Cation Exchange Properties of Zeolites in Hyper Alkaline Aqueous

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Cation exchange properties of zeolites in hyper alkaline aqueous media Leen Van Tendeloo, Benny de Blochouse, Dirk Dom, Jacqueline Vancluysen, Ruben Snellings, Johan A. Martens, Christine E.A. Kirschhock, Andre Maes, and Eric Breynaert Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es505345r • Publication Date (Web): 08 Jan 2015 Downloaded from http://pubs.acs.org on January 23, 2015

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Environmental Science & Technology

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Cation exchange properties of zeolites in hyper

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alkaline aqueous media

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Leen Van Tendelooa, Benny de Blochousea, Dirk Doma, Jacqueline Vancluysena, Ruben

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Snellingsb, Johan A. Martensa, Christine E.A. Kirschhocka, André Maesa and Eric Breynaerta,*.

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a

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Heverlee, Belgium.

7

b

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KEYWORDS

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Cation exchange, 137Cs, zeolite, selectivity, alkaline media, chabazite, clinoptilolite

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Centre for Surface Chemistry and Catalysis, KU Leuven, Kasteelpark Arenberg 23, 3001

Sustainable Materials Management, VITO, Boeretang 200, 2400 Mol, Belgium

ABSTRACT

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Construction of multi-barrier concrete based waste disposal sites and management of alkaline

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mine drainage water requires cation exchangers combining excellent sorption properties with a

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high stability and predictable performance in hyper alkaline media. Though highly selective

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organic cation exchange resins have been developed for most pollutants, they can serve as

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growth medium for bacterial proliferation, impairing their long-term stability and introducing

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unpredictable parameters into the evolution of the system. Zeolites represent a family of

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inorganic cation exchangers, which naturally occur in hyper alkaline conditions and cannot serve

ACS Paragon Plus Environment

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as electron donor or carbon source for microbial proliferation. Despite their successful

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application as industrial cation-exchanger in near neutral conditions, their performance in hyper

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alkaline, saline water remains highly undocumented. Using Cs+ as a benchmark element this

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study aims to assess the long term cation exchange performance of zeolites in concrete derived

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aqueous solutions. Comparison of their exchange properties in alkaline media with data obtained

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in near neutral solutions demonstrated that the cation exchange selectivity remains unaffected by

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the increased hydroxyl concentration; the cation exchange capacity did however show an

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unexpected increase in hyper alkaline media.

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INTRODUCTION

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Environmentally relevant cation exchange applications in highly alkaline aqueous media are

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mostly related to the management of abandoned mines releasing contaminated alkaline mine

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drainage water, to the control of pollutants released in cement derived pore water from concrete

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based waste disposal facilities, or to the use of waste containing concrete in construction

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applications. Since most of these applications involve long-term processes (up to several

32

decades), cation exchangers exhibiting long-term chemical stability and highly predictable

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behavior are desired. While organic exchangers can exhibit cation exchange behavior and short

34

term stability in hyper alkaline media, they have limited radiation stability,[1] and they can serve

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as growth medium for bacterial proliferation,[2-4] thereby introducing unpredictable parameters

36

into the system.

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Zeolites naturally occur in hyper alkaline conditions and cannot serve as an electron donor or a

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carbon source for microbial proliferation. Next to their use in construction,[5] and as a work-

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horse for the chemical industry,[6] zeolites have been exploited successfully as cation-


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exchangers for water softening, [7, 8] ammonia recovery,[9, 10] sewage water treatment,[11, 12]

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and removal of radio-isotopes such as 137Cs and 90Sr from nuclear waste water. [13, 14]

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The excellent pozzolanic reactivity of natural zeolites stimulated their use as supplementary

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cementitious material in so-called blended Portland cements.[15] Partial replacement of the

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portland clinker in so-called blended Portland cements by pozzolans such as natural zeolites

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allows reduction of the economic and environmental cost of the energy-intensive cement

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production.[15] Moreover, zeolitic additions were shown to impart technical improvements in

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concrete durability such as increased resistance to corrosive salt solutions,[16] and lower

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susceptibility to expansion due to alkali–silica reactions.[17, 18] Recent investigations into the

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fate of zeolites incorporated as finely ground supplementary cementitious material in Portland

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cement based concrete blends have demonstrated the consumption of zeolites by the pozzolanic

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reaction with the portlandite formed in the hydrating cement, producing sparsely soluble calcium

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silicate hydrates and calcium aluminate hydrates.[19] (Justification SI 1). This observation led to

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the assumption that zeolites could not be used as cation exchangers to enhance the safety of

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concrete based waste disposal sites. The search for commercially available materials, suitable to

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serve as extra engineered barriers in the embankment or inspection rooms of concrete based

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surface disposal facilities, shifted zeolites back into the focus for such applications. Such a

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zeolite based engineered barrier should be physically separated from the first cementitious

