Biosurfactant Complexation of Metals and Applications for Remediation - Biosurfactants: Research Trends and Applications - page 295

two species of earthworm after the washing (Slizovskiy et al., 2011). Taken together,
these studies suggest that the contamination age of a soil along with its physical and
chemical characteristics must be carefully considered to identify suitable extractants
and extraction conditions. Further examination of historically contaminated soils is
warranted, not only for biosurfactant application, but for the technology as a whole.
Sediments
Sediments have different characteristic properties than soil, for example, higher clay
and organic matter content, and they too have been studied for surfactant-enhanced
soil washing process efficacy. Mulligan et al. (2001) compared the ability of sur-
factin, rhamnolipid, and sophorolipid for removal of metals from a zinc- and cop-
per-contaminated sediment. Results showed that rhamnolipid was most effective;
a single washing of a 0.5% rhamnolipid solution removed 65% Cu and 18% Zn. A 4%
sophorolipid solution removed 25% of Cu and 60% of Zn, while a 0.25% surfactin
solution removed only 15% of Cu and 6% of Zn (Mulligan et al., 2001). In a sec-
ond study that examined metal removal in continuous flow columns, 5% rhamnolipid
was able to remove 37% of Cu, 7.5% of Zn, and 33% of Ni. Interestingly, the addition
of 1% NaOH to 0.5% rhamnolipid increased the removal efficiency of Cu to levels
near the 5% (10-fold higher) rhamnolipid treatment (Dahrazma and Mulligan, 2007).
This increase in Cu recovery was due to the solubilization of organic matter by
NaOH since Cu tends to partition into the organic fraction of soils and sediments.
These results again suggest that the physicochemical characteristics of the system
(sediment, soil, or water) must be carefully considered to allow extraction conditions
to be optimized.
Mine Tailings
Mine tailings are the leftover waste material generated from mining processes. The
materials, especially in older legacy mine tailings sites, can be highly contaminated
with metals such as Pb and As. Surfactant-enhanced soil washing has recently been
examined for use in the remediation of mine tailings. Aniszewski et al. (2010) exam-
ined the removal of Zn and Cd from a waste generated from mining zinc. They
isolated four Microbacterium spp. with bioemulsifying activity and tested both cell-
free supernatants and partially purified preparations for metal recovery. Cadmium
removal ranged from 17% to 41% and Zn removal ranged from 14% to 68%, depend-
ing on the Microbacterium strain and the growth substrate used to produce the
bioemulsifier.
Wang and Mulligan (2009a) used rhamnolipid to remove As from tailings, find-
ing that the mobility of As increased with increasing rhamnolipid concentration and
increasing pH. Since As is generally present as an anion, the authors discussed the
potential mechanism of removal. The addition of rhamnolipid was found to lower
the zeta potential of the tailings, suggesting that rhamnolipid was adsorbed to the
tailings particle surfaces. It was hypothesized that this enhanced the mobilization
of As anions due to increased charge density (negative) and repulsive electrostatic
interactions (Wang and Mulligan, 2009a). In a second study, 70 pore volumes of
a 0.1% rhamnolipid solution were applied to a column containing oxidized mine
tailings with elevated levels of As, Cu, Pb, and Zn (Wang and Mulligan, 2009b).