CATION EXCHANGE MECHANISM OF DIVALENT METALS IONS IN SYNTHETIC NaX AND LTA ZEOLITES: EFFICIENCY AND SELECTIVITY

In this investigation, the removal of divalent heavy metals by two types of zeolites has been studied. The mechanism of cation exchange for Cu, Ni, Cd and Pb on NaX Faujasite and Linde type A (LTA) zeolites was studied. Both zeolites are very efficient in divalent heavy metals removal from water. Ion exchange isotherms were performed and the thermodynamic parameters were calculated as well. Kielland plots showed that the order of selectivity is not the same for both the zeolites. For NaX it followed the following order Pb > Cu > Cd > Ni while for LTA the order is Pb > Cd > Cu > Ni. Exchange velocity (kw) and the radii of the hydrated metals ions are involved in determining this order of selectivity.


Introduction
Zeolites are cation exchangers that occur naturally, the Swedish mineralogist Axel Fredrick Crønstedt first discovered them in 1756. 1 The framework of zeolites is a three-dimensional arrangement of tetrahedral SiO4 and AlO4 -units.The aluminate units in the framework are responsible for its net negative charge which is balanced by the presence of extra framework cations located on defined crystallographic positions.These cations are responsible for zeolite exchange properties and the rich ion-exchange chemistry of these materials.The elevated cation exchange capacity made zeolite attractive for wastewater treatment especially for removing heavy metals.
Nowadays, many water resources are contaminated due to the direct release of industrial influents.Heavy metals are one of the most serious environmental pollutants since they accumulate in living organisms even at low concentration causing dangerous diseases. 2Natural zeolites have been used to remove heavy metal from aqueous solutions because they are inexpensive and highly abundant.4][5] One of The most frequently studied natural zeolites is Clinoptilolite. 6,7The cation exchange capacity is usually between 0.6 and 2.3 meq g -1 . 8Recently, a great deal of attention has been paid to synthetic zeolites due to their enhanced properties including higher cation exchange capacity compared to natural ones.Synthetic Faujasite zeolite, NaX (Na88Al88Si104O384 and Si/Al ratio = 1.18) and Zeolite A, LTA (Na96Al96Si96O384 and Si/Al ratio=1) are more favourable for heavy metals adsorption since they contain more exchangeable Na + ions in their framework.
There are several studies dealing with divalent heavy metals adsorption onto synthetic zeolites, 9,10 but no detailed analysis for the cation exchange mechanism is available.The current study highlights the theory of ion exchange in synthetic NaX and LTA zeolites.The efficiency and selectivity of divalent heavy metals binding (Cu 2+ , Ni 2+ , Cd 2+ and Pb 2+ ) were analyzed in detail based on the cation exchange theory including calculation of thermodynamic parameters which allows better understanding the selectivity of these zeolites toward metal ions.

Experimental Materials
The French Institute of Petroleum (IFP Energie Nouvelle) provided the synthetic NaX Faujasite and LTA zeolite samples.These samples were used as received without any further chemical modification.Copper

Batch adsorption experiments
Metal ions solutions (Cu 2+ , Ni 2+ , Pb 2+ and Cd 2+ ) were prepared from their corresponding nitrate salts in ultrapure water to obtain solutions of different concentrations.In batch studies, 20 mg of the solid zeolite powder was placed in 100 mL of a metal solution of desired concentration.For obtaining the isotherms, the initial metal ions concentrations were varied between 0.17 mmol L -1 and 5 mmol L -1 .The solutions were stirred for 120 min at 25 °C then filtered by a 0.45 μm syringe filter and the remaining metal ions were measured by AAS (Perkin Elmer AA200).The adsorption capacity (mmol g -1 ) of the adsorbent at equilibrium was calculated by eqn.(1).

𝑞 e =
( 0 - e )  (1)   where C0 is the initial concentration Ce is the concentration at equilibrium, V is the volume of metal solution in litre and m is the mass of the adsorbent in g.

