Advances in the Adsorption Capacity, Rupture Time and Saturation Curve of Natural Zeolites

Abstract

Reviewing the bibliography, it is found that the amount of heavy metals that natural zeolites are capable of adsorbing under normal conditions is 30%, +/- 10, 20, with respect to the weight of the zeolite used as an adsorbent material in the best cases, highlighting the family of clipnoptilolite, it has been proven that with physical/chemical modifications, as well as, in non-normal conditions of pressure and temperature, superior adsorption results can be achieved. The present study analyzes the capacity of a certain family of natural zeolites that, by presenting a different chemical configuration, that is, instead of having 1,2,3 interchangeable bases such as Ca, K, Mg, has a compound such as (O Mg) and therefore a reorganized unit cell with the capacity to adsorb heavy metals up to 80% with respect to the total weight that is used as adsorbent material, this would be a new parameter to be considered in the adsorption of heavy metals by natural zeolites. According to the scientific literature, it is precisely the amount of exchangeable bases, diameter, and the weight and size of the zeolite pore that largely determines the adsorption of heavy metals.

1. Introduction

According to the FAO 2018. There are 110 million mines and explosives scattered in approximately 64 nations in the world that contaminate water resources with heavy metals that could seriously affect the food chain, ecosystems, therefore, public health could be seriously threatened [1]. On the Ecuador-Colombia border is one of the red points of this contamination of water resources by heavy metals in Latin America, until 2013 more than 200 illegal open-pit gold extraction mines were established, according to E. Rebolledo et al.’s [2] studies carried out using the rapid index of water quality B.M.W.P [3] found the presence of heavy metals in water, sediments and aquatic species and the concentrations of heavy metals found are above the permissible limits of environmental regulations, [4], even in very superficial studies there are already cases that affect the health of the population due to heavy metals. A simple and quick method to evaluate biological quality of running freshwater based on Hellawell [3]: A new approach to the original B.M.W.P. [3] have been performed. Most of the macroinvertebrate families living in the Iberian Peninsula have been added to the original index and some of the scores have been changed. By comparison with some others biotic and diversity indexes, the different values of the new approach (B.M.W.P') have been correlated with quality classes and rate of pollution. The situation worsens when more than half of the population, in rural areas, on this border do not have drinking water and sewage services. The UN “in resolution 64/292 approved in 2010 establishes that water and sanitation are basic rights for life and for the dignity of all people.” In addition, it is established that industry, mining and agriculture are the main polluters of water resources [5].

Natural zeolites are presented as an alternative with experimental scientific support as means of environmental remediation in this and other fields [6]. Natural zeolites are chemically hydrated aluminum silicates, and structurally they belong to the group of technosilicates. This chapter corresponds to a review of the adsorption capacity of natural zeolites considering the chemical formula of the zeolite. The efficiency of adsorption of heavy metals, making use of natural Zeolites, depends on several indicators such as pH, ionic strength, conductivity, initial concentration of cations and anions, the mass of the zeolite used, the particle size of zeolite, the rate of adsorption. This work intends to have experimental information if the chemical structure of the zeolites has any influence on the adsorption capacity. With the exception of the chemical formula of the zeolite, the other indicators can be potentiated by pre-treating the zeolite: ZNSP, ZNAT, ZNAA or ZNATA [7], the following samples were used: Clinoptilolite, Haulandite and a mixture between Haulandite and Clinoptilolite, Morante F. [8], and a fourth sample that differs in its chemical structure and unit cell (X), using fixed-bed concentration models with zeolites (ZNAA), that is, natural zeolites activated in an acid medium, with granulometries of 0.25 mm–1 mm, solutions with known concentrations (0.032 N of ZnSO4 H₂O) are prepared and the fractions are collected in 100 ml volumetric flasks that are analyzed by atomic absorption to determine the concentration in ppm of cation zn2+, the analysis finished when the concentration of the Zn2+ cation in the zeolite column effluent is close to or similar to the initial concentration of the Zn2+ cation. The columns have the same conditions (sample mass in grams g of zeolites, height in cm, volume in cm³, diameter in cm, density in g/cm3, flow rate in cm³/h, and T^o).

