Environmental Engineering Reference
In-Depth Information
heavy metals necessitate the development of technically and economically feasible
processes for the removal of these metals from wastewater (Keskinkan et al., 2003).
As one of the most promising techniques for the removal of heavy metals from
electroplating wastewaters, adsorption technology has been used for many years and the
effectiveness of various adsorbents was demonstrated. With hazardous waste
minimization regulated by the Congress of the United States in 1984, adsorption process
highlighted its importance on hazardous waste minimization. Many kinds of adsorbents
for wastewater treatment have been commercialized or are being developed (Kannan and
Sundaram, 2001; Yu et al., 2001). Activated carbon as the most common adsorbent used
in adsorption process shows its higher efficiency for the adsorption of organic than
inorganic matters. The spent activated carbon is either landfilled or regenerated at rather
high temperatures commonly used by commercial regenerators (Manuel et al., 1995).
However, the regeneration loss may be as much as 10%, even with well-operated
systems. Also, regeneration usually affects the properties of the carbon. Generally, the
capacity of carbons is expected to be around 90% of the original capacity after
regeneration (Cooney, 1999). As a result, the regeneration of this kind adsorbent is
limited because of its high-cost, loss of capacity, and operating difficulties. Besides the
activated carbon, sawdust and peat also have been commonly used for the adsorption
purpose. In most cases, these commercialized adsorbents are highly porous materials,
providing adequate surface area for adsorption. However, the existence of intraparticle
diffusion may lead to the decrease in the adsorption rate and available capacity,
especially for macromolecules. Thus, developing an adsorbent with large surface area
and small diffusion resistance is of great significance in practical engineering
applications.
With the developments in nanotechnology, nanoscale materials have gained
attention to treat wastewater and soils by accelerating the coagulation of sewage,
removing radionuclides, adsorbing organic dyes, and cleaning up contaminated soils
(Masciangioli et al., 2003). The high surface area and high active surface sites of
nanomaterials may lead to superior adsorption capacity and selective removal of specific
pollutants. For example, nanotubes have been suggested as “a superior sorbent” for
dioxins. The adsorption capacity of dioxin on carbon nanotubes was found to be almost
3 times that of activated carbon. The Langmuir constant b, a parameter characterizing
the sorption affinity, was many orders of magnitude higher for nanotubes than for
activated carbon (Long and Yang, 2001). Nanosized zeolites were developed to
selectively oxidize hydrocarbons, such as toluene and benzaladehyde. Selectivity for
benzaladehyde using the nanostructured zeolite was 87%, as compared to 35% for the
same reaction with conventional zeolite material (Panov et al., 2000). Moreover, it is
possible that the adsorption reaches equilibrium in a short time when compared to the
porous adsorbents. The iron sulfide nanoparticles were used effectively in the removal of
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