Analysis for the Sorption Kinetics of Ag Nanoparticles on Natural Clinoptilolite

1Centro de Fı́sica Aplicada y Tecnoloǵıa Avanzada, Universidad Nacional Autónoma de México, Boulevard Juriquilla 3001, 76230 Santiago de Querétaro, QRO, Mexico 2Instituto de Ciencias Biomédicas, Universidad Autónoma de Ciudad Juárez, Envolvente del PRONAF y Estocolmo s/n, 32300 Ciudad Juárez, CHIH, Mexico 3Instituto de Investigaciones Metalúrgicas, UMSNH, Edificio U, Ciudad Universitaria, 58000 Morelia, MICH, Mexico


Introduction
Natural zeolites have been extensively studied due to their structural characteristics and physicochemical properties.These minerals present great porosity and a significant cation-exchange capacity [1].Because of this, natural zeolites offer a wide range of applications; one of the most notable is the removal of contaminant ions in water and soil [2,3].It is reported that natural zeolites, specifically clinoptilolite, have been used in the removal of Mn, Cu, Ar, Pd, Cd, and Zn [4][5][6].However, it is important to study the rate of adsorption of these species, in order to provide the optimum conditions for the adsorption process.The nature of the sorption process will depend on physical or chemical characteristics of the adsorbent systems and the system conditions.The most commonly used kinetic expressions to explain the solid/liquid adsorption processes are the pseudo-first-order kinetics and pseudo-second-order kinetic model [7][8][9][10].A good number of works were reported where the modifications of these natural clays with nanoparticles were produced to carry out the adsorption of metals from aqueous solutions [11,12].These minerals offer low cost effective supports for nanoscale entities.The reactivity of nanosized zero valent metals like Ag and the high cation-exchange capacity of the zeolites represent a potential alternative for remediation applications [5,11].However, specific studies about kinetics of adsorption of nanoparticles on clinoptilolite have not been explored thoroughly.In this research, the chemical characterization and the synthesis methodology of the composite of silver nanoparticles AgNPs-zeolite are presented.Additionally, we present a detailed analysis of the kinetics of adsorption of Ag nanoparticles in the clinoptilolite, where the study of the adsorption efficiency under optimized conditions will be carried out and subsequently it will be evaluated in relation to the removal of heavy metals and their antibacterial effects.

Experimental Procedure
The zeolite type clinoptilolite was collected from Oaxaca, Mexico.It was milled and sieved to −120 +60 mesh (2 mm).Subsequently, it was washed with deionized water to remove any impurities and dried at 80 ∘ C for 24 hours.Ag nanoparticles solutions were previously prepared according to the chemical reduction method reported in previous work [13].The composites preparation was carried out by immersing of 1 g of zeolite in 1 mL of Ag nanoparticles with concentrations of 1, 2, 3, and 4 mg L −1 .Each sample was magnetically stirred for different times in intervals of 30 min from 0 to 180 min.In order to identify the Ag structure in the composite, DRX patterns were obtained using a Rigaku Ultima IV diffractometer.The experimental data of the adsorption behavior in zeolites and their chemical characterization were conducted by an atomic absorption spectrophotometry (AAS) technique using Perkin Elmer 3100 equipment.In the case of chemical characterization, the sample was previously subjected to a digestion and dilution process using lithium metaborate, LiBO 2 .The characterization was complemented by an energy dispersive (EDS) analysis and the AgNPs distribution on the zeolite was studied using a Philips Tecnai F20 transmission electron microscope with a field-emission gun attachment.

Results and Discussion
The natural zeolite obtained from Oaxaca was structurally characterized and presented in a recent paper [13].However, the chemical characterization of the material is just presented in the following lines of text.Figure 1(a) shows a chemical analysis obtained by EDX, which identified typical elements as Si, Al, O, Mn, K, and Ca present in the clinoptilolite.
Figures 1(b), 1(c), and 1(d) show bright field images corresponding to the morphologies of the clinoptilolite, mordenite, and feldspars phases reported on the structural characterization of the mineral [13].
It is important to mention that the chemical analysis was obtained by EDX technique.However, experimentally, the chemical characterization of the zeolite was performed using the metaborate method as digestion and dilution mechanism.Subsequently, the absorption spectrophotometry   [14].Therefore, it can be affirmed that zeolite exhibits an excellent behavior in cation-exchange capacity.This result will be evaluated through the study of the adsorption kinetics presented in this research.
Figure 2(a) shows the experimental XRD pattern corresponding to the zeolite.The main phase present in the natural zeolite is the clinoptilolite; the higher intensities observed are attributed to the mineralogical phase.In Figure 2(b), the simulated XRD can be observed where concordance between the experimental and simulated intensities is verified.Therefore, the three phases (clinoptilolite, mordenite, and feldspar) proposed for the XRD pattern simulation are present in the zeolite [13].Additionally, individual XRD patterns for each phase are presented in Figures 2(c)-2(e).
Figure 3 shows the AgNPs adsorption by clinoptilolite.The highest rates of adsorption are observed in sample with concentration of 2 mg L −1 .The value of maximum adsorption is 90% and is conducted in a time of 150 min.In subsequent values to 150 min and 4 mg L −1 , no relevant changes are presented in the adsorption rates.This result suggests that zeolite saturation depends directly on the effective diameter of the nanoparticles and consequently the volume of AgNPs used for adsorption.Some theoretical models are presented below to investigate the mechanism of AgNPs sorption onto clinoptilolite.

