Surface Potential as the Parameter of the Effective Control of Sorption Properties of Zeolites

doi:10.21203/rs.3.rs-4828144/v1

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Surface Potential as the Parameter of the Effective Control of Sorption Properties of Zeolites

https://doi.org/10.21203/rs.3.rs-4828144/v1

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The natural sorbents are undergoing physical and chemical modification for the improvement of their sorption and ion-exchange properties. In the presented paper the effect of physical modification on the sorption ability of zeolites has been studied and the methodology of the evaluation of the sorption ability of physically modified zeolites has been suggested based on the value of surface potential. The physical modification of zeolites has been carried out by the influence of infrared, ultraviolet, X-ray waves and by the beam of electrons. It has been established that for sorption of metals’ ions-isotopes the more effective is modification by the beam of electrons. By the computer simulation it has been shown that the surface potential of the sorbent is the summary parameter for the evaluation of the modification effectiveness and the sorption ability of zeolites and that it is related to topography of the distribution of thepotential on the surface.

Zeolit

beam of electrons

modification

surface potential

sorption

Zeolites, having unique properties, are widely used in different industries and techniques. As for sorbents, theyare used for purification of wastewaters of different sources, including purification of radioactive wastewaters [1, 2, 15, 17–19]. In order to improve the sorption properties of natural zeolites, they undergo chemical modification and influencing by physical methods. For the modification of zeolites chemically the acidic and basic treatment [3, 4, 20, 21] and modification by surfactants [5, 13, 14] are widely used. Among physical treatment methods the thermal treatment is more common which is mainly used in complex with the chemical modification [13, 20, 22, 23]. The works in the literature about other methods of physical modification of zeolites are rare. In the [10] has been established the effectiveness of electromagnetic influence as the method of physical modification of the natural sorbents. Also, the sorption properties of the zeolites, activated by ultraviolet rays and radiation influence, have been studied [7, 12, 16, 24]. In that works the sorption properties of the zeolites have been evaluated based on the experimentally obtained results of the metals sorption.

In the presented paper the physical parameters identified, which makes possible to evaluate the effectiveness of physical modification of zeolite, as well as other natural sorbents, for the sorption of the ion-isotopes of the metals. With this goal the fact that the carcass of the zeolite has negative charge and the positive charge is in the indoor of tetrahedron has been applied [3]. It means that the surface potential of the zeolite is also negative, which is additional factor for the effectiveness sorption of the metals positive ions.

The rationality of the radiation influences consists inidentifying of the optimal regimes of treatment at which the targeted changes of zeolites occur. During works the possibility of the using of the treatment by the accelerated electrons for increasing the sorption properties of the samples has been studied. As the source of the radiation influences of the electrons the accelerator of the electrons of the electrograph EG-100 has been used.

The simplest variant of the widely known Kelvin’s method of the measurement of potentials differences has been used [6]. The principle of this method is as follows: in the gap between two galvanic connected elements, which form flat condensate, the electrical field rises, the value of which is determined as the differences of electrodes’ surface potentials. At the change of the condensate gap size the voltage of the field is changed, which leads to the appearance of the current in the outer conductor, to which are connected the electrodes. By turning on the voltage source to the open circuit of the capacitor and regulating it, it is possible to ensure that the current disappears when one of the electrodes is moved. The required quantity of the contacted difference of potentials has been obtained by measuring of the voltage on the source at the zero current of the capacitor.

The dynamic flat capacitor is formed by the fixed studied sample and standard vibrating electrode. The mechanical calibration at the 1000 Hz is transmitted to the vibrating electrode by a vibrator, which is connected with generator of the audio frequency. The constant voltage on the dynamic capacitor is regulated by resistor, which is connected with the two sources of the constant electrodriving force which makes possible to change the polarity of the voltage. The modulation voltage, with the frequency of 30 Hz, is supplied to the dynamic capacitor with the balancing voltage regulator, which is connected to the audio frequency generator, and is measured with a voltmeter. One of the known variations of the Kelvin’s method scheme are given in the Fig. 1 [6], which is adapted for the measuring of the surface potential of the studied samples.

Amplitude-modulating signal, arising in the chain of the dynamic capacitor, is amplified by a linear amplifier that converts the dynamic capacitor current into voltage, and supplies to the Y-input of the vertical deflection of the oscilloscope beam. The scanning voltage is supplied to the X-input of the horizontal deflection of the oscillograph beam from a voltage regulator connected to the generator.

In the above-mentioned experimental method, the used probe has a diameter at times larger than the diameter of the zeolite pores. Thus, the measured potential is integral which doesn’t make possible precise determination the distribution of the potential change on the surface. For the precise determination of the potential, it is necessary to have a probe with a diameter comparable with the diameter of the zeolite pores. As we do not have the appropriate probe, computer simulation has been used for the evaluation of the distribution of the potential change on the surface.

The program Hyperchem-8.5, methods of the geometric optimization and molecular dynamics (MD) have been applied. The pair-additive potentials have been considered as a force field, which is widely used for simulation of the zeolite by the MD method [9, 11]. With this goal the model of the fragment of the zeolite has been structured (Fig. 2). The clinoptilolite of the Noyemberyan deposit, Armenia, has been used as zeolite.

