1. Introduction

The technologies for ammonia removal from wastewater are based on physicochemical and biochemical treatment methods [1]. One of these treatment methods is adsorption, which is a low-cost process. Different adsorbents, such as wheat straw biochars, pine sawdust or zeolites, can be effective in adsorbing ammonium in wastewater [2–12].

Several studies have reported on the adsorption of ammonium ions by natural or synthetic zeolitic material adsorbents as well [9–12]. Zeolite - aluminosilicate hydrate minerals with a porous, three-dimensional crystal structure are considered an excellent ion-exchange material because of their high selectivity for NH₄⁺ due to their microstructure [2]. An adsorbent of natural zeolites possesses a polar surface and is therefore able to attract ammonium ions quickly and effectively [1]. The removal of ammonia from water was carried out by using natural and synthetic zeolites. In this research three types of natural zeolites (clinoptilolite, mordenite and ferrierite), and synthetic zeolite A were used. The different forms of zeolites such as sodium, potassium and calcium forms were investigated [9]. It was concluded that natural zeolites show high selectivity for ammonium ions with respect to other monovalent ions despite the much higher theoretical exchange capacity of zeolite A. In the study 11, the clinoptilolite was fused with sodium hydroxide prior to a hydrothermal reaction, and it was transformed to a modified zeolite Na–Y. The results were acceptable, showing that modified zeolite Na–Y exhibited a much higher uptake capacity compared with that of clinoptilolite. At an initial concentration of 250 mg/L NH₄⁺, the ammonium ion uptake value of sample 2 was 19.29 mg/g NH₄⁺ adsorbed, while that for sample 1 was only 10.49 mg/g1 NH₄⁺ adsorbed.

The emergence of high-energy complexes in the adsorption of NaX zeolite ammonia is dependent on the interaction of ammonia molecules in the S_III' and S_II voids in two adjacent states, two abutting sodium cations. S_III' generates heat of adsorption on cations in the void, when the extrapolation of ammonia is Н⁺ equal to 110 kDj/mol, ~ 90 kDj / mol, relative to the Q_d curve to zero saturation [13-14].

In our research, the adsorption of ammonia molecules in various synthetic zeolites was carried out by the adsorption microcalorimetric method in a high-vacuum adsorption device, and the full thermodynamic properties of the adsorption processes were described [13-38].

2. Materials and Methods

The composition of the zeolite obtained for the study is Na₉₆(AlO₂)₉₆(SiO₂)₉₆. The adsorption-calorimetric tip used in this study allows to reveal the detailed mechanism of adsorption processes occurring in adsorbents and catalysts, as well as obtaining adsorbents and catalysts, moreover, obtaining high-precision molar thermodynamic characteristics.

Adsorption measurements and doses of adsorbate were performed using a universal adsorption device, in the working part of which only mercury valves were used and the valves were replaced with vacuum grease [39]. The device allows to dose the adsorbate by both gas-volume and liquid-volume methods. As a calorimeter, a modified DAK 1-1 calorimeter with high accuracy and reliability was used.

3. Result and Discussion

Ammonia adsorption isotherms to NaLSX zeolite were carried out in a volumetric manner at a temperature of 303 K, Figure 1 provides the results of the experiment (1) and the re-described characteristics (2) using the equation of the volumetric saturation theory of micropores. The isotherm does not change in the same plane as the pressure increases, but rather each of them reflects the transition of one type of centers to another. The isotherm (abscissa) axis consists of a logarithmic (ln), adsorption (N) (N-supervoid and 1/8 NH₃ molecule number of elementary cell) (ordinal) axis, which allows us to imagine the adsorption process over the entire pressure equilibrium range. In the adsorption of the three ammonia molecules, the isotherm line goes vertically at the initial filling of the zeolite micropores, where the isotherm ranges from ln(p/p°)=-18 to ln(p/p°)=-16,8, and the adsorption is N=3 C₆H₆/1/8 u.c. Na + cations in the S_III′ void rise sharply when adsorbed, then turns to the adsorption axis and grows to 6 NH₃/1/8 u.c (S_I’). In the adsorption after 6 NH₃/1/8 u.c, the isotherm 18 NH₃/1/8 u.c. increases with a slope to the adsorption axis and is partially saturated (S_II).

