Mechanisms of Humic Acid Fouling on Capacitive and Insertion

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Remediation and Control Technologies

Mechanisms of Humic Acid Fouling on Capacitive and Insertion Electrodes for Electrochemical Desalination Xitong Liu, Jay F. Whitacre, and Meagan S Mauter Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 21 Sep 2018 Downloaded from http://pubs.acs.org on September 21, 2018

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Environmental Science & Technology

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Mechanisms of Humic Acid Fouling on Capacitive and Insertion Electrodes for

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Electrochemical Desalination

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Revised: Sept 4, 2018

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7 Xitong Liu,1 Jay F. Whitacre,2,3,4 and Meagan S. Mauter1,2,4*

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1. Department of Civil & Environmental Engineering, Carnegie Mellon University, 5000 Forbes

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Ave., Pittsburgh, PA, 15213, United States

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2. Department of Engineering and Public Policy, Carnegie Mellon University, 5000 Forbes Ave.,

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Pittsburgh, PA, 15213, United States

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3. Department of Material Science and Engineering, Carnegie Mellon University, 5000 Forbes

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Ave., Pittsburgh, PA, 15213, United States

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4. The Scott Institute for Energy Innovation, Carnegie Mellon University, 5000 Forbes Ave.,

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Pittsburgh, PA, 15213, United States

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*Authors to Whom Correspondence Should be Addressed:

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M. S. Mauter: [email protected]

412-268-5688

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1

ACS Paragon Plus Environment


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Abstract

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Though electrochemical deionization technologies have been widely explored for brackish water

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desalination and selective ion removal, their sustained performance in the presence of foulants

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common to environmental waters remains unclear. This study investigates the fundamental

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mechanisms by which carbonaceous electrodes used in capacitive deionization and insertion

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electrodes used for high-capacity selective ion removal are affected by the presence of humic

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acid (HA). We evaluate HA adsorption behavior and the resulting impact on the ion storage

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capacity and cycling stability of the electrode materials. We find that HA is primarily adsorbed

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to the mesopores of two carbonaceous electrodes with distinctly different pore structures, but that

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the ion storage and transport properties of the electrodes are not significantly impacted by HA

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adsorption. In contrast, HA adsorption resulted in sharp capacity decay for the insertion

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(Na4Mn9O18) electrode. We attribute this decay to both hindered Na+ ion diffusion to the

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insertion interface in the presence of adsorbed HA, as well as HA mediated electrode dissolution.

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These findings highlight the contrasting mechanisms for HA fouling of capacitive and insertion

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electrodes and suggest that insertion electrodes may be more susceptible to performance decline

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in electrochemical deionization of environmental waters.

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Introduction

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The use of carbon electrodes to remove salts from water dates back to the work of

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“electrochemical water demineralization” pioneered by Murphy et al. in 1960s.1,

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decades, capacitive deionization (CDI) using porous carbon electrodes has attracted renewed

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interest for brackish water desalination.3, 4 Recent thermodynamic analyses have shown that CDI

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has the potential improve upon the energy efficiency of reverse osmosis for low salinity

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In recent

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feedstreams, so long as the energy recovery is high.3,

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advantages, including high recovery rates that minimize brine disposal volume and low pressure

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operating conditions that minimize capital costs for very small systems.3, 6, 7

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CDI may also provide operational

In conventional CDI, salt ions are stored in and released from the electrical double layer

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(EDL) of porous activated carbon electrodes during charge and discharge processes.

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capacitive process enables energy storage and recovery between desalination cycles, but suffers

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from low round trip efficiency due to water splitting8 and low salt adsorption capacity due to the

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low capacitance of carbon electrodes.8 In addition, electrosorption in the EDL is nonspecific, so

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energy is expended in removing ions that may not be of concern.

This

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To circumvent these limitations, insertion compounds capable of accepting sodium and

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sometimes other cation species, including Na4Mn9O18 (NMO), NaTi2(PO4)3, and Na2FeP2O7,

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have been explored as alternative electrode materials in electrochemical desalination.9-13 Ion

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storage in these cation insertion compounds involves redox reactions of the metal atoms in

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concert with cation insertion/transport into well-defined tunnels in the crystal structure.14, 15 As

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such, insertion compounds can offer intrinsic selectivity in electrochemical removal of ions with

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different sizes, even when those ions are of similar charge.12 The ion selectivity of insertion

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electrodes can also be achieved through modulating the electrochemical window or surface

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chemistry of the electrodes.16, 17 Additionally, the high specific capacity of insertion compounds

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enables significantly higher salt removal per unit mass and volume of electrode9 and may extend

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the salinity range over which electrochemical removal processes are energetically advantageous.

