Catalytic Hydrogel Membrane Reactor for Treatment of Aqueous

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Catalytic hydrogel membrane reactor for treatment of aqueous contaminants Randal P Marks, Joseph Seaman, Patricia Perez-Calleja, Junyeol Kim, Robert Nerenberg, and Kyle Doudrick Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.9b01667 • Publication Date (Web): 14 May 2019 Downloaded from http://pubs.acs.org on May 19, 2019

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Environmental Science & Technology

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Catalytic hydrogel membrane reactor for treatment of aqueous contaminants

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Randal Marks,a Joseph Seaman,b Patricia Perez-Calleja,a Junyeol Kim, a Robert Nerenberg, a Kyle Doudrick

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Affiliations:

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aUniversity

of Notre Dame, Department of Civil and Environmental Engineering and Earth Sciences

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bUniversity

of Notre Dame, Department of Chemical and Biomolecular Engineering

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*Corresponding Author Address:

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Kyle Doudrick, Department of Civil and Environmental Engineering and Earth Sciences, University of Notre Dame, 156 Fitzpatrick Hall, Notre Dame, Indiana, USA, 46556 Phone: 574-631-0305 e-mail: [email protected]

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Journal: Environmental Science & Technology

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Revision Date: 5/8/19

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1 ACS Paragon Plus Environment


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Abstract

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Heterogeneous hydrogenation catalysis is a promising approach for treating oxidized contaminants in

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drinking water, but scale-up has been limited by the challenge of immobilizing the catalyst while

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maintaining efficient mass transport and reaction kinetics. We describe a new process that addresses this

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issue – the catalytic hydrogel membrane (CHM) reactor. The CHM consists of a gas-permeable hollow-

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fiber membrane coated with an alginate-based hydrogel containing catalyst nanoparticles. The CHM

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benefits from counter-diffusional transport within the hydrogel, where H2 diffuses from the interior of the

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membrane and contaminant species (e.g., NO2-, O2) diffuse from the bulk aqueous solution. The reduction

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of O2 and NO2- were investigated using CHMs with varying palladium catalyst density, and mass transport

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of reactive species in the catalytic hydrogel was characterized using microsensors. The thickness of the

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“reactive zone” within the hydrogel affected the reaction rate and by-product selectivity, and it was

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dependent on catalyst density. Continuously mixed flow reactor tests using groundwater showed the CHM

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was stable over a 3-day period. Outcomes of this study illustrate the potential of the CHM as a scalable

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process for treating aqueous contaminants.

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Graphical Abstract

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Keywords: catalytic hydrogenation, oxyanions, nitrite, catalytic hydrogel, microsensors, counterdiffusional membrane reactor, drinking water treatment

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Introduction

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Heterogeneous hydrogenation catalysis (HHC) is a promising advanced reduction process for treating

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oxidized contaminants using hydrogen gas (H2).1-6 For water treatment applications, target reactions occur

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at the interface between solid (catalyst), liquid (water), and gas (H2) phases. Palladium (Pd) is the most

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widely studied catalyst for HHC due to high activity and desirable selectivity.7, 8 While attempts have been

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made to replace Pd and other platinum group metals (PGMs) with more earth-abundant materials (e.g.,

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metals sulfides and phosphides), their activities are orders of magnitude lower than PGMs,9 they are

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unstable in aqueous environments,10 and/or they require high operating pressures and temperatures.11

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Thus, PGMs still remain the optimal choice,12 and though they are costly, economic analysis has shown

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recent advances in nanotechnology have made PGM catalysts cost competitive compared to other

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treatment technologies (e.g., ion exchange, air stripping, activated carbon adsorption).13-16 However, full-

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scale HHC implementation has yet to come to fruition due to mass transfer limitations, costs, catalyst

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instability, and scalability of current treatment options.

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Conventional slurry reactors use catalysts that are supported on an inorganic substrate, e.g., activated

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carbon, alumina, or silica. These supported catalysts are suspended in the aqueous phase and H2 is

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bubbled directly into the aqueous phase.17 While simple to operate, this approach suffers from

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disintegration of powdered catalysts due to particle abrasion during mixing, post filtration requirements,

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inefficient H2 consumption, and/or H2 safety concerns.4 Using fixed (or packed) bed catalytic reactors

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alleviates the need for both rapid stirring of the catalyst solution and post-treatment filtration.4, 7 For

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example, the trickle-bed reactor is effective for treating aqueous nitrate,13,

