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Extremely Rapid Uptake of Perchlorate with Release of an Environmentally Benign Anion: Silver Bipyridine Acetate Susan C Citrak, Derek Popple, Kevin Delgado-Cunningham, Katherine Tabler, Kareem Bdeir, Allen G. Oliver, Peter B Kvam, and Scott R. J. Oliver Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01797 • Publication Date (Web): 31 Jan 2018 Downloaded from http://pubs.acs.org on February 8, 2018

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Crystal Growth & Design

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Extremely Rapid Uptake of Perchlorate with Release of an

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Environmentally Benign Anion: Silver Bipyridine Acetate

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Susan C. Citrak,a Derek Popple,a Kevin Delgado-Cunningham,a Katherine Tabler,a Kareem Bdeir,a

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Allen G. Oliver,b Peter B. Kvamc and Scott R. J. Olivera,*

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a

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95064; bDepartment of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, IN

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46556; cAerojet Rocketdyne, P.O. Box 13222, Sacramento, CA 95813

Department of Chemistry and Biochemistry, University of California, Santa Cruz, Santa Cruz, CA

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ABSTRACT

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We report the selective removal of perchlorate from water by a cationic coordination polymer in

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highest observed capacity and rate to date. Silver bipyridine acetate [Ag(4,4’-bipy)+][CH3CO2–

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]6H2O (bipy = bipyridine) is easily synthesized in water under ambient conditions (60 min with

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99.1% yield). The material releases its environmentally benign acetate anions upon perchlorate

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uptake to 99.9% mol/mol completion in Millipore water. Actual contaminated industrial water from

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an underground plume (53 ppm initial perchlorate concentration) was treated to 96% mol/mol in 60

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min. The uptake capacity is near-record high at 310 mg/g and is 94% complete within 30 min.

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Perchlorate exchange was also conducted at 1 to 30 ppm perchlorate typical of underground

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contaminated source sites, with up to 99.7% mol/mol completion.

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INTRODUCTION

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Development of materials to trap toxic oxo-anions from water has expanded in the last decade due to

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increased need for environmental remediation of pollutants.1–6 Perchlorate is still the only oxidant

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used in rockets and is often present in products such as flares and fireworks.7–10 If introduced into the

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human body, perchlorate can block iodine uptake by the thyroid gland. This blockage can lead to

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hypothyroidism, negatively affecting metabolic processes in adults and possible improper neural

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development in children.10,11 Currently, the established material for ion trapping is ion exchange

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resins.8,12 The uptake is relatively slow compared to other materials, with limited capacity and

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reversibility.4,13 The perchlorate exchange capacity is only ~ 0.6 meq/g and the standard brine

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regeneration method decreases capacity but moreover is too costly due to its generation of waste.

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[Ag(4,4’-bipy)+][X–] coordination polymer crystal structures (X– = NO3–, ClO4–, BF4–, –O2C-R-CO2–,

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potential use as ion exchangers. Two of these structures, silver bipyridine nitrate (SBN,

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[Ag(4,4’-bipy)+][NO3–]) and SLUG-21 ([Ag(4,4’-bipy)+][–O3SCH2CH2SO3–]4H2O) show excellent

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uptake and preference for toxic anions over potential competitors commonly found in water such as

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NO3–, CO32–/HCO3– and SO42–.4,14,15 To date, SBN has been our most promising material for removal

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of ClO4– from water.4,14 With the very fast and effective removal of ppm level ClO4–, we are currently

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studying actual underground contaminated water from the Aerojet Rocketdyne site.

O3S-R-SO3–, CrO42–, MnO4– or ReO4–) have been studied by our group3,4,14,15 and others16–19 for their

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Although the above-mentioned materials demonstrate exceptional performance, they exchange for an

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undesirable anion. Ethanedisulfonate is an expensive reagent and toxic. Nitrate is an EPA pollutant,

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albeit with a much higher maximum concentration limit (MCL) of 10 ppm level compared to 6 ppb

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for perchlorate. In 2000, Sampanthar et al. reported silver bipyridine acetate (SBA, [Ag(4,4’-

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bipy)+][CH3CO2–]6H2O) when studying the effect of the counter anion of silver-triphenylphosphine

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coordination polymers.20 The structure was determined by single crystal X-Ray diffraction (SCXRD)

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Crystal Growth & Design

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but no properties were reported. Additionally, the synthesis route was slow evaporation of the volatile

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organic compounds (VOCs) acetonitrile and methanol and gave only a 34% yield. Rapid, high yield

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synthesis using aqueous “green chemistry” and release of acetate instead of nitrate or

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ethanedisulfonate would be an exciting move forward for the environmental remediation of toxic

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oxo-anions.