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With the continuous improvement of electrode performance and system design,

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electrochemical deionization processes are becoming viable candidates for brackish groundwater

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and brackish agriculture drainage treatment. In addition to salt concentrations between 1,000

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and 10,000 mg/L, naturally occurring organic macromolecules are present at concentrations of 1

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to 21 mg/L as DOC (dissolved organic carbon).18-21 These macromolecules can adsorb to the

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electrode surface and may negatively impact desalination performance. To date, organic fouling

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in CDI processes has received only sporadic attention.7, 22-25 For example, Mossad and Zou7

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reported that the salt removal in an activated-carbon-based CDI process decreased over time in

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the presence of HA.

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Designing approaches to mitigate the fouling impacts documented in these observational

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studies will require a fundamental understanding of the mechanisms underlying performance

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decline. Specifically, there are research gaps in relating electrode properties, such as pore size

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distribution, to fouling propensity and performance decline.

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understanding of how the mechanism of ion storage, whether in the EDL or in the bulk structure

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of the electrode material as for insertion compounds, will influence the rate, extent, and

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mechanism of performance decline.

There are also gaps in the

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This study addresses those gaps by elucidating the effects of HA adsorption on the

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specific capacity and cycling stability in both capacitive-based and insertion-based electrodes in

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half-cell experiments. We studied two activated carbon electrodes with distinctly different pore

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size distributions to elucidate the structural dependence of performance decline mechanisms.

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We also selected NMO as the representative insertion compound in this study on the basis of its

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high stability, ability to insert/de-insert sodium,12 and successful application in electrochemical

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desalination devices.9 This study is the first to compare organic fouling between capacitive and

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insertion electrodes in the same environment and will provide important guidance for the design

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and deployment of real-world electrochemical desalination systems.

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Materials and Methods

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Synthesis and Characterization of Active Material. Two activated carbons were

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selected as representative materials for the carbonaceous capacitive electrode. According to

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manufacturer specifications, YEC-8 (Fuzhou Yihuan, China) and Darco S-51 (Cabot) activated

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carbons have surface areas of 2000–2500 m2/g and 650 m2/g, respectively, with the former being

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specifically manufactured for use in EDL capacitors.

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characterized by nitrogen and CO2 gas adsorption to determine the contribution of micropore and

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mesopores to total pore volume using a Quantachrome Autosorb 1-C analyzer. The samples

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were degassed at 350 ºC overnight under vacuum prior to measurements. Nitrogen and CO2

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adsorption were conducted at −196.15 and 0 ºC, respectively.

The two activated carbons were

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NMO powder was synthesized via a solid-state reaction as reported in previous

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publications.12, 26 Briefly, Na2CO3 (Fisher) and Mn2O3 (Aldrich) was mixed at a molar ratio of

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0.55:1. The mixture was ball milled (8000M Mixer/Mill, SPEX SamplePrep) for 1 h, followed

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by calcination in a box furnace (NEY 6-160A) at 750 ºC for 8 h with heating and cooling ramp

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rates of 5 and 1 ºC/min, respectively. The resulting powder was characterized using an X-ray

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diffractometer (X’Pert Pro MPD, PANalytical) and the obtained pattern was compared with

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ICDD standard pattern of Na4Mn9O18.

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Electrode Preparation.

To prepare Darco, YEC, and NMO electrodes, the active

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material was mixed with carbon black (Super-P) and polytetrafluoroethylene (PTFE, Alfa Aesar)

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at an 80:10:10 mass ratio. The volume fractions of the active material in the carbonaceous and

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NMO electrodes were 75% and 23%, respectively. The volume fractions of carbon black in the

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carbonaceous and NMO electrodes were 23% and 72%, respectively, adequate to ensure charge

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percolation throughout the electrode structures.27 The mixture was sheeted using an automatic

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mortar grinder for 10 min. The resulting electrode material was pressed onto one end of a

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titanium mesh (Unique Wire Weaving Co.) strip of size 0.7 ´ 5 cm. Each electrode has an area

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of 0.2–0.3 cm2 and a thickness of 0.55 mm. The typical electrode mass of activated carbon and

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NMO electrodes was 3–4 and 5–7 mg, respectively.

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HA Adsorption on Electrodes. HA from Alfa Aesar (purity >95%) was used without

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further purification. The adsorption of HA on Darco, YEC, and NMO electrodes was carried out

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in the absence of electrical field. Duplicate adsorption experiments were performed in 200 mM

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NaCl and pH 7.0 ± 0.5 (pH maintained by 1 mM phosphate buffer) in glass scintillation vials

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(VWR) at 25°C. Deionized (DI) water (18 M W, Millipore) was used for the preparation of all

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solutions. After equilibrating the electrodes with HA solution for 4 days, the concentration of

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HA in the supernatant was measured using a UV-Vis spectrophotometer (Cary Series, Agilent

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Technologies) at a wavelength of 254 nm. The amount of HA adsorbed on the electrodes was

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calculated through material balance. To investigate the influence of electrical field on HA

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adsorption on the electrodes, adsorption experiments were also conducted in the absence and

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presence of repeated cyclic-voltammogram (CV) cycles over the course of 26 h.