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reactivity due to H2 solubility limits and poor mass transfer to catalyst surface as well as biofilm fouling

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during long-term operation.19 Reactors that use highly structured immobilized catalyst supports (e.g.,


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but it has limited


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monoliths,22-24 cloth,25-29 foam,30, 31 and forced-flow membranes32, 33) can address mass transfer limitations

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on reactivity caused by large particle sizes used in traditional packed bed reactors. 18, 25, 34

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Three-dimensional interfacial catalytic membranes are an exciting approach to immobilized catalyst

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reactors that allow for a dense loading of nano-sized catalysts and delivers H2 through a counter-

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diffusional pathway. In a typical interfacial membrane reactor configuration, catalyst particles are loaded

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onto the surface of a gas- or aqueous-permeable membrane. Gaseous and aqueous phases are introduced

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on opposite faces of the membrane and interact as gas diffuses through the membrane toward the

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catalyst. This configuration enables independent control of aqueous and gaseous operating conditions,

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which allows for improved control of the reaction, operational safety, and H2-consumption efficiency

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compared to systems based on H2 bubbling into the aqueous phase.35, 36 In membrane biofilm reactors,

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the gas-transfer efficiency has been shown to approach 100%.37-40 Membrane-type reactors have

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previously been demonstrated for hydrogenation reactions using alumina tubular membranes loaded

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with catalyst particles for NO2- reduction; ammonia selectivity was controlled through modification of H2

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partial pressure within the reactor.36, 41-43 However, such reactors suffer from complex catalyst synthesis

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procedures, poor control of catalyst loading, and low contaminant reaction rates. New types of catalyst

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support are needed to address these challenges.

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In this study, a novel type of HHC architecture was developed and characterized – the catalytic hydrogel

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membrane (CHM) reactor. The CHM consists of a gas-permeable hollow-fiber membrane (HFM) coated

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with an alginate-based hydrogel containing catalyst nanoparticles. The CHM operates using counter-

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diffusional transport of reactive species within the hydrogel, where H2 diffuses from the interior of the

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membrane and dissolved target species (e.g., dissolved oxygen) diffuse from the bulk aqueous solution.

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Counter-diffusional delivery of H2 reduces consumption of H2 and allows the reaction to be “tuned” 4 ACS Paragon Plus Environment

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towards desired by-products. The CHM reactor is the first to use hydrogels for catalyst immobilization in

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conjunction with counter-diffusional membrane techniques. Herein, the CHM concept is developed with

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a model catalyst (Pd) and model contaminants (O2 and NO2-). Specifically, this study (i) develops and

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physically characterizes a CHM with Pd nanoparticle catalysts, (ii) investigates the counter-diffusional

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behavior of the CHM using O2 as a model contaminant, (iii) provides a proof-of-concept study assessing

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the reduction of a water contaminant, NO2-, as a function of Pd loading, (iv) demonstrates the effects of

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H2 delivery mode and mass transport limitations on the activity and by-product selectivity of NO2-

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reduction, and (v) establishes the short-term stability of the CHM reactor for reduction of NO2- over a

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three-day period.

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Materials and Methods

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2.1 Assembly of the catalytic hydrogel membrane

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The CHM consists of a gas-transfer HFM coated with a calcium-alginate hydrogel that is embedded with

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catalytic nanoparticles (Figure 1). The Ca-alginate hydrogel was selected as a catalyst matrix due to its low

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cost, simple synthesis process, and desirable properties in aqueous solution. Alginate hydrogels have been

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reported as effective matrices for catalytic metal nanoparticles including gold,44, 45 silver,46-48 platinum49

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and palladium.50 Ca-alginate hydrogels form when the guluronate regions of the alginate biopolymer

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cross-link with Ca2+ ions to form a three-dimensional structure.51 The cross-linked nature of the polymer

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allows for effective immobilization of catalyst nanoparticles within the alginate matrix. Further, the highly

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aqueous nature of the hydrogel provides limited resistance to the transport of dissolved species, e.g., H2,

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O2 and NO2-.52

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The CHM was constructed using a novel method involving the in-situ reduction of Pd2+ ions enmeshed in

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a calcium-alginate hydrogel. Briefly, 2% (m/m) alginic acid, sodium salt (low viscosity, #01469, Chem5 ACS Paragon Plus Environment


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Implex Inc.) was dissolved in ultrapure water (18.2 MΩ-cm) with stirring until homogeneity was achieved.

