Study of the Process of Sorption of Iron and Copper from Sulfuric Acid in Their Joint Presence by Natural Zeolite

Abstract

The most promising method for the purification of concentrated technical sulfuric acid is the purification sorption method, which is the most effective and innovative, using a natural sorbent. Study of the process of sorption of iron and copper cations from concentrated technical sulfuric acid by a natural zeolite. The specific surface area of the zeolite isolated from reactive sulfuric acid is 4.781 m²/g. The true absorption volume in the zeolite after the purification of sulfuric acid decreases to a value of 147.0068 mL/g for a zeolite sample. The adsorption pore volume for the zeolite after the acid purification calculated from the obtained results is 0.229 mL/g. The physicochemical methods of analysis (NGR, IR, X-ray diffraction, DTA, porosimetry, electron microscopy) and chemical methods revealed that in concentrated sulfuric acid the Fe–O bonds of octahedrons and SiO bonds of tetrahedrons of the zeolite framework are stable. The sorption process was carried out under conditions of a room temperature of T = 25 °C, the ratio “zeolite: H₂SO₄” of 10:100, and a process time of 5–50 min. The specified concentration of the Fe and Cu cations was created by introducing the calculated amount of FeSO₄·7H₂O and CuSO₄·5H₂O, in order to identify the patterns of the sorption process of copper and iron in their joint presence (C_Fe > C_Cu; C_Fe = C_Cu). The regularities of sorption of iron and copper cations by zeolite in their joint presence on the model system “H₂SO₄–zeolite–Fe–Cu” were studied and selective sorption capacity of zeolite with respect to iron cations was revealed. The maximum degree of sorption of iron cations in concentrated sulfuric acid is achieved in 10–15 min and makes up 95% and that of copper 30.6%. The process of iron sorption from sulfuric acid occurs according to the types of ion isomorphism and ion exchange, as indicated by a very high number of sorbed Fe ions and the absence of their release (desorption) from the zeolite into the solution. The Cu cations are sorbed by zeolite from acid by the ion exchange method, which is confirmed by the physicochemical analysis methods.

1. Introduction

Many chemical industries require large quantities of sulfuric acid. If waste sulfuric acid is not recycled, it will cause significant damage to the environment. In recent years, methods for cleaning and processing waste sulfuric acid have been introduced in our country and abroad. The feasibility of treating waste sulfuric acid with microfluidic technology is analyzed [1]. In most chemical processes, concentrated sulfuric acid is polluted and diluted, resulting in large amounts of waste sulfuric acid impurities [2]. Thus, sulfuric acid produced in Kazakhstan is heavily contaminated with various impurities, the removal of which from concentrated sulfuric acid is difficult. This is especially true for the Fe, Cu, Pb cations and the substances that restore KMnO₄. This is due to the fact that these cations are prone to complex formation with sulfate ions. The impurities most difficult to remove are iron compounds (II-III), that difficulty being associated with their complexing ability in sulfuric acid solutions, as well as copper (II) cations [3]. To improve the quality of sulfuric acid, it is necessary to pre-purify it from impurity cations. The existing methods of purification of technical sulfuric acid, including from impurity metals, can be divided into reagent-free, extraction and sorption methods [4,5,6,7,8,9]. The settling method is most often used for preliminary purification of sulfuric acid from the suspended impurities and iron [10]. It has been established that the maximum sorption of copper (II) and iron (III) ions is observed in monocomponent model aqueous solutions. The combined presence of Fe³⁺ and Cu²⁺ in the system leads to a slight decrease in the absorption capacity of clay with respect to each of them. At the same time, the total sorption capacity of the studied montmorillonite-containing clay in the combined presence of ions of the specified heavy metals is quite high and amounts to 0.287 mmol/L [11]. The possibility of extracting copper from a 95% concentrate solution obtained by roasting in the presence of NaCl at 450 °C for one hour, and leaching the roasting product with sulfuric acid by an extraction method, using LIX 984N solution, was investigated. The process of re-extraction of the copper solution to obtain 99% copper and extractant was studied [12,13].