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Characterization of Fouled Electrodes and Activated Carbon Particles. To confirm

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the fouling of electrodes by HA after undergoing CV cycles in the presence of HA, we

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characterized the morphology of both fresh and HA-exposed Darco, YEC, and NMO electrodes

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using a Phillips field emission gun XL-30 SEM at an acceleration voltage of 10 kV.

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electrodes were rinsed with DI water and vacuum dried prior to imaging. The NMO electrodes

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were sputter coated with a platinum layer with a thickness of 2 nm to avoid sample charging. In

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addition, we characterized both fresh and HA-exposed electrodes using a Fourier transform

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All

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infrared spectroscopy equipped with a diamond attenuated total reflectance crystal (ATR-FTIR,

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Perkin Elmer Frontier).

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The zeta potentials of Darco and YEC activated carbons were measured using a Zetasizer

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(Zetasizer Nano, Malvern). First, the zeta potential of fresh Darco and YEC activated carbons

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were measured at 25 mM NaCl and pH 7. Next, the carbon particles were mixed with 200 or

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1000 mg/L HA at 25 mM NaCl and pH 7 overnight, and the zeta potentials of HA-exposed

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carbon particles were measured directly in the HA solution.

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calculated from electrophoretic mobilities using the Smoluchowski equation.28

Zeta potential values were

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Electrochemical Testing. Electrochemical testing was carried out on a potentiostat (Bio-

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Logic Science Instruments) with Darco, YEC, or NMO electrodes as the working electrode,

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Ag/AgCl (saturated KCl, Koslow Scientific Company) as the reference electrode, and a platinum

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wire (Sigma) as the counter electrode. Prior to electrochemical tests, NMO electrodes were

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charged by chronoamperometry to +0.7 V (vs. Ag/AgCl) and held at this potential for 10 min to

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discharge Na+ ions from NMO. The tests include CV, linear sweep voltammetry (LSV), and

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electrochemical impedance spectroscopy (EIS). Unless otherwise noted, the CV scan windows

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for carbonaceous and NMO electrodes were 0 to 0.4 V and −0.15 to 0.7 V (vs. Ag/AgCl),

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respectively. LSV from 0.7 to −0.15 V (vs. Ag/AgCl) was performed on NMO electrodes. EIS

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data were recorded for carbonaceous electrodes with a 10 mV amplitude sinusoidal potential

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perturbation over a frequency range of 100 kHz to 50 mHz at open circuit potential (0.18 V vs.

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Ag/AgCl), and recorded for NMO electrodes from 100 kHz to 2 mHz at 0.4 V vs. Ag/AgCl. The

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test electrolytes were 200 mM NaCl at pH 7.0 ± 0.5 (pH maintained by 1 mM phosphate buffer).

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Unless otherwise noted, the test solutions were not deaerated in order to reflect the conditions in

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real-world desalination processes.

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Measurement of NMO dissolution. The dissolution of NMO during CV scans in the

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absence and presence of HA was investigated. Aliquots of solutions were sampled at regular

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intervals during CV scanning and filtered through 0.2 µm polypropylene syringe filters (VWR).

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The concentration of dissolved manganese in the samples was measured using an inductively

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coupled plasma mass spectrometer (ICP-MS 7700, Agilent Technologies).

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Results and Discussion

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HA Adsorption on Electrodes. We first investigated the adsorption of HA on Darco,

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YEC, and NMO electrodes in the absence of electrical field (Figure 1a). The equilibrium

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adsorption density normalized to the mass active electrode material (excluding carbon black and

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PTFE binder), qe, increased in the order of NMO < YEC < Darco. The low adsorption of HA on

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NMO is likely due to its low surface area. The BET surface area of NMO was determined in our

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previous publication to be 1.85 m2/g,12 which is much lower than that of Darco (650 m2/g) or

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YEC (2000–2500 m2/g) activated carbon.

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Although YEC activated carbon possesses higher BET surface area than Darco, the YEC

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electrode exhibited considerably lower adsorption of HA compared to Darco.

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determined the mesopore and micropore volume of the two activated carbons using N2

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adsorption measurements (Figure 1b). The YEC mesopore (2–50 nm) volume was substantially

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lower than that of the Darco activated carbon, whereas the micropore (< 2 nm) volume manifests

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the opposite trend.

We further

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Together, these results suggest that the mesopores in activated carbon are primarily

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responsible for HA adsorption. In previous studies, the size of HA was determined to be 0.5–13

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nm using atomic force microscopic imaging.29-31 Due to size-exclusion effects, HA larger than 2

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nm will not access the micropores. Liu et al.32 reported that the adsorption of both soil and coal

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HA was substantially higher on a synthesized mesoporous carbon than on a commercial

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microporous activated carbon, consistent with our present observation.

