Uptake of nickel by synthetic mackinawite

The uptake of aqueous Ni(II) by synthetic mackinawite (FeS) was examined in anaerobic batch experiments at near-neutral pH (5.2 to 8.4). Initial molar ratios of Ni(II) to FeS ranged from 0.008 to 0.83 and maximum Ni concentrations in mackinawite, expressed as the cation mol fraction, were as high as X_Ni = 0.56 (Fe_{1 - x} Ni _x S; 0 ≤ x ≤ 1). Greater than 99% Ni removal from solution occurred when Ni loading remained below 0.13 ± 0.03 (1σ) mol Ni per mol FeS due to sorption of Ni at the mackinawite surface. Characterization of experimental solids using X-ray diffraction and Raman spectroscopy showed patterns characteristic of nanocrystalline mackinawite; no evidence of nickel monosulfide (α-NiS or millerite), polydymite (Ni₃S₄), or godlevskite [(Ni,Fe)₉S₈] formation was indicated regardless of the amount of Ni loading. Slight expansion of the c-axis correlated with increasing Ni content in synthetic mackinawite, from c = 5.07 ± 0.01 Å at X_Ni = 0.02 to c = 5.10 ± 0.01 Å at X_Ni = 0.38. Ni K-edge extended X-ray absorption fine structure (EXAFS) spectra of synthetic Ni-bearing mackinawite are similar in phase and amplitude to the Fe K-edge EXAFS spectrum of Ni-free mackinawite, indicating that the molecular environment of Ni²⁺ in Ni-bearing mackinawite is similar to that of Fe²⁺ in Ni-free mackinawite. EXAFS data fitting of Ni-bearing mackinawite with X_Ni = 0.42 indicated a coordination number of 4.04 ± 0.30 and an average Ni_S bond distance of 2.28 Å, in good agreement with the Fe_S bond distance of 2.26 Å in mackinawite, tetrahedral Fe coordination, and slight lattice expansion along the c-axis. At lower Ni loadings (X_Ni = 0.05-0.11), EXAFS analysis showed a decrease in Ni_S coordination towards CN = 3, which reflects the influence of sorbed Ni. Continued Ni uptake, past the maximum amount of sorption, was accompanied by proportional molar release of Fe to solution. Interstitial occupancy of Ni within the mackinawite interlayer may be transitional to structural substitution of Fe. The Ni-mackinawite solid-solution is described by a one-site binary mixing model: $\text{Ln}{\text{K}}_{\text{d}}=\text{ln}{\text{K}}_{\text{e}}-\left(\text{W/RT}\right)\left(1-2{\text{X}}_{\text{Ni}}\right)$ where K_d is the distribution coefficient, K_e is the ratio of equilibrium constants for Ni-mackinawite and mackinawite (14.4 ± 1.3), W is an ion interaction parameter, and X_Ni is the mole fraction of end-member NiS in the solid solution. The experimentally determined value of W is 17.74 ± 1.15 kJ/mol and indicates significant non-ideality of the solid solution. Transformation processes were evaluated by aging Ni-mackinawite with polysulfides and solutions saturated with air. Reaction of Ni-mackinawite with polysulfides led to the formation of pyrite (FeS₂) and Ni retention in the solid phase. When Ni-mackinawite was aged in the presence of dissolved oxygen, transformation to goethite (FeOOH) and violarite (FeNi₂S₄) was observed.