often been found biphasic due to nutrient and carbon limitations, typically with faster
initial phases followed by slower declines. When switching to field studies,
14 C-labelled compounds are not allowed, and additional dissipation processes of
compounds can occur simultaneously including leaching to deeper soil layers with
irrigation water and photodegradation at the surface of soil due to an upward water
flow movement by capillarity [76]. In field studies, the loss in microbial activities
due to nutrient limitation is not an issue. However, biphasic degradation kinetics are
still often observed. This is due to the decrease in compound bioavailability as the
result of compound concentration decrease due to their biodegradation or plant
uptake [77]. Biphasic degradation may also be found with chiral compounds because
the individual stereoisomers are often degraded at different rates. If the degradation
rates of two enantiomers in a racemic mixture are very different, the overall decline
of the compound, when determined with a non-enantioselective analytical method,
will be biphasic [78]. Consequently, the behavior and fate of PhACs in soil are very
complex due to interconnected processes. It is often impossible to discriminate
between abiotic and biotic processes, so the development of an in situ molecular
marker of biodegradation would be desirable. This will be relevant because biodegradation is probably the most important dissipation pathway, which can lead to
PhAC elimination.
In order to develop a molecular marker of biodegradation, several approaches
have been considered. Isotopic fractionation (i.e., compound stable isotope analysis
(CSIA)) is considered the best molecular marker of biodegradation. Stable isotope
fractionation relies on the observation of ratio shifts of stable isotope mainly C, H,
and N, prompted by the breaking or generation of chemical bonds during chemical
transformations [79]. The mathematical description of the relationship between the
extent of degradation and isotopic composition of a targeted compound can be
expressed by the Rayleigh equation (see Fig. 1a).
E t and E 0 represent the initial and conversion-dependent isotope ratio, Conc t /
Conc 0 is the residual fraction of the contaminant, and ε represents the isotope
enrichment factor. ε can be obtained as the slope of the linear regression line of
the natural log of the isotopic enrichment against the natural log of the extent of the
degradation. It is usually expressed in per mill unit, requires very accurate measurements to obtain, and is very specific to a biotransformation reaction. This approach
has already been applied to quantify in situ contaminant biodegradation and to
investigate mechanisms of biodegradation [80]. However, CSIA require specific
instrumentations (e.g., GC-IRMS), are still difficult to achieve in a routine way, and
are very often limited to GC amenable compounds. Limits of detection (LODs) are
not compatible with the occurrence and levels of compounds in soil (i.e., ng g
À1
level), and analyte enrichment and purification steps are needed during which
isotopic fractionation might be induced.
In this context, a novel approach applied to investigate biodegradation of PhACs
in environmental compartments is enantiomeric fractionation, which measures the
enantiomeric ratio (ER). Enantioselective process occurs when one enantiomer of a
chiral compound is favored over the other during biotransformation. This approach
makes sense because more than 50% of PhACs are actually commercialized as
racemic mixtures, which are mixtures of two enantiomers at equal concentrations.
Soil Sorption and Degradation Studies of Pharmaceutical Compounds Present in. . .
161
initial phases followed by slower declines. When switching to field studies,
14 C-labelled compounds are not allowed, and additional dissipation processes of
compounds can occur simultaneously including leaching to deeper soil layers with
irrigation water and photodegradation at the surface of soil due to an upward water
flow movement by capillarity [76]. In field studies, the loss in microbial activities
due to nutrient limitation is not an issue. However, biphasic degradation kinetics are
still often observed. This is due to the decrease in compound bioavailability as the
result of compound concentration decrease due to their biodegradation or plant
uptake [77]. Biphasic degradation may also be found with chiral compounds because
the individual stereoisomers are often degraded at different rates. If the degradation
rates of two enantiomers in a racemic mixture are very different, the overall decline
of the compound, when determined with a non-enantioselective analytical method,
will be biphasic [78]. Consequently, the behavior and fate of PhACs in soil are very
complex due to interconnected processes. It is often impossible to discriminate
between abiotic and biotic processes, so the development of an in situ molecular
marker of biodegradation would be desirable. This will be relevant because biodegradation is probably the most important dissipation pathway, which can lead to
PhAC elimination.
In order to develop a molecular marker of biodegradation, several approaches
have been considered. Isotopic fractionation (i.e., compound stable isotope analysis
(CSIA)) is considered the best molecular marker of biodegradation. Stable isotope
fractionation relies on the observation of ratio shifts of stable isotope mainly C, H,
and N, prompted by the breaking or generation of chemical bonds during chemical
transformations [79]. The mathematical description of the relationship between the
extent of degradation and isotopic composition of a targeted compound can be
expressed by the Rayleigh equation (see Fig. 1a).
E t and E 0 represent the initial and conversion-dependent isotope ratio, Conc t /
Conc 0 is the residual fraction of the contaminant, and ε represents the isotope
enrichment factor. ε can be obtained as the slope of the linear regression line of
the natural log of the isotopic enrichment against the natural log of the extent of the
degradation. It is usually expressed in per mill unit, requires very accurate measurements to obtain, and is very specific to a biotransformation reaction. This approach
has already been applied to quantify in situ contaminant biodegradation and to
investigate mechanisms of biodegradation [80]. However, CSIA require specific
instrumentations (e.g., GC-IRMS), are still difficult to achieve in a routine way, and
are very often limited to GC amenable compounds. Limits of detection (LODs) are
not compatible with the occurrence and levels of compounds in soil (i.e., ng g
À1
level), and analyte enrichment and purification steps are needed during which
isotopic fractionation might be induced.
In this context, a novel approach applied to investigate biodegradation of PhACs
in environmental compartments is enantiomeric fractionation, which measures the
enantiomeric ratio (ER). Enantioselective process occurs when one enantiomer of a
chiral compound is favored over the other during biotransformation. This approach
makes sense because more than 50% of PhACs are actually commercialized as
racemic mixtures, which are mixtures of two enantiomers at equal concentrations.
Soil Sorption and Degradation Studies of Pharmaceutical Compounds Present in. . .
161