The adsorption methods for removing impurities from sulfuric acids, using inorganic sorbents, are widely used. The most efficient sorbent in sorption processes for natural sorbents is zeolite, which has a microporous framework structure, high adsorption activity and molecular-sieve properties, and which is also acid-resistant. The selectivity of natural zeolite in relation to heavy metal ions determines its potential for use in extracting these elements from aqueous solutions, industrial wastewater and drinking water [14], and a property like chemical resistance in aggressive environments. There are many examples of the use of zeolites in various fields to clean heavy metal ions from wastewater. For example, the sorption of heavy metals (Ni, Co, Cr, Zn, Cu) from surface water on natural and modified clinoptilolite of the Kholinskoe deposit was studied in [15]. In [16], the sorption of cobalt by clinoptilolite from a Mexican deposit was studied. The mechanism of sorption of heavy metals (Pb, Cu, Ni, Co) from individual and multicomponent solutions by clinoptilolite from a deposit in the Ukraine was considered in [17]. A series of experimental studies were carried out on the extraction of metal ions from polluted waters by Bulgarian natural and modified zeolites [18]. The sorption of non-ferrous metals from solutions of low concentrations by natural zeolites was studied in [19], and their use was shown to be promising. The adsorption method found application for purification of 95% sulfuric acid from a number of cations (Cu, Fe, Pb, Mg, etc.) and volatile substances that reduce KMnO₄ by passing it through a layer of heated solid porous packing of the silica gel type at a temperature 60–120 °C higher than the boiling point of the acid [20]. In [21], the process of extraction of Cu²⁺, Ni²⁺, Fe³⁺, and Co²⁺ ions by natural zeolite from aqueous solutions in a dynamic mode in their joint presence was experimentally studied. The obtained results show that natural zeolite can be used as an effective sorbent for the extraction of the indicated ions from polluted waters.

The most diluted reserves of zeolites in the south of Kazakhstan include the zeolite of the Shankanai deposit, which makes its use in the processes of purification of mineral acids economically feasible. Sulfuric acid contaminated with impurity compounds goes into the production of acid, which is strictly regulated by the content of iron, copper, lead, and reducing substances. In this regard, there is a need for its additional purification. Possessing selectivity and stability, they also change activity under the influence of temperature and acidic environments. This allows for deep purification of aggressive acidic liquids, as well as environments containing aggressive impurities. The Shankanai zeolite belongs to the acid-resistant group and is characterized by the cation-exchange properties and anion-exchange properties. All this determines the possibility of its use for the purification of aggressive sulfuric acid. In this work, the natural Kazakhstani zeolite is used for the first time as a sorbent for the purification of concentrated technical sulfuric acid. Therefore, there has been a need to study its sorption capacity in relation to iron and copper in sulfuric acid.

The aim of the work is the study of the process of sorption of iron and copper cations from concentrated sulfuric acid by natural zeolite.

2. Materials and Methods

Zeolite from the Shankanai deposit (Taldykorgan, Kazakhstan) and concentrated sulfuric acid (95% “chemically pure” grade, Taraz, Kazakhstan) was used to carry out the process of sorption of iron and copper cations. Zeolite is a high-silica mineral (Si/Al = 5.75), with the following composition, wt.%: K₂O—1.38; Na₂O—0.95; Fe₂O₃—0.16; Al₂O₃—10.81; CaO—2.32; MgO—0.93; SiO₂—65.28, p.p.p.—18.15. Natural zeolite has sorption properties (E—3.67 mg-eq/g, cation exchanger SEC—0.997 mg-eq/g) and good sorption capacity for a number of individual cations in mineral acids [22]. The process was carried out under conditions of aroom temperature T of 25 °C, the ratio “zeolite: H₂SO₄” of 10:100, and a process time of 5–50 min. The specified concentration of the Fe and Cu cations (C_Fe, C_Cu) was created by introducing the calculated amount of iron sulfates FeSO₄·7H₂O and copper CuSO₄·5H₂O. In order to identify the patterns of the sorption process of copper and iron in their joint presence, experiments were carried out in several versions: C_Fe > C_Cu; C_Fe = C_Cu.

The IR spectra of the samples were recorded on a PerkinElmer Spectrum 100 FT-IR Spectrometer, Waltham, MA, USA. The decoding and identification of the IR spectra of the samples were carried out in accordance with [22].

X-ray diffraction patterns were taken on a DRON-3 diffractometer using Cu Kα radiation, with a current strength of 20 mA and voltage of 25 kV, 1000i, 2.5 s.

The thermogravimetric analysis (TGA) was carried out on a SKZ1053 thermogravimetric analyzer manufactured by SKZ Industrial Co., Limited (Jinan, China) in the temperature range of 20–900 °C in an air atmosphere with a heating rate of 10 degrees per minute. The sample weight varied from 200 mg to 1500 mg. The resistance in the differential heating circuit of the DTA, the DTG circuit and the sensitivity of the weighing system were set depending on the composition of the samples.

The NGR spectra were recorded on a Mössbauer Cu 2201 spectrometer with a Co57 source in a chromium matrix and an activity of 100 millicurie. The spectra were processed, using a computer program, using the least squares method. The obtained isomer shifts are given relative to the center of the α-Fe spectrum.

