Title: Quarterly Geo Insights 7: Performance and Persistence Differences Between Abiotically- and Biotically-Generated Mackinawite-Structured Iron Sulfides

Webinar Presented By:

Derek Pizarro
AST Environmental

Gary Meyer
REDOX Solutions

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The introduction of reactive iron species for treatment of inorganic contaminants is well known, yet the efficiencies of these various irons – zero valent iron (ZVI), ferrous or ferric sulfate, and iron sulfides – differ greatly in reactivity, efficiency, and cost. ZVI, which is widely used these days, may have its own set of constraints, limitations, and variabilities in successfully meeting remediation goals when deployed at a particular site. One group of reactive iron species are “reactive iron sulfides” (FeS) which have been successfully used for the reduction and co-precipitation of inorganic contaminants including as chromium, arsenic, and mercury. Compared with several specific forms of FeS, ZVI is less chemically efficient (due to passivation) and persistent in the environment than other FeS.

To counteract some of these challenges, during the past decade, ZVI reagent providers have begun to sulfidate (sulfonate) their ZVI, chiefly with the intent is to increase the ZVI’s reactivity, selectivity, and longevity for various reductive processes. Although these sulfidated or sulfonated ZVIs (S-ZVI) have become a more commonly used product, for both inorganic and organic contaminant reduction applications, consistently meeting a site’s long-term remediation goals has remained elusive, even with repeat injections.

While the formation of a stable, highly reactive FeS is possible in both abiotic and biotic scenarios, there are significant performance differences between chemically synthesized FeS (abiotic FeS) and biogeochemical generated FeS (biotic FeS). It has become common to enhance this biotic formation of FeS by adding additional nutrients and kinetic additives to condition the aquifer and promote more optimal geochemical conditions that improve the speed, efficiency, and quantity of FeS produced biogeochemically.

Even with these advancements mentioned, one of the greatest challenges to overcome in these types of biogeochemical systems is the amount of time (and timing) required to create the desired environment for successful remediation. Deploying an abiotically, manufactured (chemically synthesized) FeS presents several advantages and resolutions to the limitations of the biogeochemically generated version. A chemically manufactured version (S-FeS) has also been utilized with prevalence in recent years in similar deployments. It has proven to be effective immediately and demonstrate subsurface persistence.

The differences in these two FeS materials, outside of performance and timeframe, are also realized in project cost and timeline. The intent of biotic FeS is to reduce the cost of the injectate on a per pound basis. Yet, 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, which is difficult to control – longer than weeks or months in most cases. The agglomerate structure is also not conducive to high-capacity reduction or degradation. With S-FeS, the weight of available irons and sulfides is straightforward to calculate and evaluate on a stoichiometric and cost basis. Examples of S-FeS injected in situ to remediate hexavalent chromium, arsenic, and other heavy metals will be provided, and lessons learned from the design and distribution of reactant at these sites will be discussed.

Following the presentation, 30 minutes are reserved for answering questions.

Attendees must keep cameras on, microphones muted, and answer all questions throughout presentation in order to receive credit.