Absorption mechanisms of lead in aqueous solutions with hydrous manganese dioxide

Hydrous manganese dioxide (HMO) has been synthesized through a facile method. It is found that HMO has an excellent Pb(II) adsorption performance, and an equilibrium adsorption capacity of 127 mg g^-1 is expected under room conditions. The sorption capacity is strongly dependent on pH when pH < 7.0, but is not affected by ionic strength, suggesting a mechanism of inner-sphere surface complexation confirmed by X-ray photoelectron analyses (XPS) and Fourier transform infrared spectroscopy (FTIR). The adsorption capacity of Pb(II) in the presence of coexisting negative ions is higher than positive ions. The best fitting of adsorption kinetics and isotherm data are simulated by pseudo-second order equation and Langmuir model, respectively. An endothermic and spontaneous nature in the adsorption process of Pb(II) can be deduced from the obtained thermodynamic data (ΔG < 0, ΔH > 0, ΔS > 0).