kinetic study of the sorption process with cu(ii) ions … no2... · the sorption process of the...

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Chemical Industry & Chemical Engineering Quarterly 15 (2) 63−67 (2009) CI&CEQ

63

CLAUDIA COBZARU1 CRISTINA CIBOTARU2

ADRIANA ROTARIU1

ADRIANA MARINOIU3 SPIRIDON OPREA1

1Technical University ”Gh. Asachi” Iasi, Faculty of Chemical

Engineering and Environmental Protection, D. Mangeron Ave. 71,

70050, Romania 2Antibiotic S.A., 1, 707410, Iasi,

Romania 3S.C. OLTCHIM-Research Center,

1 Uzinei St., Rm Vâlcea, 1000, Romania

SCIENTIFIC PAPER

UDC 546.56:544.354:549.6/.67

KINETIC STUDY OF THE SORPTION PROCESSWITH Cu(II) IONS ON CLINOPTILOLITE AND ANALCIME. EFFECTS OF TEMPERATURE AND PARTICLE SIZE

The sorption process of the Cu(II) ions on the clinoptilolite and analcime con-taining volcanic tuff was studied at different temperatures like 40, 60 and 80 °C. The effects of temperature and a particle size on the sorption capacity of the used zeolites were analysed.

Key words: clinoptilolite; analcime; sorption process.

The Cu(II) ion is a polluting agent frequently found in the composition of the waste waters resulting from different activities. High concentrations of Cu2+ have negative effects on the aquatic life and compli-cate the purification process of the residual waters. The removal of Cu2+ was intensively studied using dif-ferent materials like: natural zeolites [1-16], synthetic zeolites [17], organic resins [18,19], etc. It was shown that the natural zeolites possess a very good selecti-vity for Cu2+ from wastewaters. The sorption capacity is influenced by many factors like: composition, porosity, pre-treatment, temperature of the sorption process, ions’ concentrations, time, solution/zeolite ratio, etc.

Among natural zeolites, the most known are cli-noptilolite and analcime which could be found in vol-canic rocks at concentrations: 60-90%, 50-60% res-pectively. Such volcanic tuffs having in composition 70% clinoptilolite and 60% analcime [20] were sub-jected to investigation. The temperature effect on the retaining capacity of Cu2+ from residual waters by the sorption process shown by the two zeolites is pre-sented in this study.

Corresponding author: C. Cobzaru, Technical University ”Gh. Asachi” Iasi, Faculty of Chemical Engineering and Environmen-tal Protection, D. Mangeron Ave. 71, 70050, Romania. E-mail: [email protected] Paper received: 9.07.2008 Paper revised: 7.01.2009 Paper accepted: 17.01.2009

EXPERIMENTAL

Materials

The chemical compositions of the two types of volcanic tuff, containing 70% clinoptilolite and 60% analcime, are presented in Table 1 [20].

Table 1. Chemical composition (% of oxides) of the two types of volcanic tuff

Chemical composition Clinoptilolite Analcime

SiO2 69.98 68.38

Al2O3 12.60 12.2

Fe2O3 1.04 1.39

CaO 4.80 3.24

MgO 0.80 0.52

K2O 2.20 2.03

Na2O 0.40 2.52

TiO2 0.18 0.25

Samples of particles size within 0.5-0.6 mm and 0.12-0.25 mm were prepared from each material.

The sorption process

In the first step, the samples were washed with distilled water, dried, ground and sieved. In the se-cond step, the products were brought in contact with 0.10 N CuSO4⋅5H2O solution in a volcanic tuff/solution ratio of 1/10 at 80, 60 and 40 °C for several hours.

C. COBZARU et al.: KINETIC STUDY OF THE SORPTION PROCESS… CI&CEQ 15 (2) 63−67 (2009)

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The pH value of the exchange solution before the sorption process was up to 6.0. The samples for the analysis were collected after certain time periods. By the completion of the sorption process, the samples were washed several times with distilled water and dried at 100 °C. In this way, clinoptilolite- and anal-cime-based volcanic tuffs containing Cu ions were prepared. The pH value of the exchange solution after the sorption process was up to 6.2. The sorption capacity of the tuff was determined by the analysis of the Cu amount left in the solution after the sorption process by the iodometric titration method [21].

