Nanoparticles as Sorbents for Removal of Heavy Metal Ions from Aqueous Solutions - Nanotechnologies for Water Environment Applications - page 148

nitrogen-donor atoms (e.g. amine groups). Yantasee, et al . (2007) obtained surface-
modified iron nanoparticles carrying dimercaptosuccinic acid, via the ligand-
exchange method. The modified nanoparticles have a high adsorption capacity of 227
mg/g nanoadsorbent for Hg(II). High heavy metal affinity of the modified
nanoparticles also promote a more rapid adsorption kinetics, for instance, 99 wt% of
1 ppm Pb(II) was removed within a minute.

Another type of hybrids iron-based nanoparticles was obtained by
encapsulating pre-synthesized iron nanoparticles with polymers which carry desired
heavy metal-selective binding sites. Shih and Jang (2007) utilized Fe 3 O 4
nanoparticles as the seeds and allowed the precursor monomer
ethylenedioxythiophene to adsorb onto the seed nanoparticles under acid etching-
mediated conditions, followed by polymerization. The poly(3,4-
ethylenedioxythiophene) (PEDOT) polymers encapsulated the parent seeds, and the
resulting hybrids nanoparticles displayed remarkable heavy metal sequestration
capacity. For instance, the maximum adsorption capacity for heavy metal could
exceed 400 mg/g of the modified nanoparticle.

6.3 Polymeric Nanoparticles for Removal of Heavy Metal Ions

6.3.1 Introduction

Polymeric nanoparticles (10-500 nm) represent an extremely wide category of
macromolecules or molecular aggregates that have been researched and developed
over the last few decades. Polymeric nanoparticles, or otherwise known as latexes or
polymeric microgels, share some similarities to surfactant micelles (e.g., polymeric
nanoparticles possessing amphiphilic properties too). Other than that, polymeric
nanoparticles differ greatly from surfactant micelles often utilized in the micellar-
enhanced ultrafiltration (MEUF) process (Klepac et al . , 1991), in terms of physical
dimension as well as their microstructures. One of the distinct dissimilarities is that
the surfactant-based micelles would only maintain their structure provided that the
critical micellization concentration (CMC) is reached or exceeded; whereas the size
and shape of polymeric nanoparticles will remain stable after manufacturing.

Due to the enormous technical and economical importance, various types of
polymeric nanoparticles have been synthesized via a vast array of “heterophase
polymerization” routes, which have been established and continually advanced in
polymer-emulsions industry (Ma, 1999; Tauer, 2004). One of the most important
commercial processes is emulsion polymerization, in which the emulsion-polymers
could be synthesized efficiently in water (as solvent) with various process
configurations (i.e., batch, semibatch and continuous). Emulsion polymerization
could be further sub-divided, depending on the types of polymerization initiators and
stabilizers utilized. For examples, the (macro-)emulsion polymerization employs a
water-soluble initiator with all kinds of stabilizers, and often yields polymeric
nanoparticles from 5 nm up to 10 m, with good size monodispersity.