Kinetics and Thermodynamic of Sorption of Chromate by HDTMA –Exchanged Zeolite
Vandana Swarnkar and Radha Tomar*
S.O.S. in Chemistry, Jiwaji University, Gwalior M.P. 474011.
*Corresponding Author E-mail: sonivandana20@gmail.com
ABSTRACT:
Zeolites are hydrated crystalline aluminosilicate containing exchangeable alkaline and alkaline earth cations in their structural frameworks. Since zeolite have permanent negative charges on their surfaces, they have no affinity for anions. However recent studies have shown that modification of zeolites with certain surfactants or metal cations yield sorbents with a strong affinity for many anions. In this paper, modification of zeolites ERIONITE (E), COWLESITE (C), WILLHENDERSONITE (W) were performed by exchange of naturally occurring cations with HDTMA-Br ion. The exchanged zeolites were used to sorbs chromate from aqueous solution. Among parameters investigated were effect of pH, chromate initial concentrations, contact time and temperature. The maximum exchange capacity of HDTMA exchanged zeolite was obtained when using solution with initial pH of 5. Zeolite (C) gives highest HDTMA exchanged capacity compared to other zeolites. The results showed that Cr(VI) sorption by surfactant modified zeolites occurred at about pH 5 and increased as pH increased and reaching maximum at equilibrium pH increased and reaching maximum at equilibrium pH about 7. On the other hand, almost no chromate sorption occurred unexchanged zeolites. This indicates that HDTMA ion exchanged zeolites is taking part on Cr (VI) sorption via surface precipitation. The results also showed that the sorption capacities of 0.41, 0.21, and 0.19 mmol/g at 25oC for HDTMA – exchanged zeolite (ERI- SMZ) HDTMA-E, HDTMA – exchanged zeolite(COW-SMZ) HDTMA–C and HDTMA – exchanged zeolite (WILL- SMZ) HDTMA-W .The amounts of sorption of Cr(VI) by HDTMA exchanged zeolite increased as temperature increased from 15oC to 35oC indicating that the process is endothermic. The free energy change (ΔGo) for the sorption at 25oC were -1.01, -24.06 and -24.56 KJ/mol for HDTMA- E, HDTMA-C and HDTMA-W, respectively. The negative values of (ΔGo) meant that the sorption of Cr(VI) ion on HDTMA–exchanged zeolite was spontaneous perhaps because the HDTMA-Br had high affinity towards the chromium ion as indicated by a low Ksp value of HDTMA chromate. A slightly positive entropy change for sorption of Cr (VI) ion on HDTMA–exchanged Zeolite could be due to fixation of the ions on the HDTMA exchanged sites that was randomly distributed on the sorbents. The kinetic study showed that the Cr (VI) sorption followed first–order kinetic constants for the sorption are 11.7 x 10-3, 56.9 x 10-3 and 8.1 x 10-3min-1 for HDTMA-E, HDTMA-C, and HDTMA- W respectively.
KEYWORDS: HDTMA-exchanged zeolite, sorption, thermodynamic , kinetics
In the last years the levels of several toxic metals in the superficial waters have increased gradually due to pollution caused by the discharges of industrial and municipal waste waters. The chromium is a heavy metal that can be very frequently found in a high proportion in waste water discharges from different industries1.
There are greater concerns about heavy metal contamination2,3 in the receiving water system and land . High levels of heavy metals can damage soil fertility and may affect productivity4,5 .Chromium exist in oxidation states +2, +3, +4, +5 and +6,but The most common, stable and abundant forms are Cr (III) and Cr (VI)6. The Cr (VI) species is more toxic and carcinogenic than Cr (III), however, it is possible that Cr (III) may be oxidized to Cr (VI) in the appropriate conditions hence the toxicity of Cr (VI) takes place usually, Cr (III) is readily oxidized to hexavalent state at high pH7. Leather and chromium planting industries are the major causes for environmental influx of chromium8,9. In aqueous solutions, Cr (VI) is very soluble and exists in the form of chromic acid (H2CrO4) and in the form of dichromate (Cr2O7-2) while in neutral solutions, Cr (VI) is present in the form of HCrO-4 and CrO4-2(10). Cr (III) is readily oxidized to the hexavalent state at high pH11.Cr (VI) has been recognized as a probable agent of lung cancer and it also produces gastrointestinal disorders, dermatitis and ulceration of skin in man12. The world health organization (WHO) recommends a maximum level of 50μg/L (PPb) for Cr(VI) in drinking water13 and the national institute for occupational safety and health (NIOSH) recommends that the levels of chromium should be reduced to 10-3mg/m314.There are a number of methods employed(15-19) for removal of hexavalent chromium from industrial waste water such as the use of various types of adsorbents. In the last years the adsorption of Cr(VI) on several low coast biosorbents has been investigated. Some of the biosorbents tested include Eucalyptus bark, Agave lechugilla biomass, yonimbe bark and grap stalk20-22. Recently the ion exchange resin in the cationic form23, Zeolite24 betonites25 and activated carbon26 have been used to remove Cr. The main industrial activities that cause chromium pollution are mining, metal finishing and leather tanning. In aqueous systems the chromium can be mainly found as Cr(III) can be considered a bioelementry since it plays an important role in the metabolism of plants and animals. On the other hand, Cr (VI) is very toxic even at very low concentrations, and causes mutagenic effects in plants and animals.
