**1. Introduction**

As is known, the cation exchange capacity (CEC) is the property of a solid to adsorb cations from the liquid phase, exchanging them for an equivalent amount of other cations. In the solid-liquid system, a dynamic equilibrium is established between the cations of the solution and those adsorbed on the active points of the surface of solid [1]. Similarly, a cation exchange capacity (CEC) can be defined as the measurement of the adsorption equilibria of anions. Cations retained in clays can be replaced by other cations; therefore, they are interchangeable.

There are many kinds of materials used for ion exchange of metals; firstly, the natural and synthetic inorganic minerals such as zeolites, clay minerals, and permutites can be considered; secondly, a great variety of organic materials like synthetic

resins, polysaccharides, and proteins can be found; and finally, carbonic materials can be found. Among all the above mentioned materials used for ion exchange, natural clays are of great interest because they are cheaper than the synthetic and organic materials; therefore, the evaluation of the ion exchange capability of some clays and natural materials could be of great interest to recover heavy and precious metals and also rare earths elements easily.

Particularly, the word "rare earth" comes from some oxides that are classified as "rare," not by the difficulty of finding them, but because it was difficult to separate the elements from their minerals. Although rare earths are found in hundreds of minerals, only about 20 have favorable conditions to process them, such as bastnasite, monazite, aluminous clays, xenotima, loparita, and parisita [2]. A group of rare earths belong to a series of elements that are very difficult to separate by traditional techniques, since they have very similar chemical properties. Currently, rare earths are in great demand, because in the study of their properties, and have been discovered multiple industrial and technological applications, which currently have placed them as strategic chemical elements for the development of high-tech products and in high demand [2, 3].

The increasing demand for these elements in modern industry, especially for various advanced technologies, has required obtaining them in a very pure state, impossible to achieve with conventional techniques, which is why a novel researchers and efforts have been carried out, into important development in the different fields of obtaining and recovering rare earths, from the different mineral resources that contain them [3–7]. For the recovery of these elements, there are different processes of separation of rare earths, such as photochemical separation, supercritical extraction, and extraction with solvents and ion exchange [8].

According to heavy metals, almost all of them are too toxic due to their industrial sources and the permitted levels in environment for their discharge. Toxicity of heavy metals depends on their bioaccumulation in environment, that also, depends about the chemical speciation, persistence and tendency of accumulation or bioaccumulation [9]. The classification of the techniques for the treatment of heavy metals in wastewaters depends on different factors, and these technique are conventional and nonconventional, principally. Among the conventional techniques, filtration by membrane, electrodialysis, inverse osmosis, nanofiltration, ultrafiltration, ionic exchange, chemical precipitation, and others can be found [10]. So, ion exchange using natural minerals could be of importance, due to the low cost of these minerals and the possibility of finding a better way to remove heavy metals from contaminated effluents.

According to the precious metals ions, their recovery traditionally was carried out by chemical precipitation; however, the process showed some disadvantages like the formation of toxic subproducts which avoid a complete recovery of the precious metals such as Ag and Au [11]. For the abovementioned, some processes of ion exchange have been developed, using resins and activated carbon, interchanging Ag and Au ions contained in residues or dissolved solutions from cyaniding [12].

On the other hand, various works on the ion exchange of metals with natural adsorbents (aluminosilicates such as zeolites, mineral clays, feldspars, and zeolites) show that these can be good candidates for the recovery of rare earths, as well as heavy and precious metals, taking into consideration some factors that affect the ion exchange such as pH, temperature, contact time, particle size, nature of the cations (size, ionic charge, shape, and concentration), anions associated with the cations in solution, solvent (water, organic solvents) and material selectivity, as well as the ion exchange rate [13–16].

