Sorption mechanism of some heavy metal ions from aqueous waste solution by polyacrylamide ferric antimonate

Incorporation of a polymer material into an inorganic ion exchanger provides a class of hybrid ion exchangers with a good ion exchange capacity, high stability and high selectivity for heavy metals. We can see these properties in the present study. The kinetic of Fe (III), Pb (II), Cd (II), Cu (II), and Zn (II) ions on polyacrylamide ferric antimonate has been studied. The thermodynamic parameters such as activation energy (Ea), entropy of activation ∆S* and diffusion coefficient (Do) have been evaluated and a correlation has been made of these parameters with the ion exchange characteristics of the material.


INTRODUCTION
In recent years, the search for a new class of high performance and high functional organic-inorganic composite ion-exchangers were developed by the incorporation of organic conducting polymers into inorganic precipitates (Khan et al., 2003(Khan et al., , 2003. Heavy metals contamination exists in aqueous waste stream from many industries such as metal plating, mining, tanneries, and painting, car radiator manufacturing, as well as agricultural sources where fertilizers and fungicidal spray were intensively used. The removal of heavy metal in an effective manner from water and waste water is, thus, ecologically very important. There are many reported and established technologies for the recovery of metals from waste water, which include chemical precipitation, flotation, electrolytic recovery, membrane separation and activated carbon adsorption. In recent years, the search for a new class of high performance and high functional organic-inorganic composite ion-exchangers were developed by the incorporation of organic conducting polymers into inorganic precipitates . These materials were found selective for heavy toxic metal ions and utilized for analysis of water pollution as such materials have a great deal of attention because of their special mechanical and chemical stabilities . The newly developed composite offered a high capacity and Faster sorption kinetics for the metal ions such as Fe (III), Pb (II), Cd (II), Cu (II), and Zn (II) ions.

Materials
All chemicals used in this work were of analytical grade and used *Corresponding author. Email: mabdelamid@yahoo.com. Fax: 202-462-0608. Author(s) agree that this article remain permanently open access under the terms of the Creative Commons Attribution License 4.0 International License without further purification.

Preparation of polyacrylamide
Polyacrylamide was prepared by mixing equal volume of 20% acrylamide prepared in distilled water with 0.1 M potassium persulfate (K 2 S 2 O 8 ) prepared in 1 M HCl, a viscous solution was obtained by heating the mixture gently at 70±5°C with continuous stirring.

Preparation of ferric antimonate
Ferric antimonate prepared from 0.5 M of ferric chloride was dissolved in distilled water and adding the same volume of 0.5 M antimony metal dissolved in aqua regia slowly with constant stirring using a magnetic stirrer at a temperature of 70±5°C. The resulting solution was precipitated by using ammonia solution drop by drop until the pH was about 0.13. This results in formation of red brown precipitate.

Preparation of polyacrylamide ferric antimonate
Polyacrylamide ferric antimonate was formed by adding a precipitate of ferric antimonate to polyacrylamide with stirrer by using magnetic stirrer to obtain homogenous precipitate. The precipitates were left to age in the mother liquor overnight, the precipitate was washed with distilled water several times. The supernatant liquid was decanted and the gel filtered using a centrifuge (about 10 4 rpm) and dried at 50±1°C. The product was crashed and sieved to obtain different mesh sizes of 0.12 to 1.00 mm. The weight loss of polyacrylamide ferric antimonate in the different forms such as H(I), Pb (II), Fe (III), Cd (II), Cu (II) and Zn (II) ions at 1000°C were determined using thermal analysis technique (TG and DTA) to be 34.9, 17.33, 19.81, 21.55, 21.22 and 21.33% w/w, respectively.

Chemical stability
Chemical stability of polyacrylamide ferric antimonate ion exchangers were studied in water, nitric and hydrochloric acid [1, 2, 3, 4, 5 and 6 M], as well as in potassium and sodium hydroxide (0.1, 1 M) by mixing 50 mg of ion exchanger samples and 50 ml of the desired solution with intermittent shaking for approximately three weeks at 25±1°C.