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barrier to avoid consumption of the zeolites by pozzolanic reactions. In Belgium for example, the

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national agency responsible for nuclear waste management and disposal (ONDRAF/NIRAS) has

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recently proposed a concrete based vault for surface disposal of low- and medium-level, short-

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lived conditioned radioactive waste (category A waste in the Belgian context).[20] This

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repository (cAt project) has been designed to ensure passive safety for a timeframe far beyond


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300 years. In order to limit the release of radionuclides, the ONDRAF/NIRAS design relies on a

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cement-based primary barrier. To increase the defense in depth, this primary system could be

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complemented with a redundant engineered barrier in the inspection rooms of the disposal

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facility. Preferably, such a complementary barrier is a substrate unsuitable for bacterial growth,

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consists of a material different from the primary barrier system and should be able to adsorb and

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retain the radionuclides during the lifespan of the disposal facility. As result of water leaching

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through the cement-based primary barrier, the pH of the system is expected to vary between

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alkaline (state I porewater, pH 13.5) and near-neutral conditions. Since zeolites naturally occur in

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such conditions, exhibit promising sorption properties for the nuclides predominantly

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contributing the final dose to man (e.g.

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they are added to the short list of potentially suitable and commercially available sorption sinks,

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despite their degradation when used as pozzolan in fresh concrete.

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Cs, 137Cs, …) and are unsuitable for bacteria growth,

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This study aims at providing essential information to allow assessing the feasibility of using

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zeolitic materials as a sorption sink in inspection rooms or embankment layers below concrete

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modules similar to those outlined in the cAt project.[20]

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Considering the cAt surface storage scenario the zeolitic buffer material or its degradation

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products should guarantee the retention of Cs+ for several centuries in presence of concrete pore

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water that slowly evolves in composition. The chemical composition of these rain based pore

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waters is expected to evolve as a function of time from a young pore water (state I, 13,5 > pH >

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12,5) rich in K and Na, via pore water controlled by the solubility of portlandite (state II, pH =

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12.5), an aged CSH concrete pore water (state III, 12.5 > pH > 10), to a calcite buffered state IV

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pore water with a pH < 10. [21, 22] In a first phase, the research is focused on zeolite ion

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exchange properties in the state I young pore water.


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EXPERIMENTAL SECTION

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Materials. Both synthetic and natural zeolites were used. The synthetic zeolite samples used

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were Na-A (Si/Al = 0.94), Na-X (Stock Vetikon ’78, Suisse), K-chabazite (synthesized as

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described in [23], Si/Al = 2), and Na-mordenite (Zeolon 100, Norton, Si/Al = 5). The natural

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zeolite samples were chabazite (Christmas, Arizona, Si/Al = 4.54 [24]), clinoptilolite (Pyramid

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Lake, Nevada, Washoe County) and clinoptilolite (Crooked Creek, Rome, Oregon (Malheur

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County)).

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Zeolite preconditioning. The natural zeolite samples were ground using a mortar and ball mill

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(Fritsch, 10 min.) to obtain a size fraction below 50 µm. These powders were subsequently wet

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sieved over a 50 µm stainless steel Retsch sieve (DIN-ISO: 3310/1) using a pH 8.5 NaOH

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solution prepared from double distilled water. The zeolite fraction passing the sieve was then

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transferred to 250 mL centrifuge cups while adjusting the total volume to 250 mL with the pH

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8.5 NaOH solution. In a next step, the suspension was centrifuged with a cut-off of 1 µm (JA-14,

99

5 min, 2000 rpm) to obtain the zeolite fraction with dimensions between 1 and 50 µm. After

100

centrifugation 200 mL of the supernatant solution was discarded, followed by addition of 200

101

mL of a 1 N NaNO3 solution titrated to pH 8.5 with NaOH. Following re-suspension of the

102

pellet, the system was equilibrated overnight on an end-over-end shaker and subsequently

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centrifuged with a cut-off of 1 µm, while discarding the supernatant solution. This washing step

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was repeated 3 times to obtain a well-defined sodium exchanged zeolite fraction with dimensions

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between 1 and 50 µm. The resulting Na-form zeolite materials were then desalinated by three

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additional washing steps of 15 minutes: initially with 0.1 N NaNO3 solutions at pH 8 and

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afterwards twice with MilliQ water, titrated to pH 8 with NaOH. The resulting standardized

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material was dried at 65°C and subsequently stored in a desiccator over a saturated LiCl solution.


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Young Concrete Water (YCW). Simulated state I concrete pore water (pH 13) was obtained

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by mixing 500 mL Milli-Q water, 0.0296 g Ca(OH)2 (4·10-4 mol), 10 mL Na2CO3 solution (10-2

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M), 180 mL KOH (1 M), 70 mL NaOH solution (1 M), and 0.189 g CaSO4.2 H2O (1.1·10-3 mol)

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in this order. Upon complete dissolution of all components, the solution was topped up to 1 L

113

with Milli-Q water.

