1,4-Dioxane (14D) is a heterocyclic synthetic organic chemical that contains two ether bonds, is miscible with water, does not strongly sorb to natural or engineered materials, and does not biodegrade in many environments, all of which results in its persistence and rapid transport in aqueous environments. 14D is a suspected human carcinogen, which has resulted in relatively strict standards for drinking water sources. Advanced oxidation processes (AOPs) are the most well-developed technologies for removal of 14D from potable water supplies. Several treatment technologies are being developed for in situ treatment of 14D including chemical oxidation, cometabolic bioremediation, and thermally enhanced soil vapor extraction.
Properties, Fate and Transport
1,4-Dioxane (14D) is a heterocyclic organic compound, and the most commonly encountered of the three dioxane isomers (1,2-, 1,3- and 1,4-dioxane). Key physical and chemical properties of 14D are listed in Table 1. The heterocyclic, polar structure of 1,4-dioxane causes it to be miscible with water and also unable to strongly sorb or partition to aquifer solids or engineered sorbents (i.e. activated carbon). While 14D has a moderate vapor pressure, the high aqueous solubility results in limited volatilization.
|Chemical Abstracts Service (CAS) Number||123-91-1|
|Appearance||Colorless Liquid||NIOSH, 2007|
|Molecular Weight||88.105 g/mol||NIST, 2018|
|Water Solubility||Miscible||NIOSH, 2007|
|Specific Gravity||1.03||NIOSH, 2007|
|Vapor Pressure at 25° C||38.1 mm Hg||US EPA, 2017|
|Henry’s Law Constant at 25° C||4.80 X 10-6 atm-m3/mol||US EPA, 2017|
|Octanol-Water Partition Coefficient (log Kow)||-0.27||US EPA, 2017|
|Organic Carbon Partition Coefficient (log Koc)||1.23||Lyman et al., 1990|
Biodegradation of 14D is limited in many environments including surface water, groundwater and wastewater treatment plants. Currently, there is little evidence that 14D biodegrades under anaerobic conditions. In contrast, under aerobic conditions, several bacteria and fungi can grow on 14D as a sole source of carbon and energy, including well-characterized strains such as Pseudonocardia dioxanivorans CB1190 and other closely related actinomycetes. However, growth of these organisms on 14D is slow, and as a consequence, 14D-metabolizing microorganisms are generally not effective in degrading 14D at the lower concentrations (≤100 μg/L) commonly observed in surface water and groundwater .
While 14D is not commonly used as a microbial growth substrate at ambient concentrations, there are a wide variety of microorganisms that can cometabolically biodegrade 14D when grown on a different (primary) substrate. During cometabolism, the primary substrate supports production of active biomass and induces the appropriate enzymes that can then fortuitously biodegrade 14D. Primary substrates that have been shown to support cometabolic biodegradation of 14D include tetrahydrofuran (THF), toluene, methane, ethane, propane, and isobutane. Recent field studies have demonstrated that propane-stimulated cometabolic biodegradation can reduce 14D to below 2-3 µg/L under appropriate conditions.
As a result of the low sorption, low volatilization and low biodegradation, attenuation of 14D is often limited in groundwater, surface water and traditional potable and wastewater treatment systems. In groundwater, the limited sorption of 14D results in relatively low concentrations in the source area and large dilute plumes. Under these conditions, much of the 14D mass will be present in the downgradient plume and aggressive treatment of the source area is generally less effective in reducing 14D migration.
Toxicity and Regulatory Standards (updated 2018)
The U.S. Department of Health and Human Services classifies 14D as a reasonably anticipated human carcinogen, and the International Agency for Research on Cancer regards 14D as possibly carcinogenic to humans (Group 2B). These classifications are primarily based on research on mice, rats and guinea pigs. One human epidemiological study reported no increased deaths from cancer in workers exposed to 14D.
There is currently no maximum contaminant level (MCL) for 14D. However, tap water screening (0.46 µg/L) and drinking water equivalent levels (1 mg/L) have been established. For soil contamination, the US EPA has calculated a residential soil screening level (SSL) of 5.3 milligrams per kilogram (mg/kg), an industrial SSL of 24 mg/kg and leach-based SSL of 9.4 x 10-5 mg/kg . Many states have established drinking water and groundwater standards, which range from 0.25 (New Hampshire) to 9.1 µg/L (Texas).
