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Capacitive membrane stripping for ammonia recovery (CapAmm) from dilute wastewaters Changyong Zhang, Jinxing Ma, Di He, and T. David Waite Environ. Sci. Technol. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.estlett.7b00534 • Publication Date (Web): 21 Dec 2017 Downloaded from http://pubs.acs.org on December 24, 2017

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Environmental Science & Technology Letters

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Capacitive membrane stripping for ammonia recovery (CapAmm) from

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dilute wastewaters

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Changyong Zhang,a Jinxing Ma,a Di He,b,c and T. David Waite*a

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a

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Australia

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b

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Guangzhou 510006, China

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c

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Science and Engineering, Guangdong University of Technology, Guangzhou 510006, China

School of Civil and Environmental Engineering, University of New South Wales, Sydney, NSW 2052,

Institute of Environmental Health and Pollution Control, Guangdong University of Technology,

Guangzhou Key Laboratory of Environmental Catalysis and Pollution Control, School of Environmental

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Email addresses: [email protected] (Changyong Zhang); [email protected]

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(Jinxing Ma); [email protected] (Di He); [email protected] (T. David Waite)

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Environmental Sciences & Technology Letters

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__________________________________

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*Corresponding authors: Professor T. David Waite; [email protected] 1 ACS Paragon Plus Environment


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ABSTRACT

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A novel cost-effective flow-electrode capacitive deionization unit combined with a

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hydrophobic gas-permeable hollow fiber membrane contactor (designated as “CapAmm”) is

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described here and used for efficient ammonia recovery from dilute synthetic wastewaters.

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During operation, ammonia undergoes migration across a cation exchange membrane and

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selective accumulation in the cathode chamber of a flow electrode followed by

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transformation to dissolved NH3 with subsequent stripping via membrane contactor and

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recovery as ammonium sulfate. Our results demonstrate that the CapAmm process can

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achieve an ammonia removal efficiency of ~90% and recovery efficiency of ~60%. At current

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densities of 5.8 and 11.5 A m−2 (normalized by effective cation exchange membrane area) and

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hydraulic retention time of 1.48 min, the energies required for ammonia recovery were 9.9 and

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21.1 kWh Kg−1 N respectively with these values comparable with other similar electrochemical

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ammonia recovery systems. These findings suggest that the CapAmm technology described

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here has the potential for the dual purposes of cost effective salt removal and ammonia

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recovery from wastewaters, with greater stability, better flexibility and greater energy

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efficiency than other methods.

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KEYWORDS

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Ammonia recovery, flow electrode capacitive deionization, membrane contactor, nitrogen

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fertilizer, CapAmm

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2 ACS Paragon Plus Environment

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INTRODUCTION

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A rapidly increasing world population and improvement in living standards have

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resulted in the demand for greater food production with this production strongly dependent

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on the utilization of fertilizers rich in nitrogen (N) and phosphorus (P) to maintain soil

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fertility and increase crop yields.1, 2 However, the traditional Haber-Bosch process for the

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production of ammonia, the key component in the synthesis of most popular N fertilizers

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(including urea, ammonium nitrate and ammonium sulfate), is responsible for 1.5-2.5% of the

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annual global energy consumption and more than 1.6% of global CO2 emissions.3-5

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Considering that the demand for nitrogen fertilizer is expected to increase at an average

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annual growth rate of 1.5% over the next decade,6, 7 the issue of how to produce ammonia in

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a sustainable manner represents a global challenge.

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While expenditure on ammonia production for fertilizer use is increasing, water

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contamination has become increasingly severe, especially in recently industrialized countries

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such as China and India, with ammonia nitrogen recognized to be one of the principal

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contaminants. It has been reported that 2.5 million tonnes of ammonia nitrogen is discharged

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annually to waterbodies in China2 with the injection of this major nutrient resulting in severe

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eutrophication, acute and chronic toxicity to aquatic organisms and production of toxic

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byproducts during drinking water disinfection.8,

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technologies such as nitrification/denitrification, electrochemical oxidation and breakpoint

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chlorination have been developed to remove ammonia from wastewaters,10-14 this practice