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A moderate pH counterpart of the state I concrete pore water (pH 8) was prepared in a 1 L

115

volumetric flask by dissolving respectively 0.189 g of CaSO4.2 H2O [1.1x10-3 moles] and 0.0945

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g of Ca(NO3)2.4 H2O in 500 mL of MilliQ water, followed by addition of 10 mL of a 10-2 M

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NaHCO3 solution, 180 mL of a 1M KNO3 solution, 70 mL of a 1M NaNO3 solution. After

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complete dissolution of all components the total volume was made up to 1 L by addition of

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MilliQ water, immediately followed by a transfer of the solution to a closed 1 L polypropylene

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bottle.

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The mono-ionic pH 13 electrolyte solution in the Na-form was prepared similar to the state I

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YCW by replacing Ca(OH)2 and KOH by equivalent molar amounts of NaOH, and replacing

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CaSO4.2 H2O by Na2SO4 (anhydrous). To obtain the K-form of the YCW, Ca(OH)2 and NaOH

124

were replaced by KOH, Na2CO3 by K2CO3 and CaSO4.2 H2O by KHSO4 (anhydrous).

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Zeolite pre-equilibration in YCW (pH 13 and pH 8). 2 g of standardized zeolite in its Na-

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form was suspended three times for 24 h at 25 °C in 20 ml of its respective concrete pore water.

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After each equilibration step the zeolite was separated from the supernatant by centrifugation

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(Beckman, J2-HS, JA-17, 5 min, 2000 rpm, 25°C) and 17 ml of the supernatant solution was

129

exchanged with new YCW solution. Following pre-equilibration, the zeolite material was

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washed twice with MilliQ water (15 minutes) to remove the interstitial solutions, centrifuged at

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7000 rpm for 10 minutes to allow removing the maximum amount of supernatant solution and


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dried at 65°C (72 h). The dried material was stored in a desiccator over a saturated LiCl solution

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until further use.

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Cs sorption. Cs sorption experiments were run using 137

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Cs spiked solutions.

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Cs was

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purchased from Polatomas carrier-free

CsCl dissolved in 0.1 M HCl. Upon arrival, this

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solution (0.1 ml; 925 MBq cm-3; 3220 GBq g-1; 2.1x10-3 M Cs) was diluted 10 times with MQ

137

water in the conical bottom vial in which the spike was delivered. For the sorption experiments,

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this initial stock solution was further diluted with stable CsNO3 solutions prepared in the

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respective concrete pore water or mono-ionic electrolyte solution to reach a final

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concentration of 3.1x10-12 M.

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Cs

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In the Cs sorption experiments, pre-equilibrated zeolite in respectively the Na, K or concrete

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pore water form, was transferred to Oak Ridge centrifuge tubes containing 20 mL of either a Na

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or K containing (0.25 M) pH 13 electrolyte solution or one of the concrete pore water solutions,

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yielding a solid/liquid ratio of 1.5x10-3 and 5x10-3 respectively in the binary sorption

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experiments and the (screening) sorption experiments in YCW. After equilibration for 24 h on an

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end-over-end shaker, the systems were centrifuged using a cut-off of 85 nm. (Beckman, J2-HS,

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JA-17 rotor, 20min 10000 rpm, 25°C). After this centrifugation step, 16 mL of the supernatant

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solution was replaced by 16 mL of the corresponding alkaline electrolyte solution containing

149

varying concentrations of

150

equilibrated together with their respective blanks (Cs containing electrolyte solution without

151

zeolite). After equilibration for 1 or 4 days at 25 °C on a rotary shaker for respectively the

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screening study and the isotherm experiments, the systems were centrifuged and sampled for

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radioactive assay. Aliquots of 1 mL were transferred to liquid scintillation vials containing 2 mL

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scintillation cocktail (Ultima Gold XR, Packard) and counted in a Tricarb 2800 TR liquid

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Cs spiked Cs [10-4 to 10-10 M]. These resulting systems were


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scintillation counter. After sampling, the systems were remounted on the end-over–end shaker.

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The Cs concentration in solution was then followed as function of time by resampling the

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supernatant solution and analyzing the samples using liquid scintillation counting. Sorption

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isotherms are shown in their default representation ([Cs+]S versus [Cs+]L ) in Figure SI 1, and as

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KD ([Cs+]S/[Cs+]L) versus [Cs+]L in Figure 1 and Figure 2.

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Acid microwave digestion. 100 mg zeolite material was transferred to PTFE digestion cups

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and mixed in order of appearance with 1 mL HClO4 (70%), 3 mL HNO3 (65%) and 1 mL HCl

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(37%). After microwave digestion in a Milestone mls 1200mega unit using the following

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programmation: 1) 1 min (250 W), 2) 2 min (0 W) 3) 5 min (250 W), 4) 5 min (400 W), 5) 5 min

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(600 W), 6) 45 min (0 W, ventilate), the acid solution was transferred quantitatively to 50 ml

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volumetric flasks and diluted with milliQ water. The resulting solutions were analysed for K, Na,

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Mg and Ca with AAS (Unicam Solaar 969; Software: Solaar 969; Universal Burner).

