Utilization of m -Phenylenediamine-Furfural Resin for Removal of Cu(II) from Aqueous Solution-A Thermodynamic Study

: m -Phenylenediamine was condensed with furfural in absence of catalyst at room temperature. The produced m -phenylenediamine-furfural resin was used for the removal of Cu(II) from aqueous solution. The pH for the optimum removal of Cu(II) was 6. The negative values of Gibbs free energy at low concentration of Cu(II) (20, 30 ppm) indicative of the spontaneous adsorption process, while, at higher Cu(II) concentration (40,50 ppm) the positive and weak values of ∆ G o indicate that the process is feasible but non spontaneous. The values of ∆ H° were positive indicating that the sorption process is endothermic. On the other hand, the values of activation energy (Ea) were inconsistent with the values of ∆ H° both are positive and lie in the range of physisorption. The entropy ∆ S° of the process was positive indicative of the randomness of the Cu(II) ions at the solid / liquid interface. The values of sticking probability S* were less than one which indicate a preferable adsorption process and the mechanism is physisorption.


Introduction
Industrial and mining wastewaters are important sources of pollution of heavy metals 1 . Copper is used extensively by electrical industries, in fungicides and in anti -fouling paints. When copper is ingested at high concentrations it can become toxic to humans, causing cancer and promoting oxidation. The present method for removal of Cu is to precipitate copper hydroxide by liming process. But with this process, residual Cu remain a problem 2 . Owing to their wide use, the efficient removal of toxic metal ions from wastewater is an important and widely studied research area where a number of technologies have been developed over the years 3 . The most important of these methods include filtration, chemical precipitation, ion exchange, adsorption, electrodeposition and membrane systems. All these approaches have their inherent advantages and limitations. Although filtration and chemical precipitation are low -cost and effective in removing large quantities of metal ions quickly, neither is capable of removing trace levels of metal ions. Adsorption is also ineffective at very low concentration of metal ions. Ion exchange can be used to reduce metal concentrations to region of part per million. Chelating resins provide a dramatic improvement in ion selectivity relative to traditional ion exchange resins but still suffer from two distinct drawbacks: slow metal uptake kinetics and limited operating pH range. On the other hand, chelating polymer resins are found to be more selective by nature 4,5 as compared to other conventional techniques [6][7][8][9][10][11][12] in the removal of metal ions. The objective of the present work is to evaluate the thermodynamic parameters like ∆H°, ∆G o , ∆S° and activation energy Ea for the adsorption of Cu(II) onto m-phenylenediamine-furfural resin and prediction the mechanism of the adsorption process.

Preparation of m-phenylenediamine-furfural resin(phdm-fu)
The condensation of m-phenylenediamine and furfural was carried out in the absence of catalyst, according to the procedure for preparation of m-aminophenol-furfural resin 13 as follows: A mixture of m-phenylenediamine (0.04 mol, 4.32 g), freshly distilled furfural (0.04 mol, 3.84 g) and water (1 mL) was stirred at room temperature for 15 minutes. The mixture was converted to a solid mass, washed with 100 mL of distilled water then crushed and purified by dissolving in dimethyl sulfoxide (DMSO) and precipitated by addition to distilled water. The solid resin was filtered and dried under reduced pressure at 40 o C for 24 h.

Metal solutions
Metal stock solutions containing Cu(II) with a concentration of 500 ppm was prepared by dissolving 1.9005 g of Cu(NO 3 ) 2 in 1 L of deionized water. This solution was used for further experimental solution preparation. Analytical grade reagents were used throughout this study. The pH values of Cu(II) solutions were measured by pH meter type Hana, 301 instruments. The residual copper in the sorption solutions was determined by atomic absorption spectrophotometer type (A Analyst 200 Perkin Elmer).

Thermodynamic study
The thermodynamic experiments were carried out at different temperatures (15,25,35 and 45 0 C) by shaking the phdm-fu resin 1 g/L with 50 mL of Cu(II) 20, 30, 40 and 50 ppm solutions for 120 minutes at pH 6. The amount of Cu(II) adsorbed onto the resin, q e mg/g was calculated using the following equation: Where: V = Volume of equilibrated solution. M = The weight of the resin used C i = Initial concentration of Cu(II). C e = Equilibrium concentration of Cu(II).