The microstructure of the obtained samples was studied on an electron probe microanalyzer “JXA-8230” by JEOL (Tokyo, Japan) at an accelerating voltage of 25 kV and an electron beam current of up to 100 nA and on an FEI “QUANTA 250 FEG” (Veldhoven, Netherlands). The main feature of the SEM contrast in the backscattered electron mode is the well-known fact of a brighter contrast of particles with a higher atomic number, compared to the particles constituting the general background [23,24]. The Brunauer–Emmett–Teller (BET) method was used in the work to determine the specific surface area and pore sizes of natural and modified zeolites. The specific surface area of the samples was determined on a specific surface analyzer, using the BET method on a Quadrasorb SI-KR/MP device (Quantachrome Instruments, Boynton Beach, FL, USA).

3. Results and Discussion

The Physicochemical Study of the Natural Zeolite from the Shankanai Deposit and Treated in Reactive Sulfuric Acid

The natural zeolite from the Shankanai deposit, treated in reactive sulfuric acid, was subjected to a physicochemical study. The IR spectroscopic analysis of zeolites after the contact with sulfuric acid showed that all spectra contain characteristic absorption bands in the region of 463–467 cm⁻¹, 592–594 cm⁻¹ and 760–800 cm⁻¹, related to the deformation vibrations of its framework, as well as an intense frequency at 1037 cm⁻¹, characterizing the valence oscillations of the Al, Si–O bond of the zeolite framework (Figure 1, curves 2, 3). That is, the aluminum–oxygen framework and silicon–oxygen skeleton of the zeolite in highly concentrated sulfuric acid are not destroyed in the range of temperatures studied (25–50 °C). A small part of sulfates is sorbed and is displayed in the broadening of the band 594–595 cm⁻¹ and the transition to the shoulder. As can be seen, the zeolite is indifferent to sulfate ions [25,26].

In the IR spectrum of the natural Shankanai zeolite, the intense frequency of 1400 cm⁻¹ and the weak band of 1635 cm⁻¹ belong to the deformation vibrations of the OH group, located at the apex of the tetrahedron.

An increase in the intensity of the band at 1400 cm⁻¹, corresponding to the deformation vibrations of the OH group, and frequencies in the region of 1626–1636 cm⁻¹ were observed in the IR spectrum of the zeolite treated with acid at 25 °C. The additional formation of the OH groups in the zeolite, bonded to aluminum in the Al₃(OH) tetrahedron, is observed due to the exchange reaction between the calcium and aluminum cations of the zeolite and the acid protons. The region of stretching vibrations of water of 2500–3700 cm⁻¹ is also subject to noticeable changes under the influence of temperature. In the spectrum of acid-treated zeolites, the additional frequencies of 2850 and 2920 cm⁻¹ appear, related to OH groups, as well as a clearly defined shoulder of 3180 cm⁻¹, which are not found in the natural zeolite and are related to the valence vibrations of water of a different nature. The revealed changes in the spectrum in the low-frequency region indicate the appearance of compounds with a slightly different nature of the stretching vibrations of water. The increase in the intensity of the deformation and stretching vibrations of water, as well as the appearance of the additional frequencies on a wide band of stretching vibrations of water, may be due to the formation of a defective zeolite structure after the decationization and dealumination, but the structure of the main zeolite mineral itself is not destroyed [27]. That is, the IR spectroscopic analysis of zeolites treated with sulfuric acid has revealed the presence of several forms of the OH groups, apparently occupying different structural positions in the aluminosilicate framework. Comparative studies of natural and acid-treated zeolites by infrared spectroscopy have shown the difference in the IR spectra of the latter from the spectra of natural zeolite in the region of stretching and deformation vibrations of water.

The X-ray phase analysis shows the presence of a number of diffraction maxima near 5.28; 4.44; 2.20; 2.18; 1.96; 1.85; 1.7 Å in the X-ray diffraction pattern of the zeolite treated in the reactive sulfuric acid that indicate the process of its hydroxylation (Figure 2b). In the bar diagram of the natural zeolite, the maximum of the most intense frequency falls on 3.19 Å (I/I0 = 100%) (Figure 2a); in the zeolite treated in reactive acid, its intensity decreases to 90.4% (Figure 2b). An intense diffraction maximum with dα = 3.35 Å (I/I0 = 83%) is recorded for the natural zeolite and the zeolite isolated from reactive sulfuric acid (100%).

Thus, the zeolite treated in reactive sulfuric acid has a band with dα = 4.27 Å and an intensity of 7.4%, while the natural zeolite has an intensity of 28.9% and 13%, respectively. This also applies to the diffraction maxima near 3.62 Å; 3.00 Å; 2.81 Å; 2.70 Å, etc. The X-ray diffraction pattern of the zeolite, treated in reactive sulfuric acid, shows diffraction maxima at 7.9 Å; 7.76 Å; 6.42 Å; 5.15 Å, etc. The indicated changes in the X-ray diffraction pattern of zeolite, treated in reactive sulfuric acid, testify to the formation of an amorphous region due to the process of dealumination and decationization.

