Reactive iron sulfides can be used for the reduction and precipitation of inorganic contaminants, including chromium, arsenic, and mercury. The introduction of reactive iron species for treatment of these noted inorganics is well known. Yet the efficiencies of various irons – zero valent iron (ZVI), ferrous or ferric sulfate, and iron sulfide compounds, differ greatly in reactivity, efficiency, and cost. Comparatively, the use of mackinawite structured iron sulfide for the reduction and treatment of chromium and arsenic is inexpensive compared with ZVI ($/wt.) and more chemically efficient and persistent in the environment than ferrous sulfide or calcium polysulfide – another common product that produces reducing conditions in situ.
The goal – a stable, highly-reactive FeS – is determined as possible in both abiotic and biotic generation scenarios; in reality, there is a significant performance difference between biogeochemical generation of FeS (biotic FeS) and chemical synthesis of FeS (abiotic FeS). The introduction of bioremediation components with S-ZVI to generate FeS in situ is a demonstrated technology, but requires a specific set of in situ conditions in a certain sequence, and because of many in situ factors, it is difficult to estimate the total mass of FeS that will be generated even if conditions remain constant and optimal. The biotically-generated agglomerate structure is also not conducive to high-capacity reduction or degradation.
A chemically manufactured (synthesized) version, trade name FerroBlack, has been utilized with prevalence and in similar in situ deployments. It has proven to be effective immediately and demonstrates subsurface persistence; it is not constrained by in situ hydrogeochemical parameters. The differences in these two FeS materials – performance and timeframe – are realized in project cost and schedule. Plus, with FerroBack, the percent weight of available iron and sulfide is straightforward to calculate and evaluate on a stoichiometric and cost basis, and can be tailored (reformulated stoichiometric ratio) to match contaminant demands of the site.
Learn more about how AST uses FerroBlack for heavy metals treatment of groundwater
