Page 260 - Soil and water contamination, 2nd edition

Chemical transformation 247

Use Equation (13.16) to calculate the dissolved PCB concentration after 4 hours:
C 172 172 e 2 . 0 4 172 154e 0 .. 8 172 169 103 ng l -1
18
w

13.3 BIOLOGICAL PRODUCTION AND DEGRADATION
Organic chemicals and some inorganic compounds (e.g. NH , NO , H S) are susceptible
4 3 2
to biological transformation processes. Microorganisms, especially bacteria and fungi, obtain
their energy by oxidation (primarily organic matter , but also organic pollutant chemicals
and sulphide species such as FeS , H S). Under oxic conditions, the biological degradation
2 2
processes involves oxygen , but under anoxic conditions, nitrate , sulphate , and reduced
organic carbon act as respective oxidants as the conditions become increasingly reducing.
Some organic chemicals in which the carbon is in a fairly oxidised state (e.g. chlorinated
hydrocarbons) are reduced under reducing conditions. The microorganisms responsible for
the biological transformation processes form biofilms on the solid surfaces (e.g. the river bed
and aquatic vegetation in the case of surface waters, and the aquifer matrix in the case of
groundwater. A biofilm consists of the microorganisms attached by an extracellular ‘glue’
made of polysacharides. By forming a biofilm, the microorganisms remain at one place
and take advantage of the supply of nutrients by advective flow, instead of relying solely
on diffusive transport, as happens when they are suspended in the water. Because most
biochemical transformation processes take place in biofilms, they play a key role in the
environmental fate of biodegradable substances in both surface water and groundwater.
The changes in chemical concentrations that result from biochemical reactions are

usually modelled using first-order kinetics . The first-order rate constant is commonly
-3
assumed to be proportional to the active cell density in the biofilm [cells L ]. Although, as
mentioned above, biofilms are very important for biotransformation, they are rarely explicitly
accounted for in distributed transport models, but the influence of the cell density or
number of microorganisms in the biofilm is commonly incorporated in the first-order rate
constants. The advantage of this approach is that the model becomes simpler and requires
fewer data. The disadvantage is that the rate constants become dependent on the system.
In addition to the cell density, the total microbial activity and, accordingly, the first-order
rate constants depend on environmental factors such as the redox regime, temperature, and
concentration of the substrate. The substrate consists of nutrients (reductants, including
organic carbon ) needed for energy supply and the growth of the organisms. In addition,
an oxidant (mostly oxygen , but some microorganisms use nitrate or sulphate ) is needed to
oxidise the nutrients. If the oxidant or one or more of the substrate components is in limited
supply, the organism’s growth and the biodegradation process are hampered. A pollutant
may be the principal substrate, a co-substrate, or oxidant. The dependence on temperature
is particularly relevant in surface waters where the water temperature may vary considerably
throughout the day and the year. In groundwater, the effects of the water temperature are
of minor importance, since groundwater temperatures are relatively constant throughout the
year. The mathematical expressions used to account for the effects of substrate limitation and
temperature in the first-order rate constants are elaborated further below.
Table 13.2 demonstrates that the first-order rate constants for the degradation of organic
chemicals in soil and groundwater may vary by up to two orders of magnitude. Likewise, the
denitrification rates in groundwater are subject to substantial variation. In sandy aquifers in
the Netherlands, where nitrate is the main oxidant for the degradation of sediment organic
-5
-1
matter , the first-order denitrification rate constant ranges from less than 3.5 10 d to about
-4
-1
4 10 d (Uffink, 2003). The organic matter content of these aquifers is very low (usually
less than 0.5 percent). The value of the denitrification rate may increase significantly – up to

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