Zn2+ adsorption results: Clinoptilolite 6.3 mg of Zn2+/g, Haulandite-Clinoptilolite 22.26 mg of Zn2+/g, Haulandite 5.5 mg of Zn2+/g, Morante [8] and sample X 45.1 mg of Zn2+/g. When applying X-ray diffraction (XRD), significant differences were observed in the chemical and structural structure of natural zeolites and sample X [6]. Regarding the rupture time tb and saturation time tsat, the following results were obtained in the respective order 0.7 h–25 h, 2.0 h–44 h, 1.6 h–11.67 h and for sample X 5.0 h–21 h, it is suggested that if there is an influence by the chemical structure of the zeolites with respect to the adsorption capacity and the amplitude of the breaking time.

2. The adsorption process

Experimentally now zeolites are “clartrates” or also called inclusion group due to the ability of zeolites to adhere or attract various substances in their structure as a guest. According to Armbruster and M Gunter they explain: “A zeolite mineral is a crystalline substance with a structure characterized by a tetrahedral molecular structure, which consists of four oxygen atoms surrounding a cation. This molecular structure contains open cavities in the form of channels and portals. These are generally occupied by water molecules and structural cations that are commonly exchangeable. These channels are large enough to allow invited species to pass through. In stages of hydration and dehydration they occur at temperatures almost always below 4000 C and vice versa it is higher. The atomic structure can be interrupted by groups (OH, F), these are placed externally on the tetrahedron that is not shared with an adjacent tetrahedron.

Figure 1 establishes that the basic structure of the natural zeolite is made up of Silicon⁴⁺ atoms, surrounded by four Oxygen atoms; Al³⁺ is replacing Si, creating a deficiency of positive charges or an increase in negative charges, which is compensated by the castions Ca²⁺, Mg²⁺, Mg²⁺ and Na⁺, thus maintaining the balance of the zeolite.

Figure 1.

FONT: F. Morante. Natural zeolites of Ecuador: Geology, Characterization and Applications. ISBN:978-9978-310-90-8. ESPOL 2014. Basic structure of a zeolite [8].

The figure with a yellow background shows the basic structure of a natural zeolite and the location of the elements that form it together with their functions. The figure to the right with a white background shows the cation exchange process (CEC).

Adsorption is a process by which atoms, ions, and molecules are trapped or included on the surface or interior of an adsorbate, unlike absorption, which represents a volume. In chemistry, the adsorption of a species; cation, ion, molecule, among others, is its inclusion in a part of the interfacial adsorbate between two phases [9]

In this sense, zeolites are also called molecular sieves for their ability not only to trap cations, but also molecules. The purpose of this work is to investigate if there is a difference in adsorption capacity when zeolites have interchangeable bases such as K, Ca, Mg or when they have a compound such as O Mg.

Figure 2 establishes the way in which physical adsorption and chemical adsorption develop.

Figure 2.

FUENTE: Molina Vergaray . Physicochemical adsorption.

Physical adsorption is based on Van der Waals forces and chemical adsorption is established through chemical bonds. Ortega [10], Corona [11].

3. Chemical composition of natural zeolites

In natural zeolites, the presence of the trivalent aluminum element instead of the tetravalent silicon element detracts from the positive charges, increasing the negative charges. The remaining negative charges are compensated by monovalent and divalent cations, such as Na+, K+, Ca2+ elsewhere in the structure, so the empirical formula of zeolite is established as follows.

$${\mathbf{M}}_{\mathbf{2}}/\mathbf{n}\mathbf{O}-{\mathbf{Al}}_{\mathbf{2}}{\mathbf{O}}_{\mathbf{3}}-\mathbf{x}{\mathbf{SiO}}_{\mathbf{2}}–\mathbf{y}{\mathbf{H}}_{\mathbf{2}}\mathbf{O}$$E1

Where M corresponds to any alkali/alkaline earth cation, n is the valence of that cation, x is a number from 2 to 10 and y is a number from 2 to 7.

