Biosurfactant Complexation of Metals and Applications for Remediation - Biosurfactants: Research Trends and Applications - page 301

∼95% Zn when collecting ferric hydroxides (pH 6). Lichenysin, however, was inef-
fective for Zn removal (Zouboulis et al., 2004).
r eCovery of m etal from B iosurfaCtant -m etal C omPlexes
It is worth noting that once a biosurfactant-metal complex has been formed and used
to remove the metal from a contaminated soil, sediment, or solution, the complex can
be separated again through a simple pH adjustment. This allows recovery and recy-
cling of both the metal and the biosurfactant. For anionic surfactants such as rham-
nolipid or surfactin, the solution can be acidified (∼pH 2) to precipitate the surfactant
and release the metal ion. The biosurfactant can then be removed by centrifugation
and recycled for reuse (Herman et al., 1995; Mulligan et al., 2001). For a nonionic
surfactant, such as saponin, a pH adjustment to 11 will precipitate the complexed
metals; the metals can then be removed by centrifugation and the biosurfactant col-
lected for reuse (Gao et al., 2012).
CONCLUSION
As shown in this chapter, the body of literature on biosurfactant-metal interactions
is large and continuing to grow. The potential for green, economical remediation
technologies based on these interactions is enormous, but there remain several chal-
lenges that must be resolved before biosurfactants are viable alternatives for use in
remediation technologies.
The first challenge is material cost. There is still no biosurfactant that can be com-
petitively produced, in terms of cost, when compared to synthetic surfactants. The
second challenge is in understanding the chemical properties of individual biosur-
factant congeners so that they can be optimized for application in remediation tech-
nologies. For example, up to 60 rhamnolipid congeners are produced by wild-type
bacteria, but these congeners can have very different chemical properties (e.g., CMC,
interaction with metals). The ability to genetically manipulate bacteria to produce
single congeners or alternatively, to chemically synthesize single congeners, has the
potential to increase the efficacy of biosurfactants dramatically. The third challenge
is to move research from the bench scale and into field testing. Optimally, this can
be done by creating academic-industry partnerships, which can be implemented in
two stages: first to demonstrate effectiveness, and second to scale-up production of
biosurfactants for commercial use.
To close, it is clear humanity's impact on the biogeochemical cycling of metals
is significant and resulting in increasing risks to public health due to metal expo-
sures. As we continue to increase global consumption of new technologies, includ-
ing cell phones and computers, the need for a variety of metals increases along with
the resulting impacts of mining and metal consumption. As our demand for metals
increases, we must find a way to reduce emissions and the impacts related to their
use. The foundation for preserving the health and stability of our environment while
encouraging innovation and economic growth lies in the development of green tech-
nologies and green chemicals; in the realm of metals, biosurfactants are one of the
most promising possibilities available today.