Lead and Cadmium Sorption Onto Magnetite: Similarities and Differences

Abstract

Pb(II) and Cd(II) sorption onto synthetic magnetite (mixed valence iron oxide Fe304) with specific surface of 3 m2/g (BET) are studied at room temperature in reductive conditions (Ar/CH4: 90/ 10), in 0.1 M NaNO3 electrolyte. The studied parameters before and after reactions are: lead and cadmium concentrations and their speciations in solutions, iron concentrations variations in solutions, pH, E(h) and z (oxidation number of magnetite to maghemite). The sorption processes are studied as a function of pH (4.5-9.4), SOl-ption density (0.6 nmol-8.0 gmol/m 2 for Pb(II); 2.0 umol-2.0 gmol/ m 2 for Cd(II)) and [Pb]e q (80 nM-45 mM) or [Cd]eq (2 n M 6 raM). Recent study by Peterson et al. (1996) about chromium sorption on magnetite has shown heterogeneous redox reaction at the aqueous chromium/solid interface. They observed a reduction of mobile Cr(VI) to strongly-chemico-adsorbed Cr(III) in agreement with an oxidation in the solid of Fe z+ to Fe 3+ by electron transfer. Very few other studies on sorption of metallic cations onto magnetite have been published. However this mixed valence iron oxide with its sorption capacities, its electrochemical characteristics (fast electronic transfer without structure changes) and its magnetic properties should be in the future an important element for the heavy metals decontamination of industrial and urban sludges and waters. Magnetite has the advantage on usual solid phases used for heavy metals sorption that water can be very efficiently cleared from a magnetite suspension by the use of magnetic field (Dixon (1985)).