ASTM D7991-22

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Designation: D7991 − 15 D7991 − 22
Standard Test Method for
Determining Aerobic Biodegradation of Plastics Buried in
Sandy Marine Sediment under Controlled Laboratory
Conditions
This standard is issued under the fixed designation D7991; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 This test method determines the biodegradation level of plastic materials exposed to laboratory conditions that simulate the
environment found in the sandy tidal zone.
1.2 The tidal zone, that is, the part of the coast affected by the tides and movement of the waves, is the borderline between sea
and land, frequently a sandy area that is kept constantly damp by the lapping of the waves. Stony and rocky shorelines also exist.
1.3 Plastic marine debris is frequently washed up in this habitat where it must be removed in order to restore the original
landscape.
1.4 It is of interest to know the biodegradation behavior of plastics when exposed to conditions simulating this habitat, because
this information can help in predicting the time needed for the biodegradation of the litter.
1.5 Biodegradation is determined by measuring the CO evolved by the plastic material when exposed to a sediment kept wet with
salt-water in a reactor, to simulate the tidal zone.
1.6 Marine fresh-water habitats (for example, those found in brackish waters and estuaries) are not considered by this standard.
1.7 Reports shall clearly state the percentage of net CO generation for both the test and reference samples at the completion of
the test. Furthermore, in the laboratory reports, the results shall not be extrapolated beyond the actual duration of the test.
NOTE 1—There is no known ISO equivalent to this standard.
1.8 Units—The values stated in SI units are to be regarded as standard. No other units of measurement are included in this
standard.
1.9 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use.
This test method is under the jurisdiction of ASTM Committee D20 on Plastics and is the direct responsibility of Subcommittee D20.96 on Environmentally Degradable
Plastics and Biobased Products.
Current edition approved Sept. 1, 2015May 1, 2022. Published September 2015May 2022. Originally approved in 2015. Last previous edition approved in 2015 as
D7991 - 15. DOI: 10.1520/D7991–15.10.1520/D7991–22.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D7991 − 22
1.10 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
D5988 Test Method for Determining Aerobic Biodegradation of Plastic Materials in Soil
2.2 ISO Standards:
ISO 8245 Water quality Guidelines for the determination of total organic carbon (TOC) and dissolved organic carbon (DOC)
3. Terminology
3.1 Definitions:
3.1.1 tidal zone, n—the part of the marine environment that extends from the high tide line, which is rarely inundated with water,
to the low tide line, which is typically always covered with water.
3.1.1.1 Discussion—
Synonyms are: eulittoral zone, midlittoral zone, mediolittoral zone, intertidal zone, foreshore.
4. Summary of Test Method
4.1 This test method consists of the following:
4.1.1 Selection of plastic material for the determination of aerobic biodegradation in a controlled laboratory system.
4.1.2 Obtaining sediment and seawater from the shoreline.
4.1.3 Exposing the plastic material to the wet sediment under controlled conditions.
4.1.4 Measuring CO evolved as a function of time.
4.1.5 Assessing the degree of biodegradation by determining the percentage of organic carbon in the plastic material that is
converted to CO during the duration of the test. This percentage represents the percentage of mineralization and will not include
the amount of carbon converted to cell biomass that is not in turn metabolized to CO during the course of the test.
4.1.6 Estimating the qualitative disintegration of the test material by visual inspection at the end of the test.
5. Significance and Use
5.1 Plastic is sometimes carried by rivers or accidentally discharged by ships into the sea; this plastic can then reach different parts
of the marine environment. Tides and waves also frequently deliver plastic marine debris into the sandy tidal zones.
5.2 This test method simulates the environmental conditions found in the tidal zone. Plastic debris that reaches the sandy tidal zone
can settle there and become partially or totally buried by sand and kept wet by waves or tides. It is of interest to assess the
biodegradation behavior of plastic materials under these conditions to predict the removal time of this waste in the environment.
5.3 This test method is applied to determine the extent of biodegradation of a plastic exposed in the laboratory to a sandy sediment
kept wet with seawater. Both sediment and seawater are collected from a sandy beach in the tidal zone. If the natural microbial
population present in the sediment is able to biodegrade the plastic, there will be an evolution of CO as a consequence of the
aerobic microbial respiration. The level of biodegradation at any given time is the ratio between the cumulative amount of the
evolved net carbon dioxide and the theoretical amount produced in the case of total conversion of the organic carbon present in
the plastic into carbon dioxide.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Available from American National Standards Institute (ANSI), 25 W. 43rd St., 4th Floor, New York, NY 10036, http://www.ansi.org.
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5.4 This test method does not measure the amount of organic carbon that is converted into biomass, but only the biodegradation
that leads to mineralization (that is, the formation of CO ).