The unit cell of the clinoptilolite zeolite would look like this:

$$\left({\mathbf{Na}}_{\mathbf{4}}{\mathbf{K}}_{\mathbf{4}}\right)\mathbf{O}–\left({\mathbf{Al}}_{\mathbf{8}}{\mathbf{Si}}_{\mathbf{40}}\right)–{\mathbf{O}}_{\mathbf{96}–}\mathbf{24}{\mathbf{H}}_{\mathbf{2}}\mathbf{O}$$E2

In the Figure 3 the empirical formula of the zeolites is observed, the formation of the geometric figure of the tetahedr is observed, where the Silicate is the central cation of the figure, the structure is electrically neutral as occurs in quartz (SiO₂), however in some zeolites, some tetravalent silicons are replaced by trivalent aluminum, decreasing the positive charges Chiappim [12].

Figure 3.

FONT: Terkimitda. Substructural units of zeolites.

According to Morante F. [13] The ions located within the first parenthesis of the unit cell formula are called exchangeable cations; those in the second parentheses are called structural cations, because they build the tetrahedral structure with oxygen. In natural zeolites water is in free form and represents 10–20% of the hydrated phase, the entire volume of this water can be extracted continuously or reversibly through temperature around 3500 C. According to Mumpton and Ormsby. The dehydration or activation of a zeolite is an endothermic process; conversely, rehydration is exothermic. The following figure shows the chemical structure of the most widely used natural zeolites Georgiev [14].

4. Types of adsorption

The adsorption of cations using natural zeolites is possible by the exchange of the zeolite's own cations or also called exchangeable bases.

The adsorption surface area of ​​zeolites is considered to be approximately 10 m²/g, unlike sand which is 0.01 m²/g, this allows a larger contact area for the adsorption of suspended solids, microorganisms, and other materials in solution. Considering these characteristics of zeolites, the following types of adsorption are proposed: physical and chemical, in active as well as passive treatments, an active treatment is considered when continuous energy and reactive consumption methods are used, passive when neither energy nor reactives are used. In a broader sense, the adsorption of pollutants is also present in environmental remediation alternatives such as: Geological, Microbiological, Reactive barriers. According to [15], zeolites can be modified in order to increase their adsorption efficiency through the use of acids, bases, inorganic salts or hydrothermal treatment, however, the most appropriate modification is the one that agrees to specific pollutants [16, 17].

5. Significant advances in adsorption with natural zeolites

According to Wang, Xu and Sheng [18], Hawash et al. [19] they experienced advances in the purification of polluted water resources, as well as the adsorption of total phosphorus through CW (constructed wetlands).

According to Rahimi and Mahmoudi [20], Choi [21], Mahmood Ibrahimi [7] they obtained significant advances by modifying zeolites with (Sodium Hydroxide and Magnesium Oxide) to remove (Lead, Cobalt, Chromium and Zinc) from an aqueous solution, obtaining an adsorption result of 98.38%, 89.51 %, 81.07% and 78.24%, of Pb2+, Co2+, Cr2+, Zn2+ respectively, concluding that the capacity of the zeolite modified with MgO, in the adsorption of lead was more significant than that modified with NaOH, under similar conditions. Bezerra et al. [22].

Other significant advances with modified zeolites are; [23]; De-La-Vega et al. [24] Experimented hydrothermally modified zeolites from residual quartz sand and calcium fluoride, doped with copper and synthetic faujasite type to remove heavy metals present in aqueous solutions, the results with respect to the percentage of lead removal was 93%, 95.9% and 70% respectively, they concluded that the removal process with modified zeolite is a viable, profitable and efficient alternative for adsorption processes Goñi [25].

Other significant tests were carried out with modified zeolite with high and low calcium fly ash for the removal of heavy metals in aqueous media, lead removal results were around 80%, they concluded that they are very significant advances [26, 27]. Zheng [28] in this same order of modifications, zeolites enriched with another cation in their chemical structure increase heavy metal removal by 40% [29].

On natural zeolites mixed with hexadecyltrimethylammonium bromide (HTAB), it was found that the HTAB-modified zeolite showed significant advances with respect to the natural zeolite, and that the zeolite is effective for the removal of colorants in aqueous solutions. In more significant advances, according to Yurekli, [30]; The removal and filtration processes were analyzed from a nanoparticle (zeolite membrane) combined with polysulfone (Psf) in the removal of lead and nickel in laboratory solutions [31].

