Biodegradation of poly(propylene glycol)s under the conditions of the OECD screening test

In silico approaches for the prediction of the breakthrough of organic contaminants in wastewater treatment plants

The removal efficiency (RE) of organic contaminants in wastewater treatment plants (WWTPs) is a major determinant of the environmental impact of chemicals which are discharged to wastewater. In a recent study, non-target screening analysis was applied to quantify the percentage removal efficiency (RE%) of more than 300 polar contaminants, by analyzing influent and effluent samples from a Swedish WWTP with direct injection UHPLC-Orbitrap-MS/MS. Based on subsets extracted from these data, we developed quantitative structure-property relationships (QSPRs) for the prediction of WWTP breakthrough (BT) to the effluent water. QSPRs were developed by means of multiple linear regression (MLR) and were selected after checking for overfitting and chance relationships by means of bootstrap and randomization procedures. A first model provided good fitting performance, showing that the proposed approach for the development of QSPRs for the prediction of BT is reasonable. By further populating the dataset with similar chemicals using a Tanimoto index approach based on substructure count fingerprints, a second QSPR indicated that the prediction of BT is also applicable to new chemicals sufficiently similar to the training set. Finally, a class-specific QSPR for PEGs and PPGs showed BT prediction trends consistent with known degradation pathways.

Identification of more than 100 new compounds in the wastewater: Fate of polyethylene/polypropylene oxide copolymers and their metabolites in the aquatic environment

100 ethylene oxide (EO)/propylene oxide (PO) copolymer precursor and metabolites were detected in wastewater effluents. The homopolymers of EO and PO as well as the EO/PO copolymers are widely used as surfactants, e.g., for the production of cosmetics, pharmaceuticals and lubricants. Concomitantly, these compounds are discharged into the wastewater and the environmental fate of the PO homopolymers, also called polypropylene glycols (PPGs), and EO/PO copolymers is mostly unknown. In the present study, we identified hitherto unknown copolymer EO/PO homologous series and their metabolites in wastewater effluent. The identified compounds occur in homologous series and consist of PPGs and EO/PO copolymers, and their carbonylated, carboxylated and dicarboxylated metabolites. MBBR lab incubations of PPGs and EO/PO copolymers showed the successive degradation by cleavage of individual PO and EO groups, with high removal (>90%) in the initial 8 h for most of the copolymers. Carbonylated and carboxylated metabolites were degraded within 40 h. EO/PO copolymers with a higher number of EO and PO units showed a higher removal in MBBR and conventional activated sludge wastewater treatment plants. Polymers with lower molecular weight were initially formed by degradation of the EO/PO polymers. The mono-carboxylated metabolites were also detected in surface waters. Overall, our results provide new knowledge about degradation pathways of PO containing compounds and show the hitherto unnoticed occurrence of EO/PO copolymers and metabolites in the water cycle.

Mechanistic Study of Isotactic Poly(propylene oxide) Synthesis using a Tethered Bimetallic Chromium Salen Catalyst

Initial catalyst dormancy has been mitigated for the enantioselective polymerization of propylene oxide using a tethered bimetallic chromium(III) salen complex. A detailed mechanistic study provided insight into the species responsible for this induction period and guided efforts to remove them. High-resolution electrospray ionization-mass spectrometry and density functional theory computations revealed that a μ-hydroxide and a bridged 1,2-hydroxypropanolate complex are present during the induction period. Kinetic studies and additional computation indicated that the μ-hydroxide complex is a short-lived catalyst arrest state, where hydroxide dissociation from one metal allows for epoxide enchainment to form the 1,2-hydroxypropanolate arrest state. While investigating anion dependence on the induction period, it became apparent that catalyst activation was the main contributor for dormancy. Using a 1,2-diol or water as chain transfer agents (CTAs) led to longer induction periods as a result of increased 1,2-hydroxyalkanolate complex formation. With a minor catalyst modification, rigorous drying conditions, and avoiding 1,2-diols as CTAs, the induction period was essentially removed.

