1,2,3-Trichloropropane

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1,2,3-Trichloropropane (TCP) is a chlorinated volatile organic compound (CVOC) that has been used in chemical production processes, in agriculture, and as a solvent, resulting in point and non-point source contamination of soil and groundwater. TCP is mobile and highly persistent in soil and groundwater. TCP is not currently regulated at the national level in the United States, but maximum contaminant levels (MCLs) have been developed by some states. Current treatment methods for TCP are limited and can be cost prohibitive. However, some treatment approaches, particularly in situ chemical reduction (ISCR) with zero valent zinc (ZVZ) and in situ bioremediation (ISB), have recently been shown to have potential as practical remedies for TCP contamination of groundwater.

Related Article(s):

Contributor(s):

Key Resource(s):

  • Prospects for Remediation of 1,2,3-Trichloropropane by Natural and Engineered Abiotic Degradation Reactions. Strategic Environmental Research and Development Program (SERDP), Project ER-1457.[1]
  • Verification Monitoring for In Situ Chemical Reduction Using Zero-Valent Zinc, A Novel Technology for Remediation of Chlorinated Alkanes. Strategic Environmental Research and Development Program (SERDP), Project ER-201628.[2]

Introduction

Figure 1. Ball and stick representation of the molecular structure of TCP (Salter-Blanc and Tratnyek, unpublished)

1,2,3-Trichloropropane (TCP) (Figure 1) is a man-made chemical that was used in the past primarily as a solvent and extractive agent, as a paint and varnish remover, and as a cleaning and degreasing agent.[3]. Currently, TCP is primarily used in chemical synthesis of compounds such as polysulfone liquid polymers used in the aerospace and automotive industries; hexafluoropropylene used in the agricultural, electronic, and pharmaceutical industries; polysulfide polymers used as sealants in manufacturing and construction; and 1,3-dichloropropene used in agriculture as a soil fumigant. TCP may also be present in products containing these chemicals as an impurity[3][4]. For example, the 1,2-dichlropropane/1,3-dichloropropene soil fumigant mixture (trade name D-D), which is no longer sold in the United States, contained TCP as an impurity and has been linked to TCP contamination in groundwater[5][4]. Soil fumigants currently in use which are composed primarily of 1,3-dichloropropene may also contain TCP as an impurity, for instance Telone II has been reported to contain up to 0.17 percent TCP by weight[6].

TCP contamination is problematic because it is “reasonably anticipated to be a human carcinogen” based on evidence of carcinogenicity to animals[7]. Toxicity to humans appears to be high relative to other chlorinated solvents[6], suggesting that even low-level exposure to TCP could pose a significant human health risk.

Environmental Fate

TCP’s fate in the environment is governed by its physical and chemical properties (Table 1). TCP does not adsorb strongly to soil, making it likely to leach into groundwater and exhibit high mobility. In addition, TCP is moderately volatile and can partition from surface water and moist soil into the atmosphere. Because TCP is only slightly soluble and denser than water, it can form a dense non-aqueous phase liquid (DNAPL) as observed at the Tyson’s Dump Superfund Site[8]. TCP is generally resistant to aerobic biodegradation, hydrolysis, oxidation, and reduction under naturally occurring conditions making it persistent in the environment[1].

Table 1. Physical and chemical properties of TCP[9]
Property Value
Chemical Abstracts Service (CAS) Number 96-18-4
Physical Description
(at room temperature)
Colorless to straw-colored liquid
Molecular weight (g/mol) 147.43
Water solubility at 25°C (mg/L) 1,750 (slightly soluble)
Melting point (°C) -14.7
Boiling point (°C) 156.8
Vapor pressure at 25°C (mm Hg) 3.10 to 3.69
Density at 20°C (g/cm3) 1.3889
Octanol-water partition coefficient
(logKow)
1.98 to 2.27
(temperature dependent)
Organic carbon-water partition coefficient
(logKoc)
1.70 to 1.99
(temperature dependent)
Henry’s Law constant at 25°C
(atm-m3/mol)
3.17x10-4[3] to 3.43x10-4[10]

Occurrence

TCP has been detected in approximately 1% of public water supply and domestic well samples tested by the United States Geological Survey. More specifically, TCP was detected in 1.2% of public supply well samples collected between 1993 and 2007 by Toccalino et al[11] and 0.66% of domestic supply well samples collected between 1991 and 2004 by DeSimone[12]. TCP was detected at a higher rate in domestic supply well samples associated with agricultural land-use studies than samples associated with studies comparing primary aquifers (3.5% versus 0.2%)[12].