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would seem unsustainable in view of the lifecycle of ammonia mentioned above. In light of

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both the problems associated with removal of ammonia from waters and the increasing

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demand for ammonia-based fertilizers, there is increased interest in ammonia recovery (and

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production) from dilute wastewaters in view of the economic and environmental benefits.15-17

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Though a great number of robust



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The first step in the recovery of ammonia from low-strength wastewaters involves

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pre-concentration of the ammonia. While ion exchange adsorption by zeolites and membrane

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separation (e.g., reverse osmosis and nanofiltration) may be used to generate ammonia-rich

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solutions,18,

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quantities of chemicals for regeneration purposes and, as a result, are less feasible for

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ammonia recovery from dilute waste streams. Recently, flow-electrode capacitive

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deionization (FCDI) has attracted interest given its potential applications in seawater

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desalination and energy storage.19-22 FCDI is particularly effective in accumulating ions in the

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flow electrodes with cations such as Na+ and NH4+ concentrated in the cathode chamber and

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anions such as Cl− concentrated in the anode chamber. In addition to this capacitive

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mechanism, electron exchange at the surface of the flow electrodes may well result in the

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consumption and/or production of protons.23 An increase in pH in the cathode chamber (as is

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typical as a result, in part, of the reduction of oxygen)23,

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speciation from NH4+ dominance at pH < pKa (of 9.3) for the NH4+-NH3 acid-base pair to

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NH3(aq) dominance at pH > pKa. Equilibrium will rapidly be established between dissolved

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and gaseous ammonia in the cathode chamber with the possibility of subsequent removal of

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NH3(g) by interfacing the solution with a gas permeable membrane.

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these technologies either suffer from high energy cost and/or require large

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would result in change in

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In this study, we develop a process for capacitive membrane stripping for ammonia

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recovery (CapAmm) from dilute wastewaters (Scheme 1), in which ammonia undergoes

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migration across a cation exchange membrane and selective accumulation in the cathode

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chamber of a flow electrode followed by transformation to dissolved NH3 with subsequent

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stripping via membrane contactor and recovery in acid solution (for example, as (NH4)2SO4).

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A number of operational parameters associated with the performance of CapAmm are

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investigated with specific consideration given to the mass flow of ammonia from the influent

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wastewater to the acidic receiving solution. An overall evaluation of the operating cost and


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economic benefit of the ammonia produced highlights the opportunities and challenges of

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CapAmm in sustainable water, resource and energy management.

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MATERIALS AND METHODS

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Experiment Setup. The structure of the CapAmm apparatus is shown in Scheme 1a and

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Figure S1. A silicone gasket and a nylon sheet (100-mesh) were bounded by two ion-

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exchange membranes (CEM-Type I/AEM-Type I, FUJIFILM Europe) with the gap between

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the ion exchange membranes acting as a spacer chamber (thickness of ~500 µm) through

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which the wastewater flowed (Scheme 1b).21,

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channels for flow electrodes were placed against the graphite current collectors (Scheme 1a).

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The cross-section of each channel was 3 mm × 3 mm with an effective contact area between

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the ion-exchange membrane and the flow electrodes of 34.9 cm2. These parts were held

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together with the use of acrylic end plates (Scheme 1a and Figure S2). The flow electrodes

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were continuously cycled between the flow channels and the circulation tanks, with the

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polypropylene (PP) hollow-fiber membrane contactor (Schemes 1a and c) placed in the

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negatively charged flow electrode circulation tank. The total length of the membranes used in

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this work was 30 cm, with outer diameter of 2 mm, wall thickness of 0.1 mm, pore size of

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0.45 µM and total effective surface area of 18.8 cm2.

