Evidence of 1,4-dioxane attenuation at groundwater sites contaminated with chlorinated solvents and 1,4-dioxane

Occurrence of Rhodococcus sp. RR1 prmA and Rhodococcus jostii RHA1 prmA across microbial communities and their enumeration during 1,4-dioxane biodegradation

1,4-Dioxane, a likely human carcinogen, is a co-contaminant at many chlorinated solvent contaminated sites. Conventional treatment technologies, such as carbon sorption or air stripping, are largely ineffective, and so many researchers have explored bioremediation for site clean-up. An important step towards this involves examining the occurrence of the functional genes associated with 1,4-dioxane biodegradation. The current research explored potential biomarkers for 1,4-dioxane in three mixed microbial communities (wetland sediment, agricultural soil, impacted site sediment) using monooxygenase targeted amplicon sequencing, followed by quantitative PCR (qPCR). A BLAST analysis of the sequencing data detected only two of the genes previously associated with 1,4-dioxane metabolism or co-metabolism, namely propane monooxygenase (prmA) from Rhodococcus jostii RHA1 and Rhodococcus sp. RR1. To investigate this further, qPCR primers and probes were designed, and the assays were used to enumerate prmA gene copies in the three communities. Gene copies of Rhodococcus RR1 prmA were detected in all three, while gene copies of Rhodococcus jostii RHA1 prmA were detected in two of the three sample types (except impacted site sediment). Further, there was a statistically significant increase in RR1 prmA gene copies in the microcosms inoculated with impacted site sediment following 1,4-dioxane biodegradation compared to the control microcosms (no 1,4-dioxane) or to the initial copy numbers before incubation. Overall, the results indicate the importance of Rhodococcus associated prmA, compared to other 1,4-dioxane degrading associated biomarkers, in three different microbial communities. Also, the newly designed qPCR assays provide a platform for others to investigate 1,4-dioxane biodegradation potential in mixed communities and should be of particular interest to those considering bioremediation as a potential 1,4-dioxane remediation approach.

Cometabolism of Chlorinated Volatile Organic Compounds and 1,4-Dioxane in Groundwater

This article provides an overview of the bioremediation of groundwater plumes containing admixtures of chlorinated volatile organic compounds (CVOCs) and 1,4-dioxane. The remediation of these plumes has historically focused on the reductive dechlorination of the CVOCs. Many of the remaining plumes are relatively large, and contaminant concentrations are diluted below the concentrations that can sustain reductive dechlorination. Cometabolic processes can decrease contaminant concentrations below the thresholds needed to support direct metabolism but typically require the addition of a substrate, such as high-purity propane. Relatively intensive site characterization and monitoring is necessary to implement bioremediation.

Characterization of 1,4-dioxane degrading microbial community enriched from uncontaminated soil

1,4-Dioxane is a contaminant of emerging concern that has been commonly detected in groundwater. In this study, a stable and robust 1,4-dioxane degrading enrichment culture was obtained from uncontaminated soil. The enrichment was capable to metabolically degrade 1,4-dioxane at both high (100 mg L^-1) and environmentally relevant concentrations (300 μg L^-1), with a maximum specific 1,4-dioxane degradation rate (q_max) of 0.044 ± 0.001 mg dioxane h^-1 mg protein^-1, and 1,4-dioxane half-velocity constant (K_s) of 25 ± 1.6 mg L^-1. The microbial community structure analysis suggested Pseudonocardia species, which utilize the dioxane monooxygenase for metabolic 1,4-dioxane biodegradation, were the main functional species for 1,4-dioxane degradation. The enrichment culture can adapt to both acidic (pH 5.5) and alkaline (pH 8) conditions and can recover degradation from low temperature (10°C) and anoxic (DO < 0.5 mg L^-1) conditions. 1,4-Dioxane degradation of the enrichment culture was reversibly inhibited by TCE with concentrations higher than 5 mg L^-1 and was completely inhibited by the presence of 1,1-DCE as low as 1 mg L^-1. Collectively, these results demonstrated indigenous stable and robust 1,4-dioxane degrading enrichment culture can be obtained from uncontaminated sources and can be a potential candidate for 1,4-dioxane bioaugmentation at environmentally relevant conditions. KEY POINTS: •1,4-Dioxane degrading enrichment was obtained from uncontaminated soil. • The enrichment culture could degrade 1,4-dioxane to below 10 μg L^-1. •Low K_s and low cell yield of the enrichment benefit its application in bioremediation.

Mechanistic Considerations in 1,4-Dioxane Cancer Risk Assessment

The risk assessment of many carcinogens involves extrapolation across large exposure differences between the dose levels used in animal studies and the much lower human exposures. This is true for 1,4-dioxane which has a consistent liver carcinogenic effect in both genders of rats and mice. These data have been applied to risk assessment assuming a linear low dose extrapolation in some cases but non-linear or threshold models have been used in others. This choice hinges on our understanding of the 1,4-dioxane cancer mechanism. While 1,4-dioxane is not genotoxic in standard test batteries and has non-linear toxicokinetics, the mechanism for its carcinogenic effect remains unknown and is an active area of research. This review summarizes the possible modes of action for this chemical, data gaps and application to risk assessment. We find that the cytotoxicity/hyperplasia and metabolic saturation hypotheses do not explain the carcinogenic response and do not take into account 1,4-dioxane's induction of its own metabolism, leading to less likelihood for saturation during chronic exposure. There is evidence for other mechanisms, especially oxidative stress associated with the induction of CYP2E1 and in vivo genotoxicity that is not seen in vitro. The dose response for these effects needs further exploration compared to the time course and dose response for 1,4-dioxane-induced carcinogenesis. An additional consideration is the manner in which these 1,4-dioxane effects may augment naturally occurring and disease-related processes that contribute to the increasing rate of human liver cancer. These factors add to the rationale for using a non-threshold linear approach for extrapolating to low dose for this carcinogen, which is consistent with the default for carcinogens which do not have a clearly defined mode of action.

Remedial strategies for abating 1,4-dioxane pollution-special emphasis on diverse biotechnological interventions

1,4-dioxane is a heterocyclic ether used as a polar industrial solvent and are released as waste discharges. 1,4-dioxane deteriorates health and quality, thereby attracts concern by the environment technologists. The need of attaining sustainable development goals have resulted in search of an eco-friendly and technically viable treatment strategy. This extensive review is aimed to emphasis on the (a) characteristics of 1,4-dioxane and their occurrence in the environment as well as their toxicity, (b) remedial strategies, such as physico-chemical treatment and advanced oxidation techniques. Special reference to bioremediation that involves diverse microbial strains and their mechanism are highlighted in this review. The role of macronutrients, stimulants and other abiotic cofactors in the biodegradation of 1,4-dioxane is discussed lucidly. We have critically discussed the inducible enzymes, enzyme-based remediation, distinct instrumental method of analyses to know the fate of intermediates produced from 1,4-dioxane biotransformation. This comprehensive survey also tries to put forth the different toxicity assessment tools used in evaluating the extent of detoxification of 1,4-dioxane achieved through biotransforming mechanism. Conclusively, the challenges, opportunities, techno-economic feasibility and future prospects of implementing 1,4-dioxane through biotechnological interventions are also discussed.

Oxidative stress, glutathione, and CYP2E1 in 1,4-dioxane liver cytotoxicity and genotoxicity: insights from animal models

1,4-Dioxane (DX) is an emerging drinking water contaminant worldwide, which poses a threat to public health due to its demonstrated liver carcinogenicity and potential for human exposure. The lack of drinking water standards for DX is attributed to undetermined mechanisms of DX carcinogenicity. This mini-review provides a brief discussion of a series of mechanistic studies, wherein unique mouse models were exposed to DX in drinking water to elucidate redox changes associated with DX cytotoxicity and genotoxicity. The overall conclusions from these studies support a direct genotoxic effect by high dose DX and imply that oxidative stress involving CYP2E1 activation may play a causal role in DX liver genotoxicity and potentially carcinogenicity. The mechanistic data derived from these studies can serve as important references to refine the assessment of carcinogenic pathways that may be triggered at environmentally relevant low doses of DX in future animal and human studies.

