Thermodynamic Study on the Adsorption of Pb 2 + and Zn 2 + From Aqueous Solution by Human Hair

Adsorption of Pb(II) and Zn(II) ions from aqueous solutions was studied in a batch system using modified human hair. The optimum conditions for the adsorption of Pb(II) and Zn(II) ions from aqueous solution by human hair were investigated by considering the extent of adsorption with respect to contact time, initial metal ion concentration and temperature. The results obtained indicates that the extent of metal ions removed decreases with increasing contact time but increased with increase in the initial metal ion concentration. The adsorption equilibrium data best fitted Freundlich adsorption isotherm. The adsorption of Pb(II) and Zn(II) ions onto human hair is endothermic, spontaneous and is characterised by increasing degree of orderliness.


Introduction
Lead and zinc are among the toxic heavy metals that have been deeply studied [1][2][3][4][5] .The environmental significant of these metals is partly attributed to their presence in industrial effluent and other sources of waste.Therefore, the removal of these metal ions is necessary in order to forestall the manifestation of their toxic impact through bioaccumulation and biomagnification along the food chain.
Technologies available for the removal of heavy metals from aqueous solutions include oxidation and reduction, chemical precipitation, filtration, electrochemical treatment, ion exchange, membrane separation, reverse osmosis, evaporation and electrolysis [6][7][8] .However, adsorption process has is one of the best options available for the removal of heavy metals from aqueous solution 9 .Several works have been carried out on the removal of heavy metal ions from aqueous media using suitable adsorbents [10][11][12][13] .In spite of the large volume of works published on the adsorption of heavy metal, literature on the use of human hair as an adsorbent for lead and zinc ions is scanty.Interestingly, human hair are normally disposed as waste after hair cutting implying that their utilization for the removal of heavy metal ions from aqueous solution will reduced the environmental problems associated with their disposal.Therefore, the objective of our study is to investigate the adsorption potentials of human hair for the removal of Zn 2+ and Pb 2+ from aqueous solution.

Experimental
Samples of human hair collected from different barbing saloon were thoroughly washed with distilled water, grounded to powdered form and modified by treating with HNO 3 at 60 °C for 15 h.The acid modified sample was re-washed with distilled water to neutral pH, re-dried in an oven and used for the study.The reagents used were analytical grade purchased from BDH chemicals.These included Pb(NO 3 ) 2 and ZnSO 4 .7H 2 SO 4 .Serially diluted solutions (10 to 50 mg/L) of these salts were prepared from standard solutions of their respective salt.
The adsorption study was conducted by mixing 1 g of the sample with 100 mL solution of the respective metal in a plastic bottle.In each case, the mixture was stirred in a thermostated water bath (maintained at 288 K) for a contact period of 2 h.The solution was centrifuge at the speed of 240 rpm, filtered and the supernatant was analysed for heavy metal concentration using inductive couple plasma spectrophotometer (ICPS-7000).The experiment was repeated for various concentrations of Pb and Zn salts at different temperatures (288, 298, 308 and 323 K).From the measured concentration of Zn 2+ and Pb 2+ , the amount of sorption per unit mass of adsorbent (x/m) was calculated using equations 1.
x/m = (C i -C)/C i x V/m (1) Where C i and C are initial and final (outlet or effluent) concentrations of Zn 2+ /Pb 2+ , m is the mass of the adsorbate (in g) and V (in cm 3 ) is the volume of solution added.

