**4.1. Characterization and preparation**

The XRD analysis of the ashes used for zeolite preparation is shown in figure 1. In according to the results of X ray diffraction, the sample ash is mainly formed by mullite phases (Al6Si2O13) and quartz (SiO2) with some traces magnetite (Fe2O3), shown in Figure 1. Chemical analysis carried content was 56.8% of SiO2, 24.5% of Al2O3 and SiO2/ Al2O3 ratio of 2.32.

Figure 2 shows the XRD of all products of the hydrothermal treatments. It can be observed that treatments resulted in formation of zeolitic phases as zeolite A, cubic analcime, philipsite, hydroxycancrinite and Na8(AlSiO4)6(OH)2 .2H2O. Table 2 shows the sorption capacity obtained for the zeolitic products. The results show that a hydrothermal treatment can increase 10–25 times the % sorption when comparing with the value of the original ashes. The sorption of Mn+2 is higher than 85% and the absorption of Cu+2 reaches up to 99%. In the case of a solution with 100 ppm of Mn the sorption increased about 25 times when compared with the results of the test for the same Mn concentration performed with ashes with no hydrothermal treatment. The increasing was about 16 times for Cu+2. The tests 1 and 2 showed small sorption capacity for cations tested. In figure 2, these tests did not show zeolitic phases.

**Figure 1.** XRD pattern of coal fly ash. Mineral abbreviations: M, mullite; Q , Quartz.


**Table 2.** Results of Cu+2 and Mn+2 sorption

290 Ion Exchange Technologies

**3.3. Sorption experiments** 

cations sorption onto the sorbents.

dried at 60 ° C for 24 hours.

**4. Experimental results** 

2.32.

**4.1. Characterization and preparation** 

zeolitic material from test 8 (Table 1) was used as sorbent.

**3.4. Investigation of temperature effect – Thermodynamic study** 

The bottles were shaken for 2 h at 180 rpm in a KS501 IKA shaker and the solids were filtered with Whatman filter paper. The concentrations of metal ions of all tests were

Sorption capacity (%) = ((Ci - Ce )\ Ci ) x 100 (1)

determined in a Varian Atomic Absorption Spectrometer - model Spectra 50B.

The sorption capacity was defined as a percentage and calculated by the equation (1):

Ci and Ce are respectively initial and final concentrations of the metal ion in solution.

The sorption experiments for the heavy metals, Cu2+, Pb2+, Zn2+, and Mn2+ with synthesized zeolite were carried out using the shaking device and methodology described previously in Section 3.2. A concentration range from 100 to 3000 mg/l was used for each cation. Only the

The obtained data were plotted and adjusted with isotherm sorption models to analyze the

Samples of 30 g ash were mixed with 150 ml of a 3 mol/L NaOH solution. This mixture was reacted in an autoclave Parr - 4562, made of nickel alloy 200, equipped with a turbine impeller for 2 hours and 350 rpm agitation and temperatures of 50, 100, 150, 200 and 250 ° C. The product formed after cooling was filtered, washed with 2 liters of distilled water and

The XRD analysis of the ashes used for zeolite preparation is shown in figure 1. In according to the results of X ray diffraction, the sample ash is mainly formed by mullite phases (Al6Si2O13) and quartz (SiO2) with some traces magnetite (Fe2O3), shown in Figure 1. Chemical analysis carried content was 56.8% of SiO2, 24.5% of Al2O3 and SiO2/ Al2O3 ratio of

Figure 2 shows the XRD of all products of the hydrothermal treatments. It can be observed that treatments resulted in formation of zeolitic phases as zeolite A, cubic analcime, philipsite, hydroxycancrinite and Na8(AlSiO4)6(OH)2 .2H2O. Table 2 shows the sorption capacity obtained for the zeolitic products. The results show that a hydrothermal treatment can increase 10–25 times the % sorption when comparing with the value of the original ashes. The sorption of Mn+2 is higher than 85% and the absorption of Cu+2 reaches up to 99%. In the case of a solution with 100 ppm of Mn the sorption increased about 25 times when compared with the results of the test for the same Mn concentration performed with ashes