Thermogravimetric analysis gives important results in determining the dehydration curve of zeolites [28]. On the DTA curve of natural zeolite (Figure 3, curve 1), the first clearly expressed endothermic effect in the range of 90–250 °C with a clear minimum at 145 °C is due to the removal of the adsorbed and crystallization water from the zeolite. The mass loss is 1.6%.

A weakly expressed and extended endothermic effect in the range of 350–400 °C with a minimum at 390 °C is associated with the onset of zeolite water release and is accompanied by a mass loss of 4.65%. Up to 400 °C, 6.75% of water is lost. The endothermic effect at 550 °C characterizes the loss of residual zeolite water. In the range from 400 °C to 600 °C, another 1.5% of the zeolite mass is lost. A weak endothermic effect at 660 °C apparently characterizes the beginning of deformation of the zeolite crystal lattice [28,29,30]. The thermographic analysis of the natural zeolite shows that the total weight loss is 13.3% of its initial mass.

An analysis of the weight loss curves (Figure 3, curves 1,2,3) shows that for the natural zeolite the total weight loss is 13.3%, whereas for the acid-treated one at 25 °C it is 11.0%, and for the zeolite sample, isolated from sulfuric acid at 50 °C, the weight loss is 6.1%. The zeolite treated with concentrated sulfuric acid at 25 °C loses water stepwise, unlike the sample, isolated from the acid at 50 °C. It is possible that at a higher temperature the structure of the zeolite in concentrated sulfuric acid, as already noted in the IR spectroscopic study of zeolites, is strengthened and approaches that of the natural zeolite. This is expressed in the identical nature of the curves of weight loss of the natural zeolite and the zeolite acid-treated at 50 °C. In the case of sulfuric acid treatment of the zeolite at 25 °C, the total weight loss is 4.9% greater than in the zeolite sample treated at 50 °C. That is, in concentrated sulfuric acid, when heated, the zeolite is subject to greater dehydration, and at 25 °C to hydroxylation, but in the both cases with the preservation of the crystalline structure.

The porometric studies of zeolites were carried out using the BET method for low-temperature nitrogen adsorption. Figure 4 shows the curves of pore volume distribution by effective radii of natural zeolite (Figure 4a) and zeolite treated in sulfuric acid (Figure 4b). The comparison of the curves shows that they are similar in nature and are characterized by the presence of the weakly expressed and distinct maxima. However, on the curve of the pore volume distribution of the natural zeolite, the maxima are recorded at 11 Å and 50 Å and for the zeolite isolated from the reactive acid at 10 Å and 42 Å. That is, after the purification of sulfuric acid, the maxima on the pore volume distribution curve in the zeolite shift to the region of larger pore values. Changes in the distribution of micro- and transition pores in the zeolite after purification of sulfuric acid indicate their filling.

The specific surface area of the zeolite (Figure 5), isolated from concentrated technical sulfuric acid, is 4.781 m²/g. The specific surface area of the natural zeolite (Figure 5, curve 1) is 4.781 m²/g, which is 0.158 m²/g less than that of zeolite treated in reactive sulfuric acid (4.939 m²/g) (Figure 5, curve 2). The true absorption volume in the natural zeolite is 118.5995 mL/g, while for the zeolite sample, treated in reactive sulfuric acid, it is147.0068 mL/g, so the difference is 28.4073 mL/g. The adsorption pore volume calculated from the obtained results for the natural zeolite is 0.185 mL/g, and for the zeolite, treated in the reactive sulfuric acid, respectively, it is equal to 0.229 mL/g. Based on the obtained results, it follows that the porosity of the natural zeolite is less than for the zeolite isolated from the reactive acid.

Since concentrated technical sulfuric acid is contaminated with the impurity compounds of iron and copper, which reduce the quality of the acid, we have studied the process of sorption of iron and copper from sulfuric acid by the Shankanai deposit zeolite.

To exclude the influence of the impurity compounds, the sorption capacity of the zeolite with respect to iron and copper in their combined presence was studied on the model system “H₂SO₄–zeolite–Fe–Cu”.

In the first version, the process of sorption of Fe and Cu was studied under the condition of C_Fe > C_Cu. At the same time, the effect of the quantitative content of copper and iron in the acid on the sorption capacity of the zeolite was considered; that is, the experiments were carried out at high and low values of the concentration of iron and copper.

Astudy of the process of iron and copper sorption from sulfuric acid by zeolite at their low content (C_Fe = 0.002%; C_Cu = 0.001%) in acid, when C_Fe is two times greater than C_Cu, revealed that the iron cations are sorbed by 95% in 10 min of the process (

Table 1. Sorption of iron and copper from sulfuric acid at C_Fe 2 times > C_Cu, C_Fe = 0.002%.

