242
Heavy Metal Removal
Oxidized CNTs have high adsorption capacity for metal ions with fast kinetics. The
surface functional groups (e.g., carboxyl, hydroxyl, and phenol) of CNTs are the
major adsorption sites for metal ions, mainly through electrostatic attraction and
chemical bonding [19, 20]. As a result, surface oxidation can significantly enhance
the adsorption capacity of CNTs. Several studies show that CNTs are better adsorbents than activated carbon for heavy metals (e.g., Cu
2+
, Pb
2+
, Cd
2+
, and Zn
2+
) and
the adsorption kinetics is fast on CNTs due to the highly accessible adsorption sites
and the short intraparticle diffusion distance [16, 21].
Overall, CNTs may not be a good alternative for activated carbon as widespectrum adsorbents. Rather, as their surface chemistry can be tuned to target specific contaminants, they may have unique applications in polishing steps to remove
recalcitrant compounds or in pre-concentration of trace organic contaminants for
analytical purposes. These applications require small quantity of materials and
hence are less sensitive to the material cost.
Produced by exfoliating graphite with strong acids and oxidizers, graphite oxide
is a potentially low-cost adsorbent. It was recently reported that sand granules
coated with graphite oxide were efficient in removing Hg
2+
and a bulky dye molecule (rhodamine B); its performance was comparable to commercial activated carbon [22, 23].
Regeneration and Reuse
Regeneration is an important factor that determines the cost-effectiveness of adsorbents. Adsorption of metal ions on CNTs can be easily reversed by reducing the
solution pH. The metal recovery rate is usually above 90% and often close to 100%
at pH <2 [16, 24]. Moreover, the adsorption capacity remains relatively stable after
regeneration. Lu et al. reported that Zn
2+
adsorption capacity of SWNT and MWNT
decreased less than 25% after ten regeneration and reuse cycles, while that of activated carbon was reduced by more than 50% after one regeneration [16, 25]. A
statistical analysis based on the best-fit regression of Zn
2+
adsorption capacity and
the number of regeneration and reuse cycles suggested that CNT nano- adsorbents
can be regenerated and reused up to several hundred times for Zn
2+
removal while
maintaining reasonable adsorption capacity [26, 27].
Metal-Based Nano-Adsorbents
Metal oxides such as iron oxide, titanium dioxide, and alumina are effective, lowcost adsorbents for heavy metals and radionuclides. The sorption is mainly controlled
by complexation between dissolved metals and oxygen in metal oxides [28, 29]. It is
a two-step process: fast adsorption of metal ions on the external surface, followed by
the rate-limiting intraparticle diffusion along the micropore walls [30, 31]. Their
13 Wastewater
Heavy Metal Removal
Oxidized CNTs have high adsorption capacity for metal ions with fast kinetics. The
surface functional groups (e.g., carboxyl, hydroxyl, and phenol) of CNTs are the
major adsorption sites for metal ions, mainly through electrostatic attraction and
chemical bonding [19, 20]. As a result, surface oxidation can significantly enhance
the adsorption capacity of CNTs. Several studies show that CNTs are better adsorbents than activated carbon for heavy metals (e.g., Cu
2+
, Pb
2+
, Cd
2+
, and Zn
2+
) and
the adsorption kinetics is fast on CNTs due to the highly accessible adsorption sites
and the short intraparticle diffusion distance [16, 21].
Overall, CNTs may not be a good alternative for activated carbon as widespectrum adsorbents. Rather, as their surface chemistry can be tuned to target specific contaminants, they may have unique applications in polishing steps to remove
recalcitrant compounds or in pre-concentration of trace organic contaminants for
analytical purposes. These applications require small quantity of materials and
hence are less sensitive to the material cost.
Produced by exfoliating graphite with strong acids and oxidizers, graphite oxide
is a potentially low-cost adsorbent. It was recently reported that sand granules
coated with graphite oxide were efficient in removing Hg
2+
and a bulky dye molecule (rhodamine B); its performance was comparable to commercial activated carbon [22, 23].
Regeneration and Reuse
Regeneration is an important factor that determines the cost-effectiveness of adsorbents. Adsorption of metal ions on CNTs can be easily reversed by reducing the
solution pH. The metal recovery rate is usually above 90% and often close to 100%
at pH <2 [16, 24]. Moreover, the adsorption capacity remains relatively stable after
regeneration. Lu et al. reported that Zn
2+
adsorption capacity of SWNT and MWNT
decreased less than 25% after ten regeneration and reuse cycles, while that of activated carbon was reduced by more than 50% after one regeneration [16, 25]. A
statistical analysis based on the best-fit regression of Zn
2+
adsorption capacity and
the number of regeneration and reuse cycles suggested that CNT nano- adsorbents
can be regenerated and reused up to several hundred times for Zn
2+
removal while
maintaining reasonable adsorption capacity [26, 27].
Metal-Based Nano-Adsorbents
Metal oxides such as iron oxide, titanium dioxide, and alumina are effective, lowcost adsorbents for heavy metals and radionuclides. The sorption is mainly controlled
by complexation between dissolved metals and oxygen in metal oxides [28, 29]. It is
a two-step process: fast adsorption of metal ions on the external surface, followed by
the rate-limiting intraparticle diffusion along the micropore walls [30, 31]. Their
13 Wastewater