How ‘ Hydrophilic Sites ’ Work in Water Adsorption / Desorption by Natural Clinoptilolite

The kinetic mechanism of water adsorption/desorption on samples of natural clinoptilolite-K has been investigated by using an electrical method based on measurements of variation of AC current intensity during the time. In particular, a high-frequency sinusoidal voltage (5kHz) was applied to the sample (high frequency was required to avoid sample/electrode interface polarization phenomena) and the resulting AC micro-current intensity was monitored during the time. The sample was hydrated by exposition to a 75% humidity atmosphere, while dehydration was achieved by exposing the sample to activated silica gel in a close container or simply taking it in air. The hydration reaction followed a pseudo-zero-order kinetics, while the dehydration reaction followed a first-order kinetics both in air or dry atmosphere. The observed kinetic behaviors can be explained on the basis of a 'catalytic effect' of cations in both water adsorption and desorption from the 3D-framework walls.


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Abstract-The kinetic mechanism of water adsorption/desorption on samples of natural clinoptilolite-K has been investigated by using an electrical method based on measurements of variation of AC current intensity during the time.In particular, a high-frequency (5kHz) sinusoidal voltage was applied to the sample (high frequency was required to avoid sample/electrode interface polarization phenomena) and the resulting AC micro-current intensity was monitored during the time.The sample was hydrated by exposition to a 75% humidity atmosphere, while dehydration was studied by exposing the sample to activated silica gel in a close container or simply taking it in air.The hydration reaction followed a pseudo-zero-order kinetics, while the dehydration reaction followed a first-order kinetics both in air or dry atmosphere.The observed kinetic behaviors can be explained on the basis of a cation ability to promote both adsorption and desorption of water from the 3D-framework walls.

I. INTRODUCTION
The zeolite crystalline structure consists of windows, cages, and supercages that causes the formation of a natural microporosity, made of variously sized channels, characterized by uniform pore size [1].Consequently, zeolite behaves like a molecular sieve and it can be used as desiccant/dryer [2], [3], selective gas adsorber [4], air pollutant adsorber [5], gas concentrator [1], etc. Zeolite adsorption has the unique characteristic of an extremely high capacity even at very low gas concentrations and at high temperatures [1], in addition molecules to be adsorbed need to have a diameter inferior to the pore windows (c.a.1.7nm).Since zeolite is a cationic conductor [6], its use in the fabrication of electrical chemosensors has been proposed too [7].Also for this application, selectivity is a strictly required characteristic.As a consequence, the comprehension of the cation role in the adsorption mechanism becomes very important for enhancing the molecular selectivity of devices based on such materials, which could be only partially achieved by the molecular sieving characteristics.
It is well-known that polar and polarisable molecules are preferentially adsorbed in zeolites, especially at low partial pressures, in the order: H2O>NH3>H2S>CO2>N2>butane [6].However, owing to the hydrophobic nature of ≡O-Si-O≡ groups present in the 3D-framework, only the 'hydrophilic centers' contained in the 3D-framework should be involved in the process of adsorption of water and other small polar molecules [8].The 'hydrophilic centers' in zeolites correspond to hydroxyl groups (external and acid silanol groups, Si-OH) and cation sites (i.e., the cation-tetrahedrally coordinated aluminum system).The last being the most abundant type of hydrophilic sites and the most important for technological applications.In principle, water molecules can both (i) bridge the oxygen atoms in the 3D-framework by hydrogen bonds and (ii) associate directly with cations by cation-dipole forces (in fact, very strong ionic electric fields are present in zeolites).In both cases, the resulting electrostatic shielding may facilitate cation movement in the crystal pores under the effect of an electric field.So far, the exact way the adsorption of water and other polarizable molecules takes place and influence electrical conduction in the zeolite structure has been still in debate and the persistence of research in this area is strongly justified by the need for developing selective absorber and sensor materials for industrial and everyday uses.
If adsorption in zeolite involves mainly cations, then the way cations influence the hydration/dehydration processes can be directly investigated by studying the temporal evolution of electrical transport in zeolite exposed to water vapor.In fact, the adsorption of water molecules in the areas of cation sites influences the cation ability to move under the effect of an applied electric field, which reflects in the behavior of current intensity under a constant applied voltage.Consequently, an exact picture of mechanism involved in water adsorption process, that is the role of cations and the 3D-framework surrounding these cations in the adsorption, can be deduced by analyzing the electrical behavior of zeolite during water adsorption.Electrical measurements require the application of AC signals of a convenient frequency to avoid sample/electrode interface polarization phenomena.Usually, zeolites show an electrical behavior equivalent to a RC series circuit, and therefore, to remove the reactive contribution to the material impedance, a high-frequency signal must be used.At high frequency (e.g., 5kHz), there is not sample/electrode interface polarization, and consequently the current intensity variation can be directly correlated to variations in the carrier concentration, under constant applied voltage conditions.The temporal behavior of the current intensity represents exactly the variation of the hydrated cations concentration.These electrical data can produce kinetic information on the water adsorption and desorption processes, useful for mechanism comprehension.
In this work, a natural clinoptilolite sample has been morphologically and structurally characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectrometry (EDS), X-ray powder diffraction (XRD), How 'Hydrophilic Sites' Work in Water Adsorption/Desorption by Natural Clinoptilolite Gianfranco Carotenuto infrared spectroscopy (FT-IR), and thermogravimetric analysis (TGA), to establish its exact nature, the amount of impurities, the type of porosity, the water content, and the type of contained cations.The electrical behavior of this clinoptilolite sample under AC voltage, with frequency ranging from 10Hz to 2MHz, has also been investigated for a sample in equilibrium with a 75% humidity atmosphere.Then, the kinetic of hydration and spontaneous dehydration reactions have been investigated at room temperature by monitoring the temporal evolution of the intensity of the AC current present in the sample during exposition to a wet (75% of water) and a dry atmosphere, respectively.

