Adsorption of Some Heavy Elements on Surface of Activated Carbonized Cellulose from Aqueous Solution

In this study the activated carbonized cellulose in (350 ºC) by citric acid have been used to remove some heavy metals (Hg Cd, Cu, Pb) from its aqueous solution at room temperature. The adsorbed metals data applied on three adsorption isotherm models, Freundlich, Langmuir and Temkin isotherms. The adsorption results were very good fitted with isotherm models by the (R2) meaningful value. The removal metals adsorbed on surface of adsorbent from high to the less remove arranged according to its ability depending on the nature and size of metals. The free energy (ΔG) and constants of the adsorption process (Ɵ, n, kf, kT, b, bT) for copper, lead and cadmium were measured from isotherm curves, infra-red spectrums of the activated carbonized cellulose and cellulose itself were measured by FTIR spectrophotometer.


Introduction
Adsorption is a phenomenon of binding atoms, molecules or ions on the surface of substances physically or chemically by means of hydrogen bonds, Vander Walz forces, valence dispersion forces, and bipolarization interventions [1], [2]. The adsorption process is one of the most important processes used to remove organic and inorganic pollutants of low concentrations that cannot removed by other methods. Activated charcoal distinguished by its high adsorption capacity due to the large pore size and high surface area, and it can have recovered or activated after use by heat or concentration [3]. Industrial activities are the main source of water pollution due to chemical wastes that contain heavy elements that affect health and the environment such as mercury, copper, lead and cadmium [4]. To increase the adsorption capacity of some materials, they activated with basic or acid solutions such as citric acid [5]. The relationship between the amount of adsorbent on the material surface and the equilibrium pressure or concentration at a given temperature called the adsorption isotherm. Such as Freundlich, Langmuir and Temkin isotherms [6]. Suppose Freundlich Most solid surfaces are not homogeneous, and the change in potential energy is not uniform due to a difference in Adsorption sites for their energy levels [7]. The Freundlich equation expressed by the following relationship: ln qe = ln kf + 1/n ln ce Where (n, kf) Freundlich constants and (qe) the amount of adsorbent at equilibrium in unit (mg /g) and (ce) the amount of concentration at equilibrium in (mg / l). The curve (ln qe) against (ln ce) gives a straight line with slope (1 / n) which represents a measure of the adsorption intensity and with a section (ln k) which represents a function of adsorption capacity. In low concentrations, but in high concentrations a slight curvature of the straight line occurs, especially at high temperatures [8], [ 9].
The Langmuir equation expressed by the following relationship by straight line with slope 1/q and section 1/aq: 1/qe = 1/ qo + 1/ qo kL ce Where (qe) the amount of metal adsorbed per gram of the adsorbent at equilibrium (mg/g) and (qo) is maximum adsorption capacity and (kL) Langmuir isotherm constant [10]. Through the Temkin isotherm equation for adsorption, the reaction between the adsorbent and the adsorbate can calculated as follows [11]:

Procedure
A quantity of cellulose pelleted and made as compressed balls by hand and placed in the Furnace at temperature (350ºC) for ten minutes to get carbonized cellulose (100) gm. of this cellulose took and soaked in 0.6 M citric acid for half an hour at room temperature (25°C) and dried at temperature (40°C) to the next day. It washed several times until PH (6.5 -7) obtained, dried and left inside the desiccator until the next day. The functional groups of cellulose and activated carbonized cellulose were diagnosed using an American-made FTIR Spectrophotometer [13]. The solutions of the studied elements (Hg, Cd, Pb, Cu) were prepared at concentration of (20, 30, 40, 50, 100) ppm from the standard solutions of these elements at concentration 1000 ppm. Thermal adsorption experiment was carried out in the form of a onetime experiment for the heavy elements under study, (1.5g) of adsorbent material was taken from activated carbonized cellulose and put in sealed glass containers with a capacity of (100) ml, solutions of elemental ions (Cd, Pb, Cu, Hg) were added. At the indicated concentrations and final volume 50 ml to glass containers and closed tightly and the vessels were shaken at a temperature of (25°C) for a period of time (2,4,8) hours. The solutions filtrated and filtrates collected, and the concentrations of heavy elements evaluated by the volumetric methods [12].
The adsorption (removed elements) percentage (%Ɵ) was calculated from the following law: Where C i is an initial concentration of the elements (mg/L), C f is the final concentration of the elements (mg/L) the adsorbed amount of the element calculated from the following formula [14]:

Results and Discussion
The infrared spectrum (FTIR) showed on the surface of the adsorbent (activated carbonized cellulose) absorption beams for the free (OH) group and the (COO) group in (3782) cm -1 and (2347) cm -1 respectively in figure (2) which contains a pair of electrons, leads to the binding of heavy elements on the adsorbent surface through polar and basic or acidic groups [15]. The nonactivated cellulose does not contain these beams, as shown in Figure ( (3) and (4) show the percentage of adsorption (% Ɵ) and the amount of elements that were removed from the aqueous solution in (2,4,8) hr. at concentration (100) ppm. The amount of adsorbed elements on the surface of adsorbents depends on the equilibrium among the adsorption competitions of the size of the ions, the stability of the bonds of element ions on adsorbents, the nature of the adsorbents and to the distribution of the active groups on the surface of adsorbents. Despite of the high size of lead, but mercury was more absorbable, followed by cadmium, and lead then copper depending on molar mass of the elements is more important at adsorption process [2].and to the strength of the bond with transition metals that receive pair of electrons from the surface of the adsorbent material.          Where ( K f ) is an indicator of adsorption capacity and ( n ) is strength of adsorption of elements on the surface of adsorbent if (n) = 1 then the adsorption process is independent on concentration ,at normal adsorption (favorable adsorption) ( n) will be above ( 1 ) and if ( n ) is below of ( 1 ) the adsorption be cooperative adsorption [18] (5) show's Temkin constants (b, k T , b T ) for (Cd,pb,Cu) the data as this isotherm show the heat of sorption and adsorption without depending on the value of concentration [18]. The results in tables (2-4) for free energy (ΔG) of adsorption on the surface of the activated carbonized cellulose ranged between (6.395 -10.438) k j /mole, which indicates the physical adsorption process [19]. The results in Table (6) indicate that the adsorption decreases with increasing concentration due to the saturation of the pores available on the surface of the activated carbonized cellulose and Mercury has been reduced at low concentrations of less than (100ppm) from the solution for adsorption all mercury material on the surface of activated carbonized cellulose.

Conclusion
In this research some heavy elements (Hg, Cd, Cu, Pb) have been removed from aqueous solution at room temperature using activated carbonized cellulose and the results were fitted on three of adsorption isotherm models Freundlich, Langmuir and Temkin isotherms. As all mercury removed at a concentration below (100ppm), so it can have considered as a standard method for removing mercury from its aqueous solutions.