**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 To ).

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

### *Advances in the Adsorption Capacity, Rupture Time and Saturation Curve of Natural Zeolites DOI: http://dx.doi.org/10.5772/intechopen.110008*

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.

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 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].*

**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.

$$\mathbf{M}\_2/\mathbf{n}\,\mathbf{O}-\mathbf{A}\mathbf{l}\_2\mathbf{O}\_3-\mathbf{x}\,\mathbf{Si}\mathbf{O}\_2-\mathbf{y}\,\mathbf{H}\_2\mathbf{O}\tag{1}$$

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:

$$(\mathbf{Na}\_4\mathbf{K}\_4)\mathbf{O} - (\mathbf{Al}\_8\mathbf{Si}\_{40})\mathbf{-O}\_{96-}\mathbf{24}\mathbf{H}\_2\mathbf{O} \tag{2}$$

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].

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].

**Figure 3.** *FONT: Terkimitda. Substructural units of zeolites.*

*Advances in the Adsorption Capacity, Rupture Time and Saturation Curve of Natural Zeolites DOI: http://dx.doi.org/10.5772/intechopen.110008*

### **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 m<sup>2</sup> /g, unlike sand which is 0.01 m<sup>2</sup> /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%.
