Determining the Temperature Dependency of Biodegradation Kinetics for 34 Hydrocarbons while Avoiding Chemical and Microbial Confounding Factors

Influence of Season on Biodegradation Rates in Rivers

Biodegradation plays a key role in the fate of chemicals in the environment. The variability of biodegradation in time can cause uncertainty in evaluating the environmental persistence and risk of chemicals. However, the seasonality of biodegradation in rivers has not yet been the subject of environmentally relevant testing and systematic investigation for large numbers of chemicals. In this work, we studied the biodegradation of 96 compounds during four seasons at four locations (up- and downstream of WWTPs located on two Swedish rivers). Significant seasonality (ANOVA, p < 0.05) of the first-order rate constant for primary biodegradation was observed for most compounds. Variations in pH and total bacterial cell count were not the major factors explaining the seasonality of biodegradation. Deviation from the classical Arrhenius-type behavior was observed for most of the studied compounds, which calls into question the application of this relationship to correct biodegradation rate constants for differences in environmental temperature. Similarities in magnitude and seasonality of biodegradation rate constants were observed for some groups of chemicals possessing the same functional groups. Moreover, reduced seasonality of biodegradation was observed downstream of WWTPs, while biodegradation rates of most compounds were not significantly different between up- and downstream.

Closed aerobic biodegradation kinetics test with activated sludge and low concentration chemical mixtures

The activated sludge process at wastewater treatment plants is important to prevent discharge of organic pollutants to the environment. Determination of biodegradation kinetics in activated sludge is challenging for mixtures that cover a diverse range of structures. The aims of this study were to (1) design a closed aerobic biodegradation batch test with activated sludge and (2) develop a sample preparation procedure that is compatible with LC-MS and Solid Phase Microextraction (SPME) coupled to GC-MS. A headspace:sludge ratio of 4:1 was sufficient to ensure aerobic conditions in activated sludge for 7 days at co-solvent concentrations <0.01%. Ethanol was added to sub-samples (50%) to stop biodegradation, extract sorbed chemicals and allow storage at -18 °C without ice formation. The ethanol extracted the chemicals from the sludge before filtration (0.2 μm). The filtrate was diluted in ultrapure water to <12% ethanol before analysis by SPME GC-MS/MS and was suitable for direct injection on LC-MS/MS. Biodegradation was distinguished from sorption through abiotic controls using autoclaved poisoned sludge. Linalool, naphthalene, α-isomethylionone, phenanthrene, citronellol, drometrizole, 2-ethylhexyl 4-methoxycinnamate, dicyclohexyl phthalate, BP-1, BP-3, methyl-, ethyl-, propylparaben, alkyl sulfates and isethionates degraded within 48 h in activated sludge, while musk ketone, tonalide and 1,3,5-trichlorobenzene did not. A 10 times reduction of sludge density did not markedly affect the microbial diversity but slowed biodegradation kinetics (partly explained by theory). This study demonstrated a 'cold' alternative to an OECD 314b test and how biodegradation kinetics can be determined for mixtures of diverse chemicals in closed batch tests with activated sludge.

Biodegradation of chemicals tested in mixtures and individually: mixture effects on biodegradation kinetics and microbial composition

Biodegradation in the aquatic environment occurs in the presence of many chemicals, while standard simulation biodegradation tests are conducted with single chemicals. This study aimed to investigate the effect of the presence of additional chemicals on (1) biodegradation kinetics of individual chemicals and (2) the microbial composition in test systems. Parallel mixture and single substance experiments were conducted for 9 chemicals (phenethyl benzoate, oxacycloheptadec-10-en-2-one, α-ionone, methyl 2-naphthyl ether, decan-5-olide, octan-2-one, 2'-acetonaphthanone, methyl N-methylanthranilate, (+)-menthone) using inoculum from a Danish stream. Biotic and abiotic test systems were incubated at 12 °C for 1-30 days. Primary biodegradation kinetics were then determined from biotic/abiotic peak area ratios using SPME GC/MS analysis. The effect of the mixture on biodegradation varied with test chemical and was more pronounced for chemicals with lag-phases above 14 days: two chemicals degraded in the mixture but not when tested alone (i.e., positive mixture effect), and two degraded when tested alone but not in the mixture (i.e., negative mixture effect). Microbial composition (16S rRNA gene amplicon sequencing) was highly affected by 14 days incubation and the presence of the mixture (significant carbon source), but less by single chemicals (low carbon source). Growth on chemical mixtures resulted in consistent proliferation of Pseudomonas and Malikia, while specific chemicals increased the abundance of putative degraders belonging to Novosphingobium and Zoogloea. The chemical and microbiological results support (1) that simulation biodegradation kinetics should be determined in mixtures at low environmentally relevant concentrations and (2) that degradation times beyond some weeks are associated with more uncertainty.