Time, min.	Fe		Cu
	Residual Content in Acid, mg/L	Sorption Degree (Ks), Rel.%	Residual Content in Acid, mg/L	Sorption Degree (Ks), Rel.%
CFe = 0.002% (36.234 mg/L); CCu = 0.001% (18.119 mg/L)
5	1.811	95.00	11.754	35.13
10	1.811	95.00	12.578	30.58
20	3.623	90.00	13.187	27.22
30	3.623	90.00	12.883	28.90
40	3.623	90.00	12.900	28.81
50	3.623	90.00	13.172	27.30
CFe = 0.01% (183.125 mg/L); CCu = 0.001% (18.110 mg/L)
5	1.831	99.00	12.619	30.32
10	1.831	99.00	12.107	33.15
20	1.831	99.00	11.764	35.04
30	1.831	98.80	13.483	25.55
40	1.831	99.00	13.689	24.41
50	1.831	99.00	14.482	20.03

).

With an increase in the sorption time from 10 to 20 min, the sorption of iron cations somewhat decreases, and the degree of acid purification is 90%. With an increase in the duration of the contact of the sorbent with the acid up to 50 min, the degree of acid purification from iron remains constant.

As for the sorption of the copper cations, the process is most intensive during the first 5 min of the contact between zeolite and acid. In this case, sulfuric acid is purified from copper by 35.13%. With an increase in the contact time of zeolite with acid up to 10 min, the residual content of the Cu²⁺ cations in the solution increases up to 7.025 mg/L, respectively, and the degree of the purification of sulfuric acid decreases to 30.58%. Over 30 min of the process, there is no decrease in the degree of acid purification (Table 1). A decrease in the sorption capacity of zeolite with respect to Cu over time is associated with its desorption from zeolite into the sulfuric acid solution. Based on the obtained results, it follows that under the studied conditions, zeolite has a preferential sorption capacity with respect to the iron cations. In this case, over 10 min of the contact of the sorbent with the acid, the Fe³⁺ and Cu²⁺ cations are apparently released from zeolite into the acid solution. However, this applies to a greater extent to the copper ions. Thus, after 50 min of the contact of zeolite with sulfuric acid, the degree of the purification of concentrated sulfuric acid from the Fe cations decreases by 5%, and from the Cu²⁺ cations by 7.83%.

Next, we studied the sorption of iron and copper from concentrated sulfuric acid by zeolite at the content of the Fe cations significantly higher (10 times) than copper; i.e., the concentration of iron in the solution is high (C_Fe = 0.01%), and that of the copper cations islow (C_Cu = 0.001%). As can be seen from Table 1, under the studied conditions, the selective sorption capacity of zeolite with respect to the iron ions is preserved. In almost 5 min of the process, the maximum degree of sorption of the Fe cations (99%) is achieved, remaining unchanged with the duration of the process. Unlike the previous version (C_Fe is two times greater than C_Cu), there is a small maximum at 20 min of the process (Ks = 35.04%). Over 20 min, the degree of copper sorption decreases, apparently due to its desorption from zeolite into acid, and at 50 min Ks becomes equal to 20.03%. It should be noted that with a significantly higher concentration of the iron cations in the acid, the absorption capacity of zeolite for copper increases more slowly. At the same time, in both cases under study, the maximum degree of the copper absorption by the zeolite does not exceed 35%.

The further studies are related to the study of the process of the iron and copper sorption under the condition of their equal concentrations in the acid (C_Fe = C_Cu). In this case, the process was carried out under conditions of their low (0.002%) and high (0.01%) content in sulfuric acid. The obtained data are presented in

Table 2. Sorption of iron and copper from sulfuric acid under the condition of C_Fe = C_Cu.

Time, min.	Fe		Cu
	Residual Content in Acid, mg/L	Sorption Degree (Ks), Rel.%	Residual Content in Acid, mg/L	Sorption Degree (Ks), Rel.%
CFe = 0.002% (36.190 mg/L); CCu = 0.002% (36.315 mg/L)
5	7.2199	80.05	25.5621	29.81
10	7.2163	80.06	28.5726	21.32
20	7.1837	80.15	29.1857	19.41
30	7.1946	80.12	32.0516	11.74
40	7.2561	79.95	32.0516	10.15
50	7.2452	79.98	32.8614	9.51
CFe = 0.01% (183.240 mg/L); CCu = 0.01% (183.167 mg/L)
5	14.5676	92.05	180.7126	1.34
10	14.6409	92.01	173.4591	5.30
20	18.1408	90.10	165.7478	9.51
30	19.8815	89.15	165.1617	9.83
40	18.3240	90.00	165.7295	9.52

and Figure 6.