Theoretical Models
The theoretical models reveal the solid/liquid adsorption processes.These processes are generally due to a cation-exchange reaction between adsorbent and adsorbate related to masstransport processes or particles diffusion in macropores or micropores [15].Pseudo-first-order, pseudo-second-order, Elovich, and intraparticle diffusion models are presented to describe and support the adsorption kinetic behavior of AgNPs.

Lagergren Pseudo-First-Order Model.
The pseudo-firstorder adsorption model is defined by the Lagergren equation [16,17]: where   and   are the amounts of AgNPs sorbed at equilibrium and a given time .The pseudo-first-order adsorption rate coefficient is  1 (min −1 ).Solving the differential equation for boundary conditions,  = 0,   = 0,  = , and   =   , this equation can be expressed in the lineal form as ln (  −   ) = ln   − . (2)

Pseudo-Second-Order Model.
The pseudo-second-order kinetic equation may be expressed as Using the integration limits employed in the first-order equation, this equation can be written in the following way [18]: In this case,  2 represent the rate constant for pseudo-secondorder sorption (g mg −1 min −1 ).

Elovich
Model.This mathematical model has been widely used in description of the kinetics of adsorption of a solute in a liquid phase from a solid sorbent.The mathematical expression that governs the behavior of this model is Integrating (3) and using the boundary conditions pseudofirst-order model, Elovich or Roginsky and Zeldovich equation would be [19] where  is the initial adsorption rate (mg⋅g −1 ⋅min) and  is the desorption constant (g⋅mg −1 ).In this case,  is described in terms of surface area covered and the activation energy derived from the chemisorption by the adsorbent.

Intraparticle Diffusion Model. The other kinetic equation is the intraparticle diffusion model, given by
where   describes the amount per unit mass of nanoparticles adsorbed by the zeolite at time ,   (mg⋅g −1 ⋅min −0.5 ) is the rate constant of intraparticle diffusion, and   is the intercept [20,21].Figure 4(a) shows the Lagergren kinetics adsorption model of AgNPs into clinoptilolite.In this case, a significant correlation between the experimental results and  2 factor is observed (Table 2).The highest correlation values for the first-order Lagergren model are presented in the concentrations of 3 and 4 mg L −1 .This result supports the experimental values shown in Figure 4 where the adsorption efficiency values are similar in both cases and their behavior is linear.Of the employed theoretical models, the pseudo-second-order model (Figure 4(b)) presents the highest correlation values which are of the order of 0.99.Therefore, the second-order model describes with greater accuracy the adsorption process.This represents chemisorption or chemical adsorption due to the formation of chemical bonds between adsorbent and adsorbate in a monolayer on the surface [16,22].
In this sense, the Elovich model (Figure 4(c)) also represents a chemisorption phenomenon.Nonetheless, this model is applied to systems with heterogeneous surfaces and different activation energies [7,23].Consequently, the correlation coefficients obtained by the Elovich model (0.88164 <  2 < 0.93829) exhibit less agreement with the experimental values obtained.The intraparticle diffusion model (Figure 4(d)) presents  2 values in the range of 0.911 <  2 < 0.9523 (Table 2).This result can be attributed to the homogeneous porous structure of the zeolite.Therefore, this model also describes the transport and diffusion mechanism of solute through the internal structure of the pores of adsorbent (clinoptilolite).
Figures 5(a) and 5(b) show a bright field TEM image, in which the homogeneous distribution of the AgNPs on the zeolite surface can be appreciated.This result verifies that the chemisorption phenomenon describes the adsorption process carried out by the mineral.This is due to the AgNPs interaction with the active sites of the zeolite and the formation of a unimolecular layer uniformly distributed on the surface of the zeolite is observed (Figure 4(c)).Finally, Figure 5(d) shows the chemical analysis made from the composite, in which the incorporation of silver nanoparticles   is found.In this sense, it can be deduced that the process of impregnation of the particles was carried out efficiently since the adsorbate was distributed throughout the zeolite surface.

Conclusions
The interaction between the zeolite and the AgNPs is described through a chemisorption process.In this research, the characteristics of this process are observed and verified by adsorption theoretical models.Pseudo-second-order and intraparticle diffusion models present higher correlation factors with respect to the experimental values obtained.This fact supports the formation of chemical bonds between adsorbent and adsorbate in a monolayer on the zeolite surface.The homogeneous mineral porosity promotes this behavior.On the other hand, high adsorption efficiencies observed verified the affinity of the zeolite and the AgNPs.Minor particle concentrations (1 and 2 mg L −1 ) present high adsorption rates, because the pore saturation is lower, promoting the adsorption phenomenon.

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Advances in Condensed Matter Physics

Figure 2 :
Figure 2: XRD patterns of the natural zeolite obtained (a) experimentally and (b) by simulation of the crystalline phases.(c)-(e) Simulated XRD patterns of the clinoptilolite, mordenite, and feldspar, respectively.

Figure 3 :
Figure 3: Efficiency adsorption plot of AgNPs on the zeolite.

Figure 5 :
Figure 5: ((a) and (b)) Bright field TEM and (c) STEM images corresponding to the AgNPs distribution onto the zeolite surface.(d) EDS analysis of the zeolite-AgNPs material.

Table 1 :
Chemical composition of natural zeolite.