The adsorption of Cr (VI) on various adsorbents has been studied extensively as an alternative process for removing Cr (VI) from water solutions. The activated carbon is the most studied adsorbent and his capacity for adsorbing Cr (VI) is dependent upon the solution pH. In the last years the adsorption of Cr (VI) on several low cost biosorbents has been investigated. Zeolites are crystalline, hydrated aluminosilicate minerals containing exchangeable alkaline and alkaline earth metal cations normally of group I and group II elements, in particular, magnesium, calcium, strontium, and barium, as well as water molecules in their structural frameworks structurally, they are complex porous, crystalline inorganic polymers, enclosing interconnected cavities in which the metal ions and water molecules are contained. They are based on an infinitely extending three dimentional network of AlO4 and SiO4 tetrahedra linked to each other by sharing all the oxygen ions27.
The most naturally occurring materials have negative charges on their external surfaces, which will prevent sorption and retention of anions, efforts were made to convert the surface charges from negative to positive by surface modification to increase their anoin retention28 due to strong sorption on negatively charged materials, cationic surfactants were used extensively for modification of clay minerals and zeolites 29-31.
The surfactant – modified zeolites were initially intended as sorbents to remove hydrophobic organic compounds from water because of their higher organic carbon contents after modification. Subsequently, researchers found that in the presence of surfactants (i.e. beyond 100% of external cation exchange capacity of zeolites). The sorbed surfactant molecules from an admicelle formation, resulting in strong sorption of anionic contaminants such as arsenate and nitrate32. The removal of anionic contaminants from water by SMZ was attributed to surface anion exchange as indicated by the stoichiometry between anion sorbed and counterions desorbed most of these studies were limited to batch tests, even through a preliminary pilot scale test was performed on using SMZ to remove chromate from water.
The perpose of this study was to conduct the sorption of chromate by HDTMA- exchanged zeolite at different parameter like kinetics and thermodynamic.
2. MATERIAL AND METHOD:
2.1Chemicals and reagents:
Sodium nitrate, potassium nitrate, calcium nitrate, aluminium nitrate were purchased from merk, surfactant modifier HDTMA, potassium chromate, sulphuric acid were procude from Qualigens. A Cr+6 complexing reagent solution of 1, 5 diphenylcarbazide (DPC) HDTMA was prepared by dissolving 0.25 gm of DPC in 100 ml of acetone, and stored in amber –colored bottle. All reagents were analytical grade and the solution was prepared in DDIW.
2.2 Surfactant modification:
The surfactant used is hexadecyltrimethyl ammonium HDTMA, a cationic surfactant that as bromide as the balancing counter ion. The synthesize zeolite (HDTMA-E, HDTMA-C, HDTMA-W) 5gm were loaded with cationic surfactant HDTMA 0.25 gm these mixtures were removed. Zeolites were washed with to portions of distilled water and then air-dried.
2.3 Characterization:
Powder X-ray diffraction (XRD) was recorded on diffractometer with Cu Kά radiation (Y= 1.54059). FTIR spectra of E and HDTMA-E, C and HDTMA- C, and W and HDTMA- W were recorded on a FTIR - spectrometer (NICOLET-410 spectrometer).
2.4 Batch mode sorption studies:
Batch mode sorption studies were carried out by shaking 100 ml conical flasks containing 0.1gm of ECW- HDTMA and 50ml of chromate solution of desired concentration of on thermostate water bath shaker at 150 rpm, 25oC and at an initial pH 1.0. The solution pH was adjusted with 0.1N HCL and 0.1N NaOH solutions. At the end of the adsorption period the supernatant was separated by centrifugation 3000 rpm for 30 minute. Then the concentratration of the residual chromate ion was determined by UV-visible spectrophotometer. The amount of chromate adsorbed was calculated from the concentration in the solution before and after adsorption. Effect of pH on adsorption of chromate on to HDTMA-ECW was studied for the chromate concentrations. Effect of contact time was studied by withdrawing the sample from the shaker at predetermined time intervals and residual chromate concentration was analysed as above. Effect of adsorbent dose was studied with different adsorbate doses (50-250mg 50 mL-1) for the chromate concentration. Langmuir isotherm was used to analyse the equilibrium adsorption data.
