This group comprises a number of well known contaminants e.g. Tetrachloroethylene (PCE), Trichloroethene 
(TCE), 1,1,1-Trichloroethane (TCA), 1,1-Dichloroethene (DCE), Carbon tetrachloride, Chloromethane, 
Dichloromethane, and Tetrachloromethane. 
 

Chlorinated aliphatics have been a major concern across Europe, both because they are widespread soil and groundwater contaminants and because of the carcinogenic properties of some of these compounds (1,2-Dichlorethane, vinylchloride and maybe TCE). Chlorinated aliphatics are found at many different sites due to the fact that they have been applied for multiple industrial purposes, including metal degreasing, dry cleaning, leather manufacturing, paint manufacturing, etc.

 

In Table 1, the most common chlorinated aliphatics are shown.

 

Table 1. Chlorinated aliphatics

 

Compound	Abbreviation	Formula	Other names
Chloromethane	CM	CH3Cl	Methylchloride
Dichloromethane	DCM	CH2Cl2	Methylenchloride
Trichloromethane	TCM	CHCl3	Chloroform
Tetrachloromethane	TeCA	CCl4
Chloroethane	CE	CH3-CH2Cl	Ethyl chloride
1,1-Dichloroethane	1,1-DCA	CH3-CHCl2	Ethylendichloride
1,2-Dichloroethane	1,2-DCA	CH2Cl-CH2Cl	Ethylenchloride
1,1,1-Trichloroethane	1,1,1-TCA	CH3-CCl3
1,1,2-Trichloroethane	1,1,2-TCA	CHCl2-CH2Cl	Vinyltrichloride
Chloroethene	CE	CH2=CHCl	Vinylchloride
1,1-Dichloroethene	1,1-DCE	CCl2=CH2	Dichloroethylene
cis-1,2-Dichloroethane	c-1,2-DCE	CHCl=CHCl	Dichloroethylene, Acetylenchloride
trans-1,2-Dichloroethane	t-1,2-DCE	CHCl=CHCl	Dichloroethylene, Acetylenchloride
Trichloroethene	TCE	CHCl=CCl2	Trichloroethylene
Tetrachloroethene	TeCE	CCl2=CCl2	Tetrachloroethylene

 

 

These compounds are relatively volatile and water soluble and they are quite different from many other organic soil and groundwater contaminants in terms of biodegradability, since they are less biodegradable under aerobic conditions than under anaerobic conditions. For these reasons they are found in numerous groundwater aquifers beneath industrial areas, and they are probably the compounds which have given rise to closing down of most groundwater wells for drinking supply in the Western world, maybe only outcompeted by certain widespread pesticides.  

 

 

Table 2. Physio-chemical properties of selected chlorinated compounds

 

Compound	Mole weight g mole-1	Den-sity g ml-1	Boiling point °C	Water solubility mg l-1	Vapor pressure mmHg	Log Kow
Chloromethane	50.49	0.92	-23.7	5235	3756	0.91
Dichloromethane	84.94	1.34	40	13200	438	1.25
Trichloromethane	119.38	1.50	61.7	8700	151	1.97
Tetrachloromethane	153.82	1.58	76.7	780	91	2.64
Chloroethane	64.52	0.9	12.3	5700	1.5	1.43
1,1-Dichloroethane	98.96	1.18	57.3	4767	226	1.79
1,2-Dichloroethane	98.96	1.26	83.5	8606	82	1.48
1,1,1-Trichloroethane	133.41	1.35	74.1	1250	100	2.49
1,1,2-Trichloroethane	133.41	1.44	114	4394	23	2.38
Chloroethene	62.5	0.92	-14	2763	2660	1.38
1,1-Dichloroethene	96.94	1.22	32	3344	500	2.13
c-1,2-Dichloroethene	96.94	1.27	60	3500	206	1.86
Tr-1,2-Dichloroethene	96.94	1.25	48	6260	300	1.93
Trichloroethene	131.39	1.47	87	1400	74	2.53
Tetrachloroethene	165.83	1.63	121	240	19	2.88

 

 

Fate of chlorinated aliphatics in the soil and groundwater environment

 

Transports of chlorinated aliphatics in the subsurface is governed by the fact that these compounds are denser than water and as such belong to the group of DNAPLs (Dense Non-Aqueous Phase Liquids). They also have a relatively low viscosity and can be characterised as fluent. That means that free phase contaminants will tend to sink downwards through the aquifer and settle in pools on top of the bottom impermeable layers. During their transport some of the mass will be stuck to the porous medium as smears, and these smears will together with the pools constitute a source for continuous dissolution of contaminants to the water phase, in some cases resulting in contamination with an effect for decades (Johnson & Pankow, 1992).

 

Degradation of the chlorinated aliphatics in the soil and groundwater environment can take place through two different processes, abiotic transformation or biodegradation. The chemical transformation can be either abiotic dechlorination/hydrolysis with half-life rates in the order of 0.5, 0.7 and 0.9 years at 20°C for 1,1,1-TCA, PCE and TCE respectively (Vogel et al. 1987) or metal catalysed degradation with half-life rates in the order of 5, 18, and 14 hours for the same compounds (Gillham et al. 1993). Biodegradation of the compounds is often faster than chemical transformation, but heavily depends on the redox conditions. Most of the compounds are not degraded without availability of primary substrates (Oldenhuis et al. 1989; Freedman & Gossett, 1989; Tandol et al. 1994). In Table 3, the degradability of selected chlorinated aliphatics is shown.