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Here, we report the first room temperature, aqueous synthesis of SBA in 1 h with a 99.1% yield, as

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well as a hydrothermal synthesis of SBA crystals in 3-days with a 62.3% yield. We additionally show

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that this material has potential as a water remediation tool with incredibly fast uptake of perchlorate

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in near record-high capacity. The successful exchange of an environmentally benign anion such as

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acetate for toxic perchlorate is an exciting step forward in the development of these types of materials

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as ion exchangers.

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EXPERIMENTAL

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Reagents. Silver acetate (AgCH3CO2, Fisher Chemical, 98%) and 4,4'-bipyridine [(NC5H4)2, Acros

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Organics, 98%] were used as-received for the synthesis. Sodium perchlorate monohydrate

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(NaClO4·H2O, Fluka Analytical, 98%) was used as-purchased for the anion exchange reactions.

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Sodium hydroxide (NaOH, Fisher Chemical, 97%) and sulfuric acid (H2SO4, Fisher Chemical, 50%)

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were used as-purchased for pH dependence experiments. Purolite A532E exchange resin was

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obtained from Aerojet Rocketdyne and was used as-received for anion exchange experiments.

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Synthesis. A microcrystalline powder with a needle morphology approximately 150 µm x 800 µm in

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size (Figure S1) was synthesized at room temperature. In a typical synthesis, silver acetate (0.10 g,

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0.60 mmol) was combined with 4,4’-bipyridine (0.10 g, 0.64 mmol) and 10 mL of Millipore water

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and gently stirred in a sealed beaker for 1 h. The powder was then collected via vacuum filtration and

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rinsed with deionized water and acetone (yield: 0.19 g, 99.1% based on silver acetate). Colorless

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plate crystals approximately 50 x 50 µm2 in size (Figure S2) were synthesized hydrothermally. In a

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typical synthesis, the same ratios as above were used and gently stirred for 10 min. The mixture was

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then transferred in to a Teflon liner and placed in a 150 °C oven under autogenous pressure over 3 d

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with slow cooling (ramp rate 99 °C /min; soak time 72 h; cooling rate 0.1 °C/min). The resulting

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crystals were then collected via vacuum filtration and rinsed with deionized water and acetone (yield:

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0.12 g, 63.2% based on silver acetate).

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Anion exchange experiments. Batch experiments for anion exchange were conducted at ambient

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pressure and temperature. For general anion exchange and loading capacity, 43.5 mg (0.31 mmol, 1

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eq) of perchlorate sodium salt was added to 50 mL Millipore water in a 100 mL beaker and allowed

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to dissolve. After dissolution, a 2 mL zero-timepoint aliquot was taken from the solution. 100 mg

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(0.31 mmol, 1 eq) of SBA was then added, the beaker sealed and the solution stirred gently for 48 h,

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at which point a final aliquot was taken. For kinetics experiments in Millipore water, separate batch

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experiments were performed so the most accurate data could be obtained with respect to time (see

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Supporting Information). For pH studies, solutions of Millipore water with pH levels of 4, 7 and 9

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were prepared using sulfuric acid and sodium hydroxide to adjust the pH. 12.6 mg (0.09 mmol, 1 eq)

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of perchlorate was added to 50 mL Millipore water in a 100 mL beaker and a 2 mL zero-timepoint

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aliquot was taken after dissolution. 29.1 mg (0.09 mmol, 1 eq) SBA was then added to the solution, a

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timer was started, the beaker was sealed and the solution was stirred gently for the allotted amount of

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time. For the contaminated groundwater, 30.3 mg (0.09 mmol) of SBA was added directly to 50 mL

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of the groundwater and the procedures outlined above were followed. The groundwater was pumped

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from a contaminated underground plume and provided by one of the co-authors (P. B. K.). For both

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Crystal Growth & Design

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Millipore and industrial water, the final solution was vacuum filtered and the product was collected

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and analyzed via PXRD. The extent of exchange of ClO4– was analyzed by ion chromatography (IC).

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Low ppm anion exchange. Conditions were the same as above. 50 ml solutions of 1, 5, 10, 15 and

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30 ppm ClO4– in Millipore water were prepared and an initial 2 mL aliquot was taken. 15 mg (0.046

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mmol) of SBA was then added. The solutions were stirred gently for 2 d, at which point a final 2 mL

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aliquot was taken.