Acid mine drainage (AMD) is one of the environmental challenges that has an urgent character for radiation, according to C Ayora et al., [32]. Caustic magnesia (MgO) maintains the pH between 8.5 and 10, allowing complete elimination of divalent metals. And according to Zendelska in her research, she used natural zeolite as an effective sieve for the removal of heavy metals from acid mine drainage (AMD) focusing on zinc ions, obtaining a removal percentage of 74%.

6. Advances in adsorption of heavy metals between natural zeolites with different chemical formulas

The following figure shows the main formulas of the unit cell of the main natural zeolites used in the world. It is observed that all these structural characteristics have an incidence or directly influence the adsorption of contaminants. But it is observed that the ion exchange capacity is related in a special way to the number of exchangeable cations of the first parenthesis (LINDEA, NATROLITE, ANALCIMA, LAUMONTITE).

6.1 Physicochemical characteristic of the main zeolites

Figure 4 shows the physical and specific characteristics of most zeolites, especially the ionic exchange capacity for those zeolites that have a single exchangeable base, which would be in accordance with the present work in which the study is presented. where greater adsorption of heavy metals was obtained with a zeolite that did not have interchangeable bases, but rather a binary compound (O Mg).

Figure 4.

FONT: F. Morante. Zeolitas naturales del Ecuador: Geología, Caracterización y Aplicaciones. ESPOL. 2014 ISBN:978-9978-310-90-8.

Observing that Figure 5 of natural zeolites had a different adsorption activity, a characterization was carried out by means of X-Ray Diffraction (XRD).

Figure 5.

FONT: C. Montaño 2022.

It is observed that Figure 5 under the same adsorption conditions as Figure 6, but with different chemical formulas in its unit cell, have a different adsorption capacity, favorable for Figure 5. It is also observed that depending on the chemical formula it is possible that the spatial configuration or shape of the unit cell has changes that favor adsorption.

Figure 6.

FONT: C. Montaño 2022.

In this work, a sample with the compound formula (O Mg) corresponding to Figure 5 and another sample in which the chemical formula has interchangeable bases (K, Na, Mg) which corresponds to Figure 6.

In the Figure 5 (Sample 14-0078), shows a chemical structure with an amorphous material that corresponds to 86%, this indicates that this sample space does not behave as a crystalline structure, on the other hand, unlike the chemical structure of other zeolites, the cations in the first bracket of sample X is a compound: MgO, in the other zeolites it is a cation, in this sense, it is considered that this structure favors the adsorption process due to the presence of oxygen as well as the large space amorphous (86%). In addition, this property of this sample (Figure 5), allows to have a longer rupture time and saturation time with respect to sample 2 (Figure 6).

In the Figure 6 (Sample 14-0079), Meets the standard of a chemical structure of zeolites, that is, it follows a crystalline model; the amorphous space is only 3.3%. and its exchangeable cations that corresponding to the first bracket is not a compound, and the other natural zeolites behave like this. It is suggested that this is the reason for the decrease in its adsorption process when compared to Figure 5.

7. Fixed bed concentration models

According to Morante, F., [13], Hartini et al. [33], Erdem [34] in the fixed bed concentration models, the concentrations in the fluid phase and the solid phase vary with time and the position of the bed, the greatest mass transfer takes place near the inlet. of the bed, where the fluid comes into contact with the adsorbent. If the solid initially has no adsorbate, the concentration in the fluid decreases exponentially with distance to almost zero before reaching the far end of the bed. After a few minutes, the solid near the inlet is nearly saturated, and most of the mass transfer takes place away from the inlet. The concentration gradient assumes an S shape.

Figure 7 represents the physical process of a fixed bed concentration model based on natural zeolites or also called molecular sieves. Using fixed-bed concentration models with zeolites (ZNAA), that is, natural zeolites activated in an acid medium, with granulometries of 0.25 mm–1 mm, solutions with known concentrations (0.032 N of ZnSO₄ H₂O) are prepared and the fractions are collected in 100 ml volumetric flasks that are analyzed by atomic absorption to determine the concentration in ppm of cation Zn²⁺, the analysis finished when the concentration of the Zn²⁺ cation in the zeolite column effluent is close to or similar to the initial concentration of the Zn²⁺ cation. The columns have the same conditions (sample mass in grams g of zeolites, height in cm, volume in cm³, diameter in cm, density in g/cm³, flow rate in cm³/h, and T^o).