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Figure 1. (a) Adsorption isotherms of HA on Darco, YEC, and NMO electrodes in the absence of electrical field. Solution chemistry: [NaCl] = 200 mM, pH = 7.0 ± 0.5. (b) Micropore and mesopores volume of Darco and YEC activated carbon derived from N2 and CO2 adsorption. (c) CV curves of fresh Darco, YEC, and NMO electrodes in 200 mM NaCl (solid lines) and that of HA-fouled electrodes in 200 mM NaCl and 200 mg/L HA (dashed lines). Potential windows for CV curves: 0 – +0.4 V for Darco and YEC; −0.15 – +0.75 V for NMO.

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Characterization of HA-Fouled Electrodes. To confirm the fouling of electrodes by

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HA, we performed SEM imaging of the electrodes before and after undergoing CV cycles in HA

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solutions (Figure 2). It is noteworthy that the electrodes comprise active material (activated

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carbon or NMO) as well as carbon black and PTFE, so HA adsorption by the bulk electrode may

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be affected by all three components. No obvious change in the surface of Darco electrode was

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observed after HA adsorption (Figure 2a and d). Figure 2b shows that the YEC electrode

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features micro-sized activated carbon particles with smooth surfaces. After exposure to HA,

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discrete dark patches were observed on the YEC carbon particles (Figure 2e), likely due to the

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deposition of HA aggregates on the surface. The SEM image of the fresh NMO electrode

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(Figure 2c) shows needle-like NMO particles. After exposure to HA, most of the NMO particles

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were coated by HA (Figure 2f).

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Figure 2. SEM images of pristine A) Darco, B) YEC, and C) NMO electrodes as well as SEM images of HA-fouled D) Darco, E) YEC, and F) NMO electrodes.

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We further confirmed the fouling of the electrodes by HA using FTIR spectroscopy

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(Figure S1). The band features of HA (Figure S1a) include alcohol/phenol O–H stretching at

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3350 cm-1, carboxylate C=O stretching at 1560 cm-1, in-plane O–H bending at 1370 cm-1, and

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aliphatic (i.e., polysaccharide or alcohol) C–O stretching at 1090 cm-1 and 1030 cm-1.33-35 The

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peak intensity at 1560 and 1030 cm-1 in the FTIR spectra of HA-exposed Darco and YEC

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electrodes was greater than that of the fresh electrodes (Figure S1b and c). The FTIR spectrum

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of the HA-exposed NMO electrode exhibit adsorption bands at 3275, 1370, and 1030 cm-1,

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which were absent in the spectrum of the fresh NMO electrode (Figure S1d). These observations

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further confirm the fouling of the electrodes by HA.

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Possible driving forces for the adsorption of HA on activated carbon include hydrophobic

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and p-p interactions between the graphitic surface of carbon and aromatic rings in HA, hydrogen

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bonding between oxygen-containing functional groups (e.g., carboxylic acid) from both carbon

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and HA, and van der Waals (dispersion) forces between carbon and HA.36 In addition to

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hydrogen bonding and dispersion forces, the adsorption of HA on NMO likely involves the

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formation of complexes between MnIII/IV and oxygen-containing functional groups in HA.37, 38

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Impact of HA Fouling on Ion Removal Capacity of Activated Carbon and NMO

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Electrodes. After adsorbing HA in the absence of an electric field, we examine the change in

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electrode ion removal capacity using CV. In a previous study, deionization capacities of carbon

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electrodes were shown to be a linear function of their capacitance.39 For NMO electrodes, their

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charge and discharge capacities are directly attributable to insertion and deinsertion of sodium

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ions. Therefore, we use charge capacity as a proxy for evaluating the desalination performance

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for both carbon and NMO electrodes.

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The CV curves of both Darco and YEC electrodes (Figure 1c) exhibited the rectangular

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shape characteristic of an electrochemical double-layer capacitor.

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electrode was higher than Darco electrode, consistent with the theory that micropores contribute

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the majority of ion storage capacity.3 The CV curve of the NMO electrode displayed the three

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characteristic sodium insertion peaks (+0.36, +0.12, and −0.09 V vs. Ag/AgCl) and de-insertion

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peaks (+0.17, +0.43, and +0.61 V vs. Ag/AgCl). After HA adsorption, the CV curves of both

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Darco and YEC electrodes showed marginal change, suggesting an insignificant effect of HA

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The capacity of YEC


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adsorption on sodium ion storage capacity. Despite adsorbing the least amount of HA (Figure

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1a), the NMO electrode experienced an appreciable decrease in the height of the three sodium

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insertion peaks and three sodium de-insertion peaks. This observation demonstrates a decline in

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the ion storage capacity of NMO electrodes after HA adsorption.

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We further investigated the influence of HA on the capacity retention of the electrodes

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over extended CV cycling (Figure 3). The two activated carbon electrodes were cycled between

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0 and +0.4 V (vs. Ag/AgCl) to avoid carbon oxidation at potentials greater than +0.5 V40 and the

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NMO electrode was cycled between -0.15 and +0.7 V. The adsorption of HA on the three

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electrodes was enhanced by CV cycling (Figure S2a), which we attribute to greater electrostatic

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attraction between the electrodes and HA when the electrodes are positively polarized. The HA

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adsorption normalized to BET surface area was considerably higher on Darco than on YEC

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(Figure S2b), again confirming our previous hypothesis that mesopores are primarily responsible

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for HA adsorption.