RESULTS AND DISCUSSIONS

The behavior of the volcanic tuff, based on cli-noptilolite and analcime, during the sorption process with Cu2+ at 80 °C, was followed by the variation of the sorption capacity in time (Fig. 1).

Fig. 1. Kinetic curves of the sorption process for Cu2+ with the samples of the particle size within 0.5-0.6 mm range at 80 °C.

As Fig. 1 presents, the sorption process for the two zeolites reached an equilibrium after aproximately

1 h when considerable amounts of Cu2+ were retained by zeolites. Nevertheless, the clinoptilolite based-tuff showed a higher capacity of ion exchange in compa-rison with the analcime-based one. Another situation was observed when the sorption process took place at lower temperatures. As shown in Fig. 2, at 60 °C the process was faster (aproximatelly 30 min) but the amounts of retained Cu2+ were decreased.

As expected, under identical conditions, at even lower temperature like 40 °C, the process reached the equilibrium after only 10 min and the amounts of sorbed Cu2+ were very small (Fig. 3).

The main difference at this temperature was the inverse situation with regard to the amounts of the sorbed Cu2+ by the two tuffs. At 40 °C, analcime-ba-sed tuff contained more Cu2+ than the clinoptilolite– -based one. For a better comparison, the variation of the sorption capacity at different temperatures of each volcanic type for Cu ions is represented in Fig. 4.

Copper cations in an aqueous solution are hyd-rated with water molecules generating cationic com-plexes [22-26]. It was already shown in the literature that between the simple Cu2+ and their complex form an equilibrium exists. As indicated by numerous lite-rature studies [8,27-31] the processes like outer- or inner-sphere complexation, Cu precipitation in a low- -solubility phase or salt adsorption take place thus leading to a higher content of retained copper on the zeolite. The precipation could be explained based on the presence of the hydroxyl groups adsorbed on the zeolites, groups that are then liberated during the sorption process and reacted with Cu2+ thus genera-ting probably Cu(OH)2 as precipitate. Moreover, the possible hydrolysis of the Cu aqueous complex ions could lead to the precipation of the copper.

The literature data indicate that metals form either outer- or inner-sphere complexes with surface




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sites depending on its concentration. The following reactions represent the two mechanisms for Cu2+ [8]: inner-sphere complex,

S OH + Cu2+ OCu ++ H+S

inner-sphere complex,

S OH+ Cu2+ +H+S2 O( )2 Cu 2

and outer-sphere complex

+ Cu2+ +S O( )2 Cu 2-S O( )

2-

.......

where the symbol S corresponds to the surface cen-tral metal (i.e. Si and Al).

The outer-sphere complex is associated with the ion-exchange reactions between metal ions and sur-face counterbalance cations whereas the inner-sphere complex is formed when the metal concentration in-creases and metal ions are forced into internal sur-face sites. The latter type of the complexation leads to more stable surface groups due to the formation of covalent bonds. Moreover, hydrogen cations are re-leased as products and the process causes a total decrease in the solution pH.

In our case, it is possible for outer-sphere com-plexes between Cu2+ and surface sites to be formed, following the ion-exchange process.

It is also worth noting that the amount of Cu2+ removed by the two zeolites at 80 °C was much higher than the quantities removed at 60 and 40 °C (Fig. 1). In this case, we consider that, with the in-crease in temperature, the kinetic energy of Cu ions

increases and thus they are capable to move faster and to reach more sites (also internal) more effici-ently. It is this energy that inhibits equilibrium to be reached fast since surface complexes are not so stable under the increased temperature. However, the analcime-based tuff contains more Cu2+ than the cli-noptilolite–based one at 40 °C (Fig. 3). Ahmed et al. [32] reported that ions with low charge densities were present in a less hydrated environment and so inter-acted more strongly with the zeolite framework. Bar-rer et al. [33] also found that larger, less strongly hydrated cations tended to concentrate in the zeolite in contrast to smaller, more hydrated ions, which ten-ded to remain in the solution. Analcime shows less preference for the smaller divalent cations because of their stronger ability to hold water molecules around them. Based on this, we could explain the behaviour of the Cu2+ on analcime as being caused by the cation hydration energy and by the cation size.