Isotactic Poly(propylene oxide): A Photodegradable Polymer with Strain Hardening Properties

Leakage and accumulation of highly stable commercial plastics has led to substantial contamination of the environment. Highly isotactic poly(propylene oxide) (iPPO) was investigated as a potential high-strength thermoplastic with greater susceptibility toward degradation under ambient conditions. Various stereoregular forms of iPPO including enantiopure, enantioenriched, racemic, and stereoblock were synthesized with a single catalyst architecture in the presence of chain transfer agents. These materials were found to possess the same approximate ultimate tensile strength (UTS) via uniaxial tensile elongation analysis (∼75 MPa). A serrated tensile response corresponding to stress oscillations was observed in all forms of iPPO. An investigation on strain rate dependence showed that an increase in strain rate results in the decay and disappearance of the serrated response. Further evaluation of iPPO revealed its dramatic strain hardening afforded an UTS comparable to that of nylon-6,6. Exposing iPPO to UVA light (365 nm) resulted in photolytic degradation. Following 30 days of continuous exposure at 250 μW cm^-2, the M_n decreased from 93 kDa to 21 kDa, while samples not exposed to UVA light remained unchanged. Through selective stabilization with antioxidant additives, we believe iPPO could be a suitable replacement for nylon-6,6 in environmentally susceptible applications.

Degradation of polyethylene glycols and polypropylene glycols in microcosms simulating a spill of produced water in shallow groundwater

Polyethylene glycols (PEGs) and polypropylene glycols (PPGs) are frequently used in hydraulic fracturing fluids and have been detected in water returning to the surface from hydraulically fractured oil and gas wells in multiple basins. We identified degradation pathways and kinetics for PEGs and PPGs under conditions simulating a spill of produced water to shallow groundwater. Sediment-groundwater microcosm experiments were conducted using four produced water samples from two Denver-Julesburg Basin wells at early and late production. High-resolution mass spectrometry was used to identify the formation of mono- and di-carboxylated PEGs and mono-carboxylated PPGs, which are products of PEG and PPG biodegradation, respectively. Under oxic conditions, first-order half-lives were more rapid for PEGs (<0.4-1.1 d) compared to PPGs (2.5-14 d). PEG and PPG degradation corresponded to increased relative abundance of primary alcohol dehydrogenase genes predicted from metagenome analysis of the 16S rRNA gene. Further degradation was not observed under anoxic conditions. Our results provide insight into the differences between the degradation rates and pathways of PEGs and PPGs, which may be utilized to better characterize shallow groundwater contamination following a release of produced water.

Exploration of polyelectrolyte incorporated with Triton-X 114 surfactant based osmotic agent for forward osmosis desalination

Selection of a proper osmotic agent is important to make the forward osmosis (FO) feasible. The objective of this study was to enhance FO by lowering reverse solute flux and maintaining high water flux. Poly(propylene glycol) with molecular weight of 725 Da (PPG-725) was found to possess high osmolality, making it a strong candidate for using as a draw agent. In addition, to reduce the partial leakage of draw solute, a non-ionic surfactant (Triton X-114) has been incorporated. Typically, when the hydrophobic tails of Triton X-114 interacted with the membrane surface, a layer on the surface of membrane is produced to constrict the pores and thus minimize the reverse solute flux. In this study, different concentrations of PPG-725 incorporated with different concentrations of Triton X-114 (0.2-0.8 mM) were used to evaluate their osmotic potentials as draw solute. The specific reverse solute flux (J_s/J_w) of 40% PPG-725 doped with Triton X-114 was found to be 0.01 g/L, considerably much lesser than the conventional inorganic draw agents. Finally, membrane distillation operation was utilized as the recovery system in which solute rejection of 97% was achieved for 40% PPG-725/Triton X-114. Therefore, the overall performance supported PPG-725/Triton X-114 as being an efficient draw agent for forward osmosis-membrane distillation hybrid process.