II. EXPERIMENTAL PART
A commercial sample of natural clinoptilolite, provided by T.I.P. (Technische Industrie Produkte GmbH), was used in the as received form for both impedance spectroscopy and time-resolved measurements.Specimens for electrical characterization were fabricated by cutting the raw zeolite pieces in little slabs (5.0x5.0x3.0 mm) by a diamond saw.Electrical measurements were performed with the twocontacts method.In particular, a curable silver paste (ENSON, EN-06B8), dried in air for 2 days and then backed in an oven at 140°C for 30min, was used to electrically contact the slab surface.The experimental set-up used for electrical measurements is schematically shown in Fig. 1.To avoid sample/electrode interface polarization phenomena, the electrical transport in natural clinoptilolite samples was investigated by using high frequency alternate current measurements.A sinusoidal signal, produced by a DDS signal generator (GW Instek, AFG-2112), was used and the effective current intensity (Ieff) and the effective voltage (Veff) were measured by a 100kHz bandwidth True-RMS digital multimeter (Brymen, BM869s).A sinusoidal signal of 20Vpp and 5kHz was applied to natural clinoptilolite sample for time-resolved measurements.In order to record current intensity during the time, a devoted multimeter data logger was used (software: Bs86x Data Logging System Ver. 6.0.0.3s).Measurements were done at room temperature and coaxial cables (BNC) were used for connections.To expose zeolite samples to a constant humidity environment, a plastic self-made cell was used.The saturated salt solution technique [9] was used for controlling humidity content in this cell.In particular, in order to achieve an atmosphere with 75% of humidity, which allowed easily measurable current intensity values (micro-amperes) an atmosphere with 75% of humidity was generated by using wet NaCl crystals placed at cell bottom (distilled water was used to wet NaCl).The cell contained a small hole to keep always the sample at 1 atm.A similar cell, containing freshly activated silica gel at bottom, was used to study the spontaneous dehydration process.
Material porosity and type of contained ions was established by SEM (FEI Quanta 200 FEG microscope) and EDS analysis, respectively.The crystalline nature of the commercial clinoptilolite sample was investigated by XRD (X'Pert PRO, PANalytical).The clinoptilolite was also spectroscopically characterized by Furier-transform infrared spectroscopy (FT-IR), using a Nicolet apparatus in the ATR configuration.The thermal gravimetric analysis (TGA) was performed on the clinoptilolite sample by using a TA Instruments Q500, operating in nitrogen flow with a constant rate of 10°C/min.