**Figure 2.** XRD pattern of modified coal fly ash. Phases abbreviations: M, mullite; Q, quartz; Z, zeolite Na8(AlSiO4)6(OH)2 . 2H2O; C, hydroxycancrinite; A, zeolite A; An, analcime; P, zeolite P. the legends LC01 to LC08 are related with tests 1-8 from Table 1.

Figure 3 and 4 show the results of preliminary statistical analysis and the effects of the variables. The *F*-test was used to identify the most significant variables in the hydrothermal process. The significance level (p-value) adopted was 0.05. Temperature and time were the most significant variables in the synthesis of the zeolites. An increase of the level of these variables tends to increase the sorption capacity for Mn and Cu.

**Figure 2.** XRD pattern of modified coal fly ash. Phases abbreviations: M, mullite; Q, quartz; Z, zeolite Na8(AlSiO4)6(OH)2 . 2H2O; C, hydroxycancrinite; A, zeolite A; An, analcime; P, zeolite P. the legends

Figure 3 and 4 show the results of preliminary statistical analysis and the effects of the variables. The *F*-test was used to identify the most significant variables in the hydrothermal process. The significance level (p-value) adopted was 0.05. Temperature and time were the most significant variables in the synthesis of the zeolites. An increase of the level of these

LC01 to LC08 are related with tests 1-8 from Table 1.

variables tends to increase the sorption capacity for Mn and Cu.

**Figure 3.** Pareto chart of standardized effects to fatorial planning - % Mn sorption ( *p*-level = 0.05)

**Figure 4.** Pareto chart of standardized effects to fatorial planning - % Mn sorption ( *p*-level = 0.05)

Several authors suggest that changes in synthesis temperatures lead to different zeolitic phases. The differences among sorption capacities of the various zeolites synthesized may be credited to the different zeolitic phases present in the products of the hydrothermal synthesis [25,31-32]. An increase in the reaction time tends to promote a better crystallization of the phases formed, which it might also explain the increase in the of sorption capacity [33].

#### **4.2. Sorption studies**

The sorption of different metal ion concentrations onto synthetic zeolite at 250 C was studied for Cu+2 , Pb+2, Zn+2, and Mn+2 in the range 100-3000 mg\L keeping all other variables constant. The results are shown in figure 5. The sorption for Cu+2,, Pb+2, Zn+2, and Mn+2 increases with increasing metal concentration in aqueous solutions. These results indicate that energetically less favorable sites become involved when the concentration of metal in solutions increases. The metal uptake can be credited to different mechanisms of both ionexchange and sorption. During the ion-exchange process, metal ions have to move through the pores of the zeolite, but also through channels of the lattice, and they have to replace exchangeable cations (mainly sodium and calcium). Difusion is faster through the pores and is retarded when ions move through channels of small diameter. In this case the metal ion uptake can mainly be credited to ion- exchange reactions in the porous of the zeolitic samples [34].

**Figure 5.** Sorption isotherms of de Mn+2 ,Cu+2 ,Zn+2 , Pb+2 .

The preferred order observed for sorption was Pb>Cu>Zn>Mn. In the literature, similar results were obtained when the sorption capacity of a large variety of zeolite minerals for cadmium, copper and zinc and revealed that zinc had the lowest sorption for all zeolites synthesized [31,35-36]. Zeolites obtained under same conditions as philipsite and chabazite had limited sorption capacity (CA) for zinc as compared to copper [37]. The sorption characteristics of Zn (II) onto pure fly ash showed that the solution pH was the key factor affecting the sorption characteristics [38-39].

### **4.3. Investigation of temperature results – Thermodynamic study**

294 Ion Exchange Technologies

sorption capacity [33].