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Characterization. SCXRD was collected on a Bruker APEX-II diffractometer using a combination

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of ω- and φ-scans of 0.5°.21 Data were corrected for absorption and polarization effects and analyzed

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for space group determination.22 The structure was solved by dual methods and expanded routinely.23

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The model was refined by full-matrix least-squares analysis of F2 against all reflections.24 PXRD was

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measured on a Rigaku Americas Miniflex Plus diffractometer, scanning from 2 to 50° (2θ) at a rate of

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2°·min–1 with a 0.04° step size under Cu-Kα radiation (λ = 1.5418 Å). IC analysis was performed to

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assess perchlorate concentration using a Dionex ICS-3000 with an IonPac AS20 column and a

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detection limit of 3 µg/L (ppb). A standard curve for perchlorate was prepared for all IC experiments

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to quantify the concentration of the samples. Scanning electron microscopy (SEM) images were

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collected with a FEI Quanta 3D Dualbeam microscope. Inductively coupled plasma optical emission

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spectrometry (ICP-OES) measurements were taken on a Perkin Elmer Optima 4300DV, run in radial

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mode using Y as an internal standard for the solubility experiments. A standard curve for silver(I)

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was prepared to quantify the amount of free silver in the samples.

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RESULTS & DISCUSSION

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Synthesis. A white microcrystalline powder was synthesized under aqueous room temperature (RT)

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conditions in 60 min. Hydrothermal synthesis at 130 °C and 150 °C also yielded pure-phase SBA,

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although single crystals were not obtained at 130 °C. Hydrothermal synthesis at 175 °C and 200 °C

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both yielded as-yet unknown patterns (Figure S3). Previously, SBA was synthesized in acetonitrile

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and methanol by evaporation over 2 d, yielding block crystals and a 35% yield.20 Our 1 h RT

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synthesis is much faster, has far higher yield of 99.1% and does not require VOCs.

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Structural Characterization. Given our alternate synthesis route, we have also solved the structure

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of our crystals by SCXRD. There are two silver centers, two acetate anions, two 4, 4’-bipyridine

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linkers and six water molecules of crystallization in the asymmetric unit of the primitive,

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centrosymmetric, monoclinc space group P21/c. The asymmetric unit is shown in Figure 1. Most of

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the [Ag(4,4’-bipy)+][X–] structures have the guest anions sandwiched between layers of π-

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stacked polymeric chains, where the latter contain Ag+ chelated by the nitrogens of two neutral 4,4’-

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bipyridine units.3,14,25–28 Here, we again see this layout except four silver atoms are co-linear and

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arranged into two dimers [3.1347(8) Å], slightly reducing the amount of π-stacking but leading to

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greater covalent connectivity within the layer. These sheets are further stabilized by π-stacking of the

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bipyridine rings [3.425(8) to 3.699(8) Å]. The acetates charge-balance the polymers and µ-2 bridge

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two Ag centers of adjacent polymers (Figures 2, 3, S4). The Ag(1) coordinated by the nitrogens of

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two 4,4’-bipyridine units and one oxygen from two separate CH3CO2– units define a distorted square

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planar geometry. The interstitial water molecules occupy the interlayer space between the silver

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complex chains and form an extensive hydrogen-bonded network [1.9271(8) to

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2.0970(8) Å].21 Hydrophobic pockets exist in the region of the acetate methyl group

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and bipyridne rings (Figure S4). One of the pyridine moieties has been modeled with positional

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disorder over two sites; the disorder is a rocking of the pyridine anchored at the nitrogen and para-

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Crystal Growth & Design

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carbon atom. There is a second pyridine moiety that has elongated displacement parameters and is

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tending towards some disorder but was not further modeled as such.

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Figure 1. Crystal data for C24H34Ag2N4O10: Mr = 754.29; monoclinic; space group P21/c; a =

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11.464(3) Å; b = 18.299(5) Å; c = 14.010(4) Å; α = 90°; β = 102.275(4)°; γ = 90°; V = 2871.8(13)

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Å3; Z = 4; T = 120(2) K; λ(Mo-Kα) = 0.71073 Å; µ(Mo-Kα) = 1.424 mm–1; dcalc = 1.745 gcm–3;

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66324 reflections collected; 7211 unique (Rint = 0.0340); giving R1 = 0.0237, wR2 = 0.0572 for 6102

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data with [I > 2σ(I)] and R1 = 0.0326, wR2 = 0.0610 for all 7211 data. Residual electron density (e–

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Å–3) max/min: 0.427/–0.505.

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Figure 2. Adjacent linear chains seen end-on in this a-projection are bridged vertically by Ag-Ag

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pairs and acetate molecules.