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In the presence of HA, the capacity of the two activated carbon electrodes decreased by 5%

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over 30 CV cycles (Figure 3), comparable to their capacity loss in a pure NaCl solution (Figure

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S3). In contrast, the NMO electrode experienced over 25% capacity loss in the presence of HA

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over 30 CV cycles relative to capacity decline in a pure NaCl solution. To rule out the effects of

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different scan potential windows and duration of HA exposure between activated carbon and

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NMO electrodes on their capacity stability, we conducted CV cycles on the Darco electrode

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within a potential window of −0.35 – +0.65 V vs Ag/AgCl, similar to that for NMO, under

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constant argon sparging to minimize carbon oxidation (Figure S4). Again, the presence of HA

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exhibited negligible influence on the capacity stability of the Darco electrode over 40 cycles.

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Collectively, these results reveal the substantially different mechanisms by which HA impacts

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the ion storage capacities of activated carbon and NMO electrodes.

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Figure 3. Percentage of initial capacity as a function of cycle number for Darco, YEC, and NMO electrodes. The potential windows for the CV cycles for Darco and YEC was 0 – 0.4 V (vs. Ag/AgCl). The potential window for the CV cycles for NMO electrode was −0.15 – +0.7 V (vs Ag/AgCl). [NaCl] = 200 mM, pH = 7.0 ± 0.5, [HA] = 200 mg/L. Scan rate of 1 mV/s was used for all three electrodes. The solutions were constantly stirred during experiments. Duration of experiment: 7 h for Darco and YEC; 14 h for NMO.

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Small Impact of Humic Acid on Ion Storage and Transport in Capacitive-based

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Electrodes. Influence of HA on Ion Storage. We have demonstrated that HA adsorption had

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limited impact on the ion adsorption capacity of the two activated carbon electrodes. Capacitive

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ion storage has been modeled under the framework of either the Gouy-Chapman-Stern model,41

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which describes non-overlapping EDL structures (in large pores or at high salt concentrations),

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or the modified Donnan model,3, 42 which describes ion storage in micropores where EDLs fully

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overlap. In Gouy-Chapman-Stern model, the stored charge density normalized to surface area, s

283 284

(in C/m2), is calculated as41

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285

𝜎=4

$% &

𝑠𝑖𝑛ℎ

+ ,

Δ𝜙/ 𝐹

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(1)

286

where 𝑐∞ is the bulk salt concentration, 𝜅 is the Debye parameter, Δ𝜙/ is the dimensionless

287

diffuse layer potential, and F is the Faraday constant. In the modified Donnan model, the stored

288

charge density normalized to micropore volume, smi (in C/m3), is calculated as3, 5 𝜎34 = −2𝑐7 𝑠𝑖𝑛ℎ Δ𝜙8 𝐹

289 290

(2)

where ∆𝜑8 is the dimensionless Donnan potential within the micropores.

291

Based on Eqs. 1 and 2, the potential mechanisms for HA impacting ion storage in

292

capacitive electrodes include: 1) obstruction of the micropores by HA and a resulting decrease in

293

surface area or micropore volume; 2) change in the diffuse layer potential Δ𝜙/ or Donnan

294

potential Δ𝜙8 (which are both related to Stern potential Δ𝜙; )3, 5 as a result of HA adsorption. In

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addition, adsorbed HA can alter the ion storage capacity of the carbon material by introducing

296

redox-active groups.43, 44

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Our results suggest that the pore blockage mechanism is not a dominant mechanism of

298

capacity decline for the electrodes and foulant composition/concentration combinations

299

employed in this study. Given the much higher micropore volume of YEC than Darco (Figure

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1b), the YEC electrode is expected to experience a greater capacity loss than Darco electrode

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when their micropores are blocked by adsorbed HA. Figures 1c and 3 show that HA had

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similarly negligible impact on the capacity of YEC and Darco electrodes, inconsistent with a

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mechanism of capacity decline due to micropore blockage by HA. Our hypothesis is consistent

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with the study by Yang et al.,45 which reported that the total pore volume of activated carbon

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powder decreased by only 8% and the ratio of micropore volume to total pore volume decreased

306

by only 0.8% upon contacting 1000 mg/L HA for 24 h.

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The influence of HA on the Stern potential is challenging to quantify since there is no

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direct method for measuring Δ𝜙; .28 We instead measured the change in zeta potential, which is

309

often used to approximate Δ𝜙; , of both Darco and YEC carbon before and after HA exposure

310

(Figure 4a). The zeta potentials of both Darco and YEC carbon were negative at pH 7. After

311

exposure to 200 or 1000 mg/L HA, the zeta potential of the two carbons became more negative,

312

with an increase in magnitude of less than 30 mV. This change in zeta potential, however, is

313

small compared with the applied voltage of 0.4 V or higher in this study. Further, we performed

314

CV on the Darco electrode in diluted (5 mM) NaCl solution under Argon sparging to investigate

315

the influence of HA adsorption on the potential of zero charge (EPZC) of the electrode (Figure S5).

