The dynamic behavior caused by hydro-adsorption process of materials based on a rich mineral clinoptilolite together with their acidic, basic and calcinated forms has been studied by the dynamic laser speckle (DLS) technique. We propose a modified Peleg’s equation to improve fitting of DLS data. Textural (BET), structural (XRD) and spectroscopic (FTIR) properties were also studied and compared. We demonstrate d that DLS was the most sensitive, simple and inexpensive method for comparing the performance of adsorptive materials with slightly modified surfaces. It also allowed the correlation with physicochemical properties.
The minerals based on aluminosilicates such as clays (kaolinite, montmorillonite) and zeolites (clinoptilolite) are abundant and inexpensive [
Natural and modified zeolites and their applications have been widely studied, so determination of textural and structural properties has been of great interest. Usually, the hygroscopic properties of the zeolites can be characterized by the well-known TG-DTA, XRD, FTIR and BET methods [
The present work addresses the implementation of dynamic laser speckle technique to the hydro-adsorption analysis of natural zeolite, clinoptilolite, and their acidic, basic and thermally treated forms. This technique can be very useful to estimate its hygroscopic capacity.
In this paper, experimental DLS data were fitted using the Hawkes and Flink [
In view of the versatile importance of the DLS technique, we here report its application together with physicochemical analysis such as XRD, FTIR and BET method.
The natural granular clinoptilolite zeolite used in this study was obtained from La Rioja (northwestern province of Argentine). A sample with the chemical composition given in
The natural zeolite was ground manually in order to obtain a granular form as a solid powder, then 1 g of the solid powder was treated with 10 mL of ammonium hydroxide (1 M) solution for basic modification and 10 mL of concentrated nitric acid (1 M) for the acid one. Both samples were stirred during 12 hours and heated at 70˚C.
Solid samples were separated from the solution by filtration and washed with distilled water several times till the effluent became neutral to pH paper.
On the other hand, two samples of 1g of original zeolite were thermally treated at 250˚C and 500˚C for 2 h in air. The zeolites (Z, in what follows) natural and the zeolites modified: Z-H+ (acid form), Z-NH4+ (basic form); Z-250 and Z-500 (zeolites heated at different temperatures). Characterization of original and treated zeolites was performed by Electronic microscopy using a Philips SEM 505 combined with semiquantitative analysis by energy dispersive X-ray analysis (EDS) by an analyzer EDAX 9100. Diagrams of X-ray diffraction powders (XRD) were registered using a Philips PW 1714 with a CuKα radiation and Ni filter from 2θ = 5˚ to 60˚. Infrared FTIR Spectra were obtained by a Equinox 50 FTIR equipment, from 4000 to 400 cm−1 wave-numbers. Surface areas and porosity of samples were determined by physical N2 adsorption at 77 K (BET method [
A typical granular interference pattern named “speckle” is observed [
If this surface presents some type of local movement, then the intensity pattern evolves in time. This phenomenon, known as “dynamic laser speckle” (DLS), can be observed in biological samples [
SiO2 | TiO2 | Al2O3 | Fe2O3 | FeO | MnO | MgO | CaO | Na2O | K2O | P2O5 | LOI |
---|---|---|---|---|---|---|---|---|---|---|---|
61.8 | 0.31 | 13.21 | 1.51 | 0.00 | 0.01 | 0.89 | 3.73 | 1.94 | 1.34 | 0.06 | 14.08 |
LOI: Loss on ignition.
We propose a DLS method to characterize the water adsorption of the zeolite. We employed a 10 mW He-Ne laser to illuminate the samples. Dynamic speckle pattern were recorded with a CCD camera connected to a frame grabber to digitize the images as shown in
We used Oulamara et al. method [
The speckle activity of the sample changes their intensity in the horizontal direction. Therefore, when a phenomenon shows low activity, time variations of the speckle pattern are slow and the THSP shows elongated shape. When the phenomenon is very active, the THSP resembles an ordinary speckle pattern. See
The moment of inertia of the co-occurrence matrix method [
where
In our case, the initial value of I decreases as the water is adsorbed by the zeolite surface until reaching the steady state, thus it is directly related to the adsorbed water amount.
