Biogeochemical Cycles - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 160

Adsorption isotherms are used to define the equilibrium relationship of sorption
between organic chemicals and metals and particulate matter at constant temperature.
Several models have been proposed to mathematically represent these isotherms. 45
Among the most popular are the Langmuir and Freundlich isotherms, which are
mentioned in Section 4.2.4.
Examples of models of especially toxic organic compounds in water include the
Mirex behavior model developed by Halfon; 153 the model of hydrophobic organic
pollutants by Schwarzenbach and Imboden; 154 and the model of toxins in the Tamar
Estuary, U.K., developed by Harris et al. 155 The effects of toxic substances on aquatic
biota are often determined by the concentration of the dissolved fractions. It is,
therefore, of some significance to correctly formulate the equilibrium between the
sorbed and the dissolved components. 120 This was clearly demonstrated in the model
developed by Schwarzenbach and Imboden. 154
The extent of adsorption is also proportional to the sorbent surface area. To
be able to compare different adsorbents, a specific surface area, defined as that
portion of the total surface area available for adsorption per unit of adsorbent, is
used. This means that the adsorption capacity of a nonporous adsorbent should
vary inversely with the particle diameter, whereas for highly porous adsorbents
the capacity should be almost independent of the particle diameter. Because
flocculation and consequently increasing floc size and pore characteristics cannot
occur at salinities below 2%, adsorption will indirectly be affected by salinity.
This is an important phenomenon for coastal lagoons in which salinities vary
over in a wide range. The nature of the adsorbate also influences adsorption. In
general, an inverse relationship can be anticipated between the adsorption of a
solute and its solubility in the solvent (water) from which adsorption occurs. This
is the so-called Lundilius' rule, which may be used for the semi-quantitative
prediction of the effect of the chemical character of a solute on its uptake from
solution (water). Ordinarily, the solubility of any organic compound in water
decreases with increasing chain length because the compound becomes more
hydrophobic as the number of carbon atoms increases. This is Traube's rule.
Together, these rules also suggest that increasing ionization means decreasing
adsorption (when water is the solvent).

4.2.3.4 Hydrolysis

Hydrolysis of organic compounds in the aquatic environment is of major environ-
mental interest since it is one of the most important mechanisms for the breakdown
of pollutants. Organic pollutants can undergo reactions with water, resulting in the
introduction of a hydroxyl group into the chemical structure:

RX + H O ROX + HX (4.41)

2

RCOX + H O RCOOH + HX (4.42)

2

Oxonium and/or hydroxyl ions catalyze these reactions. Hydrolysis refers to the
reaction of an organic compound (RX) or (RCOX) with water, resulting in a net
exchange of group X for the OH group from the water at the reaction center.