 

Table 3. Degradability of selected chlorinated aliphatics

 

Compound	Aerobic degradability	Aerobic degradability with primary substrate1)	Anaerobic degradability with primary substrate2)
Chloromethane	-	+	+
Dichloromethane	+	+	+
Trichloromethane	-	+	++
1,2-Dichloroethane	+	+	+
1,1-Dichloroethane	-	+	+
1,1,1-Trichloroethane	-	+	++
Vinylchloride	+	+	+
1,2-Dichloroethene	-	+	+
Trichloroethene	-	++	++
Tetrachloroethene	-	-	++

1) Analogue gas, toluene, phenol (Nelson et al. 1988)

2) Easy degradable carbon substrate

 

Toxic effects

 

The chlorinated aliphatics are most known for the carcinogenic properties of some of the compounds. Severe acute ecotoxicological effects have not been shown for any of the compounds, but phytotoxicity, which is considered relevant in this context, is shown for some of them. In Table 2, the carcinogenity and the phytotoxicity against poplar for selected chlorinated aliphatics are shown.

 

Table 2. Geno- and phytotoxicity of chlorinated aliphatics

 

Compound	Carcinogenity	Phytotoxicity, Zero growth conc (mM) 1)
Chloromethane	-	Ne
Dichloromethane	-	Ne
Trichloromethane	+	Ne
Tetrachloromethane	+	Ne
Chloroethane	-	ne
1,1-Dichloroethane	-	10.7
1,2-Dichloroethane	+	2
1,1,1-Trichloroethane	-	ne
1,1,2-Trichloroethane	-	2.3
Chloroethene	+	ne
1,1-Dichloroethene	-	5.6
c-1,2-Dichloroethene	-	6
tr-1,2-Dichloroethene	-	4.8
Trichloroethene	(+)	0.9
Tetrachloroethene	-	0.3

ne: not estimated; 1) Dietz & Schnoor, 2001)

 

Management and remediation

These contaminants have attracted much attention for many years by the authorities because of their carcinogenity and because they are been spread rapidly from the source to the surroundings. The traditional remediation technique was for many years so-called Pump-and treat. But this method has been shown to have significant limitations with respect to free phase contaminations with chlorinated aliphatics (Mercer et al. 1990; Haley et al. 1991; MacDonald & Kavanaugh, 1994). In addition to the formation of smears and pools of free phase, these compounds will have a tendency to diffuse into fractures and areas of low permeability in the aquifer, where they can be very difficult to remobilise and can only be removed very slowly. The kinetics of diffusion and dissolving from the free phase therefore govern the rate of which it can be removed and pumped out of the aquifer. One can theoretically expect a typical pattern in the concentration of the chlorinated aliphatics in the water, which is pumped up, with a rapid initial decrease in the beginning of the pumping period followed by a stabilisation of the concentration, often at a level above the acceptable concentration value of the contaminant. This phenomenon is called tailing. If the pump-and-treat is operated at intervals, often rebound of the concentration will be observed in the effluent water after stop of pumping, due to the relocation of diffusion to the water from low-producing to high-producing deposits. For these reasons other remediation techniques have been replacing pump-and-treat in many cases. The most frequent techniques used are SVE, thermal desorption, reductive dechlorination with iron in reactive barrier schemes, but also bioremediation techniques based on biostimulation have been suggested (Semprini et al. 1992). 

 

Publications

 

Dietz, AC, Schnoor, JL (2001) Phytotoxicity of chlorinated aliphatics to hydrid poplar (Populus deltoids x nigra DN34). Environ. Toxicol. Chem. 20(2), 389-393.

 

Freedman, DL, Gossett, JM (1989) Biological reductive dechlorination of tetrachloroethylene and trichloroethylene to ethylene under methanogenic conditions. Appl. Environ. Microbiol. 55, 2144-2151

 

Gillham, RW, O’Hannesin, SF, Orth, WS (1993) Metal enhanced degradation of chlorinated aliphatics: laboratory tests and field trials. In proceedings of the 1993 HazMat Central Conference, March 9-11, 1993, Chicago, US.

 

Haley, JE, Hanson, B, Enfield, C, Glass, J (1991) Evaluating the effectiveness of ground water extraction systems, GWMR, winter 1991, 119-124.

 

Johnson, RL, Pankow, JF (1992) Dissolution of dense chlorinated solvents into groundwater. 2. Source functions for pools of solvent. Environ. Sci. Technol. 26, 896-901.

 

Macdonald, JA, Kavanaugh, MC (1994) Restoring contaminated groundwater: An achievable goal? Environ. Sci. Technol. 28.

 

Mercer, JN, Skipp, DC, Giffin, D (1990) Basics of pump-and-treat groundwater remediation technology. USEPA-600/8-90/003, Robert S. Kerr Environmental Researcj laboratory, Ada, OK, 31 p.

 

Oldenhuis, R, Vink, RLJM, Janssen, D, Witholt, B (1989) Degradation of chlorinated hydrocarbons by Methylosinus trichosporium OB3b expressing soluble methane monooxygenase. Appl. Environ. Microbiol. 55, 2819-2826.

 

Semprini, L, Hopkins, GD, McCarty, PL, Roberts, PV (1992) In-situ transformation of carbon tetrachloride and other halogenated compounds resulting from biostimulation under anoxic conditions. Environ. Sci. Technol. 26, 2454-2461.

 

Tandoi, V, DiStefano, TD, Bowser, PA, Gossett, JM, Zinder, SH (1994) Reductive dehalogenation of chlorinated ethenes and halogenated ethanes by a high rate anaerobic enrichment culture. Environ. Sci. Technol. 28, 973-979.

 

Vogel, TM, Criddle, CS, McCarthy, PL (1987) Transformation of halogenated aliphatic compounds. Environ. Sci. Technol. 21, 722-736.