Local wheat peel as a solid surface to remove Azure B dye from aqueous solution:Equilibrium isotherms and thermodynamic study

In this research local wheat peel was used as an adsorbent surface for removal of Azure B (AB) dye from the aqueous solution. The adsorption process was performed at different experimental parameters, equilibrium time, temperature, ionic strength and solution pH. The isotherms of adsorption are of H-type as compared with Giles curves and the adsorption data were coincide with Freundlich equation. The adsorption kinetic data were analyzed using pseudofirst and second order kinetic models. The effect of temperature was studied and the amount of dye adsorbed was found to increase with the increasing of temperature from 25 to 50 o C. The values of thermodynamic functions like enthalpy and entropy have been estimated. The quantity of adsorbed dye on the wheat peel increase according to the sequence follows: pH 9.3˃ 8 ˃ 7 ˃6 ˃5.2.The adsorption phenomenon is influenced by the sodium chloride concentration of solution. The obtained data refer to a decrease in amount of Azure B adsorbed in the existence of electrolyte.


Introduction
The pollution of the water is known to the changes in its chemical, biological and physical properties that which may be effect the organisms life.The contamination of water is occur when materials and energy are released to the water sources, and this cause in degrading the nature and purity of the water that use in different kinds of users.It implicates every of the contaminant materials which is not destroyed by the water naturally [1].There are much of the several types of methods may be physical or chemical that were used widely for the treatment of the contaminated of water, one of these methods is change of phase, the separation of gravity, the processes of oxidation or the reduction, ion exchange of the ions of water and adsorption [2].Adsorption can be defined is the phenomenon in which the atoms of the matter or the ions of this matter and may be the molecules which accumulate on the surface of another solid material by different kinds of binding forces, it's usually can be used as a economical method to treat the pollution of water and to remove the toxically soluble compounds that may be in any kind, organic or inorganic materials [3].One of the most popular organic compounds are dyes, which are excessively used in various types of industries like industry of plastic, leaf, textiles, dwelling and fur dyeing [4,5]."The discharge of the effluents of textile manufacture that which contain high colored into the water sources makes the nature of water was not suitable to use in domestic, industrial and the processes of agricultural" [6].Usually the most dyes used in the textile are of highly toxicity to the human and animal cells and may cause cancer, mutagen of the cells and teratogenicity effect on the health of human and animal [4,7].Literatures recorded a common use of technique of adsorption to treat the wastewater inflow from the textile factories using activated charcoal prepared from various agricultural plants [8],metals of clay [9] and variety types of wastes agricultural such as , rice peel [10], saw dust [11], bean husk [12], pistachio shell [13],etc.
In this research the ability to use the waste of agricultural ,wheat peel, as a cheap surface to remove the Azure B dye from waste water was investigated in different experimental factors of pH temperature, and the ionic strength of solution.The equilibrium data were analyzed in terms of thermodynamic functions of adsorption process.

Materials and Methods: Preparation Solutions of Dye
The azure B was purchased from sigma company; the chemical formula of this dye was seen in Scheme (1).

Scheme (1). The chemical formula of the Azure B dye
The wavelength at maximum absorbence (ƛ max ) was estimated for Azure B and was found to be 646 nm.The concentration of dye at equilibrium was determined at this wavelength value.The dye solution was prepared with different concentration and the absorbance quantity of these diluted dye solutions at the 646 nm was draw versus the values of concentration.The applicability of Beer-Lambert's was verified from the straight line of the plot.

The preparation of solid surface
To remove the dust and other polluted materials that may affect the properties of the solid surface, the wheat peel was cleaned by washing with distilled water for more than one time.The washing surface was then left to dry for 1.5 hr in an oven at 120 o C, and kept until using in a container.The surface in powder form was sieved to get a size of particle 150 μm and this particle grain was used to performed the adsorption experiments.

Effect of Equilibrium Time
The kinetic of dye adsorption was performed by using 0.1g of surface with 10 ml of fixed concentration of dye solution (60 ppm).The suspensions were centrifuged at different times and the equilibrium dye concentration was estimated by the UV-Vis spectroscopy (Shimadzu.PC1650,Japan).

Adsorption Isotherm
The ten ml of the solutions of AB dye of varies concentrations were mixed with the weight of 0.1g of wheat peel surface in stopper flasks.The flasks were put in water bath controlled by the temperature thermostat at 25 o C (shaking bath type Scientific K&K, Korea) until the adsorption process is attained equilibrium (90 min).then, the suspension solutions were centrifuged for 10 min at 3000 rpm.The equilibrium concentration of dye was determined from the calibration plot of AB dye.The equation used to calculated the amount of the dye adsorbed is [14]:- Where C o , C e are the initial concentration and the equilibrium concentration of AB dye (mg/L).m is the weight of wheat peel surface (g) and V is the volume solution of the AB dye (L).

Temperature Effect
The effect of temperature on the amount of dye removal was studied at 25, 35 and 50 0 C to estimate the enthalpy, entropy and Gibbs free energy of removal process.

Electrolyte Effect
The solution of knowing concentration of the AB dye and sodium chloride with various weight of (0.05-0.3g) were mixed with 0.1 g of wheat peel surface in a conical flask.The adsorption experiment was performed to obtain the amount of AB dye removed by the adsorbent in the presence of electrolyte.