Decontamination of water co-polluted by copper, toluene and tetrahydrofuran using lauric acid

Co-contamination by organic solvents (e.g., toluene and tetrahydrofuran) and metal ions (e.g., Cu²⁺) is common in industrial wastewater and in industrial sites. This manuscript describes the separation of THF from water in the absence of copper ions, as well as the treatment of water co-polluted with either THF and copper, or toluene and copper. Tetrahydrofuran (THF) and water are freely miscible in the absence of lauric acid. Lauric acid separates the two solvents, as demonstrated by proton nuclear magnetic resonance (¹H NMR) and Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy (ATR-FTIR). The purity of the water phase separated from 3:7 (v/v) THF:water mixtures using 1 M lauric acid is ≈87%v/v. Synchrotron small angle X-Ray scattering (SAXS) indicates that lauric acid forms reverse micelles in THF, which swell in the presence of water (to host water in their interior) and ultimately lead to two free phases: 1) THF-rich and 2) water-rich. Deprotonated lauric acid (laurate ions) also induces the migration of Cu²⁺ ions in either THF (following separation from water) or in toluene (immiscible in water), enabling their removal from water. Laurate ions and copper ions likely interact through physical interactions (e.g., electrostatic interactions) rather than chemical bonds, as shown by ATR-FTIR. Inductively coupled plasma—optical emission spectrometry (ICP-OES) demonstrates up to 60% removal of Cu²⁺ ions from water co-polluted by CuSO₄ or CuCl₂ and toluene. While lauric acid emulsifies water and toluene in the absence of copper ions, copper salts destabilize emulsions. This is beneficial, to avoid that copper ions are re-entrained in the water phase alongside with toluene, following their migration in the toluene phase. The effect of copper ions on emulsion stability is explained based on the decreased interfacial activity and compressional rigidity of interfacial films, probed using a Langmuir trough. In wastewater treatment, lauric acid (a powder) can be mixed directly in the polluted water. In the context of groundwater remediation, lauric acid can be solubilized in canola oil to enable its injection to treat aquifers co-polluted by organic solvents and Cu²⁺. In this application, injectable filters obtained by injecting cationic hydroxyethylcellulose (HEC +) would impede the flow of toluene and copper ions partitioned in it, protecting downstream receptors. Co-contaminants can be subsequently extracted upstream of the filters (using pumping wells), to enable their simultaneous removal from aquifers.

Activation of ozone by peroxymonosulfate for selective degradation of 1,4-dioxane: Limited water matrices effects

The presence of 1,4-dioxane in various water streams poses a threat to the health of human beings. In this study, the oxidative combination of ozone with peroxymonosulfate (PMS) was for the first time used to remove 1,4-dioxane from water. Near complete abatement of 1,4-dioxane was achieved by ozone-PMS after reaction of only 15 min and the degradation kinetics was found to be positively correlated with doses of PMS and ozone. Ozone-PMS oxidation had the optimum performance at slight base pH values. Both sulfate radicals and hydroxyl radicals were generated in ozone-PMS oxidation and these radicals resulted in the degradation of 1,4-dioxane. The effects of common water constituents and real water matrices were investigated. It was found that bicarbonate ions with a concentration up to 10 mM had a slight promoting effect, while either chloride ions or natural organic matter inhibited only slightly the degradation. Meanwhile, no obvious difference in the degradation of 1,4-dioxane was found among the real water matrices and deionized water, which demonstrates that ozone-PMS oxidation has high tolerance and stability. The results from this study demonstrate that ozone-PMS may be a promising technology for the removal of 1,4-dioxane from various water matrices.

Evaluation of 1,4-dioxane attenuation processes at the Gelman Site, Michigan, USA

1,4-Dioxane released at the Gelman Site in Washtenaw County, Michigan, produced a series of contaminant plumes migrating up to 3 km through a heterogenous glacial aquifer system. An analysis of 1,4-dioxane concentrations in the Eastern Area of the Gelman Site between 2011 and 2017 documented a mass balance deficit of 2200 kg in excess of 2100 kg of 1,4-dioxane removed via remediation. Five mechanisms were evaluated to account for the mass deficiency: sorption, matrix diffusion, biodegradation, surface discharge, and bypass of the existing monitoring well network. The mass of 1,4-dioxane sorbed to aquifer and aquitard materials and the mass of 1,4-dioxane diffused into low permeability zones were estimated. However, decreasing aqueous concentrations across most of the contaminated area between 2011 and 2017 are expected to induce desorption and back diffusion during this period. Surface water discharge to a storm drain in the downgradient portion of the site was analyzed using concentration measurements and stream gage data. Results suggest that 1,4-dioxane mass entering the drain during the period between 2011 and 2017 was insufficient to account for the mass deficiency. Although available geochemical measurements indicate predominantly anaerobic aquifer conditions at the Gelman Site, biodegradation of 1,4-dioxane was estimated using first order decay rate constants from other sites where conditions may be more favorable. Results suggest that biodegradation could explain some but not all of the missing mass. Bypass of the downgradient monitoring well network is the most parsimonious explanation for the 1,4-dioxane mass deficit. This conclusion is supported by documented flow path complexity through the aquifer system and the sparse density of monitoring wells in the downgradient Eastern Area. These findings underscore the importance of characterizing aquifer heterogeneity when modeling and remediating persistent groundwater contaminants such as 1,4-dioxane.

Response surface algorithm for improved biotransformation of 1,4-dioxane using Staphylococcus capitis strain AG

The present investigation reports the biotransformation of an endrocrine disrupting agent; 1,4-dioxane through bacterial metabolism. Initially, potential bacterial isolates capable of surviving with minimum 1,4-dioxane were screened from industrial wastewater. Thereafter, screening was done to isolate a bacteria which can biotransform higher concentration (1000 mg/L) of 1,4-dioxane. Morphological and biochemical features were examined prior establishing their phylogenetic relationships and the bacterium was identified as Staphylococcus capitis strain AG. Biotransformation experiments were tailored using response surface tool and predictions were made to elucidate the opimal conditions. Critical factors influencing bio-transformation efficiency such as tetrahydrofuran, availability of 1,4-dioxane and inoculum size were varied at three different levels as per the central composite design for ameliorating 1,4-dioxane removal. Functional attenuation of 1,4-dioxane by S. capitis strain AG were understood using spectroscopic techniques were significant changes in the peak positions and chemical shifts were visualized. Mass spectral profile revealed that 1.5 (% v/v) S. capitis strain AG could completely (∼99%) remove 1000 mg/L 1,4-dioxane, when incubated with 2 μg/L tetrahydrofuran for 96 h. The toxicity of 1,4-dioxane and biotransformed products by S. capitis strain AG were tested on Artemia salina. The results of toxicity tests revealed that the metabolic products were less toxic as they exerted minimal mortality rate after 48 h exposure. Thus, this research would be the first to report the response prediction and precise tailoring of 1,4-dioxane biotransformation using S. captis strain AG.

Oxidative degradation of commingled trichloroethylene and 1,4-dioxane by hydroxyl radicals produced upon oxygenation of a reduced clay mineral

Improper disposal of chlorinated solvents such as trichloroethylene (TCE) and its stabilizer 1,4-dioxane has resulted in extensive contamination in soils and groundwater. Oxidative degradation of these contaminants by strong oxidants has been proposed recently as a remediation strategy, but specific mechanisms and degradation efficiencies are still poorly understood, especially in commingled systems. In this study, a reduced iron-bearing clay (RIC), nontronite (rNAu-2), was oxygenated to produce hydroxyl radicals (•OH) for degradation of TCE and 1,4-dioxane under circumneutral and dark conditions. Results showed that TCE and 1,4-dioxane could be effectively degraded during oxygenation of rNAu-2 in both single and commingled systems. Compared with the single compound system, the degradation rates and efficiencies of TCE and 1,4-dioxane decreased in the commingled system. The negative effect was more significant for TCE than 1,4-dioxane. The commingled TCE and 1,4-dioxane impacted the degradation pattern of each other, due to their difference in •OH scavenging efficiency, surface affinity to rNAu-2 and solubility. Moreover, solution pH, buffer type, rNAu-2 dosage, and dissolved organic matter all affected •OH production and contaminant degradation efficiency. Our findings provide new insights for investigating the natural attenuation of commingled chlorinated solvents and 1,4-dioxane by RIC in redox-fluctuating environments and offer guidance for developing possible in-situ remediation strategies.

Establishing the prevalence and relative rates of 1,4-dioxane biodegradation in groundwater to improve remedy evaluations

Options for remediating 1,4-dioxane at groundwater sites are limited due to the physical-chemical properties of this compound. The relevance of natural attenuation processes for 1,4-dioxane was investigated through data from field, lab, and modeling efforts. The objectives were to use multiple lines of evidence for 1,4-dioxane biodegradation to understand the prevalence of this activity and evaluate convergence between lines of evidence. A ¹⁴C-1,4-dioxane assay confirmed 1,4-dioxane biodegradation at 9 of 10 sites (median rate constant of 0.0105 yr^-1 across wells). Site-wide rate constants were established using a calibrated fate and transport model at 8 sites (median = 0.075 yr^-1). The ¹⁴C assay constants are likely more conservative, and variability in rates suggested that biodegradation at sites may be localized. Stable isotope fractionation was observed at 7 of 10 sites and served as another direct line of evidence of in situ biodegradation of 1,4-dioxane. This includes sites where indirect lines of evidence, including geochemical conditions or genetic biomarkers for degradation, would not necessarily have been supportive. This highlights the importance of collecting multiple lines of evidence to document 1,4-dioxane natural attenuation, and the widespread prevalence of biodegradation suggests that this process should be part of long-term management decisions.

Evaluation of natural attenuation of 1,4-dioxane in groundwater using a 14C assay

Monitored Natural Attenuation (MNA) is a preferred remedy for sites contaminated with 1,4-dioxane due to its low cost and limited environmental impacts compared to active remediation. Having a robust estimate of the rate at which biodegradation occurs is an essential component of assessing MNA. In this study, an assay was developed using ¹⁴C-labeled 1,4-dioxane to measure rate constants for biodegradation based on accumulation of ¹⁴C products. Purification of the ¹⁴C-1,4-dioxane stock solution lowered the level of ¹⁴C impurities to below 1% of the total ¹⁴C activity. This enabled determination of rate constants in groundwater as low as 0.0021 yr^-1, equating to a half-life greater than 300 years. Of the 54 groundwater samples collected from 10 sites in the US, statistically significant rate constants were determined with the ¹⁴C assay for 24. The median rate constant was 0.0138 yr^-1 (half-life = 50 yr); the maximum rate constant was 0.367 yr^-1 (half-life = 1.9 yr). The results confirmed that biodegradation of 1,4-dioxane is occurring at 9 of the 10 sites sampled, albeit with considerable variability in the level of activity. The specificity of the assay was confirmed using acetylene and the absence of oxygen to inhibit monooxygenases.

Management of large dilute plumes of chloroethenes and 1,4-dioxane via monitored natural attenuation (MNA) and MNA augmentation

Management of large, dilute groundwater plumes of comingled chlorinated volatile organic compounds (CVOCs) and 1,4-dioxane (dioxane) is problematic due to chemical, hydrogeologic and economic concerns. The US Environmental Protection Agency (US EPA) has conducted research on the management of CVOC plumes for many years, and more recently dioxane. US EPA research on monitored natural attenuation (MNA) of CVOC plumes was reviewed by a science advisory board in 2001. Specific additional research was recommended and has been addressed in a series of US EPA reports produced over almost two decades. These reports are summarized in this document along with supporting information including evidence of biological degradation of dioxane. Based on the summarized reports, US EPA work documented elsewhere, and the work of others, under appropriate conditions MNA or augmented MNA remain viable management options for these plumes. Unlike MNA of plumes containing only CVOCs, however, MNA of large dilute comingled plumes should be expected to occur by cometabolic oxidation rather than direct metabolic processes.

Effect of biostimulation and bioaugmentation on biodegradation of high concentrations of 1,4-dioxane

1,4-Dioxane is a pervasive and persistent contaminant in numerous aquifers. Although the median concentration in most contaminant plumes is in the microgram per liter range, a subset of sites have contamination in the milligram per liter range. Most prior studies that have examined 1,4-dioxane concentrations in the hundreds of milligrams per liter range have been performed with industrial wastewater. The main objective of this study was to evaluate aerobic biodegradation of 1,4-dioxane in microcosms prepared with soil and groundwater from a site where concentrations range from ~ 1500 mg·L^-1 in the source zone, to 450 mg·L^-1 at a midpoint of the groundwater plume, and to 6 mg·L^-1 at a down-gradient location. Treatments included biostimulation with propane, addition of propane and a propanotrophic enrichment culture (ENV487), and unamended. The highest rates of biodegradation for each location in the plume occurred in the bioaugmented treatments, although indigenous propanotrophs also biodegraded 1,4-dioxane to below 25 µg·L^-1. Nutrient additions were required to sustain biodegradation of propane and cometabolism of 1,4-dioxane. Among the unamended treatments, biodegradation of 1,4-dioxane was detected in the mid-gradient microcosms. An isolate was obtained that grows on 1,4-dioxane as a sole source of carbon and energy and identified through whole-genome sequencing as Pseudonocardia dioxivorans BERK-1. In a prior study, the same strain was isolated from an aquifer in the southeastern United States. Monod kinetic parameters for BERK-1 are similar to those for strain CB1190.

A Synergistic Platform for Continuous Co-removal of 1,1,1-Trichloroethane, Trichloroethene, and 1,4-Dioxane via Catalytic Dechlorination Followed by Biodegradation

Groundwater co-contaminated with 1,4-dioxane, 1,1,1-trichloroethane (TCA), and trichloroethene (TCE) is among the most urgent environmental concerns of the U.S. Department of Defense (DoD), U.S. Environmental Protection Agency (EPA), and industries related to chlorinated solvents. Inspired by the pressing need to remove all three contaminants at many sites, we tested a synergistic platform: catalytic reduction of 1,1,1-TCA and TCE to ethane in a H₂-based membrane palladium-film reactor (H₂-MPfR), followed by aerobic biodegradation of ethane and 1,4-dioxane in an O₂-based membrane biofilm reactor (O₂-MBfR). During 130 days of continuous operation, 1,1,1-TCA and TCE were 95-98% reductively dechlorinated to ethane in the H₂-MPfR, and ethane served as the endogenous primary electron donor for promoting 98.5% aerobic biodegradation of 1,4-dioxane in the O₂-MBfR. In addition, the small concentrations of the chlorinated intermediate from the H₂-MPfR, dichloroethane (DCA) and monochloroethane (MCA), were fully biodegraded through aerobic biodegradation in the O₂-MBfR. The biofilms in the O₂-MBfR were enriched in phylotypes closely related to the genera Pseudonocardia known to biodegrade 1,4-dioxane.

Determination of 1,4-dioxane in water samples using freeze-assisted liquid-liquid extraction and gas chromatography-mass spectrometry with select reaction monitoring

In this study, we developed an analytical method for the determination of 1,4-dioxane in aqueous solutions using freeze-assisted liquid-liquid extraction, also known as frozen microextraction, and gas chromatography with triple quadrupole mass spectrometry with select reaction monitoring. The method is capable of quantifying 1,4-dioxane across a broad range of concentrations (1-10 000 μg/L) relevant to contaminated sites, with an instrument detection limit and method detection limit experimentally verified as 2.1 and 2.2 μg/L, respectively. In contrast to methods with similar detection limits that require 50 to 500 mL volume of sample, our method uses only 200 μL of sample. The method presented here facilitates field and laboratory applications where small sample volumes and high precision are required and could be extended to other strongly water-soluble GC-amenable analytes.

Sulfate radical-induced destruction of emerging contaminants using traces of cobalt ions as catalysts

Cobalt is part of vitamin B12, which is essential to maintain human health, and trace levels of cobalt ions are ubiquitous in water and soil environments. In this study, the destruction of 1,4-dioxane (1,4-D) by peroxymonosulfate (PMS) under the catalysis of trace levels of Co²⁺ was investigated under buffered conditions. The results showed that near 100% removal of 1,4-D was achieved after reaction for 6 and 10 min with 50 and 25 μg/L Co²⁺, respectively, in the presence of 5 mM phosphate ions. Mechanism studies revealed that radicals mediated the destruction of 1,4-D and sulfate radicals were the primary reactive species. The traces of Co²⁺ had the greatest reactivity for the catalysis of PMS in neutral environments (pH 7.0). However, pH 5.5 was observed to be the best condition for 1,4-D destruction, which was probably caused by the involvement of phosphate radicals. Common water components including chloride ions and bicarbonate ions were observed to have promoting and inhibiting effects, respectively, on the removal of 1,4-D. To further demonstrate the potential of Co²⁺-PMS in practical applications, we explored the simultaneous degradation of 20 antibiotics using trace levels of Co²⁺. The results showed that all the investigated antibiotics, except for lomefloxacin, could be efficiently degraded by Co²⁺-PMS with removal rates of greater than 97%. The findings from this study demonstrate the promise of using trace levels of cobalt for environmental remediation applications, even when high concentrations of phosphate ions are co-present.

Carbon quantum dots-decorated TiO2/g-C3N4 film electrode as a photoanode with improved photoelectrocatalytic performance for 1,4-dioxane degradation

In this study, carbon quantum dots (CQDs) were used to decorate a TiO₂/g-C₃N₄ (TCN) film electrode. The morphological, optical, and electrochemical properties of the TiO₂/g-C₃N₄/CQDs nanorod arrays (TCNC NRAs) film were investigated using transmission electron microscopy (TEM), scanning electron microscopy (SEM), UV-vis diffuse reflectance spectroscopy (DRS), photoluminescence (PL), and electrochemical impedance spectroscopy (EIS). The improved optical properties, photoelectrochemical properties and photoelectrocatalytic (PEC) performance of photoanode can be observed by doping CQDs onto the TCN NRAs film. Compared with TiO₂ NRAs and TCN NRAs, the narrower band gap of 2.47 eV and longer lifetime of photoinduced electron-hole pairs were observed in the TCNC NRAs. Under visible light irradiation and a bias voltage of 1.2 V, the photocurrent density and 1,4-dioxane (1,4-D) removal rate of PEC process with TCNC NRAs electrode reached 0.16 mA/cm² and 77.9%, respectively, which was 2.5 times and 1.5 times of that with TCN NRAs electrode. TCNC NRAs electrode could keep >75% of the 1,4-D removal rate during five cycles tests. High PEC performance with TCNC NRAs electrode could be attributed to the enhanced charge separation and the change of electron transfer mechanism from typical heterojunction to Z-scheme, which may increase the active species production and change the dominant reactive species from O₂·^- to ·OH. Our experimental results should be useful for studying the degradation of 1,4-D and developing efficient PEC materials.

Enhanced long-term attenuation of 1,4-dioxane in bioaugmented flow-through aquifer columns

Long term natural attenuation of 1,4-dioxane (dioxane) and its enhanced biodegradation after bioaugmentation with Pseudonocardia dioxanivorans CB1190 were assessed using flow-through aquifer columns. Natural attenuation of dioxane was not observed even after 2 years of acclimation. However, dioxane removal was observed in the bioaugmented columns (34% when the influent was 200 µg/L and 92% for 5 mg/L). The thmA gene that encodes the tetrahydrofuran monooxygenase that initiates dioxane degradation by CB1190 was only detected at the inoculation port and persisted for months after inoculation, implying the resiliency of bioaugmentation and its potential to offer long-term enhanced biodegradation capabilities. However, due to extensive clumping and limited mobility of CB1190, the augmented catabolic potential may be restricted to the immediate vicinity of the inoculation port. Accordingly, bioaugmentation with CB1190 seems more appropriate for the establishment of biobarriers. Bioaugmentation efficiency was associated with the availability of oxygen. Aeration of the column influent to increase dissolved oxygen significantly improved dioxane removal (p < 0.05), suggesting that (for sites with oxygen-limiting conditions) bioaugmentation can benefit from engineered approaches for delivering additional oxygen.

Impact of groundwater quality and associated byproduct formation during UV/hydrogen peroxide treatment of 1,4-dioxane

In this study, a semi-batch, bench-scale UV/hydrogen peroxide (UV/H₂O₂) advanced oxidation process system was used to investigate how typical groundwater quality parameters (pH, alkalinity, natural organic matter (NOM), nitrate, and iron) influence the treatment of 1,4-dioxane. Deionized (DI) water spiked with 1,4-dioxane (100 μg L^-1), treated using H₂O₂ (10 mg L^-1) in a commercially available UV system (40 W low-pressure lamp) showed an UV fluence-based first-order rate constant (k') and electrical energy-per-order (EEO) of 4.32✕10^-3 cm²-mJ^-1 and 0.15 kWh-m^-3-order^-1, respectively. The most abundant byproduct generated in spiked-DI water was oxalic acid (up to 55 μg L^-1), followed by formic and acetic acids. The k' showed no significant difference at pH ranging from 5 to 7 and at low alkalinity concentrations (<20 mg-CaCO₃ L^-1), typical of sandy aquifers. The k' declined by up to 85% with increasing NOM concentration. Elevated production (up to ∼400% increase) of aldehydes and organic acids was observed in NOM-spiked water, implying that NOM is a significant byproduct precursor during UV/H₂O₂ treatment. High NO₃^- concentration (10 mg-N L^-1) in source water reduced the k' by 25%, while no significant impact was observed at lower concentrations (<2 mg-N L^-1). Addition of Fe(II) at 0.5 mg-L^-1 resulted in an instantaneous Fenton-reaction-assisted removal of ∼10% 1,4-dioxane in the presence of H₂O₂, but did not enhance the performance of UV/H₂O₂ treatment over time. In contrast, both Fe(II) and Fe(III) addition lowered the k' by 15-27%. The decline of k' observed in these experiments was attributed to reduced UVT (Fe), ^.OH radical scavenging (pH), or both (NO₃^-, NOM). Treatment of groundwater samples collected from three 1,4-dioxane-contaminated wells located in Long Island, NY, showed k' values of 13-40% lower than what was observed for DI water due to radical scavenging from a combination of high NO₃^- and NOM in the samples. A multiple linear-regression model, developed using water quality data as model input, showed good agreement with field observations (paired t-test: p > 0.05) in predicting k' for the removal of 1,4-dioxane from groundwater. This study provides the first systematic evaluation of the impacts of groundwater quality on UV/H₂O₂ process to remove environmentally relevant levels of 1,4-dioxane and reports standardized performance-related parameters to aid in the design and evaluation of full-scale systems.

Monitoring, assessment, and prediction of microbial shifts in coupled catalysis and biodegradation of 1,4-dioxane and co-contaminants

Microbial community dynamics were characterized following combined catalysis and biodegradation treatment trains for mixtures of 1,4-dioxane and chlorinated volatile organic compounds (CVOCs) in laboratory microcosms. Although a few specific bacterial taxa are capable of removing 1,4-dioxane and individual CVOCs, many microorganisms are inhibited when these contaminants are present in mixtures. Chemical catalysis by tungstated zirconia (WO_x/ZrO₂) and hydrogen peroxide (H₂O₂) as a non-selective treatment was designed to achieve nearly 20% 1,4-dioxane and over 60% trichloroethene and 50% dichloroethene removals. Post-catalysis, bioaugmentation with 1,4-dioxane metabolizing bacterial strain,Pseudonocardia dioxanivorans CB1190, removed the remaining 1,4-dioxane. The evolution of the microbial community under different conditions was time-dependent but relatively independent of the concentrations of contaminants. The compositions of microbiomes tended to be similar regardless of complex contaminant mixtures during the biodegradation phase, indicating a r-K strategy transition attributed to the shock experienced during catalysis and the subsequent incubation. The originally dominant genera Pseudomonas and Ralstonia were sensitive to catalytic oxidation, and were overwhelmed by Sphingomonas, Rhodococcus, and other catalyst-tolerant microbes, but microbes capable of biodegradation of organics thrived during the incubation. Methane metabolism, chloroalkane-, and chloroalkene degradation pathways appeared to be responsible for CVOC degradation, based on the identifications of haloacetate dehalogenases, 2-haloacid dehalogenases, and cytochrome P450 family. Network analysis highlighted the potential interspecies competition or commensalism, and dynamics of microbiomes during the biodegradation phase that were in line with shifting predominant genera, confirming the deterministic processes guiding the microbial assembly. Collectively, this study demonstrated that catalysis followed by bioaugmentation is an effective treatment for 1,4-dioxane in the presence of high CVOC concentrations, and it enhanced our understanding of microbial ecological impacts resulting from abiotic-biological treatment trains. These results will be valuable for predicting treatment synergies that lead to cost savings and improve remedial outcomes in short-term active remediation as well as long-term changes to the environmental microbial communities.

Solvent extraction and gas chromatography-mass spectrometric determination of probable carcinogen 1,4-dioxane in cosmetic products

In the present work, a method based on solvent extraction and gas chromatography-mass spectrometry (GC-MS) has been validated for the determination of 1,4-dioxane in cosmetics. Various solvents including ethyl acetate, hexane, methanol, dichloromethane and acetone have been used for the extraction of 1,4-dioxane, among them the ethyl acetate was found to be the most efficient extracting solvent. This method has offered excellent quality parameters for instance linearity (R² > 0.9991), limit of detection (LOD, 0.00065-0.00091 µg/mL), limit of quantification (LOQ, 0.00217-0.00304 µg/mL) and, precision intra-day (1.65-2.60%, n = 5) and inter-day (0.16-0.32%, n = 5) in terms of relative standard deviation (RSD%). A total of thirty-nine cosmetic samples of different brands and origin have been studied. Among them, the 1,4-dioxane was found in twenty-three samples (FB₁-FB₇, MC₁-MC₄, MC₆-MC₈, HS₃, HS₅, BL₁-BL₃, BL₅ and PLD₁-PLD₃) at the levels between 0.15 µg/mL and 9.92 µg/mL, whereas in sixteen samples (MC₅, HS₁, HS₂, SG₁-SG₅, BL₄ and HP₁- HP₆) was found to be not detected. The recovery values were achieved between 93% and 99% in both low and high level of spiked samples. In comparison to the traditional analytical techniques, the proposed method was found to be very sensitive and cost-effective for the routine analysis of 1,4-dioxane at low concentration in cosmetics.

Contaminant Degradation by •OH during Sediment Oxygenation: Dependence on Fe(II) Species

It has been documented that contaminants could be degraded by hydroxyl radicals (•OH) produced upon oxygenation of Fe(II)-bearing sediments. However, the dependence of contaminant degradation on sediment characteristics, particularly Fe(II) species, remains elusive. Here we assessed the impact of the abundance of Fe(II) species in sediments on contaminant degradation by •OH during oxygenation. Three natural sediments with different Fe(II) contents and species were oxygenated. During 10 h oxygenation of 200 g/L sediment suspension, 2 mg/L phenol was negligibly degraded for sandbeach sediment (Fe(II): 9.11 mg/g), but was degraded by 41% and 52% for lakeshore (Fe(II): 9.81 mg/g) and farmland (Fe(II): 19.05 mg/g) sediments, respectively. •OH produced from Fe(II) oxygenation was the key reactive oxidant. Sequential extractions, X-ray diffraction, Mössbauer, and X-ray absorption spectroscopy suggest that surface-adsorbed Fe(II) and mineral structural Fe(II) contributed predominantly to •OH production and phenol degradation. Control experiments with specific Fe(II) species and coordination structure analysis collectively suggest the likely rule that Fe(II) oxidation rate and its competition for •OH increase with the increase in electron-donating ability of the atoms (i.e., O) complexed to Fe(II), while the •OH yield decreases accordingly. The Fe(II) species with a moderate oxidation rate and •OH yield is most favorable for contaminant degradation.

Distinct Catalytic Behaviors between Two 1,4-Dioxane-Degrading Monooxygenases: Kinetics, Inhibition, and Substrate Range

Monitored natural attenuation (MNA) and engineered bioremediation have been recognized as effective and cost-efficient in situ treatments to mitigate 1,4-dioxane (dioxane) contamination. Dioxane metabolism can be initiated by two catabolic enzymes, propane monooxygenase (PRM) and tetrahydrofuran monooxygenase (THM), belonging to the group-6 and 5 of soluble di-iron monooxygenase family, respectively. In this study, we comprehensively compared catalytic behaviors of PRM and THM when individually expressed in the heterologous host, Mycobacterium smegmatis mc²-155. Kinetic results revealed a half-saturation coefficient (K_m) of 53.0 ± 13.1 mg/L for PRM, nearly 4 times lower than that of THM (235.8 ± 61.6 mg/L), suggesting that PRM has a higher affinity to dioxane. Exposure with three common co-contaminants (1,1-dichloroethene, trichloroethene, and 1,1,1-trichloroethane) demonstrated that PRM was also more resistant to their inhibition than THM. Thus, dioxane degraders expressing PRM may be more physiologically and ecologically advantageous than those with THM at impacted sites, where dioxane concentration is relatively low (e.g., 250 to 1000 μg/L) with co-occurrence of chlorinated solvents (e.g., 0.5 to 8 mg/L), underscoring the need of surveying both PRM and THM-encoding genes for MNA potential assessment. PRM is also highly versatile, which breaks down cyclic molecules (dioxane, tetrahydrofuran, and cyclohexane), as well as chlorinated and aromatic pollutants, including vinyl chloride, 1,2-dichloroethane, benzene, and toluene. This is the first report regarding the ability of PRM to degrade a variety of short-chain alkanes and ethene in addition to dioxane, unraveling its pivotal role in aerobic biostimulation that utilizes propane, isobutane, or other gaseous alkanes/alkenes (e.g., ethane, butane, and ethene) to select and fuel indigenous microorganisms to tackle the commingled contamination of dioxane and chlorinated compounds.

Mechanisms of 1,4-Dioxane Biodegradation and Adsorption by Bio-Zeolite in the Presence of Chlorinated Solvents: Experimental and Molecular Dynamics Simulation Studies

The use of bioaugmented zeolite (bio-zeolite) can be an effective technology for irreversibly removing recalcitrant organic pollutants in aqueous mixtures. Removal of 1,4-dioxane by a bio-zeolite (Pseudonocardia dioxanivorans CB1190-bioaugmented ZSM-5) in the presence of several chlorinated volatile organic compounds (CVOCs) was superior to removal by adsorption using abiotic zeolite. Mixtures containing 1,1-dichloroethene (1,1-DCE) were an exception, which completely inhibited the bio-zeolite system. Specific adsorption characteristics were studied using adsorption isotherms in single-solute and bisolute systems accompanied by Polanyi theory-based Dubinin-Astakhov (DA) modeling. Adsorption behavior was examined using characteristic energy (E_a/H) from modified DA models and molecular dynamics simulations. While the tight-fit of 1,4-dioxane in the hydrophobic channels of ZSM-5 appears to drive 1,4-dioxane adsorption, the greater hydrophobicity of trichloroethene and cis-1,2-dichloroethene cause them have a greater affinity over 1,4-dioxane for adsorption sites on the zeolite. 1,4-Dioxane was desorbed and displaced by CVOCs except 1,1-DCE because of its low E_a/H value, explaining why bio-zeolite only biodegraded 1,4-dioxane in 1,1-DCE-free CVOC mixtures. Understanding the adsorption mechanisms of solutes in complex mixtures is crucial for the implementation of sorption-based treatment technologies for the removal of complex contaminant mixtures from aquatic environments.

1,4-Dioxane as an emerging water contaminant: State of the science and evaluation of research needs

1,4-Dioxane has historically been used to stabilize chlorinated solvents and more recently has been found as a contaminant of numerous consumer and food products. Once discharged into the environment, its physical and chemical characteristics facilitate migration in groundwater, resulting in widespread contamination of drinking water supplies. Over one-fifth of U.S. public drinking water supplies contain detectable levels of 1,4-dioxane. Remediation efforts using common adsorption and membrane filtration techniques have been ineffective, highlighting the need for alternative removal approaches. While the data evaluating human exposure and health effects are limited, animal studies have shown chronic exposure to cause carcinogenic responses in the liver across multiple species and routes of exposure. Based on this experimental evidence, the U.S. Environmental Protection Agency has listed 1,4-dioxane as a high priority chemical and classified it as a probable human carcinogen. Despite these health concerns, there are no federal or state maximum contaminant levels for 1,4-dioxane. Effective public health policy for this emerging contaminant requires additional information about human health effects, chemical interactions, environmental fate, analytical detection, and treatment technologies. This review highlights the current state of knowledge, key uncertainties, and data needs for future research on 1,4-dioxane.

Carbon sources that enable enrichment of 1,4-dioxane-degrading bacteria in landfill leachate

1,4-Dioxane (DX) is a recalcitrant cyclic ether that has gained attention as an emerging pollutant in the aquatic environment. Enrichment of indigenous DX-degrading bacteria, which are considered to be minor populations even in DX-impacted environments, is the key for efficient biological DX removal. Therefore, this study aimed to explore carbon sources applicable for the enrichment of DX-degrading bacteria present in landfill leachate, which is a potential source of DX pollution. Microorganisms collected from landfill leachate were cultivated on six different carbon sources (DX, tetrahydrofuran (THF), 1,3,5-trioxane (TX), ethylene glycol (EG), diethylene glycol (DEG), and 1,4-butanediol (BD)) in a sequential batch mode. Consequently, enrichment cultures cultivated on THF in addition to DX improved the DX degradation ability compared to that of the original leachate sample, while those on the other test carbon sources did not. The results indicated that THF can be an alternative carbon source to enrich DX-degrading bacteria, and that TX, EG, DEG and BD are not applicable to concentrate DX-degrading bacteria in complex microbial consortia. In addition, sequencing analyses of 16S rRNA and soluble di-iron monooxygenase (SDIMO) genes revealed notable dominance of thm/dxm genes involved in group 5 SDIMO both in DX- and THF-enrichment cultures. The analysis also showed a predominance of Pseudonocardia in THF-enrichment culture, suggesting that Pseudonocardia harboring thm/dxm genes contributes to enhanced DX degradation in THF-enrichment culture.

1,4-Dioxane cosolvency impacts on trichloroethene dissolution and sorption

Solvent stabilizer 1,4-dioxane, an emerging recalcitrant groundwater contaminant, was commonly added to chlorinated solvents such as trichloroethene (TCE), and the impact of co-disposal on contaminant transport processes remains uncertain. A series of batch equilibrium experiments was conducted with variations of 1,4-dioxane and TCE composition to evaluate aqueous dissolution of the two components and their sorption to aquifer sediments. The solubility of TCE increased with increasing amounts of 1,4-dioxane, indicating that 1,4-dioxane acts as a cosolvent causing solubility enhancement of co-contaminants. The solubilization results compared favorably with predictions using the log-linear cosolvency model. Equilibrium sorption coefficients (K_d and K_f) were also measured for different 1,4-dioxane and TCE compositions, and the findings indicate that both contaminants adsorb to aquifer sediments and TCE K_d values increased with increasing organic matter content. However, the K_d for TCE decreased with increases in 1,4-dioxane concentration, which was attributed to cosolvency impacts on TCE solubility. These findings further advance our understanding of the mass-transfer processes controlling groundwater plumes containing 1,4-dioxane, and also have implications for the remediation of 1,4-dioxane contamination.

Dioxanes and dioxolanes in source waters: Occurrence, odor thresholds and behavior through upgraded conventional and advanced processes in a drinking water treatment plant

Over the last years, the human probable carcinogen 1,4-dioxane and alkyl-1,3-dioxanes and dioxolanes have been detected and identified as the cause of several pollution episodes in the Llobregat River (Catalonia, NE Spain) and its aquifer. It is an issue of major concern to study these compounds which are released to the environment by resin manufacturing plants' spills and wastewater discharges spread along rivers and reach drinking water treatment plants (DWTPs) in order to protect the environment and public health. In this study four seasonal sampling campaigns were carried out over a year to determine the removal efficiency of the dioxanes and dioxolanes at each step of a DWTP including ozonation, granular activated carbon filters, ultrafiltration and reverse osmosis step's treatments. Additionally, a weekly sampling monitoring of 1,4-dioxane and alkyl-1,3-dioxanes and dioxolanes in raw water, groundwater and finished water was performed at a DWTP over more than two years. Aqueous odor concentration thresholds (OTCs) were established by the three-alternative forced choice method (3-AFC). Following a previous published methodology, samples were analyzed and results showed that the advanced treatment (Ultrafiltration followed by reverse osmosis) line removes more efficiently 1,4-dioxane, alkyl dioxanes and dioxolanes (80 ± 6% for 1,4-dioxane, 97 ± 7% for 5,5-DMD and 100 ± 0% for 2,5,5-TMD) than the upgraded conventional treatment line (ozonation followed by granular activated carbon filters) (-12 ± 50%, 25 ± 62% and 50 ± 51% respectively), where some desorption processes were eventually observed. From the monitoring study, results suggest that the presence of 1,4-dioxane is not only due to spills, but also from other sources of contamination. Whereas dioxolanes almost completely disappeared in time, 1,4-dioxane's concentrations remained low and fluctuant. A background concentration of 1,4-dioxane in surface waters (∼1 μg/L) has been determined with a relevant concentration up to 11.6 μg/L of 1,4-dioxane in groundwater. The perception values for some of the studied compounds were extremely low (few ng/L only), which confirms the relevancy of this group of compounds as malodorous agents in waters.

Microbial Community Analysis Provides Insights into the Effects of Tetrahydrofuran on 1,4-Dioxane Biodegradation

Tetrahydrofuran (THF) is known to induce the biodegradation of 1,4-dioxane (dioxane), an emerging contaminant, but the mechanisms by which THF affects dioxane biodegradation in microbial communities are not well understood. To fill this knowledge gap, changes in the microbial community structure in microcosm experiments with synthetic medium and landfill leachate were examined over time using 16S rRNA gene amplicon sequencing and functional gene quantitative PCR assays. The overarching hypothesis being tested was that THF promoted dioxane biodegradation by increasing the abundance of dioxane-degrading bacteria in the consortium. The data revealed that in experiments with synthetic medium, the addition of THF significantly increased the abundance of Pseudonocardia, a genus with several representatives that can grow on both dioxane and THF, and of Rhodococ cus ruber, a species that can use THF as the primary growth substrate while cometabolizing dioxane. However, in similar experiments with landfill leachate, only R. ruber was significantly enriched. When the THF concentration was higher than the dioxane concentration, THF competitively inhibited dioxane degradation since dioxane degradation was negligible, while the dioxane-degrading bacteria and the corresponding THF/dioxane monooxygenase gene copies increased by a few orders of magnitude.IMPORTANCE Widespread in groundwater and carcinogenic to humans, 1,4-dioxane (dioxane) is attracting significant attention in recent years. Advanced oxidation processes can effectively remove dioxane but require high energy consumption and operation costs. Biological removal of dioxane is of particular interest due to the ability of some bacteria to mineralize dioxane at a low energy cost. Although dioxane is generally considered recalcitrant to biodegradation, more than 20 types of bacteria can degrade dioxane as the sole electron donor substrate or the secondary electron donor substrate. In the latter case, tetrahydrofuran (THF) is commonly studied as the primary electron donor substrate. Previous work has shown that THF promotes dioxane degradation at a low THF concentration but inhibits dioxane degradation at a high THF concentration. Our work expanded on the previous work by mechanically examining the effects of THF on dioxane degradation in a microbial community context.

Cyclodextrin-enhanced 1,4-dioxane treatment kinetics with TCE and 1,1,1-TCA using aqueous ozone

Enhanced reactivity of aqueous ozone (O₃) with hydroxypropyl-β-cyclodextrin (HPβCD) and its impact on relative reactivity of O₃ with contaminants were evaluated herein. Oxidation kinetics of 1,4-dioxane, trichloroethylene (TCE), and 1,1,1-trichloroethane (TCA) using O₃ in single and multiple contaminant systems, with and without HPβCD, were quantified. 1,4-Dioxane decay rate constants for O₃ in the presence of HPβCD increased compared to those without HPβCD. Density functional theory molecular modeling confirmed that formation of ternary complexes with HPβCD, O₃, and contaminant increased reactivity by increasing reactant proximity and through additional reactivity within the HPβCD cavity. In the presence of chlorinated co-contaminants, the oxidation rate constant of 1,4-dioxane was enhanced. Use of HPβCD enabled O₃ reactivity within the HPβCD cavity and enhanced 1,4-dioxane treatment rates without inhibition in the presence of TCE, TCA, and radical scavengers including NaCl and bicarbonate. Micro-environmental chemistry within HPβCD inclusion cavities mediated contaminant oxidation reactions with increased reaction specificity.

The natural activation ability of subsurface media to promote in-situ chemical oxidation of 1,4-dioxane

The ability of soils and sediments to promote in-situ activation of persulfate and persulfate combined with hydrogen peroxide was investigated for treatment of 1,4-dioxane (dioxane). Experiments were conducted with both batch-reactor and column systems to examine reaction rates and activation mechanisms. Four soils and aquifer sediments were used. ICP-MS and XRD analyses were used to characterize geochemical properties of the solutions and sediments, while EPR spectroscopy was used to characterize radical formation. For the batch experiments, degradation of dioxane was significantly greater in the presence of each of the four subsurface geomedia compared to the controls with no geomedia. This indicates that all four geomedia induced oxidant activation, thereby enhancing dioxane degradation. Dioxane degradation was significantly enhanced by the addition of peroxide to the persulfate solution. It is hypothesized that iron associated with the geomedia is primarily responsible for activation, and that the degree of degradation enhancement relates in part to dissolved-phase iron content. EPR results indicate that manganese oxides and soil organic matter may also have contributed to some degree to persulfate activation, and that manganese oxides enhanced activation of peroxide under the study conditions. Approximately 10% of dioxane was degraded in the miscible-displacement experiments, consistent with the short residence time compared to dioxane's half-life. The pseudo first-order rate coefficients obtained from the batch and column experiments were similar. The results of this study indicate that subsurface geomedia can induce activation of persulfate and peroxide to enhance in-situ chemical oxidation applications.

Response and recovery of microbial communities subjected to oxidative and biological treatments of 1,4-dioxane and co-contaminants

Microbial community dynamics were characterized following combined oxidation and biodegradation treatment trains for mixtures of 1,4-dioxane and chlorinated volatile organic compounds (CVOCs) in laboratory microcosms. Bioremediation is generally inhibited by co-contaminate CVOCs; with only a few specific bacterial taxa reported to metabolize or cometabolize 1,4-dioxane being unaffected. Chemical oxidation by hydrogen peroxide (H₂O₂) as a non-selective treatment demonstrated 50-80% 1,4-dioxane removal regardless of the initial CVOC concentrations. Post-oxidation bioaugmentation with 1,4-dioxane metabolizer Pseudonocardia dioxanivorans CB1190 removed the remaining 1,4-dioxane. The intrinsic microbial population, biodiversity, richness, and biomarker gene abundances decreased immediately after the brief oxidation phase, but recovery of cultivable microbiomes and a more diverse community were observed during the subsequent 9-week biodegradation phase. Results generated from the Illumina Miseq sequencing and bioinformatics analyses established that generally oxidative stress tolerant genus Ralstonia was abundant after the oxidation step, and Cupriavidus, Pseudolabrys, Afipia, and Sphingomonas were identified as dominant genera after aerobic incubation. Multidimensional analysis elucidated the separation of microbial populations as a function of time under all conditions, suggesting that temporal succession is a determining factor that is independent of 1,4-dioxane and CVOCs mixtures. Network analysis highlighted the potential interspecies competition or commensalism, and dynamics of microbiomes during the biodegradation phase, in line with the shifts of predominant genera and various developing directions during different steps of the treatment train. Collectively, this study demonstrated that chemical oxidation followed by bioaugmentation is effective for treating 1,4-dioxane, even in the presence of high levels of CVOC mixtures and residual peroxide, a disinfectant, and enhanced our understanding of microbial ecological impacts of the treatment train. These results will be valuable for predicting treatment synergies that lead to cost savings and improved remedial outcomes in short-term active remediation as well as long-term changes to the environmental microbial communities.

Co-contaminant effects on 1,4-dioxane biodegradation in packed soil column flow-through systems

Biodegradation of 1,4-dioxane was examined in packed quartz and soil column flow-through systems. The inhibitory effects of co-contaminants, specifically trichloroethene (TCE), 1,1-dichloroethene (1,1-DCE), and copper (Cu²⁺) ions, were investigated in the columns either with or without bioaugmentation with a 1,4-dioxane degrading bacterium Pseudonocardia dioxanivorans CB1190. Results indicate that CB1190 cells readily grew and colonized in the columns, leading to significant degradation of 1,4-dioxane under oxic conditions. Degradation of 1,4-dioxane was also observed in the native soil (without bioaugmentation), which had been previously subjected to enhanced reductive dechlorination treatment for co-contaminants TCE and 1,1-DCE. Bioaugmentation of the soil with CB1190 resulted in nearly complete degradation at influent concentrations of 3-10 mg L^-1 1,4-dioxane and a residence reaction time of 40-80 h, but the presence of co-contaminants, 1,1-DCE and Cu²⁺ ions (up to 10 mg L^-1), partially inhibited 1,4-dioxane degradation in the untreated and bioaugmented soil columns. However, the inhibitory effects were much less severe in the column flow-through systems than those previously observed in planktonic cultures, which showed near complete inhibition at the same co-contaminant concentrations. These observations demonstrate a low susceptibility of soil microbes to the toxicity of 1,1-DCE and Cu²⁺ in packed soil flow-through systems, and thus have important implications for predicting biodegradation potential and developing sustainable, cost-effective technologies for in situ remediation of 1,4-dioxane contaminated soils and groundwater.

Electrochemical filtration process for simultaneous removal of refractory organic and particulate contaminants from wastewater effluents

Versatile electrochemical reactions are effective for removing a wide range of water contaminants. This study focuses on the development and testing of bifunctional electrocatalytic filter anodes as reactive and separating media for the simultaneous removal of refractory dissolved organic and particulate contaminants from real wastewater effluents. The results show that the TiO₂ particle interlayers formed between the Ti fiber support and the top composite metal oxide catalyst layers assist in reducing filter pores to an effective size range that enables removal of most particulates within the wastewater. The double-sheet design, which is a sandwich-structured module with an internal void space for permeate, prevents filter fouling, and transmembrane pressure can be maintained at a very low level of <5 kPa during batch and continuous flow reactor operations. Substantive and simultaneous removal of dissolved organics (e.g., chromophores, fluorophores, 1,4-dioxane, chemical oxygen demand, and total organic carbon) and particulate matter (i.e., turbidity) are achieved, although removal rates and efficacies differ depending on the electric current density applied. Decolorization and particulate rejection occur swiftly and immediately, but 1,4-dioxane degradation is relatively slow and quite time-dependent. Possible 1,4-dioxane degradation pathways during electrocatalysis are also proposed. Electrochemical filtration technology shows considerable promise for use in the next generation of advanced wastewater treatment solutions.

Aerobic biodegradation kinetics for 1,4-dioxane under metabolic and cometabolic conditions

Biodegradation of 1,4-dioxane has been studied extensively, however, there is insufficient information on the kinetic characteristics of cometabolism by propanotrophs and a lack of systematic comparisons to metabolic biodegradation. To fill in these gaps, experiments were performed with suspended growth cultures to determine 16 Monod kinetic coefficients that describe metabolic consumption of 1,4-dioxane by Pseudonocardia dioxanivorans CB1190 and cometabolism by the propanotrophic mixed culture ENV487 and the propanotroph Rhodococcus ruber ENV425. Maximum specific growth rates were highest for ENV425, followed by ENV487 and CB1190. Half saturation constants for 1,4-dioxane for the propanotrophs were one-half to one-quarter those for CB1190. Propane was preferentially degraded over 1,4-dioxane, but the reverse did not occur. A kinetic model was used to simulate batch biodegradation of 1,4-dioxane. Propanotrophs decreased 1,4-dioxane from 1000 to 1 μg/L in less time than CB1190 when the initial biomass concentration was 0.74 mg COD/L; metabolic biodegradation was favored at higher initial biomass concentrations and higher initial 1,4-dioxane concentrations. 1,4-Dioxane biodegradation was inhibited when oxygen was below 1.5 mg/L. The kinetic model provides a framework for comparing in situ biodegradation of 1,4-dioxane via bioaugmentation with cultures that use the contaminant as a growth substrate to those that achieve biodegradation via cometabolism.

1,4-Dioxane pollution at contaminated groundwater sites in western Germany and its distribution within a TCE plume

An effective and sensitive method for the analysis of 1,4-dioxane in water has been available since 2008 (EPA 522). This method is increasingly being applied to investigate the distribution of 1,4-dioxane in the aquatic environment. However, there is a need for more information about the possible occurrence of 1,4-dioxane in groundwater in Europe in general, and in Germany in particular, where virtually no data have been collected so far. The possible contamination of groundwater with 1,4-dioxane is of relevance to Germany because up to 70% of Germany's drinking water is obtained from groundwater and about 17% from river bank filtrate, which contains variable proportions of groundwater. The aim of the present study is to investigate selected and representative groundwater sites in Germany that have suspected occurrences of 1,4-dioxane. Five of the sites are well known for their volatile chlorinated hydrocarbon contamination, two sites have representative landfill leachate characteristics, and one site is negatively impacted by a detergent manufacturing plant. The presence of 1,4-dioxane was observed at each of these sites. Measured maximum concentration values ranged from 0.15μg/L to 152μg/L. An aquifer containing a trichloroethylene (TCE) plume with 1,4-dioxane as a co-contaminant was investigated in more detail. A perfect match was found between the concentrations of 1,4-dioxane and TCE in the vertical and horizontal distribution profiles. The results indicate the necessity for investigating groundwater contamination by 1,4-dioxane at sites with known 1,1,1-trichloroethane (TCA) and TCE contaminations, in landfill leachates, and at sites of detergent production.

Spectroscopic methods for aqueous cyclodextrin inclusion complex binding measurement for 1,4-dioxane, chlorinated co-contaminants, and ozone

Recalcitrant organic contaminants, such as 1,4-dioxane, typically require advanced oxidation process (AOP) oxidants, such as ozone (O₃), for their complete mineralization during water treatment. Unfortunately, the use of AOPs can be limited by these oxidants' relatively high reactivities and short half-lives. These drawbacks can be minimized by partial encapsulation of the oxidants within a cyclodextrin cavity to form inclusion complexes. We determined the inclusion complexes of O₃ and three common co-contaminants (trichloroethene, 1,1,1-trichloroethane, and 1,4-dioxane) as guest compounds within hydroxypropyl-β-cyclodextrin. Both direct (ultraviolet or UV) and competitive (fluorescence changes with 6-p-toluidine-2-naphthalenesulfonic acid as the probe) methods were used, which gave comparable results for the inclusion constants of these species. Impacts of changing pH and NaCl concentrations were also assessed. Binding constants increased with pH and with ionic strength, which was attributed to variations in guest compound solubility. The results illustrate the versatility of cyclodextrins for inclusion complexation with various types of compounds, binding measurement methods are applicable to a wide range of applications, and have implications for both extraction of contaminants and delivery of reagents for treatment of contaminants in wastewater or contaminated groundwater.

Associating potential 1,4-dioxane biodegradation activity with groundwater geochemical parameters at four different contaminated sites

1,4-Dioxane (dioxane) is a groundwater contaminant of emerging concern for which bioremediation may become a practical remediation strategy. Therefore, it is important to advance our heuristic understanding of geochemical parameters that are most influential on the potential success of intrinsic bioremediation of dioxane-impacted sites. Here, Pearson's and Spearman's correlation and linear regression analyses were conducted to discern associations between 1,4-dioxane biodegradation activity measured in aerobic microcosms and groundwater geochemical parameters at four different contaminated sites. Dissolved oxygen, which is known to limit dioxane biodegradation, was excluded as a limiting factor in this analysis. Biodegradation activity was positively associated with dioxane concentrations (p < 0.01; R < 0.70) as well as the number of catabolic thmA gene copies (p < 0.01; R = 0.80) encoding dioxane monooxygenase. Thus, whereas environmental factors such as pH, temperature, and nutrients may influence dioxane biodegradation, these parameters did not exert as strong of an influence on potential biodegradation activity as the in situ concentration of substrate dioxane at the time of sampling. This analysis infers that aerobic sites with higher dioxane concentrations are more likely to select and sustain a thriving population of dioxane degraders, while sites with relatively low dioxane concentrations would be more difficult to attenuate naturally and may require alternative remediation strategies.

