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

The reactive iron monosulfides including mackinawite are known for their ab ility to scavenge trace metals. Oxidation and reduction reactions in sedime nts and the types of metal bonding with mackinawite determine both the stab ility of the metal bonds and the susceptibility of the material to oxidatio n. Metal retention is important because it influences the availability of t oxic metals to aquatic organisms. In this study, Pb and Cd interactions wit h mackinawite were investigated, and two major retention mechanisms were su ggested. 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 mackinaw ite 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 a re more insoluble and more stable than the pure mackinawite, while surface adsorption is a relatively weak and labile retention mechanism. Both reacti ons contributed toward a drop in pH, although the mackinawite containing Pb and Cd was stable at this lower pH.