Initial pH Effect
The effect of the pH on the adsorption process can be followed by the use of the solutions of dye with known concentration and varied pH (5.2-9.3).0.1M hydrochloric acid and 0.1M of sodium hydroxide that were used to adjust the acidic function of the dye solution.

Results and Discussion:
The removal of AB dye from the solution by wheat peel surface was determined at 25˚C and at higher temperatures at (35 and 50 o C) with the pH of solution  6.
The equilibrium isotherm for adsorption of dye was indicated in Figure (1), the amount of dye adsorbed on the biosorbent surface was plotted against the concentration of equilibrium at 25 o C.

Fig.(1). The isotherm curve of AB dye on wheat peal at 25 o C
As appear from the results, the capacity of the surface of wheat peel increase as the increasing of concentration of AB dye.The physicochemical properties of the wheat peel as a kind of agricultural plants consist of different functional groups, fibers of abundant floristic, and series of proteins that make the adsorption process occur easily [15].
As reported by Giles et al, [16] the isotherms of AB dye were found to take shape of H-curve.The H-curve refer to the more affinity between the structure of dye and surface of the solid adsorbent.
The equilibrium of removal process of dye can be determined by applying the experimental equation Freundlich [17]: q e = kC e 1/n   The Table (1) appeared the values of the thermodynamic of adhesion process of dye to the surface.The van der Waals bonding is the common forces that controls the attachment process.The positive enthalpy value (ΔH o ) and the negative free energy value(∆G o ) indicated that AB dye-wheat peel adsorption system is occur spontaneously with endothermic nature.In addition the adsorption process of dye is in accompanied with the increase in randomity of the system.

Table (1): the calculations of thermodynamic functions of the process of adsorption
Figure (5) showed that adsorption of dye on wheat peel was slightly affected by pH solution.

Fig.(5): The effect of pH solution on adsorption of AB dye on wheat peel at 25 o C
The soluble AB dye take the cationic form in the aqueous solution, so that in the minimum pH the forces of repulsion between charge with the positive sign of the wheat peel surface and the positive charges of the functional groups of dye may be dominant.
The effect of electrolyte concentration on the adsorption of AB dye by wheat peel was demonstrated at six weights of the salt of sodium chloride (0.05-0.3 g).As shown from Figure (6), the effect of the concentration of electrolyte on the amount of AB dye adsorbed on the biosorbent surface.

Fig.(6):The effect of electrolyte concentration on the amount of AB dye adsorption
The extent of adsorption was decreased in the existence of sodium chloride.This result can be refer to that hydrated cations (sodium positive ions and the cationic groups of dye) compete to adsorbed on the active sites of the solid surface [19].Figure (7) indicates the change in removal percent of the AB with the time.At the initial stages of adsorption process, the percent of dye removal is change linearly with the time; which refers to that more active sites presence on the surface of the wheat peel.The analysis of the kinetic of adsorption reaction was demonstrated by the pseudo-first order and pseudosecond order kinetic equations, the linear equation of the first order kinetic [20] is: ln (q e -q t ) = ln q ek 1 t Where q t and q e are the amounts of adsorption (mg dye/gm of adsorbent) at time t and at equilibrium (min), respectively.The slope of plot of the above equation (Figure (8)) was used to determine the value of rate constant (k 1 ).

Fig.(8): The first order kinetic curve of dye adsorption
The linear kinetic equation of pseudo second order [21] is: The initial rate of adsorption is h and was equal to: h = k 2 q e 2 , k 2 is the rate constant of pseudo-second order.The straight line of the curve t/q t against t was given in Figure ( 9) and the constants of pseudo-second order can be calculated.

Fig.(9):The pseudo-second order curve of dye adsorption system
The kinetic constants for the attachment of dye are indicated in Table (3).As appered from the values of theoretical qe and the correlation coefficient of the curve, the experimental data was good demonstrate with the second order equation as compared with that of first order equation.Pseudo-first order equation Pseudo-second order equation k1(min -1 ) qe(mg/g) r 2 k2 (g.mg -1 .min - ) qe(mg/g) h(mg.g -1 .min - ) r 2 0.0420 0.3890 0.9400 0.4650 9.8130 44.8400 1.000

Conclusions:
Chaff biosorbent has high capacity for removal of Azure B from solution.
-The isotherms of adsorption of Azure B dye obeyed Freundlich model.
-Azure B-wheat peel interaction give positive and low value of enthalpy (endothermic adsorption system).
-The adsorption of Azure B dye on the surface is influence by the pH of dye solution.
-The extent of adsorption of Azure B dye decrease with the increasing of electrolyte concentration in solution.
-The kinetic of removal of Azure B dye is demonstrate by pseudo second-order equation.
Figure(2).The magnitudes of correlation coefficient and the Freundlich parameters was determined (r 2 = 0.985, n=2.68, k=10 0.77 ).The applicability of Freundlich equation to the experimental data, refer to the physical adsorption of AB dye on the biosorbent surface.The high n value, indicate the higher affinity between the components of adsorption system.

Fig
Fig.(3): Isotherms curves of adsorption of AB at various temperatures

Fig.( 7 )
Fig.(7): The change in removal percent of AB dye with the time at 25 o CThe analysis of the kinetic of adsorption reaction was demonstrated by the pseudo-first order and pseudosecond order kinetic equations, the linear equation of the first order kinetic[20] is: