Geoscience Reference
In-Depth Information
sludge collected from a wastewater treatment plant in Teheran (Iran) were dis-
posed of on four experimental plots containing silty loam soils from the Varamin
plain area, and heavy metal transport with depth was determined. The metal dis-
tribution of Cr, Cu, and Pb under various treatments is shown in Fig.
12.9
. It can
be seen that under both wastewater and sludge disposal treatments, heavy metals
are transported with depth, relative to the control case where the contaminants
remain in the soil upper layer.
12.1.2 Metalloids
Arsenic (As) is a highly toxic metalloid with various oxidation states. For example,
arsenite [As(III)] may be oxidized to arsenate [As(V)] as a result of oxidation by
either mineral or microbial environments (Jones et al.
2012
). A conceptual model
showing biotic and abiotic processes of arsenic fate in the soil-subsurface system
is depicted in Fig.
12.10
.
The mobility of arsenic in soil-subsurface systems is affected by its oxidation
state, the constituents of the adsorbing phase, and the pH of pore water. An
example of metalloid redistribution in such systems is given by Williams et al.
(
2003
), who report results of a laboratory study on coupled adsorption and
transport of As(V) in a heterogeneous soil containing iron oxide. In contact with
the soil, As(V) was subject to reactive interactions that affected the oxyanion
arsenate mobility. Adsorption experiments showed that As(V) adsorbed strongly
and nonlinearly to the soil, with natural pH of 4.5 (Fig.
12.11
), indicating that
arsenate subsurface transport would be significantly retarded. In addition to time
and concentration, it was found that As(V) adsorption was affected by pH, ionic
strength, and competing anions. For example, Williams et al. (
2003
) found that
higher adsorption at low pH and a dramatic decrease at pH between 9 and 10.
The mobility of As(V) was determined by measuring arsenate concentrations in
effluent versus relative concentrations during leaching with up to 4,000 pore
volumes of aqueous solution. The breakthrough curves were asymmetrical, indi-
cating nonlinear and rate-limited adsorption, with significant retardation due to
chemical interactions with the subsurface materials. Detectable As(V) concen-
trations were not determined in the column effluent until aqueous leaching of about
200 pore volumes (Fig.
12.12
). The experiment also illustrated the presence of an
irreversibly adsorbed and/or a slowly desorbing fraction. After the appearance of
the As(V) peak, the concentration of the arsenate began to decrease as the influent
concentration became an As(V)-free solution. At this point, the amount of As(V)
recovered was only 44 % of the total arsenate input. As a consequence, a sig-
nificant fraction of As(V) was irreversibly adsorbed on the soil, affecting the
pathway of this contaminant in the soil-subsurface system.
Arsenate concentration affects its transport in the soil-subsurface region.
Yolcubal and Akyol (
2008
) found that the degree of arsenate retardation in a
carbonate-rich
soil
decreased
with
increasing
As(V)
concentration.
The