For the evaluation of the contribution of the surface potential in the process of sorption by the usage of the modified zeolite, natural zeolite sample was being irradiated by IR, UV, X-ray rays and electrons beam. Based on the determined surface potential valueit has been shown that the surface potential of the zeolite, by staying negative, slightly increases in the absolute values due to IR and UV irradiation. The observed growth of the potential at the UV irradiation is 1.5-2 times higher than in case of the IR irradiation. It can be explained by the fact that IR rays, having the energy comparable with the energy of the hydrogen bond, destructed those bonds on the surface of the solid, due to which the surface partially exempt by polarized water and the screening of the surface potential by the water layer is reduced. The quantum energy of the UV is enough not only for the destruction of the hydrogen bonds but also for the separation of OH group and deviation of metal-ion from its position in the pore of zeolite. This is the reason of the greatest increase of the surface potential of the zeolite in case of the UV irradiation compared with the IR influence. Another behavior has been observed in case of the X-ray irradiation: the surface potential decreases which can be explained by the high energy of the X-ray quantum. It leads to the separation of the electron from the solid surface, due to which the negative potential of the surface decreases. Also the possibility of the influence of the polarization of the studied samples on the process of the reduction of the surface potential is not excluded. The similar results have been also obtained by other authors [2, 12, 18, 24]. The reduction of the sorption capacity can be explained by the decreasing of the surface potential of the zeolite due to the X-ray irradiation which is related to the separation of OH group from the surface of the zeolite and ionization of the surface. In case of irradiation of the surface by the beam of electrons the potential is increased. The reasons of the increasing of the surface potential can be different-implantation of the electrons, structural changes of zeolite, the local heating of the sample due to the adsorption of the accelerated electrons, etc. In the result of the studies, it has been established that the accelerated electrons have significant effect on the granulometric composition of the sample, which depends on the dose of the irradiation: the maximum changes are observed in the high doses range (more than 5 kGr). The initial processes, occurring by the influence of the electrons beam, mainly depend on parameters of the beam and conditions of the irradiation. The defining parameters are the density of the current and the energy of the electrons beam. The increase of the surface potential depends on the cross section of the electron beam and the irradiation exposure time. In case of the low adsorption of energy (up to 1 kGr) the main result of the treatment by accelerated electrons is the radiation charging of the sample surface without significance changes of the physical properties of the zeolites. At the increasing the quantity of the energy adsorption up to 10 kGr the cumulation of the charge is possible which can lead to the power outages. The charge accumulation process occurs differently for strong and weak conductive samples. In case of the strong conductors due to the established equilibrium between in flow and drain of charge, the accumulation of charge doesn’t occur. In the weak conductors the charge is cumulating and its excess is discharged through the breakdown channels. The formation of breakdown channels is one of the important features of the treatment by the accelerated electrons and therefore, also the appearance of the cracks along the boundaries of grain fusion, which leads to the fragmentation of the zeolite. The studies have shown that at the treatment of zeolite by the accelerated electrons its surface properties- electrochemical potential, composition of the sorption layer and so on change.

Thus, it is possible to regulate the sorption properties of the zeolite by the surface potential and reaching the adsorption high indicators. From the above-mentionedit follows that the surface potential can be the effective parameter for the evaluation of the degree of the heavy metals ions-isotopes removing due to the rays modification.

For the effective modification of the sorbents not only the sign and value of the surface potential is important, but also its distribution on the surface.

The surface potential near the pores has been evaluated based on computer simulation. As the sensitive parameter in terms of the influences the changes of the zeolite’s carcass charge have been observed. By this approach the equipotential lines on the surface have been obtained. Figure 3 gives the distribution of the potential of the non-modified zeolite, from which it is seenthat the potential of the electrical field is greater near the pores edge.

The distribution of the equipotential of the surface of the fragment of the model of the zeolite modified by electrons beam is presented in the Fig. 4. From the obtained data it can be suggested that the voltage of the electrical field created the trap for the positive ions and the ion, falling into that field, falls into the pores with the probability proportional to the value of the filed potential.

It leads to the increasing of the negative charge of the carcass, thus, by staying negative, the potential of the surface increases, which, in its turn, leads to the positive ions adsorption increases. It is possible to insure that by studying the spectrum of the zeolite before and after irradiation by electrons (Fig. 5,6).

Figure 5 Electronic spectrum of the non-modified zeolite model fragment surface

The electron orbitals stay maximum replaced, but the energy of the electron increases in absolute value, which increase the probability of the occurring of the redox and donor-acceptor processes, and therefore, also the probability of the ion-exchange. On the other hand, the high population of the orbitals leads to the increase of the surface potential (Fig. 4): the lines of the surface potential become thicken which indicates the increasing of the voltage of the electrical field near the pores. The obtained data indicates that the potential near the pores is higher than the measured potential which leads to the increasing of the field strength near the pores by several orders, compared with other parts of the sample. As the potential is changed from 0.1 to 1 V and the size of the pores is measured in nanometers, the near the pores reaches about 10⁷÷10⁸V/m. In that case these regions act as traps of the positive ions by that increasing the probability of the effectiveness of sorption and ion-exchange.

Based on the obtained data it can be concluded that the surface potential can be the effective parameter for the evaluation of the sorption ability of the positive ions due to the physical modification of the zeolites.

It has been obtained, that

For the removing of positive charged ions more efficient are zeolites, which are modified by the electrons beam.

In case of the optimal distribution of the potential on the surface the pores of the zeolites act as traps of the metals ions.

The value of the surface potential can be used as effective parameter for the evaluation of the modification of natural sorbents by X-ray, IR, UV rays and electron beam.

Funding : no funding

Ethical approval:no applicable.

Consent to participate:all authors have read and agreed to the published version of the manuscript

Author Contribution

H.G. Badalyan was computer modelind end experimants of modification by electron beam and wrote the main manuscript.H.H. Sargsyan was omputer modelind end experimants of modification by IR, UV rays.L.R. Harutyunyan prepared figures 1-6 and take part in disscationR.S. Harutyunyan was computer modelind end experimants of modification by X-ray

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No competing interests reported.

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Surface Potential as the Parameter of the Effective Control of Sorption Properties of Zeolites

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