Biosurfactant Complexation of Metals and Applications for Remediation - Biosurfactants: Research Trends and Applications - page 281

Soil Environments
The primary natural source of trace metals in soil environments is derived from
pedogenesis, and metal content depends on the parent material. Like water, soils may
also accumulate metals from atmospheric deposition. Deposited aerosols may con-
tain background or elevated levels of metals; elevated levels can be generated from
industrial or contaminated site point sources or from nonpoint sources, which might
simply be a highly populated region. Such atmospheric contributions are important
especially in soils found in rural and remote areas (Nriagu, 1990).
Anthropogenic inputs of trace elements to soils come from many different
sources including fertilizers, pesticides, biosolids, metal mining and process-
ing, and industrial wastes (Nriagu, 1990; Nriagu and Pacyna, 1988; Wuana and
Okieimen, 2011). In 1990, antimony, Cu, Pb, and Zn anthropogenic soil emissions
exceeded natural weathering flux threefold, and for Hg, ten-fold (Nriagu, 1990).
Anthropogenic metal inputs to soil are several times greater than inputs into both
air and water (Callender, 2003).
Summary
It is difficult to separate the atmosphere, hydrosphere, and pedosphere when exam-
ining the movement and cycling of trace elements considering their innate connect-
edness and interdependence. Overall, data reveal that anthropogenic trace element
emissions are greater than natural emissions and humanity has now become the
driving influence over the biogeochemical cycling of these elements globally
(Callender, 2003; Nriagu, 1989, 1990; Nriagu and Pacyna, 1988). In fact, it has been
shown that Pb, V, As, Sn, Zn, Cd, Hg, Sb, Cu, Ag, and Se emissions have caused
perturbation of natural geochemical cycles at the global level, i.e., intercontinen-
tally (Pacyna et al., 1995).
BIOSURFACTANT COMPLEXATION OF METALS
Biosurfactants are surface-active compounds derived from natural, biological
sources; they are amphiphilic molecules consisting of two parts: a hydrophilic head
group and a hydrophobic tail group. The hydrophilic group can be composed of a
mono-, oligo- or polysaccharide, peptide, citric acid with two cadaverines, or a pro-
tein. The hydrophobic group is composed of saturated, unsaturated, and/or hydrox-
ylated fatty acids or fatty alcohols (Bodour et al., 2004; Lang, 2002). Due to their
amphiphilic nature, these molecules accumulate at interfaces. At the interface of
immiscible fluids, surfactants increase the solubility and mobility of the hydrophobic
or insoluble organic phase within the aqueous phase (Singh et al., 2007). Surfactants
are characterized by their physical and chemical properties: ability to reduce surface
tension, critical micelle concentration, hydrophile-lipophile balance, aggregate for-
mation, charge, chemical structure, and source (Van Hamme et al., 2006).
The study of metal interactions with biosurfactants is a relatively new field of
study. To the best of our knowledge, the first reports of biosurfactants interacting
with metals involved the lipopeptide surfactin and some of the group I and II metals
from a medical perspective (Thimon et al., 1992, 1993). Tan et al. (1994) was the first
report of the interactions between a glycolipid, rhamnolipid, and a heavy metal, Cd,