Biodegradation of Surfactants in Seawater

Abstract

Biodegradation is the process in which biological activity breaks down organic compounds (e.g., water, CO₂, and biomass). Therefore, biodegradability can be a desirable property when considering the scientific and regulatory environmental fate of ingredients. “Biodegradation of surfactants in seawater” is a scientifically published study commissioned by ERASM to describe the extend and kinetics of biodegradation of 18 surfactants in seawater. The surfactants tested included polymeric and non-polymeric compounds from a diverse chemistry array (see Appendix for details). The sorption of two surfactants, an alcohol ethoxylate (AE) and a quaternary ammonium compound (QAC) to marine particles (algae and clay particles), and primary biodegradation of two non-ionic AE were also investigated. This study demonstrated extensive seawater biodegradation across various surfactant chemistries, and that the presence of Suspended Particulate Matter (SPMs) did not hamper the primary biodegradation of the tested surfactants.

Background

Although surfactants are extensively removed in wastewater treatment plant processes with 90-99% efficiencies, small fractions of surfactants may be released and eventually end up in the marine environment. Existing biodegradation data are mainly available from freshwater environments (e.g., from lab studies conducted in accordance with OECD 301/310 test guidelines or equivalent). Thus, there was a lack of systematic biodegradation information across various surfactant types in seawater up until the execution of this project.

Surfactants can be classified into four distinct classes, depending on the properties of the hydrophilic head:

1. Amphoteric surfactants (g., betaines), which can be used as bactericides, corrosion inhibitors, in fabric softeners, in cosmetics, and in detergent formulations.
2. Negatively charged anionic surfactants (alkyl sulphates, alkylether sulphates, alkylether carboxylates), often used in detergent production (for instance in household and laundry formulas, hand dishwashing liquids and shampoos).
3. Uncharged non-ionic surfactants (g., fatty acids, alcohol ethoxylates), mainly used in detergents, emulsifiers, wetting and dispersing agents, as industrial cleaners, in textiles, and in pulp and paper processes.
4. Positively charged cationic surfactants (g., QACs), often used as biocides, softeners, in metal industry as corrosion inhibitors, in pigments as dispersants.

EU legislation, namely the REACH regulation (Registration, Evaluation, Authorisation and Restriction of Chemicals – Regulation (EC) No 1907/2006), requires the inclusion of biodegradation tests to determine the environmental fate and persistence (P) properties of chemicals. Screening tests for ready biodegradability in freshwater (FW) and seawater (SW) are used to determine if chemicals are readily biodegradable in FW, or ‘likely to fulfil the criteria for ready biodegradability in SW’. In combination with other biodegradation tests, these screening tests are also important in the assessment of persistence, according to EU REACH criteria for Persistent Bioaccumulative and Toxic (PBT) and very persistent very bioaccumulative (vPvB) assessment.

Outcome

The biodegradability screening testing in SW was performed according to the OECD 306 Guideline and showed that 12 of the 18 surfactants tested reached >60% ultimate biodegradation (complete mineralization) after 28 days of incubation, and these were therefore considered to fulfil the criteria for ready biodegradability in SW. One surfactant reached >60% ultimate biodegradation at day 75. For the other 5 substances, >20% ultimate biodegradation was determined after 28 days, which is indicative of a potential for primary biodegradation in the marine environment.

Tab. 1: No. surfactants that achieved ultimate biodegradation >60% after 28 days, >60% between 28 and 75 days, and between 20% and 60% after 28 days.

Comparison of the data between SW and FW screening tests, revealed generally slower biodegradation in SW than FW tests, assumed to be attributed to lower bacterial concentrations.

Marine particles may form sinking aggregates, and surfactant attachment to particles may therefore be relevant for the marine fate of the surfactants. Incubation of low concentrations of two surfactants, a non-ionic AE and a cationic QAC in SW with marine algae (Skeletonema pseudocostatum) and clay particles (kaolin) showed rapid primary biodegradation of both surfactants. The surfactants were measured primarily in a fraction <20μm, not in collected particles/aggregates >20μm, when compared on volumetric basis.

Primary biodegradation studies of two polymeric AEs with different distributions of ethylene oxide (EO) groups (respectively with a mean of 9 or 50 mol EO) showed different biodegradation kinetics in SW. Very fast depletion of the lower molecular weight AE polymeric surfactant with mean average of 9 EO was observed, with corresponding occurrences of PEG degradation products. However, the higher molecular weight AE polymeric surfactant with mean average of 50 mol EO was degraded much slower in SW, without PEGs significantly measured. The sizes and bioavailability of polymeric surfactants may therefore be of importance for surfactant biodegradation rates in SW.

In summary, this study indicates that most tested surfactants presented high biodegradability rates in seawater. They will likely degrade quickly if entering the marine compartment via sewage runoffs or direct release. The main findings of this study and further details have been published in the peer reviewed scientific journal Environmental Toxicology and Chemistry (Brakstad et al 2023¹) which provides a more comprehensive technical overview of the study methods and results.

Appendix

Overview of surfactants included in the present study.

1 Brakstad, O.G., Sarno, A., Geerts, R., Dawick, J., Machado, A. and Hopp, P. (2023), Ultimate and Primary Biodegradation of a Range of Nonpolymeric and Polymeric Surfactants in Seawater. Environ Toxicol Chem. https://doi.org/10.1002/etc.5632