316

The EPZC corresponds to the potential where the specific capacitance of the electrode was

317

minimum in dilute electrolyte solutions.46, 47 The EZPC of the electrode was ca. +0.14 V vs.

318

Ag/AgCl before and after HA adsorption, again demonstrating the minor influence of HA on the

319

surface potential of the electrode.

320

In addition to the preceding two mechanisms, the adsorbed HA can alter the ion storage

321

capacity of the carbon material by introducing redox-active groups.43, 44 Walkowiak and co-

322

workers48 demonstrated that the capacitance of activated carbon electrodes was enhanced after

323

adding > 20 g/L HA to the electrolyte (6 M KOH), which they attributed to the redox-active

324

quinone moieties in HA. In our study, the increase in ion storage capacity of Darco and YEC

325

electrodes was not observed when transferring the electrodes from HA-free to HA-containing

326

NaCl solutions (Figure 3, cycle number 1). Although the possibility of introducing additional

327

redox-active groups after HA adsorption is not ruled out, we do not consider this mechanism to

328

be important at the lower concentrations of HA employed in this study.

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329 330 331 332 333 334 335 336

Figure 4. Zeta potential of Darco and YEC carbon particles in the absence and presence of HA. Solution chemistry: 25 mM NaCl, pH 7.4 (A). Nyquist plots of fresh and HA-fouled Darco (B) and YEC (C) electrodes in 200 mM NaCl and 200 mg/L HA (pH 7.0 ± 0.5). Self discharge of Darco and YEC electrodes in 200 mM NaCl as well as that of HA-fouled electrodes in a 200 mg/L HA and 200 mM NaCl solution (D). In all cases, the electrodes (or carbon particles) were stirred with HA solutions for at least 7 h to facilitate HA adsorption.

337

Influence of HA on Ion Diffusion in Carbonaceous Electrodes. We further investigated

338

the impact of HA fouling on the diffusion of ions in the two carbonaceous electrodes using EIS.

339

The Nyquist plot of the fresh Darco electrode exhibits a semicircle in the mid-frequency region

340

and an almost vertical line in the low-frequency region (Figure 4b). The semicircle is attributed

341

to interfacial charge transfer between the electrode and the electrolyte.49 The steep slope of the

342

low frequency regime in the Nyquist plot of the Darco electrode is indicative of a capacitive

343

system, consistent with the predominance of mesopores and low diffusion resistance. In contrast,

344

the YEC electrode manifests a 45º line in the Nyquist plot between 5 and 0.2 Hz (Figure 4c).

16


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345

This 45º line is characteristic of Warburg impedance (or diffusion impedance)47, 50 associated

346

with the diffusion of ions within the micropores in the YEC electrode.51 After the electrodes

347

were fouled by HA, no obvious change in the Nyquist plots of either Darco or YEC electrodes

348

were observed, indicating that HA adsorption did not appreciably impact the ion diffusion within

349

the two electrodes.

350

We further investigated the self-discharge of Darco and YEC electrodes before and after

351

HA adsorption (Figure 4d). After being charged to +0.8 V, the fresh and HA-fouled electrodes

352

were left at open circuit for 12 h to allow for self-discharge. The potential decayed over time

353

due to the diffusion of ions back into the bulk solution.52 The discharge curves for both Darco

354

and EC electrodes were similar before and after HA fouling, again suggesting the limited impact

355

of HA on ion diffusion in the electrodes.

356

Mechanisms for HA Mediated Decreases in the Ion Storage Capacity of Insertion

357

Electrodes. We hypothesize that HA may influence the ion storage of insertion electrodes

358

through 1) changing the crystalline structure of NMO, 2) reducing available binding sites for Na+

359

by facilitating manganese dissolution, or 3) hindering Na+ diffusion into the bulk electrode.

360

NMO Crystalline Structure Unaffected by HA. We verified that the XRD patterns of

361

NMO electrodes cycled in the absence and presence of HA matched the standard pattern of

362

Na4Mn9O18 (Figure S6), thereby ruling out the detrimental effect of HA on the crystal structure

363

of NMO. This result is consistent with the presence of well-defined channels in NMO which are

364

too narrow to accommodate the bulky HA macromolecules.

