Biogeochemical Cycles - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 161

Dominant reaction pathways can be determined quantitatively from data on hydrol-
ysis rate constants and half-lives. Hydrolysis data will generally be important in
assessing risks from organic chemicals that have hydrolyzable functional groups
(e.g., esters, amides, alkyl halides, epoxides, and phosphoric esters).
As stated previously, a hydrolysis reaction can be catalyzed by acidic or basic
species, including H 3 O + (H + ) and OH − . The promotion of hydrolysis by H + or OH −
is known as specific acid or specific base catalysis. Hydrolysis rates are the same
in natural freshwaters and in buffered distilled water at the same temperature and
pH. Thus, only specific acid or base catalysis, together with the neutral reaction,
needs to be considered. Although other chemical species can catalyze hydrolysis
reactions, the available concentrations of these species in the environment are usually
too low to have any effect and are not expected to contribute significantly to the rate
of hydrolysis. 45,156 Hydrolysis can be simulated using rates that are first order for
the neutral chemical and second order for its ionic forms. The second-order rates
are pH and temperature dependent. 78 Hydrolysis and chemical oxidation of natural
organic matter are usually insignificant compared to microbiological decomposition.
The influence of hydrolysis on the oxygen budget is, therefore, mostly unimportant.
However, these processes can be of great importance for specific, toxic organic
compounds. Therefore, many fate transport and exposure models of toxic substances
in aquatic ecosystems include these processes. 120,157 The hydrolysis rate equations
commonly used are presented in Section 4.2.4.
In the environment, hydrolysis of specific organic chemicals (pollutants) occurs
in dilute solution. Under these conditions, water is present in excess and the con-
centration of chemicals is essentially constant during hydrolysis. Hence, the kinetics
of hydrolysis is rather pseudo first order at a fixed pH. Many environmental factors
influence the rate of hydrolysis, such as temperature, pH, solubility, sunlight, adsorp-
tion or absorption, and volatility. 137
Mabey and Mill, 158 Wolfe et al., 159 and Tinsley 160 have studied the mechanisms
of hydrolysis and provide predictive methods to estimate the kinetic rates of hydrol-
ysis of various compounds. Table 4.40 gives examples of the hydrolysis rates of
selected halogenated compounds.
Some chemicals, including alkyl halides, were found to have hydrolysis rates
that were independent of pH over the usual environmental pH range of 4-9, while
other chemicals, such as carboxylic acid esters, had hydrolysis rates that were highly
dependent on pH ( see Figure 4.9) . 131
Recent studies have shown that the hydrolysis of organic compounds actually
proceeds in the sediment-sorbed state. It has been reported that on sediment surfaces
alkaline hydrolysis is retarded, neutral hydrolysis is unaltered, and acid hydrolysis is
unaltered or accelerated as compared to the water phase process. It is difficult to predict
the magnitude of rate constants for hydrolysis reactions in the sediment-sorbed state. 121

4.2.3.5 Oxidation

All organic matter will undergo oxidation if present in a natural aerobic aquatic
environment for a sufficiently long period of time. If reduced material is sufficiently
abundant, however, the oxygen dissolved in interstitial water and/or in the lower