Abstract-Disordered mackinawite, FeS, is the first formed iron sulfide in ambient sulfidic environments and has a highly reactive surface. In this study, the solubility and surface chemistry of FeS is described. Its solubility in the neutral pH range can be described by K s app ϭ {Fe 2ϩ } · {H 2 S(aq)} · {H ϩ } Ϫ2 ϭ 10 ϩ4.87Ϯ0.27 . Acid-base titrations show that the point of zero charge (PZC) of disordered mackinawite lies at pH ϳ7.5. The hydrated disordered mackinawite surface can be best described by strongly acidic monocoordinated and weakly acidic tricoordinated sulfurs. The mono-coordinated sulfur site determines the acid-base properties at pH Ͻ PZC and has a concentration of 1.2 ϫ 10 Ϫ3 mol/g FeS. At higher pH, the tricoordinated sulfur, which has a concentration of 1.2 ϫ 10 Ϫ3 mol/g FeS, determines surface charge changes. Total site density is 4 sites nm Ϫ2 . The acid-base titration data are used to develop a surface complexation model for the surface chemistry of FeS.

Arsenate, As(V), sorption onto synthetic iron(II) monosulfide, disordered mackinawite (FeS), is fast. As(V) sorption decreases above the point of zero surface charge of FeS and follows the pH-dependent concentration of positively charged surface species. No redox reaction is observed between the As(V) ions and the mineral surface over the time span of the experiments. This observation shows that As(V) dominantly forms an outer-sphere complex at the surface of mackinawite. Arsenite, As(III), sorption is not strongly pH-dependent and can be expressed by a Freundlich isotherm. Sorption is fast, although slower than that of As(V). As(III) also forms an outer-sphere complex at the surface of mackinawite. In agreement with previous spectroscopic studies, complexation at low As(V) and As(III) concentration occurs preferentially at the mono-coordinated sulfide edge sites. The K d (L g Ϫ1 ) values obtained from linear fits to the isotherm data are ϳ9 for As(V) and ϳ2 for As(III). Stronger sorption of As(V) than As(III), and thus a higher As(III) mobility, may be reflected in natural anoxic sulfidic waters when disordered mackinawite controls arsenic mobility.

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