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Quantitative phase analysis. The phase composition of the natural zeolite starting materials was

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quantified using X-ray powder diffraction (XRD) analysis by the Rietveld method. To enable the

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quantification of the total X-ray amorphous phase fraction a 10 wt.% ZnO internal standard was

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added to the hand-ground sample. The sample and internal standard were intimately mixed and

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finely ground in a McCrone Micronising Mill, methanol was used as a grinding agent to prevent

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grinding amorphization. XRD data were collected on a Phillips PW1830 diffractometer. CuKα

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radiation was generated at 45 kV and 30 mA, data were collected in flat-plate reflection

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geometry over an angular range of 5-70 °2θ, using a 0.02 °2θ step size and 2 s counting time per

175

step. Phase identification was carried out using the DiffracPlus EVA software, and subsequent

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Rietveld analysis using the Topas Academic package. The Rietveld quantitative phase analysis

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was carried out according to the strategy described in [25].


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RESULTS AND DISCUSSION

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Screening study

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A screening study was designed to assess the sorption behavior of Cs+ in the conditions

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prevailing in young concrete pore water (YCW; hyper-alkaline pH and high ionic strength). To

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assess the effects of pH on the exchange properties, the experiment combined four zeolite

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frameworks with two electrolyte solutions mimicking the cation content in YCW, but with

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different pH of 8 and 13, respectively (See Table SI 1 for speciation). The four zeolites selected

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for this scoping experiment included natural clinoptilolite (Pyramid Lake, HEU (Justification SI

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2)), natural chabazite (Christmas Creek, CHA), synthetic zeolite A (LTA) and synthetic zeolite X

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(FAU). The efficiency of the zeolites for cesium removal is typically expressed by the Cs+

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distribution coefficient (KD), defined by:

189

190

where subscripts S and L are respectively indicating solid and liquid.

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The KD values shown in Table 1 readily reveal a higher affinity of Cs+ for chabazite and

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clinoptilolite as compared to zeolites X and A. The affinity series observed is independent of pH

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and reads: chabazite ≈ clinoptilolite >> zeolite A > zeolite X. This order is in accordance with

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the selectivity information reported in literature.[26-28] In addition, Table 1 demonstrates an

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increased uptake by chabazite and clinoptilolite in hyper alkaline conditions (pH 13) as

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compared to pH 8. Within the equilibration timeframe of 180 days, no obvious trends were

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observed in the values of KD versus time.


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Table 1. Distribution coefficients [l kg-1] observed in the screening experiment. The Cs concentration used was 3.5 x 10-10 M. While the propagated 1 σ uncertainty resulting from the radioactive assay was lower than 1%, the overall uncertainty on the reported values typically falls between 5 and 10 %. Name

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pH 8 pH 13 24 days 95 days 180 days 24 days 95 days 180 days chabazite (Christmas) 208 188 208 239 233 257 clinoptilolite (Pyramid Lake) 120 106 121 179 139 152 zeolite A 53 43 40 46 43 44 zeolite X 20 20 34 13 13 16 + To verify these observations for a much wider range of Cs occupancies and increased number

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of zeolite frameworks, Cs sorption isotherms were constructed for two natural clinoptilolites

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(Pyramid Lake and Crooked Creek), natural and synthetic chabazite, zeolite A and mordenite

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(Figure 1 and Figure 2). The Cs sorption isotherms (Figure 1 and Figure SI 1) confirm the

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affinity trends observed in the screening study, indicating these trends remain valid for an

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extended concentration range (up to 5 x 10-5 M). As expected, KD values decrease for most of the

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zeolites with increasing equilibrium concentration. In case of mordenite and clinoptilolite

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(Crooked Creek), the KD values abruptly drop for equilibrium concentrations exceeding 10-6 M

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Cs+. This should be interpreted as a strong indication for the existence of a limited number of

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Cs+-selective sites with a capacity estimated around 10-7 and 10-6 moles Cs+ per gram for

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mordenite and Crooked Creek clinoptilolite (containing 17% of phillipsite), respectively. In

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mordenite, Sinha et al. found indications for the presence of two energetically different sites for

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Cs+ sorption in presence of Na+.[29] This can be explained from structural considerations since

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mordenite contains wide channels constructed from 12-membered rings (12 RMc, Ø=6.5Å) in

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addition to smaller 8-ring channels. The large Cs+ can diffuse and adsorb more easily, and

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energetically more favorably, in the larger channels compared to the smaller ones.[30-32]


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Figure 1. Sorption isotherms for different zeolite types after 14 days in YCW (pH 13). While the propagated 1 σ uncertainty resulting from the radioactive assay was lower than 1%, the overall uncertainty on the reported values typically falls between 5 and 10 %.