Theory of cation exchange in zeolites
The sodium-divalent metal ion exchange process in zeolite can be expressed by eqn.( 2) The bar above the symbols represents the ion exchanger phase.The thermodynamic equilibrium constant, K, of the reversible ion-exchange reaction is defined by eqn.(3). 11 [Na + ] and [M 2+ ] are the molarities of the ions in solution, γi and fi are the activity coefficients in the aqueous phase and in the ion-exchanger phase, respectively.The standard states for the ion-exchanger phase are taken as the exchanger in its pure Na + and M 2+ forms, and the activity coefficients, fNa and fM, are chosen as unity when the exchanger is at the standard state of the Na + form and the M 2+ form, respectively. ̅ i is an equivalent fraction of ion i in the ionexchanger phase, defined by Eqns.( 4) and (5).
The equivalent fractions of the ions in the solution (Xi) can replace the molarities [Na + ] and [M 2+ ] leading to eqns.(6 -8).
where TN represents the total normality of the solution.By using a corrected selectivity coefficient,  Na M , the thermodynamic equilibrium constant can be rewritten eqn.(9). 12

𝐾 Na
where, When the total normality, TN= [Na + ] + 2[M 2+ ], is lower than 0.01 N, γ 2 Na/γM will be close to unity. Na M depends on the total normality.When the corrected selectivity coefficient is larger than unity (ln  Na M >0), the zeolite is selective for the ion M 2+ .If  Na M is smaller than unity (ln  Na M <0), Na + ions are more preferred.When  Na M is equal to unity (ln  Na M =0), there will be no preference between these ions. Na M is also related to the Kielland coefficient as given below.Kielland plots show the details of the ion exchange selectivity as a function of the equivalent fraction  ̅  .
The coefficient, Cm, is called generalized Kielland coefficient.These plots often give linear relationships with a slope 2C1.
If the Gibbs-Duhem equation is applied to the ionexchange reaction, the thermodynamic equilibrium constant, K, is given by the integration of Kielland plot from  ̅ M =0 to  ̅ M =1.
Thus, the thermodynamic equilibrium constant is determined by the valences of the exchanged and exchanging cations, the generalized Kielland coefficient C1 and the intercept of Kielland plot, ( Na M ) X ̅ M→0 .
The Gibbs standard free energy change ΔG° can be calculated by eqn.(14).

Exchange equilibrium curves for NaX Zeolite
The exchange isotherms are shown in Figure 1 and are represented as the equivalent fraction of the divalent metal ions fraction ( ̅ M ) inside the NaX zeolite as a function of the equivalent ion fraction in the solution ( M ).
The obtained exchange isotherms almost have the same shape, they showed a steeper increase in the initial stage of XM 2+ <0.2 and then the isotherms increased more gently.This shape is characteristic of a favourable exchange process.
For 2Na + →Pb 2+ exchange reaction, the isotherm proceeds more steeply up to  ̅  ~0.6 which indicates higher selectivity for Pb 2+ than for the other three heavy metals (Figure 1).The theoretical exchange capacity was calculated from the Na + ions contained in NaX (Na88Al88Si104O384). 1 g of NaX zeolite contains 6.54 mmol Na + , so theoretically, it has a maximum adsorption capacity for divalent ions equals to 3.27 mmol g -1 since every two Na + ions are exchanged for one divalent metal cation so this value was supposed to be the theoretical exchange capacity of NaX.The ion exchange capacities for all metal ions were determined from their corresponding isotherms and were found to be 93, 87, 95, and 96% of the theoretical exchange capacity for Cu 2+ , Ni 2+ , Cd 2+ and Pb 2+ respectively.These values were calculated from the equivalent fraction of cations in zeolite when the maximum exchange capacity was reached.

Exchange Equilibrium Curves for LTA Zeolite
The exchange reactions of the four heavy metals ions Cu 2+ , Ni 2+ , Cd 2+ and Pb 2+ were done for LTA zeolite as well under the same conditions.The corresponding isotherms are shown in Figure 2. As in the case of NaX zeolite, the exchange isotherms for LTA showed a favourable exchange process with a steep increase in the initial stage of XM 2+ < 0.2 that proceeds more gently afterwards.Similarly, for 2Na + → Pb 2+ exchange reaction, the isotherm proceeded more steeply up to  ̅  ~0.6 which indicates higher selectivity for Pb 2+ than for the other three heavy metals.The theoretical exchange capacity was calculated from the Na + ions contained in LTA (Na96Al96Si96O384). 1 g of LTA zeolite contains 7 mmol Na + .So theoretically, it has a maximum adsorption capacity for divalent ions equals to 3.5 mmol g -1 since every two Na + ions are exchanged for 1 metal divalent cation so this value was supposed to be the theoretical exchange capacity of LTA.The ion exchange capacities for all were determined from their corresponding isotherms and were found to be 88, 82, 96, and 95% of the theoretical exchange capacity for Cu 2+ , Ni 2+ , Cd 2+ and Pb 2+ respectively.These values were calculated from the equivalent fraction of cations in zeolite when the maximum exchange capacity was reached.

