Interaction between Kaolin and Urea in Organoclay and Its Impact on Removing Methylene Blue from Aqueous Solution

Interaction between kaolin (particle size 53 and 106 μm) and urea was studied by infrared spectroscopy and powder X-ray diffraction. Interaction was found to be dependent on the particle size of kaolin raw material. Nature of interaction achieved through the formation of hydrogen bonds between urea and both AlOH and Si-O surface of kaolinite. Effect of temperature on equilibrium adsorption of methylene blue (MB) from aqueous solution using kaolin also studied, the results were analyzed by Langmuir and frendlich isotherms. Thermodynamic parameters such as ΔG, ΔH and ΔS were calculated. Results suggested that the MB adsorption on kaolin was spontaneous and exothermic process.


Introduction
Kaolin is one of the clay materials widely used in a large number of applications such as in ceramics, paper coating, paper filling, paint extender rubber filler, cracking catalyst or cements, oil refinery and water treatment (adsorption of dyes and other pollutant) [1]- [4] with the chemical composition Al 2 Si 2 O 5 (OH) 4 .For each application the engineering properties of the clays must be carefully designed to obtain the desired result.Clays are usually defined as natural materials presenting fine granulometry.Often, these materials exhibit a lamellar structure as a consequence of the crystalline arrangement formed by the silicon and aluminum oxides, which are the main components of clays.These structures are displayed by these materials.Kaolinite is a common 1:1 dioctahedral phyllosilicate (clay) mineral found throughout the world in highly-weathered environments.Being a 1:1 mineral, it has one silica tetrahedral layer and one aluminum octahedral layer combine to form a unique structural arrangement in which sheets of tetrahedral and octahedral overlap each other, leading to structural changes such as 2:1 (one octahedral sheet between two tetrahedral sheets) and 1:1 (one tetrahedral sheet to one octahedral sheet) that characterize the various clay minerals [5] [6].
Kaolinite is a 1:1 tetrahedral aluminosilicate with two distinct basal cleavge faces.One of them consist of tetrahedral siloxane surface formed by very chemically inert Si-O-Si bonds, while the other constituted by an Octahedral sheet Al(OH) 3 can be distributed and broken bands have the ability to accommodate OH group.The layers are bonded by hydrogen bonds.Hydrogen bonds occur between oppositely charged ends of a permanent dipole [7] [8].
Interaction between clays and organic compounds have received increase attention due to the wide ranges of applications especially in chromatography separations [9], to remove organic pollutants from air [10], and water [11], and to develop improved formulation for pesticides and as chemical sensor and molecular sieves [12].
This research primarily studies the nature intercalations between kaolinite and urea by using FTIR and XRD.Furthermore, it also studies the impacts to adsorption capacities made by the interaction, the kineticsod adsorption and application to remove dye from aqueous solution.

Experimental
Kaolinite used in this study was hydrated aluninum silicate, which was provided from general company for the manufacture of glass and ceramic (ceramic factory) in Ramadi.Chemical analysis of kaolin is shown in Table 1.Urea powder with a melting point of 132˚C -135˚C, and density of 1.33 g/ml was obtained from sigma Aldrich.

Preparation of Kaolin-Urea Organoclay (Granular Size 53 μm and 106 μm)
1. 70 gm of grinded kaolin of granular size 53 μm was weighed and placed in a Beaker (capacity of 500 ml).2. 35 gm of Urea was weighed and then added to the clay on the same Beaker.3. The mixture was mixed by an electrical mixer in its dry form.4. Suitable amount of water then added to the mixture with keeping continuous stirring, till getting a solution of kaolin-urea.5.The mixture then placed at a porcelain crucible and heat in an oven at 90˚C till dryness.6.The products, finally was grinded and became ready to the required tests (FTIR, XRD and Adsorption of methylene blue (MB).7. Same procedure was used on kaolin (partical size 106 µm).

Preparation of Methylen Blue Solution
1 gm of MB dye was dissolved in one liter of double distilled water to obtain 1000 ppm MB dye solution.UV-Vis spectra of this solution appeared an absorption band at λ max = 660 nm.

Steps of Adsorption
-0.5 gm of the prepared organoclay was weighed, each alone and placed at 25 ml volumetric flask.
-10 ml of methylen blue solution dye of the required concentration was added and stirred, very well to the clay.-The flasks were placed at shaker water bath at different temperatures (10˚C, 30˚C, 40˚C and 50˚C) and stirred for 1 hour each.-The solutions were filtered.
-The absorption was measured for each filtrate at 660 nm.
-The adsorption required calculation according to (Langmuir and Freundlich isotherms), from which the thermodynamic constant can be obtained (ΔG, ΔH and ΔS).

Preparation of Nano Organoclays
1) Weigh 5 g of prepared organoclay and placed in a glas Baker 250 ml.
2) Added 200 ml of a solution of urea concentration 2 M and shake well.
3) The solution is placed on the ultrasonic (probe ultrasonic) and placed the amount of ice around the beaker for one hour.
4) Separating the precipitate from the filtrate using a centrifuge.5) Dried the pricipitate and then grinds it and conducted the tests required.6) Returned the same previous steps for (Thiourea, Acetamide, DMSO and DMF).