History of Use and Release to Environment
The most common use of 14D has been as a solvent stabilizer for 1,1,1-Trichloroethane (TCA) utilized in metal degreasing operations. The addition of 14D to degreasing solvents such as TCA inhibits the formation of metal salts, which can result in solvent breakdown. Approximately 90% of 14D produced in the 1980s was used as a stabilizer for TCA. Following the phase-out of ozone depleting chemicals including TCA, the production and use of 14D has declined. Beyond its use as a solvent stabilizer, 14D has also been used in textile, paint and ink, cellulose acetate membrane and adhesive manufacturing operations.
14D is an un-intended byproduct of some organic chemical synthesis processes. 14D can be produced during manufacture of alcohol ethoxylates which are used in a wide variety of consumer products including soaps and detergents, cosmetics and food. In 2001, ethoxylated raw materials were found to contain up to 1.4% 14D. Once this issue was recognized, measures were put into place to reduce 14D levels. 14D levels in the final product can be reduced by controlling the production process and by treatment of finished products by several approaches including vacuum stripping, steam stripping, and drying. 14D is produced as a byproduct during production of polyethylene terephthalate (PET), polyester and strong surfactants, as well as many other products. Treated industrial wastewater containing 14D may be released to surface water or groundwater.
14D has been detected in air, potable water, wastewater and groundwater. 14D was detected in ambient air in several studies). In a survey of 4,864 public drinking water systems, 14D was detected in 21% of the systems and exceeded the health-based reference concentration of 0.35 μg/L in 6.9%. Municipal wastewater and landfill leachates also frequently contain 14D.
In groundwater, 14D is frequently observed as a co-contaminant with TCA. As 14D was often utilized as an additive to degreasing solvents, its presence is positively correlated with other chlorinated organics such as TCA, trichloroethene (TCE), and 1,1-Dichloroethene (1,1-DCE). In one study, TCA and/or TCE were observed in 94% of the monitoring wells with detectable 14D.
A comprehensive evaluation of a large multi-site dataset found that 14D was detected, when analyzed for, at 52% of sites containing TCE, 70% of sites with 1,1,1-TCA, and 69% of sites with 1,1-DCE. The spatial extent of the 14D plumes typically fell within a similar or smaller footprint than the co-occurring chlorinated solvent plumes. The more limited migration of 14D, compared to chlorinated solvents, may be due to the timing of the 14D release. TCE was used at many sites prior to the use of TCA. As a result, TCE (and its by-products) may have been released before TCA and 14D and could have a “head-start”. The results were somewhat surprising given the high migration potential of 14D, as explained in the video shown in Figure 1. The maximum historical dioxane concentrations were below 365 μg/L for half of 194 sites with detectable 14D. Overall, 14D plumes are generally so dilute that source zones may be difficult to identify.
Also, modeling results indicate that matrix diffusion of 14D mass in and out of lower-permeability soils (i.e. silts and clays) can be an important fate and transport process for this compound as discussed in more detail in the video shown in Figure 2. The contribution of matrix diffusion processes to 14D concentrations within the source zone and the downgradient plume should be accounted for during site management.
The common presence of 14D in potable water supplies is a concern because 14D removal is low in many unit processes commonly employed in publicly owned treatment works (POTW) due to the high solubility, low volatility, and relative resistance to biodegradation (see section on physical/chemical treatment). 14D removal was not observed in several physical/chemical water treatment processes including coagulation, oxidation and air stripping processes. Activated carbon adsorption is somewhat more successful, with reported removal efficiencies ranging from 50 to 67 percent. Stepien reported that 14D removal was not observed in four domestic wastewater treatment plants in Germany. In a study of North Carolina water treatment facilities, 14D removal was not observed at two of the facilities, but was reduced by 65% at a third which employed ozonation of raw and settled water.
Above Ground Treatment
Air stripping is generally not effective for removing 14D from contaminated water due its low Henry’s Law constant. While 14D can be removed from water by sorption to granular activated carbon (GAC), sorption capacities are low resulting in high capital and operating costs. In contrast, polymeric sorbents (e.g. AmbersorbTM 560) are reported to have 5 to 10 times the sorption capacity of GAC, making sorption a more attractive alternative, though with additional operational requirements of adsorbent regeneration and proper disposal of spent waste. While 14D can be degraded by high energy UV radiation in high purity water, direct ultraviolet photolysis is not economically viable due to operational challenges and high costs.
