Environmental Engineering Reference
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solution. Former et al. (2005) reported that aggregation of nano-Ceo (the prototypical
carbon-based nano-particle) is the major reason that nano-Ceo concentrations of > 100
mg/L are able to remain stable in solution below ionic strengths of 0.05 /, concentrations
that are ~11 orders of magnitude greater than the estimated molecular solubility of Ceo
when solvation forces are ignored. The formation of a donor-acceptor complex with
water was proposed to explain the surface charge by which the NPs are stabilized.
NPs containing adsorbed or grafted nonionic surfactant or polymer layers
possess an additional mechanism of stabilization, steric stabilization. In drinking water
treatment practices, it is known that polymer bridging allows particle attachment at
distances larger than the thickness of the electrostatic repulsive layer surrounding the
particles. However, the effects of adsorbed nature organic matters (NOMs), Ca 2+ , and
trace metals on NP release, transport, and deposition in natural porous media have not
been studied in much detail. Heidmann et al. (2005) reported that the addition of Cu + ,
Pb + , or Ca + results in strongly increased aggregation rates for kaolinite-fulvic acid
pH was increased from 4 to 6. In the presence of Cu 2 + and Pb 2+ , however, the opposite
trend was observed; the aggregation rates tended to increase slightly with an increase in
pH. Chen et al. (2006) reported that, in the presence of Ca + ions, an alginate gel
network forms, and this network plays a critical role in enhancing the aggregation of
hematite NPs (~75 nm in diameter). The aggregate structures that formed in this latter
study were interestingly similar to NOM-inorganic colloidal particle clusters naturally
occurring in lake waters, suggesting that low concentrations of alginate/polysaccharide
are sufficient for enhanced aggregation of NMs.
particles. In the presence of Ca , the aggregation rates were found to decrease as the
Adsorbed humic substances are known to add high-affinity sorption sites for
trace metal cations on colloidal clay surfaces. Kretzschmar et al. (1995) reported that
removal of humic substances from soil clays strongly decreased colloid mobility in
granitic saprolite, a highly porous and strongly weathered rock residuum. The effect was
reversed by adding small amounts of humic acid to the clay. Several processes are
potentially operant in the effect that humic and other NOM materials have on NP
mobility. Higher molecular weight humic acid was more effective in stabilizing the
colloids than were lower molecular weight fulvic acid fractions, possibly due to steric
interactions. Kretzschmar and Sticher (1997) investigated the influence of colloid-
bound humic acid, solution Ca + concentration, and the presence of adsorbed trace
metals (Cu + and Pb + ) on the transport and deposition kinetics of nanoscale hematite
colloids (a-Fe2C>3; 122 nm in diameter) in a natural soil matrix. They used eq. 15.38 to
describe the transport of colloids through porous media with RTR = - kC (k = the colloid
deposition rate coefficient). They reported that adsorption of humic acid to hematite
caused reversal of surface charge from positive to negative, leading to a decrease in the
deposition rates of NPs by 2 orders of magnitude (aggregation attachment efficiency a =
0.01). Their results indicated that pure Fe oxide NPs (with a ZPC around pH 9) are very
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