Nanoparticles as Sorbents for Removal of Heavy Metal Ions from Aqueous Solutions - Nanotechnologies for Water Environment Applications - page 141

heavy metal ions often evokes their Lewis acidity/basicity as separation based on ionic-
charges is no longer effective in this regime. Heavy metals are typically classified as soft
acids [e.g., Hg(II), Cd(II)] or borderline acids [e.g., Cu(II), Ni(II), Zn(II), Pb(II)]
according to the Hard and Soft Acids and Bases (HSAB) principles of Pearson (1963).
Selective complexation of these heavy metal ions and, hence, their removal from
aqueous phase onto solid adsorbents is to utilize chelating ligands based on their
chemical composition, structure as well as binding affinity that are effective towards the
target metal ions (Hancock and Martell, 1989). Many advances in polymer resins'
synthesis and manufacturing have enabled polymer resins, or polymers in general, to be
modified and engineered with ease for the desired properties. Specialty polymer resins
that are tailored for sequestration of heavy metals or even precious metals (e.g.,
palladium, gold) have been made commercially available in recent decades
(Alexandratos and Crick, 1996; Szlag and Wolf, 1999; Sherrington, 2001). Polymers
have since become one of the workhorses for water/wastewater treatment and other
remediation applications.
6.1.4 Nanotechnology and its Implications
Recent advances in nanoscale science and engineering have provided the ability
to create objects measuring between 1 to 100 nm in at least one dimension. Many novel
material properties have been discovered as the particle sizes are reduced and enter the
nanoscale regime, mainly attributed to the quantum effects (Klabunde et al . , 1996 and
references therein). For example, highly reactive calcium oxide nanoparticles were
synthesized and proven to be more effective than its bulk analogue in destructive
adsorption of chlorinated hydrocarbons (Koper et al . , 1993). Because many of the
environmental remediation or water treatment techniques related to the removal of heavy
metals may rely on adsorption at the solid-liquid interface, it is, therefore, logical to
explore nanotechnology for new generation of adsorbents for removal of heavy metal
ions, as nanoparticles exhibit much greater specific surface areas in comparison with
their bulk or micron-scale analogues. This could be translated to more surface reactive
sites and thus faster kinetics for adsorption/separation (Tratnyek and Johnson, 2006).
To better enhance either a conventional end-of-pipe mitigation or POU treatment
in water supply, the key characteristics of an ideal environmental remedial agent should
possess fast sequestration kinetic as well as good selectivity and high capacity. Clearly,
higher treatment throughput could be achieved if the remedial agent could reduce the
concentration of target pollutants within a very short time scale. Selectivity is especially
vital if an engineer would like the remediation process to remove specific target
pollutants such as a trace level of arsenic, in the presence of various competing
compounds, such as phosphate or perchlorate, that are a thousand-fold more abundant.
In addition, nanoparticles with high remedial capacity could reduce the dosage and
therefore lower the material cost. All of these demanding requirements lead the