Environmental Engineering Reference
In-Depth Information
stability enhancement experiments involving magnetorheological fluids (Fang et al.,
2005; Wu et al., 2006). Its low cost and “green” attributes are appealing in a
nanoparticle dispersant (Tiraferri et al., 2008).
Saleh et al.
(2005) showed that amphiphilic triblock copolymers with an A-B-C
triblock microstructure are effective delivery systems for nZVI. High molecular weight
amphiphilic polymers show essentially irreversible absorption and, thus, are more
suitable as a delivery system for groundwater remediation (Velegol and Tilton, 2001;
Braem et al., 2003). The triblock copolymers were produced using atom transfer radical
polymerization (ATRP) in conjunction with post-polymerization ester-hydrolysis and
sulfonation steps. The results of the research demonstrated enhanced colloidal stability
and an increased affinity for a water/organic interface provided by the amphiphilic
triblock copolymer. The poly(methacrylic acid)-block-poly(methylmethacrylate)-block-
poly(styrenesulfonate) contains an anchoring block (polymethacrylic acid), hydrophobic
block (polymethylmethacrylate), and hydrophilic block (polystyrenesulfonate). The
hydrophilic group imparts good colloidal stability, and the hydrophobic group gives the
particle better affinity for the organic contaminant and resists the access of water to the
nZVI surface. The novel polymer architecture creates a thermodynamic affinity of the
modified nZVI for the water/contaminant interface. However, kinetic studies showed a
decrease in the rate of contaminant degradation by the polymer-modified nZVI as
compared to unmodified nZVI
(Saleh et al., 2007). The reduction in the contaminant
degradation rate was attributed to low permeability of the contaminant through the film
absorbed onto the nZVI. The synthesis method used by this research group to produce
the triblock copolymer structure is sensitive to impurities and oxygen. The process is
relatively slow. Further, polymerization catalyst residues can be difficult to remove from
the copolymer.
Recently, the same research group studied the effect of ionic strength on the
mobility of the modified nZVI (Saleh et al., 2008). They measured the change of surface
charge as a function of concentration of Na
+
and Ca
2+
ions with three different surface
modifiers. They used the high molecular weight (MW) (125 kg mol
-1
) poly(methacrylic
acid)-
b
-(methyl methacrylate)-
b
-(styrene sulfonate) triblock copolymer (PMAA-
PMMA-PSS), low MW (23 kg mol
-1
) polyaspartate biopolymer, and sodium
dodecylbenzene sulfonate surfactant (SDBS, MW = 348.5 g mol
-1
) to determine the
effect of electrosteric and electrostatic repulsions on particle stabilization. The negative
surface potential of nZVI increased due to surface modification, and the particle
hydrodynamic diameter also increased moderately. The group conducted column
studies to simulate a groundwater environment. They concluded that, in typical
groundwater containing 0.5-1.0 mM Ca
2+
or Mg
+
, polyaspartate and SDBS (both low
MW) will not enhance mobility of nZVI in the aquifer. However, the triblock
copolymer, PMAA-PMMA-PSS, would provide the electrosteric repulsions to inhibit
attachment to sand grains under typical groundwater conditions. Electrosteric repulsion
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