**4.2. Sorption studies** 

samples [34].

**Figure 5.** Sorption isotherms of de Mn+2 ,Cu+2 ,Zn+2 , Pb+2 .

Several authors suggest that changes in synthesis temperatures lead to different zeolitic phases. The differences among sorption capacities of the various zeolites synthesized may be credited to the different zeolitic phases present in the products of the hydrothermal synthesis [25,31-32]. An increase in the reaction time tends to promote a better crystallization of the phases formed, which it might also explain the increase in the of

The sorption of different metal ion concentrations onto synthetic zeolite at 250 C was studied for Cu+2 , Pb+2, Zn+2, and Mn+2 in the range 100-3000 mg\L keeping all other variables constant. The results are shown in figure 5. The sorption for Cu+2,, Pb+2, Zn+2, and Mn+2 increases with increasing metal concentration in aqueous solutions. These results indicate that energetically less favorable sites become involved when the concentration of metal in solutions increases. The metal uptake can be credited to different mechanisms of both ionexchange and sorption. During the ion-exchange process, metal ions have to move through the pores of the zeolite, but also through channels of the lattice, and they have to replace exchangeable cations (mainly sodium and calcium). Difusion is faster through the pores and is retarded when ions move through channels of small diameter. In this case the metal ion uptake can mainly be credited to ion- exchange reactions in the porous of the zeolitic

According to figure 6, near the temperature of 100°C, the X-ray diffraction indicates that no phase transformation occurs. According to literature, the reaction kinetics is highly temperature dependent, and it is believed that in this case, a larger reaction time would be required.

**Figure 6.** XRD analysis of samples of gray (blue) and the products of the tests at 50 and 100°C (green and black, respectively)

From a temperature of 150ºC (Figure 7), zeolite P was identified in the X-ray diffractograms have been replaced by analcime 200°C (Figure 8), which was later replaced by hydroxycancrinite phase at 250° C (figure 9).

**Figure 7.** XRD analysis of the product for testing at 150 ° C

**Figure 8.** XRD analysis of the test product at 200 ° C

**Figure 9.** XRD analysis of the product for testing at 250 ° C

**Figure 7.** XRD analysis of the product for testing at 150 ° C

**Figure 8.** XRD analysis of the test product at 200 ° C

From the experimental results, we performed a thermodynamic study, to investigate the effect of temperature on the synthesis of these zeolite phases. For the species treated zeolite in this work, whose thermodynamic properties were not known, estimates have been proposed. Was used for this study a variation of the equation developed by [40] for estimating enthalpies of formation for the species treated in this work, to enable the calculation of free energies at several temperatures.

The proposed equation for calculating the enthalpy of formation of species (kJ/mol) is shown in equation 2, in which 1 is a parameter calculated by nonlinear regression of data made from enthalpy of known species.

$$\begin{aligned} \text{DH}^{\text{o}}\_{\text{f}} &= \text{n}\_{\text{Na}} \text{DH}^{\text{o}}\_{\text{f}} \left[ \text{NaOH} \right] + \text{n}\_{\text{Al}} \text{DH}^{\text{o}}\_{\text{f}} \left[ \text{Al} \left( \text{OH} \right)\_{\text{g}} \right] + \\ &+ \text{n}\_{\text{Si}} \text{DH}^{\text{o}}\_{\text{f}} \left[ \text{Si} \left( \text{OH} \right)\_{4} \right] - \left( \text{y} - \text{x} \right) \text{DH}^{\text{o}}\_{\text{f}} \left[ \text{H}\_{2} \text{O} \right] - \text{b}\_{1} \text{n}\_{\text{Na}} \text{R}\_{\text{Na}} \end{aligned} \tag{2}$$

The magnitudes nNa, nAl, NSi, are the stoichiometric coefficients of the respective Na, Al and Si present in the molecular formula of each species. The quantity y is the coefficient of oxygen stoichiometric, x the stoichiometric coefficient of water incorporated and RNa the radius of sodium ion (0.102 nm). The values of enthalpies of formation (Hof) and the standard molar entropies (Sof) species used in the calculations were taken from the database program HSC Chemistry 7.0

The calculation of nonlinear regression allowed us to estimate 1 equal to 613.231, with a correlation coefficient of 0.999. In Table 3 are the values estimated by the model and the


waste from the values taken from the database. One can observe that the maximum residue was 3.08% for the species andalusite.