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Acrylic sheets with carved serpentine

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Operating Conditions. Synthetic wastewater containing 1000 mg NaCl L−1 and 40 mg

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NH4+-N L−1 was fed into the spacer chamber in single-pass mode. The effect of different

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wastewater flow rates (of 0.85 to 2.55 mL min−1 with corresponding hydraulic retention time

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(HRT) of 2.94 to 0.98 min) was evaluated in this study. The electrical conductivity of this

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stream was continuously measured at the outlet with the use of a conductivity meter (CON-

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BTA, Vernier) connected to a data acquisition system (SensorDAQ, Vernier). ). The flow

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electrodes were composed of 10 wt% 100-mesh DARCO activated carbon (which, based on

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SEM images, exhibited an average particle size on the order of 18.5 µm) and 90 wt% Milli-Q



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water and were prepared in an identical manner to that described in our earlier studies.21 In all

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experiments, the 65 mL flow electrodes were recirculated respectively using a dual-head

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peristaltic pump (Longer pump, Baoding, China) at a constant flow rate of 50 mL min−1. In

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one operating cycle, electrosorption was carried out at a constant current density (3.0-17.2 A

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m−2) using a DC power supply (MP3840, Powertech). For ammonia recovery, the membrane

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module was immersed in the cathode flow-electrode slurry while 65 mL of receiving solution

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(0.5 M sulfuric acid) was continuously circulated (at 50 mL min−1) inside the hollow fibre

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membranes.26, 27 The ammonia dissolved in the flow electrode slurry evaporates and diffuses

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through the pores of the gas permeable membrane then reacts with sulfuric acid resulting in

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formation of ammonium sulfate (the detailed mechanism is shown in SI text 1 and Figure S3).

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The analytical methods and calculations are presented in Supporting Information.

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RESULTS AND DISCUSSION

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Ammonia Removal from the Dilute Wastewater. Results for salt removal

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performance in the CapAmm system at different charging current densities and HRTs are

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summarized in Figure S4 and Figure 1. Relatively stable effluent conductivity was observed

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under all experimental conditions with higher current densities and longer HRTs resulting in

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greater reduction in feed water conductivity (Figure S4). Meanwhile, increases in both

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applied current densities and HRTs also exert positive effects on NH4+ and Na+ removal. It is

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also revealed that CapAmm has a higher selectivity for NH4+-N compared to Na+ removal,

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particularly at lower current density and/or HRT (Figure 1a and b). For instance, the ratio of

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removal efficiency between NH4+-N and Na+ (i.e., RE(NH4+-N)/RE(Na+)) was 2.5 at a current

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density of 3.0 A m−2 but gradually decreased to 1.8, 1.34 and 1.0 at higher current densities of

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5.8, 11.5 and 17.2 A m−2. A similar trend was observed for the effect of change in HRT with

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RE(NH4+-N)/RE(Na+) declining from 1.6 to 1.0 on increase in HRT from 0.98 to 2.94 min. The

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change of current efficiency is described in the Supporting Information (see Figure S5).


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It has previously been reported that ions with a smaller hydrated radius are transported

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more rapidly from the feed water into the cathodic flow electrode under a fixed electrical field,

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provided that they have the same valence charge.28 Additionally, smaller ions are more readily

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electrosorbed by activated carbon particles than larger ions because of their easier access to the

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micro-pores of these particle electrodes.29, 30 The fact that NH4+ has a slightly smaller hydrated

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radius (3.31 Å) than Na+ (3.58 Å)31 may account for the observation that the electrosorption

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process in our CapAmm system exhibits a higher selectivity towards NH4+ compared to Na+ at

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lower current densities/shorter HRTs.

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Ammonia Transformation in the Flow Electrode. Figure 1c and d indicate that the pH

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of the cathode gradually changed from neutral to alkaline during operation. The shaded areas

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represent the region where the pH was higher than the pKa of NH4+ (9.3). In this region,

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almost all of the NH4+ will be deprotonated and transformed into dissolved NH3(aq), thereby

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providing ideal prerequisites for ammonia separation and recovery from the bulk solution via

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membrane stripping.

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According to recent studies, Faradaic reactions such as oxygen reduction take place at

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the cathode even when the cell voltage is relatively low (e.g. < 1.0 V), consuming H+ and

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resulting in pH increase.23, 32-34 It is to be expected that higher current densities (and higher

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charging voltages) will facilitate oxygen reduction leading to a more rapid rise in pH in the

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cathode. In contrast, changing HRTs does not influence the duration for which the flow

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electrode is exposed to a particular applied potential (as the flow electrode is recirculated at a

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constant rate) and, as such, would not be expected to significantly influence the pH.