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The KD values observed for natural chabazite and clinoptilolite are similar. Comparing the

223

natural and synthetic samples, the KD values for the synthetic samples far exceed those for the

224

natural samples. This observation still holds after correcting the sorption isotherms of the natural

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samples for the zeolite mass fraction (Figure SI 2). All three natural samples contain around 65%

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zeolite phase, next to significant fractions of X-ray amorphous material (Table SI 2). The XRD

227

pattern analysis also reveals the presence of chabazite and phillipsite in clinoptilolite,

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respectively from Pyramid Lake and Crooked Creek. The clinoptilolite sample with 19 wt% of

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chabazite exhibited significantly lower Cs+ sorption. This was unexpected in view of literature

230

data suggesting a slightly higher selectivity of chabazite for Cs+ as compared to phillipsite.[33-

231

35]


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Sorption isotherms as function of time provide a sensitive probe to assess zeolite stability as

233

function of time. Within the timeframe of 180 days, no decrease in Cs+ sorption was observed for

234

the natural minerals. On the contrary a slight increase in KD is observed over time at both pH

235

values, which is probably related to very slow exchange processes, slow dissolution of impurities

236

blocking a fraction of the exchange sites, or the slow creation of new sites (Figure 2).

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This was interpreted as a strong indication for the stability of the natural zeolites in the highly

238

alkaline concrete pore water conditions. Hydroxyl induced structure alteration is considered as

239

the most feasible explanation for the pH induced change in distribution coefficients. Over the

240

studied timeframe, the XRD analysis provided no indication for zeolite amorphization or phase

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transformation of crystalline phases in hyper alkaline pore water conditions at room temperature

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(Figure SI 3). Long-term stability will have to be evaluated over an extended period of time

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using sorption data, insights in zeolite formation, and material characterization techniques to

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allow formulating a well-supported conclusion on the stability of these zeolites in high pH

245

concrete pore water.[36]


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247 248 249 250 251

Figure 2. Cs+ sorption isotherms in pH 8 and pH 13 concrete pore water after different equilibration times. While the propagated 1 σ uncertainty resulting from the radioactive assay was lower than 1%, the overall uncertainty on the reported values typically falls between 5 and 10 %

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After 2 weeks, most zeolites, reveal a trend towards higher Cs+ sorption in pH 13 concrete pore

253

water as compared to its pH 8 counterpart. In addition, the sorption isotherms recorded in pH 13

254

and pH 8 electrolytes seem to be almost parallel. This effect is most distinct in the case of

255

synthetic chabazite, exhibiting parallel sorption isotherms at pH 8 and pH 13 (Figure 2, bottom

256

left), for which the variation within each isotherm as function of time and occupancy is

257

dramatically lower than the difference observed between pH 8 and pH 13. The potential

258

hypothesis that the observed difference was induced by a changing Ca-speciation was disproved.


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In the pH 13 YCW, the Ca2+ concentration is lower, therefore, an adjusted pH 8 pore water with

260

identical Ca2+ concentration was made. As using the adjusted pH 8 pore water had an opposite

261

effect (leading to even lower KD values), competition effects cannot explain the increased Cs+

262

sorption.

263 264

At trace concentrations of

137

Cs used in this work the relation between KD and CEC for

univalent-univalent exchange is approximated by: [30]

265

266

where M is an univalent cation.

267

As and the nature and speciation of the cations in the systems at pH 8 and pH 13 were identical,

268

and hydronium competition cannot explain this effect (See Justification SI-3)[37], the

269

observation of increased KD values at high pH thus has to be related to either the creation of

270

additional exchange sites exhibiting a selectivity for Cs+, or to the conversion of existing sites

271

into sites with a higher selectivity for Cs+.

272

Total cation compositions. Table 2 shows solid state concentrations of the relevant alkali and

273

earth-alkaline cations for the synthetic and natural zeolite samples used in this study. The results

274

in Table 2 indicate K, Mg and Ca are still present in the Na-exchanged form of the studied

275

zeolites. They also demonstrate the dominant presence of K+ on chabazite, clinoptilolite and

276

mordenite equilibrated with any of the simulated concrete pore waters. This was expected as

277

these zeolite types were described as having the Eisenman selectivity series I for monovalent

278

cations (Cs > Rb > K> Na > Li). [38] As zeolites A and Y exhibit less selectivity for K+, a more

279

evenly distributed surface composition containing mainly Na+ and K+ was observed. Comparison

280

of the cation concentrations upon equilibration with pH 8 and pH 13 pore waters indicates a


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281

higher total concentration at the higher pH (except for zeolite A). This provides additional

282

indications for the unexpected KD increase with increasing pH. This effect could be related to

283

several mechanisms such as siloxane bridge hydrolysis, partial Al or Si leaching leading to

284

silanol nests or the formation of calcium silicate hydrate or calcium aluminate hydrate surface

285

layers. Due to the absence of significant changes in the XRD patterns, it can be concluded these

286

changes do not result in significant structural changes.