Synergistic Treatment of Mixed 1,4-Dioxane and Chlorinated Solvent Contaminations by Coupling Electrochemical Oxidation with Aerobic Biodegradation

Biodegradation of the persistent groundwater contaminant 1,4-dioxane is often hindered by the absence of dissolved oxygen and the co-occurrence of inhibiting chlorinated solvents. Using flow-through electrolytic reactors equipped with Ti/IrO₂-Ta₂O₅ mesh electrodes, we show that combining electrochemical oxidation with aerobic biodegradation produces an overadditive treatment effect for degrading 1,4-dioxane. In reactors bioaugmented by Pseudonocardia dioxanivorans CB1190 with 3.0 V applied, 1,4-dioxane was oxidized 2.5 times faster than in bioaugmented control reactors without an applied potential, and 12 times faster than by abiotic electrolysis only. Quantitative polymerase chain reaction analyses of CB1190 abundance, oxidation-reduction potential, and dissolved oxygen measurements indicated that microbial growth was promoted by anodic oxygen-generating reactions. At a higher potential of 8.0 V, however, the cell abundance near the anode was diminished, likely due to unfavorable pH and/or redox conditions. When coupled to electrolysis, biodegradation of 1,4-dioxane was sustained even in the presence of the common co-contaminant trichloroethene in the influent. Our findings demonstrate that combining electrolytic treatment with aerobic biodegradation may be a promising synergistic approach for the treatment of mixed contaminants.

1,4-Dioxane drinking water occurrence data from the third unregulated contaminant monitoring rule

This study examined data collected from U.S. public drinking water supplies in support of the recently-completed third round of the Unregulated Contaminant Monitoring Rule (UCMR3) to better understand the nature and occurrence of 1,4-dioxane and the basis for establishing drinking water standards. The purpose was to evaluate whether the occurrence data for this emerging but federally-unregulated contaminant fit with common conceptual models, including its persistence and the importance of groundwater contamination for potential exposure. 1,4-Dioxane was detected in samples from 21% of 4864 PWSs, and was in exceedance of the health-based reference concentration (0.35μg/L) at 6.9% of these systems. In both measures, it ranked second among the 28 UCMR3 contaminants. Although much of the focus on 1,4-dioxane has been its role as a groundwater contaminant, the detection frequency for 1,4-dioxane in surface water was only marginally lower than in groundwater (by a factor of 1.25; p<0.0001). However, groundwater concentrations were higher than those in surface water (p<0.0001) and contributed to a higher frequency of exceeding the reference concentration (by a factor of 1.8, p<0.0001), indicating that surface water sources tend to be more dilute. Sampling from large systems increased the likelihood that 1,4-dioxane was detected by a factor of 2.18 times relative to small systems (p<0.0001). 1,4-Dioxane detections in drinking water were highly associated with detections of other chlorinated compounds particularly 1,1-dichlorethane (odds ratio=47; p<0.0001), which is associated with the release of 1,4-dioxane as a chlorinated solvent stabilizer. Based on aggregated nationwide data, 1,4-dioxane showed evidence of a decreasing trend in concentration and detection frequency over time. These data suggest that the loading to drinking water supplies may be decreasing. However, in the interim, some water supply systems may need to consider improving their treatment capabilities in response to further regulatory review of this compound.

Potential for cometabolic biodegradation of 1,4-dioxane in aquifers with methane or ethane as primary substrates

The objective of this research was to evaluate the potential for two gases, methane and ethane, to stimulate the biological degradation of 1,4-dioxane (1,4-D) in groundwater aquifers via aerobic cometabolism. Experiments with aquifer microcosms, enrichment cultures from aquifers, mesophilic pure cultures, and purified enzyme (soluble methane monooxygenase; sMMO) were conducted. During an aquifer microcosm study, ethane was observed to stimulate the aerobic biodegradation of 1,4-D. An ethane-oxidizing enrichment culture from these samples, and a pure culture capable of growing on ethane (Mycobacterium sphagni ENV482) that was isolated from a different aquifer also biodegraded 1,4-D. Unlike ethane, methane was not observed to appreciably stimulate the biodegradation of 1,4-D in aquifer microcosms or in methane-oxidizing mixed cultures enriched from two different aquifers. Three different pure cultures of mesophilic methanotrophs also did not degrade 1,4-D, although each rapidly oxidized 1,1,2-trichloroethene (TCE). Subsequent studies showed that 1,4-D is not a substrate for purified sMMO enzyme from Methylosinus trichosporium OB3b, at least not at the concentrations evaluated, which significantly exceeded those typically observed at contaminated sites. Thus, our data indicate that ethane, which is a common daughter product of the biotic or abiotic reductive dechlorination of chlorinated ethanes and ethenes, may serve as a substrate to enhance 1,4-D degradation in aquifers, particularly in zones where these products mix with aerobic groundwater. It may also be possible to stimulate 1,4-D biodegradation in an aerobic aquifer through addition of ethane gas. Conversely, our results suggest that methane may have limited importance in natural attenuation or for enhancing biodegradation of 1,4-D in groundwater environments.

A novel concept in ground water quality management: Towards function specific screening values

This paper is meant to initiate and feed the discussion on a more sophisticated procedure for the derivation and use of groundwater screening values (GSVs). To this purpose, the possibilities and tools for the derivation of function specific GSVs, i.e., GSVs that depend on the actual contact of humans and ecosystems with groundwater and groundwater-related mediums, are elaborated in this study. Application of GSVs geared to the specific use and function of specific groundwater volumes could result in a more effective and cost-efficient groundwater quality management, without compromising the protection of human health and the ecosystem. Therefore, a procedure to derive function specific GSVs was developed. For illustrative purposes, risk limits have been derived for human health and ecological protection targets, for arsenic, benzene, methyl tert-butyl ether (MTBE) and vinylchloride. Agriculture and Nature reserves (combined), Residential and Industrial land uses have been considered and two different groundwater management purposes, i.e., curative and sustainable groundwater management. For each of the four contaminants, this results in a series of risks limits for each function and land use combination. It is shown that for all four contaminants higher groundwater screening values are considered appropriate for less sensitive combinations of function and land use. In the process towards (policy) implementation of these function specific GSV, it is recommended to evaluate the selection of protection targets, the scientific basis of the risk assessment procedures applied and the methodology to assess the time factor for groundwater quality assessment, given the fact that groundwater is a dynamic medium. Moreover, protection levels must be harmonized with national or regional groundwater quality standards and correspond with the requirements of the Groundwater Daughter Directive of the European Union Water Framework Directive. Groundwater plumes that are judged as 'no need for remediation' are not compatible with the Water Framework Directive requirement to take actions to prevent or limit inputs of contaminants, even when no receptor is present. However, the European Commission formulated a series of exemptions, to avoid that the "prevent" requirement would imply an onerous and sometimes unfeasible task. The function specific GSVs derived in this study could be used to identify the groundwater volumes that do not result in an unacceptable risk.

Hindrance of 1,4-dioxane biodegradation in microcosms biostimulated with inducing or non-inducing auxiliary substrates

A microcosm study was conducted to assess two biostimulation strategies (relative to natural attenuation) to bioremediate 1,4-dioxane contamination at a site in west Texas. Dioxane concentrations were relatively low (<300 μg/L), which represents a potential challenge to sustain and induce specific degraders. Thus, biostimulation was attempted with an auxiliary substrate known to induce dioxane-degrading monooxygenases (i.e., tetrahydrohyran [THF]) or with a non-inducing growth substrate (1-butanol [1-BuOH]). Amendment of 1-BuOH (100 mg/L) to microcosms that were not oxygen-limited temporarily enhanced dioxane biodegradation by the indigenous microorganisms. However, this stimulatory effect was not sustained by repeated amendments, which might be attributed to i) the inability of 1-BuOH to induce dioxane-degrading enzymes, ii) curing of catabolic plasmids, iii) metabolic flux dilution and catabolite repression, and iv) increased competition by commensal bacteria that do not degrade dioxane. Experiments with the archetype dioxane degrader Pseudonocardia dioxanivorans CB1190 repeatedly amended with 1-BuOH (500 mg/L added weekly for 4 weeks) corroborated the partial curing of catabolic plasmids (9.5 ± 7.4% was the plasmid retention ratio) and proliferation of derivative segregants that lost their ability to degrade dioxane. Addition of THF (300 μg/L) also had limited benefit due to competitive inhibition; significant dioxane degradation occurred only when the THF concentration decreased below approximately 160 μg/L. Overall, these results illustrate the importance of considering the possibility of unintentional hindrance of catabolism associated with the addition of auxiliary carbon sources to bioremediate aquifers impacted with trace concentrations of dioxane.