365

HA Adsorption Facilitates Manganese Dissolution. To investigate the role of HA in the

366

dissolution of NMO, we compared the concentration of released Mn to the aqueous solution

367

during CV cycling of NMO with and without HA. Approximately 2.5% of the total mass of Mn

17



Page 18 of 27

368

in NMO was released during 40 CV cycles in the presence of HA, whereas no dissolution was

369

detected in the absence of HA (Figure S7). In their study of the reaction between Mn(III/IV)

370

oxides and phenolic compounds, Stone et al. proposed that Mn(III/IV) cations can be reduced by

371

phenolic groups, resulting in the release of Mn2+ to the aqueous phase.37,

372

mechanism, the MnIII+ and MnIV+ cations in NMO14, 15 that are exposed to the solution can be

373

reduced to Mn2+ by the phenolic groups in HA, thereby reducing the available binding sites for

374

sodium and leading to a decay in sodium storage capacity. It is noteworthy, however, that the

375

percentage of Mn dissolution (3%) was much less than the percentage of NMO capacity loss

376

(33%), indicating that other mechanisms are also contributing to the capacity fade.

53

Based on this

377

Adsorbed HA Hinders Sodium Diffusion into the NMO Electrode. We further suggest that

378

the adsorption of a layer of HA macromolecules on NMO surface hinders sodium ion transport

379

into the NMO electrode. To test this proposition, we performed linear sweep voltammetry (LSV)

380

from +0.7 to +0.3 V for both fresh and HA-fouled NMO electrodes at different scan rates in

381

quiescent solutions. In a diffusion controlled system, the dependence of peak current, ip, for a

382

reversible electron transfer reaction on scan rate, v, is described by the Randles-Sevcik

383

equation:47 +/,

𝑖< = (2.69×10D )𝑛F/, 𝐴𝐷J 𝐶J∗ 𝜈+/,

384

(3)

385

where n is the number of electrons per species reaction (1 for the Mn4+/Mn3+ redox couple), A is

386

the electroactive area of the electrode (cm2), DO is the apparent diffusion coefficient of Na+

387

(cm2/s), and 𝐶J∗ is the amount of Na+ in unit volume of NMO particles (mol/cm3).

388

For fresh NMO electrodes, the peak current for Na+ insertion determined from LSV

389

(Figure 5a) depended linearly on the square root of the scan rate (Figure 5b). Using Eq. 3, the

390

apparent diffusion coefficient of Na+ was estimated to be 6.3 ´ 10-14 cm2/s (see Text S1 in the SI

18


Page 19 of 27


391

for detailed calculation), consistent with the speculation that Na+ diffusion in NMO was the rate-

392

limiting step for Na+ insertion (diffusion coefficients of Na+ in NMO have been reported as 10-

393

14

394

decreased appreciably compared with that of fresh electrodes. Such decrease in slope indicates a

395

reduction in the apparent diffusion coefficient of Na+.

–10-16 cm2/s).54, 55 After the NMO electrodes are fouled by HA, the slope of the ip-v1/2 plot

396

397 398 399 400 401 402 403 404 405 406

Figure 5. A) LSV from +0.70 to +0.30 V showing the peak current for sodium insertion at +0.41 V. B) Peak current of both fresh and HA-fouled NMO electrodes normalized to electroactive area of the electrodes as a function of the square root of scan rate. LSV of fresh NMO was performed in 200 mM NaCl at pH 7.0 ± 0.5. LSV of HA-fouled NMO was performed in 200 mM NaCl and 200 mg/L HA (pH 7.0 ± 0.5) after 20-h stirring. Dash lines represent linear regression. C) Schematic showing free diffusion of + Na ions into NMO in the absence of HA, as well as hindered diffusion in the presence of adsorbed HA.

407

Previous studies have demonstrated that the apparent diffusion coefficients of ions in

408

bulk electrodes are heavily influenced by the nature of the electrolyte-electrode interfaces.56, 57

409

For example, Kim et al.57 reported that the apparent diffusion coefficient of Na+ within NMO

410

was remarkably greater in aqueous electrolytes than in organic electrolytes, which they attribute

411

to the formation of a solid electrolyte interphase (SEI) in the organic electrolytes. We speculate

412

that the presence of adsorbed HA layers on NMO hinders Na+ transport into the bulk NMO

413

electrode, akin to the role of SEI in organic electrolytes. In the porous structure of adsorbed HA

414

layers, the presence of HA macromolecules causes the diffusion trajectory of Na+ to deviate from

19



415

straight lines58 and thus reduces the apparent diffusion coefficient (Figure 5c). Consequently, the

416

transport of Na+ into the crystalline structure of NMO is hindered, and the ion storage capacity of

417

NMO is not fully utilized during electrode charging. Similar observations have been made in

418

membrane-based systems, where the NaCl permeability coefficient in forward osmosis

419

membranes decreased after membrane fouling by humic acid.59

420

To further verify the hindrance of Na+ diffusion into NMO by adsorbed HA, we recorded

421

EIS spectra of NMO electrodes before and after HA fouling (Figure S8). The Nyquist plot of the

422

HA-fouled NMO electrode manifests a longer tail in the low-frequency region compared to the

423

fresh electrode (Figure S8a). In the Bode plots, the HA-fouled electrode exhibits higher absolute

424

value of impedance in the frequency range lower than 0.1 Hz (Figure S8b). We further fit the

425

EIS data to an equivalent circuit including a Bisquert element which represents anomalous

426

diffusion (Figure S8c).60-62 The fitted resistance Rm, which is related to diffusion resistance,

427

increased from 254 ± 60 ohm for fresh to 663 ± 288 ohm (n = 3) for HA-fouled NMO electrode.

