Chlorinated Solvents

The Chlorinated Solvents family includes sixteen molecules in the IsoFind catalog, divided into five subfamilies that reflect their base structure: chlorinated ethylenes (PCE, TCE, DCE, VC), chlorinated methanes (chloroform, DCM, CCl₄), chlorinated ethanes, chloroethanes, and chlorobenzenes. This is the best-documented family in the catalog: eighteen tabulated degradation pathways, twenty-one isotopic fractionations covering carbon and chlorine, and eighteen parent-metabolite relationships forming complete cascades. Historically, CSIA was developed primarily on chlorinated solvents, and IsoFind reflects this scientific maturity.

Five Subfamilies

The classification by subfamily follows classical chemical nomenclature: ethylenes for molecules with a C=C double bond, ethanes for saturated two-carbon chains, methanes for C₁ compounds, and chlorobenzenes for aromatics. This granularity is necessary because environmental behaviors differ significantly between these classes.

Subfamily	Count	Molecules
Chlorinated Ethylenes	6	PCE, TCE, cDCE, tDCE, 1,1-DCE, VC
Chlorobenzenes	4	Chlorobenzene, 1,2-DCB, 1,4-DCB, HCB
Chlorinated Methanes	3	Chloroform, DCM, CCl₄
Chlorinated Ethanes	2	TCA (1,1,1-trichloroethane), 1,2-DCA
Chloroethanes	1	1,1,2-TCA

Four molecules remain under free access (simple chlorobenzenes and HCB, plus 1,1,2-TCA as a TCE precursor). The other twelve, including all chlorinated ethylenes and chlorinated methanes, are part of the Pro access.

Regulatory Thresholds

Regulatory thresholds cover a wide range depending on the molecule and its toxicity. Vinyl chloride carries the strictest threshold (0.5 µg/L Dir. 98/83/EC, 2 µg/L EPA MCL) due to its IARC Group 1 status (liver angiosarcoma). TCA, once a massive industrial solvent, has a high threshold (200 µg/L EPA) reflecting its relatively low toxicity, despite its ban for ozone depletion reasons.

Molecule	Water Threshold	Main Framework	IARC Status
HCB (hexachlorobenzene)	0.010 µg/L	WFD 2013/39/EU Priority Hazardous; Stockholm Convention	Group 2A
Vinyl Chloride (VC)	0.5 µg/L	Dir. 98/83/EC; EPA MCL 2 µg/L	Group 1
Chloroform	0.1 µg/L	Dir. 98/83/EC sum of THMs; WHO 2022 0.30 µg/L	Group 2A
CCl₄ (carbon tetrachloride)	2 µg/L	Dir. 98/83/EC; Montreal Protocol (Banned ODS)	Group 2B
1,2-DCA	3 µg/L	Dir. 98/83/EC; EPA MCL 5 µg/L	Group 2B
TCE	10 µg/L	Dir. 98/83/EC sum of TCE+PCE; EPA MCL 5 µg/L	Group 1
PCE (tetrachloroethylene)	10 µg/L	Dir. 98/83/EC sum of TCE+PCE; EPA MCL 5 µg/L	Group 2A
DCM	20 µg/L	WHO 2022; Banned for EU general public since 2010	Group 2A
1,1-DCE	30 µg/L	EPA MCL 7 µg/L; WHO 2022 30 µg/L	-
cDCE / tDCE	50 µg/L	EPA MCL 70 and 100 µg/L; WHO 2022 50 µg/L	-
TCA (1,1,1-trichloroethane)	200 µg/L	EPA MCL; Montreal Protocol (Banned ODS)	Group 3

Directive 98/83/EC groups TCE and PCE under a single combined threshold of 10 µg/L. On a site contaminated by sequential dechlorination (PCE → TCE), this sum remains conserved until the cascade proceeds to cDCE. Compliance with this common threshold therefore provides no information on the degradation state, only on the total load of the first two terms.

The Anaerobic Dechlorination Cascade

The flagship case study for chlorinated solvents is the anaerobic reductive dechlorination cascade by Dehalococcoides and Dehalobacter. It transforms PCE into TCE, then cDCE, VC, and finally ethene, removing one chlorine at each step. All four steps are tabulated in the database with their maximum molar yields.

Step	Target Molecule	Typical t½ (d)	Dominant Metabolite	Max Yield
1	PCE	500	TCE	0.90
2	TCE	300	cDCE	0.85
3	cDCE	600	VC	0.90
4	VC	900	Ethene (non-toxic)	0.85

The cascade slows down as it progresses: PCE dechlorination is relatively fast with active strains, but VC dechlorination to ethene becomes the rate-limiting step on most sites. The accumulation of cDCE and VC intermediates is a typical signature of mature plumes, where all four compounds appear simultaneously with ratios that inform on the progress of the reaction.

