Biomedical Engineering Reference
In-Depth Information
products, biomass, and polymer remnants require a combination of procedures
to separate and detect different fractions. The protein content of the insoluble
fraction is usually determined to estimate the amount of carbon converted to
biomass, using the assumptions that dry biomass consists of 50% protein, and
that the carbon content of dry biomass is 50% [90-92].
11.5.4.3
Suitability
Gas evolution tests are popular test methods because they are relatively simple to
perform and sensitive. A direct measure for mineralization is determined, and
water-soluble or -insoluble polymers can be tested as fi lms, powders, or objects.
Furthermore, the test conditions and inoculum can be adjusted to fi t the applica-
tion or environment in which biodegradation should take place. Aquatic synthetic
media are usually used, but also natural sea water [93, 94] or soil samples [86, 88,
89, 95] can be applied as biodegradation environments. A prerequisite for these
media is that the background CO
2
evolution is limited, which excludes the applica-
tion of real composting conditions. Biodegradation under composting conditions
is therefore measured using an inoculum derived from matured compost with low
respiration activity [76 - 78, 96, 97] .
A drawback of using complex degradation environments such as mature
compost is that simultaneous characterization of intermediate degradation prod-
ucts of determination of the carbon balance is diffi cult due to the presence of a
great number of interfering compounds. To overcome this, an alternative test was
developed based on an inoculated mineral bed-based matrix [98, 99].
11.5.5
Radioactively Labeled Polymers
11.5.5.1
Principle and Applications
Some materials tend to degrade very slowly under stringent test conditions without
an additional source of carbon. However, if readily available sources of carbon are
added, it becomes impossible to tell how much of the evolved carbon dioxide can
be attributed to the decomposition of the plastic. The incorporation of radioactive
14
C in synthetic polymers gives a means of distinguishing between CO
2
or CH
4
produced by the metabolism of the polymer, and that generated by other carbon
sources in the test environment. By comparison of the amount of radioactive
14
CO
2
or
14
CH
4
with the original radioactivity of the labeled polymer, it is possible to deter-
mine the percent by weight of carbon in the polymer which was mineralized during
the duration of the exposure [51, 100-102]. Collection of radioactively labeled gasses
or low-molecular-weight products can also provide extremely sensitive and repro-
ducible methods to assess the degradation of polymers with low susceptibility to
enzymes, such as polyethylene [8, 103] and cellulose acetates [104, 105] .
11.5.5.2
Drawbacks
Problems with handling the radioactively labeled materials and their disposal are
issues on the down side to this method. In addition, in some cases, it is diffi cult
