Tetrachloroethene (PCE)

About PCE Contamination

Tetrachloroethene (PCE) is one of the most widely studied and most commonly detected groundwater contaminants in the United States. It was the standard dry-cleaning fluid for most of the twentieth century and a common metal degreaser and textile-processing solvent, so it is found beneath former dry cleaners, strip malls, laundries and manufacturing plants. It is a dense non-aqueous phase liquid (DNAPL) with a density around 1.62 g/cm³, heavier than TCE and far heavier than water, which is why PCE source zones are hard to find and hard to treat: the product sinks below the water table and settles on clay layers and bedrock, where it dissolves slowly into the groundwater passing over it. Its solubility is low, roughly 150 mg/L, but that is still tens of thousands of times the drinking-water limit.

PCE is the parent of the chlorinated ethene family. Under reducing conditions it degrades to TCE, then cis-1,2-dichloroethene (DCE), then vinyl chloride, and finally to harmless ethene. Each step is slower than the last, and a plume that starts as PCE frequently ends up as a vinyl chloride problem unless the remedy is designed to finish the sequence or avoid it altogether.

Regulatory limits

The federal drinking-water MCL for PCE is 5 µg/L (0.005 mg/L), the same as TCE. State groundwater standards are often lower, and vapor intrusion from PCE beneath occupied buildings is a frequent driver at former dry-cleaner sites. EPA classifies PCE as likely to be carcinogenic to humans.

Treatment Options for PCE

Source zone treatment

PCE source zones carry high concentrations and often residual or pooled DNAPL. Two Hepure products are built for that setting. Sodium permanganate (40% liquid) delivers the highest permanganate concentration available for injection, which matters when the oxidant has to overwhelm a large mass in a small volume. Ferox Target zero valent iron is engineered for source-area injection and soil mixing in saturated source zones. Ferox Plus (eZVI) combines ZVI with an emulsified vegetable oil carrier, which helps the iron distribute and leaves a long-lived electron donor behind for biological follow-up.

In situ chemical oxidation (ISCO)

Permanganate oxidizes PCE at the double bond to carbon dioxide, chloride and manganese dioxide, with no chlorinated intermediates. PCE is the slowest of the four chlorinated ethenes to react with permanganate, but at field concentrations the reaction still completes within hours, and permanganate’s persistence in the subsurface (weeks to months) gives it time to find the mass. The stoichiometry is the lightest in the family: complete oxidation consumes four-thirds of a permanganate ion per PCE molecule, about 1.3 lb of potassium permanganate (or about 1.1 lb of sodium permanganate as the pure salt) per lb of PCE, before the aquifer’s own oxidant demand is added. Activated sodium persulfate is the alternative oxidant where PCE is mixed with chlorinated ethanes or petroleum hydrocarbons.

Plume treatment with zero valent iron

For the dissolved plume downgradient of the source, Ferox Flow injected into the plume or granular Ferox PRB placed in a permeable reactive barrier provide in situ chemical reduction that keeps working for 3 to 15+ years. ZVI reduces PCE abiotically, mainly by beta-elimination through acetylene to ethene and ethane, so it does not generate the TCE, DCE and vinyl chloride that a biological sequence produces on the way down. Hepure’s Ferox ZVI has been used at more than 150 sites since 2000.

Enhanced anaerobic bioremediation

PCE is not degraded aerobically; it is only degraded by reductive dechlorination under anaerobic conditions. Emulsified vegetable oil and sodium lactate supply the hydrogen that Dehalococcoides and related bacteria use to strip chlorine from PCE step by step. The remedy succeeds when the sequence runs all the way to ethene, which requires sustained electron donor, suitable geochemistry and, at some sites, bioaugmentation. Where it stalls, the result is a DCE or vinyl chloride plume, which is why many PCE programs pair EVO with ZVI (as Ferox Plus or as separate placements) so the abiotic pathway backs up the biological one.

Daughter product management

The question to ask of any PCE remedy is what it does with the daughters. Permanganate destroys all four ethenes, vinyl chloride fastest. ZVI treats all four without producing them. Electron donor alone produces them on purpose and relies on biology to finish; it is the right choice at the right site, and the wrong one where the organisms or the geochemistry are not there.

Common Questions

Why is a dry-cleaner site so hard to close?

Because PCE DNAPL hides. Small volumes released through a floor drain decades ago can sit on a clay lens as residual product, invisible to monitoring wells, dissolving slowly for decades. The dissolved plume responds to treatment; the source keeps replenishing it. Source-zone chemistry (sodium permanganate, Ferox Target) aimed at where the mass actually is, not where the wells are, is what breaks that cycle.

Permanganate or ZVI for PCE?

Permanganate for speed and for source zones with high mass; ZVI for long-term plume control, for barriers, and for sites where daughter-product accumulation is the concern. Many sites use both, oxidant first, with enough time between them that the reductant is not consumed by residual oxidant.

Can PCE be treated in the vadose zone?

Yes. Potassium permanganate dry-blended into excavated or mixed soil and Ferox Target mixed into saturated source soils both treat PCE in place. See soil remediation for the soil-side chemistry.

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