The cascade produces a toxicological paradox: VC, a degradation intermediate, is classified as IARC Group 1, whereas the parent PCE is only Group 2A. Partial degradation stalled before ethene can therefore increase the overall health risk. This property makes isotopic monitoring essential to confirm the actual progress of the cascade through to the non-toxic terminal product.

Tabulated Degradation Pathways

Eighteen degradation pathways are tabulated for this family, the highest count in the catalog. They cover anaerobic biological pathways (dominant reductive dechlorination), aerobic biological pathways (oxidation by mono- and dioxygenases), abiotic hydrolysis pathways, and remediation pathways via Fe-ZVI reactive barriers.

Anaerobic Biological Pathways

Molecule	Eh (mV)	Typ t½ (d)	Metabolite	Strain
PCE	−250 to +50	500	TCE	Dehalococcoides / Dehalobacter
TCE	−250 to +50	300	cDCE	Dehalococcoides mccartyi
cDCE	−250 to 0	600	VC	Dehalococcoides
VC	−300 to −50	900	Ethene	Dehalococcoides (limiting step)
1,2-DCA	−250 to 0	300	Ethene	Dehalorespiration
TCA	−250 to 0	120	1,1-DCA	Dehalobacter sp.
Chloroform	−250 to 0	150	DCM	Dehalobacter sp. CF (e⁻ acceptor)

Aerobic Biological Pathways

Molecule	Eh (mV)	Typ. t½ (d)	Metabolite	Mechanism
DCM	+100 to +400	20	Formaldehyde → CO₂	Dichloromethane dehalogenase
VC	+100 to +400	30	CO₂	Alkene monooxygenase (Mycobacterium)
Chlorobenzene	+100 to +400	45	Catechol	Chlorobenzene dioxygenase (Pseudomonas)
cDCE	+100 to +400	60	CO₂	Polaromonas (carbon source)
1,2-DCA	+100 to +400	100	2-chloroethanol → glycolaldehyde	Xanthobacter autotrophicus GJ10
TCE	+100 to +400	700	TCE-epoxide → glyoxylate	Co-metabolism, requires co-substrate
Chloroform	+100 to +400	700	CO₂	Aerobic co-metabolism

Abiotic Pathways and Remediation

Molecule	Pathway	Environment	Typ. t½ (d)	Metabolite
1,2-DCA	Chemical Hydrolysis	Water, pH 5-9	23,000	2-chloroethanol
TCA	Chemical Hydrolysis (elimination)	Water, pH 5-9	400	1,1-DCE (more toxic!)
TCE	Fe-ZVI (reactive barrier)	PRB, Eh −400 to −100 mV	15	Ethene / Ethane
PCE	Fe-ZVI (reactive barrier)	PRB, Eh −400 to −100 mV	30	Ethene / Ethane via radicals

The chemical hydrolysis pathway of TCA deserves particular attention: it produces 1,1-DCE, which is more toxic than the parent compound and relatively persistent. While this pathway is slow on a human timescale (t½ ≈ 400 days), it is inevitable at TCA-contaminated sites. The accumulation of 1,1-DCE upstream of a biotic degradation signature is a recognizable abiotic fingerprint.

Abundant CSIA Data

Twenty-one isotopic fractionations are tabulated for this family, representing more than one-third of the entire catalog. They systematically cover carbon (¹³C/¹²C), and for several molecules also chlorine (³⁷Cl/³⁵Cl), enabling a particularly powerful dual-isotope diagnosis for organochlorines.

Carbon Fractionation

Molecule	Context	ε ¹³C (‰)	Range	AKIE
Chlorobenzene	Laboratory Pseudomonas	-0.4	[-0.8 ; -0.2]	-
TCA	Laboratory	-4.9	[-6.0 ; -3.5]	1.005
PCE	Biotic culture laboratory	-5.5	[-7.1 ; -3.5]	1.006
PCE	Abiotic Fe-ZVI	-7.8	[-13.2 ; -5.7]	1.009
1,2-DCA	Xanthobacter degradation	-3.5	[-4.3 ; -2.5]	1.007
1,2-DCA	Laboratory (dechlorination)	-33.0	[-41.6 ; -27.5]	1.034
VC	Dechlorination culture laboratory	-22.4	[-31.1 ; -14.8]	1.024
VC	Laboratory Mycobacterium (aerobic)	-8.2	[-12.5 ; -5.6]	1.008
TCE	Consensus on 20 studies (biotic)	-8.8	[-14.0 ; -6.7]	1.0095
TCE	Pure culture laboratory	-18.2	[-22.5 ; -13.0]	1.020
TCE	Abiotic Fe-ZVI	-15.0	[-25.6 ; -8.6]	1.016
cDCE	Culture laboratory	-14.1	[-21.1 ; -9.0]	1.015
cDCE	Polaromonas (aerobic)	-6.5	[-9.0 ; -4.2]	-
Chloroform	Dehalobacter	-27.5	[-33.0 ; -22.0]	1.028
DCM	Methylobacterium extorquens	-42.0	[-66.0 ; -22.0]	1.045

