Equilibrium, kinetics, and mechanism of lead adsorption using zero-valent iron coated on diatomite

Abstract

The lead adsorption performances of zero-valent iron coated on diatomite (ZVI-D) were investigated. ZVI-D was prepared by impregnation of diatomite with iron sulfate solution, and followed by reducing with sodium borohydride. The zero-valent iron of ZVI-D was confirmed by XPS. The adsorption performances were tested in a batch reactor with an initial lead concentration in the range of 100–1,250 mg/L. It was found that a lead adsorption isotherm could be described by Langmuir equations with a maximum adsorption capacity of 158.73 mg/g at 298 K. The mean adsorption energy (E) of 54.78 kJ/mol was determined by using Dubinin–Radushkevich isotherm. The kinetics of Pb2+ adsorption was fitted with a pseudo-second-order model. From the thermodynamic study, it was found that the adsorption process was endothermic and spontaneous. An adsorption mechanism was proposed by using XPS data. Three steps of the adsorption were suggested, (i) Pb2+ ions were oxidized to Pb0 on ZVI active sites, (ii)ferrous ions reacted with the Pb2+ to form FeOPbOH, PbO–Fe, PbO2-–Fe2O3, and PbO–FeOOH species on ZVI surface, and (iii) Pb2+ turned to PbO2–Si, PbO–Si, Pb(OH)2–Al, and PbO2–Al2O3 on diatomite surface.