Materials and methods

Mineralogical and Physicochemical Characterization

The analysis by X-ray diffractometer showed mineralogical phases such as; heulandite; mordenite; smectite; quartz; cristobalite and feldspar, evidencing a natural zeolite with the presence of impurities. Figure No. 1, shows the mineralogical species.

Figure No 1. X-ray diffractogram showing the main mineralogical species present in samples 

Figure No 2. SEM micrograph showing heulandite and mordenite embedded in clay 

Table No. 1, shows the chemical composition determined by X-ray analysis.

Table No 1. Chemical composition of the as received material determined by means of X-ray spectrometry 

The chemical analysis indicates that the zeolite contains alumina silicates with a high content of silica and is strongly alkaline.

The following laboratory determinations were performed:

Specific gravity = 2.32 g/cm³, Specific surface area = 33.56 m²/g, Cation exchange capacity (CEC) = 50 meq/100 g. Low specific surface area indicates low porosity due to low zeolitic crystallinity (20).

NH₄ ⁺, Zn⁺² cation exchange tests by varying time

Cation uptake by the zeolite was calculated as the difference between the amount in the starting and final solutions. Figure No. 3, shows cation uptake for each of the ions examined, expressed as milligrams per gram of zeolite versus contact time.

Figure 3 clearly reveals that after roughly 20 hours in the operating conditions the amount of cation exchanged remains practically constant. Thus, in subsequent characterization tests, in line with experiments reported in the literature, the contact time between the zeolite material and the cation containing solutions was set at 24 hours to ensure the steady-state condition (21).

Tests by varying cation concentrations in the starting solution

Tests were performed following the above described procedures and for a contact time of 24 hours on the untreated and Na-exchanged material and using solutions containing different cation concentrations, examined singly. Figures No. 4, 5 and 6 show the NH₄ ⁺, Pb⁺², Cd⁺², Cu⁺², Zn⁺² and Mn⁺² uptake respectively for the untreated and Na-exchanged material, expressed as milligrams per g of zeolite, and as percentage with respect to the starting solution, versus cation content in the starting solution (mg/L)

Figure No 3. NH₄ ⁺ and Zn⁺² uptake (mg per g of zeolite material) versus time 

Figure No 4. Uptake by untreated and Na-exchanged zeolite: (a) NH₄ ⁺ and (b) Pb⁺² 

Correlation of experimental data with the Freundlich and Langmuir isotherms

Tables No. 2 and 3 show for the untreated and Na-exchanged zeolite material respectively the values of the Freundlich and Langmuir isotherm constants and the corresponding correlation coefficients (R) for the experimental equilibrium values.

Table No. 2. Untreated zeolite material. Values of Freundlich and Langmuir isotherm constants and corresponding correlation coefficients (R) for experimental equilibrium values

Figure No 5. Uptake by untreated and Na-exchanged zeolite: (c) Cd⁺² and (d) Cu⁺² 

Figure No 6  . Uptake by untreated and Na-exchanged zeolite: (e) Zn⁺² and (f) Mn⁺² 

Table No 3. Na-exchanged zeolite material. Values of Freundlich and Langmuir isotherm constants and corresponding correlation coefficients (R) for experimental equilibrium values 

Examination of the results obtained for the ion exchange tests on the untreated material with size class in the -5+0.7 mm range using solutions containing the single cations examined reveals that: The exchange equilibrium relationships are well described by the Freundlich isotherm. It can be seen that the values ​​of R, are close to unity and that the values ​​of n are greater than 2, which indicates a favorable adsorption process on the surface of the zeolite (22).

The selectivity of the zeolitic material towards the cations examined differs in the order: Pb⁺²> Cd⁺²> Cu⁺²> Zn⁺²> Mn⁺²> NH₄ ⁺, in agreement with the findings reported by other researchers for metallic ions. Zamzow [23] (cited by Kraljevic Pavelic [24, p.2]) aligned the selectivity of clinoptilolite zeolite as follows: Pb⁺²> Cd⁺²> Cs⁺> Cu⁺²> Co⁺²> Cr⁺³ >Zn⁺²> Ni⁺²> Hg⁺². This depends on the fact that adsorption phenomena are influenced by charge density as well as by hydration energy and ionic radius of the cations.

For the metal ions examined here, exchange capacity decreases with ionic radius, greater exchange capacity corresponding to greater ionic radius (Pb⁺²). Charge density being equal, the smaller ions (Mn⁺²) exhibit higher electric charge as they hydrate more readily, encountering greater difficulty in penetrating the zeolite pores to reach the available exchange sites. Note that exchange capacity towards NH₄ ⁺, though not negligible, is inferior not only for Pb⁺², as occurs for other zeolitic materials, but also to the other metal ions examined. Presumably the significant amounts of clay contained in the zeolitic material tested here contribute to uptake of the metal ions examined.

For the material Na-exchanged with a 2N NaCl solution, the results obtained show that: The metal cation exchange equilibrium relationships are better described by the Langmuir isotherm than by the Freundlich isotherm and denotes that adsorption is multilayer and that there is surface heterogeneity in the adsorbent (25).

This can be explained by the fact that during pretreatment of the zeolite material with a strongly alkaline solution (2N NaCl) the ions present are replaced by Na in the exchange sites. Subsequently, when the Na-exchanged material comes into contact with solutions containing bivalent metal cations, exchange occurs more readily with Na and greater exchange capacity is achieved for lower cation concentrations in the starting solution. Only for NH₄ ⁺ did the Freundlich isotherm provide a better correlation with the experimental results. For this cation, the selectivity of the zeolite is strong due to the lower hydrated ionic radius that it presents. Petrous [26] (cited by Santiago [27, p. 14]) confirmed that selectivity of clinoptilolite is higher for NH₄ ⁺ due to its lower hydrated radius ion 3.3 Å. Similarly to the untreated zeolitic material, the Na-exchanged zeolite also exhibits different selectivity in the order: Pb⁺²> Cd⁺²> Zn⁺²> Cu⁺²> Mn⁺²> NH₄ ⁺, though in this case the positions of Cu⁺² and Zn⁺² are inverted. Further investigation is warranted to explain this behavior, which can presumably be attributed to changes in the properties of the Na-exchanged material. Under the same operating conditions, the Na-exchanged material proved to be more effective in removing the cations than the untreated material. As an example, for a cation concentration of 1000 mg/L in the feed solution, uptake by the Na-exchanged material was 32%, 55%, 47%, 25%, 53% and 42% higher for NH₄ ⁺, Pb⁺², Cd⁺², Cu⁺², Zn⁺² and Mn⁺² respectively than the untreated material. The improvement of the Na-exchanged removal capacity can be explained by the enrichment of Na⁺ cations on the natural material and create new available sites and that are accessible for incoming cations [27].