DCM carries the highest fractionation in the database (ε = −42 ‰, AKIE = 1.045): this is the largest ¹³C fractionation reported for biodegradation in literature. Variability is significant depending on the strain (ranging from −66 to −22 ‰), which serves as a reminder that a single value is not suitable for site interpretation: the range encompasses the actual mechanistic uncertainty.

Chlorine and Dual Isotope Fractionation

Molecule	ε ³⁷Cl (‰)	Λ C/Cl	Diagnosis
1,2-DCA	-1.0	0.28	Dechlorination
Chloroform	-4.3	0.16	Reductive dechlorination
DCM	-5.0	0.12	Biotic dechlorination
cDCE	-3.0	0.21	Reductive dechlorination
TCE	-3.9	0.44	Λ ≈ 0.4 biotic vs ≈ 0.7 chemical
PCE	-2.7	0.49	Dual CSIA distinguishes oxidative vs reductive

The Λ parameter (the slope in a dual isotope plot of Δδ³⁷Cl against Δδ¹³C) is the key mechanistic discriminant for chlorinated solvents. Two pathways can produce the same apparent ¹³C enrichment but diverge in the dual plot. The most important bibliographic note in the database for this family concerns TCE: "Λ ≈ 0.4 biotic signature vs ≈ 0.7 chemical." This distinction is why double-element CSIA remains the standard diagnostic tool for PCE/TCE plumes.

Metabolites and Full Cascades

Eighteen parent-metabolite relationships are tabulated, allowing the propagation of full cascades within the IsoFind simulation engine. Below is the consolidated table for the three primary cascades.

Parent	Metabolite	Max Yield	Stability	Toxic
PCE	TCE	0.90	intermediate	yes
	cDCE	0.70	intermediate	yes
	VC (IARC Class 1 carcinogen)	0.50	intermediate	yes
	Ethene (mineralization)	0.40	stable	no
TCE	cDCE	0.85	intermediate	yes
	tDCE (minor pathway)	0.05	intermediate	yes
	VC	0.60	intermediate	yes
	Ethene	0.50	stable	no
cDCE	VC	0.90	intermediate	yes
VC	Ethene	0.85	stable	no
Chloroform	DCM	0.80	intermediate	yes
DCM	Formaldehyde (to CO₂)	0.30	intermediate	no
TCA	1,1-DCE (more toxic!)	0.80	intermediate	yes
TCA	1,1-DCA	0.70	intermediate	yes
Chlorobenzene	Catechol	0.70	intermediate	no

Standard Case Study: A Legacy PCE Plume

Former dry cleaning or industrial degreasing activities have generated a PCE plume that has partially degraded over decades. The instantaneous state of the plume integrates all steps of the cascade.

Observation	IsoFind Interpretation
Residual PCE + significant TCE + dominant cDCE	Active mature cascade, degradation in progress
High cDCE/TCE ratio	Advanced progress on Step 2
Significant presence of VC	Step 3 partially reached, local health risk
Ethene detected	Functional complete cascade
δ¹³C enrichment of residual parent	Confirms degradation (not just dilution)
Λ TCE/Cl ≈ 0.4	Biotic signature (Active Dehalococcoides)
Λ TCE/Cl ≈ 0.7	Abiotic Fe-ZVI signature (if barrier installed)

API Access

Endpoint	Usage
GET /api/molecules/reference/catalogue?famille=Chlorinated solvents	List of the 16 reference chlorinated solvents
POST /api/molecules/catalogue/seed/Chlorinated solvents	Pre-fills the project with this family
GET /api/molecules/csia/PCE/pathways	Tabulated degradation pathways for PCE
POST /api/molecules/csia/resolve	CSIA resolution with local geochemical conditions
POST /api/molecules/csia/dual	C/Cl dual isotope with Λ calculation, mechanistic discriminant