A water-sediment screening tool for measuring biodegradation of organic chemicals

Improved closed test setup for biodegradation testing of slightly volatile substances in water-sediment systems (OECD 308)

Standardized biodegradation testing methods, like the OECD 308 Aerobic and Anaerobic Transformation in Aquatic Sediment Systems, generate data on biodegradation required during environmental risk and hazard assessment of chemicals under different European and international regulations. However, difficulties arise when applying the OECD 308 guideline for testing hydrophobic volatile chemicals. Especially the use of a co-solvent (like acetone) as a measure to facilitate the application of the test chemical in combination with a closed setup to reduce losses due to volatilization tend to deplete/restrict the amount of oxygen in the test system. The result is a low oxygen or even anoxic water column in the water-sediment system. Thus, the degradation half-lives of the chemical generated from such tests are not directly comparable to the regulatory half-life values for Persistence assessment of the test chemical. The aim of this work was to further develop the closed setup to improve and maintain aerobic conditions in the water phase of the water-sediment systems for testing slightly volatile hydrophobic test chemicals. This improvement was attained by optimizing the test system geometry and agitation technique to maintain aerobic conditions in the water phase in a closed test setup, investigating appropriate co-solvent application strategy, and trialing the resulting test setup. This study shows that when using a closed test setup for OECD 308 tests, agitation of the water phase overlaying the sediment and the test item application using low co-solvent volume is critical for maintaining an aerobic water layer.

Compartment-Specific Screening Tools for Persistence: Potential Role and Application in the Regulatory Context

The persistence assessment under the European Union regulation Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH) relies on compartment-specific degradation half-lives derived from laboratory simulation studies with surface water, aquatic sediment, or soil. Although these data are given priority, they are not available for most of the compounds. Therefore, according to the Integrated Assessment and Testing Strategy (ITS) for persistence assessment, results from ready biodegradability tests (RBTs) are used within a persistence screening to decide whether a substance is considered as "not persistent" or "potentially persistent." However, ready biodegradability is currently tested only in water. Consequently, there is a lack of approaches that include the soil and sediment compartments for persistence assessment at the screening level. In previous studies, compartment-specific screening tools for water-sediment (Water-Sediment Screening Tool [WSST]) and soil (Soil Screening Tool [SST]) were developed based on the existing test guideline Organisation for Economic Development and Co-operation (OECD TG 301C [MITI (Ministry of International Trade and Industry, Japan) test]). The test systems MITI, WSST, and SST were successfully applied to determine sound and reliable biodegradation data for 15 test compounds. In the present study, these results are used within the scope of a new alternative persistence screening approach, the Compartment-Specific Persistence Screening (CSPS). Compared to the persistence screening under REACH, the CSPS is a more conservative approach that provides additional reasonable results, particularly for compounds that sorb to sediment and soil, and for which the current standard persistence screening might be insufficient. Thus, the CSPS can be used to identify potentially persistent and nonpersistent compounds in the regulatory context by a comprehensive assessment that includes water, soil, and sediment. Moreover, experimentally determined half-lives from the compartment-specific screening tools can be used as input for multimedia models that estimate, for example, overall persistence (P_ov ). The application of fixed half-life factors to extrapolate from water to soil and sediment, which is here demonstrated to be inappropriate, can thereby be avoided. Integr Environ Assess Manag 2019;00:000-000. © 2019 SETAC.

Environmentally Relevant Inoculum Concentrations Improve the Reliability of Persistent Assessments in Biodegradation Screening Tests

Standard OECD biodegradation screening tests (BSTs) have not evolved at the same rate as regulatory concerns, which now place an increased emphasis on environmental persistence. Consequently, many chemicals are falsely assigned as being potentially persistent based on results from BSTs. The present study increased test duration and increased inoculum concentrations to more environmentally relevant levels to assess their impact on biodegradation outcome and intratest replicate variability for chemicals with known environmental persistence. Chemicals were assigned to potential persistence categories based on existing degradation data. These more environmentally relevant BSTs (erBSTs) improved the reliability of persistence assignment by reducing the high variability associated with these tests and the occurrence of failures at low inoculum concentrations due to the exclusion of specific degraders. Environmental fate was determined using a reference set of ¹⁴C-labeled compounds with a range of potential environmental persistences, and full mass balance data were collated. The erBST correctly assigned five reference chemicals of known biodegradabilities to their appropriate persistence category in contrast to a standard OECD Ready Biodegradation Test (RBTs, P < 0.05). The erBST was significantly more reproducible than an OECD RBT (ANOVA, P < 0.05), with more consistent rates and extent of biodegradation observed in the erBST.

Pesticide behavior in modified water-sediment systems

The standardized laboratory water-sediment study in darkness is utilized as primary information on pesticide behavior to assess its ecotoxicological impacts in the edge-of-field water bodies. The half-lives of pesticide in water and sediment are key parameters to predict its environmental concentration, and its metabolic profiles help to avoid overlooking unexpected toxicological impacts from metabolites. However, no consideration of environmental factors such as sunlight and aquatic macrophytes is included, and this may lead to a conservative assessment. We review the experimental factors in the existing standardized design and then the effects of illumination and aquatic macrophytes introduced to the water-sediment system. The effects of temperature and the water-sediment ratio should be investigated in more detail and the pesticide behavior is possibly modified by illumination via photodegradation and/or metabolism in phototrophic microorganisms. Aquatic macrophytes play a major role as an additional sorption site and in further pesticide metabolism.