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Ammonia Recovery and Energy Consumption. As shown in Figure 2, it can be

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concluded that ammonium ions removed from the feed stream were effectively transferred to the

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acidic receiving solution, with the recovered NH4+-N concentrations finally reaching 51.1 ± 1.2,

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100.1 ± 9.6, 147.2 ± 0.6 and 174.9 ± 11.4 mg L−1 at 3.0, 5.8, 11.5 and 17.2 A m−2 respectively



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(Figure 2a and b). The average ammonia recovery rate increased from 5.7 to 19.5 g N m−2 d−1

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when the current density increased from 3.0 to 17.2 A m−2. In the first hour, the NH4+-N

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concentration in the acid solution increased relatively slowly, presumably because the initial low

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pH in the cathode chamber (Figure 1c and d) hindered the transformation of NH4+ into NH3(aq)

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and the subsequent diffusion of NH3(g) across the gas membrane. On analysing the NH4+-N

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concentration in the cathode solution, a sharp increase was observed in the first hour and then

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reached a relatively stable concentration of 35-40 mg L−1 in the following three hours (Figure 2a).

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Similar trends were observed when operated under different HRTs (Figure 2c and d).

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From Figure 3a, we can clearly see that the applied current density plays an important role in

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ammonia recovery. At current densities higher than 11.5 A m−2, 55-65% ammonia can be

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recovered by the membrane contactor as ammonium fertilizer, while about 15% of the ammonia is

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still present in the cathode and 20% was not detected (presumably either because it is electro-

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absorbed on the carbon particles or because it has been air-stripped). It would be particularly

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advantageous if the ammonia remaining in the cathode chamber (~35% of the total) could be

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recovered. As is clear from Figure 3b, HRTs have a minor influence on the ammonia recovery

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efficiency, especially when the HRT is longer than 1.48 min, with a similar recovery rate at these

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long HRTs of 50~60%.

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An overview of the effluent water quality, NH4+-N recovery efficiency, energy

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consumption and average ammonia removal/recovery rate over a range of operating conditions is

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provided in Table S1. Overall, the energy required for water desalination ranged from 0.05-1.95

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kWh m−3, depending on the applied current densities and HRTs.

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At applied current densities of 3.0, 5.8, and 11.5 A m−2 (for HRT fixed at 1.48 min), the

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energies consumed for ammonia recovery were 6.1, 9.9 and 21.7 kWh Kg−1 N with recovery

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efficiencies (respectively) reaching 18.7%, 38.1% and 55.1% and calculated average recovery

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rates (respectively) of 5.7, 11.3 and 16.5 g N m−2 d−1. The energy consumption of the


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CapAmm system is comparable with other electrochemical ammonia recovery systems. For

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instance, an integrated system composed of a hydrogen gas (H2) recycling electrochemical cell

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(recycling of the H2 generated at the cathode to the anode) and a gas permeable membrane

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contactor was found to achieve an ammonia recovery efficiency of 60-73% and an average

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ammonia recovery rate 110.2 g N m−2 d−1 at an energy requirement of 10.0 kWh kg−1 N. 35

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However, this system required the injection of additional H2 and the application of a noble catalyst

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to enable stable operation with these components representing an additional energy cost.

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Another ammonia recovery system that combined an electrochemical cell (EC) and air-stripping

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was able to recover 96% ammonia at a recovery rate of 120 g N m−2 d−1 and exhibited an energy

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consumption of 20 kWh kg−1 N (with 13 kWh kg−1 N for EC and 9 kWh kg−1 N for air-stripping).

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focussed on harvesting ammonia from high ammonia concentration (~3400 mg L−1 N) streams

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(such as urine) with relatively low concentrations of competing ions (~1400 mg L−1 K+ and ~1600

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mg L−1 Na+) with the competing cation and NH4+-N mole ratio as low as 0.43. However, the

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influent of our CapAmm system contains 40-50 mg L−1 NH4+-N (~80 times lower than that in

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urine) and ~390 mg L−1 Na+ (~1000 mg L−1 NaCl) resulting in a high Na+/NH4+-N mole ratio

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of 5.7. This means that a larger part of the energy will be consumed for water desalination

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(salt removal) thereby resulting in relatively lower ammonia recovery rate. In our future

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studies, we will examine the applicability (and energy usage) of the CapAmm process to

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recovery of ammonia from medium/high ammonia concentration waste streams.

