Aerobic biodegradation of plastic materials in marine environment – ASTM D6691
The standard ASTM D6691 is a test method carried out under controlled laboratory conditions, to characterise the polymer material that are potentially biodegradable, and compare to a positive reference material (i.e. cellulose film), in an aerobic marine environment, to determine the rate of biodegradation.
The determination of biodegradability is paramount for the evaluation of the ecological behaviour of materials. Biodegradability has a key parameter to ensure the degradable substance will cause no long-term risk in the environment. In the world we currently live in, plastic materials cannot always be recycled due to pollution and plastic waste. Plastic waste is a problem with ships at sea, where plastic materials are becoming more commonplace on ships, as well as the material being disposed of at sea. As biodegradation processes may occur in a variety of natural environments, manufacturers are developing plastics that biodegrade in natural conditions, such as a marine environment.
This test may involve natural sea water with the population (required microorganisms) already present or a solution with at least ten populations of pre-grown aerobic marine microorganisms. These pre-grown marine microorganisms are of known genera or are indigenous population existing in natural sea water. The standard ASTM D6691 is applicable to any polymer materials that contain a minimum of 20% carbon content that are not inhibitory to the microorganisms already present in the marine environment. The acceptable forms of test specimen include, powders, films, pieces, fragments, formed articles, or aqueous solutions. This is to optimise the exposure of the test specimen to the marine solution.
How to test for biodegradability of plastics in marine environments?
A test sample will be tested for characterisation of carbon content and molecular weight prior to testing. Preparing the marine environment for the test by ensuring that it has the correct microorganisms present and then the polymer sample will be submerged in the inoculum (marine solution) in a Biochemical Oxygen Demand (BOD) system. The BOD system comprises test bottles and CO2 sensor and together creates a closed system. The microorganism in the marine solution will slowly consume the polymer sample and simultaneously develop carbon dioxide. As a result, the it creates a pressure drop in the BOD system. The measuring principle of the BOD is to detect such pressure drop and the BOD sensor and shown in the directly in the display as a BOD value in mg/l O2.
In accordance with ASTM D6691, the biodegradability of the sample material will be determined by the released CO2 (biogas) measured, which is measured through the use of a respirometer. The biogas is the conversion of our polymer to CO2, as noted the percentage of conversion. The measured biogas produced is used to determine biodegradability, by comparison to a known reference.
It should be noted that a typical respirometer type system can run between 10-90 days. Nevertheless, if the specimens are extremely biodegradable, then it is acceptable for the experiment to be terminated when cumulative CO2 production plateaus. At the same time, the experiment can be extended if no biodegradation has occurred in a specific timeframe.
