**2.4. Hydrothermal synthesis of zeolites**

The synthesis of zeolites from some mineral phases has been investigated by several researchers. It is well established that the coal ash composed mainly of phases such as quartz (SiO2), mullite (2SiO2.3Al2O3), hematite (Fe2O3) and magnetite (Fe3O4) can produce zeolites from alkaline hydrothermal treatment. By doing the necessary adjustments in the Al / Si, the ashes can be used for the synthesis of zeolites such as Zeolite Na-A, faujazita, zeolite Na-P and philipsita [18] . [19] report the synthesis of zeolite-type Na from coal ash. The synthesis methodology is focused on so-called "activation" of gray, or was more popularly known as "hydrothermal synthesis" in closed systems for solutions of NaOH or KOH.

Pure zeolites are synthesized from supersaturated solutions of silicates and aluminates under stringent experimental conditions [20] .

It is therefore possible to synthesize zeolite from coal ash by the hydrothermal treatment or in aqueous solution at elevated pressure and temperature [14] .Several articles have proposed various methods for the hydrothermal activation of coal ash in zeolite synthesis, using for this variation of several parameters involved in this synthesis [12,21-24].

The zeolites that have been obtained by hydrothermal treatment of the coal ash with sodium hydroxide (NaOH) are analcime, zeolite A, zeolite P (gismondina), zeolite X and Y (faujasite), sodalite, chabazite, gmelinite and cancrinite .

All methods are developed based on the dissolution of the phases of fly ash supported by Al and Si with alkaline solutions and subsequent precipitation of the zeolitic material [12].

According to [25], the hydrothermal reaction mechanism involves stages of dissolution, condensation or gelation and crystallization.

The dissolution of the ash is between 20-120° C. The amorphous silica, quartz and mullite, which are the phases of Al and Si are present in the ash dissolved. The kinetics is very dependent on the concentration of OH- , which is responsible for dissolving the reagents and keeps them in solution, providing its saturation to occur the formation of zeolites.

The next step is the condensation or gelation which is the phase where the reaction occurs between aluminate and silicate ions with the formation of aluminosilicate gel (hydrogel). In the final stage of crystallization the hydrogel is transformed into a zeolite crystal (the reaction speed is dependent on the amount of Na+). The crystallization of zeolites generally results in a final product containing 40-60% of zeolite.

The zeolite sinthesis from the coal ash is therefore an equilibrium reaction between the alkaline solution and solid phase. By raising the temperature, the solubility of silica and alumina ions increases and condensation reactions for the formation of crystal nuclei are initiated. The crystal growth leads to a complete dissolution of the original material for the formation of amorphous phases zeolite.

Depending on the experimental conditions and the chemical composition of the ash used, one obtains different zeolites. The most important parameters in the processes of zeolitização by hydrothermal process are the chemical composition of ash concentration and type of agent activation, the ratio liquid / solid, temperature, reaction time and intensity of agitation [9].

Each zeolitic material obtained has application in accordance with its characteristics and properties. At least 15 different types of zeolites can be produced from a gray by varying the activation parameters. The activation conditions may be optimized to maximize the adsorptive capacity of the product obtained and the production costs [12] .

Most studies on the use of zeolites derived from coal ash describes a major potential applications of this zeolites as the sorption of toxic metal ions from solutions of pollutants under laboratory conditions [12].

### **2.5. Sorption**

286 Ion Exchange Technologies

application in various sectors [15] .

**2.4. Hydrothermal synthesis of zeolites** 

under stringent experimental conditions [20] .

(faujasite), sodalite, chabazite, gmelinite and cancrinite .

In nature, are usually found in deposits associated with the activation of alkaline volcanic rocks. As naturally occurring normally have a high content of impurities and / or do not possess the properties required for their use, therefore, be synthesized zeolites started [14] .

Zeolites exhibit features as high selectivity, high stability and high exchange capacity, which together with the properties of sorption and catalysis, provide their technological

One advantage of synthetic zeolites is to provide uniformity in size and shape of the channel, another is its chemical composition pre-defined in relation to the purposes intended. Considering, however, its high cost, synthetic zeolites are reserved for applications that require features more uniform structure and composition, as in the catalysis of hydrocarbons and in the detergent industry. The zeolites type A, X and Y are the

Among various uses, natural zeolites can be applied to wastewater treatment for removal of toxic metals, in removing odors in the air purification and in the conditioning of soil [17].