A. Material Characterization
Since natural zeolites usually contain large amount of impurities, the different crystalline phases present in the zeolite sample were identified by using X-ray powder diffraction (XRD).XRD of the zeolite sample is shown in Fig. 2. According to this XRD characterization, the zeolite sample mostly consisted of clinoptilolite (48.4% by weight) but some anortite (42% by weight) and quartz impurity (8.9% by weight) were also contained.Quartz is typically found in most of the clinoptilolite deposits [10].The microscopical structure of the natural zeolite samples was investigated by Scanning Electron Microscopy (SEM).As visible in the SEM-micrograph shown in Fig. 3a, this material has a compact structure, characterized by the absence of macro-and meso-porosity.(EDS) of the natural zeolite samples was used to determine: (i) the approximate value of the Si/Al ratio and (ii) the type and concentration of contained extra-framework cations.
The EDS analysis was performed on three different samples by analyzing an area of ca.900mm 2 .The Si/Al atomic ratio resulted of 5.3, which is a typical value for the clinoptilolite class.As visible in Fig. 2, four types of cations were present in the material: potassium, calcium, iron, and magnesium.
The average percentages of these extra-framework cations are given in Table I.According to these results, the investigated zeolite sample contains mostly clinoptilolite-K.
According to the literature [1], [4], these high siliceous zeolites are quite hydrophobic, show scarce water adsorption properties, and have low electrical conductivity.The FT-IR spectrum of the natural zeolite sample is shown in Fig. 4 and it has been interpreted according to the literature information available for clinoptilolite [11], [12].The FT-IR spectrum of natural zeolite evidences a broad adsorption band of medium intensity in the 3800-3000 cm -1 region, this adsorption is produced by the hydroxyl groups (OH) present in the sample.In particular, the OH stretching modes at wavenumber of 3420 cm -1 (with a shoulder at 3236 cm -1 ) and that at wavenumber of 3610 cm -1 should belong to two different Brönsted sites, which could be Si-OH and Al-OH, respectively.The weak and sharp peak at 1628 cm -1 belongs to bending vibration of water molecules trapped into the zeolite microporosity.The FT-IR spectrum of natural zeolite sample in the 1300-600 cm -1 region is generated by the internal vibration of the framework TO4 (T = Si, Al) tetrahedron, the primary building units (PBU) in all zeolite 3D-frameworks.The strongest vibration in this region is assigned to a T-O stretch involving motion primarily associated with oxygen atoms.The broad band around 1018 cm -1 has been assigned to the asymmetric stretching of SiO4 tetrahedra (←OT→←O).The adsorption peak at 800 cm -1 should correspond to the symmetric stretching vibration of SiO4 groups (→←OTO→←).A typical TGA-thermogram for the natural zeolite sample heated under fluxing nitrogen is shown in Fig. 5.The dashed red lines are extensions of linear portions of the curve, which contains three inflection points of transitions.According to the literature [13], the low temperature portion (below 100°C) of the curve represents fast desorption of water from the surface of the grains in the powdered sample.The middle portion, between 200°C and 300°C, represents the slow desorption of internally adsorbed water, usually referred to as 'loosely-bound water'.The high temperature portion, from 450°C and 500°C, represents the desorption of a small amount of internally adsorbed water, usually referred to as 'tightly-bound water'.From 500°C to 800°C the weight of the zeolite sample was practically constant.Therefore, according to these TGA results: (i) this natural zeolite is a slightly hygroscopic material, (ii) the percentage of water present in the sample was of ca.10% by weight, and (iii) this water can be completely removed from the sample by heating it above 600°C.

