دراسة حركية – ثرموديناميكية لامتزاز أيونات الفوسفات والخلات على سطوح الخارصين/الالمنيوم Zn/Al)) النانوية ثنائية الهيدروكسيد

Kinetic- Thermodynamic Studies of Adsorption for Phosphate and Acetate Ions on the (Zn/Al) Nano Double Hydroxide Surfaces

Abstract

The pollution of the water is the most important reason that effects on human health. To eliminate this problem nanocomposite (Zn/Al-LDH) was synthesized, it has the ability to detention the anionics (PO43-, CH3COO-) between their layer (66)
This study dealt with the adsorption of some negative ions polluting water on the surfaces of (Zn/Al-LDH). These nanocomposites were diagnosed through the infrared spectroscopy technique for the zinc aluminum layers, as well as the atomic force microscope (AFM) to determine the atomic clusters of these layers.
In this study the effect of experimental factors which are,initial concentration of anions (10-50ppm) adsorbent dosage and contact time to investigate the efficiency of synthesized nanocomposite (Zn/Al-LDH) in removal of these anions. The equilibrium data were adequately represented by Langmuir and Freundlich models. using UV-visible spectroscopy and the results indicate that these anions adsorption onto the surface of (Zn/Al-LDH) nanocomposite follow to the Freundlich isotherm.
As for the effect of temperature on adsorption, which falls within the temperature range( 298-318 K), the results showed that the amount of adsorption increases with increasing temperature for the negative ions, phosphate and acetate, and by calculating the values of the (∆Gad ,∆Had , ∆Sad) thermodynamic functions, it became clear from the results that the ∆Had values were positive when the temperature increased, and this means the reaction is endothermic for the negative ions of phosphate and acetate. .
As for the values of (∆Gad), the results showed positive values when adsorption of phosphate ions This means that the adsorption is not spontaneous. As for the negative values that appeared, they indicate the spontaneity of adsorption at those temperatures. As for (∆Sad) for phosphate, all values were positive for MR=6, MR=3 ratios, and this means that adsorption is more random for these ratios, MR=2 ratio contains negative values, meaning that adsorption is less random
As for the (∆Gad) values for acetate ions, it was found that all values were negative for MR= 6 and MR= 3 ratios, and this indicates the spontaneity of adsorption for these ratios, as for MR= 2 ratio most of its values are positive, meaning that the adsorption on their surfaces is not spontaneous. The (∆Sad) values for acetate all showed positive values, meaning that the adsorption is random and irregular.
The effect of the pH solution (4,7,10) on the adsorption process was studied, and the results was found followed the order :
pH = 4 ˃ 7 ˃ 10
The results of adsorption of phosphate ions as the follow:
MR= 2 11.106 ˃ 10.518 ˃ 9.753
MR=3 22.212 ˃ 20.800 ˃ 19.506
MR=6 12.871 ˃ 11.576 ˃ 10.635
The results for acetate ions were as the follow:
MR= 2 18.895 ˃ 18.193 ˃ 17.667
MR=3 20.325 ˃ 19.886 ˃ 19.711
MR=6 13.725 ˃ 13.491 ˃ 13.374
As for the adsorption kinetics data of phosphate and acetate ions onto the surface of zinc aluminum dihydroxide layers when using different concentrations and fixed weights of the adsorbent surface and at 298K and pH = 7, it was found that the value of the adsorption rate constant kad for negative ions increases as the concentration of the solution decreases, as shown.
CO (ppm) = 10 ˃ 20 ˃ 30 ˃ 40 ˃ 50
The kad values at different concentrations of phosphate solution were as the follow:
MR= 2 0.5040 ˃ 0.4292 ˃ 0.3173 ˃ 0.1967 ˃ 0.0893
MR=3 0.5190 ˃ 0.4395 ˃ 0. 3761 ˃ 0.2369 ˃ 0.0047
MR=6 0.6310 ˃ 0.6023 ˃ 0.4997 ˃ 0.3911 ˃ 0.0951
Also kad values varied with different concentrations of acetate, with the highest value given at the lowest concentration, as shown:
MR=2 (0.7738 , 0.5806 , 0.6257 , 0.5277 , 0.5626 )
MR=3 (0.8039 , 0.5915 , 0.6407 , 0.7728 . 0.6333 )
MR=6 (0.6984 , 0.5491 , 0.6944 , 0.6603 , 0.5868 )
As for the change in the value of kad with increasing temperature and the concentration of the adsorbent being constant, it increases with increasing temperature and its value was as the follow:
T(K) = 318 ˃ 308 ˃ 298
Values of the adsorption rate constant when adsorption of phosphate ions at different temperatures:
MR=2 0.6448 ˃ 0.5053 ˃ 0.3603
MR=3 0.6726 ˃ 0.6437 ˃ 0.4353
MR=6 0.8574 ˃ 0.6401 ˃ 0.4046
Values of the adsorption rate constant when adsorption of acetate ions at different temperatures:

MR= 2 0.8565 ˃ 0.5988 ˃ 0.4737
MR=3 0.6038 ˃ 0. 4207 ˃ 0.4151
MR=6 0.7816 ˃ 0.5795 ˃ 0.4710
As for the activation energy values, Ea, they varied according to the molar ratios when adsorption of phosphate and acetate ions, where MR = 6 when immobilizing phosphate ions gave the best rate of adsorption, and its activation energy was equal to 29.643J/mole, while the best rate for adsorption of acetate ions was MR = 2, and its activation energy was 23.259 J/mol
But with regard to the limited X-ray ratio of the nanocomposites, it turns out that this dihydroxide nanocomposite has a crystalline structure and a crystalline unit with a size for each ratio of MR=2 (5.93nm), MR=3 ( 4.43nm) , MR=6 (4.35nm).
It turned out that the surfaces of the nanocomposite, as revealed in the atomic force microscope (AFM), were in the form of peaks, and some of them were in the form of spherical aggregates of different dimensions.