Application of Zero-valent Iron Nanoparticles for Environmental Clean Up - Advanced Materials for Agriculture, Food and Environmental Safety - page 395

media such as dissolved oxygen or water, resulting in for-
mation of passive layers over the surface of nFe 0 . h is pas-
sive layer hinders the access of target contaminants to nFe 0 ,
resulting in the loss of reactivity and degradation ei ciency.
Furthermore, due to size ef ects and high surface energy,
nanoparticles tend to aggregate rapidly, resulting in forma-
tion of much larger clusters which may or may not rel ect the
properties of their nanoconstituents. In addition, aggrega-
tion also decreases the surface area, which in turn impedes
the overall reactivity of the nanoparticle. To address these
issues, several strategies such as use of stabilizing agent, sup-
ports, immobilization/encapsulation, etc., have been devel-
oped, which not only prevent aggregation but also resist
oxidation, prolonging the life of reactive nFe 0 [24, 25, 35, 65].

• pH: h e pH plays a detrimental role in corrosion of iron and

thus indirectly controls the degradation of contaminants.
h e lower pH is responsible for (a) acceleration of corrosion
of iron, and (b) an increase in the aqueous solubility of fer-
rous (Fe 2+ ) and ferric (Fe 3+ ) ions, yielding less iron hydroxide
precipitates on the surface of nFe 0 and more exposed reac-
tive sites [35]. However, an extremely low pH does not favor
the contaminant reduction reaction. h is can be explained
by intensive corrosion of iron at extremely acidic pH, which
results in an abrupt production of hydrogen which may pro-
duce a blanket of gas bubbles around the surface of nFe 0 ,
inhibiting the contact of contaminant species to nanopar-
ticles; hence the overall degradation rate decreases [66]. At
higher pH, Fe 2+ and Fe 3+ from iron surface and OH ions in
the alkaline solution react to precipitate iron oxides/hydrox-
ides on the surface of iron occupying the reactive sites, thus
hindering the access of contaminant species to the reactive
sites, reducing the reaction rate [67, 68].

• Dosage/Loading: h e reactivity of nanoparticles is a func-

tion of concentration of reactive sites. With the increase in
nFe 0 dosage, there is a corresponding increase in the num-
ber of reactive sites. h e greater the number of reactive sites,
the greater the number of target molecules accessing the site,
hence the rate of reaction is greater [20, 38, 69]. Alternatively,
if the concentration of the target contaminant exceeds the
number of reactive sites available for reaction, there will be a
decrease in the rate of reaction.