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The alginate solution was transferred to a custom-built half-tube reactor for coating. A 25-cm length of

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the silicone HFM (0.037” OD x 0.025” ID, #025BRA002, Braintree Scientific, Saint Gobain) was completely

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submerged in the alginate solution. A 20-mL cross-linking solution (100 mM total concentration) was

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prepared by dissolving calcium chloride (CaCl2, anhydrous, #C77, Fisher Scientific) and palladium nitrate

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dihydrate (Pd(NO3)2 · 2H2O, #76070, Sigma Aldrich) in aqueous solution, which was then poured into a

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separate half-tube reactor. The alginate-coated membrane was removed from the alginate solution and

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rapidly transferred to the Ca2+/Pd2+ cross-linking solution. The membrane was soaked in the cross-linking

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solution for 30 min to allow for complete cross-linking. The resulting Ca-alginate hydrogel had a vibrant

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orange color from Pd2+ intrusion. The alginate coating and cross-link procedure was repeated once to

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increase the hydrogel thickness and stability. Finally, the reduction of the embedded Pd2+ to Pd

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nanoparticles was induced by submerging the membranes in 2.5 mM sodium borohydride (NaBH4, 98%,

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#13432, Alfa-Aesar), during which the hydrogel turned a dark gray color. The completed catalytic hydrogel

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membrane was stored in ultrapure water until use.

Figure 1. Growth process for catalytic hydrogel film on an HFM. A clean HFM is dipped in an aqueous solution of sodium alginate, then rapidly transferred to a solution of Pd(NO3)2 and CaCl2 to cross-link. After cross-linking is complete, Pd2+ ions are reduced in a sodium borohydride solution to form Pd nanoparticles. The coated membrane is then installed in a glass reactor for hydrogenation catalysis experiments.

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2.2 Hydrogenation of NO26 ACS Paragon Plus Environment

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To conduct the NO2- hydrogenation experiments, the CHM was installed in a tubular glass reactor with

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ports for aqueous and gaseous supplies, which formed the CHM reactor assembly (Figure S1). The

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aqueous solution was composed of ultrapure water (18.2 MΩ-cm) and 0.35 mM of sodium nitrite (NaNO2,

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reagent grade, #0535, VWR). For all counter-diffusional experiments, 100% H2 (ultra-high purity, #HY

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UHPT, American Gas and Welding) was supplied through the lumen of the HFM (operated in closed mode,

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constant lumen pressure of 3 psi). The majority of experiments were conducted in a recirculating batch

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system, where the aqueous solution (volume = 60 mL) was recycled along the exterior of the CHM using

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a peristaltic pump. A single experiment using groundwater was conducted using a completely mixed flow

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reactor consisting of eight CHMs bundled. This reactor setup and the groundwater characterization are

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described in more detail in the SI (Figure S2, Table S1). During hydrogenation experiments, bulk aqueous

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aliquots were removed periodically for subsequent analysis of NO2- and NH4+ by ion chromatography (IC;

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Dionex ICS 5000+; AS-23 and CS-12A analytical columns). The sole by-products were assumed to be N2

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and NH4+, so N2 formation was calculated from the mass balance between measured NO2- and formed

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NH4+.

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2.3 Materials Characterization of the CHM

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Optical Coherence Tomography (OCT; Ganymede II Spectral Domain OCT System, THORLABS, Inc.) was

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used to obtain optical 2D sections of the internal structure of the catalytic hydrogel while submerged in

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ultrapure water. Pd nanoparticles were extracted from the hydrogel by dissolving it in a solution of

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ethylenediaminetetraacetic acid (0.1 M) and sodium citrate (0.2 M). The isolated particles were then

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centrifugally washed and resuspended in ethanol. A drop of this solution was added to a transmission

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electron microscopy (TEM) grid, dried, and then analyzed by TEM (Titan 80-300, 300 kV) to obtain the

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primary particle size. The Pd crystal structure was characterized using selected area electron diffraction

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(SAED). The total Pd mass loading in each CHM was determined using inductively coupled plasma-optical 7 ACS Paragon Plus Environment


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emission spectroscopy (ICP-OES; Perkin-Elmer Optima 8000). To prepare the samples for ICP-OES, the

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hydrogel was first stripped from the HFM, dried in air at room temperature overnight, massed, and then

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digested in concentrated nitric acid (68%, redistilled, GFS) using microwave digestion (210oC for 45 min,

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110 mL MarsXpress vessel, Mars6 Microwave Digester).

