Chemistry of Freshwaters - Principles of Environmental Chemistry - page 97

Diffuse layer metal retention and outer sphere complex formation involve
electrostatic attractive forces, which are characteristically weaker than
co-ordinative interactions leading to inner sphere surface complex for-
mation. A number of factors influence metal interactions with surfaces,
including the chemical composition of the surface, surface charge, and
the nature and speciation of the metal ion. The importance of the pH
of the aqueous phase in these interactions will be discussed further in
Section 3.2.4.1.

3.2.3.5 Organic Complexes. Dissolved organic matter consists of a
highly heterogeneous mixture of compounds, including low molecular
weight acids and sugars as well as the high molecular weight coloured
compounds termed humic substances. Humic substances are secondary
synthetic products derived from decaying organic debris. Although they
are structurally poorly defined, it is accepted that large numbers of
mainly oxygen-containing functional groups are attached to a flexible,
predominantly carbon backbone. Individual organic molecules, in par-
ticular those of humic substances, can provide more than one functional
group for complex formation with a hydrated metal ion. Ligands which
provide two and three functional groups are termed bidentate and
tridentate, respectively. Complexes where more than one functional
group from the same organic molecule is involved are more stable than
those where functional groups are from discrete organic molecules. The
concentration of dissolved organic matter in freshwaters is generally low
(2-6 mg C L 1 ) and humic substances, comprising molecules which
possess large numbers of functional groups in numerous different
chemical environments, are implicated as the component of natural
organic matter most important in metal binding. 31

3.2.4 Species Distribution in Freshwaters
3.2.4.1 pH as a Master Variable. pH is one of the key parameters
which influences species distribution in aqueous systems. Many equili-
brium expressions contain a hydrogen ion activity term, e.g. acidity
constants, complexation constants. It is therefore useful to consider the
relationship between the activity of the species of interest (e.g. con-
taminant metal ions, organic pollutants, naturally occurring inorganic
and organic solutes, and weakly acidic functional groups on mineral
surfaces) and the activity of hydrogen ions. For example, an acid -
conjugate base pair can be represented as HA and A , respectively.

A þ H 1

HA " (3.36)