Trichloroethene (TCE)

About TCE Contamination

Trichloroethene (TCE, also written trichloroethylene) is the most common chlorinated solvent in contaminated groundwater in the United States. For decades it was the degreaser of choice in machining, electronics, aerospace and metal finishing, and it was handled with the care of the era, which is why it turns up under so many former industrial properties. It is a dense non-aqueous phase liquid (DNAPL): heavier than water, with a density around 1.46 g/cm³, so spilled product sinks through the water table and pools on low-permeability layers, where it feeds a dissolved plume for years. Its solubility, about 1,100 mg/L, is high enough that a small amount of DNAPL can hold a large plume above regulatory limits for a very long time.

TCE sits in the middle of the chlorinated ethene family. Under reducing conditions it is produced from tetrachloroethene (PCE) and degrades in turn to cis-1,2-dichloroethene (DCE) and then vinyl chloride, which is more toxic and more strictly regulated than TCE itself. A remedy for TCE has to account for what it will turn into.

Regulatory limits

The federal drinking-water MCL for TCE is 5 µg/L (0.005 mg/L). Many states apply the same or stricter groundwater standards, and TCE vapor intrusion screening levels are frequently the controlling number at sites with buildings over the plume. EPA classifies TCE as carcinogenic to humans.

Treatment Options for TCE

Abiotic reduction with zero valent iron

Hepure’s Ferox zero valent iron powders treat TCE abiotically at the iron surface. The dominant pathway is beta-elimination through chloroacetylene to acetylene, ethene and ethane, which bypasses the sequential chain that produces DCE and vinyl chloride. That is the practical reason engineers reach for ZVI at TCE sites where a biological remedy has stalled at the daughter products. Since 2000, Ferox iron powders have been used at more than 150 sites across North America.

Detailed kinetics studies by Dr. Paul G. Tratnyek at Oregon Health & Science University found that Ferox Flow gave the most favorable TCE degradation results of the iron powders tested, with more than twice the activity of leading competitor powders when normalized to surface area. ZVI is delivered as injectable powder (Ferox Flow for plumes, Ferox Target for source areas), as granular Ferox PRB in a permeable reactive barrier, or combined with vegetable oil as Ferox Plus (eZVI). A single placement typically stays reactive for 3 to 15+ years, and because iron does not react with soil organic matter it is not consumed by natural oxidant demand.

In situ chemical oxidation (ISCO)

Permanganate is the classic oxidant for TCE. It attacks the carbon-carbon double bond directly and carries the molecule through to carbon dioxide, chloride and manganese dioxide, with no chlorinated intermediates. The stoichiometry is simple: complete oxidation of TCE consumes two permanganate ions per molecule, which works out to about 2.4 lb of potassium permanganate per lb of TCE (about 2.2 lb of sodium permanganate as the pure salt). Real demand is higher because the aquifer consumes oxidant too; the permanganate page explains how to estimate it.

Both potassium permanganate (crystalline, dissolved on site to about 5%) and sodium permanganate (40% liquid) deliver the same permanganate ion. Sodium permanganate is favored for high-concentration injection into source zones; potassium permanganate for soil mixing and cost-sensitive programs. Activated sodium persulfate is the alternative where TCE shares the plume with compounds permanganate handles poorly, such as chlorinated ethanes or petroleum hydrocarbons.

Enhanced anaerobic bioremediation

Where biology is the right tool, TCE is degraded by reductive dechlorination: bacteria, most importantly Dehalococcoides, strip chlorine atoms one at a time using hydrogen as the electron donor. Hepure’s emulsified vegetable oil (EVO) provides a slow-release hydrogen source that lasts years; sodium lactate provides a fast-acting one. The chain runs TCE to DCE to vinyl chloride to ethene, and it must be driven all the way; a plume that stops at DCE or vinyl chloride has traded one regulated compound for a worse one. Ferox Plus pairs ZVI with EVO so the abiotic and biological pathways run together.

Choosing between them

• Source zone, high mass, DNAPL suspected: permanganate (sodium permanganate for concentration) or Ferox Target, often both in sequence.
• Dissolved plume, long-term control: a Ferox PRB barrier or injected Ferox Flow; EVO where the geochemistry and microbial population favor biology.
• Daughter products accumulating: ZVI, which does not make them, or an oxidant to destroy them outright.
• Oxidant first, then reductant: works, with time between them. A reductant placed into a freshly oxidized zone spends itself on the residual oxidant.

Common Questions

Will ZVI produce vinyl chloride?

Not as a significant product. ZVI degrades TCE mainly by beta-elimination, which skips the DCE and vinyl chloride steps. Small amounts can form through a minor hydrogenolysis pathway, but they are also degraded at the iron surface. This is the central difference between abiotic and biological treatment of TCE.

How much permanganate for a TCE plume?

Start from the TCE mass (dissolved plus sorbed plus any DNAPL, not the concentration in one well), multiply by 2.4 for potassium permanganate, then add the natural oxidant demand of the soil in the treatment volume, which usually dominates. A bench NOD test tells you that number; we can supply bench quantities and turn the result into a product quantity and packaging.

What about the DCE and vinyl chloride already there?

Permanganate oxidizes all three ethenes, vinyl chloride fastest. ZVI treats all three. EVO drives the sequence to ethene if the right organisms are present. Any of the three chemistries addresses the daughters; the difference is how fast and for how long.

Related contaminants: PCE, vinyl chloride and the broader chlorinated solvent (CVOC) family.