An analysis of the results shows that under the studied conditions, zeolite also has a preferential sorption capacity with respect to the Fe cations. In this case, the sorption curve for iron, as in the above-described experiments, has a linear character overtime. That is, the iron sorption occurs in the first 5–10 min of the process, remaining almost constant up to 50 min of the process. However, the concentration of the sorbed cations affects the degree of their purification and the nature of the sorption curve of copper depending on time. In the case of a low concentration of the Fe and Cu cations (0.002%), zeolite absorbs up to 80% of iron from the solution in 5 min of the process, and the degree of purification remains practically constant until the end of the process (Table 2, Figure 6a, curve 1).

The sorption capacity of zeolite in relation to copper is noticeably weaker than in relation to iron. In 5 min, the acid is purified from only 29.81% of the Cu²⁺ cations. Moreover, with an increase in the contact time of the sorbent with the acid, the degree of its purification from copper decreases. For example, in 10 min, zeolite sorbs 21.32% Cu, in 20 min 19.41% Cu, and in 50 min 9.51% Cu (Table 2, Figure 6a, curve 2). An increase in the concentration of the copper cations in the acid solution with the duration of the process is due to desorption from the zeolite into the liquid phase of the system.

An increase in the concentration of Fe and Cu causes an extreme dependence of the copper sorption curve on time (Figure 6a,b). The increase in the sorption of the copper ions occurs from 5 to 30 min, and then the sorption curve falls, and the degree of the purification of concentrated sulfuric acid from copper decreases. This is due, as in the previous cases, to the desorption of the copper cations from zeolite into the acid. That is, at 30 min, zeolite absorbs the largest amount of the Cu²⁺ cations. Based on the obtained results, it follows that with the simultaneous presence of the Fe and Cu cations in sulfuric acid, zeolite apparently sorbs the maximum amount of the Fe cations first, and the Cu cations are extracted only due to the exchange positions that remain unfilled in zeolite. The maximum degree of the purification of the acid from copper is achieved after 30 min of the process (Table 2). Over this time, the process of desorption of copper into solution begins.

A comparative analysis of the sorption capacity of zeolite with respect to the iron and copper cations, provided that their concentrations in the acid are equal, shows that with practically the same high degree of purification of the acid from iron, the amount of iron sorbed by zeolite also increases with an increase in the concentration of the sorbed cations (

Table 3. The amount of Fe and Cu sorbed by zeolite from the process time. C_Fe = C_Cu.

Time, min	Initial Concentration of the Fe and Cu Cations in Acid
	0.002% (36 mg/L)		0.005% (90 mg/L)		0.01% (183 mg/L)
	Amount of the Sorbed Cations, mg/L
	Fe	Cu	Fe	Cu	Fe	Cu
5	28.9701	10.7529	85.7851	13.7628	168.6724	2.4544
10	28.9737	7.7424	85.8753	18.3625	168.5991	9.7079
20	29.0063	7.1293	85.7851	22.4823	165.0992	17.4192
30	28.9954	4.2634	85.7851	27.3808	163.3585	18.0053
40	28.9339	4.2634	85.8753	16.9772	164.916	17.4375
50	28.9448	3.4536	85.8753	8.4388	152.4740	16.1187

). In the studied conditions, zeolite also has a preferential sorption capacity in relation to the Fe cations, which is due to its smaller radius (0.67 Å) compared to the copper cation (0.80 Å).

The amount of copper sorbed by zeolite increases up to the concentration of 0.005% and then decreases (Table 3). For example, in 20 min from 100 g of sulfuric acid, 10 g of zeolite at C_Fe = C_Cu = 0.002% absorbs 29.00 mg of Fe and 7.13 mg of Cu, at C_Fe = C_Cu = 0.005%—85.78 mg of Fe and 22.48 mg of Cu, and at C_Fe = C_Cu=0.01%—165.1 mg of Fe and 17.42 mg of Cu.

The Mössbauer spectroscopy of zeolites has found wide application in recent years. The NGR method shows the charge of the iron ions, the symmetry of the electric field at their locations, their interaction with the neighboring ions, the nature of the environment of these ions, and makes it possible to both identify the compounds and see the change in the position of iron in zeolite [31].

Therefore, the zeolite samples were studied using the NGR method. Figure 7 shows the Mössbauer spectra of zeolite, isolated after the 10 min contact with sulfuric acid at 25 °C, in comparison with the NGR spectrum of the natural zeolite and the zeolite isolated from sulfuric acid with the low (0.014%) iron concentration.

The NGR spectrum analysis of the natural zeolite (Figure 7a,

Table 4. The main parameters of the interaction in zeolite.

