Nanoparticles as Sorbents for Removal of Heavy Metal Ions from Aqueous Solutions - Nanotechnologies for Water Environment Applications - page 139

0.3 ~ 1.0 x 10 6 tons per year (Nriagu and Pacyna, 1988). Over the years, the regulation
on heavy metal discharge into the aquatic environment has become increasingly more
stringent, due to the ever-growing concern of their negative impacts on the drinking
water supply. The World Health Organization (WHO) has set comprehensive guidelines
for drinking-water quality (WHO, 2006), from which the guideline values for some of
the heavy metals of industrial and ecological significance, together with their basic
information on their origins, are summarized in Table 6.1.
6.1.2 Mitigation Strategies for Heavy Metals' Pollution
The general strategies to mitigate heavy metal pollution include both preventive
and corrective measures. The preventive measures aim at reducing the use of these
heavy metals at their sources and thus their escape into the aquatic environment.
Alternative manufacturing processes and more environmentally benign substitutes have
been explored and implemented to augment the “reduction-at-source” strategy. Once
released, heavy metals will not be degraded and will remain in the environment until
effective containment is carried out. The common technologies for treating heavy metals
laden liquid effluents from either industry or municipality sources are chemical
precipitation, followed by solid-liquid separation or further polishing of the effluents by
ion-exchange process or activated carbon adsorption. However, a lack of adequate
centralized facilities for the treatment as well as the inability of existing facilities to meet
the growing treatment demands are not uncommon now. For further protection of water
quality, installation of point-of-use (POU) small scale water treatment facilities has been
recommended (USEPA, 2004).
6.1.3 Adsorption and Adsorbents for Removal of Heavy Metals
Generally, the simplest form of heavy metals found in contaminated water or
polluted industrial discharge is their cations. Heavy metals in aqueous solution may also
exist in other forms such as metal anions, metal-organic complexes and even nonionic
species. Hydrolysis reactions are common to most heavy metal cations in the aquatic
environment, which gives rise to various forms of hydrolysis by-products such as metal
oxides and hydroxides (Baes and Mesmer, 1977). Complexation between heavy metal
cations or anions and naturally occurring organic materials such as humic substances
also determine, to a large extent, the bioavailability and mobility of heavy metals, which
further complicates their fate and transport as well as their removal (Ritchie and Sposito,
2002). Various separation processes targeted at heavy metal removal is briefly listed in
Table 6.2. Each process has its own strengths and shortcomings as different heavy
metals have different properties.
Adsorption-based separation processes have been widely applied in
chemical/petroleum industries and water/wastewater treatment plants. Partial lists of