Environmental Engineering Reference
In-Depth Information
table 17.1
guideline values for chemicals that have health significance in drinking water [1]
chemical
Guideline value (mg/l)
1,1,1-trichloro-2,2′bis(p-chlorophenyl)ethane (DDT)
0.001
2,4,6-trichlorophenol (Tcp)
0.2
Arsenic (As)
0.01
cadmium (cd)
0.003
chromium (cr)
0.05
copper (cu)
2
lead (pb)
0.01
Mercury (Hg)
0.006
nickel (ni)
0.07
nitrate (as nO
3
−
)
50
Tetrachloroethene (c
2
cl
4
)
0.04
17.2
nanoparticles in Water remediation
nanoparticles have shown great potential in water remediation because their small sizes (1-100 nm) allow them to cover larger
surface areas with no resistance on internal diffusion [8]. They can also be anchored onto a solid matrix, such as activated
carbon (Ac) or zeolite, to enhance their effectiveness in water remediation [9], thereby presenting opportunities to resolve or
greatly ameliorate water remediation problems [3, 10]. Water remediation can be achieved through adsorption [11-13], photo-
catalytic degradation [14-17], and disinfection [18-20] using nanoadsorbents, nanophotocatalysts, and bioactive
nanoparticles.
17.2.1
Water remediation by nanoadsorbents via the adsorption technique
The adsorption technique using nanoadsorbents is one of the most common and widely used methods in removing certain
classes of water contaminants. In principle, the adsorption technique is beneficial for the removal of water contaminants as well
as for recovery purposes [21]. The adsorbed toxic materials are recovered in a concentrated form for disposal, and the valuable
adsorbed materials are recycled for industrial purposes [22]. The adsorption technique is a simple, low-cost, and effective tech-
nique for removing organic and inorganic contaminants. In addition, dyes [23] and phenolic contaminants [24, 25] with low
biodegradability, as well as heavy metal ions, such as As [12, 23, 26-29], cu [11], Hg [11], cd [11], pb [11, 30], cr [11, 23,
30], and cobalt (co) [24], can also be effectively removed by nanoadsorbents.
nanoadsorbents with high surface area to volume ratios are required to achieve high adsorption capacity for water contam-
inants [10]. nanosized particles are more efficient than larger-sized particles as adsorbents in water remediation because the
former are thermodynamically metastable and tend to transform into more stable forms by adsorbing substances, such as mol-
ecules, onto their surfaces to reduce the total free energy and approach the equilibrium state. There are several types of nanopar-
ticles that are suitable and can be used as nanoadsorbents for the removal of water contaminants, such as Fe
0
[9, 25-27, 29-32]
and nanosized metal oxides [8, 11, 12, 21, 23, 33]. Other than the selection of suitable adsorbents, the adsorption mechanisms
of nanoadsorbents should be studied to improve adsorption efficiency during water remediation [27].
17.2.1.1 Nanoscale Particles
Fe
0
is a nontoxic, inexpensive, highly stable, and durable substance. Hence, it is suitable for
water remediation and can be sustained for a long period of time without any loss in activity [25]. Furthermore, nanoscale
Fe
0
is an effective reductant in treating a wide variety of water contaminants, such as chlorinated benzenes, chlorinated eth-
enes, pesticides, organic dyes (Orange II, acid orange, acid red, chrysoidine, and tropaeolin O), heavy metal ions (Hg, cd,
ni, and silver (Ag)), organic contaminants (
N
-nitrosodimethylamine and trinitrotoluene(TnT)), and inorganic anions (As,
dichromate (cr
2
O
7
2−
), perchlorate (clO
4
−
), and nO
3
−
) [9]. The mechanism of nanoscale Fe
0
in water remediation often
involves the iron (Fe) oxidation process. The oxidation stoichiometry of nanoscale Fe
0
is given by equations 17.1 and 17.2.
In the presence of oxygen and water, nanoscale Fe
0
is rapidly oxidized to form free Fe ions through the release of electrons
[9]. Therefore, highly reactive nanoscale Fe
0
should be stored in ethanol, not in water, because water contains higher levels
of dissolved oxygen than ethanol. Zhang et al. [9] reported that the electron released from the oxidation of nanoscale Fe
0
is
accepted by c
2
cl
4
in water, and then reduced to ethane (c
2
H
4
) in accordance with equation 17.3, as given in Table 17.2.