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2.4 Microsensor Analysis of Concentration Profiles

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To measure the concentration profiles of dissolved species O2 and H2 during catalytic O2 reduction, a

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single-coated CHM was installed in a custom-built tubular glass reactor (6-mm inner diameter and 25-cm

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length) containing two ports for microsensor measurements (Figure S3). During operation, 100% H2 (ultra-

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high purity, #HY UHPT, American Gas and Welding) was supplied through the lumen of the HFM (open

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mode, 1 psi), while 100% O2 (ultra-high purity, #OX UHPT, American Gas and Welding) was constantly

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bubbled into the aqueous reservoir to achieve a high dissolved O2 concentration. The aqueous solution

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was recycled through the reactor at a flow velocity of 2.7 cm s-1. Prior to analysis the system was operated

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for 0.5 hr to obtain steady-state conditions. As a control, in separate experiments, 100% N2 (high purity,

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#NI-300-HP, American Gas and Welding) was bubbled either into the lumen or the aqueous solution.

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H2 and O2 microscale electrodes, or microsensors, with a 25-µm tip diameter (H2-25 and OX-25, Unisense

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A/S, Denmark) were used to measure the dissolved H2 and O2 profiles within the catalytic hydrogel.

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Microsensors were calibrated according to manufacturer’s instructions, and data was collected using

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Unisense Logger 2.7 software. The microsensors, controlled with a motorized micro-manipulator (Model

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MC-232 and MM33, Unisense A/S), were inserted into the catalytic hydrogel perpendicular to the

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membrane until contact was established with the outer wall of the HFM. Measurements of species

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concentrations were then taken at 20-µm intervals as the microsensors were withdrawn through the


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hydrogel. Measurements were continued another 400 µm into the bulk aqueous layer. The total

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measurement time for each profile was approximately 5 minutes.

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Results and Discussion

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3.1 Material characterization of the CHM

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As observed in the TEM micrographs (Figure 2A), the average primary particle size of the Pd particles was

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4.59 ± 1.02 nm (Figure 2B). SAED analysis revealed the presence of numerous Pd facets, including the

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[111], [200], [220], [311], and [300] (Figure S4). Using the TEM micrographs, an average d-spacing of 0.23

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nm was measured, which matches the [111] crystal facet for Pd and indicates it is the dominate facet.

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The color of the hydrogel of the CHM was dark grey and consistent throughout with no visible particles or

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aggregates. OCT images (Figure 2C) confirmed uniform dispersion of the Pd particles as no large

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aggregates were observed, and they showed the mean thickness of the hydrogel for a single-layer and

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double-layer coated HFM was 304 ± 37 µm and 988 ± 119 µm, respectively. Longitudinal uniformity of Pd

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was investigated by analyzing the Pd-loading for 10 equivalent 2.5 cm segments of a single CHM. The Pd-

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loading was found to be consistent with an average Pd mass of 0.184 ± 0.0206 mg for one segment. With

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regards to the hydrogel film stability (i.e., adherence to the HFM and consistency within the gel), Pd-

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loadings greater than a theoretical Pd wt./ dry alginate wt. of 12.5% resulted in stability loss for the

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alginate viscosity/concentration used herein. The stability loss was presumably due to the decrease of

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alginate cross-linking sites as Pd2+ was reduced to Pd0. Consequently, 12.5% theoretical Pd wt./ dry

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alginate wt. was the maximum loading used in this study.



Figure 2. A) TEM image of Pd nanoparticles isolated from hydrogel support. B) Histogram of particle diameters measured from TEM images showing a normal distribution. C) OCT cross-sectional image of hydrogel on HFM. The lower two white lines delineate the HFM wall, with the hydrogel above. 192 193 194

3.2 Microsensor analysis of H2 and O2 transport through the CHM

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To investigate the effects of counter-diffusional delivery of H2 and the reactant on CHM behavior,

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microsensor analysis was used to map the concentration profiles of relevant species during

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hydrogenation. CHMs provide a unique opportunity for the direct observation of concentration profiles

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within the catalyst support. These profiles have been proposed previously for similar inorganic membrane

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supports, but they have not been quantified.41, 42 Microsensors have been established in the field of

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biofilms for assessment of concentration profiles within the films and they can be simply applied to the

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CHM due to the similar structural characteristics of the biofilm and alginate hydrogel.53 Pd is known to

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catalyze the reduction of O2 in the presence of H254 and it was used as a model reaction because the


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reactants can be measured by commercially available microsensors and steady-state conditions of the

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reaction are easily maintained within the CHM.