It is well-known that the retention of Cu2+ on zeolites is mainly controlled by the internal diffusion of the exchanging ions through the zeolite pores. Thus, the small ions possess a faster acces to the internal chanels of the zeolite and could easily replace the larger ones. The ionic radii of the cations existing in the structures of the two zeolites that could be ex-changed with Cu2+ (ionic radius of 0.96 Å) from the solution are indicated in Table 2 [34].

The clinoptilolite-based zeolite consists of a bidi-mensional system of chanels of 8 and 10 tetrahedral cavities with an effective diameter of the access win-dows into the chanels and cavities of about 4.4×3 Å and 7.9×3.5 Å, respectivelly. The analcime-based zeolite consists of a cubic structure consisting of rings

(a)

(b)

Fig. 4. The variation of the sorption capacity of the two zeolites, a) clinoptilolite and b) analcime, as a function of temperature.


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of 4-6 and 8 tetrahedral cavities that form 16 larger interconnected cavities and a secondary series of 24 smaller cavities. The big cavities are filled up with wa-ter molecules and the small ones are partially filled up with Na+ [20]. In order to verify the above-mentioned afirmations with regard to the limitations due to the internal diffusion, we performed the ion exchange pro-cesses on the two zeolites of the particle size within 0.12-0.25 mm at 80 °C (Fig. 5).

Table 2. The ionic radii of the cations existing in the two studied zeolites

Ions Na+ Ca2+ K+

Ionic radius, Å 1 1.06 1.33

Fig. 5. Kinetic curves of the sorption process for Cu2+ with samples (clinoptilolite and analcime) of the particle size

within 0.12-0.25 mm range at 80 °C.

As shown in Fig. 5, the samples of particle size within 0.12-0.25 mm range reached the equilibrium of the sorption process faster than in the case of the zeolites of a higher particle size (0.5-0.6 mm). In Fig. 6, the difference in the metal uptake for the samples with particle sizes 0.12-0.25 mm and 0.5-0.6 mm, respectively, is depicted.

No significant difference in sorbed Cu concen-trations with the decrease of a particle size was ob-served since the differences in sorbed amounts are within the statistical error. Although the decrease in a particle size is expected to affect metal sorption in a positive way, thus it is also known that the particle size does not significantly affect batch experiments as it affects sorption experiments in columns [35].

Since clinoptilolite and analcime samples were washed prior to measurements, differences in specific surface are attributed to the structural status (Table 3). It is obvious that smaller particles (0.12-0.25 mm) have a smaller specific surface than that of larger particles (0.5-0.6) for both zeolites. Nevertheless, the difference in the specific surfaces seems not to be big enough to lead to a clear difference on sorbed Cu concentrations.

Table 3. Total specific surface of clinoptilolite and analcime samples

Sample Particle size range, mm S / m2 g-1

Clinoptilolite 0.12-0.25 30.73

0.5-0.6 34.69

Analcime 0.12-0.25 14.64

0.5-0.6 20.58

(a)

(b)

Fig. 6. Kinetic curves of the sorption process for Cu2+ for both clinoptilolite and analcime with the particle size 0.12-0.25 mm and 0.5-0.6 mm respectively, at 80 °C.


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CONCLUSION

We studied the kinetics of the ion exchange process with Cu2+ on clinoptilolite and analcime-ba-sed zeolites with the particle size within 0.5-0.6 mm and 0.12-0.25 mm range at different temperatures. The obtained results indicate that the temperature favors the sorption process of zeolites with Cu2+. Thus, the amounts of Cu2+ retained on the zeolites are much higher at 80 °C than at lower temperatures like 60 and 40 °C. Instead, the equilibrium of the sorption process is faster reached at lower tempe-ratures than at 80 °C. The samples of the particle size within 0.12-0.25 mm range reach the equilibrium of the sorption process faster than in the case of the zeolites of higher particle size (0.5-0.6 mm) and the amount of Cu2+ retained on the two zeolites is small.

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kinetic study of the sorption process with cu(ii) ions … no2... · the sorption process of the...

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