The fly ash are sources of aluminum and silicon, which are the main elements in the composition of the zeolites, because of this, the high content of reactive stages of ash and fine particle size thereof, are considered excellent feedstock for the synthesis of zeolites[12] .

The synthesis of zeolites from some mineral phases has been investigated by several researchers. It is well established that the coal ash composed mainly of phases such as quartz (SiO2), mullite (2SiO2.3Al2O3), hematite (Fe2O3) and magnetite (Fe3O4) can produce zeolites from alkaline hydrothermal treatment. By doing the necessary adjustments in the Al / Si, the ashes can be used for the synthesis of zeolites such as Zeolite Na-A, faujazita, zeolite Na-P and philipsita [18] . [19] report the synthesis of zeolite-type Na from coal ash. The synthesis methodology is focused on so-called "activation" of gray, or was more popularly known as "hydrothermal synthesis" in closed systems for solutions of NaOH or KOH.

Pure zeolites are synthesized from supersaturated solutions of silicates and aluminates

It is therefore possible to synthesize zeolite from coal ash by the hydrothermal treatment or in aqueous solution at elevated pressure and temperature [14] .Several articles have proposed various methods for the hydrothermal activation of coal ash in zeolite synthesis,

The zeolites that have been obtained by hydrothermal treatment of the coal ash with sodium hydroxide (NaOH) are analcime, zeolite A, zeolite P (gismondina), zeolite X and Y

All methods are developed based on the dissolution of the phases of fly ash supported by Al and Si with alkaline solutions and subsequent precipitation of the zeolitic material [12].

using for this variation of several parameters involved in this synthesis [12,21-24].

predominant for commercial use as ion exchangers and adsorbers [16] .

The sorption process is the separation of components of a mixture through a mass transfer phenomenon when a component in the mixture which may liquid or gaseous called adsorbate is in contact with a solid called sorbent. When the two phases are in contact, the dilute component in the mixture is transfered the liquid phase to the surface of the sorbent [2] .The sorption process depends on several factors such as nature and concentration of the adsorbate, the characteristics of sorbent and the sorption conditions (pH, temperature, ratio solid\liquid).

The sorption phenomena are classified into two types: physical sorption and chemisorption. In physical sorption occurring weak interactions like Van der Waals, featuring a reversible process . By not having any sort of change in the nature of the species involved, the physical sorption is a non-specific, and can occur for different adsorbates [26] .In the chemical sorption or chemisorption, the chemical species involved are altered, as there is an effective exchange of electrons between the solid and the adsorbed molecule, resulting in the formation of a monolayer on the solid surface [27] .

If the sorption of one or more ionic species is accompanied by simultaneous desorption of an equivalent amount of ionic species, this process is regarded as ion exchange. The shapes of sorption usually encountered in environmental studies are the physical sorption, chemical sorption and ion exchange [2] .Being essentially a phenomenon of sorption surface, that has an sorption capacity of sorbent significant, must have large surface area, which implies a highly porous structure.

The sorbent materials, in turn, are natural substances or synthetic crystalline structure whose inner surface of the pores is accessible by a selective combination between the solid and solution [26] .

The sorption isotherms are the main way of studying the capacity to remove toxic metals from different solid sorbents [28] .

## **2.6. Sorption isotherms**

To obtain the sorption isotherms is the first step for quantitative evaluation of the sorption mechanisms. The data obtained from the isotherms can be used to project and define the operating conditions of industrial equipment that are based on the principle of sorption [1].

Isotherms relates the concentration of the adsorbate fluid phase and solid phase at a certain temperature and are represented in graphic form, presenting in various ways that reflect the behavior of the mechanism of sorption. In general, the isotherms convex represent usually the solid microporous sorbent. The more complex shapes may be associated with multilayer sorption or the varying sizes of pores of the sorbent material.

We can cite as examples, the isotherms of Henry, Langmuir, Freundlich, BET and Radke-Prausnitz. The Langmuir and Freundlich isotherms are the most commonly used to demonstrate the balance of removing a metal ion sorption, and are the most used models to describe the mechanisms of sorption on zeolites [29].

To provide us with data on the mechanisms of sorption isotherms can be linearized and applied to mathematical models.