B. Electrical Measurements
The electrical properties of clinoptilolite-K were preliminarily studied in order to establish: (i) the electrical behavior of this material and (ii) the frequency and voltage conditions adequate for kinetic monitoring of water adsorption and desorption processes.The behavior of conductance, G, for the clinoptilolite-K sample (at equilibrium with a 75% moisture atmosphere) with angular frequency (2f), is shown in Fig. 6a for a 100Hz-10kHz frequency range.As visible, the logarithm of conductance increases linearly with logarithm of angular frequency, and therefore the conductance seems to follow the well-known "Universal Power Law" (i.e., log(G)=log(A)-n•log() [14]), with n=0.79 and A=-2.61 typically observed with ceramics.The impedance spectrum of the same sample is shown in Fig. 6b, according to this curve profile, the clinoptilolite-K sample has a reactive nature (its equivalent circuit is a RC series model, with: C~490pF and R~100k).Because the reactive contribution to the total impedance rapidly reduces with increasing of frequency, the sample reactance (XC=1/•C) can be considered as negligible (i.e., Z=R) at frequencies higher than 5kHz.As a consequence, while at low and medium frequencies the applied electric field causes both electric transport in the material and sample/electrode interface polarization, at high frequencies (e.g., above 5kHz) all produced hydrated cations are involved only in the electrical transport, and, consequently, the measured electrical current can be considered as directly proportional to the concentration of hydrated cations.This experimental condition is of a fundamental importance for the electrical monitoring of the cation hydration/dehydration reactions.
Fig. 6 Conductance spectrum of clinoptilolite-K sample (75% of humidity) in a log-log scale (a), and impedance spectrum of the same sample (b).At frequencies higher than 5kHz impedance is practically coincident with sample resistance.
The I-V characteristic for this clinoptilolite-K sample, at equilibrium with a 75% humidity atmosphere, is shown in Fig. 7.As visible, using a sinusoidal waveform with a frequency of 5kHz, this material behaves like an ohmic conductor up to a voltage of 7Veff (20Vpp), and its surface resistance is of 93k.

