Legendary musician Neil Young coined the phrase “Rust Never Sleeps” in the late 70’s referring to relentless personal and societal decay. Neil wasn’t likely thinking about site remediation at the time but as Professor Paul Tratnyek pointed out in his May 2021 webinar, corrosion is what occurs when zero-valent Iron (ZVI) is used to treat contaminants. In this case “rust” is cleaning up our environment; fostering renewal, not decay.
When metals lose electrons, corrosion occurs. For ZVI, corrosion converts Fe0 to Fe2+ and Fe3+ with the liberated electrons reducing and degrading chlorinated solvents such as 1,1,1-trichloroethane (TCA) or perchloroethene (PCE), explosives such as TNT, pesticides such as DDT or promote immobilization of metals such as arsenic, chromium and selenium. The ability of ZVI to degrade a wide range of contaminants, and relatively low cost, has made ZVI the most popular in situ chemical reduction (ISCR) amendment. Other popular ISCR amendments include ferrous iron and sulfur salts, hydrogen sulfide and zero valent zinc (US EPA).
ZVIs Many Forms and Many Uses
ZVI was first deployed in the early 1990s in granular form in trench-based permeable reactive barriers (PRBs) for chlorinated solvent remediation. ZVI is often derived from milling cast iron products and metallurgic powders and has been deployed in remedial applications in a variety of forms and formulations including:
- Millimeter to nanoscale sized ZVI materials for applications in trench based PRBs, soil mixing and direct injections ;
- Sulfidated ZVI, which is iron reacted with reduced sulfur salts to produce ferrous sulfides on ZVI surface and thus increasing efficiency of electrons used for contaminant reduction;
- Emulsified ZVI (eZVI), ZVI encapsulated in a hydrophobic oil droplet, to enhance DNAPL treatment;
- Combinations of ZVI and solid organic carbon for enhancing both biotic and abiotic pathways; and
- Mixtures of activated carbon and ZVI that leverage both sorption and degradation.