Electrochemical monitoring of metal ions removal in Fe0/H2O systems: competitive effects of cations Zn2+, Pb2+, and Cd2

Metallic iron (Fe 0 ) is a reactive material that is widely used for industrial water treatment. The course of the metal ion removal process using Fe 0 (iron powder) was monitored electrochemically (differential pulse polarography). As probe species, Zn 2+ , Pb 2+ , and Cd 2+ were selected for their different (1) adsorptive affinity to iron corrosion products (FeCPs), (2) redox properties, (3) precipitation ability at various pH. Batch experiments were carried out with binary (Zn 2+ / Pb 2+ and Zn 2+ / Cd 2+ ) and ternary (Zn 2+ /Cd 2+ /Pb 2+ ) systems to reveal the mutual interference of these cations. Detailed time monitoring of iron aging for up to 14 days as well as concentration decay of individual removed cations represent important data for mechanistic discussions. The aqueous concentration of Fe 2+ was also monitored. FeCPs were characterized using X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Results showed that the presence of Pb 2+ delays the Zn 2+ removal whereas the presence of Cd 2+ in solution accelerates its removal. The removal of Pb 2+ by FeCPs was not affected by the presence of Zn 2+ and Cd 2+ , moreover, the Pb 2+ inhibited the effect of Cd 2+ on the removal of Zn 2+ . XPS has proven existence of Fe 2 O 3 and hydrated Fe oxidic phase, whilst the SEM showed that the original Fe grains were partly dissolved into buffered ambient under formation of fine particles of FeCPs. Results confirm that reductive transformation of any contaminant in a Fe 0 /H 2 O system is the consequence and not the cause of iron corrosion. Graphic abstract