Biosurfactant Complexation of Metals and Applications for Remediation - Biosurfactants: Research Trends and Applications - page 300

solution that passes through the filter (Fillipi et al., 1999). Pores sizes can range from
1,000 to 50,000 molecular-weight-cutoff (MWCO). Biosurfactants, because they are
biodegradable, are particularly suited for this application; surfactant monomers can
leak through the membrane leaving low levels of surfactant in the aqueous solution
which can thereafter be biodegraded. The efficacy of this technique with biosurfac-
tants has been examined. Micellar-enhanced ultrafiltration was tested with a spicu-
lisporic acid derivative 2-(2-carboxyethyl)-3-decyl maleic anhydride (DCMA-3Na)
(Hong et al., 1998). Results showed that DCMA-3Na removed 99%, 99%, and 93%
of Cd, Cu, and Zn, respectively, when at equimolar concentrations using a 3000
MWCO membrane. DCMA-3Na exhibited a metal binding affinity of Cd 2+ > Cu 2+ ∼
Zn 2+ > Ni 2+ with Ni removal reaching only 65%.
A second study examined the use of rhamnolipid in micellar-enhanced
ultrafiltration (El Zeftawy and Mulligan, 2011). Optimized conditions were deter-
mined for use of rhamnolipid to remove metals from six wastewater samples from
the metal-refining industry. The wastewater samples contained Zn (60-130 mg L −1 );
Cd (10-30 mg L −1 ); and Pb, Cu, and Ni, which were all below 10 mg L −1 . The optimal
conditions identified were as follows: membrane pressure, 69 ± 2 kPa; rhamnolipid:
metal molar ratio, 2:1; temperature, 21 ± 1°C; and pH, 6.9 ± 0.1. Operating under
these conditions, the level of all metals in the six wastewater samples was reduced
to below 1.2 mg L −1 meeting Canadian Federal discharge limits (El Zeftawy and
Mulligan, 2011).
A second approach to removing biosurfactant-metal complexes from solution is
ion flotation, also known as dissolved air flotation or foam fractionation. This tech-
nique employs air sparging into a surfactant solution. The surfactant will absorb
to the air bubbles generating a foam that can be harvested and removed from the
solution. In the presence of metal ions, the surfactants can complex the ions and
carry them into the foam (Chen et al., 2011). Using this technique, Chen et al. (2011)
showed surfactin was able to remove 45% of the Hg from a 2 mg L −1 Hg solution.
To achieve this removal, the optimal conditions were determined to be surfactin
applied at 10 times the critical micelle concentration (10 × CMC) at a pH of 8 or 9.
Overnight mixing prior to removal also helped increase Hg removal. In a second
study, a 3:1 molar ratio of saponin to metal was used to remove lead, copper, and
cadmium from wastewater with efficiencies of 90%, 81%, and 71%, respectively
(Yuan et al., 2008). A third study used surfactants produced by Candida (likely
sophorolipids) to achieve a removal of ≥98% of Fe (62 mg L −1 ) and Mn (4 mg L −1 )
from neutralized acid mine drainage using a 0.02% (200 mg L −1 ) surfactant solution
(Menezes et al., 2011).
Adsorbing colloid flotation is a method that combines the use of a colloidal
sorbent material (e.g., clays and goethite) with the flotation technique (Zouboulis
et al., 2004). This method essentially follows sorbent treatment of wastewater with
surfactant flotation. Since the sorbent is present as a colloid, the surfactant foam
will collect the metal ions adsorbed to the colloid particles by floating the sorbent.
Similarly, compounds like Fe +3 can be used to coprecipitate metals for subsequent
flotation. Using this technique, surfactin and lichenysin were used to collect either
goethite (pH 4-7) or ferric hydroxide (pH 4) colloids that had sorbed Cr, resulting
in the removal of Cr with almost 100% efficiency. Surfactin could also remove