Geoscience Reference
In-Depth Information
density of Fe(II) at the mineral surfaces, the concentration of Fe(II) in solution, the
solution pH, and the contact time between dissolved Fe(II) and the oxide surfaces.
16.1.2 Surface-Induced Transformation on Clays
Transformation of organic contaminants adsorbed on clay materials is a surface-
mediated process controlled by the molecular structure, the type of clay, and the
clay-saturating cation, with the rate of contaminant conversion affected by the
hydration status of the system. Studies on surface interactions of clays with
organophosphorous pesticides (Yaron
1978
; Yaron and Saltzman
1978
) are used
here to illustrate this mechanism. Of the many organophosphorus pesticides
studied, parathion-clay compatibility is presented as an example.
Parathion, a heavily used pesticide with strongly adverse environmental effects,
is a member of the phosphoric acid esters group with a general formula of the type
O(S)
RO
ð
16
:
2
Þ
P
OX
(s)
RO
where R is an alkyl group and X is an organic radical. These esters are stable at
neutral or acidic pH but are susceptible to hydrolysis in the presence of alkalines
when the P-O-X ester breaks down. The rate of the process is related to the nature
of the constituent X, the presence of catalytic agents, pH, and temperature. The
mechanism of hydrolysis involves an attack on a relatively positive site, the
phosphorus, by the negatively charged OH
-
group. In the case of parathion, X in
Eq. 16.2 is p-nitrophenol. This group increases the positive character of the
phosphorus and, thus, its susceptibility to a nucleophilic attack. In contact with
clay surfaces, nonionic organic molecules such as parathion are retained at the
surface by different mechanisms, mainly by the ion-dipole or coordination inter-
action. The size of the adsorptive molecule, the nature of the clay-saturating
cation, and the amount of water associated with the cation are determining factors
in the adsorption and subsequent conversion of organic molecules.
Saltzman et al. (
1974
) compare the persistence of parathion on a glass surface
and adsorbed on an oven-dried Ca
2+
-kaolinite clay (Fig.
16.13
). Parathion is rel-
atively stable on a glass surface, but it breaks down partially in an aqueous solution
with pH 8.5 and degrades much more when adsorbed on dry Ca
2+
-kaolinite. The
differences in degradation of parathion in water and on the clay surface suggest a
strong catalytic activity of the Ca
2+
-kaolinite.
Parathion molecules react with dissociated water, and hydrolysis may occur at
the clay surface according to
