Biomedical Engineering Reference
In-Depth Information
tested individually and in a mixture. An 80 mM monorhamnolipid (4%) solution
removed 40% of Zn
+2
and Pb
+2
and nearly 60% of Cd
+2
(Herman et al., 1995).
A second study showed 99% removal of Pb, Ni, Cu, and Cd after flushing with a
1% glycolipid solution for 30 d (Parthasarathi and Sivakumaar, 2011). Yet a third
study showed that 0.1% rhamnolipid removed 92% of Cd and 88% of Pb during a
36 h column leaching study, with no impact on culturable counts of bacteria or fungi
(Juwarkar et al., 2007). Using a different approach, Wang and Mulligan (2004) exam-
ined whether a rhamnolipid foam could be used as a soil washing agent. Interestingly,
a 0.5% rhamnolipid foam was 11% more effective at removing Cd and 17% more
effective at removing Ni from a sandy soil than a 0.5% rhamnolipid solution. The
authors hypothesized this was due to an increase in the homogeneity of the washing
solution flow and surface contact when it was applied as a foam.
Saponin has also been studied as a soil washing agent. Saponin removed nearly
100% of Cd, Zn, and Cu from three soils with the removal efficiency of the soils being
sandy loam > loam > silty clay. These batch studies showed that removal efficiency
improved with multiple washings and that triplicate washes yielded optimal effi-
ciency (Gusiatin and Klimiuk, 2012). Saponin was also shown to have high removal
efficiencies for Cd (90%-100%) and Zn (85%-98%) from an Andosol, Cambisol,
and Regosol when applied at a 3% concentration (Hong et al., 2002). Both Cu and Pb
were also removed but with lower efficiencies (30%-60%).
Finally, surfactin has been demonstrated as a soil washing agent (Mulligan et al.,
1999). Results of this study showed that metal removal by 0.25% surfactin solution
was 25% for Cu and 6% for Zn. When the surfactin concentration in the washing
solution was increased to 1%, the amount of metal removed decreased. Removal
efficiency then increased again as surfactin was increased from 1% to 4%. As shown
previously for rhamnolipids, multiple washes with surfactin improved metal recov-
ery. For example, a 0.25% surfactin solution was used in five successive washes,
removing 70% of Cu, 25% of Zn, and 15% of Cd.
Soil Washing Efficacy of Biosurfactants on Aged Contamination
While the earlier results sound promising, each of the aforementioned studies
described used an artificially contaminated soil. Soil washing is not nearly as effi-
cient in aged contaminated soils. When two historically contaminated soils were
studied, one from a mining site and one from an abandoned army depot, rhamnolipid
only removed a small fraction of the metals. Focusing on Pb which was present in
both soils, after 10 washings under batch conditions, rhamnolipid removed 14.2%
and 15.3% of total Pb from the soils, a relatively small amount. This was attributed
to the association of the Pb with stable carbonate and oxide fractions in these soils.
Rhamnolipid was effective in removing the soluble and exchangeable fraction of Pb
from the soils but was much less effective at removing Pb bound to carbonates and
amorphous iron oxides. This significantly reduced removal efficiency suggests that
the longer a contaminant is in the soil, the more recalcitrant and associated with
recalcitrant fractions of the soil that contaminant becomes (Neilson et al., 2003).
A historically contaminated acidic soil from the Palmerton Zinc Pile Superfund
site was more successfully treated to remove Zn, Cu, Pb, and Cd (39%, 56%, 68%,
and 43%, respectively) with rhamnolipid. The soil was reported to be less toxic to
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