**2. Materials and methods**

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 described.

#### **2.1 Materials**

*Trace Metals in the Environment - New Approaches and Recent Advances*

cal extraction, and extraction with solvents and ion exchange [8].

According to heavy metals, almost all of them are too toxic due to their industrial

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

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

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 contami-

metals and also rare earths elements easily.

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

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, supercriti-

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nated effluents.

well as the ion exchange rate [13–16].

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.

#### **2.2 Ion exchange methods**

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

#### *Trace Metals in the Environment - New Approaches and Recent Advances*


#### **Table 1.**

*Chemical composition of the leaching liquors of the SEDEX mineral, done by ICP.*

dilution factor. With this, all the elements fell in a range of concentration below 1 ppm for the case of Au, Ce, Nd, L, Tb, Ge, and Yb. For the rest of the elements, a direct sample was taken using standards between 1 and 5 ppm (without dilution).

## **2.3 Analytical methods**

## *2.3.1 Characterization of minerals*

The analytical methods used for the characterizations of the different mineral used to the ion exchange stage were characterized by dry granulometric analysis using a standard sieves (Tyler series); X-ray diffraction using an INEL equinox 2000 equipment was located at the Autonomous University of the State of Hidalgo (AUSH), and the spectra was obtained with a Cu Kα1 radiation of wave length of 1.50056 Å, a voltage of 30 kV, an intensity of 20 mA, and a sweep speed of 22 θ/ min, which were then treated with the software MATH to identify the mineral species contained in each material.

#### *2.3.2 Chemical analysis*

Then, to determine the chemical composition of standard solutions and leached liquors (before and after ion exchange), an inductively coupled plasma spectrophotometer (ICP-OES) Perkin Elmer brand/model 2100 located at the AUSH was used. In the same way, scanning electron microscopy (SEM) was used to determine morphology and particle type of minerals, using a JEOL brand microscope model JMS6300, having an Energy Dispersive Spectrometer of X-ray (EDS) for chemical microanalysis and also located at the AUSH. And finally, X-ray fluorescence (XRF) was executed to determine the chemical composition of minerals before and after the ion exchange procedure, and for this analysis, a portable spectrophotometer of X-ray brand BRUKER, model S1 TITAN was used, also using the GeoChem calibration software (equipment located at the National Autonomous University of Mexico).

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**Figure 1.**

*X-ray diffractogram of bentonite.*

**Table 2.**

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

Bentonite is a rock composed essentially of minerals from the group of smectites, regardless of any genetic connotation. Additionally, it has the ability to inflate and increase its volume by weight several times on contact with water, to form thixotropic gels when added in small quantities. Finally, it can be said that its main

, Mg2+, Li+

**Table 2** shows the results obtained by ICP and FRX of the elements contained in the bentonite studied, of which silicon, aluminum, iron, sodium, and potassium are

Likewise, in **Figure 1**, the mineral species identified by X-ray diffraction are observed, in which the presence of majority of mineral phases, such as quartz,

**Element (oxide) ICP (wt.%) FRX (wt.%)** Na2O 2.66 0.92 MgO 2.71 2.76 Al2O3 14.10 14.92 SiO2 73.10 73.41 K2O 1.32 1.67 CaO 3.11 3.93 TiO2 0.37 0.35 Fe2O3 2.63 2.60

, and H+ .

, Ca2+, K+

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

*3.1.1 Bentonite before ion exchange experiments*

**3. Results and discussion**

**3.1 Characterization of minerals**

interchangeable cations are Na<sup>+</sup>

present in the majority among others.

*Chemical composition of natural bentonite.*