Preparation and characterization of phdm-fu resin
The condensation of m-phenylenediamine with furfural was carried out without catalyst at room temperature for 15 minutes due to high reactivity of m-phenylenediamine.
The FTIR Spectrum of phdm-fu is shown in Figure 1, the figure shows an adsorption band at 3348 cm -1 belong to stretching vibration NH 2 group and absorption band of 3117 cm -1 for (C-H) stretching vibration of furan ring. The two adsorption bands at 1595 and 1626 cm -1 belong to the aromatic (C=C). The UV-Visible spectrum of Phdm-Fu resin was another support for the structure of Phdm-Fu resin, the absorption peak of 286 nm belong to π-π * transition for aromatic groups and that at 319 nm belong to n-π * transition for amine group. Nitrogen analysis: Calculated for C 11 H 12 O 2 N 2 ; N , 15.05 found 14.5%. The resin was soluble in DMSO, the intrinsic viscosity [η]=5.1*10 -2 dL./g, softening point, 112-120, color, dark brown .The maximum pH of adsorption was 6.0 and the time required to attain the equilibrium was 120 minutes.

Thermodynamic study
In order to determine the thermodynamic feasibility and the thermal effects of the sorption, the Gibbs free energy (∆G o ), the entropy (∆S o ) and the enthalpy (∆H o ) were calculated using the following equation [14][15][16][17][18] .
[  Where, K o is the equilibrium constant, [Cu] solid is the concentration of Cu, ppm, at the phdm-fu resin at equilibrium, [Cu] liquid is the liquid phase concentration of Cu, ppm, T is the absolute temperature (K) and R is the ideal gas constant. ∆S 0 and ∆H 0 can be obtained from the application of equation (5). The plot of log K o as a function of 1/T (Figure 2) should give a liner relationship with slope of ∆H 0 /2.303R and an intercept of ∆S 0 /2.303R. ∆G 0 is obtained at any temperature from equation (3). The plot of log K o versus 1/T (Figure 2) gives effectively a liner relationship; the values of ∆S 0 , ∆H 0 and ∆G 0 are presented in Table 1. The positive and weak values of ∆G 0 indicates that the process is feasible but non spontaneous specially at higher concentration of Cu (40 and 50 ppm), but at lower concentration of Cu (20 and 30 ppm) the adsorption process was spontaneous. On the other hand, the values of ∆H 0 is positive, indicating that the sorption process is endothermic, the range of ∆H 0 values ( Table 1), indicate that the mechanism of adsorption is physical and the positive values of ∆S 0 indicate that the adsorption lead to randomness at the solid/liquid interface.  The other parameters which can be calculated from the experimental data are, the activation energy (Ea) and sticking probability (S * ), these two parameters can give further support for the involvement of physical adsorption mechanism in the removal of Cu(II) by phdm-fu resin. They were calculated using modified Arrhenius type equation related to surface coverage (θ) as follows [14][15][16][17][18] . Ce The sticking probability (S * ), is a function of the adsorbent / adsorbate system under investigation, its value lies in the range 0< S * <1 for preferable process and is dependent on the temperature of the system 19,20 . The plot of ln (1-θ) against 1/T, (Figure 3) should give a straight line with slope of Ea/R and an intercept of ln S*. The values of Ea were comparable with ∆H 0 which are also positive, (Table 2), indicate the endothermic nature of the adsorption process and the values lie in the range of physisorption.
The values of sticking probability for the adsorption of Cu(II) onto phdm-fu resin were less than one, ( Table 2), indicates that the probability of Cu(II) ions to stick on surface of phdm-fu resin is high as S * <<1 these values confirm that the sorption process is physisorption.

Conclusion
This study confirm that the phdm-fu resin can remove Cu(II) ions from aqueous solution specially at low concentrations. The pH for optimum removal was 6. The values of ∆H o and E a were positive and lie in the range of physical adsorption. Sticking probability values were less than one which indicates a preferable process and physisorption mechanism. The free energy values indicate that the process is spontaneous at low Cu(II) concentration and non spontaneous at high concentration but the process is feasible.