Regulation

The United States Environmental Protection Agency (USEPA) has not established an MCL for TCP, although guidelines and health standards are in place[9]. TCP was included in the Contaminant Candidate List 3[13] and the Unregulated Contaminant Monitoring Rule 3 (UCMR 3)[14]. The UCMR 3 specified that data be collected on TCP occurrence in public water systems over the period of January 2013 through December 2015 against a reference concentration range of 0.0004 to 0.04 μg/L[15]. The reference concentration range was determined based on a cancer risk of 10-6 to 10-4 and derived from an oral slope factor of 30 mg/kg-day, which was determined by the EPA’s Integrated Risk Information System[16]. Of 36,848 samples collected during UCMR 3, 0.67% exceeded the minimum reporting level of 0.03 µg/L. 1.4% of public water systems had at least one detection over the minimum reporting level, corresponding to 2.5% of the population[15]. While these occurrence percentages are relatively low, the minimum reporting level of 0.03 µg/L is more than 75 times the USEPA-calculated Health Reference Level of 0.0004 µg/L. Because of this, TCP may occur in public water systems at concentrations that exceed the Health Reference Level but are below the minimum reporting level used during UCMR 3 data collection. These analytical limitations and lack of lower-level occurrence data have prevented the USEPA from making a preliminary regulatory determination for TCP[17].

Some US states have established their own standards including Hawaii which has established an MCL of 0.6 μg/L[18]. California has established an MCL of 0.005 μg/L[19], a notification level of 0.005 μg/L, and a public health goal of 0.0007 μg/L[20], and New Jersey has established an MCL of 0.03 μg/L[21].

Transformation Processes

Figure 2. Figure 2. Summary of anticipated primary reaction pathways for degradation of TCP. Oxidation, hydrolysis, and hydrogenolysis are represented by the horizontal arrows. Elimination (dehydrochlorination) and reductive elimination are shown with vertical arrows. [O] represents oxygenation (by oxidation or hydrolysis), [H] represents reduction. Gray indicates products that appear to be of lesser significance[1].

Potential TCP degradation pathways include hydrolysis, oxidation, and reduction (Figure 2). These pathways are expected to be similar overall for abiotic and biotic reactions[22], but the rates of the reactions (and their resulting significance for remediation) depend on natural and engineered conditions.

The rate of hydrolysis of TCP is negligible under typical ambient pH and temperature conditions but is favorable at high pH and/or temperature[1][22]. For example, ammonia gas can be used to raise soil pH and stimulate alkaline hydrolysis of chlorinated propanes including TCP[23]. Thermal Conduction Heating (TCH) may also produce favorable conditions for TCP hydrolysis[1][22].

Treatment Approaches

Compared to more frequently encountered CVOCs such as trichloroethene (TCE) and tetrachloroethene (PCE), TCP is relatively recalcitrant[24][1]. TCP is generally resistant to hydrolysis, bioremediation, oxidation, and reduction under natural conditions[1]. The moderate volatility of TCP makes air stripping, air sparging, and soil vapor extraction (SVE) less effective compared to other VOCs[24]. Despite these challenges, both ex situ and in situ treatment technologies exist. Ex situ treatment processes are relatively well established and understood but can be cost prohibitive. In situ treatment methods are comparatively limited and less-well developed, though promising field-scale demonstrations of some in situ treatment technologies have been conducted.

Ex Situ Treatment

The most common ex situ treatment technology for groundwater contaminated with TCP is groundwater extraction and treatment[25]. Extraction of TCP is generally effective given its relatively high solubility in water and low degree of partitioning to soil. After extraction, TCP is typically removed by adsorption to granular activated carbon (GAC)[24][26].

TCP contamination in drinking water sources is typically treated using granular activated carbon (GAC)[27].

In California, GAC is considered the best available technology (BAT) for treating TCP, and as of 2017 seven full-scale treatment facilities were using GAC to treat groundwater contaminated with TCP[28]. Additionally, GAC has been used for over 30 years to treat 60 million gallons per day of TCP-contaminated groundwater in Hawaii[29].

GAC has a low to moderate adsorption capacity for TCP, which can necessitate larger treatment systems and result in higher treatment costs relative to other organic contaminants[9]. Published Freundlich adsorption isotherm parameters[30] indicate that less TCP mass is adsorbed per gram of carbon compared to other volatile organic compounds (VOCs), resulting in increased carbon usage rate and treatment cost. Recent bench-scale studies indicate that subbituminous coal-based GAC and coconut shell-based GAC are the most effective types of GAC for treatment of TCP in groundwater[29][31]. To develop more economical and effective treatment approaches, further treatability studies with site groundwater (e.g., rapid small-scale column tests) may be needed.