For modeling the amount of adsorbed water on solids vs. time, different models have been reported [
Hawkes and Flink [
where
Azuara et al. [
In Equation (3a), WFL is the water lost by the foodstuff at time t,
Finally, Peleg [
where
In this paper, we try to use the expressions of the reported models applied to the data (moment of inertia) obtained by dynamic speckle technique.
Taking into account the above considerations, Equations (2)-(4) must be modified and turned into:
“
where “
So, in dynamic speckle technique we use the expression:
where
Azuara’s et al. model equation became:
where
Peleg’s model equation became:
where
Clinoptilolite used in this work is a heulandite-type, with chemical composition (Na,K)6(Al6Si30O72)∙20H2O previously reported [
The framework topology of this mineral consists of a two dimensional pore system formed by 8-ring channels linking together [
The cation exchange capacity (CEC) of this material has been previously determined and a value of 330 meq/100 g was found [
Chemical and physical properties of the zeolite can be modified with either inorganic basic or acid solutions. This fact mainly depends on the Si/Al ratio.
The acid treatment conduces to the alumination process from the alumino-silicate structure removing Al3+-ions progressively. On the other hand, an increase of the temperature or alkalinity reduces the silica content, due to dehydration processes and SiO2 dissolution. This treatment also promotes the process of ion exchange cation (Na+ or K+)-H+. In basic treatment using, amoniacal solution, the silanol groups surface (hydroxyl) SiO2∙OH, acquires a great concentration of -OH groups. Therefore, under this condition, the NH4+ cations can be easily exchanged by other ions like Na+ or K+. All these reactions produce chemical surface modifications that are evidenced by changes in the composition, in textural and structural properties, which could be analyzed by different physical-chemical analysis techniques.
Textural properties, surface area, volume and pores size, have been analyzed with nitrogen physic-sorption isotherms by BET model. The measure was carried out for pure and modified samples with acid, basic treatment included the two calcinated samples. The results found together with some SEM-EDS chemical data are reported in
The values obtained are typical for natural zeolites, however a noticeable change for zeolite calcinated at 500˚C was observed. In this case the SBET value was almost double and the pore size was lower than those the other samples.
Sample | SBET [m2/g] | Pore Vol. [cm3/g] | Pore Size [Å] | Na% | K% | Si/Al |
---|---|---|---|---|---|---|
Z-N | 8.04 [0.04] | 0.027 | 133.35 | 6.57 [0.67] | 2.62 [0.25] | 4.50 |
Z-H+ | 6.80 [0.03] | 0.021 | 123.09 | 2.20 [0.19] | 2.47 [0.90] | 5.33 |
Z-NH4+ | 8.76 [0.06] | 0.027 | 122.08 | 1.70 [0.36] | 1.08 [0.02] | 5.70 |
Z-250 | 8.86 [0.09] | 0.027 | 124.01 | 6.91 [0.21] | 2.58 [0.16] | 5.98 |
Z-500 | 12.80 [0.03] | 0.028 | 88.46 | 6.94 [0.29] | 2.36 [0.46] | 6.38 |
modes of internal tetrahedral. The bands at 1053 and 794 cm−1 are associated with the asymmetric and symmetric stretching modes of external linkages [
The spectra of the acid form of the zeolite show that the external asymmetric mode νa T-O-T is lightly shifted to higher wave numbers respect to the natural zeolite. In this spectrum two new bands appear. One of them, located at 1384 cm−1 is attributed to asymmetric stretching modes of the NO3 bonds from the surface nitrate group which was generated by the HNO3 treating. The other one, observed at 955 cm−1 is assigned to Si-OH stretching mode of typical Brønsted acid site.
