCl2). Inorganic chloramine formation using BDD is possible from the reaction between RCS and TAN (Garcia-Segura et al., 2019), but no TAN was detected in either the divided or undivided reactors. While the rapid oxidation of TAN to inorganic chloramines such that it is undetectable is still possible, inorganic chloramines will further oxidize form N2 and N2O gases (Dbira et al., 2015a, 2015b). These results implicate Cl
as a more effective electrolyte for

more rapid removal of urea and production of N-gases as a final oxidation byproduct. However, the presence of Cl
under oxidizing conditions has the potential to form unwanted Cl byproducts such
as chlorate (ClO
3 ) and perchlorate (ClO4 ) (Cho and Hoffmann, 2014; Dbira et al., 2015a; Perez et al., 2012a). In this study, ClO
3 and ClO
4 were measured at the end (i.e., 140 min) of the NaCl experiments, corresponding to >90% removal of urea. Approximately
1 57 mg L
1 ClO
ClO
3 and 35 mg L 4 were produced in the undi
1 vided reactor, and approximately 26 mg L
1 ClO
3 and 27 mg L ClO
4 were produced in the divided reactor. Though the concen
trations of ClO
3 and ClO4 are high, their Cl selectivity is low (<5%) compared to the starting concentration of Cl
(1 M). Cl
reactions at the cathode must also be considered because it has been shown to diminish cathodic production of H2 (Park et al., 2009), but this

8

A. Schranck, K. Doudrick / Water Research 168 (2020) 115130

would not an issue if using a divided reactor.

Anode: COðNH2 Þ2 þ HOCl

4. Conclusion

oxidation

/

Chlormaines

oxidation

/

N2 ðmajorÞ

þ Additional byproducts

Data presented herein on undivided and divided reactors with various operational conditions including electrolyte selection, pH, anolyte matrix, and cell voltage presents valuable insights into reaction kinetics and byproduct selectivity for urea oxidation using a BDD anode. The most significant differences between undivided and divided reactors were the byproduct selectivity and final pH. The fate of N was dependent on whether NO
3 species can migrate to the cathode and be reduced to TAN. The working electrode voltage (up to 4 V vs. SCE) did not change the N byproducts nor did increasing the applied current (up to 0.5 A or 167 mA/cm2) alter the reaction pathway or current efficiency. In the presence of Cl
, which is found in urine, the N byproducts will be shifted away from NO
3 toward chloramines and N-gases in undivided and divided reactors. Though, chloramine selectivity will be small in real urine solutions that contain competing compounds and lower Cl
concentration than that tested here (Garcia-Segura et al., 2019). Below are the possible overall reaction pathways proposed for undivided and divided reactors without and with Cl
.

4.1. Undivided reactor

oxidation

þ Anode: COðNH2 Þ2 þ 7H2 O / 2NO
3 þ CO2 þ 18H

oxidation

Anode: COðNH2 Þ2 þ HOCl

/

N2 þ Cl
þ 2H2 O þ Hþ ðminorÞ

There are many routes to using electrochemical reactors to manage source-separated urine. If an application such as fertilizer production is preferred, a divided reactor could be used to produce NO
3 , and this would also allow simultaneous H2 production in the cathode chamber. Or if ammonia was the preferred N product, an undivided reactor could be used and ammonia could be stripped and recovered (Tarpeh et al., 2018). While similar to the natural enzymatic formation of ammonia, an electrochemical system would be faster and allow more control over system operation. If only N removal is desired, then an undivided reactor with RCS species is likely the best option due to the ability of RCS to remove N from urine quickly with high N-gas selectivity. Though, this results
in ClO
3 and ClO4 byproduct formation that must be addressed by subsequent treatment. The results of this study support further development of electrochemical advanced oxidation processes to address nitrogen management in source-separated urine. Future research directions should continue to focus on optimal reactor design as it pertains to these applications. There is a need to evaluate these electrochemical systems using real urine, which contains numerous compounds not found in synthetic urine (e.g., micropollutants) that may alter reaction kinetics and byproduct selectivity.

þ 16e
ðmajorÞ Declaration of competing interest
reduction

þ Cathode: NO
3 þ 10H þ 8e

/

NH þ 4 þ 3H2 O ðmajorÞ

þ Cathode: NHþ 4 #NH3ðaqÞ þ H ðmajor; pKa ¼ 9:25Þ

Cathode:NH3ðaqÞ #NH3ðgÞ ðHpm ¼ 0:0171 atm=MÞ Anode: COðNH2 Þ2 Anode:NH3ðaqÞ

hydrolysis

/

oxidation

/

NH3ðaqÞ ðslow; minorÞ

N2 þ 6Hþ þ 6e
ðmajorÞ

4.2. Divided reactor

oxidation

þ Anode: COðNH2 Þ2 þ 7H2 O / 2NO
3 þ CO2 þ 18H

þ 16e
ðmajorÞ Anode: COðNH2 Þ2

hydrolysis

/

NHþ 4 ðslow; minorÞ

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was made possible through financial support provided by the University of Notre Dame and Andrew Schranck’s scholarship funding from the Patrick and Jana Eilers Graduate Student Fellowship for Energy Related Research (Center for Sustainable Energy at Notre Dame), the Rothblatt Memorial Scholarship (Lake Michigan States Section of the Air and Waste Management Association), and the Post Graduate Scholarship (National Collegiate Athletic Association (NCAA)). The authors thank the Center for Environmental Science and Technology (CEST) for providing instrumentation for IC and urea analysis. Special rez, and John Loftus for thanks to Randal Marks, Angela Abarca-Pe their help with analytical protocols for sample analysis and instrument troubleshooting. The authors thank Rob Roberts of BioLogic for technical support with electrochemical experiments, and Brent Bach, Kiva Ford, and Leon Hluchota for help machinging parts for the electrochemical reactors. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.watres.2019.115130.

4.3. Undivided or divided reactor with Cl
(RCS)

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