In Situ Treatment

In situ treatment of TCP to concentrations below current regulatory or advisory levels is difficult to achieve in both natural and engineered systems. However, several in situ treatment technologies have demonstrated promise for TCP remediation, including chemical reduction by zero-valent metals (ZVMs), chemical oxidation with strong oxidizers, and anaerobic bioremediation[24][1].

In Situ Chemical Reduction (ISCR)

Reduction of TCP under conditions relevant to natural attenuation has been observed to be negligible. Achieving significant degradation rates of TCP requires the addition of a chemical reductant to the contaminated zone[24][1]. Under reducing environmental conditions, some ZVMs have demonstrated the ability to reduce TCP all the way to propene. As shown in Figure 2, the desirable pathway for reduction of TCP is the formation of 3-chloro-1-propene (also known as allyl chloride) via dihaloelimination, which is then rapidly reduced to propene through hydrogenolysis [24][1][32]. ZVMs including granular zero-valent iron (ZVI), nano ZVI, palladized nano ZVI, and zero-valent zinc (ZVZ) have been evaluated by researchers[24][1].

ZVI is a common reductant used for ISCR and, depending on the form used, has shown variable levels of success for TCP treatment. The Strategic Environmental Research and Development Program (SERDP) Project ER-1457 measured the TCP degradation rates for various forms of ZVI and ZVZ. Nano-scale ZVI and palladized ZVI increased the TCP reduction rate over that of natural attenuation, but the reaction is not anticipated to be fast enough to be useful in typical remediation applications[22].

Commercial-grade zerovalent zinc (ZVZ) on the other hand is a strong reductant that reduces TCP relatively quickly under a range of laboratory and field conditions to produce propene without significant accumulation of intermediates[22][33][34][24]. Of the ZVMs tested as part of SERDP Project ER-1457, ZVZ had the fastest degradation rates for TCP[1]. In bench-scale studies, TCP was reduced by ZVZ to propene with 3-chloro-1-propene as the only detectable chlorinated intermediate, which was short-lived and detected only at trace concentrations[32].

Navy Environmental Sustainability Development to Integration (NESDI) Project 434 conducted bench-scale testing which demonstrated that commercially available ZVZ was effective for treating TCP. Additionally, this project evaluated field-scale ZVZ column treatment of groundwater impacted with TCP at Marine Corps Base Camp Pendleton (MCBCP) in Oceanside, California. This study reported reductions of TCP concentrations by up to 95% which was maintained for at least twelve weeks with influent concentrations ranging from 3.5 to 10 µg/L, without any significant secondary water quality impacts detected[34].

Following the column study, a 2014 pilot study at MCBCP evaluated direct injection of ZVZ with subsequent monitoring. Direct injection of ZVZ was reportedly effective for TCP treatment, with TCP reductions ranging from 90% to 99% in the injection area. Concentration reduction downgradient of the injection area ranged from 50 to 80%. TCP concentrations have continued to decrease, and reducing conditions have been maintained in the aquifer since injection, demonstrating the long-term efficacy of ZVZ for TCP reduction[2].

Potential in situ applications of ZVZ include direct injection, as demonstrated by the MCBCP pilot study, and permeable reactive barriers (PRBs). Additionally, ZVZ could potentially be deployed in an ex situ flow-through reactor, but the economic feasibility of this approach would depend in part on the permeability of the aquifer and in part on the cost of the reactor volumes of ZVZ media necessary for complete treatment.

In Situ Chemical Oxidation (ISCO)

Chemical oxidation of TCP with mild oxidants such as permanganate or ozone is ineffective. However, stronger oxidants (e.g. activated peroxide and persulfate) can effectively treat TCP, although the rates are slower than observed for most other organic contaminants[1][26]. Fenton-like chemistry (i.e., Fe(II) activated hydrogen peroxide) has been shown to degrade TCP in the laboratory with half-lives ranging from 5 to 10 hours[1], but field-scale demonstrations of this process have not been reported. Treatment of TCP with heat-activated or base-activated persulfate is effective but secondary water quality impacts from high sulfate may be a concern at some locations.

Aerobic Bioremediation

No naturally occurring microorganisms have been identified that degrade TCP under aerobic conditions[25]. Relatively slow aerobic cometabolism by the ammonia oxidizing bacterium Nitrosomonas europaea and other populations has been reported[35][25], and genetic engineering has been used to develop organisms capable of utilizing TCP as a sole carbon source under aerobic conditions[36][25][37].

