Urbanization and the Geoenvironment - Sustainable Practices in Geoenvironmental Engineering - page 258

(a)

60

Average % Pb held by the
various soil components of
the 9 samples shown

50

40

30

20

10

0

Exch Carbonates Oxide Organics Residual

SSE fractions

(b)

80

Average % Zn held by the
various soil components of
the 9 samples shown

60

40

20

0

Exch Carbonates Oxide Organics Residual

SSE fractions

FIGURE 7.20
Selective sequential extraction characterization of nine different soil samples for Pb (a) and Zn (b) retention by
the different soil components and mechanisms. Note that Exch represents Pb or Zn held by cation exchange
mechanisms, and carbonates, oxides, and organics represent soil components responsible for retaining the metals.
Residuals represent “remaining” in the soil matrix. (Data from Huang Y.-T., Heavy Metals in Urban Soils , MASc
thesis, Concordia University, Montreal, Canada, 2005.)

fraction. Metals bound to the exchangeable fraction of soil are mostly physically adsorbed
(by electrostatic force) to the soil surfaces, and thus, the bonding is weaker compared with
other binding mechanisms. The moderate to high degree of leaching by rainfall and the
competition from other cations present in the leachate solution possibly explains why only
a limited amount of Pb and Zn were retained by this soil fraction. There is a high degree
of association of Pb and Zn with soil oxides and organic matter. The metals associated
with oxides are particularly susceptible to oxidation-reduction reactions and solubiliza-
tion upon a decrease in pH by acid rain.
Sequential extraction was performed on the untreated soil from a site slated for reha-
bilitation and construction (Okoro, 2006). The results (Figure 7.21) showed that copper
existed mainly more in the organic fraction (50%), whereas zinc was mainly in the oxide
fraction (36%) and nickel more in the exchangeable and carbonate fractions (50%). Some
soil washing tests were performed with a series of ive washings with biosurfactants,
which enabled the oxide fraction of zinc, organic fraction of copper, exchangeable and
carbonate fractions of nickel to be substantially reduced, compared with the control and
the untreated soil. These results indicated the feasibility of removing heavy metals and the
total petroleum hydrocarbon content of a mixed contaminated soil with the anionic bio-
surfactants tested.