Lead and Cadmium Interactions with Mackinawite:  Retention Mechanisms and the Role of pH

Abstract

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 Pb 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.

Immobilization of aqueous Hg(II) by mackinawite (FeS)

“…The mackinawite in Fig. 1B fits the description as noted by others [24]. Previous studies [22] have demonstrated that the primary FeS precipitate formed from the reaction between Fe(II) and S(-II) in aqueous solutions at ambient temperatures and pressures is nanoparticulate stoichiometric mackinawite, Fe 1.00±0.01 S, thus the prepared FeS should be mackinawite.…”

“…FeS has a high adsorptive capacity for various divalent metals [23][24][25][26][27], but in-depth studies of Hg(II) sorption to FeS are rare. Metals whose sulfide phases are less soluble than FeS exhibit an increasing surface affinity with decreasing solubility [27].…”

Sorption Mechanism of Cadmium from Aqueous Solution on Iron Sulphide

“…Similar drop in pH with Cd 2＋ loading was observed by Coles et al 13 and Parkman et al 14 The amount of iron released accompanying the sorption of Cd 2＋ ions is given in Table 2. As observed, the amount of iron released from the surface increases with the increase in temperature.…”

“…The low values of n point towards the possibility of the formation of mixed sulphide of Cd and Fe, as suggested elsewhere. 2,13,14 This is also confirmed by the absence of iron release from the surface at 303 K, as shown in Table 2.…”

Aerobic oxidation of mackinawite (FeS) and its environmental implication for arsenic mobilization

“…Due to its chalcophilic nature, FeS can sequester a range of divalent metals (e.g., Mn(II), Co(II), Ni(II), Cd(II), Hg(II), and Pb(II)) by forming surface complexes, insoluble metal sulfides, and solid solutions Morse and Arakaki, 1993;Coles et al, 2000;Billon et al, 2001;. This phase is also capable of converting Cr(VI), a toxic and highly mobile form of Cr, to the very insoluble and less toxic Cr(III) (Patterson et al, 1997;Boursiquot et al, 2002;Mullet et al, 2004).…”