Vinyl Chloride

What vinyl chloride is and where it comes from

Vinyl chloride (chloroethene, C₂H₃Cl, VC) is a colorless gas at room temperature with a molecular weight of 62.5 g/mol and a water solubility of about 2,700 mg/L. It is the monomer from which PVC plastic is made, and historic releases occur around PVC and chemical manufacturing, but at most groundwater sites it was never spilled at all. It forms in the aquifer when PCE and TCE are reductively dechlorinated one chlorine at a time (PCE → TCE → cis-DCE → VC → ethene) and the sequence stalls at the second-to-last step. It also forms in landfills from the breakdown of chlorinated solvents in the waste. Because it is volatile and only weakly sorbed, it moves with groundwater, partitions readily into soil gas, and is a frequent driver of vapor-intrusion concerns.

The U.S. EPA classifies vinyl chloride as a known human carcinogen. The federal drinking water standard is 2 µg/L (0.002 mg/L), with a health goal of zero; by comparison the standard for PCE and TCE is 5 µg/L. Many state groundwater standards are comparable or tighter, and vapor-intrusion screening levels for VC are among the lowest for any chlorinated compound. That is why a site can report falling PCE and TCE while its regulatory problem is getting worse.

Compound	Federal MCL	Where it sits in the chain
Tetrachloroethene (PCE)	5 µg/L	Parent
Trichloroethene (TCE)	5 µg/L	First daughter
cis-1,2-Dichloroethene	70 µg/L	Second daughter; where many plumes pile up
Vinyl chloride	2 µg/L	Third daughter; the slowest step and the strictest limit
Ethene, ethane	Not regulated	The end products that signal complete dechlorination

Why plumes stall at vinyl chloride

Each dechlorination step gives up a chlorine atom more reluctantly than the last. PCE and TCE are reduced by many anaerobic bacteria; cis-DCE and vinyl chloride are reduced to ethene only by Dehalococcoides strains carrying the right reductive dehalogenase genes (vcrA and bvcA), and only while an electron donor is available and the geochemistry stays strongly reducing. When the donor is used up, when sulfate or nitrate competes for it, when the pH drifts below about 6, or when the organisms are simply absent, the chain stops at cis-DCE or VC and the daughter accumulates. The signature is unmistakable in the data: parents down, VC up, little or no ethene.

Treatment options

Zero valent iron: abiotic reduction

Ferox zero valent iron degrades chlorinated ethenes at the iron surface mainly by β-elimination, a pathway that goes through chloroacetylene and acetylene to ethene and ethane rather than through the sequential daughter products, so it does not make new vinyl chloride and it treats the VC already there. One design point matters: on iron, VC reacts more slowly than TCE or PCE, so the residence time in a barrier or the iron mass in an injection is set by the VC, not by the parent compound. Granular Ferox PRB in a permeable reactive barrier intercepts a VC plume for years; Ferox Flow and Ferox Target powders are injected into plumes and source zones; Ferox Plus pairs iron with emulsified vegetable oil so the abiotic and biological pathways run together. A field account of a direct-push iron barrier stopping a cis-DCE and VC plume is in our ZVI PRB case study, and particle sizing and delivery are covered in the zero valent iron remediation guide.

Permanganate: chemical oxidation

Permanganate oxidizes vinyl chloride quickly and completely to carbon dioxide and chloride, with no daughter products, which makes it the tool for a VC hot spot or a source zone where fast mass destruction is the goal. Complete oxidation takes 10/3 moles of permanganate per mole of VC, about 8.4 lb of potassium permanganate or 19 lb of 40% sodium permanganate solution per lb of VC, before the soil’s natural oxidant demand, which is usually the larger number. Permanganate and reducing remedies do not mix: oxidant applied to a zone where bioremediation or iron is working undoes the reducing conditions, so the two are sequenced in space or time rather than combined.

Anaerobic bioremediation: finishing the chain

Where biology is the right tool, a stalled plume is restarted by restoring what the Dehalococcoides need: a sustained electron donor, emulsified vegetable oil for the long haul and sodium lactate to establish reducing conditions quickly; pH held in the neutral range with sodium hydroxide where fermentation has acidified the aquifer; and, where the vcrA-carrying organisms are absent, a bioaugmentation culture from the bioremediation amendments line. Ethene appearing in the monitoring wells is the proof that the chain is running to the end.