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More importantly, it should be noted that most of these ammonia recovery systems were

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In contrast, bioelectrochemical systems (BES), which take advantage of electrochemically

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active bacteria to degrade organic matter and generate electricity, can recover ammonia at much

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lower energy demand compared to traditional EC systems and the CapAmm system introduced

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here.37-40 In BES systems however, the ammonia recovery rate is relatively slow (3-5 g N m−2

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d−1)17, 37 due to the limited current density produced by the anaerobic bacteria at the anode.



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Additionally, the biological process is very sensitive to a range of environmental factors including

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pH, carbon loading, chemical toxicity (e.g., ammonia present in the influent) and temperature,

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restricting the practical application of these technologies. Indeed, the CapAmm process may have

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an advantage over bioelectrochemical systems in view of its greater stability, predictability and

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flexibility and the opportunity for automatic control of the reactor.

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IMPLICATIONS

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Our study demonstrates that the CapAmm process can be a very effective alternative for

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ammonia removal and recovery from dilute wastewaters. Compared with other ammonia recovery

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systems that require additional chemical dosing to adjust the pH and which use energy-intensive

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air stripping to extract ammonia, CapAmm allows for the selective removal of ammonia from the

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wastewater stream and preconcentration in the cathode chamber, with the elevation in pH that

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occurs as a result of intrinsic Faradaic processes favoring the conversion of NH4+ into dissolved

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NH3(aq). Given the rapidity with which solution phase ammonia equilibrates with gaseous

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ammonia, NH3(g) will readily diffuse through a gas-permeable membrane into an acid solution

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and may be recovered as an ammonium salt which can, potentially, be readily marketed as a

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fertilizer.

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configurations that utilize energy intensive air-stripping processes or require the use of multiple

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absorption vessels. While sulfuric acid was used in this study for ammonia recovery, alternative

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acids (such as phosphoric acid or nitric acid) could be used in order to produce higher-value

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fertilizers.

In addition, our experimental setup is less complex than other experimental

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AUTHOR INFORMATION

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CORRESPONDING AUTHORS

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*E-mail: [email protected]. Tel: +61-2-9385-5060;

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ACKNOWLEDGEMENT

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Support for Dr Jinxing Ma through a UNSW Vice Chancellor’s Postdoctoral Fellowship is

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gratefully acknowledged.

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SUPPORTING INFORMATION

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Detailed information on the construction of the CapAmm cell, analytical methods and

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calculations, mechanism of ammonia recovery by gas permeable membranes, variation of

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effluent conductivity, variation of current efficiency and comparison of effluent water quality,

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ammonia recovery efficiency, energy consumption and ammonia removal/recovery rate over a

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range of operating conditions is available in the Supporting Information. The Supporting

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Information is available at http://pubs.acs.org.

