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01.Activated Carbons as Catalyst Supports (pp.169-204)
02.Combination of Ozone and Activated Carbon for Water and Wastewater Treatment (pp.433-474)
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Combination of Ozone and Activated Carbon for Water and Wastewater Treatment (pp.433-474) $100.00
Authors:  (F. Beltran and P.M. Alvarez, Departamento de Ingeniería Química y Química Física, Universidad de Extremadura, Badajoz, Spain)
Abstract:
One the major problems wastewater and drinking water treatment plants, (WWTPs and
DWTPs, respectively) will likely have to face in the near future is the removal of emergent
contaminants coming from the abundant use of pharmaceutical and personal care products
(PPCPs). These compounds, as well as others already catalogued as priority pollutants, are
usually encountered in influents and effluents of WWTPs (Halling-Sorensen et al., 1998,
Ternes, 1998; Gómez et al., 2007, Joss et al., 2004, Segura et al., 2007) and even of DWTPs
(Stakelberg et al., 2004, Jones et al., 2005) at low but potentially hazardous concentrations for
humans and other living beings. It has been clearly established that many of these pollutants
go through the classical primary and secondary treatments in water plants without being
properly removed and only most costly tertiary treatments technologies are able to completely
remove them from water. Among these treatments chemical oxidation, particularly advanced
chemical oxidation (ACO), and activated carbon adsorption processes (ACAPs) present high
efficiency to remove these pollutants. Also, among ACO processes, those involving ozone
have already shown their applicability in this field (Ikehata et al., 2008).
The use of activated carbon as adsorbent in water treatment dates back from the 19th
century and today ACAPs are considered as the most cost-effective methods for removal of a
number of pollutants from water and wastewater. Thus, adsorption onto activated carbon has
been designated as best available technology for the adsorption of many synthetic chemicals
from water and wastewater by the United States Environmental Protection Agency (USEPA)
and the European Union. Today about two thirds of the activated carbon world production at
industrial scale is marketed for the removal of pollutants from water and wastewater streams
(Cooney, 1999). Its usefulness derives mainly from their textural and chemical properties
(high surface area per mass which is provided with large micropore and mesopore volumes
and different surface functional groups). Activated carbons can be modeled as a set of
defective hexagonal carbon layer planes crosslinked to aliphatic bridging groups where
heteroatoms are incorporated. These heteroatoms give the character of the functional groups
typically found in aromatic compounds (Yang, 2003). In the adsorption process the
adsorbates are fixed on the carbon surface by physical interactions (i.e., electrostatic and
dispersive forces) and/or chemical bonds (i.e., chemisorption). In the water treatment field,
activated carbon is extensively used as adsorbent in a wide variety of physical forms such as
granular, powdered, fibers, nanotubes, etc. In DWTPs is used to remove taste and odor
compounds, synthetic organic chemicals and natural organic matter in order to control the
formation of some disinfection by-products (DBPs). In WWTPs activated carbon is mainly
used in granular form to remove specific organic micropollutants. Nevertheless, specific
activated carbon grades can be prepared with other particular purposes in water treatment. In
water treatment, adsorption of organics on activated carbon is a complex process where both
water and the organics are simultaneously adsorbed to different extents. There are already
excellent works published on the activated carbon adsorption of organics from water,
including large compilations of adsorption isotherms of priority pollutants such as pesticides,
phenols and chlorinated organic compounds (Faust and Aly, 1987). Main general properties
of activated carbons as adsorbents are their capability to act without being previously dried,
which is one of the reasons of their application in water treatment, their high adsorption
capacity for non-polar and weakly polar organic molecules and their low heat of adsorption
compared to those of other adsorbents. As catalysts, activated carbons can act in many
different gas and liquid reactions processes such as oxidative dehydrogenation, dehydration
and dehydrogenation of alcohols and NOx reduction (gas phase) or hydrogen peroxide, ozone
and wet air oxidation reactions (liquid phase) where their surface chemistry play a key role
(Figueiredo and Pereira, 2010).
Ozone is a powerful oxidant and disinfectant gas that, as activated carbon, started to be
applied in water treatment in the 19th century although then only as disinfectant. However, its
use declined during the first decades of the 20th century due to chlorine advantages and lack
of knowledge of ozone chemistry. It was not until the end of the seventies of the past century
when ozone application in water rose, especially due to its reaction with precursors of DBPs
(i.e., trihalomethane compounds) (Langlais et al., 1991). Years later, knowledge of the ozone
chemistry and kinetics of the processes it undergoes in water also increased and ozone
advanced oxidation processes commenced to be considered as an important tertiary treatment
method to remove priority pollutants (pesticides, polynuclear aromatic hydrocarbons, etc)
from water (Glaze et al., 1987). Nowdays, it is well known that ozone reacts with organic
matter in water through two different ways, the so-called direct and indirect reactions. Direct
reactions are mainly cycloaddition reactions (Criegge mechanism) to double or triple carbon
bonds and electrophilic substitution reactions on nucleophilic points (Bailey, 1958, von
Gunten, 2003) while indirect reactions develop through hydroxyl radicals coming from ozone
reactions with substances called promoters or initiators (Staehelin and Hoigné, 1985).
Hydroxyl radicals are more powerful oxidant species than ozone itself and they are able to
react in an unselective way with organic and inorganic matter present in water. In fact, rate
constants of the reactions between these free radicals and most of the species in water are
around 107-1010 M-1s-1 (Buxton et al., 1988). First initiators to be studied were hydrogen
peroxide and UVC radiation that decompose ozone through fast reactions to yield hydroxyl
radicals (Staehelin and Hoigné, 1982, Peyton and Glaze, 1988, Beltrán, 2003). Hydrogen
peroxide, particularly, constitutes a key agent of many advanced oxidation processes and also
for the activated carbon-ozone reactions (see later). Among initiators, catalysts of different
nature (metals or metal oxides on supported materials, perovskites, etc, are lately subject of
study in a process called catalytic or photocatalytic ozonation depending on the absence or
presence of some radiation source (UVC, UVA, solar radiation, etc) (Legube and Vel Leitner,
1999, Kasprzyk-Hordern et al., 2003, Agustina et al., 2005, Beltrán, 2008). These catalysts
(and radiation, in some cases) trigger the ozone decomposition into hydroxyl radicals
favoring the indirect way of oxidation which is particularly important when refractory
compounds to direct ozonation are present in water (Beltrán et al., 2004). Activated carbons
are one of the possible catalysts to improve the indirect way of ozonation though, so far, the
mechanism of this process is not yet well understood. Ozone and activated carbon are
currently applied in water treatment not only as single processes but as a consecutive process
which is normally called biological activated carbon process (BACP), constituted by
ozonation followed by activated carbon filtration. Also, they can be simultaneously used in a
catalytic ozonation process where activated carbon accelerates the decomposition of ozone in
hydroxyl radicals usually through hydrogen peroxide formation. This latter process is
presently at research and development level. Then, in this chapter these two processes are
reviewed. In addition, since ozone strongly reacts with some functional groups of the
activated carbon surface, the study of the effects of these interactions and how to take
advantage of them to produce activated carbons with special adsorptive properties is also
presented. Finally, this chapter also deals with the activated carbon regeneration process with
ozone, a step that is also subject of research due to the high reactivity of ozone at ambient
conditions compared to other more costly energetic methods for activated carbon
regeneration. 


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Combination of Ozone and Activated Carbon for Water and Wastewater Treatment (pp.433-474)