RESULTS AND DISCUSSION
The ion exchange capacity of polyacrylamide ferric antimonate for Pb (II), Fe (III), Cd (II), Cu (II) and Zn (II) ions were found to be 5.33, 4.31, 3.12, 2.51 and 2.35 respectively. It is indicated that the affinity sequence for all cations is Pb 2+ >Fe 3+ > Cu 2+ >Cd 2+ >Zn 2+ . This trend may be due to the electronegativity of Pb 2+ >Fe 3+ , Cu 2+ , Cd 2+ and Zn 2+ and may be due to the hydrated ionic radii  according to the fact that increasing atomic number results in decreasing hydrated ionic radii (Abou-Mesalam and El-Naggar, 2003). The effect of particle sizes on the rate of exchange of Pb (II), Fe (III), Cd (II), Cu (II) and Zn (II) ions on polyacrylamide ferric animonate was studied at 25±1°C. Straight lines passing through the origin are obtained, which were taken as indication of a particle diffusion mechanism as shown in Figure 1. The results and figures of Lead were taken as example for the sake of brevity), a relation between F and Bt against time. Similar trend was observed by El-Naggar et al. (2007. The plots of Bt and F against t for the exchange of Pb (II), Fe (III), Cd (II), Cu (II) and Zn (II) ions at different reaction temperatures (25, 45 and 65 ±1°C) on polyacrylamid ferric antimonate are presented in Figure  2. The results and figures of Lead were taken as example for the sake of brevity). straight lines were observed to be passing through the origin. This confirms that the phenomenon is particle diffusion controlled, and the rate of exchange increases by increasing the reaction temperatures from 25 to 65±1°C. The relatively small activation energy values (E a ) obtained in Table 1, for Pb (II), Fe (III), Cd (II), Cu (II) and Zn (II) ions, indicated that the rate of exchange is particle diffusion mechanism (El-Naggar et al., 2012).

Adsorption isotherm
The Langmuir adsorption isotherm assumes that adsorption takes place at specific homogeneous sites within the adsorbent and has found successful applications in many adsorption processes of monolayer adsorption. For the case of adsorption in solution, the equation is represented by the following (Langmuir, 1916).   for Pb(II), Fe(III), Cd(II), Cu(II) and Zn(II) ions on polyacrylamide ferric antimonate and represented in the Table 2. These values indicate the exothermic behavior of polyacrylamide ferric antimonate. The linearized form of the Freundlich isotherm equation (Freundlich, 1906) is.
e F e C n k q log 1 log log   Where K F (dm 3 g -1 ) and n (dimensionless) are Freundlich adsorption isotherm constants, being indicative of the extent of the adsorption and the degree of nonlinearity between solution concentration and adsorption, respectively .
The free energy ∆G° were associated to the adsorption process and were determined using the following equation The results given in Table 2 show that the change of free energy for physisorption is generally between -20 and 0 kJ mol -1 ; the physisorption together with chemisorption is at the range of -20 to -80 kJ mol -1 and chemisorptions is at a range of -80 to -400 kJ mol -1 (Jaycock and Parfitt, 1981). The negative value of the enthalpy change of Pb(II) ion indicates an exothermic behavior; the positive value of the enthalpy change indicates that the adsorption process is endothermic and this value also indicates that the adsorption follows a physisorption mechanism in nature involving weak forces of attraction between the adsorbed [Pb(II), Fe(III), Cd(II), Cu(II) and Zn(II)] ions and composite, thereby demonstrating that the adsorption process is stable energetically (Yu et al., 2001). The Freundlich constant n is a measure of the deviation from linearity of the adsorption. If a value for n is below unity, this implies that adsorption process is governed by a chemical mechanism; however, if a value for n is above unity, adsorption is favorably a physical process. The K F and n were calculated from the slopes of the Freundlich plots as shown in Figure 4. The results and figures of Lead were taken as example for the sake of brevity) and were found to be (1.64 to 2.57) and (2.70 to 4.76) respectively. The magnitudes of K F and n show easy separation of heavy metal ion from wastewater and high adsorption capacity (Saifuddin and Raziah, 2007). The value of n, which is related to the distribution of bonded ions on the sorbent surface, represent beneficial adsorption if it is between 1 and 10 (Kadirvelu and  Solener et al., 2008). The values of n at equilibrium represents favorable adsorption at studied temperatures and therefore this would seem to suggest that a physical mechanism, referred to as the adsorption bond becomes weak (Jiang et al., 2002) and conducted with van der Waals forces. Table 2 gives the isotherm parameters for both Langmuir and Freundlich isotherms. From these parameters of the adsorption isotherm, it was noted that the Freundlich isotherm model exhibits better data than the Langmuir isotherm model.

Conclusion
The present study shows that polyacrylamide ferric antimonate is an effective adsorbent for the removal of Pb(II), Fe(III), Cd(II), Cu(II) and Zn(II) ions from aqueous solutions. The following results have thus been obtained: 1) Polyacrylamide ferric antimonate has a good ion exchange capacity, high stability and high selectivity for Pb(II) and Fe(III) than Cd(II), Cu(II) and Zn(II) ions.
2) The adsorption follows a physisorption mechanism in nature involving weak forces of attraction between the adsorbed [Pb(II), Fe(III), Cd(II), Cu(II) and Zn(II)] ions and polyacrylamide ferric antimonite.
3) It was noted that the Freundlich isothermmodel exhibits better data than the Langmuir isotherm model.