**Table 3.** First estimation results for the model enthalpy of formation

The estimation of standard molar entropy (Sof) was performed using a simple regression, taking into account only the stoichiometric coefficients of each species (Equation 3). The best results were achieved with a correlation coefficient of 0.97 for the model.

$$\text{S}^{\circ} = 27.148 + 46.608 \,\text{n} \text{u} - 11.321 \,\text{n} \text{u} + 5.111 \,\text{n} \text{s} + 16.147 \,\text{n} \text{o} + 43.487 \,\text{n} \text{zo} \tag{3}$$

The thermodynamic data used for the calculation and the values estimated by the model Sof and residues, are shown in Table 4.


**Table 4.** Results estimated of Sof by linear regression model.

To check the validity of the models surveyed, we compared the values of the standard energies of formation (Gof) of some species sodium zeolite, whose values of Gof were already known. Using the values of Hof estimated by the model developed Sof the value at 298 K calculated by linear regression is calculated to Gof values for each species to be compared (Table 5).


**Table 5.** Comparison between the values of Gof at 298K (in kJ/mol) calculated from the models.

These values of Gof were estimated using the program HSC Chemistry 7.0, using the values of enthalpy of formation and standard molar entropy of the species included in the required database program, and using the balanced chemical equation as the model reaction represented by Equation 4:

$$\text{CaNa} + \text{bAl} + \text{cSi} + \text{dOn(g)} + \text{eHz(g)} \rightleftharpoons \text{estimated species} \tag{4}$$

Where a, b, c, d and e are the respective stoichiometric coefficients. The model of the reacting species (more stable forms of each element) were already in the database program HSC Chemistry 7.0. The maximum error obtained was slightly higher than 3%, showing that the estimate used seems appropriate.

#### **4.4. Calculation of equilibrium**

298 Ion Exchange Technologies

was 3.08% for the species andalusite.

and residues, are shown in Table 4.

**Species Hof**

**(kJ/mol)** 

**Table 3.** First estimation results for the model enthalpy of formation

**Species Na Al Si O H2O Sof**

**Table 4.** Results estimated of Sof by linear regression model.

results were achieved with a correlation coefficient of 0.97 for the model.

waste from the values taken from the database. One can observe that the maximum residue

**Hof (model) (kJ/mol)** 

Cianite -2581.097 -2604.19 0.894 23.091 Caulinite -4119.599 -4085.29 0.832 -34.313 Pirofilite -5637.900 -5618.33 0.347 -19.568 Paragonite -5944.209 -5901.83 0.712 -42.374 Analcime 1 -3282.348 -3286.31 0.120 3.958 Jadeita -3032.760 -3011.82 0.690 -20.943 Albita -3927.659 -3921.25 0.163 -6.404 Analcime 2 -3309.841 -3297.65 0.368 -12.194

Metasilicate de sodium -1561.511 -1600.57 2.501 39.058

Andalusite -2686.965 -2604.19 3.080 -82.777

The estimation of standard molar entropy (Sof) was performed using a simple regression, taking into account only the stoichiometric coefficients of each species (Equation 3). The best

 Sof = 27.148 + 46.608 nNa – 11.321nAl + 5.111nSi + 16.147 nO + 43.487 nH2O (3) The thermodynamic data used for the calculation and the values estimated by the model Sof