C. Kinetic Investigation of Water Adsorption/Desorption Process
A time-resolved study of clinoptilolite-K electrical conductivity, based on a high frequency (5kHz) sinusoidal signal applied to the sample, during the hydration and dehydration processes can be used to clarify the mechanism involved in the water adsorption/desorption.
According to the literature [4], the adsorption characteristics of zeolites are strongly dependent upon their cation composition and both the equilibrium and kinetic properties of adsorption can be altered by ion-exchange [4].For such a reason, cations should be involved in the water adsorption reaction.With good approximation, cations involved in the water adsorption kinetics are principally those present in the supercages because they form channels.In the case of clinoptilolite-K, potassium cations are contained in the supercages [15] and therefore they should play a leading role in the water adsorption process.The process of hydration of cations present on the open porosity walls of zeolite (supercages) can be chemically represented by the addition of one or more water molecules to the cation to transform it to a solvated cation.For a generic Me + cation (in this case, K + ), the hydration reaction is the following: Differently from the unsolvated cations, hydrated cations may move under the effect of an electric field, and therefore the measurement of the current intensity relative variation (ΔI/I0) during the hydration process can provide useful kinetic information on the solvated cation formation reaction, provided that the voltage applied to the sample is constant during the full experiment (to avoid changes in the cation speed).In fact, from the law of direct proportionality between current density and carrier (hydrated cations) concentration (J=A•[Me(OH2) + ], with A=q•e•v, q is the number of charges on the cation, e is the elementary charge (1.6x10 -19 C), and v is the cation speed [16]) it follows that the temporal evolution of the relative current density increase is coincident with relative increase of the hydrated cations concentration: The clinoptilolite-K electrical conductivity, and therefore the possibility for cations to move, was experimentally found to be promptly affected by the exposition to wet atmosphere, thus confirming that cations are involved in the water adsorption mechanism, according to the reaction scheme (1).In addition, a linear increase of the hydrated cation concentration was found (see Fig. 8) by measuring the current relative variation in an already hydrated/dehydrated sample of clinoptilolite-K, exposed to 75% moisture (sinusoidal signal with a frequency of 5kHz and amplitude of 20Vpp).Only the first hydration of a never electrically tested sample showed a diffusive temporal behavior (parabolic) of current intensity (probably because of the concomitant water absorption on acid OH groups), which was not observed in all the successive hydrations of the same sample.This experimental behavior is compatible only with a pseudo zero-order kinetic reaction.Considering that the water content in the environment is a constant (equal to 75% by weight for wet NaCl), if also the concentration of unsolvated cations is constant during the time, it follows that whatever the kinetic order relative to these dry carriers (n) is, the reaction has a pseudo-zero order.In particular, the quantity [Me + ] n can be considered as a constant because at beginning of the reaction the amount of cations contained in the material is very large and therefore the kinetic expression simplifies as follows: , where the quantity [Me(OH2) + ] has been neglected.Actually, ( 1) is a gas-solid heterogeneous phase reaction and the initial non-hydrated cation concentration into the clinoptilolite channels is a characteristic constant of the material and it depends on its Si/Al ratio.Thus, by separation of variables and integrating the differential equation between 0 and t, it results: from which: Obviously, with progress of clinoptilolite hydration, the linear behavior goes slowly to saturation because the nonhydrated cations terminate, and it takes place when the addition of further water molecules do not influence the cation mobility.This experimentally observed pseudo zero-order kinetic control suggests a reaction rate independent of the water and the unsolvated extra-framework cations concentrations.The independence of reaction rate on the water concentration is due to the sample placement in a constant humidity environment.Whereas, the observed independence of reaction rate on the unsolvated extra-framework cation concentration can be explained on the basis of a quite high initial concentration of unsolvated cations.According to these kinetic results, probably the water molecules are caught by the strong electric field of cations and then quickly organize between the cations surface (cation site) and the water adsorption sites present in the channels of the 3D-framework located in the surroundings of the cations.Then, more water molecules, catch by the cations electric field, progressively distribute between the cation and the four water sites surrounding that cation site in the 3Dframework channels, because much more effective cationdipole and short range interactions (i.e., hydrogen bridges among water and oxygen atoms) become possible.Such adsorption model is corroborated by the fact that: (i) the cation electric field is much stronger than the total electric moment of Si-O dipoles in the 3D-framework, (ii) the alkaline and alkaline earth cations cannot coordinate water molecules by dative bonds, and (iii) water molecules may interact with oxygen atoms in the 3D-framework by intensive hydrogen bonds.Therefore, the complete hydration reaction scheme becomes: Me + + H2O(g) → Me(OH2) + (slow) Me(OH2) + → Me + -H2O(ad) (fast) It must be pointed out that, depending on the supercage hydration state (more or less water molecules occupying the water sites), different freedom levels are possible for the extra-framework cations.In particular, cations close to empty water sites have a high charge density (one or two charges in the ionic volume) and simultaneously interact with the partial negative charges on the aluminum and two oxygen atoms (see Fig. 9).Such multiple and intensive electrostatic interactions strongly limits movement of these cations under an alternate electric field.Differently, cations with water molecules in the surrounding water sites are quite free to move and may undergo the effect of the AC electric field.In fact, the cation-site electrostatic interaction is much lower because distance among different charges increases.Finally, the clinoptilolite-K hydration may deeply modify the electrical behavior of this material under an AC electric field.
The inverse reaction, which is the spontaneous desorption of water molecules adsorbed on clinoptilolite-K in presence of activated silica gel, was monitored by using the same electrical technique, based on recording the temporal variation of current intensity inside the material (sinusoidal wave with a frequency of 5kHz and an amplitude of 20Vpp).If cations have a role also in the water desorption it is possible to write the following reaction scheme: Me(OH2) + → Me + + H2O(g) (6) In this case, water molecules first move from the cation environment (water sites) to the cation surface (fast process) and then desorbs from the cation (slow process).The measured current intensity is proportional to the total concentration of hydrated cations present in the clinoptilolite-K sample.In fact, there is always a small fraction of hydrated cations in clinoptilolite-K that cannot promptly desorb water molecules (water molecules trapped in sodalite cages, for example, cannot be promptly released to the environment).Therefore, for a kinetic analysis based on the measurement of current intensity, the kinetic expression needs to be written in terms of the total concentration of hydrated cations (that is, Ctot(t)=C(t)+Ceq) because Ctot(t) is related to the current intensity.According to the following equation: C(t) = Ctot(t) -Ceq (7) and for the definition of reaction rate: v = -dC(t)/dt = -dCtot(t)/dt (8) because Ceq does not change with time.Now, if the water desorption reaction (equation ( 6)) follows a first-order kinetics, the rate expression is the following: and it can be rewritten as: that gives by integration: and it can be also rewritten in terms of current intensity in the following form: ln[(I(t)-Ieq)/(I0-Ieq)] = -k•t (12) By using this equation, it is possible to measure the kinetic constant (k) for the cations dehydration reaction by using high-frequency AC current intensity measurements (see Fig. 10).In particular, it was found a value for k of 0.00414 s -1 for dehydration in presence of activated silica gel.Fig. 10.Temporal evolution of quantity in (12) for the natural clinoptilolite-K dehydration in presence of activated silicagel (pure dehydration).
When the water desorption process is performed simply in air, the inverse reaction (hydration) becomes possible, consequently the reaction kinetics is still of the first-order (see Fig. 11) but the kinetic constant has a lower value.In particular, the kinetic analysis for dehydration in air gives a kinetic constant value of 0.00218 s -1 , and therefore the dehydration reaction was twice faster in presence of activated silica gel.According to the achieved results, water molecules are adsorbed on the surface of channels in the surroundings of extra-framework cations, and their presence allows cation mobility under an applied AC electric field.Water adsorption/desorption processes are promoted by the extraframework cations, that participate to these reactions, in fact they capture the water molecules by long range cationdipole forces and then share these molecules with the walls of the 3D-framework channels where they may form short range hydrogen bonds with partially charged oxygen atoms.Cations may promote also water desorption, because when the surface of cation is free from water molecules, the cation electric field is not shielded.Therefore, the cation electric field act on the electric dipoles of adjacent water molecules, adsorbed on the surface of channels close to the cation site, attracting them to the cation surface.Then, these molecules are released to the environment because they are not chemically bonded to the cation surface and are in equilibrium with environmental moisture.
Owing to the significant ion movement, repeated electrical treatments in wet conditions and at high frequency and voltage were able to modify the zeolite porosity.Probably, the application of an alternated electric field caused an inner erosion of 3D-framework with formation of a mesoporosity in the original microporous structure.In fact, a modification of the original electrical behavior was observed in repeated testing of the same zeolite samples.With formation of a bimodal porosity (micro/mesoporosity), all cations present in the material participate to the transport mechanism and during the electrical monitoring of the adsorption process the original linear behavior (single carrier conduction) was replaced by several linear steps (multi-carrier conduction).