Natural Attenuation of Nonionic Surfactants Used in Hydraulic Fracturing Fluids: Degradation Rates, Pathways, and Mechanisms

Hydraulic fracturing fluids are injected into shales to extend fracture networks that enhance oil and natural gas production from unconventional reservoirs. Here we evaluated the biodegradability of three widely used nonionic polyglycol ether surfactants (alkyl ethoxylates (AEOs), nonylphenol ethoxylates (NPEOs), and polypropylene glycols (PPGs)) that function as weatherizers, emulsifiers, wetting agents, and corrosion inhibitors in injected fluids. Under anaerobic conditions, we observed complete removal of AEOs and NPEOs from solution within 3 weeks regardless of whether surfactants were part of a chemical mixture or amended as individual additives. Microbial enzymatic chain shortening was responsible for a shift in ethoxymer molecular weight distributions and the accumulation of the metabolite acetate. PPGs bioattenuated the slowest, producing sizable concentrations of acetone, an isomer of propionaldehyde. Surfactant chain shortening was coupled to an increased abundance of the diol dehydratase gene cluster (pduCDE) in Firmicutes metagenomes predicted from the 16S rRNA gene. The pduCDE enzymes are responsible for cleaving ethoxylate chain units into aldehydes before their fermentation into alcohols and carboxylic acids. These data provide new mechanistic insight into the environmental fate of hydraulic fracturing surfactants after accidental release through chain shortening and biotransformation, emphasizing the importance of compound structure disclosure for predicting biodegradation products.

Characterization of the chemicals used in hydraulic fracturing fluids for wells located in the Marcellus Shale Play

Hydraulic fracturing, coupled with the advances in horizontal drilling, has been used for recovering oil and natural gas from shale formations and has aided in increasing the production of these energy resources. The large volumes of hydraulic fracturing fluids used in this technology contain chemical additives, which may be toxic organics or produce toxic degradation byproducts. This paper investigated the chemicals introduced into the hydraulic fracturing fluids for completed wells located in Pennsylvania and West Virginia from data provided by the well operators. The results showed a total of 5071 wells, with average water volumes of 5,383,743 ± 2,789,077 gal (mean ± standard deviation). A total of 517 chemicals was introduced into the formulated hydraulic fracturing fluids. Of the 517 chemicals listed by the operators, 96 were inorganic compounds, 358 chemicals were organic species, and the remaining 63 cannot be identified. Many toxic organics were used in the hydraulic fracturing fluids. Some of them are carcinogenic, including formaldehyde, naphthalene, and acrylamide. The degradation of alkylphenol ethoxylates would produce more toxic, persistent, and estrogenic intermediates. Acrylamide monomer as a primary degradation intermediate of polyacrylamides is carcinogenic. Most of the chemicals appearing in the hydraulic fracturing fluids can be removed when adopting the appropriate treatments.

Development of a Dispersive Liquid-Liquid Microextraction Procedure for Biodegradation Studies on Nonylphenol Propoxylates Under Aerobic Conditions

Aerobic biodegradation behavior of nonylphenol propoxylates was investigated using dispersive liquid-liquid microextraction as a simple and fast technique for sample preparation. The developed method proved to be efficient for the isolation and concentration of nonylphenol propoxylates before their quantification with the use of high performance liquid chromatography. The primary biodegradation of nonylphenol propoxylates was approximately 80 % by 10 days after the beginning of the test. However, the biodegradation products which were identified with the use of mass spectrometric detection persisted for many days.

Comparison of Biodegradation of Nonylphenol Propoxylates with Usage of Two Different Sources of Activated Sludge

Aerobic biodegradation behaviour of nonylphenol propoxylates was investigated in two tests with different sewage sludge as inocula. The samples containing target compounds were pre-concentrated using dispersive liquid-liquid microextraction and analysed with the use of high performance liquid chromatography with tandem mass spectrometry. Both primary biodegradation and formation of different biodegradation by-products were studied. Primary biodegradation of nonylphenol propoxylates was relatively slow and reached only about 70 % in over 70 days from the start of the tests. The biodegradation by-products from both oxidative and non-oxidative pathways were found. In the non-oxidative route, shortening of the propoxy chain was observed. In the oxidative pathway carboxylic acids and ketones were identified. The biodegradation by-products identified with the use of mass spectrometric detection also persisted for many days.