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Figure 3. An alternate view of SBA with the acetates and occluded waters omitted for clarity Left:

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one layer of π-stacked [Ag(4,4’-bipy)+] polymers where two adjacent silver dimers are co-linear;

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Right: [Ag(4,4’-bipy)+] layers viewed end-on down the crystallographic a-axis. For both views, one

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of the dimerized polymers is highlighted in red.

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Anion Exchange. Considering the outstanding ion exchange capabilities found in other silver

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bipyridine structures,3,4,14,26 the kinetics and loading capacity of perchlorate by SBA were examined.

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The exchanges were followed by both IC and PXRD. The kinetics showed that 94% of perchlorate

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uptake by SBA takes place in the first 30 min (Figures 4, 5, S5), transforming SBA to silver

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bipyridine perchlorate (SBP, [Ag(4,4’-bipy)+][ClO4–]). Graphical analysis suggests the kinetics

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follows a second order reaction mechanism with k = 0.0845(1) M–1s–1 (Figure S6). When shown in

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comparison to the previously reported materials (Figure 5), SBA shows superior kinetics for

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perchlorate uptake over SBN, Amberlite IRA-400 and calcined hydrotalcite.

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Figure 4. Left: perchlorate loading capacity versus time; Right: perchlorate removal from water in

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ppm versus time.

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Figure 5. SBA compared with SBN, Amberlite IRA-400 exchange resin and hydrotalcite shows the

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decrease of perchlorate in mM ClO4– per gram of material. All data other than that of SBA was

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previously published by our group.21

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Perchlorate exchange at ppm level. Given the above record performance of SBA using equimolar

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perchlorate, ClO4– concentrations typical of underground contaminated water plumes were also

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investigated to determine the potential application of this material. Perchlorate removal with SBA

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powder was tested for 30, 15, 10, 5 and 1 ppm solutions over a period of 2 d. 15 mg of SBA

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exchanged for perchlorate to 95.7 to 99.7% completion (Table 1). Despite the low concentration, the

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uptake clearly demonstrates the potential utility of this material over a far shorter timeframe than the

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several weeks currently used industrially with single-use exchange resins.

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Crystal Growth & Design

Table 1. Summary of ppm level perchlorate exchange results initial ppm (via IC)

ppm after 2 d (via IC)

% exchange

30 ppm

28.02

0.34

98.8%

15 ppm

15.20

0.65

95.7%

10 ppm

8.92

0.02

99.7%

5 ppm

5.37

0.03

99.4%

1 ppm

1.05

0.04

96.3%

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Perchlorate removal from contaminated underground water. The performance of SBA in actual

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contaminated groundwater was also investigated to ensure that competing ions would not present

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significant issue. Through our collaboration with one of the co-authors (P. B. K.), we obtained

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samples of contaminated ground water with a perchlorate concentration of ca. 53 ppm. The water

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also contained chloride (13 ppm), sulfate (11 ppm), nitrate (1.5 ppm), orthophosphate (0.079 ppm)

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and trichloroethene (3 ppm) after analysis with their third party testing company. Initial tests show

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that after 30 min, SBA powder removed 92.5% of perchlorate, and up to 95.9% after 1 h (Figure 6).

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Figure 6. Removal of perchlorate versus time for the contaminated ground water. 11

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SBA powder was used in all kinetics exchanges (vide supra), so its surface area was greater than that

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of a resin and therefore would be expected to have more rapid perchlorate uptake. Our collaborator

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currently uses Purolite A532E exchange resin microparticles of approximately 50 µm in diameter.

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Our single crystals of SBA are closer in size to the Purolite A532E resin at approximately 50 x 50

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µm2. We compared the exchange kinetics of the Purolite to the SBA single crystals using a gentle

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top-stirrer to promote continuous exchange while limiting any mechanical breakdown of the SBA

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crystals. After 30 min, the Purolite had a perchlorate uptake of 15.5% and after 120 min an uptake of

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49.6%. The values are compared to 50.5% and 77.2% respectively using SBA crystals (Figure S7).

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Although the uptake of the SBA crystals versus the SBA powder is expectedly slower, it is still far

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more rapid than that of the currently used Purolite.

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Solubility. Initial solubility studies of SBA by ICP-OES indicate that it is more soluble in water than

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SBP, possibly due to its lower stability and/or the lower hydration energy of the perchlorate (∆Ghydr°:

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–214 kJ/mol) as compared to acetate (∆Ghydr°: –373 kJ/mol).26,27,29 The greater stability of SBP is in

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fact highly beneficial for the trapping of the toxic perchlorate while releasing the environmentally

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benign acetate. It also poses a challenge, however, for regeneration from SBP to SBA. In our

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previously published research, it was determined that after exchange, SBP could be regenerated back

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to SBN over indefinite cycles.4 We therefore believe SBA could be an excellent “first-pass” material

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for perchlorate removal from industrial waters with its fast kinetics, high loading capacity and

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exchange for an environmentally benign anion.