Results and Discussion
Figures 1 and 2 respectively illustrate the variation of the amount of Zn 2+ and Pb 2+ adsorbed (by human hair) with the concentrations of Zn 2+ and Pb 2+ in solution.From Figures 1 and 2, it is evident that the amount of heavy metal ion adsorbed by human hair increases with increasing concentration of the metal ions and with increase in temperature suggesting that the mechanism of adsorption of Zn 2+ and Pb 2+ by human hair is chemical adsorption.For a chemical adsorption mechanism, the extent of adsorption increases with increase in temperature as observed in this study.The adsorption characteristics of Zn 2+ and Pb 2+ onto human hair were studied using adsorption isotherm.Data obtained from adsorption experiments were used to fit curves for different adsorption isotherms including Langmuir, Temkin, Frumkin, Flory-Huggins and Freundlich adsorption isotherm.The tests indicated that the adsorption of Zn 2+ and Pb 2+ is best described by Freundlich adsorption isotherm.The assumptions establishing Freundlich adsorption isotherm can be written as follows 14 , x/m = KC e 1/n (2) Where x/m = Q e is the amount adsorbed per unit mass of the adsorbent, K is the adsorption equilibrium constant, C e is the equilibrium concentration of the adsorbate and n is the number of adsorption sites that must be replaced by the adsorbate.Taking logarithm of both sides of equation 2, yields equation 3. logQ e = log K + 1/n logC e (3) From equation 3, a plot of logQ e versus logC e should be linear provided the assumptions of Freundlich are valid.Figures 3 and 4 show Freundlich isotherms for the adsorption of Zn 2+ and Pb 2+ onto human hair respectively.Values of Freundlcih adsorption parameters deduced from the plots are presented in Tables 1 and 2 respectively.From the results obtained, it can be seen that the number of adsorption sites tend to increase with increase in temperature confirming the mechanism of chemical adsorption.The equilibrium constant of adsorption obtained from Freundlich isotherm is related to the free energy of adsorption as follows 15 .
∆G ads =-2.303RT log K (4) The effect of temperature on the rate of adsorption of Zn 2+ and Pb 2+ by human hair was studied using the Arrhenius equation which can be written as follows 16 , R=Aexp(-E a /RT) (5) Where R is the rate of adsorption of Zn 2+ and Pb 2+ by human hair, A is the Arrhenius or pre-exponential factor, E a is the activation energy, R is the gas constant and T is the temperature.Taking logarithm of both sides of equation 5 yields equation 6. logR = logA -E a /2.303RT (6) Using equation 6, the plots of values of logR versus 1/T (Figure 5 and 6) were linear indicating that the slope is equal to -E a /2.303R.Values of E a calculated from the slopes of the Arrhenius plots are recorded in Table 3.The activation energies ranged from 17.82 to 22.25 J/mol and from 18.02 to 40.15 J/mol for the adsorption of Zn 2+ and Pb 2+ respectively.The activation energies increased with increase in the inlet concentration of the respective metal ion indicating that there is increasing ease of adsorption of the metal ion as the inlet concentration increases.
In order to calculate thermodynamic parameters for the adsorption of Zn 2+ and Pb 2+ onto human hair (∆H ads and ∆S ads ), the transition state equation (equation 7) 17  Plots of log(CR/T) versus 1/T were linear (Figures 7 and 8) implying that the slope and intercept of the plots equate ∆H ads /2.303R and (log(R/Nh) + ∆S ads /2.303R) respectively.From the results obtained, values of ∆H ads for the adsorption of Zn 2+ ranged from 16.01 to 20.32 J/mol (mean = 38.54J/mol) indicating that the adsorption of Zn 2+ onto human hair is endothermic.However, ∆S ads values ranged from -207.21 to -196.45 J/mol (mean = -217.6647J/mol) indicating that the adsorption of Zn(II) ions by human hair is accompanied with decreasing degree of disorderliness.For the adsorption of Pb 2+ , ∆H ads values were found to range from 16.08 to 38.22 J/mol while ∆S ads ranged from -207.03 to 187.35J/mol.The positive and negative values obtained for ∆H ads and ∆S ads respectively also indicate that the activation complex might have been the rate determining step and that there is association of the adsorbates rather than dissociation.The results also reveal that the calculated thermodynamic parameters tend to increase as the inlet concentration of the metal ion increases which suggest that there is appreciable increase in the heat of adsorption and the degree of orderliness of the adsorbed layer as the inlet concentration of the metal ion increases.5.These values tend to increase with increase in temperature implying that the adsorption of Zn 2+ and Pb 2+ onto human hair is temperature dependent.The amount of heavy metals adsorbed by any materials also depend on factors such as the ionic character of the ion, surface area of the adsorbent, pH and temperature of the medium, metal content of the adsorbent and the concentration of the heavy metal 19 .In this study, since other factors were held constant, the most important factor that can be used to account for the extent of adsorption of the metal ions is the ionic character of the ions.Table 6 presents the ionic character of zinc and lead ions.From the values of the ionic characters, it can be justified why the amount of zinc ion adsorbed onto human hair is relatively greater than the amount of lead ion adsorbed.The mass to charge ratio, ionic radius and electronegativity of Zn 2+ is relatively smaller than those of Pb 2+ .As a rule, the smaller the ionic radius, the better the adsorption potential.

Conclusion
Human hair can be used as an adsorbent for the removal of Pb 2+ and Zn 2+ from aqueous solutions.The adsorption behaviour of human hair can be optimised by controlling the period of contact, initial metal ion concentration and temperature.Thermodynamic principles can adequately be used to predict the direction of the adsorption process.

Figure 1 .CFigure 2 .
Figure 1.Variation of amount of Zn 2+ adsorbed by human hair with concentration of Zn + in solution at various temperatures

Figure 5 .Figure 6 .
Figure 5. Arrhenius plot for the adsorption of various concentrations of Zn 2+ onto human hair

Figure 7 .Figure 8 .
Figure 7. Transition state plot for the adsorption of various concentrations of Zn 2+ onto human hair

Table 1 .
Freundlich isotherm for the adsorption of Pb 2+ by human hair Values of K calculated from the intercepts of Freundlich adsorption isotherms were used to compute ∆G ads values using equation 4. Calculated values of ∆G ads for Zn 2+ and Pb 2+ are recorded in Table 1 and 2 respectively.∆G ads values are negative indicating that the adsorption of Zn 2+ and Pb 2+ onto human hair is spontaneous.Freundlich parameters for the adsorption Zn 2+ onto human hair

Table 2 .
Freundlich parameters for the adsorption Pb 2+ onto human hair

Table 3 .
Thermodynamic parameters and activation energy for the adsorption of Zn 2+ onto human hair * are the degree of linearity of the Arrhenius and the transition state plots respectively .

Table 4 .
Thermodynamic parameters and activation energy for the adsorption of Pb 2+ onto human hair 2* R2and R 2* are the degree of linearity of the Arrhenius and the transition state plots respectively

Table 5 .
Values of distribution coefficients for the adsorption of Zn 2+ and Pb 2+ onto human hair at different temperature