The distribution, fate, and effects of propylene glycol substances in the environment

The propylene glycol substances comprise a homologous family of synthetic organic molecules that have widespread use and very high production volumes across the globe. The information presented and summarized here is intended to provide an overview of the most current and reliable information available for assessing the potential environmental exposures and impacts of these substances across the manufacture, use, and disposal phases of their product life cycles.The PG substances are characterized as being miscible in water, having very low octanol-water partition coefficients (log Pow) and exhibiting low potential to volatilize from water or soil in both pure and dissolved forms. The combination of these properties dictates that, almost regardless of the mode of their initial emission, they will ultimately associate with surface water, soil, and the related groundwater compartments in the environment. These substances have low affinity for soil and sediment particles, and thus will remain mobile and bio-available within these media.In the atmosphere, the PG substances are demonstrated to have short lifetimes(I. 7-11 h), due to rapid reaction with photochemically-generated hydroxyl radicals.This reactivity, combined with efficient wet deposition of their vapor and aerosol forms, lends to their very low potential for long-range transport via the atmosphere.In the aquatic and terrestrial compartments of the environment, the PG substances are rapidly and ultimately biodegraded under both aerobic and anaerobic conditions by a wide variety of microorganisms, regardless of prior adaptation to the substances.Except for the TePG substance, the propylene glycol substances meet the OECD definition of "readily biodegradable", and according to this definition are not expected to persist in either aquatic or terrestrial environments. The TePG exhibits inherent biodegradability, is not regarded to be persistent, and is expected to ultimately biodegrade in the environment, albeit at a somewhat slower rate. The apparent ease with which microorganisms and higher organisms can metabolize the PG substances, along with their low log Pow and very high water solubility values, portends them to have very low potential for bioaccumulation and/or biomagnification in aquatic and terrestrial organisms. These same properties, along with their neutral structures and lack of biological reactivity, are the reasons for which the PG substances exhibit a base-line, non-polar narcosis mode of toxicity.The PG substances have been shown here to be practically non-toxic to essentially every aquatic and terrestrial animal and plant species tested. Collectively, the available wealth of information relating to persistence, bioaccumulation, and eco-toxicity of these substances allows a definitive conclusion of their categorization as not being PBT (i.e., persistently bioaccumulative/toxic). The PBT screening and categorization of substances on the Canadian Domestic Substances List (DSL) by Environment Canada has formally concluded that each member of this substance family is "not P", "not B", and "not T' according to their associated PBT criteria.Similarly, the preceding evaluations of these high production volume substances within the OECD SIDS program concluded that MPG, DPG, and TPG are low priorities for further examination of potential impacts to humans and the environment.More extensive evaluations of potential risks to human health and the environment were recently completed by industry, as required for their registration under the European Union REACh legislation; each evaluation demonstrated that current uses, associated exposures, and controls thereof, will not result in exposures that exceed predicted no effect concentrations in the environment.

Bio-oxidation of tripropylene glycol under aerobic conditions

Aerobic biodegradation of tripropylene glycol (PG3) was investigated under the conditions of the OECD screening test 301E and the Continuous Flow Activated Sludge Simulation test (CFAS). A modified two-chamber facility with a denitrification stage was used for the CFAS test. Primary PG3 biodegradation was measured by the HPLC with fluorimetric detection and analyte derivatisation. Metabolites were identified by LC-MS with electrospray ionisation and GC-MS with electron impact ionisation, as well as semiquantitatively determined by the LC-MS technique. PG3 was found to be inherently biodegradable and it exhibits a strong poisonous effect on activated sludge after exceeding the threshold concentration (10 mg l(-1)). Metabolite accumulation onto the activated sludge is probably responsible for this poisonous effect. Probable biotransformation products of tripropylene glycol under the aerobic conditions include metabolites with a single terminal aldehyde or a ketone group and metabolites with two terminal aldehyde or ketone groups. Their concentration rises at the end of the OECD screening test.