Figure 1. Isotherm of ammonia adsorption in zeolite NаLSX at 303 K. 1- experimental data 2 – calculated data using the theory of volumetric micropore occupancy (VMOT)

Adsorption isotherm of ammonia in the NаLSX zeolites is satisfactorily described by three-term equation of the theory of volumetric micropore occupancy (VMOT) [40,41]:

N = 3,33exp[-(A/43,09)²²] + 12,32exp[-(A/22,71)³] + 3,26exp[-(A/10,2)³],

N - adsorption in micropores NH₃/1/8 u.c., А = RT ln(P^o/P) - adsorption energy, kDj / mol. The parameters of the equation for the first level of the NaLSX zeolite ammonia system are: N₀₁ = 3.33 NH₃/1/8 u.c., E₀₁ = 43.09 kDj / mol and n₁=22; for the second degree N₀₂ = 12,32 NH₃/1/8 u.c., E₀₂ = 22,71 kDj / mol and n₂ = 3; and for the third degree N₀₃ = 3.26 NH₃/1/8 u.c., E₀₃ = 10.2 kDj/mol and n₃ = 3.

There is a correlation between the isotherm and the adsorption heat. Figure 2 shows a graph of the differential heat (Q_d) change in the amount of ammonia adsorption (N) in NaLSX zeolite. Thermal condensation (_v) of ammonia adsorption is indicated by dashed lines. The differential heat (Q_d) curve has a complex wavy shape. Each fragment on the curve represents the stoichiometric ratio between the same number of adsorption centers and the number of molecules adsorbed. NaLSX zeolite contains Si/Al=1:1 ratio. According to the literature [16], the distribution of cations in the zeolite is as follows: 4 cations (in hexagonal rings connected by a cuboctahedron and a hexagonal prism) are in the 1/8 elemental cell (1/8 u.c.) of the S_I′ position, 4 cations (in hexagonal rings connecting cupoctahedrons and large voids) are in the 1/8 elemental cell (1/8u.c.) of the S_II position, and the remaining 4 cations are located in 1/8 u.c. of the S_III′ void (in four-membered rings of large void). There are 12 cations in 1/8 u.c. of the supervoids, or a total of 96 cations in the unit cell, according to calculations. As can be seen from the content, zeolites have a very high density.

Figure 2. Differential heats of adsorption of ammonia in zeolite NaLSX at 303 K. Horizontal dotted line - heat of condensation of ammonia at 303 K

The adsorption heat decreases to a stepped shape of the curve and is divided into 4 main fragments: 0-3 (fragment I), 3-7 (II), 7-15 (III) and 15-18 NH₃/(1/8) u.c.(IV). The first high-energy fragment (0-4 NH₃/(1/8)u.c.) is associated with the adsorption of ammonia to the cations in the S_III' void on the Q_d curve with heat varying from 106.4 kDj/mol to 80.20 kDj/mol. These are the most suitable cations in the void because they are bound to 2-4 oxygen atoms of the forming void and therefore the adsorption goes with high heat and lasts to 2 NH₃/1/8 u.c. It can therefore be assumed that the adsorption of ammonia at acidic centers H⁺ is in a 1: 2 ratio, then half of the number of protons and equal to 1. The total number of cations in the S_III' void is 1+3=4, which corresponds to the crystallographically determined value of the number of cations in this position. During the first molecule ammonia adsorption, 106.4 kDj/mol of heat is released. The reason for the high temperature can be explained by the fact that the cation in position S_III' is close to the entrance window, ie near the supervoid, and cations in other positions attract a small number of ammonia molecules, which takes a long time to balance the released energy. The heat of adsorption of the next molecule of ammonia decreases sharply to 90.50 kDj/mol, and we can see the stepwise decrease of 7 NH₃/(1/8) u.c., from the Q_d curve.

In the second fragment, 4 NH₃/1/8 u.c. ammonia is sorbed, which corresponds to the number of cations in the S_II or S_I' position. Given that ammonia molecules do not enter sodalite voids [42], this section indicates that ammonia is adsorbed in a large void. Given the relatively high adsorption energy, it can be assumed that these centers are sodium cations that migrate from the sodalite voids to the supervoids and are localized in the open S_III void. At this stage the adsorption is 3 NH₃/1/8 u.c. and 7 NH₃/1/8 u.c., the adsorption differential heat decreases from 80.20 kDj/mol to 51.60 kDj/mol.

