Investigating the impact of {Fe2+}:{HS-} ratio on FeS formation: preliminary results on particle size and charge

Mackinawite (FeS) is the first iron sulfide phase to form in anoxic systems containing ferrous iron and sulfide. It is a major inorganic scale in oil and gas piping and its catalytic properties make it a potential candidate for a variety of industrial applications including energy storage systems and batteries. This, together with Mackinawite’s potential for remediation through the doping of heaving metal cations, makes it an interesting subject of investigation. Since in natural conditions iron and sulfide do not generally occur in alike concentrations, investigating diverging ratios of iron:sulfide activities, improves our knowledge about iron sulfide early formation in natural and geo-engineered settings.

Here, we investigated FeS formation at a saturation index of 1.8 (~63 fold supersaturation), varying {Fe²⁺}:{HS^-} and at pH 10.2. Particle size distribution was explored using Dynamic Light Scattering measurements, surface charge of particles (Zeta potential) was measured with Electrophoretic Light Scattering and samples were imaged using Transmission Electron Microscopy.

Regarding particle charge, we observed particles that were more negatively charged when the solution had an excess of anions (HS^-), compared to solutions with more cation(Fe²⁺) which led to having particles with less negative net surface charge.

Furthermore, preliminary results indicated non-linear evolution of FeS particle size through time. Higher concentrations of iron promoted formation of larger particles, whereas having more sulfide induced the formation of  smaller particles.

Our observations reveal that FeS particle formation is sensitive to the ratio of {Fe²⁺}:{HS^-} in the solution. When there is an excess of iron, growth and/or aggregation of nuclei is enhanced and predominates over nucleation, in contrast to the other conditions(equal activities, or excess HS^-). This behavior may be explained by the zeta potential, which reflects the surface charge of the particles. At pH 10, the FeS particles are negatively charged (Wolthers et al., 2005) and more so at stoichiometric and excess-sulfide conditions. In excess Fe, the particles are less charged and therefore less physically stable and more likely to aggregate, leading to larger particle growth.