Aerobic bioremediation: the option the parents do not have

Unlike PCE and TCE, vinyl chloride is readily degraded by bacteria with oxygen, directly as a growth substrate and co-metabolically by ethene and methane oxidizers. That makes the oxygenated fringe of a plume a VC sink, and it means a dissolved VC plume that has left the reducing zone can be treated by adding oxygen: calcium peroxide for a slow release over months, or hydrogen peroxide for a fast pulse. It is the natural second stage after an anaerobic remedy has converted the parents to VC and ethene.

Plume control while treatment proceeds

Where a VC plume is moving toward a receptor faster than treatment can catch it, a colloidal activated carbon barrier holds it in place while the upgradient remedy works.

Matching the remedy to the site

Situation	Usual choice	Why
Bio remedy stalled: parents down, VC up, no ethene	More electron donor; pH check; bioaugmentation if vcrA organisms are absent	Restores the step that stopped
VC plume crossing a property line	Ferox PRB barrier, or Ferox Flow injected transect	Passive interception, no new daughters
VC hot spot or source zone	Permanganate, or Ferox Target injection	Fast mass destruction without daughter products
Dissolved VC at the aerobic fringe	Calcium peroxide	VC degrades aerobically; oxygen is the limiting input
Mixed PCE/TCE/VC plume, long timeline	Ferox Plus or iron plus EVO	Abiotic and biological pathways in one placement
Vapor-intrusion driver under a building	Treat the groundwater source with iron or permanganate; coordinate with the vapor mitigation	VC in soil gas follows the groundwater concentration

What Hepure provides

We supply the chemistry and help you size it: Ferox zero valent iron in all grades, from 1-lb samples to 2,205-lb super sacks; potassium permanganate in 55-lb pails and 330-lb drums and sodium permanganate at 40% in drums, totes and tankers; calcium peroxide in 55-lb bags and 2,000-lb super sacks; emulsified vegetable oil and sodium lactate in drums and totes; and sodium hydroxide for pH. Send Pat (West, x1) or Bob (East, x2) the chlorinated ethene concentrations including ethene, the redox and pH data, the treated volume and whether the remedy is a barrier, an injection or a bio program, and you will get a product recommendation and a quantity estimate. We do not design the remedy or run the injection; that stays with your engineer and your driller.

Common questions

Why is vinyl chloride rising while PCE and TCE decline?

Reductive dechlorination is running but incomplete; the chain is stalling at VC. Check electron donor, pH and whether ethene is being produced before deciding the remedy has failed. Usually it needs more donor or the right organisms, not a different technology.

What is the fastest way to deal with a VC exceedance?

In a source zone, permanganate or Ferox Target injection destroys it in days. In a plume heading for a receptor, an iron or carbon barrier stops the migration while treatment continues upgradient.

Does zero valent iron make vinyl chloride?

No. Iron degrades chlorinated ethenes by β-elimination through acetylene to ethene, not through the sequential daughters, which is the reason it is chosen at sites where biology has already produced VC.

Can vinyl chloride be treated with oxygen?

Yes. VC is the one chlorinated ethene that bacteria readily oxidize with oxygen. Calcium peroxide or hydrogen peroxide in the aerobic part of a plume treats dissolved VC that the anaerobic remedy produced.

How much permanganate does VC take?

About 8.4 lb of potassium permanganate per lb of vinyl chloride for complete oxidation, before natural oxidant demand. The permanganate guide has the demand arithmetic.

References

• U.S. EPA. National Primary Drinking Water Regulations: vinyl chloride (MCL 0.002 mg/L, MCLG zero); tetrachloroethylene and trichloroethylene (0.005 mg/L); cis-1,2-dichloroethylene (0.07 mg/L).
• Arnold, W. A., and Roberts, A. L. (2000). Pathways and kinetics of chlorinated ethylene and chlorinated acetylene reaction with Fe(0) particles. Environmental Science and Technology, 34(9), 1794–1805.
• Interstate Technology and Regulatory Council (ITRC). Technical and Regulatory Guidance for In Situ Bioremediation of Chlorinated Ethenes, 2008.
• AFCEE, NFESC and ESTCP (2004). Principles and Practices of Enhanced Anaerobic Bioremediation of Chlorinated Solvents.