Biomedical Engineering Reference
In-Depth Information
Cations of lowest to highest affinity for rhamnolipid were K
+
< Mg
2+
< Mn
2+
< Ni
2+
< Co
2+
< Ca
2+
< Hg
2+
< Fe
3+
< Zn
2+
< Cd
2+
< Pb
2+
< Cu
2+
< Al
3+
. The affinities were
in the same order of magnitude or higher than those of organic acids (acetic, citric,
fulvic, and oxalic acids) with metals, thus indicating the potential of the rhamnolipid
for metal remediation. Molar ratios (MRs) of the rhamnolipid to individual metals
were 2.31 for copper, 2.37 for lead, 1.91 for cadmium, 1.58 for zinc, and 0.93 for
nickel. Common soil cations, magnesium and potassium, had lower MRs of 0.84 and
0.57, resp e ct ively.
In the presence of oil contamination, rhamnolipids were added to soil (Mulligan
et al., 1999a,b) and sediments to remove heavy metals (Mulligan et al., 2001b).
Although 80%-100% of cadmium and lead can be removed from artificially con-
taminated soil, from field samples the results were more in the range of 20%-80%
due to increased bonding of the contaminants over time (Fraser, 2000). Biosurfactant
could also be added as a soil-washing process for excavated soil. Due to the foaming
property of the biosurfactant, metal-biosurfactant complexes can be removed by
addition of air to cause foaming and then the biosurfactant can be recycled through
precipitation by reducing the pH to 2.
Neilson et al. (2003) studied lead removal by rhamnolipids. A 10 mM solution
of rhamnolipid removed about 15% of the lead after 10 washes. High levels of Zn
and Cu did not have any influence on lead removal. Mulligan et al. (1999a, 2001b)
showed that lead could be removed from the iron oxide, exchangeable and carbonate
fractions. These removal levels are very low and the process could be improved if the
biosurfactants could be added in multiple cycles (Neilson et al., 2003).
Rhamnolipids have also been added to another metal-contaminated media, min-
ing residues, to enhance metal extraction (Dahrazma and Mulligan, 2004). Batch
tests using a 2% rhamnolipid concentration showed that 28% of the copper was
extracted. Although concentrations higher than 2% extracted more copper, the rham-
nolipid solution was very viscous and became difficult to work with. The addition
of 1% NaOH with the rhamnolipid enhanced the removal up to 42% at a concentra-
tion of 2% rhamnolipid but decreased removal at higher surfactant concentrations.
Sequential extraction studies were also being performed to characterize the mining
residue and to determine the types of metals being extracted by the biosurfactants.
Approximately 70% of the copper was associated with the oxide fraction, 10% with
the carbonate, 5% with the organic matter, and 10% with the residual fraction. After
washing for 6 days with 2% biosurfactant (pH 6), 50% of the carbonate fraction and
40% of the oxide fraction were removed. In summary, rhamnolipids are effective for
hydrocarbon and heavy-metal removal and could also be effective for the removal
of mixed (hydrocarbon and metal) contaminants. However, studies have not been
performed at large scale.
Other studies have also been performed to evaluate the feasibility of metal removal
by biosurfactants. Juwarkar et al. (2007) investigated the removal of cadmium and
lead by a biosurfactant produced b
y P. aeruginosa
BS2 in column tests. Cadmium
removal was more than Pb. Within 36 h, more than 92% of Cd and 88% of Pb was
removed by the rhamnolipid (0.1%) The rhamnolipid was also able to decrease toxic-
ity, allow microbial activity (
Azotobacter
and
Rhizobium
) to take place and to not
degrade soil quality. Cost-effectiveness, though, needs to be evaluated.
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