Cation exchange selectivities: 2Na + →M 2+
The selectivity of both NaX and LTA zeolites is further analyzed through Kielland plots where the logarithm of the selectivity coefficient is plotted as a function of the equivalent fraction of cations in zeolites (XM).The Kielland plots are represented in figure 3 and 4 for NaX and LTA respectively.The plots gave a linear relation for all metals except for Cu 2+ .The line at 0 indicates that the corrected selectivity coefficient is equal to unity ( Na M = 1) where there is no preference between Na + and M 2+ ions.If the plots fall below this line this means that the zeolite is more selective for Na + while if they fall above it this indicates that the zeolite is more selective for M 2+ .From these Kielland plots, it can be deduced that for Cd 2+ and Pb 2+ ions the zeolite becomes more selective for the metal ions at  ̅  > 0.2 since all Kielland plots fall above the 0 line above this value.For Cu 2+ and Ni 2+ , M 2+ ions are more preferred almost throughout the 2Na + →M 2+ exchange reaction.From the Kielland plots, the Kielland coefficient C1, and ΔG 0 values were estimated.The obtained values are listed in Table 1.
For NaX, ΔG 0 value for Pb 2+ was the smallest while that of Ni 2+ was the highest.Based on this results, the order of selectivity will be Pb 2+ > Cu 2+ > Cd 2+ > Ni 2+ which is in accordance with what was found before using the distribution coefficient Kd.The Kielland coefficient, C1, is related to the energy of the steric limitation or jumping barrier for the ions in the sodalite cages.In general, the energy term for the steric limitation is larger as the |C1| value is larger.For NaX zeolite, the energy of the steric limitation increases in the order of Cd 2+ > Pb 2+ > Ni 2+ > Cu 2+ but the order of selectivity didn't follow the same order.These findings are related to the nature of the metal ions and mainly to the exchange velocities between the ligands and the aqueous metal ions which is generally expressed as eqn.(15). 13e(H2O)m) n+ + L  (Me(H2O)m-1L) n+ +H2O (15) where L represents H2O or other ligands and Me the metal ion.The exchange velocity constant (k_w) of water molecules with metallic ions (Table 2) is the fundamental step controlling the rate of ions removal from water.The obtained selectivity results vary with the order of the exchange velocity (k_w) values which seem to be the main parameter that controls their adsorption.Pb 2+ has the greatest exchange velocity (k_w) so it is the first to be adsorbed while Ni 2+ has the lowest k_w value so its adsorption is less favorable compared to Pb 2+ , Cu 2+ and Cd 2+ .
For LTA zeolite, the energy of the steric limitation increases in the order of Pb 2+ > Cd 2+ >Ni 2+ > Cu 2+ (Table 3) but the order of selectivity didn't also follow the same order.The order of selectivity according to ΔG 0 values will be Pb 2+ > Cd 2+ > Cu 2+ > Ni 2+ .LTA is more selective for Pb 2+ and less selective for Ni 2+ like in the case of NaX zeolite but the selectivity of Cu 2+ and Cd 2+ is reversed.So in the case LTA zeolite, the selectivity might be affected by the radii of the hydrated metal ions (Table 2) in addition to exchange velocity (k_w) since the pore size of LTA are smaller than that of NaX zeolite.

Conclusions
NaX and LTA zeolites were found to be very efficient in the removal of heavy metals from water.The theory of cation exchange was applied in order to understand the mechanism of heavy metals adsorption.For both zeolites, Kielland plots showed that Cd 2+ and Pb 2+ have higher selectivity over Ni 2+ at  ̅  > 0.2 while for Cu 2+ and Ni 2+ , M 2+ ions are more preferred almost throughout the 2Na + →M 2+ exchange reaction.
For NaX, the order of selectivity is Pb 2+ > Cu 2+ > Cd 2+ > Ni 2+ while for LTA the selectivity order is Pb 2+ > Cd 2+ > Cu 2+ > Ni 2+ .This order was directly controlled by exchange velocity (k_w) for NaX and the radii of the hydrated metal ions for LTA.

Table 2 .
The exchange velocity constant (k_w) and Hydrated ionic radius (R) for the four studied metal ions.