Remediation and Management of Contaminated Soil - Sustainable Practices in Geoenvironmental Engineering - page 425

ores (Mulligan, 2002). Biohydrometallurgical techniques such as those utilizing the fungus
Aspergillus niger are thus potentially more sustainable. A. niger has shown potential for
producing citric acid and other organic acids effective for metal solubilization (Mulligan
and Kamali, 2003). Effectiveness for leaching was enhanced when sulfuric acid was added
with organic acids to the medium. Different agricultural wastes such as potato peels were
tested as growth substrates for the fungus. Maximum solubilization levels of 68%, 46%,
and 34% were achieved for copper, zinc, and nickel, respectively. Also minimal iron dis-
solution was obtained (7%), which allows for further metal puriication.
In the research study to evaluate the potential for mobilization of arsenic (As) from mine
tailings in the presence of natural organic matter (NOM), humic acid (HA) was used as
a model for NOM (Wang and Mulligan, 2009). By introducing HA at a low mass ratio
(below 2 mg HA/g mine tailings) under acidic conditions As mobilization was inhibited.
However, As mobilization increased with increasing mass ratios and under alkaline con-
ditions, As mobilization was signiicantly HA enhanced.
11.5.5 Bioconversion Processes
Microorganisms are also known to oxidize and reduce metal contaminants. Mercury and
cadmium can be oxidized, whereas arsenic and iron can be reduced by microorganisms.
This process (called mercrobes) has been developed and tested in Germany for removal
of concentrations greater than 100 ppm. Since the mobility of the metal contaminants is
inluenced by their oxidation state, these reactions can affect their mobility. It needs to be
noted that organic contaminants such as benzene can also be degraded.
Chromium conversion is also affected by the presence of biosurfactants. In the study
by Massara et al. (2007) on the removal of Cr(III) from kaolinitic soil, the effect of addition
of negatively charged biosurfactants (rhamnolipids) on chromium contaminated soil was
evaluated. The sequential extraction results showed that rhamnolipids removed Cr(III)
mainly from the carbonate, and oxide/hydroxide portions of the soil, stable forms from the
soil. The rhamnolipids were also capable of reducing close to 100% of the extracted Cr(VI)
to Cr(III) over a period of 24 days.
11.5.6 Phytoremediation
Some plants have been shown to retain contaminants in their roots, stems, and leaves via
phytoextraction, phytodegradation, phytostabilization, and biodegradation in the rhizo-
phere (Hazardous Waste Consultant, 1996) (Figure 11.12). Phytoaccumulation is the transport
of contaminants from the roots to the shoots and leaves. Contaminants are metabolized in
the plant via enzymes for phytodegradation. Phytostabilization immobilizes contaminants
by excretion of various chemicals from the roots. Around the plants root, microorganism
growth is stimulated by the nutrients in the soil. These microorganisms can then biodegrade
the contaminants in the soil. Various examples are shown in Table 11.2.
Vegetative caps consisting of grasses, trees, and shrubs can be established in shallow
freshwater. The resulting vegetative mat can hold sediments in place. The construction of
wetlands is growing for wastewater treatment, and thus, the knowledge on wetland conig-
urations is growing. However, vegetative caps have not yet been applied to the remediation
of sediments (Mulligan et al., 1999). Phytoremediation can be implemented where dredged
sediments have been placed in contained areas and a wetland is then constructed to reme-
diate and contain the sediments. Lee and Price (2003) indicated that although phytoextrac-
tion of Pb with chelates may be troublesome due to potential leaching into groundwater,