Using weight of evidence to assess degradation potential of UVCB hydrocarbon solvents

Hydrocarbon solvents are a diverse group of petrochemical substances that are identified as unknown or variable composition, complex reaction products, or biological materials (UVCBs) and may contain tens of thousands of individual chemical constituents. As such, it is generally not possible to analytically resolve every chemical constituent in a hydrocarbon solvent. This, along with the low water solubility and/or high vapor pressure of constituents, precludes the use of many standardized tests designed to determine biodegradation in the environment (e.g., Organization for Economic Co-operation and Development [OECD] 309). A weight of evidence approach may be needed to reduce uncertainty to an acceptable level such that a determination on the biodegradation of the substance can be drawn. Based on the OECD 2019 weight of evidence guidance, we present a framework using various lines of evidence that can be used to evaluate the biodegradation of a UVCB solvent in a weight of evidence approach. The lines of evidence include whole substance testing, data on representative constituents, quantitative structure activity relationship (QSAR) models, and biological plausibility. Using these lines of evidence, "Hydrocarbon, C11-C14, normal alkane, isoalkane, cyclic, <2% aromatics" (EC# 926-141-6) was evaluated in a case study. Data from three whole substance tests, 43 constituents (representing 152 data points), three QSAR models and evidence of microbial degradation pathways were evaluated. Based on the available data, it is concluded that the solvent for the case study is not expected to persist in the environment. This framework sets out a real-world example of how the weight of evidence can be used to evaluate hydrocarbon solvents. While focused on persistence, similar approaches can be used to evaluate other endpoints such as bioaccumulation and toxicity. Integr Environ Assess Manag 2023;00:1-11. © 2023 The Authors. Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC).

Technical guidance on biodegradation testing of difficult substances and mixtures in surface water

The aim of this article is to address critical challenges in the OECD 309 "Aerobic mineralization in surface water - simulation biodegradation test" for volatile chemicals, highly hydrophobic chemicals, mixtures or UVCBs (unknown or variable composition, complex reaction products or biological materials). Several modifications are presented to address technical challenges (minimize and account for losses), make testing more environmentally relevant (lower concentrations) and generate data for multiple substances (more and better aligned data):•Minimizing and accounting for test substance losses: Aqueous solutions are handled using gas tight syringes, tests are conducted in gas tight vials, and automated analysis is performed directly on unopened test vials. Abiotic losses are accounted for via concentration ratios between test systems and abiotic controls that are incubated and measured in parallel.•Testing at low environmentally relevant concentrations: Substances are tested at low concentrations to avoid toxicity and solubility artefacts and analyzed using a sensitive analytical method. Substances are added without co-solvent (using passive dosing) or with a minimum of co-solvent (using microvolume spiking).•Testing of multiple chemicals in mixtures combined with constituent specific analysis: Primary biodegradation kinetics of chemicals are determined in tests of multi-constituent mixtures or UVCBs using constituent specific analysis.