Sample	δE1, mm/s	ΔEQ, mm/s	Heff. ke.	S, %	Note
Natural zeolite	0.33	0.66	−	17	Fe3+
	1.18	2.35	−	6	Fe2+
	0.37	−0.21	512	68	αFe2O3
	0.38	−0.16	491	9	Unknown form
Acid-treated zeolite	0.34	0.59	−	16	Fe3+
	1.11	2.42	−	5	Fe2+
	0.37	−0.22	510	68	αFe2O3
	0.38	−0.21	490	11	Unknown form
Zeolite after the sorption CFe = 0.014%	0.34	0.59	−	20	Fe3+
	1.07	2.31	−	6	Fe2+
	0.37	−0.21	510	66	αFe2O3
	0.37	−0.23	486	8	Unknown form

Note: δE1—isomeric shift relative to M-Fe, mm/s; ΔEQ—quadrupole splitting, mm/s; Heff.—effective magnetic field at the iron nucleus, ke; S—area of the component relative to the total spectrum, %.

) shows the presence of the Fe³⁺ and Fe²⁺ components. The large isomer shift and large quadrupole splitting indicate their high-spin state [32,33]. This state corresponds to the paramagnetic forms of iron. Based on the isomer shift and quadrupole splitting values, it can be assumed that the iron ions are in an octahedral environment. In addition, the natural zeolite contains two magnetically ordered oxide forms of trivalent iron Fe³⁺. One of them is identified as αFe₂O₃ [34], and the other can be attributed to a hydroxyl compound of the FeOOH type or to a substituted solid solution of the ((Fe_1−xM_x)₂O₃) type [35].

In the NGR spectrum of the acid-treated zeolite, there is a decrease in the isomer shift value (δE1) for the Fe²⁺ form from a value of 1.18 mm/s in the natural zeolite down to the value of 1.11 mm/s in the acid-treated sample. In the NGR spectrum of the acid-treated zeolite, a decrease in the isomer shift (δE1) for the Fe²⁺ form occurs. If in the natural zeolite ΔEQ = 0.66 mm/s, then in the zeolite after the contact with sulfuric acid the value of ΔEQ is equal to 0.59 mm/s. Such a decrease in the isomer shift value is due to a change in the ion coordination, i.e., a decrease in the symmetry of the environment.

It should be noted that the NGR spectrum lines of the acid-treated zeolite are written more clearly compared to the spectrum lines of the zeolite not treated with acid. It is also shown that in the spectrum of the zeolite after the purification of sulfuric acid, the doublet characteristic of Fe³⁺ increases in intensity (Figure 7b), and with an increase in the concentration of Fe in the acid, the splitting at the end of the doublet line increases (Figure 7c). That is, when purifying sulfuric acid, iron cations apparently occupy different positions in the zeolite structure. Based on the above, it follows that the zeolite structure is preserved after the treatment in concentrated sulfuric acid. The NGR spectroscopic analysis of acid-treated zeolite is in good agreement with the IR spectroscopic method of analysis.

Figure 8 shows the IR spectra of zeolites, isolated from sulfuric acid with different iron content, in comparison with the natural zeolite. The contact time of zeolite with the acid was 10 min.

As can be seen from the obtained data, the additional bands appear in the spectra of zeolites, isolated from the system “H₂SO₄ reagent grade–zeolite–Fe” in the regions 404–405 cm⁻¹, 427–430 cm⁻¹, 445–467 cm⁻¹ characteristic frequencies of the valence vibrations for iron [36]. We have attributed the appearance of a new frequency in the higher-frequency region (664–670 cm⁻¹) to the vibrations of the SO_422210032 anion [35]. The presence of this anion in zeolite can explain the shift of the strongest frequency of the natural zeolite of 1027.5 cm⁻¹ to the values of 1065.8 cm⁻¹ and 1058.9 cm⁻¹ in the IR spectra of zeolites isolated from the iron-containing acid solution [29,36]. The presence of the sulfate ions in zeolite is confirmed by the fact that an additional frequency of 651.1 cm⁻¹, characteristic of SO₄²⁻ anion vibrations, appears in the IR spectrum of zeolite after 30 min of the iron sorption (Figure 8a). The presence of new bands in the region of the water deformation vibrations (1426–1650 cm⁻¹) and in the region of the hydroxyl group vibrations (3800–3920 cm⁻¹), in comparison with the IR spectrum of zeolite, isolated from sulfuric acid after 10 min of its purification process, we believe, is due to the sorption of the complex. Moreover, in case of a long-term iron sorption, zeolite absorbs three times more monohydrate (H₂SO₄), compared to a 10 min experiment.

Based on the data obtained, it can be assumed that iron cations in the process of their sorption from concentrated sulfuric acid contribute to the strengthening and preservation of the zeolite crystal lattice. This fact can be explained from the position of its isomorphic entry into the zeolite structure.