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wastewater streams by means of a hollow-fiber membrane contactor. Desalination 2012, 285, 383-392. 28. Tran, A. T. K.; Zhang, Y.; Lin, J.; Mondal, P.; Ye, W.; Meesschaert, B.; Pinoy, L.; Van der Bruggen, B., Phosphate pre-concentration from municipal wastewater by selectrodialysis: Effect of competing components. Sep. Purif. Technol. 2015, 141, 38-47. 29. Hou, C.-H.; Huang, C.-Y., A comparative study of electrosorption selectivity of ions by activated carbon electrodes in capacitive deionization. Desalination 2013, 314, 124-129. 30. Zhou, F.; Gao, T.; Luo, M.; Li, H., Preferential electrosorption of anions by C/Na 0.7 MnO 2 asymmetrical electrodes. Sep. Purif. Technol. 2018, 191, 322-327. 31. Nightingale Jr, E., Phenomenological theory of ion solvation. Effective radii of hydrated ions. The Journal of Physical Chemistry 1959, 63, 1381-1387. 32. Holubowitch, N.; Omosebi, A.; Gao, X.; Landon, J.; Liu, K., Quasi-steady-state polarization reveals the interplay of capacitive and Faradaic processes in capacitive deionization. ChemElectroChem 2017. 33. Nativ, P.; Badash, Y.; Gendel, Y., New insights into the mechanism of flow-electrode capacitive deionization. Electrochem. Commun. 2017, 76, 24-28. 34. Dykstra, J. E.; Keesman, K. J.; Biesheuvel, P. M.; van der Wal, A., Theory of pH changes in water desalination by capacitive deionization. Water Res. 2017, 119, 178-186. 35. Kuntke, P.; Rodriguez Arredondo, M.; Widyakristi, L.; Ter Heijne, A.; Sleutels, T. H.; Hamelers, H. V.; Buisman, C. J., Hydrogen gas recycling for energy efficient ammonia recovery in blectrochemical systems. Environ. Sci. Technol. 2017, 51, 3110-3116. 36. Desloover, J.; Woldeyohannis, A. A.; Verstraete, W.; Boon, N.; Rabaey, K., Electrochemical resource recovery from digestate to prevent ammonia toxicity during anaerobic digestion. Environ. Sci. Technol. 2012, 46, 12209-16. 37. Kuntke, P.; Smiech, K. M.; Bruning, H.; Zeeman, G.; Saakes, M.; Sleutels, T. H.; Hamelers, H. V.; Buisman, C. J., Ammonium recovery and energy production from urine by a microbial fuel cell. Water Res. 2012, 46, 2627-36. 38. Zamora, P.; Georgieva, T.; Ter Heijne, A.; Sleutels, T. H. J. A.; Jeremiasse, A. W.; Saakes, M.; Buisman, C. J. N.; Kuntke, P., Ammonia recovery from urine in a scaled-up Microbial Electrolysis Cell. J. Power Sources 2017, 356, 491-499. 39. Kuntke, P.; Sleutels, T. H. J. A.; Saakes, M.; Buisman, C. J. N., Hydrogen production and ammonium recovery from urine by a microbial electrolysis cell. Int. J. Hydrogen Energy 2014, 39, 4771-4778. 40. Ledezma, P.; Jermakka, J.; Keller, J.; Freguia, S., Recovering nitrogen as a solid without chemical dosing: bio-electroconcentration for recovery of nutrients from urine. Environ. Sci. Technol. Lett. 2017, 4, 119124.

354



355

TOC graphic

356 Freshwater out

+

NH4+ Cl-

Ammonium fertilizer out

High pH

Cl-

NH3

-

Cl-

Acid in

ClNa+

Na+

Gas permeable membrane module Flow electrode

357

AEM

Flow CEM electrode

Wastewater in

358


Page 14 of 18

Page 15 of 18


Anion exchange Cation exchange Graphite current membrane membrane collector Gasket Gasket Gasket

(a) End plate

Freshwater out

Ammonium fertilizer Membrane module Acid Flow electrode

Reservoir for electrically charged flow electrode (i.e., anode)

End plate

Wastewater in

+

Cl-

+

Current collector

Na+

Cl-

-

NH4

Cl-

NH4+

Na+

O2

OH−

Cl-

+

NH4+

Cl-

+

Na

+

Na

-

-

Na+

+

Cl-

-

Na +

NH4

+

Fertilizer

-

NH3 NH4

-

+

NH4

+

SO42-

+

NH3

-

Na+

Na+

+

Cl-

+

Na+

Carbo n

NH4

-

+ Cl- NH4

Gas permeable membrane

-

Cl-

Cl-

Cl-

+

359

Na+

Na+

-

Na+

+

+

Na+

Current collector

Cl-

+ Cl-

(c) -

+ Cl-

+

Cl-

Acrylic flow channel

Spacer

Graphite current Acrylic flow collector channel

(b)

Reservoir for electrically charged flow electrode (i.e., cathode)

NH4

+

NH3 NH4+

NH3 +

Na

+

Na

NH3

-

- Na+

NH3

H

SO42-

+

H

SO42-

+

H+ H

+

SO42-

Acid

360

Scheme 1 Schematic representations of (a) the capacitive membrane stripping for ammonia

361

recovery (CapAmm), (b) ammonia migration and transformation in the desalination unit and

362

(c) selective ammonia recovery from the negatively charged flow electrode.