6. Apparatus
6.1 Reactor—Glass vessel approximately 2 to 4-L internal volume that can be sealed air-tight, such as 150-mm desiccators, with
an airtight opening, large enough to allow the handling of the content. Biometer flasks are also appropriate. A suitable apparatus
is shown in Figure 1 in Test Method D5988. Reactors with higher volumes can be used, if environmental conditions are not
affected.
6.2 Container for the CO Absorber—A glass beaker to be located in the headspace of the reactor and filled with 100 ml of
Ba(OH) 0.025 N or with 30 mL of KOH 0.5 N.
6.3 Darkened Chamber or Cabinet, in which the temperature can be maintained at a constant level within a 62°C range.
NOTE 2—Incubator with either built in lights that can be programmed or else plug in lights that can be operated with a timer power strip can be used to
better simulate the environment. The lighting in that case need to be 12:12 day/night. Details on the lighting regime, light intensity, wave length, incubator
type, etc. shall be provided in the report.
6.4 Analytical Balance, to weigh the test specimen.
6.5 Technical Balance, to weigh reactors and sediment.
6.6 pH Meter.
7. Reagents and Materials
7.1 Purity of Reagents—Reagent grade chemicals shall be used in all tests. Unless otherwise indicated, it is intended that all
reagents conform to the specifications of the Committee on Analytical Reagents of the American Chemical Society where such
specifications are available. It is acceptable to use other grades provided it is first ascertained that the reagent is of sufficiently
high purity to permit its use without lessening the accuracy of the determination.
7.2 Barium Hydroxide Solution (0.025 N), prepared by dissolving 4.0 g anhydrous Ba(OH) /L in distilled water. Filter free of solid
material and store sealed as a clear solution to prevent absorption of CO from the air. It is recommended that 2 to 4 L be prepared
at a time when running a series of tests. Confirm normality by titration with standard acid before use. When using Ba(OH) ,
however, care must be taken that a film of BaCO does not form on the surface of the solution in the beaker, which would inhibit
CO diffusion into the absorbing medium. Alternatively, potassium hydroxide solution (KOH, 0.5 N) could be used and is prepared
by dissolving 28 g of anhydrous KOH/L in distilled water and proceeding in the same way as for the Ba(OH) solution.
7.3 Hydrochloric acid, 0.05 N HCl when using 0.025 N Ba(OH) or 0.3 N HCl when using 0.5 N KOH.
7.4 Sediment—Collect seawater and sediment samples from the shoreline of a sandy beach, where the sediment is submerged in
the shallow water. Collect top sediment (the layer from surface till about 20 cm depth). It is important to obtain sediment from
multiple samples from the same location (at least 3). Collect the seawater with a bucket and then collect sediment samples with
a shovel in separate containers overlain with water, then transfer all samples to a watertight container and quickly deliver it to the
laboratory. Remove any obvious plant material, shells, pieces of driftwood, petroleum tar, and other large material. Store the
sediment and seawater at approximately 4°C until use. Allow air exchange at time to avoid anaerobiosis. Use preferably within
four weeks of sampling. Report the storage times. Before use, perform gravity filtration on the sediment in a funnel with a coarse
filter paper to remove excess water. Sediment is ready for testing when seawater is no longer recovered from the filtration. Nitrogen
sources (such as NH Cl or NaNO ) can be added to the sediment if this is considered as a factor limiting biodegradation. These
4 3
additions shall be reported in the test report.
Reagent Chemicals, American Chemical Society Specifications, American Chemical Society, Washington, DC. For suggestions on the testing of reagents not listed by
the American Chemical Society, see Analar Standards for Laboratory Chemicals, BDH Ltd., Poole, Dorset, U.K., and the United States Pharmacopeia and National Formulary,
U.S. Pharmacopeial Convention, Inc. (USPC), Rockville, MD.
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NOTE 3—No data are available at this stage indicating that a specific nitrogen level is beneficial for the biodegradation process.
7.5 Plastic Material—Determine the total organic carbon both of the test material and the reference material using ISO 8245 and
report it, preferably, as grams of total organic carbon per gram of total dry solids. Alternatively, provided the materials do not
contain inorganic carbon, it is possible to determine the carbon content by elemental analysis. The test material shall have sufficient
organic carbon to yield CO in an amount suitable for the determination.
7.6 Reference Material—A cellulose filter paper for laboratory purposes. Determine the carbon content as described in 7.5.
7.7 Negative Control Material (optional)—A polyethylene film. Determine the carbon content as described in 7.5.
7.8 Test Samples—It is preferable that the plastic material is in the form of film or plate. Cut out square-shaped samples with a
dimension of approximately 5 cm. Likewise prepare square-shaped samples of reference material and negative control material.
Record the mass of each sample.
NOTE 4—It is acceptable that the test material be introduced as powder. Mix the powder homogeneously with the sediment. Refer to ISO 10210 for
preparation of powder from plastic materials. Furthermore, report data showing that milling has not changed the chemical structure of the test material.
NOTE 5—It is acceptable that the test material be introduced as a perforated film or plate in order to facilitate
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