Environmental Engineering Reference
In-Depth Information
removing surface oxide layers, thereby re-exposing the covered Pd islets,
whereas NaBH 4 was rejuvenated via reducing the iron oxide layer formed
on Pd/nFe 0 surface to Fe 0 . Of these two methods, regeneration via HCl was
found relatively more ei cient.
14.2.4.3 Stabilized and Supported Iron Nanoparticles: Improving
the Mobility and Stability of nFe 0
h e activity of nFe 0 is very high, thus stabilizing or supporting the particles
is important to preserve their chemical nature until they can be contacted
with the target contaminant. In general, two approaches are applied for sta-
bilization of nanoparticles, i.e., pre-synthesis and post-synthesis. Between
these two techniques, the pre-synthesis approach was reported to be more
ei cient in producing nanoparticles with narrow size distribution and low
sedimentation rate/long suspension time. h e most widely used stabilizers
are carboxymethyl cellulose (CMC), polyvinylpyrrolidone (PVP) and guar
gum. h ey dif er in their stabilization mechanism and thus their stabiliz-
ing ability. h e stabilization potential mainly depends on the functional
group, molecular structure and molecular weight of the stabilizing agent.
For instance, carboxylate group present in CMC forms strong complex
with Fe 2+ ions present in the solution, resulting in dispersion of the Fe 2+
ions throughout the CMC network. h ese Fe 2+ ions then undergo rapid
nucleation on addition of reducing agent, which is followed by growth of
nuclei to a critical size. At this point, steric and electrostatic hindrance
produced by negatively charged CMC molecule limits further growth of
nuclei [81]. An increase in molecular weight of CMC leads to formation
of well-dispersed nanoparticles with narrow size distribution. In the case
of PVP, carbonyl group forms weak bonds with Fe 2+ , slowing the rate of
nucleation, resulting in formation of relatively larger-sized nanoparticles.
As PVP is a neutral molecule, growth of nanoparticles is predominately
counter-checked by steric hindrance. Similar to PVP, guar gum is also neu-
tral, but hydroxyl group of guar gum forms more stable complexes with
Fe 2+ as compared to carbonyl-Fe 2+ complex, resulting in the synthesis of
nanoparticles with relatively smaller-sized nanoparticles [84]. Among
these three stabilizers, CMC produces the smallest nanoparticle. As far
as stability is concerned, guar gum exhibits maximum stability followed
by CMC and PVP. Moreover, high ionic strength (0.5 M NaCl and 3 mM
CaCl 2 ) does not af ect the stability of guar gum-stabilized nFe 0 [85]. An
increase in molecular weight of PVP increases the nanoparticles suspen-
sion stability, but it is not ef ective in the case of CMC.
Search WWH ::




Custom Search