363



+

+

+

+

NH4 -N

RE (NH4 -N)/RE (Na )

3.0

2.0

60

40

1.5

20

1.0

0

Ion removal efficiency (%)

2.5

80

0.5 3.0

5.8

11.5

+

+

RE (NH4 -N)/RE (Na )

2.5

80

2.0

60

40

1.5

20

1.0

0

0.5

17.2

0.98

-2

1.48

1.96

2.94

HRT (min)

13

13

(c)

(d)

12

12

11

11

pH

pH

+

(b)

Current density (A m )

10 9

10 9

-2

3.0 A m -2 5.8 A m -2 11.5 A m -2 17.2 A m

8

0.98 min 1.48 min 1.96 min 2.94 min

8

7

7 0

364

Na

3.0

100

(a) RE (NH4+-N)/RE (Na+)

Ion removal efficiency (%)

100

Na

RE (NH4+-N)/RE (Na+)

+

NH4 -N

Page 16 of 18

40

80

120

160

200

240

0

40

Time (min)

80

120

160

200

240

Time (min)

365

Figure 1. Average removal efficiencies for ammonium and sodium ions in the CapAmm

366

system (a) under different current densities (i.e., 3.0, 5.8, 11.5 and 17.2 A m−2) with a constant

367

HRT of 1.48 min, and (b) at a constant current of 11.5 A m−2 with HRTs of 0.98, 1.48, 1.96

368

and 2.94 min. The blue symbols are the ratio of removal efficiency of RE (NH4+-N)/RE (Na+)

369

with this ratio representing the selectivity of NH4+ compared to Na+. Temporal change of pH

370

in the cathode chamber of the CapAmm system operated (c) under different current densities

371

(i.e., 3.0, 5.8, 11.5 and 17.2 A m−2) with a constant HRT of 1.48 min, and (d) under a constant

372

current of 11.5 A m−2 with different HRTs (i.e., 0.98, 1.48, 1.96, and 2.94 min). The shade

373

area represents the pH range >9.3, indicating the deprotonation of NH4+ to NH3.

374


Page 17 of 18


200

(a) Cathode chamber -2

NH3-N (and NH4+-N) (mg L-1)

NH 3-N (and NH 4+-N) (mg L-1)

200 3.0 Am -2 5.8 Am -2 11.5 Am -2 17.2 Am

150

100

50

0

(b) Acid solution -2 3.0 Am -2 5.8 Am -2 11.5 Am -2 17.2 Am

150

100

50

0 0

40

80

120

160

200

240

0

40

Time (min)

160

200

240

200

240

160

(c) Cathode chamber



120

Time (min)

160 0.98 min 1.48 min 1.96 min 2.94 min

120

80

40

0

(d) Acid solution 0.98 1.48 1.96 2.94

120

min min min min

80

40

0 0

40

80

120

160

200

240

0

40

80

120

160

Time (min)

Time (min)

375

80

376

Figure 2. Temporal variation of ammonia concentrations in (a) the cathode chamber and (b)

377

acidic receiving solution of the CapAmm system operated under different current densities

378

(i.e., 3.0, 5.8, 11.5 and 17.2 A m−2) with a constant HRT of 1.96 min, and ammonia

379

concentrations in (c) the cathode chamber and (d) acidic receiving solution of the system

380

charged at a constant current density of 11.5 A m−2 with different HRTs (i.e., 0.98, 1.48, 1.96

381

and 2.94 min).



382 383

Figure 3. Fate and distribution of ammonia in the CapAmm system (a) at different current

384

densities (i.e., 3.0, 5.8, 11.5, and 17.2 A m−2) with a constant HRT of 1.96 min, and (b) at a

385

constant current of 11.5 A m−2 with different HRTs (i.e., 0.98, 1.48, 1.96, and 2.94 min).

386


Page 18 of 18

Capacitive Membrane Stripping for Ammonia ... - ACS Publications

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