Fig. 1. Overlay of FTIR spectra of HANP@AP after adsorption of Pb(II) ions from aqueous solution ranging from 400-4000 cm-1. FTIR peak decreased in after adsorption of Pb(II) ions were found responsible for metal binding at 3643.7, 3400.0, 2922.0 and 1038.8 cm-1.
Fig. 2. Overlay of FTIR spectra of HANP@AP after adsorption of Cd(II) ions from aqueous solution peak found responsible for metal binding were found at 3403.4, 2921.1 and 873.5 cm-1.
Fig. 3. Overlay of FTIR spectra of HANP@AP after adsorption of Ni(II) ions from aqueous solution were amide (-CONH) at 3444.4 cm-1, phenols (-OH) 3428.2 cm-1and alkenes (-CH2) 2921.4 cm-1 might involved in metal adsorption.
Equation used for adsorption and kinetics study
Adsorption Isotherms.
The adsorption isotherm models were applied in the concentration ranged from 10-300 mg/L for Pb(II), Cd(II) and Ni(II) ions, respectively. Langmuir model predicts the partioning of the adsorbate between liquid/solid phases after attaining equilibrium. It describes the monolayer adsorption and tells that each adsorbate have specific binding sites no other adsorbate can adsorbed there and once they filled it, leads to surface saturation phenomenon.
Langmuir isotherm in linear form can expressed as equation:
------(1)
Where qe (mg/g) is concentration of metal ion adsorbed per gram of adsorbent, Ce (mg/L) is concentration at equilibrium, qmax is maximum capacity in (mg/g), and b is Langmuir absorption constant (L/g). Graph is plotted between 1/qe vs 1/ce, whose slope gives the value of b (L/g) and intercept gives the value of qmax (mg/g).
Freundlich model is applicable for multilayer adsorption process takes place by chemiosorption. Freundlich equation (2), K and n represent the constants and predicts about adsorption capacity (mg/g), indication of favourability of adsorption process. Graph was plot between logqe vs logce, whose slope gives value of n and intercept gives value of K (mg/g).
------(2)
Tempkin isotherm assumes the liberation of heat during adsorption is linear rather logarithmic. Heat of sorption is decrease linearly as the adsorbate-adsorbent interaction increases.(Tempkin et al., 1940) Tempkin isotherm equation as follows:
------(3)
Tempkin isotherm equation, B and Kt signifies the Tempkin constant which indicate the heat of adsorption (J/mol) and equilibrium binding constant (L/g). Graph was plotted between qe vs lnCe, where slope gives the value of B and intercept gives Kt.
Fig.4. Comparison of adsorption capacity (mg/g) of HANP@AP for removal of Pb+2, Cd+2 and Ni+2 from aqueous solution
Kinetics Study
Kinetics models aspseudo second order kinetics (Ho et al., 2000) (6) and intraparticle diffusion (4) were applied to the adsorption data by varying time (2-60 min) and concentration (10-80 mg/L), respectively. In kinetics study, time was varied from 2-40 min for Pb(II) and Cd(II) and 2-60 min for Ni(II) ions, respectively. The flasks of each concentration (10, 20, 30, 40, 60 and 80 mg/L) were removed at time interval of (2, 5, 10, 20, 30, 40 and 60 min), respectively. At regular interval of time, flasks of each concentration were removed, filtered and analyzed for the residual metal concentration.
Pseudo second order kinetics equation as follows:
(4)
Where, K is second order constant, qe and qt are amount of metal adsorbed at equilibrium and time t, respectively. A graph between t/qt verses t was plotted and value of K and qe were obtained from intercept and slope respectively. The intraparticle diffusion model (Ho et al., 1998) (7) is a functional relationship where uptake of the metal ions varies with half power of time (t0.5) rather than t, and expressed as
(5)
Where, kiis intraparticle diffusion rate constant in mg/g min0.5. Graph is plotted in between qt vs t0.5 from where ki calculated at zero intercept.
Thermodynamics Study
Temperature varied from 293-333K to observe the effect of temperature onto adsorption process. The distribution coefficient (Kd) and change in Gibbs free energy (∆G°) were calculated by applying the equation:
(6)
Where qe, is the amount of the metal ions adsorbed and ce is the concentration of the metals ions at equilibrium.
(7)
R= universal constant (8.314) J/mol/K and T is temperature in K. The change in enthalpy () and change in entropy () calculated by using the Van’t Hoff equation (11) and plotted a graph between lnKdvs 1/T, where slope gives the value of and intercept represent for
------(8)