3. RESULTS AND DISCUSSION:
3.1 Charecterization of the adsorbents:
Fig. 1 a, b, c shows the XRD- patterns of HDTMA-E, HDTMA-C, and HDTMA-W. The XRD patterns of all zeolites gave a clear diffraction peaks, even though not sharp. X- ray powder diffraction pattern of these zeoltes have been recorded using Cu Kά – radiation in a range 2θ= 5o-120o at a scanning speed of 1 step / sec. The FTIR spectra of HDTMA-ECW are shown in fig. 2 a, b, c. It is found in the range 950-1250cm-1 and 420-500cm-1strongest vibration at 950-1250cm-1 is assigned to T-O stetching and the next strongest band at 420-500cm-1 is assigned to T-O bending mode (T= Si or Al). the reason 1800 and 3500cm-1 presents to measure absorption bands centered at 3420cm-1(hydrogen bonded –OH group) and 2921cm-1(C-H stretching of – CH2 group). The intensity of the bond at 2921cm-1 increase due to the increase in aliphatic carbon content in HDTMA-ECW which in turn is due to the adsorption of HDTMA on to zeolite surface.
Fig .1a: X-Ray diffractogram of surfactant modified Erionite
Fig.1b : X-Ray diffractogram of surfactant modified Cowlesite
Fig .1c: X-Ray diffractogram of surfactant modified Willhendersonite
Fig.2a: FTIR spectra of surfactant modified Erionite
Fig.2b: FTIR spectra of surfactant modified Cowlesite
Fig.2c : FTIR spectra of surfactant modified Willhendersonite
3.2 Cr (VI) up take by HDTMA-E, HDTMA-C and HDTMA-W:
Chromate exchanged in zeolites (HDTMA- E,C,W) increased by increasing the initial pH value20. In the present study, the highest exchange capacity of Cr (VI) was obtained when using solutions with initial pH of 1, which changed to give a final pH value of 9. The results reveal that the sorption percentage is maximum at pH 7.
The effect of amount of HDTMA exchanged zeolite on the removal of Cr(VI) is shown in (Fig-3). The results showed that sorption percentage increase as the dose of the adsorbent increases. This increasing trend is expected since the number of adsorption sites also increase.
3.3 Effect of Cr (VI) concentration on sorption:
The sorption of CrO4-2 ion on HDTMA exchanged zeolites were conducted with various initial concentrations (0.01-0.05N) at initial pH 1(equilibrium pH 7). The results show that the sorption capacities increased with increasing initial concentration (Fig-4 ). In order to establish the maximum Cr (VI) sorption capacity, the langmuir equation of the following form was applied to the sorption equilibria at different concentration:
Ce/qe = 1/ bqm + Ce/qm (1)
Where Ce is the concentration of the metal solution at equilibrium (mmol/L), qe the amount of As (V) sorbed at equilibrium (mmol/g), qm the maximum sorption capacity and b is the constant related to binding energy of sorption system. The linearity of the plots (Fig-) shows that the sorption follows the langmuir model with maximum capacities of -.036, -1.63 and -1.20 mmol/g for HDTMA-E, HDTMA-C and HDTMA-W, respectively (table.1). It appears that there is a strong correlation (R2 = 0.24) between Cr (VI) sorption capacity to exchange capacity of HDTMA in zeolite.
Table1: first order Kinetic constants for Cr(VI) onto HDTMA-exchanged zeolites.
|
Zeolites |
First order expression |
|
|
|
k |
R2 |
|
E-HDTMA |
-0.01171 |
0.06262 |
|
C-HDTMA |
-0.05698 |
0.6243 |
|
W-HDTMA |
0.00811 |
0.3676 |
3.4 Thermodynamic parameters:
The amount of sorption of Cr(VI) by HDTMA- exchanged zeolite increased as temperature increase from 15oC-35oC. The following relationship has been used to evaluate thermodynamic parameters of the standard Gibbs free energy ∆Go, enthalpy ∆Ho and entropy ∆So :
∆Go = -RT ln Kd (2)
And
Log Kd = - (∆Ho / 2.303R) 1/ T + ∆So / 2.303R (3)
Where Kd is the equilibrium constant calculated as the ratio between sorption capacity and equilibrium concentration. The change in energy (∆Go) for Cr(VI) sorption were calculated using eq. 2 and calculated Kd values were found to be 373.62, 618.06 and 273.99 KJ/ mol for HDTMA-E, HDTMA-C and HDTMA-W respectively at 25oC. The negative values of ∆Go mean that the sorption of Cr (VI) ion on HDTMA–exchanged zeolite is spontaneous. From eq.3 a plot log Kd vs 1/T (Fig-3) would give ∆Ho and ∆So which are given in table (2). The positive value of ∆Ho indicate that the endothermic nature of sorption process. A slightly positive entropy Change for sorption of Cr (VI) ion on HDTMA- exchanged zeolite is due to fixation of ions on the exchange sites.
