Plastic Seawater Degradation Testing: Standards, Methods, and Evaluation of Marine Biodegradability

Plastic seawater degradation testing is a laboratory-based evaluation that measures how plastic materials biodegrade, disintegrate, and lose physical integrity when exposed to seawater and marine sediment environments. Conducted in accordance with internationally recognized standards such as ISO 19679, ISO 18830, ISO 22403, ASTM D6691, and OECD 306, this testing provides scientifically defensible data for product development, certification, and environmental claims. This article explains the technical principles, sample requirements, test items, methods, and typical applications of seawater degradation testing for plastics.

Technical Overview

Plastic seawater degradation testing assesses the inherent biodegradability of plastic materials under conditions that simulate the marine environment, including natural seawater, seawater/sediment interfaces, and sandy marine sediments. Marine pollution caused by persistent plastics has driven significant demand for materials that can demonstrably biodegrade if they enter ocean ecosystems, and seawater degradation testing provides the objective evidence needed to support such claims.

Biodegradation in seawater is a microbial process. Microorganisms naturally present in seawater colonize the plastic surface, form a biofilm, and secrete enzymes that break the polymer chains into smaller molecules. These molecules are ultimately mineralized into carbon dioxide, water, and new biomass under aerobic conditions. Because marine environments are typically colder, nutrient-poor, and microbially less dense than composting environments, biodegradation rates in seawater are significantly slower, which is why dedicated test standards and longer test durations are required.

Seawater degradation testing distinguishes between two related but different phenomena: biodegradation, which is the biochemical conversion of organic carbon into carbon dioxide and biomass, and disintegration, which is the physical breakdown of a material into fragments. A credible marine biodegradability assessment must quantify true biodegradation, typically by measuring evolved carbon dioxide or oxygen consumption, rather than relying solely on visual fragmentation or mass loss.

The principal standards governing this field are published by ISO and ASTM International:

• ISO 19679 — Determination of aerobic biodegradation of non-floating plastic materials in a seawater/sediment interface, by analysis of evolved carbon dioxide
• ISO 18830 — Determination of aerobic biodegradation of non-floating plastic materials in a seawater/sediment interface, by measuring oxygen demand in a closed respirometer
• ISO 22403 — Assessment of the intrinsic biodegradability of plastics in an aqueous medium under mesophilic aerobic conditions
• ISO 22404 — Determination of the degree of disintegration of plastic materials in marine habitats under real field conditions
• ASTM D6691 — Aerobic biodegradation of plastic materials in the marine environment by a defined microbial consortium or natural seawater inoculum
• ASTM D7473 — Weight attrition of plastic materials in the marine environment by open system aquarium incubations
• ASTM D7991 — Aerobic biodegradation of plastics buried in sandy marine sediment under controlled laboratory conditions
• OECD 306 — Biodegradability in seawater (screening test)

Testing Samples

Biodegradable Polymer Resins and Compounds

Raw polymer materials submitted for seawater degradation testing typically include polyhydroxyalkanoates (PHA), polybutylene succinate (PBS), polybutylene adipate terephthalate (PBAT), polylactic acid (PLA) and its blends, starch-based blends, and modified cellulose or protein-based polymers. For intrinsic biodegradability assessment, resins are usually cryogenically ground into powder with a particle size below 250 micrometers to maximize surface area and ensure consistent microbial access.

Finished Products and Articles

End products such as films, carrier bags, food packaging, nonwoven fabrics, agricultural films, fishing nets and lines, ropes, aquaculture gear, straws, cutlery, coatings, and injection-molded parts can also be tested. Samples are typically cut into defined dimensions with a known surface area, dried to constant mass, and characterized for initial thickness, density, and molecular weight before exposure.

Reference and Control Materials

Every valid test series includes controls required by the standards:

• A positive reference, normally cellulose powder or thin cellulose film, which must reach a defined minimum biodegradation level to demonstrate that the inoculated seawater is biologically active
• A negative reference, normally a non-biodegradable plastic such as polyethylene, which should show negligible biodegradation
• A blank vessel containing only seawater and inoculum, used to correct for background carbon dioxide evolution

Testing Items

Depending on the selected standard and the purpose of the study, seawater degradation testing may include the following evaluation items:

• Degree of aerobic biodegradation, expressed as a percentage of theoretical carbon dioxide evolution, calculated from the carbon content of the test material
• Biochemical oxygen demand measured in a closed respirometer, expressed as a percentage of theoretical oxygen demand
• Degree of disintegration, expressed as percentage mass loss and residual fragment size distribution
• Changes in number-average and weight-average molecular weight determined by gel permeation chromatography (GPC)
• Retention of tensile strength and elongation at break after defined exposure periods
• Surface morphology changes observed by scanning electron microscopy (SEM), including biofilm formation and erosion patterns
• Chemical structural changes analyzed by Fourier-transform infrared spectroscopy (FTIR), such as carbonyl index increase
• pH, salinity, dissolved organic carbon, and nitrogen monitoring of the test medium to verify environmental conditions
• Optional ecotoxicity screening of degradation intermediates on marine organisms

Testing Methods

ISO 19679: Seawater/Sediment Interface by CO2 Evolution

Test specimens are placed on top of a layer of marine sediment in a test vessel filled with natural seawater. The system is aerated continuously, and the exhaust gas is passed through alkaline absorption traps or directed to an infrared CO2 analyzer. Evolved carbon dioxide is measured periodically over a test duration that may extend up to 24 months for slowly degrading materials. The cumulative CO2 is compared against the theoretical amount calculated from the total organic carbon of the sample, yielding the biodegradation percentage.

