Kinetics, thermodynamics and competitive adsorption of lead and zinc ions onto termite mound

The elemental composition of termite mound was determined by XRF which revealed K, Ti and Mn as minor constituents while Ca and Fe as major constituents. The dominant functional groups of termite mound are Fe– OH, Fe–O and O–H. The Pb(II) and Zn(II) adsorption capacities (mg g^-1) are in order of Pb(II) (13.07)>Zn(II) (12.40)>Pb(Pb/Zn) (11.72)>Zn(Pb/Zn) (7.62). The Langmuir adsorption isotherm fitted the adsorption data better than Freundlich isotherm. The adsorption process was best described by pseudo-second-order kinetic model for single and binary solutions, and the rate constants k₂ (g mg^-1 min^-1) are 0.036, 0.016, 0.024 and 0.015, and the calculated value of q_e (mg g^-1) is 12.33, 12.25, 11.52 and 7.84 for Pb(II), Zn(II), Pb(Pb/Zn) and Zn(Pb/Zn), respectively. The regression coefficient, R² values, for these solutions ranged between 0.9966 and 0.9978. The ΔH (kJ mol^-1) values were positive for single and binary solutions in the order Pb(II) (32.0)>Pb(Pb/Zn) (30.8)>Zn(Pb/Zn) (28.0)>Zn(II) (19.0), while ΔS (kJ mol^-1 K^-1) are in the order of Pb(II) (0.103)> Pb(Pb/Zn) (0.097)>Zn(Pb/Zn) (0.082)>Zn(II) (0.06). The ΔG value for Zn(II) is positive in both single and binary systems, while that for Pb(II) was positive between 313–333 and 323–333 K for single and binary systems, respectively. The data show that the use of neglected termite mound for Pb(II) and Zn(II) removal from aqueous solutions is economically significant in wastewater treatment.

Keywords
Termite mound; Competitive adsorption; Kinetics; Thermodynamics; Binary solution