The Next Frontier of Environmental Unknowns: Substances of Unknown or Variable Composition, Complex Reaction Products, or Biological Materials (UVCBs)

Substances of unknown or variable composition, complex reaction products, or biological materials (UVCBs) are over 70 000 "complex" chemical mixtures produced and used at significant levels worldwide. Due to their unknown or variable composition, applying chemical assessments originally developed for individual compounds to UVCBs is challenging, which impedes sound management of these substances. Across the analytical sciences, toxicology, cheminformatics, and regulatory practice, new approaches addressing specific aspects of UVCB assessment are being developed, albeit in a fragmented manner. This review attempts to convey the "big picture" of the state of the art in dealing with UVCBs by holistically examining UVCB characterization and chemical identity representation, as well as hazard, exposure, and risk assessment. Overall, information gaps on chemical identities underpin the fundamental challenges concerning UVCBs, and better reporting and substance characterization efforts are needed to support subsequent chemical assessments. To this end, an information level scheme for improved UVCB data collection and management within databases is proposed. The development of UVCB testing shows early progress, in line with three main methods: whole substance, known constituents, and fraction profiling. For toxicity assessment, one option is a whole-mixture testing approach. If the identities of (many) constituents are known, grouping, read across, and mixture toxicity modeling represent complementary approaches to overcome data gaps in toxicity assessment. This review highlights continued needs for concerted efforts from all stakeholders to ensure proper assessment and sound management of UVCBs.

Linking biodegradation kinetics, microbial composition and test temperature - Testing 40 petroleum hydrocarbons using inocula collected in winter and summer

Many factors affect the biodegradation kinetics of chemicals in test systems and the environment. Empirical knowledge is needed on how much test temperature, inoculum, test substances and co-substrates influence the biodegradation kinetics and microbial composition in the test. Water was sampled from the Gudenaa river in winter (2.7 °C) and summer (17 °C) (microbial inoculum) and combined with an aqueous stock solution of >40 petroleum hydrocarbons prepared by passive dosing. This resulted in low-concentration test systems that were incubated for 30 days at 2.7, 12 and 20 °C. Primary biodegradation kinetics, based on substrate depletion relative to abiotic controls, were determined with automated Solid Phase Microextraction coupled to GC/MS. Biodegradation kinetics were remarkably similar for summer and winter inocula when tested at the same temperature, except when cooling summer inoculum to 2.7 °C which delayed degradation relative to winter inoculum. Amplicon sequencing was applied to determine shifts in the microbial composition between season and during incubations: (1) the microbial composition of summer and winter inocula were remarkably similar, (2) the incubation and the incubation temperature had both a clear impact on the microbial composition and (3) the effect of adding >40 petroleum hydrocarbons at low test concentrations was limited but resulted in some proliferation of the known petroleum hydrocarbon degraders Nevskia and Sulfuritalea. Overall, biodegradation kinetics and its temperature dependency were very similar for winter and summer inoculum, whereas the microbial composition was more affected by incubation and test temperature compared to the addition of test chemicals at low concentrations.

Biodegradation Kinetics of Fragrances, Plasticizers, UV Filters, and PAHs in a Mixture─Changing Test Concentrations over 5 Orders of Magnitude

Biodegradation of organic chemicals emitted to the environment is carried out by mixed microbial communities growing on multiple natural and xenobiotic substrates at low concentrations. This study aims to (1) perform simulation type biodegradation tests at a wide range of mixture concentrations, (2) determine the concentration effect on the biodegradation kinetics of individual chemicals, and (3) link the mixture concentration and degradation to microbial community dynamics. Two hundred ninety-four parallel test systems were prepared using wastewater treatment plant effluent as inoculum and passive dosing to add a mixture of 19 chemicals at 6 initial concentration levels (ng/L to mg/L). After 1-30 days of incubation at 12 °C, abiotic and biotic test systems were analyzed using arrow solid phase microextraction and GC-MS/MS. Biodegradation kinetics at the highest test concentrations were delayed for several test substances but enhanced for the reference chemical naphthalene. Test concentration thus shifted the order in which chemicals were degraded. 16S rRNA gene amplicon sequencing indicated that the highest test concentration (17 mg C/L added) supported the growth of the genera Acidovorax, Novosphingobium, and Hydrogenophaga, whereas no such effect was observed at lower concentrations. The chemical and microbial results confirm that too high mixture concentrations should be avoided when aiming at determining environmentally relevant biodegradation data.