Table2: Langmuir Adsorption isotherm parameters of Cr(VI) on HDTMA-Zeolites
|
Zeolites |
Cr(VI) maximum sorption(qm) mgg-1 |
Langmuir binding energy constant, (b) Lmg-1 |
|
E-HDTMA |
-0.3633 |
-0.1553 |
|
C-HDTMA |
1.6345 |
0.62719 |
|
W-HDTMA |
1.2097 |
-0.1564 |
3.5 Sorption kinetics of Cr (VI) by HDTMA – exchange zeolites:
The kinetics of sorption of Cr (VI) by modified zeolites was studied in batch experiments. The first order rate constants for sorption of Cr (VI) ions determined using Lagergren eqn.
Log (qe-q) = log qe- (Kads / 2.303) t (4)
Table3: Thermodynamic Parameters for adsorption of Cr(VI) by HDTMA-Zeolites
|
Zeolites |
Temp(K) |
∆G0(KJ/mol) |
∆H0(KJ/mol) |
∆S0(J/Kmol) |
|
E-HDTMA |
288 |
1.18045 |
98.694 |
-24.5244 |
|
298 |
1.0177 |
|||
|
308 |
0.9065 |
|||
|
C-HDTMA |
288 |
-23.2582 |
69.485 |
80.9990 |
|
298 |
-24.0682 |
|||
|
308 |
-24.8782 |
|||
|
W-HDTMA |
288 |
-25.7253 |
98.6946 |
82.7224 |
|
298 |
-24.5685 |
|||
|
308 |
-25.3798 |
Where q and qe are the amount of metal ions adsorbed mgg-1 at time t and at equilibrium respectively, Kads is the adsorption rate constant. A straight line plot of Log (qe-q) vs t (Fig-4) and (table.3) indicates the applicability of the first order kinetic for Cr (VI) sorption. The Kads values calculated from the slope of the plot were -11.7 X10-3, 56.9 X 10-3 and 8.1 X 10-3 min-1 for the HDTMA-E, HDTMA-C and HDTMA-W.
4. CONCLUSIONS:
HDTMA–Br was used to modify the surface of zeolites (Erionite, Cowlesite and Willhendersonite). Removal of chromate ion from aqueous solution by surfactant modifier was found to be effective. HDTMA-E, HDTMA-C, HDTMA-W has been characterized using X-ray powder diffraction and FTIR techniques. Optimum pH for chromate removal was found to be pH 7.0. The results also show was that the sorption percentage and Kd value increase with increasing initial Cr (VI) concentrations. The sorption followed langmuir model with maximum Kd value of 737.62, 618.06 and -24.56 at 25 oC for HDTMA-E, HDTMA-C and HDTMA-W respectively. The amount of Cr (VI) by HDTMA-exchanged zeolite increased as temperature increase from 15oC to 35oC indicated that the process was endothermic. The free energy change ∆Go for the sorption at 25 oC were -1.01, -24.06 and -24.56 KJ/mol for HDTMA- E, HDTMA-C and HDTMA-W respectively. The negative value of ∆Go shows that the sorption of Cr (VI) ions on HDTMA exchanged zeolite is spontaneous. A slightly entropy change for sorption of Cr (VI) ion on HDTMA exchanged zeolite could be due to fixation of the ions on the HDTMA exchange site that randomly distributed on the sorbents. The kinetic study showed that Cr (VI) sorption followed 1st order kinetic model. The first order rate constant for the sorption were -11.7 X10-3, 56.9 X 10-3 and 8.1 X 10-3 min-1 for HDTMA- E, HDTMA-C and HDTMA-W respectively.
5. ACKNOWLEDGEMENT:
The authors acknowledge to Head IIT, IIT Roorkee providing necessary instrumental facilities for XRD.
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Received on 19.05.2010 Modified on 08.06.2010
Accepted on 24.06.2010 © AJRC All right reserved
Asian J. Research Chem. 4(1): January 2011; Page 44-49