The spectrum of Z-NH4+ shows in 3900 to 3200 cm−1 range, bands assigned to the N-H stretching due to the presence of NH4+ bonded to the surface. The band at 1632 cm-1 results from the superposition of the ammonium component and the mode corresponding to the bending vibrations of adsorbed H2O. The presence of ammonium FTIR bands suggest the adduct formation between the ion and Brønsted acid sites of the zeolite [
FTIR spectra of the samples do not show significant changes with the temperature in agreement with the X-ray diagrams. This fact is probably due to the high thermal stability of the structure [
Then, the experimental data were fitted using moment of inertia in Equations 6-8, and revealed the following values: a) Hawkes and Flink model [
From
Tentatively assignation | Z-N FTIR bands | Z-H+ FTIR bands | Z-NH4+ FTIR bands | Z-250 FTIR bands | Z-500 FTIR bands |
---|---|---|---|---|---|
ν OH | 3424 | 3436 | 3458 | 3469 | 3462 |
ν N-H | 3230 | 3240 | 3242 | ||
δ H2O | 1632 | 1633 | 1632 | 1639 | 1635 |
δ NO3 | 1384 | 1383 | |||
δ NH4 | 1401 | ||||
(a) νaT-O-T | 1206 | 1206 | 1207 | 1200 | 1158 |
(b)νaT-O-T | 1053 | 1080 | 1053 | 1061 | 1062 |
νsSi-OH | 955 | ||||
(a)νsT-O-T | 787 | 794 | 793 | 786 | 789 |
(b)νsT-O-T | 607 | 608 | 608 | 606 | 609 |
Network modes: “ring elongation” | 453 | 463 | 452 | 454 | 458 |
Ref.: (a) External modes; (b) Internal modes.
These mathematical approaches are equivalent and produced the same results with the experimental trend but with a high error data. This effect is probably due to the zeolite containing micro and small mesopores, which produce greater water adsorption rate, so the first-order equation did not fit the experimental results. Then, we propose a new approach modifying the equation of Peleg, extending to the second order:
where
Taking into account the good fit for Z-N, the method was also applied for Z-H+, Z-NH4+, Z-250 and Z-500 samples, in order to obtain the DLS activity (Figures 6 (a)-(d)). Data fitted to the experimental values are shown in
Derivation of the Equation (9), the I rate of change vs time was obtained.
The I rate of change for the Z-N is the highest comparing with the other samples. (see
However, the behavior is different for the other samples, especially for those thermally treated where the rate of change at small times is slow and saturation occurs at larger times for high temperature (Z-500). This effect is probably due to the decrease in the average pore size (see
Therefore, the reversible process will require the formation of new hydrogen-bridges to achieve rehydration of the pores. This process will be slower than for unheated samples. In previous report [
Parameters | Z-N | Z-H+ | Z-NH4+ | Z-250 | Z-500 |
---|---|---|---|---|---|
τ [s] | 25.830 | 31.048 | 50.294 | 87.561 | 160.221 |
1854.71 | 3184.48 | 5296.48 | 17900.51 | 73447.81 | |
−31.2984 | −57.6000 | −22.1900 | −64.6454 | −219.3426 | |
1.137 | 1.254 | 1.050 | 1.1061 | 1.2105 | |
189.5 | 700 | 2421 | 1229 | 256 |
revealed two endothermic peaks at 167 and 500˚C [
Regarding acid and base modified samples, the I rate of change behavior for Z-H+ resulted similar to Z-N. However, this value is slightly higher for Z-NH4+ (
For Z-NH4+, ammonium is also easily exchanged with Na+ and K+ ions, replacing the intra-network cations by “NH4OH” groups. This fact produces a steric effect in the pores and a slower rate of water molecules adsorption than the acid sites provided by Z-H+. This behavior is consistent with an intermediate speckle activity for Z-NH4+ (See
The dynamic speckle technique was employed in the study of natural zeolites and their thermally and chemically modified forms. This technique has been used for the first time for Zeolites material and resulted a very useful tool to compare the hydro-adsorption properties in different treatment Zeolites sample.
According the water adsorption process of the samples, the experimental results show the temporary evolution of the speckle patterns. The parameters of the optical configuration used to produce speckle have been fitted using an improved Peleg’s equation.
The good agreement between the experimental DLS results and calculated values can be considered as a potential low cost, non-destructive and simple method to study different materials of interest such as adsorbents or catalysts supports.
We are grateful to Lic. Mariela Theiler, Mrs. Graciela Valle and Eng. Edgardo Soto for their technical assistance.
The authors would like to thank the following institutions for funding this work:
CONICET (PIP 0003, 0771); CICPBA (Project 832/14) and Universidad Nacional de La Plata (Projects X606 and I172).
Mojica-Sepulveda, R.D., Mendoza-Herrera, L.J., Agosto, M.F., Grumel, E., Soria, D.B., Cabello, C.I. and Trivi, M. (2016) Hydro-Adsorption Study by Dynamic Laser Speckle of Natural Zeolite for Adsorbent and Fertilizer Applications. Advances in Chemical Engineering and Science, 6, 570-583. http://dx.doi.org/10.4236/aces.2016.65049