Lead and cadmium interactions with mackinawite: Retention mechanisms and the role of pH

The reactive iron monosulfides including mackinawite are known for their ability to scavenge trace metals. Oxidation and reduction reactions in sediments and the types of metal bonding with mackinawite determine both the stability of the metal bonds and the susceptibility of the material to oxidation. Metal retention is important because it influences the availability of toxic metals to aquatic organisms. In this study, Pb and Cd interactions with mackinawite were investigated, and two major retention mechanisms were suggested. They are, first, that Pb and Cd displace up to 29% of the Fe from mackinawite by forming (Pb,Fe)S and (Cd,Fe)S on the surface of the mackinawite and, second, that 0.91 mmol/g of Ph and 2.03 mmol/g of Cd are adsorbed on the surface of the transformed mackinawite. The mixed ferrous sulfides are more insoluble and more stable than the pure mackinawite, while surface adsorption is a relatively weak and labile retention mechanism. Both reactions contributed toward a drop in pH, although the mackinawite containing Pb and Cd was stable at this lower pH.