ISO 18830: Seawater/Sediment Interface by Oxygen Demand

This method uses the same seawater/sediment interface configuration but is performed in a closed respirometer. The oxygen consumed by microorganisms degrading the plastic is measured directly, either manometrically or electrochemically, and converted to a biodegradation percentage relative to the theoretical oxygen demand. This approach is particularly suitable for materials and configurations where continuous aeration and CO2 trapping are impractical.

ASTM D6691: Natural Seawater Inoculum

Test materials, usually as powders or films, are incubated in a mineral medium inoculated with natural seawater or a defined microbial consortium at a controlled temperature of approximately 30 degrees Celsius with agitation. Biodegradation is followed by carbon dioxide measurement. The method is widely used as a relatively rapid screening test for marine biodegradability of polymer formulations and is frequently incorporated into marine biodegradability certification schemes.

ASTM D7991: Biodegradation in Sandy Marine Sediment

Specimens are buried in sandy marine sediment saturated with seawater under controlled laboratory conditions. This method evaluates materials designed to degrade after settling to the seafloor, such as certain fishing gear components, by measuring evolved carbon dioxide over an extended exposure period.

ASTM D7473 and ISO 22404: Disintegration Assessment

ASTM D7473 uses open aquarium incubations to measure weight attrition of specimens floating or suspended in circulating seawater, while ISO 22404 assesses disintegration under real field conditions in marine habitats. These methods quantify physical breakdown and mass loss rather than mineralization and are typically combined with biodegradation tests to build a complete degradation profile.

Validity Criteria and Acceptance Thresholds

For a test to be considered valid, the cellulose positive control should generally achieve at least 60 to 70 percent biodegradation within the reference period, the blank control must remain low and stable, and replicate scatter must fall within the limits specified by the standard. For marine biodegradability claims used in certification and labeling, a common acceptance benchmark is a biodegradation level of at least 90 percent, or a level comparable to the cellulose reference, achieved within approximately six months under the specified mesophilic conditions.

Applications

Seawater degradation testing serves a wide range of industrial, regulatory, and scientific purposes:

• Research and development of inherently marine-degradable polymers, additives, and formulations, with test data guiding molecular design and grade selection
• Substantiation of marketing claims such as marine biodegradable or ocean-degradable under green claims regulations in major markets
• Support for certification and eco-labeling programs that require ISO and ASTM marine biodegradation data generated by accredited third-party laboratories
• Verification of fishing and aquaculture products, including nets, lines, traps, and gear coatings, where lost equipment inevitably enters the marine environment
• Regulatory compliance support for single-use plastic restrictions, extended producer responsibility schemes, and government procurement requirements
• Comparative benchmarking of competing materials, life cycle assessment input data, and academic research on polymer fate in marine ecosystems

FAQ

What is the difference between biodegradation and disintegration in seawater testing? Biodegradation measures the microbial conversion of polymer carbon into carbon dioxide and biomass, proven by respirometric data. Disintegration only measures physical breakdown into fragments. A material can disintegrate without truly biodegrading, so fragmentation alone does not demonstrate environmental safety.

How long does a seawater degradation test take? Screening tests such as ASTM D6691 typically run for up to six months, while interface tests such as ISO 19679 and ISO 18830 may run from six months up to 24 months, depending on the degradation rate of the material and the plateau of the biodegradation curve.

What result is required to claim a plastic is marine biodegradable? Certification-oriented schemes generally require at least 90 percent biodegradation, or equivalence to the cellulose reference, within about six months, together with valid positive, negative, and blank controls. Exact thresholds depend on the applicable scheme and jurisdiction.

Does passing the test mean the plastic can be discarded into the sea? No. A positive result demonstrates intrinsic biodegradability under controlled marine-simulated conditions. It does not justify littering or the intentional release of plastic waste into any natural environment.

What sample forms can be tested? Powders, granules, films, sheets, fibers, fabrics, and molded articles can all be evaluated. Sample geometry, thickness, and surface area strongly influence degradation rates and must be documented in the test report.

Why is natural seawater used as the inoculum? Natural seawater contains the authentic microbial communities responsible for polymer degradation in the ocean, making the test environmentally representative, whereas defined consortia offer better reproducibility for comparative screening studies.