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pH Studies. Industrial waters often have varying pH levels. Therefore, the extent of exchange over a

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range of pH levels was examined (Figure 7). At a pH as low as 4, the SBA is still able to achieve

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92% uptake of perchlorate and retain relatively good crystallinity as can be seen by PXRD. Since

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acetate is the conjugate base of a weak acid, in more acidic conditions acetate released may be

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immediately protonated, making it unavailable for potential reformation of SBA. This would result in

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an equilibrium push towards perchlorate uptake and formation of SBP. At these more acidic pH

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levels, however, some breakdown of the material would be expected, thus leading to less than 99.9%

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uptake of perchlorate seen in the previously discussed exchanges. SBA does not perform as well in

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basic conditions, possibly due competition by unprotonated acetate. It is more likely, however, that

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the material is not as stable in basic conditions. Partial breakdown as reflected in the reduced

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crystallinity by PXRD for pH 9 and 10 (Figure 7 inset) would decrease the perchlorate uptake.

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Figure 7. Removal of perchlorate from solutions of varying pH levels. Inset: corresponding PXRD

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measurements; the SBP theoretical pattern is displayed in black with lower intensity peaks (25% of

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max threshold) omitted for clarity.

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In summary, we have determined that SBA is an extremely fast and effective material for removing

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ClO4– from water. It can be synthesized in 60 min in air with no need for organic solvents. It has high

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loading capacity, record speed and releases an environmentally benign anion. In fact, acetate can be

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utilized by bacteria to facilitate the reduction of perchlorate and nitrate in water, potentially providing

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further benefit.30 The material has also shown to be effective at removing perchlorate from actual

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contaminated industrial water containing other common ions in addition to perchlorate.

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Acknowledgments

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This work was supported by the National Science Foundation under Grant No. 1603754 from the

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CBET Environmental Engineering GOALI Program. The PXRD data in this work were recorded on

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an instrument supported by the NSF Major Research Instrumentation (MRI) Program under Grant

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No. 1126845. We also acknowledge Dr. Tom Yuzvinsky and the W.M. Keck Center for Nanoscale

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Optofluidics at UC Santa Cruz for the use and assistance in image acquisition on the FEI Quanta 3D

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Dualbeam microscope.

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References

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(1) Eary, L. E.; Rai, D. Chromate removal from aqueous wastes by reduction with ferrous ion.

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Environ. Sci. Technol. 1988, 22 (8), 972–977 DOI: 10.1021/es00173a018. (2) Fei, H.; Han, C. S.; Robins, J. C.; Oliver, S. R. J. A Cationic Metal–Organic Solid Solution

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Based on Co(II) and Zn(II) for Chromate Trapping. Chem. Mater. 2013, 25 (5), 647–652 DOI:

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10.1021/cm302585r.

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(3) Fei, H.; Bresler, M. R.; Oliver, S. R. J. A New Paradigm for Anion Trapping in High Capacity and Selectivity: Crystal-to-Crystal Transformation of Cationic Materials. J. Am. Chem. Soc.

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2011, 133 (29), 11110–11113 DOI: 10.1021/ja204577p. (4) Colinas, I. R.; Silva, R. C.; Oliver, S. R. J. Reversible, Selective Trapping of Perchlorate from

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Water in Record Capacity by a Cationic Metal–Organic Framework. Environ. Sci. Technol.

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For Table of Contents Use Only

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Extremely Rapid Uptake of Perchlorate with Release of an Environmentally

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Benign Anion: Silver Bipyridine Acetate

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Susan C. Citrak,a Derek Popple,a Kevin Delgado-Cunningham,a Katherine Tabler,a Kareem Bdeir,a

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Allen G. Oliver,b Peter B. Kvamc and Scott R. J. Olivera,*

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a

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95064; bDepartment of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, IN

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46556; cAerojet Rocketdyne, P.O. Box 13222, Sacramento, CA 95813

Department of Chemistry and Biochemistry, University of California, Santa Cruz, Santa Cruz, CA

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TOC Graphic:

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Synopsis:

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Silver bipyridine acetate [Ag(4,4’-bipy)+][CH3CO2–]6H2O (bipy = bipyridine) selectively removes

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perchlorate from water in highest observed capacity and rate to date while releasing environmentally

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benign acetate. A new “green” synthesis for this material has also been discovered, thereby

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increasing the viability of this material for environmental remediation.

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