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The concentration profiles of O2 and H2 in a CHM as measured by the microsensor are shown in Figure 3.

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These profiles illustrate the concentrations of each species through the depth of the hydrogel and into

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the bulk aqueous zone. Distinct regions within the reactor were observed: the bulk aqueous region, the

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liquid diffusion layer (LDL), the reactive zone (RZ) of the hydrogel, and the non-reactive zone (NRZ) of the

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hydrogel. This bulk aqueous region is dominated by the flow of water parallel to the membrane and

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maintains a high oxygen concentration due to rapid recirculation of oxygenated water (e.g., 2.7 cm s-1)

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from the reservoir, while the bulk H2 concentrations approached zero. At the LDL, i.e., the interface

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between the hydrogel and the bulk aqueous region where diffusion is the dominant mechanism of mass

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transfer, aqueous species concentrations vary in a linear fashion per Fick’s First Law of diffusion. The

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position of the inner edge of the LDL was determined through visual observation of the microsensor

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exiting the hydrogel, while the outer edge was determined empirically by evaluation of the position of the

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change in slope of the O2 concentration gradient. LDL thickness was found to be approximately 100 µm

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under the tested conditions. After diffusion through the LDL, O2 reaches the hydrogel surface. The exterior

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region of the hydrogel closest to the LDL is the RZ, where H2 and O2 are reacting at catalyst sites. In this

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region, component concentrations profiles follow a typical diffusion-reaction shape as transport is

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controlled by both diffusion and catalyst-surface reaction rates. Finally, the interior region of the hydrogel

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closest to the HFM is the NRZ, where O2 has been depleted and no further reaction is occurring. In this

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region, H2 transport from the membrane wall is dominated by diffusion. Note the locations of the RZ and

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NRZ vary depending on the H2 and O2 concentrations at the hydrogel boundaries, the catalyst density and

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activity, and the overall hydrogel thickness.



Figure 3. Concentration profiles of O2 and H2 during catalytic reduction of O2 by a CHM as measured by microsensors. H2 diffuses out of the HFM wall (left-side) and through the hydrogel (dotted region), while O2 passes from the aqueous region (right-side) through the hydrogel. Reaction of H2 and O2 at catalytic sites results in the formation of distinct regions given at top of figure. NRZ: Non-reactive zone, RZ: Reactive zone, LDL: Liquid diffusion layer. Dashed lines indicate estimated transition points between these regions and the solid line indicates the outer edge of catalytic hydrogel. The red dot at the HFM wall represents the saturation concentration of H2 at room temperature, which is not achieved in the 226

system due to mass transfer limitations on H2 diffusion through the HFM wall.

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The transport of H2 and O2 was further investigated by measuring their respective profiles separately

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replacing H2 or O2 with N2, allowing their transport to be governed by diffusion only (Figure S5). According

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to conservation of flux, diffusivity of a species within the hydrogel can be determined from the difference

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in the concentration gradient between the hydrogel and the LDL (Table S2). The measured concentration

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gradients were used to determine effective diffusivities of O2 and H2 in the CHM, which were 6.2 x10-6 and 12 ACS Paragon Plus Environment

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11.0 x10-6 cm2 s-1, respectively. These effective diffusivities are approximately 25% of their aqueous

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diffusivities, indicating the alginate matrix does hinder diffusion. The diffusivity of dissolved O2 in the CHM

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is greater than reported for other common supports, such as alumina (3.2 x10-6 cm2/s)42 and activated

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carbon (0.58 x10-6 cm2/s).55 The effective diffusivity within the CHM could potentially be improved by

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changing the alginate properties (e.g., viscosity, density), although changes to alginate integrity due to

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these properties will require further consideration in future studies.

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3.3 Proof of Concept: NO2- Hydrogenation

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The hydrogenation of NO2- under batch recirculating conditions was investigated for a 6-hr period using a

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single CHM containing 3.68 mg of Pd (Figure 4A). Catalytic hydrogenation of NO2- over noble metal

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catalysts is a well-studied reaction can reduce aqueous NO2- under atmospheric conditions.22, 36, 56, 57 A

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multi-step process, NO2- reduction leads to formation of either N2 or NH4+ as the primary by-products.58

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Using the CHM, approximately 52% of the NO2- was reduced during the reaction period. Only a trace

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amount of nitrate (

Catalytic Hydrogel Membrane Reactor for Treatment of Aqueous

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