Environmental Engineering Reference
In-Depth Information
At the same time, catalytic activity [10], electrochemical conversion [11], and electroanalyti-
cal current signal all increase with the active surface area. Therefore, both catalytic and
electrochemical devices for contaminant destruction could use nanoporous materials. The
same is true for electrochemical sensors [12] and deionizers [13].
A porous material is a solid with free space (pores) that is illed by the luid phase (vac-
uum, gas, pure liquid, solution) and can be illed totally or partially by another solid. To
be useful in aquananotechnology, a signiicant portion of the pores must be open, that is,
accessible to the outside aqueous solution. The International Union of Pure and Applied
Chemistry (IUPAC) classiies the pores into three domains: micropores (diameter [
d
] <
2 nm), mesopores (2 nm <
d
< 50 nm), and macropores (
d
> 50 nm). The yardstick is the
size of the molecules that can be adsorbed on the pore surface. Only small molecules
(e.g., N
2
) can enter micropores, while any low molecular weight molecules can enter
mesopores. For molecular adsorption, macropores are less interesting because the spe-
ciic surface area of a common material (e.g., carbon) only having macropores will be low
(<1 m
2
/g). However, large molecules (e.g., DNA) or even nanoparticles can be adsorbed
inside macropores. Therefore, macropores could be used to immobilize nanoparticles
in macroscopic solids to avoid release into the environment. Additionally, mass trans-
port is usually hindered in mesopores and micropores, while mass transport in macro-
pores is similar to solution. Therefore, waterborne species (e.g., contaminants) will have
easy access to the nanoparticles inside the macroporous solid. On the other hand, it is
possible to prepare materials having pores of two different kinds (e.g., macropores and
mesopores). Such materials are called hierarchical and have properties related to each
pore size.
2.1.2 Historical Porous Materials
Different porous solids have been used for water remediation. Zeolites are natural or syn-
thetic microporous (subnanometric) aluminosilicate minerals that are used for ion adsorp-
tion [14]. They are used for ammonium adsorption from wastewater [15]. Ion-exchange
resins are usually made of cross-linked polymers with functional groups covalently
bonded to the polymer chains that interact with free ions present in the solution [16]. The
porosity is given by the spaces left between linked chains (microporosity) [17], and due to
the effect of porogens added during synthesis (macropores) [18]. The resins are extensively
used for adsorption of deleterious ions in water treatment [19]. Activated carbon is a widely
used material for water puriication [20]. It is produced by carbonization of cellulosic mate-
rials [21], followed by chemical or physical activation [22]. The solid has disordered poros-
ity (mesopores and micropores) with noncylindrical shapes whose tortuosity makes the
mass transport dificult [23]. The graphene-like surface of the pores promotes the physi-
sorption of organic substances (e.g., benzene) [24,25]. On the other hand, the surface can
bear different surface groups (>C=O, -COOH, etc.) [26] that can interact with ions in water
to promote chemisorption [27]. Additionally, the surface can be modiied chemically to
enhance adsorption [28], using organic chemistry reactions [29]. Nanoporous silica can be
easily produced by sol-gel chemistry [30,31]. The pore surface is highly hydrophilic and
contains mainly Si-OH groups, which could chemisorb ions from water [32]. Additionally,
the well-known silane coupling chemistry [33] can be used to modify the pore surface and
increase adsorption [34]. Other inorganic materials can be made into nanoporous solids,
such as aluminosilicates [35,36] and metallic oxides [37]. The solids have been used to
remove toxic ions from water [38]. Additionally, natural inorganic porous solids, such as
diatomaceous earth, have been extensively used for water puriication [39].