Figure 7.

Font : C. Montaño, autor.

Figure 7 also shows the operation of the liquid phase on the solid phase, the liquid phase is represented by the solution of 0.01 N Zn SO4. The solution passes through the zeolite column, the dark shading represents the adsorption of the Zn2 + cation, as it moves downward, initially the concentration in the effluent is zero (C 1), until the adsorption zone reaches the base of the column, then the breaking point (C2) is reached. The rupture time is established when the concentration of the Zn2+ cation in the effluent reaches 5% of the initial concentration (C0), from the rupture time the concentration of the Zn2+ cation grows rapidly (C3) until reaching the initial concentration (C 0), at this moment the zeolite column is totally saturated (C 4)

At the beginning of the test in these columns with time both in the liquid phase as well as in the solid phase and its limits are considered between C/C0 (concentration ratio corresponding to the fluid and the feed) is from 0.95 to 0.05.

8. Rupture curve/rupture time

Breakdown curve and breakup time are understood as the concentration curve or amount of the cation to be adsorbed in a certain time for the fluid or solution that comes out through the natural zeolite columns that act as adsorbent material.

The rupture time tb is always less than t, and the amount of adsorbed cations at the rupture point is established by integrating the rupture curve at the time tb

The graph below shows how the adsorption breakup curve is formed and when it starts. The size of the curve is useful to determine the amount of adsorbed material.

Figure 8 represents how the natural zeolites adsorb the heavy metal (Zn²⁺) over time until the saturation time, that is, when the natural zeolites do not have the capacity to adsorb more heavy metals. The rupture time tb is always less than t, and the amount of adsorbed cations at the rupture point is established by integrating the rupture curve at the time tb.

Figure 8.

FONT: F. Morante. ZEOLITAS naturales del Ecuador: Geologìa, CaracterizaciònY Aplicaciones ESPOL 2014 ISBN:978-9978-310-90-8.

9. Experimental data

9.1 Materials

1. 2 80 cm BURETTES

2. 2 VOLUMETABLE FLAKS OF 100 ML

3. 2 SAMPLES OF ZEOLITES (Figure 9)

Figure 9.

FONT: C. Montaño 2022.

In the adsorption test with fixed filters, the concentrations in the fluid phase and in the solid phase suffer variations over the time that the test lasts. As can be seen in Table 3, (Figure 6), which represents the zeolites with a chemical formula that have interchangeable bases (K, Mg, Na), had a rupture time of 1.4 h and a saturation time of 4 h in the adsorption process achieving a total Zn²⁺ adsorption of 161 Mg. Under the same conditions, (Figure 5), represents the zeolites where the chemical configuration is different and instead of interchangeable bases, they have a binary compound such as (O Mg), in the adsorption process it had a rupture time of 10 h and a saturation time of 14 h achieving a Zn²⁺ adsorption of 813 Mg (Table 1).

Zeolite (g)	Diámeter (cm)	Height (cm)	Volume (cm^3)	Density (g/cm^3)	Flow (cm^3/h)
18,0545	1	10,02	15,85	1,13	60

Table 1.

SAMPLE #1: Adsorption Column Data, Zeolite sample 1.

10. Results and discussion

The results obtained in the test are:

• Breakdown time: tb

• Mass of zinc adsorbed per gram of samples until saturation Wsat

• Equivalent length of unused bed: LUB

With the data in SAMPLE # 1 and with the feed flow rate, the superficial velocity of the fluid (dissolution) is obtained, expressed in cm/h.

$${\mathit{u}}_{\mathit{o}}\frac{\begin{array}{l}\mathit{\text{caudal}}\mathit{cm}3\\ h\end{array}}{\mathit{\text{section}}\left(\mathit{cm}2\right)}$$E3

$${\mathit{u}}_{\mathit{o}}76,40\mathit{cm}/h$$

The feed rate of the cation (Zn²⁺) per cm² of cross section, expressed in g/cm²/h, is obtained:

$${\mathbf{F}}_{\mathbf{A}}\frac{u\ast C}{1000}$$E4

Where:

$u=\frac{\mathit{cm}}{h}$ = Surface speed in centimeters per hour

C_o Initial cation concentration, expressed in mg/cm³

$${\mathbf{F}}_{\mathbf{A}}0,763\frac{\mathit{gZn}}{{\mathit{cm}}^3/h}$$E5

The breaking time of the ordinate C/CO 0.5 1.40 h.