In the third fragment, the heat decreases monotonically from 51.6 to 36.6 kDj/mol with filling. Here adsorption of 8 NH₃/1/8 u.c. ammonia occurs in cations at the S_II position. Given that there are four Na + cations for 1/8 u.c., there are two molecules for each center.

In the fourth fragment, the adsorption heat decreases from 36.6 kDj/mol to the condensation heat, i.e., 20 kDj/mol, and 3 molecules of ammonia are adsorbed. Adsorption in this fragment goes between 15 NH₃/1/8 u.c. and 18 NH₃/1/8 u.c. in the range. At this stage, it is known from the differential heat of adsorption in the increasing process that 2 molecules of ammonia are sorbed at the S_II position of the supervoid and 1 molecule of ammonia at the S_III position.

By extrapolating the Q_d curve at the initial fillings, we obtain the adsorption of ammonia is equal to 90 kDj/mol on the Na⁺ cation in the S^III'state. The average adsorption heat of ammonia is equal to 75.5 kDj/mol on Na + cations in the S^III state, while that in the S^II is ~ 50 kDj/mol.

The adsorption entropy calculated by the Gibbs-Helmholtz equation shows the state of ammonia molecules in zeolite. Molar differential entropy of ammonia (Fig. 3). It is much lower than the liquid ammonia entropy at NaLSX. This indicates that the mobility of ammonia in zeolite voids is severely limited.

Figure 3. Differential entropies of adsorption of ammonia in zeolite NaLSX at 303 K. Entropy of liquid ammonia is taken as zero. The horizontal dashed line is the mean integral entropy

Ammonia is adsorbed on NaLSX zeolites in the same amount but with different strength. β-voids are not available for ammonia molecules under standard conditions. Na⁺ cations in position S^III form four-dimensional (NH₃)₄/Na⁺ complexes in supervoids. The mobility of ammonia in the voids is lower than that of the liquid.

Figure 4 shows the equilibrium time of benzene adsorption on NaLSX zeolite. At initial fillings, the time to establish adsorption equilibrium in the NH₃-NaLSX system is greatly slowed down. This is due to the fact that the zeolite takes longer to be distributed due to the voids in the pores and the zeolite pores and the large number of cations in the pores and the small amount of ammonia molecules present. It takes longer for the ammonia adsorption to reach equilibrium. Equilibrium is set at 10-16.5 hours. This process is then accelerated, slowly accelerates the rest down for N= 8 NH₃/1/8.

Figure 4. Time to establish adsorption equilibrium depending on the adsorption of ammonia gases in NaLSX zeolite at 303 K

The thermokinetics of ammonia adsorption in NaLSX zeolite show that the initial ammonia molecules are slowly adsorbed, and then the adsorption equilibrium is reduced by 40 min.

4. Conclusions

At differential temperatures of ammonia adsorption, there are 4 fragments corresponding to the formation of monomeric complexes of ammonia with Na⁺ cations in position S_III, then S^III, the adsorption process ends with the formation of four-dimensional complexes with cations in position S^II. The temperatures of ammonia adsorption with Na⁺ cations at positions S_III', S_I' and S_II are 90, 75.5 and 50 kDj/mol, respectively. The adsorption isotherm is fully described by the three-term equation of VMOT). Na⁺ cations in the S^II position form four-dimensional (NH₃)₄/Na⁺ complexes in the supervoid. The average integral differential entropy is -59.64 J/mol*K, and the benzene molecules are adsorbed in the zeolite matrix without solid agitation. It takes a long time for the adsorption equilibrium to be established at the initial saturation. As the saturation gradually increases, the adsorption thermokinetics decreases for a few minutes.

ACKNOWLEDGEMENTS

We would like to thank the non-copyright Yakubov Y.Yu. (PhD), S.B. Lyapin for providing the necessary information in writing the article and for the doctoral students M.K. Kkhokharov and M.S. Khudoiberganov for their assistance in the experimental work. We would like to thank the authors (Rakhmatkarieva F.G., Ergashev O.K.) and the Agency for Science and Technology of the Republic of Uzbekistan for funding a total of 3 fundamental grants.