Biomedical Engineering Reference
In-Depth Information
or metal-chelating compounds in order to mobilize metals, followed by a step that
captures and reconcentrates the metals from solution. The use of strong acids has
drawbacks because it leads to the disruption of the physical, chemical, and biologi-
cal structure of soils, thus reducing possibilities for subsequent use as a soil medium
(Malandrino et al., 2006; Neilson et al., 2003; Torres et al., 2012). Metal chelators
can be effective and are less destructive. Many synthetic chelators, including some
surfactants, have been studied; the most common of which is EDTA (Malandrino
et al., 2006; Neilson et al., 2003; Torres et al., 2012). The need for an additive to
facilitate metal removal has led to the notion of examining whether there are green
additives, with reduced toxicity, that can be used in place of traditional chelators/
surfactants.
There are several approaches to the mobilization step of soil washing. In situ
soil washing is a viable technique for applications where the contaminated zone is
underlain by a nonpermeable layer. This allows the washing solution to be leached
through the contaminated zone and then pumped out and treated aboveground to
remove metals (Lestan et al., 2008). Ex situ treatment basically involves removal
and treatment of the contaminated soil. This can be done in a batch system, such
as a soil slurry reactor, or can be done using heap or column leaching where the
treatment solution is either gravimetrically percolated or pumped through the con-
taminated soil and then collected and treated. Finally, soils can be treated electroki-
netically (generally ex situ) to remove metals by applying a direct current electrical
field through a saturated contaminated soil. This causes the pore fluid to migrate
by electro-osmosis and cationic metal ions to concentrate at the cathode (Lestan
et al., 2008). For each of these approaches, in situ or ex situ soil washing or electro-
kinetic extraction, biosurfactants can be added to increase their efficacy in what is
known as surfactant-enhanced soil washing (Torres et al., 2012).
The mechanism of biosurfactant-enhanced metal removal from solid surfaces
is an interesting area of research. The sorption of metals onto mineral surfaces is
controlled by many factors including ion exchange and association with Fe and Mn
oxides, carbonates, and organic matter (precipitation/dissolution reactions). Two
hypotheses have been offered to explain the mechanism behind surfactant-enhanced
soil washing. First, direct complexation between the biosurfactant and solution phase
metal effectively removes metal from solution and increases dissolution and desorp-
tion according to Le Chatelier's principle (Miller, 1995). The second is that biosur-
factants can accumulate at the solid/liquid interface and interact with sorbed metals.
Mulligan et al. (1999) designed a series of experiments to test these hypotheses using
surfactin and a hydrocarbon-contaminated soil that was spiked with metals includ-
ing Cu, Cd, Pb, and Zn. They conclude that interaction of surfactant with the soil
allows direct interaction between the surfactant and the sorbed metal. The surfactant
also reduces the interfacial tension at the liquid-solid interface, thereby making it
easier to release the sorbed metal (Mulligan et al., 1999, 2001).
Soil Washing Efficacy of Biosurfactants on Artificial Contamination
Biosurfactants have been shown to be effective additives in soil washing technologies.
Rhamnolipid, perhaps the best studied biosurfactant for surfactant-enhanced soil
washing, was found to desorb metals from a sandy loam both when the metals were
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