FTIR Results
Vibrational spectroscopy is a key technique in the study of formation and structural characterization of kaolinite intercalates [13].FTIR of kaolin, urea and kaolin-urea complex are shown in Figure 1 and Figure 2. From these figures, one could observed that kaolin show two sharp bands at 3694 cm −1 and 3625 cm −1 .The literature however shows conflicting assignment of these bands [14], band at 3694 cm −1 belong to hydroxyl group in specific lattice sites in the layer and resulting from vibrational coupling of three surface of hydroxyl in the primitive cell and the dipole oscillation in perpendicular to the layer, while band at 3625 cm −1 in belong to hydroxyl group lie within lamellae  in plane common to both the tetrahedral and octahedral sheets.Upon intercalation with urea, the intensity of these two bands decrease and shifted to lower frequency, Also a new bands at 3503 cm −1 appeared due to the breaking of some hydrogen bonds between the kaolinite layers and formation of new band, which usually involve the inner surface OH group and change are observed in the intensities of bands assigned to vibrations of these groups [13].
Bands at 3440 and 3444 cm −1 in the Figure 1 and Figure 2 (Chart C) which appear in the results intercalation of kaolinite 53 and 106 µm with urea respectively are attributed to formation H-bond between NH 2 group from urea and Oxygen group of tetrahedral sheet for kaolinite.
The newly formed bands at 3384 and 3503 cm −1 in the intercalation of kaolinite 53 µm with urea confirmed the asymmetric and symmetric NH 2 stretching frequencies involved in weak H-bonding with the inner hydroxyls [15]- [18].
Band at 2352 cm −1 in urea chart and kaolinite 53 and 106 µm started disappear when intercalated urea with kaolinite 106 µm and happened shifted in this band to the 2356 cm −1 when intercalate urea with kaolinite 53 µm.
Also same effect appeared for the band at 1673 cm −1 , which assigned for the C═O group of urea, upon interaction with kaolin, formation a bond between C═O and OH group in Gibbsite-like layer so it shifted to 1658 and 1666 cm −1 whene kaolinite 53 and 106 µm interactions with urea respectinely (Chart C in Figure 1 and Figure 2).CN stretching of free urea appeared at 1461 cm −1 , upon interaction with kaolinite shifted to 1457 and 1454 cm −1 in the Figure 1 and Figure 2 Chart C respectively, and a new band at 1403 cm −1 appeared.This suggest urea in this system would then be considered to exist in two forms anionic and complex (ion dipole) as shown in Figure 3.

XRD Results
The XRD curves of raw kaolin chart (A), and kaolin-Urea complexes charts (B and C) are shown in Figure 4.
From this figure one could observe that the strongest three peaks and their values are recorded in Table 2.

From this table:
Peaks From the results in Table 2 and make comparison between these values, on could concluded that strong intercalation between kaolinite layers and urea as a result of appearance high intensity of peaks are due to urea and   in the same time happened shifted and decrease in the intensity of kaolinite and SiO 2 when the intercalation is event.The intercalation caused the destruction of the hydrogen bonding between the kaolinite layers [14].And from results in this table show decreasing in intensity of peaks when the kaolin 53 µm-urea intercalated with urea compared with other complex this indicates that this kaoline a granular size 53 µm is the best.

Adsorption Results
Effects of temperature on the equilibrium adsorption of methylene blue from aqueous solution using kaolin (partical size 53 and 106 µm) and kaolin-urea complex were studied.The equilibrium adsorption data were analyzed using two widely applied isotherms: Langmuir and Freundlich.The results were shown in Table 3 and Table 4. Non-linear method was used for comparing the best fit of the isotherms.Best fit was found to be Langmuir isotherm.

Thermodynamic Parameters
Thermodynamic parameters such as ΔG, ΔH and ΔS were calculated using adsorption equilibrium constant obtained from Langmuir isotherm and shown in Table 5.
Results suggested that methylene blue adsorption on kaolin was spontaneous and exothermic process.Decrease a negative value of ΔG with increase the value of ΔH (-ve) indicate that the adsorption reaction was exothermic.
Percentage of adsorption (Q%) for kaolin and kaolin-urea at conc. 100 ppm of methylen blue are shown in Table 6.

Transmission Electron Microscopy (TEM)
TEM is a microscopy technique in which a beam of electrons is transmitted through an ultra-thin specimen, interacting with the specimen as it passes through.An image is formed from the interaction of the electrons transmitted through the specimen; the image is magnified and focused onto an imaging device, such as a fluorescent screen, on a layer of photographic film, or to be detected by a sensor such as a CCD camera.
Figure 5 and Figure 6 show the TEM photographs of Kaolin (53 and 106 µm)-urea complexes.From this figures show formation of nanotubeit is also very clearly in the images.The average sizes of particles are in the range of 20.2 -24.5 nm in Figure 5 and from 20.8 -27.7 nm in Figure 6.

Table 1 .
Chemical analysis of kaolin.

Table 2 .
Values of XRD for strong peaks in

Table 3 .
Langmuir constant for adsorption at conc. 100 ppm of methylene blue.

Table 4 .
Freundlich constant for adsorption at conc. 100 ppm of methylene blue.

Table 6 .
Percentage of adsorption (Q%) for kaolin and kaolin-urea at conc. 100 ppm of methylen blue.