Cu(II) and Cd(II) Removal Efficiency of Microbially Redox-Activated Magnetite Nanoparticles

Heavy metal pollutants
in the environment are of global
concern
due to their risk of contaminating drinking water and food supplies.
Removal of these metals can be achieved by adsorption to mixed-valent
magnetite nanoparticles (MNPs) due to their high surface area, reactivity,
and ability for magnetic recovery. The adsorption capacity and overall
efficiency of MNPs are influenced by redox state as well as surface
charge, the latter of which is directly related to solution pH. However,
the influence of microbial redox cycling of iron (Fe) in magnetite
alongside the change of pH on the metal adsorption process by MNPs
remains an open question. Here we investigated adsorption of Cd2+ and Cu2+ by MNPs at different pH values that
were modified by microbial Fe(II) oxidation or Fe(III) reduction.
We found that the maximum adsorption capacity increased with pH for
Cd2+ from 256 μmol/g Fe at pH 5.0 to 478 μmol/g
Fe at pH 7.3 and for Cu2+ from 229 μmol/g Fe at pH
5.0 to 274 μmol/g Fe at pH 5.5. Microbially reduced MNPs exhibited
the greatest adsorption for both Cu2+ and Cd2+ (632 μmol/g Fe at pH 7.3 for Cd2+ and 530 μmol/g
Fe at pH 5.5 for Cu2+). Magnetite oxidation also enhanced
adsorption of Cu2+ but inhibited Cd2+. Our results
show that microbial modification of MNPs has an important impact on
the (im-)mobilization of aqueous contaminations like Cu2+ and Cd2+ and that a change in stoichiometry of the MNPs
can have a greater influence than a change of pH.