The saturation time of the ordinate C/CO 1 10 h.

In such a way that the mass of Zn²⁺ adsorbed per gram of zeolite until saturation is.

$${\mathbf{W}}_{\mathbf{sat}.}\frac{\mathit{FA}\ast .\underset{0}{\overset{\mathit{\text{tsat}}}{\int }}1-\frac{C}{C}}{d\ast h}\ast d$$E6

From where:

F_A Cation feed rate per cm² of cross section in g/m² h

d Zeolite density in g/cm³

h Height in cm of the filter bed

ʃ(1- C/C_O)ʅ * dt. Area bounded by the break curve and the ordinate C/C_O 1, expressed in hours. The upper limit of integration is the saturation time (t_sat), which corresponds to the ordinate of the curve where C/C_O

$${\mathbf{W}}_{\mathbf{sat}.}\mathbf{161}\frac{\mathit{\text{mgZn}}}{\mathit{\text{gzeolita}}}\text{is equal}\text{to}1$$E7

To calculate the fraction of the unused bed, we have.

$$\mathbf{LUB}{\mathrm{h}}^{\ast }\left(1-\frac{\mathit{Wb}}{\mathit{\text{Wsat}}}\right)$$E8

h length (height) of the filter bed, expressed in cm

Wb adsorbed mass (Zn²⁺) per gram of zeolite to rupture time

Wsat adsorber mass (Zn²⁺) per gram of zeolite at saturation point

LUB 4,8 cm (Table 2)

Sample 2 (g)	Diámeter (cm)	Height (cm)	Volume (cm^3)	Density (g/cm^3)	Flow (cm^3)
18,0606	1	9,85	15,03	1,20	60

Table 2.

SAMPLE #2: Adsorption Column Data, Zeolite sample 1.

The results with the two samples of zeolites are as follows (Table 3):

Zeolites	Break time	Saturation time	Mass of zeolite adsorbed
MUESTRA 1	1.40 H	4 H	161 MG / Zn2+
MUESTRA 2	10. 0 H	14 H	813 MG / Zn2+

Table 3.

Final results of the adsorption process of the zeolite samples.

11. Discussion

It is observed in this work that there is an influence that favors adsorption when natural zeolites have a chemical composition where there are no interchangeable bases (Na, K, Mg), if not, a binary compound (O Mg) and an upper amorphous space. at 80%. However, it is necessary to carry out other adsorption tests with other metals, especially those with a higher molecular weight than Zn²⁺, and also, it would be important to observe when there is more than one heavy metal in the solution or liquid phase.

The other scenario would be if the natural zeolites are enhanced with additives mentioned in this work: 1.1.4 (Significant advances with natural zeolites), these additives would surely improve the adsorption capacity of natural zeolites.

12. Conclusions

1. The adsorption capacity of natural zeolites is more optimal when the chemical formula has as its central axis a binary compound, (Figure 5), (O Mg) and no interchangeable bases,

   (Figure 6) (Na, K, Mg) and in the liquid phase there is only a heavy metal (Zn²⁺).

2. The second physical characteristic of natural zeolites that favors adsorption is the large amorphous space in this case, according to the RX diffraction analysis, it showed 86% (Figure 5).

3. Of course, the rupture and saturation times are observed increased in favor of (Figure 5), which is very obvious.

4. The saturation time in the test with (Figure 5) was a bit slow after the 5th hour.

5. According to Figure 4 where the physical characteristics of the main natural zeolites are, such as pore size, cation exchange capacity, among others, it agrees with the results obtained in this work, in (Figure 4) it is observed that the zeolites with 1 exchangeable base (Na, k, Mg) have higher cation exchange capacity.

6. Reviewing the literature regarding the adsorption of heavy metals, it is observed that in general all the families of natural zeolites have an average of 5–6 Mg for each gram of natural zeolite [13]. This would mean that (Figure 5) in the test achieved 44.7 Mg for each gram of natural zeolite.