According to the literature data [37], the natural zeolite contains extra-framework Al, Ca, and Mg; these cations undergo hydroxylation with the formation of a proton of the hydroxy cation, and then an isomorphic introduction of iron into its structure takes place. The stated assumption is consistent with the data given in [38,39].

The process occurs according to the well-known scheme (1) for zeolites containing di- and trivalent metals [40]:

(1)

The resulting hydroxyl groups with a covalent bond are easily replaced by the iron cations in the form of the hydroxy cations, according to the scheme (2,3):

(2)

(3)

The appearance of the additional Fe–O bonds in zeolite is confirmed by the above-conducted IR spectroscopic analysis of the zeolite samples, isolated from sulfuric acid after the iron sorption (Figure 8). However, the hydroxy cations, formed according to the scheme, are surrounded by the molecularly sorbed water and are capable of free movement. That is, the FeOH²⁺ and CuOH⁺ cations are bound by a rather weak bond and are capable of undergoing the hydrolysis process, which is due to the weakness of the crystal fields. The same thing happens during the copper sorption process. As a result, the weaker Fe–O and Cu–O bonds formed on the outer part of the tetrahedron framework should break with the removal of iron and copper into the solution. However, it is clear from the studies that even with the increase in duration of the sulfuric acid purification process, the concentrations of iron and copper in it do not increase (Table 2). We explain this fact by the fact that, apparently, cations, firstly, in the process of sorption, are localized in the tetrahedrons of the framework with the formation of the strong bonds Fe–O and Cu–O, and secondly, are sorbed in the form of a complex, the size of which is larger than the ionic radius of Fe³⁺ (0.67 Å), and the sorption of the iron complex compound, apparently, reduces the size of the channel in zeolite, preventing the release of the iron cations from it into the solution. The electron microscopic analysis of zeolites confirms this mechanism.

The results of studying zeolites under an electron microscope are shown in Figure 9.

Thus, in zeolite, treated with concentrated sulfuric acid at 25 °C (Figure 9b), the content of voids and channels, as well as their size, significantly increases compared to the natural zeolite (Figure 9a). That is, after the purification of sulfuric acid, unfilled voids and channels remain in zeolite, which is manifested in the micrograph by a noticeable number of light spots (Figure 9b). In the micrograph, after the sorption of iron and copper (Figure 9c), the picture changes; the number of voids and channels decreases, as does their size compared to the acid-treated zeolite; more light spots remain, which is associated with the sorption of iron (Fe and Cu). During the sorption, part of the iron is localized in the tetrahedron (AlO₄), and the rest is exchanged for the extra-framework cations active in the sorption processes. Copper, being sorbed by the ionic type, by means of the exchange of the extra-framework cations active in the sorption processes, fills the voids and channels in the sorbent to a greater extent, which is reflected in the microphotograph as a decrease in the number of light spots. Based on the results obtained, it follows that in concentrated sulfuric acid, zeolite undergoes hydroxylation due to the removal of the tetrahedral aluminum and extra-framework aluminum and calcium from its structure into the acid solution with the formation of the hydroxyl groups with a weak covalent bond between hydrogen and oxygen.

4. Conclusions

The natural zeolite and the zeolite treated in reactive sulfuric acid were studied by physicochemical and chemical methods. It was revealed that, in concentrated sulfuric acid, the Fe–O bonds of octahedrons and SiO bonds of tetrahedrons of the zeolite framework are stable. The IR spectroscopic analysis of the zeolites treated with the sulfuric acid has revealed the presence of several forms of the OH groups, apparently occupying different structural positions in the aluminosilicate framework.

Using the model system “H₂SO₄–zeolite–Fe–Cu”, the patterns of sorption of the iron and copper cations by zeolite in their joint presence were studied, and the selective sorption capacity of the zeolite with respect to iron cations was revealed. The maximum degree of sorption, at 10–15 min in reactive sulfuric acid in relation to iron cations, is 95%, and the copper cations 30.6%. A comparative analysis of the sorption capacity of zeolite shows that the amount of iron sorbed by the zeolite also increases with an increase in the concentration of the sorbed cations, with practically the same high degree of purification of the acid from iron. In 20 min, from 100 g of sulfuric acid, 10 g of zeolite at C_Fe = C_Cu = 0.002% absorbs 29.00 mg of Fe and 7.13 mg of Cu, at C_Fe = C_Cu = 0.01%—165.1 mg of Fe and 17.42 mg of Cu.

It has been shown that the natural zeolite in sulfuric acid exhibits molecular sieve properties. The process of iron sorption from sulfuric acid occurs by the types of ionic isomorphism and ion exchange, as indicated by the very high amount of the sorbed Fe ions and the absence of their release (desorption) from zeolite into the solution. The Cu cations are sorbed by the zeolite from the acid by the type of ion exchange, which is confirmed by the physicochemical methods of analysis.