The Effects of Molecular Properties on Ready Biodegradation of Aromatic Compounds in the OECD 301B CO2 Evolution Test

Ready biodegradation is the primary biodegradability of a compound, which is used for discriminating whether a compound could be rapidly and readily biodegraded in the natural ecosystems in a short period and has been applied extensively in the environmental risk assessment of many chemicals. In this study, the effects of 24 molecular properties (including 2 physicochemical parameters, 10 geometrical parameters, 6 topological parameters, and 6 electronic parameters) on the ready biodegradation of 24 kinds of synthetic aromatic compounds were investigated using the OECD 301B CO2 Evolution test. The relationship between molecular properties and ready biodegradation of these aromatic compounds varied with molecular properties. A significant inverse correlation was found for the topological parameter TD, five geometrical parameters (Rad, CAA, CMA, CSEV, and N c), and the physicochemical parameter K ow, and a positive correlation for two topological parameters TC and TVC, whereas no significant correlation was observed for any of the electronic parameters. Based on the correlations between molecular properties and ready biodegradation of these aromatic compounds, the importance of molecular properties was demonstrated as follows: geometrical properties > topological properties > physicochemical properties > electronic properties. Our study first demonstrated the effects of molecular properties on ready biodegradation by a number of experiment data under the same experimental conditions, which should be taken into account to better guide the ready biodegradation tests and understand the mechanisms of the ready biodegradation of aromatic compounds.

Development and application of screening tools for biodegradation in water-sediment systems and soil

Two new screening-test systems for biodegradation in water-sediment systems (WSST; Water-Sediment Screening Tool) and soil (SST; Soil Screening Tool) were developed in analogy with the water-only test system OECD 301C (MITI-test). The test systems could be applied successfully to determine reproducible experimental mineralization rates and kinetics on the screening-test level for fifteen organic chemicals in water (MITI), water-sediment (WSST) and soil (SST). Substance-specific differences were observed for mineralization compared among the three test systems. Based on mineralization rate and mineralization half-life, the fifteen compounds could be grouped into four biodegradation categories: substances with high mineralization and a half-life <28 days in (1) all three test systems, (2) only in the MITI test and in the WSST, (3) only in the SST, and (4) none of the test systems. The observed differences between the MITI results and the WSST and SST biodegradation rates of the compounds do not reflect their (reversible) sorption into organic matter in terms of experimental K(oc) values and log D values for the relevant pH range. Regarding mineralization kinetics we recommend to determine the lag-phase, mineralization half-life and mineralization rate using a 5-parameter logistic regression for degradation curves with and without lag-phase. Experimental data obtained with the WSST and the SST could be verified by showing good agreement with biodegradation data from databases and literature for the majority of compounds tested. Thus, these new screening-tools for water-sediment and soil are considered suitable to determine sound and reliable quantitative mineralization data including mineralization kinetics in addition to the water-only ready biodegradability tests according to OECD 301.

Biodegradation screening of chemicals in an artificial matrix simulating the water-sediment interface

Biodegradation is the most important attenuation process for most of organic chemicals in the environment. This process decides whether the organic substance itself or its degradation products rests in the environment and should be considered for a further risk assessment. This work presents the development of a water sediment screening test, based on OECD guideline 308, with a high significance to environmental conditions and with a good reproducibility and consistency of results. The increased reproducibility was achieved by creating an artificial and standardized medium, based on the existing OECD guidelines OECD 302C, 301D and 218. Each test consisted of five different series: blank, quality control, test, toxicity control and abiotic control. Biodegradation was assessed by measurement of pressure difference in closed vessels using the OxiTop(®) system. Aniline, diethylene glycol and sodium acetate were used to optimize and validate test conditions. Additionally, two pharmaceuticals: Acetaminophen and ciprofloxacin (CIP) were tested as an example of possible test application. Acetaminophen was mainly removed from the system by biodegradation whereas CIP was removed from water phase by sorption onto sediment. Water sediment test proved to be a promising tool for the biodegradation investigation of chemicals in the water-sediment interface.

CORAL: the prediction of biodegradation of organic compounds with optimal SMILES-based descriptors

CORAL software (http:/www.insilico.eu/coral) has been used to build up quantitative structure-biodegradation relationships (QSPR). The normalized degradation percentage has been used as the measure of biodegradation (for diverse organic compounds, n=445). Six random splits into sub-training, calibration, and test sets were examined. For each split the QSPR one-variable linear regression model based on the SMILES-based optimal descriptors has been built up. The average values of numbers of compounds and the correlation coefficients (r2) between experimental and calculated biodegradability values of these six models for the test sets are n=88.2±11.7 and r2=0.728±0.05. These six models were further tested against a set of chemicals (n=285) for which only categorical values (biodegradable or not) were available. Thus we also evaluated the use of the model as a classifier. The average values of the sensitivity, specificity, and accuracy were 0.811±0.019, 0.795±0.024, and 0.803±0.008, respectively.