287 288

Table 2. Surface composition of the untreated zeolites, after Na-exchange, and after exposure to concrete pore water. Na (mmol/g)

Sample clinoptilolite (Pyramid Lake), original Na-form pH8-form pH13-form clinoptilolite (Crooked Creek), original Na-form pH8-form pH13-form chabazite (Christmas), original Na-form pH8-form pH13-form chabazite (synth.) Na-form pH8-form pH13-form mordenite (synth.) Na-form pH8-form pH13-form

K Ca Mg (mmol/g) (mmol/g) (mmol/g)

∑ ( Na, K , Mg , Ca) (meq/g )

0.383

0.473

0.614

0.339

2.763

0.952 0.153 0.202

0.130 1.058 1.492

0.518 0.478 0.546

0.340 0.326 0.334

2.796 2.820 3.453

1.132

0.771

0.327

0.126

2.809

1.883 0.145 0.183

0.387 2.086 2.230

0.189 0.149 0.270

0.0925 0.0919 0.109

2.834 2.714 3.170

2.637 0.136 0.261

0.0760 2.330 2.434

0.081 0.0740 0.135

0.224 0.076 0.275

3.323 2.766 3.517

3.768 0.132 0.265

0.266 3.888 3.835

0 0.0307 0.112

0 0 0

4.052 4.084 4.328

2.225 0.136 0.237

0 1.971 1.917

0.0322 0.0297 0.0915

0.0513 0.0395 0.0522

2.392 2.245 2.441


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zeolite Y (synth.) Na-form 3.382 0 0.0102 0 3.407 pH8-form 1.023 2.199 0.0566 0 3.339 pH13-form 1.328 2.054 0.0708 0 3.528 zeolite A (synth.) Na-form 6.206 0 0.0168 0 6.240 pH8-form 3.325 2.404 0.0689 0 5.868 pH13-form 3.503 2.294 0.0710 0 5.940 Since the pH induced increase of the Cs KD is also observed for synthetic chabazite, originally

290

prepared in 1M KOH and subsequently pre-conditioned at respectively pH 8 and 13, reversible

291

siloxane bridge hydrolysis is currently considered as the most likely mechanism responsible for

292

this effect. A similar, unexpected increase in CEC at high pH has been observed in the

293

ECOCLAY II project.[39] The CEC of MX80 bentonite increased under alkaline attack in

294

aqueous solutions with pH above 10 in presence of concrete pore water cations. These

295

observations could be related to the formation of structures similar to calcium silicate hydrates

296

from alkali induced release of Si. These phases are the main hydration product in Portland

297

cements and exhibit cation exchange properties with significant selectivity for Cs+ in presence of

298

alkali and alkaline earth cations.[40, 41]

299

Binary ion exchange equilibria. The ion exchange experiments in simulated concrete pore

300

water solutions correspond to quaternary ion exchange equilibria. While these equilibria allow

301

verification of selective sorption of Cs+ in conditions representative for the final application,

302

binary exchange equilibria deliver selectivity coefficients more suitable for modeling. As most

303

important binary systems Na→Cs and K→Cs in alkaline (pH 13) aqueous solution were

304

considered. According to the Gaines and Thomas formalism,[42] the corrected selectivity

305

coefficient Kc is defined by:


16

Page 17 of 25


306

where NA and NB are the equivalent ionic fractions absorbed on the zeolite, aA and aB the

307

activities of the ions in solution as calculated using Phreeqc (v2) in combination with the

308

distributed llnl.dat database.[43] Figure 3 shows the binary selectivity coefficients for Cs+ on a

309

selected set of zeolites in aqueous alkaline pH 13 solutions containing Na+.

310

Synthetic mordenite, having a relatively high Si/Al, shows the highest initial selectivity for Cs+

311

over Na+ (ln Kc = 6.15, up to NCs = 2x10-3). Noteworthy is the strong decrease in selectivity

312

observed over time (Figure 3). While this zeolite exhibits the highest initial selectivity, it

313

decreases to levels in the range of clinoptilolite after 70 days.

314 315 316

Figure 3. Na→Cs selectivity coefficients for different zeolites in pH 13 aqueous solutions containing 2.5x10-1 M Na+.


17


Page 18 of 25

317

The natural zeolites exhibit a continuous decrease in selectivity with increasing fractional

318

occupation of Cs+ (measured up to 1% occupation). The strongest decrease in ln Kc with surface

319

occupation was observed for natural chabazite (Christmas). At Cs+ surface concentrations

320

exceeding 1.6x10-5 mol/g an abrupt decrease in ln Kc from 5.3 to 3.8 is observed. Since fairly

321

constant ln Kc values were observed for the synthetic chabazite, it can be expected that this

322

strong decrease in the natural chabazite is mostly due to site heterogeneity in this natural phase.

