Biomedical Engineering Reference
In-Depth Information
11.5.4
Gas (CO
2
or CH
4
) Evolution Tests
11.5.4.1
Principle
The evolution of carbon dioxide or methane from a substrate represents a direct
parameter for mineralization. Therefore, gas evolution tests can be important tools
in the determination of biodegradability of polymeric materials. A number of well-
known test methods have been standardized for aerobic biodegradation, such as
the (modifi ed) Sturm test [70-75] and the laboratory-controlled composting test
[76-79], as well as for anaerobic biodegradation, such as the anaerobic sludge test
[80, 81] and the anaerobic digestion test [82, 83]. Although the principles of these
test methods are the same, they may differ in medium composition, inoculum, the
way substrates are introduced, and in the technique for measuring gas evolution.
11.5.4.2
Applications
Anaerobic tests generally follow biodegradation by measuring the increase in pres-
sure and/or volume due to gas evolution, usually in combination with gas chro-
matographic analysis of the gas phase [84, 85]. Most aerobic standard tests apply
continuous aeration; the exit stream of air can be directly analyzed continuously
using a carbon dioxide monitor (usually infrared detectors) or titrimetrically after
sorption in dilute alkali. The cumulative amount of carbon dioxide generated,
expressed as a percentage of the theoretically expected value for total conversion
to CO
2
, is a measure for the extent of mineralization achieved. A value of 60%
carbon conversion to CO
2
, achieved within 28 days, is generally taken to indicate
ready degradability. Taking into account that in this system there will also be
incorporation of carbon into the formation of biomass (growth), the 60% value for
CO
2
implies almost complete degradation. While this criterion is meant for water-
soluble substrates, it is probably applicable to very fi nely divided moderately degra-
dable polymeric materials as well [13]. Nevertheless, most standards for determining
biodegradability of plastics consider a maximum test duration of 6 months.
Besides the continuously aerated systems, described above, several static
respirometers have been described. Bartha and Yabannavar [86] describe a two-
fl ask system; one fl ask, containing a mixture of soil and the substrate, is connected
to another chamber holding a quantity of carbon dioxide sorbant. Care must be
taken to ensure that enough oxygen is available in the fl ask for biodegradation.
Nevertheless, this experimental setup and modifi ed versions thereof have been
successfully applied in the assessment of biodegradability of polymer fi lms and
food packaging materials [87 - 89] .
The percentage of carbon converted to biomass instead of carbon dioxide
depends on the type of polymer and the phase of degradation. Therefore, it has
been suggested to regard the complete carbon balance to determine the degree of
degradation [90]. This implies that besides the detection of gaseous carbon, also
the amount of carbon in soluble and solid products needs to be determined.
Soluble products, oligomers of different molecular size, intermediates, and pro-
teins secreted from microbial cells can be measured as COD or as DOC. Solid