Cianite 0 2 1 5 0 86.680 90.353 -7.667 8.845 Caulinite 0 2 2 7 2 205.016 214.744 -6.993 3.411 Pirofilite 0 2 4 11 1 239.171 246.059 -8.905 3.723 Paragonite 1 3 3 11 1 276.833 276.235 0.379 0.137 Analcime 1 0.96 0.96 2.04 6 1 226.776 211.822 11.577 5.105 Jadeite 1 1 2 6 0 133.499 169.542 -19.060 14.277 Albite 1 1 3 8 0 226.400 206.948 11.397 5.034 Analcime 2 1 1 2 6 1 223.802 213.029 10.129 4.526 Andalusite 0 2 1 5 0 104.600 90.354 -24.169 23.106 Sillimannita 0 2 1 5 0 95.790 90.354 10.778 11.251 Nefeline 1 1 1 4 0 123.000 132.136 2.572 2.091

**(J/molK)**

**Sof (model)**

**(J/molK) Residues Residues** 

**%** 

**Residues %** 

**Residues** 

Using the estimated values of enthalpies of formation and standard molar entropy of analcime and hydroxycancrinite species, we calculated the values of free energies as a function of temperature reaction for the formation of analcime reactions (reaction A), hydroxycancrinita (reaction B) and zeolite P (reaction C) from the reaction of mullite with sodium hydroxide. Furthermore, it has been provided in the processing of analcime to hydroxycancrinite (reaction D), Zeolite P to hydroxycancrinite (reaction E) and analcime to zeolite P (reaction F).For each of these reactions studied were calculated the values of G reaction and equilibrium constant at each temperature studied.

#### **Reaction A)**

Al6Si2O13 + Na + (aq) + OH- (aq) + 8H2O = NaAlSi2O6 \* + 5Al (OH)3

#### **Reaction B)**

3Al6Si2O13+8Na+(aq) + 8OH- (aq)+17.7H2O = Na8Al6Si6O24(OH)2\*2.7H2O +12Al(OH)3

#### **Reaction C)**

5Al6Si2O13 + 6Na(+a) + 6OH(-a) + 45H2O = Na6Al6Si10O32\*12H2O + 24Al(OH)3

#### **Reaction D)**

3 NaAlSi2O6\*H2O + 5Na(+a) + 5OH(-a) + 3Al(OH)3 = Na8Al6Si6O24(OH)2\*2.7H2O + 6.3H2O

#### **Reaction E)**

3 Na6Al6Si10O32\*12H2O + 12Al(OH)3 + 22Na(+a) + 22OH(-a) = 5Na8Al6Si6O24(OH)2\*2.7H2O + 46.5H2O

#### **Reaction F)**

Na6Al6Si10O32\*12H2O = 5 NaAlSi2O6\*H2O + Al(OH)3 + Na(+a) + OH(-a) + 5H2O

It was considered the activities of solid phases equal to 1 and that the activities of Na+ and OHare equal.

**Figure 10.** Phase diagram of stability of zeolite formed from the coal ash. (Abreviations: ANA – Analcime; MUL – Mulite ; ZEOP – Zeolite P ; HCAN- Hidroxycancrinite)

It was possible to make a survey of pH versus temperature diagram as shown in Figure 10. It is possible to check, at 25 ° C, a large region of stability of the zeolite P type, ie the reaction of the mullite forming zeolite P, at pH values between 8 and 15. This region decreases as the temperature increases.Starting pH 15, the reaction hydroxycancrinite form, rather than zeolite P. The stability region analcime phase (pH between 6 and 8 to 25 ° C) maintains a constant-width around two pH units, but with increasing temperature is shifted to pH also higher. Higher values of pH, therefore higher concentrations of NaOH, promote the transformation of phases formed in hydroxycancrinite. The remaining balance for the reactions D, E and F are present in high pH values, and in all cases the phase stability hydroxycancrinite prevails.