Anaerobic Bioremediation

Like other CVOCs, TCP has been shown to undergo biodegradation under anaerobic conditions via reductive dechlorination by Dehalogenimonas (Dhg) species[24][38][39][40][41][25]. However, the kinetics are slower than for other CVOCs. Bioaugmentation cultures containing Dehalogenimonas (KB-1 Plus, SiREM) are commercially available and have been implemented for remediation of TCP-contaminated groundwater[42]. One laboratory study examined the effect of pH on biotransformation of TCP over a wide range of TCP concentrations (10 to 10,000 µg/L) and demonstrated that successful reduction occurred from a pH of 5 to 9, though optimal conditions were from pH 7 to 9[42].

As with other microbial cultures capable of reductive dechlorination, coordinated amendment with a fermentable organic substrate (e.g. lactate or vegetable oil), also known as biostimulation, creates reducing conditions in the aquifer and provides a source of hydrogen which is required as the primary electron donor for reductive dechlorination.

A 2016 field demonstration of in situ bioremediation (ISB) was performed in California’s Central Valley at a former agricultural chemical site with relatively low TCP concentrations (2 µg/L). The site was first biostimulated by injecting amendments of emulsified vegetable oil (EVO) and lactate, which was followed by bioaugmentation with a microbial consortium containing Dhg. After an initial lag period of six months, TCP concentrations decreased to below laboratory detection limits (<0.005 µg/L)[42].

Table 2. Advantages and limitations of TCP treatment technologies[2]
Technology Advantages Limitations
ZVZ
  • Can degrade TCP at relatively high and low concentrations
  • Faster reaction rates than ZVI
  • Material is commercially available
  • Higher cost than ZVI
  • Difficult to distribute in subsurface in situ applications
Groundwater
Extraction and
Treatment
  • Can cost-effectively capture and treat larger, more dilute
    groundwater plumes than in situ technologies
  • Well understood and widely applied technology
  • Requires construction, operation and maintenance of
    aboveground treatment infrastructure
  • Typical technologies (e.g. GAC) may be expensive due
    to treatment inefficiencies
ZVI
  • Can degrade TCP at relatively high and low concentrations
  • Lower cost than ZVZ
  • Material is commercially available
  • Lower reactivity than ZVZ, therefore may require higher
    ZVI volumes or thicker PRBs
  • Difficult to distribute in subsurface in situ applications
ISCO
  • Can degrade TCP at relatively high and low concentrations
  • Strategies to distribute amendments in situ are well established
  • Material is commercially available
  • Most effective oxidants (e.g., base-activated or heat-activated
    persulfate) are complex to implement
  • Secondary water quality impacts (e.g., high pH, sulfate,
    hexavalent chromium) may limit ability to implement
In Situ
Bioremediation
  • Can degrade TCP at moderate to high concentrations
  • Strategies to distribute amendments in situ are well established
  • Materials are commercially available and inexpensive
  • Slower reaction rates than ZVZ or ISCO

The 2016 field demonstration was expanded to full-scale treatment in 2018 with biostimulation and bioaugmentation occurring over several months. The initial TCP concentration in performance monitoring wells ranged from 0.008 to 1.7 µg/L. As with the field demonstration, a lag period of approximately 6 to 8 months was observed before TCP was degraded, after which concentrations declined over fifteen months to non-detectable levels (less than 0.005 µg/L). TCP degradation was associated with increases in Dhg population and propene concentration. Long term monitoring showed that TCP remained at non-detectable levels for at least three years following treatment implementation[24].

Treatment Comparisons and Considerations

When selecting a technology for TCP treatment, considerations include technical feasibility, ability to treat to regulated levels, potential secondary water quality impacts and relative costs. A comparison of some TCP treatment technologies is provided in Table 2.

Summary

The relatively high toxicity of TCP has led to the development of health-based drinking water concentration values that are very low. TCP is sometimes present in groundwater and in public water systems at concentrations that exceed these health based goals. While a handful of states have established MCLs for TCP, US federal regulatory determination is hindered by the lack of low concentration occurrence data. Because TCP is persistent in groundwater and resistant to typical remediation methods (or costly to treat), specialized strategies may be needed to meet drinking water based treatment goals. In situ chemical reduction (ISCR) with zero valent zinc (ZVZ) and in situ bioremediation have been demonstrated to be effective for TCP remediation.

References

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See Also

ATSDR Toxicological Profile: https://www.atsdr.cdc.gov/ToxProfiles/TP.asp?id=912&tid=186

EPA Technical Fact Sheet: https://www.epa.gov/sites/production/files/2014-03/documents/ffrrofactsheet_contaminant_tcp_january2014_final.pdf

Cal/EPA State Water Resources Control Board Groundwater Information Sheet: http://www.waterboards.ca.gov/gama/docs/coc_tcp123.pdf

California Water Boards Fact Sheet: http://www.waterboards.ca.gov/drinking_water/certlic/drinkingwater/documents/123-tcp/123tcp_factsheet.pdf