428

As such, the EIS results corroborate the aforementioned LSV experiments, confirming our

429

conjecture that adsorbed HA hinders Na+ diffusion into NMO and/or the NMO electrode

430

structure.

431

It is noteworthy that the lost capacity of NMO due to HA fouling was barely recovered

432

upon rinsing the electrode with 30 mM sodium dodecyl sulfate (SDS) at pH 10 (Text S2, Figure

433

S9). Only 3% of HA that had been adsorbed on NMO was released to the SDS solution, likely

434

due to the strong binding between multiple carboxylic acid groups in HA and MnIII/IV in NMO.

435

Identifying effective methods for regenerating HA-fouled NMO electrodes deserves further in-

436

depth investigation.

20


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Page 21 of 27


437

We surmise that the different roles of HA on capacitive and insertion materials originate

438

mainly from their different ion storage mechanisms. In capacitive materials, ion storage takes

439

place primarily in the EDL without involving charge transport across the electrode-electrolyte

440

interface. The main influence of HA on ion storage is the change in surface potential and the

441

reduction in available pore area. These mechanisms, however, are shown to be of minimal

442

relevance for the electrode structures and foulant size/concentration range studied here. In

443

contrast, Na+ must be transported across the NMO-electrolyte interface to be stored within the

444

crystalline lattice of NMO. The accumulation of a layer of HA on the surface of the NMO

445

crystal hinders Na+ diffusion across the NMO-electrolyte interface, thereby limiting the ion

446

storage capacity of NMO. It is possible that the negative impact of HA on the ion storage in

447

NMO can be minimized by using extremely slow charging current, though this would be

448

impractical for functional electrochemical deionization systems.

449

Implications for Electrochemical Desalination. Collectively, our results highlight the

450

contrasting effects of HA on ion storage in capacitive and insertion electrodes. Given the high

451

capacity and intrinsic selectivity for ion storage, insertion compounds hold promise for

452

electrochemically-mediated selective ion removal.

453

compared to capacitive electrode materials such as activated carbon, insertion materials are more

454

prone to capacity fade in the presence of HA foulants. The poor cycling stability of NMO in the

455

presence of HA suggests that pretreatment of feed water to remove organic foulants is required

456

to maximize the longevity of electrochemical desalination systems comprising of NMO

457

electrodes. These full cell systems deserve further investigation, which will be the focus of

458

future work. We also note that a variety of organic foulants beyond HA are present in inland

459

brackish water and agricultural drainage.

The results presented here suggest that,

Future studies on the long-term desalination

21



460

performance of insertion and capacitive electrodes in complex waters will provide importance

461

guidance on the technological feasibility of electrochemical desalination in real world

462

applications.

463 464

ASSOCIATED CONTENT

465

Supporting Information. The Supporting Information is available free of charge on the ACS

466

Publications website. Supporting information includes the following sections and figures: Text

467

S1: Calculation of diffusion coefficient of Na+ in NMO. Text S2: Method for cleaning of HA-

468

fouled NMO electrode using sodium dodecyl sulfate (SDS). Figure S1: FTIR spectra of fresh

469

and HA-fouled electrodes. Figure S2: Adsorption of HA on electrodes in the absence and

470

presence of electrical field. Figure S3: Cycling stability of carbon electrodes in the absence of

471

HA. Figure S4: Influence of HA on cycling stability of Darco electrode under Argon sparging.

472

Figure S5: Cyclic voltammetry (CV) of Darco electrode at 5 mM NaCl and pH 7, showing the

473

potential of zero charge of Darco electrode before and after HA fouling. Figure S6: XRD

474

patterns of NMO electrodes cycled in NaCl in the absence and presence of HA. Figure S7:

475

Dissolution of NMO in the absence and presence of HA. Figure S8: EIS spectra of fresh and

476

HA-fouled NMO electrodes. Figure S9: Efficacy of SDS rinsing in restoring capacity of HA-

477

fouled electrode.

478 479

AUTHOR INFORMATION

480

Corresponding Authors

481

* M.S. Mauter, [email protected], Phone +1-412-268-5688.

482

22


Page 22 of 27

Page 23 of 27


483

Notes

484

The authors declare no competing financial interests.

485 486

ACKNOWLEDGMENTS

487

This work was supported by the National Science Foundation under Award Number

488

CBET-1403826. We acknowledge use of the Materials Characterization Facility at Carnegie

489

Mellon University (CMU) supported by grant MCF-677785. We thank Sneha Shanbhag (CMU)

490

for insightful discussions. We also thank Prof. Christopher Bettinger and Xiaomin Tang (CMU)

491

for the use of FTIR as well as Jared Mitchell (CMU) for his help with electrode preparation.

492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517

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