IV. CONCLUSION
Electrical monitoring of isothermal adsorption/desorption of water on natural clinoptilolite-K samples has confirmed the role of cations in the process of hydration/dehydration.In particular, the kinetic of water adsorption and desorption reactions has been studied by a novel technique based on monitoring the relative intensity variation of a highfrequency AC current, moving in a sample exposed to wet and dry environmental conditions, respectively.The sample hydration reaction has been found to follow a pseudo-zero order kinetics, while dehydration (in air or in presence of silica gel) follows a first order kinetics.The analysis of these kinetic results obtained for the adsorption and desorption reactions allows to affirm that cations should promote both reactions.In particular, water molecules should be catch by the intensive cation electric fields and then placed in the 3Dframework channels just in the surroundings of the cations, to form bridges between cation and the negatively charged oxygen atoms of the cation site.Such an organization of water molecules in the cation site could maximize dipoledipole and dipole-cation interactions which are active in cationic sites.

Fig. 3 .
Fig. 3. SEM-micrograph of the natural zeolite sample (a) and EDS spectrum (b).The analysis by Energy Dispersive X-Ray Spectrometry

Fig. 7 .
Fig. 7. I-V characteristic of the natural clinoptilolite-K sample at a frequency of 5kHz.

Fig. 9 .
Fig. 9. Structure of the ionic sites present in the dry clinoptilolite.

Fig. 11 .
Fig. 11.Temporal evolution of quantity in (12) for the natural clinoptilolite-K dehydration in air (dehydration in presence of slight humidity).

TABLE I :
PERCENTAGE OF CATIONS CONTAINED IN THE ZEOLITE SAMPLE