Advanced oxidation processes (AOPs) using combinations of ozone (O3), hydrogen peroxide (H2O2) and ultraviolet (UV) radiation produce highly reactive hydroxyl radicals (•OH) which are effective in destroying 14D. In the peroxone process (O3 + H2O2), O3 reacts with H2O2 giving rise to hydroxyl radicals. In a variation of this approach (HiPOx™ process), H2O2 is added first followed by high pressure O3. Hydroxyl radicals are produced in UV + H2O2 systems by direct photolysis of H2O2 caused by absorption of energy from an ultraviolet lamp.
AOPs are commonly employed for 14D treatment because of their high removal efficiencies with low contact times. The degradation mechanisms and kinetics of these processes are well understood, with multiple systems in current use. All of these systems are effective in destroying 14D, but have some limitations including the potential for bromate formation (when Br is present), reduced efficiency in the presence of free radical scavengers (e.g. HCO3) and high capital and operating costs associated with energy and chemical consumption.
Biological wastewater treatment has not been widely applied for 14D removal. While high concentrations of 14D will support growth of 14D degraders growth rates on 14D are slow, and the low 14D concentrations in many systems are not sufficient to maintain an adequate level of 14D degraders for effective treatment. To overcome this problem, 14D degrading biomass was supported by addition of tetrahydrofuran (THF) to a continuously fed trickling filter for over one year. The system was effective in reducing 14D concentrations by 93–97% while influent 14D concentrations were varied from 200 to 1,200 µg/L. This basic approach was applied to full-scale treatment of leachate from the Lowry Landfill Superfund site where both THF and 14D were present as co-contaminants.
A variety of methods have been attempted for in situ treatment of 14D impacted soil and groundwater including in situ chemical oxidation (ISCO), thermally enhanced soil vapor extraction, bioremediation, phytoremediation, and monitored natural attenuation (MNA).
ISCO can be used to degrade 14D in source areas with a variety of oxidants including permanganate, persulfate, Fenton’s reagent and ozone. However, ISCO treatment of 14D in source areas is not common because much of the mass is in the downgradient plume. Dilute plumes can potentially be treated using cylinders placed in wells that slowly release oxidant over time. In a field demonstration, a system of slow release persulfate cylinders was effective in reducing 14D and chlorinated solvent concentrations by over 99% for 119 days.
Thermal remediation of source areas is not frequently utilized for 14D remediation because much of the 14D mass is in the dissolved plume. However, when applied to treat a source area, thermal remediation can be effective. Following electrical resistive heating of a chlorinated solvent source area, 14D levels in groundwater were reduced by >99%. 14D source areas can also be treated using a combination of heated air injection and focused air extraction from areas with elevated 14D concentrations. In a field demonstration, thermally enhanced soil vapor extraction reduced 14D levels by 94%. Modeling results indicate that removal increases with increasing air injection temperature and relative humidity and decreases with initial soil moisture content.
In situ bioremediation has not been commonly applied to 14D treatment due to the slow growth of bacteria using 14D as a growth substrate. However, there has been some success in stimulating cometabolic biodegradation of 14D using gaseous substrates. In a biosparging pilot test, air amended with propane was injected into a single well that was bioaugmented with the propanotroph Rhodococcus ruber ENV425. 14D levels decreased by >99% in the sparge well and two nearby monitoring wells. A propane-stimulated groundwater recirculation system has also recently been reported to effectively reduce concentrations of 14D as well as several chlorinated aliphatics such as TCE and 1,2-dichloroethane.
Phytoremediation has been reported as a viable treatment alternative for 14D. The primary removal process is via phytovolatilization, which involves transpiration of chemicals from the leaf surfaces to the atmosphere. Addition of 14D degrading bacteria (e.g. CB1190) and appropriate co-substrates can further enhance the removal of 14D in soil planted with poplar trees.
Early microcosm studies indicated that 14D did not degrade in the presence of impacted aquifer material under in situ conditions. However, a statistical analysis of multiple Air Force and California sites found evidence of 14D removal in 19% of the sites, similar to other chlorinated VOCs (TCE and 1,1-DCE). The decay rates were positively correlated to dissolved oxygen, and negatively correlated to high metals and chlorinated VOC concentrations. 14D decay has also been positively correlated to the presence of oxidizing enzymes. Native 14D degrading microorganisms have also been detected in field studies using a number of environmental diagnostic assays, such as microarray and stable isotope probing, suggesting that monitored natural attenuation (MNA) may be a feasible and cost-efficient alternative to mitigate 14D at some sites.
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