It is for this reason and due to all the efforts made for the recovery of these metals, extraordinary growth has been generated in the last two decades due to their multiple maps in vital sectors for economic development, such as those of computer

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*Use of Porous no Metallic Minerals to Remove Heavy Metals, Precious Metals...*

down its production and very serious questions for the coming years [2].

science and renewable energies, but they are also of great interest in the defense sector. However, global production of rare earths is dominated by China at an alarming 97%, as well as some derived industries such as refining, obtaining alloys, and, to a lesser extent, manufacturing new generation magnets. Currently, China has slowed

According the above, and due to the exploration and discovery of deposits with rare earths contents in the state of Hidalgo, Mexico [17, 18], that studies are proposed for the recovery of these precious and heavy metals, as well as rare earths, through the cationic exchange with natural absorbents such as non-metallic miner-

According the abovementioned, the main goal of this work is to determine the cationic exchange capacity (CEC) of different non-metallic minerals, for the recovery of precious metals, rare earths elements, and heavy metals from leached liquors. In the same way, the determination of the CEC of these minerals could be of utility for their use in the treatment of waste water containing heavy metals. Although there are some works related to the study of the CEC of diatomite, there are not many studies related with the study of the CEC for the bentonite and phosphorite,

In this section, the minerals and materials used to do the ion exchange for the recovery of heavy and precious metals, as well as rare earths elements, will be

The minerals used in this work were obtained in different regions of the State of Hidalgo, Mexico. They were collected in the grinding plants from open mines of diatomite, phosphorite, and bentonite. All the minerals were wholly characterized to describe the principal physicochemical characteristics that they have to carry out for the corresponding ion exchange. Basically, the characterization was performed preparing samples of each mineral (bentonite, phosphorite, and diatomite) and crushing and grinding them until obtain a range of particle size less than 100 μm.

To execute the ion exchange, several experiments were executed using leaching liquors of a SEDEX-type mineral located in the State of Hidalgo, Mexico, containing precious metals and rare earths [17, 18], and previously characterized by ICP-OES. The average chemical composition having these liquors is shown in **Table 1**. To carry out the ionic exchange with the natural minerals, a two-stage procedure was done as follows: 20 g of each mineral (bentonite, phosphorite, and diatomite), at a −400 mesh particle size, were weighted and then, were added to 500 ml volume of leaching solution containing the elements shown in **Table 1**. The experiments were executed at a stirring rate of 500 rpm, with a pH of 0.3, which was controlled adding NaOH constantly during all experiment, and the reaction time used was of 24 h. After ending the reaction time, solution was filtered and dried; then, both solids as residual liquors were characterized (ICP-OES, XRD, XRF, and SEM-EDS) to evaluate the capability of ion exchange of each mineral. The analysis done by ICP-OES was executed taking the final liquid after ion exchange, and then sample was diluted taking 1 ml of solution into 100 ml of distilled water having a 1/100

*DOI: http://dx.doi.org/10.5772/intechopen.88742*

als, also occurring in the state of Hidalgo.

leading to an important innovation to this work.

**2. Materials and methods**

**2.2 Ion exchange methods**

described.

**2.1 Materials**

*Use of Porous no Metallic Minerals to Remove Heavy Metals, Precious Metals... DOI: http://dx.doi.org/10.5772/intechopen.88742*

science and renewable energies, but they are also of great interest in the defense sector. However, global production of rare earths is dominated by China at an alarming 97%, as well as some derived industries such as refining, obtaining alloys, and, to a lesser extent, manufacturing new generation magnets. Currently, China has slowed down its production and very serious questions for the coming years [2].

According the above, and due to the exploration and discovery of deposits with rare earths contents in the state of Hidalgo, Mexico [17, 18], that studies are proposed for the recovery of these precious and heavy metals, as well as rare earths, through the cationic exchange with natural absorbents such as non-metallic minerals, also occurring in the state of Hidalgo.

According the abovementioned, the main goal of this work is to determine the cationic exchange capacity (CEC) of different non-metallic minerals, for the recovery of precious metals, rare earths elements, and heavy metals from leached liquors. In the same way, the determination of the CEC of these minerals could be of utility for their use in the treatment of waste water containing heavy metals. Although there are some works related to the study of the CEC of diatomite, there are not many studies related with the study of the CEC for the bentonite and phosphorite, leading to an important innovation to this work.
