DEGRADATION OF PLASTICS IN THE ENVIRONMENT - Plastics and Environmental Sustainability - page 252

1. Monitoring the accumulation of biomass (Seal, 1994). ASTM (1996)
G22-76, ASTM (2009) G21-96.
2. Monitoring the depletion of substrates either by simple “weight loss”
(Tsuji and Suzuyoshi, 2002). ASTM D6003-96 or D5247-92).
Alternatively, in oxidative biodegradation, oxygen demand can be used
as a measure (e.g., D5271-02).
3. Monitoring carbon dioxide (or methane in case of anaerobic
breakdown) evolution (e.g., D5209-92, D5210-92, D5338-92, D6691-01,
D6692-01, D6340-98).
4. Monitoring the changes in substrate properties. Changes in mechanical
or other properties of degrading plastics substrate (as in D5247-92),
which is an indirect measure of the degradation of the substrate.
Ofthese,onlythethirdapproachorrespirometryallowsaconvenientmeans
of establishing the degree of carbon conversion or mineralization. In simple
respirometry experiments, finely divided plastic materials are mixed in with
moist garden soil (~60% humidity) fortified with N and P salts and
incubated along with activated sewage sludge inoculum. The outcome of
the test is therefore to some extent dependent on variability of the biotic
composition of the inoculum. Biodegradation of the polymer and other
available soil biomass releases CO 2 . Some experimental arrangement to
sorb the CO 2 liberated in alkali for subsequent titrimetric determination is
employed. For instance, an air stream passed over the soil sample might
be bubbled into one or more sorption flasks carrying dilute alkali (see Fig.
6.13 ) . The alkali is periodically titrated to calculate the CO 2 evolved by
the biodegradation process, and fresh aliquot of alkali is substituted for it.
Continuousmeasurementoftheevolvedgasbyautomatedtitrimetry(Pagga
et al., 2001) constitutes significant recent improvements in the technique.
Alternatively infrared spectroscopic analysis (Calmon et al., 2000) or
automated gas chromatographic detection of the CO 2 might be used. A
control flask with the same amount of soil but without any polymer
substrate is used to establish background levels of CO 2 evolution.
SubtractingthebackgroundlevelfromthedataallowsthenetCO 2 evolution
attributable to the polymer to be determined. This is generally plotted as a
percentage of carbon converted versus the duration of exposure (Andrady
and Song, 1999; Narayan, 2011).