323

One of the possible explanations for the strong decrease in the natural chabazite is the presence

324

of 6% of illite which exhibits a very high selectivity for Cs+ (ln Kc ≈ 13.3).[44] Compared to the

325

natural chabazite, the natural clinoptilite sample shows a much more homogeneous sorption and

326

selectivity pattern as function of surface occupancy in the ln Kc range of 5.19 to 4.72. The figure

327

shows a limited decrease of ln Kc (Na+→Cs+) as a function of time for both natural zeolites. In

328

contrast, the synthetic mordenite exhibits a strong decrease in Cs+ selectivity as a function of

329

time.

330

Similar to the observations in the Na→Cs binary systems, the selectivity of the natural samples

331

in the K→Cs system shows a decreasing trend as a function of the surface concentration, while

332

the synthetic chabazite sample exhibits a constant selectivity (ln Kc = 3) for a wide range of Cs+

333

surface concentrations (Figure 4).


18

Page 19 of 25

334 335 336


Figure 4. K→Cs selectivity coefficients for different zeolites in pH 13 aqueous solutions containing 2.5x10-1 M K+.

337

Influence of Na+. Due to the limited surface concentration of Na+ on clinoptilolite equilibrated

338

in pH 13 concrete pore water, it was expected that the influence of Na+ on Cs+ sorption would be

339

negligible. Unexpectedly, the Cs+ sorption on clinoptilolite (Crooked Creek) is significantly

340

higher in the (pH 13) concrete pore water as compared to its sorption in a pure potassium based

341

pH 13 electrolyte solution (Figure SI 4).

342 343

Local selectivities. The presented binary sorption isotherms allow calculation of local selectivities in presence of trace concentration of Cs+ (Table 3).


19


Page 20 of 25

344

Table 3: Local selectivities (ln K) calculated for binary systems Na→Cs and K→Cs at pH 13 for

345

fractional Cs+ occupancies in the range of 10-7 to 10-4. The integral was approximated by

346

considering the curves as straight lines. Name

4 days clinoptilolite (Crooked Creek) 5.00 chabazite (Christmas) 4.72 chabazite (synth) 4.55 mordenite (synth.) 6.17

Na->Cs 12 days 4.95 4.70 4.45 5.96

6 days 2.67 2.12 3.10 /

K->Cs 15 days 2.74 2.18 3.11 /

347

The local selectivities observed in binary Na→Cs and K→Cs systems at pH 13 for trace levels

348

of Cs+ are in agreement with the limited number of data available in literature. Dyer and Zubair

349

report average ln(Kc) values

350

0.6.[45] In case of clinoptilolite, Valcke et al. found a selectivity constant

351

fractional surface occupation between 10-5 and 10-2 in a background solution with total normality

352

equal to 0.01N.[46] This value is close to the value observed for Cs+ sorption on clinoptilolite

353

(Crooked Creek) (≈2.74) in binary solutions at pH 13 (0.25 total normality).

≈ 2.85 and

≈ 4.67 for Bowie chabazite at NCs ≤

= 2.99 for

354

Using Cs+ as a benchmark element, the comparison of binary cation exchange selectivities for

355

zeolites in near neutral and highly alkaline conditions demonstrated the selectivity to be

356

unaffected by the increased hydroxyl concentration. The KD value did however show an

357

unexpected increase in hyper alkaline media, and thus implies an increase of the CEC. As cation

358

exchange selectivity of zeolites has now been demonstrated to be independent of pH, the

359

potential extension of their successful application as cation-exchanger (e.g. for water softening,


20

Page 21 of 25


360

ammonia recovery, sewage water treatment) from near neutral towards hyper-alkaline aqueous

361

media, now mostly depends on the evaluation of zeolite stability under these conditions.

362

ASSOCIATED CONTENT

363

Supporting Information. Composition of simulated young cement water. Adsorption isotherms

364

as [Cs]S versus [Cs]L. Rietveld based phase analysis of the natural zeolites. XRD patterns of

365

zeolites exposed to YCW. Cs+ KD values as function of surface concentration, corrected for

366

zeolite content of natural samples. Influence of Na content on Cs+ KD values. This material is

367

available free of charge via the Internet at http://pubs.acs.org.

368

Toc Art

369 370

AUTHOR INFORMATION

371

Corresponding Author

372

*[email protected]. KULeuven – Center for surface chemistry and catalysis.

373

Kasteelpark arenberg 23 – box 2461. B-3001 Leuven. Tel: +32 16 32 1598 Fax: +32 16 32 1998.


21


Page 22 of 25

374

Author Contributions

375

The manuscript was written through contributions of all authors. All authors have given approval

376

to the final version of the manuscript.

377

Funding Sources

378

This work was supported by ONDRAF/NIRAS, the Belgian Agency for Radioactive Waste

379

and Fissile Materials, as part of the program on surface disposal of Belgian Category A waste.

380

ACKNOWLEDGMENT

381

L. Van Tendeloo and E. Breynaert acknowledge a mandate as, respectively, an aspirant and a

382

postdoctoral fellow of FWO Vlaanderen. This work was performed in cooperation with

383

ONDRAF/NIRAS.

384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405

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28. Hulbert, M. H., Sodium, Calcium, and Ammonium Exchange on Clinoptilolite from the Fort Laclede Deposit, Sweetwater County, Wyoming. Clays and Clay Minerals 1987, 35, (6), 458-462. 29. Sinha, P. K.; Panicker, P. K.; Amalraj, R. V.; Krishnasamy, V., Treatment of Radioactive Liquid Waste containing Caesium by Indigenously Available Synthetic Zeolites: A Comparative Study. Waste Management 1995, 15, 149-157. 30. Borai, E. H.; Harjula, R.; Malinen, L.; Paajanen, A., Efficient removal of cesium from low-level radioactive liquid waste using natural and impregnated zeolite minerals. Journal of hazardous materials 2009, 172, (1), 416-22. 31. Samanta, S. K., Cesium sorption behavior of a mordenite type synthetic zeolite and its modified form obtained by acid treatment. Journal of Radioanalytical and Nuclear Chemistry 1999, 240, (2), 585-588. 32. Liang, T.-J.; Tsai, J. Y.-C., Sorption kinetics of cesium on natural mordenite. Applied Radiation and Isotopes 1995, 46, (1), 7-12. 33. Ames, L. L., Jr., Cation sieve properties of the open zeolites chabazite, mordenite, erionite and clinoptilolite. The American Mineralogist 1961, 46, (9/10), 1120-1131. 34. Adabbo, M.; Caputo, D.; de Gennaro, B.; Pansini, M.; Colella, C., Ion exchange selectivity of phillipsite for Cs and Sr as a function of framework composition. Microporous and Mesoporous Materials 1999, 28, (2), 315-324. 35. Komarneni, S., Phillipsite in Cs decontamination and immobilization. Clays and Clay Minerals 1985, 33, (2), 145-151. 36. Van Tendeloo, L.; Wangermez, W.; Kurttepeli, M.; de Blochouse, B.; Bals, S.; Van Tendeloo, G.; Martens, J. A.; Maes, A.; Kirschhock, C. E. A.; Breynaert, E., Chabazite: a stable cation-exchanger in hyper alkaline concrete pore water. Environmental Science & Technology 2015, Under review. 37. Maes, A.; Cremers, A., Transition-Metal Ion Exchange in Synthetic X and Y Zeolites. In Molecular Sieves, AMERICAN CHEMICAL SOCIETY: 1973; Vol. 121, pp 230-239. 38. Sherry, H. S., The ion-exchange properties of the zeolites. In Marcel Dekker: New York, 1969; pp 89-133. 39. Hidalgo, A.; Castellote, M.; Llorente, I.; Alonso, C.; Andrade, C., ECOCLAY II. Effects of cement on clay barrier performance—phase II, final report EC contract no FIKW-CT-200000028. European Commission 2004. 40. El-Korashy, S. A., Synthetic Crystalline Calcium Silicate Hydrate (I): Cation Exchange and Caesium Selectivity. Monatshefte für Chemie/Chemical Monthly 2002, 133, (3), 333-343. 41. El-Korashy, S. A., Characterization of Cation Exchange and Cesium Selectivity of Synthetic Beta-Dicalcium Silicate Hydrate. Journal of the Korean Chemical Society 2002, 46, (6), 515-522. 42. Gaines, G. L.; Thomas, H. C., Adsorption Studies on Clay Minerals. II. A Formulation of the Thermodynamics of Exchange Adsorption. The Journal of chemical physics 1953, 21, (4), 714-718. 43. Parkhurst, D. L.; Appelo, C. A. J. User's guide to PHREEQC (Version 2) : a computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations; Denver, Colorado, 1999. 44. Brouwer, E.; Baeyens, B.; Maes, A.; Cremers, A., Cesium and rubidium ion equilibriums in illite clay. The Journal of Physical Chemistry 1983, 87, (7), 1213-1219.


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45. Dyer, A.; Zubair, M., Ion-exchange in chabazite. Microporous and Mesoporous Materials 1998, 22, (1-3), 135-150. 46. Valcke, E.; Engels, B.; Cremers, A., The use of zeolites as amendments in radiocaesiumand radiostrontium-contaminated soils: A soil-chemical approach. Part I: Cs-K exchange in clinoptilolite and mordenite. Zeolites 1997, 18, (2-3), 205-211.


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Cation Exchange Properties of Zeolites in Hyper Alkaline Aqueous

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