12 Chlorophenols Dechlorination Water Treatment Using …
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(Gunawardana et al. 2018, 2019; Henderson and Demond 2007, 2011; Jeen 2005;
Jin suk et al. 2009; Li and Benson 2010). Among the many factors, the inherent
characteristics of the iron oxide phases cause a major influence on the reactivity of
ZVI and dechlorination potential (Gunawardana et al. 2018, 2019). Such iron oxide
phases can be the oxides initially present on the ZVI surface before reaction with
CPs as well as the oxides continuously evolving during the reaction of CPs in the
CP-ZVI system. In general, different iron oxide phases are present as a combination
on an as-received unmodified ZVI particle surface that is formed because of the
exposure of ZVI to atmospheric O 2 during many stages of ZVI processing such as
the production process and storage conditions (Cornell and Schwertmann 2003). In
addition, when ZVI is introduced to an aqueous medium, various iron oxides and/or
oxyhydroxide precipitates continuously evolve and accumulate on the ZVI surface
because of Fe
0 oxidation and ZVI corrosion occurring in the ZVI/H 2 O environment
(Matheson and Tratnyek 1994). The iron oxides initially present or form during the
reactions can also affect the ZVI performance over time. There are several competing
mechanisms by which the iron oxides formed on ZVI can either enhance or inhibit
ZVI reactivity. These oxides can act as (1) reactive or non-reactive sorption sites
for contaminant molecules, (2) incorporate the contaminants and entrap within an
oxide film during its formation, (3) affect the number of effective reactive sites and
surface area available on ZVI for dechlorination, and (4) impede the electron transfer
process, especially when these oxide phases formed on the ZVI are non-conducting
passive oxide layers (Davenport et al. 2000; Gotpagar et al. 1999; Noubactep 2007;
2013; Ritter et al. 2002; Scherer et al. 1999).
The characteristics of different iron oxide phases on the ZVI surfaces (initially
present and that are formed while reacting with the CPs) play a significant role on
the ZVI reactivity for dechlorination (Gunawardana et al. 2018). Magnetite is known
to comprise a high electrical conductivity with a low bandgap between its valence
and conduction bands (0.1 eV;) Cornell and Schwertmann (2003) and lead to an
enhancement in the electron transfer (Liu et al. 2006). In contrast, some Fe
III oxides
and oxyhydroxides claim for a higher bandgap between the valence and conduction
bands (e.g. lepidocrocite 2.06 eV; goethite 2.10 eV; maghemite 2.03 eV; and wustite
2.30 eV) (Cornell and Schwertmann 2003). These oxides result in ZVI surface passivation and hinder the ability of electrons and contaminant molecules transfer process
through the interfaces of solid and liquid (Farrell et al. 2000). ZVI surface passivation
has been observed when non-conducting oxides (haematite, 2.20 eV; lepidocrocite,
wusite) form on the ZVI surface which seems to hinder the electron transfer process
from the Fe
0 to CP molecules (Gunawardana et al. 2019). Therefore, the application of ZVI in PRBs for CP dechlorination is limited by various factors such as (1)
decrease in degradation rates during the reaction period because of reduction in ZVI
surface reactivity as a result of passive oxide phases formation, and (2) build up of
degradation products during the reaction which may be competitors simultaneously
present in the aqueous medium competing as co-contaminants for the same reactive
sites and electrons for dechlorination (Gunawardana et al. 2018).
273
(Gunawardana et al. 2018, 2019; Henderson and Demond 2007, 2011; Jeen 2005;
Jin suk et al. 2009; Li and Benson 2010). Among the many factors, the inherent
characteristics of the iron oxide phases cause a major influence on the reactivity of
ZVI and dechlorination potential (Gunawardana et al. 2018, 2019). Such iron oxide
phases can be the oxides initially present on the ZVI surface before reaction with
CPs as well as the oxides continuously evolving during the reaction of CPs in the
CP-ZVI system. In general, different iron oxide phases are present as a combination
on an as-received unmodified ZVI particle surface that is formed because of the
exposure of ZVI to atmospheric O 2 during many stages of ZVI processing such as
the production process and storage conditions (Cornell and Schwertmann 2003). In
addition, when ZVI is introduced to an aqueous medium, various iron oxides and/or
oxyhydroxide precipitates continuously evolve and accumulate on the ZVI surface
because of Fe
0 oxidation and ZVI corrosion occurring in the ZVI/H 2 O environment
(Matheson and Tratnyek 1994). The iron oxides initially present or form during the
reactions can also affect the ZVI performance over time. There are several competing
mechanisms by which the iron oxides formed on ZVI can either enhance or inhibit
ZVI reactivity. These oxides can act as (1) reactive or non-reactive sorption sites
for contaminant molecules, (2) incorporate the contaminants and entrap within an
oxide film during its formation, (3) affect the number of effective reactive sites and
surface area available on ZVI for dechlorination, and (4) impede the electron transfer
process, especially when these oxide phases formed on the ZVI are non-conducting
passive oxide layers (Davenport et al. 2000; Gotpagar et al. 1999; Noubactep 2007;
2013; Ritter et al. 2002; Scherer et al. 1999).
The characteristics of different iron oxide phases on the ZVI surfaces (initially
present and that are formed while reacting with the CPs) play a significant role on
the ZVI reactivity for dechlorination (Gunawardana et al. 2018). Magnetite is known
to comprise a high electrical conductivity with a low bandgap between its valence
and conduction bands (0.1 eV;) Cornell and Schwertmann (2003) and lead to an
enhancement in the electron transfer (Liu et al. 2006). In contrast, some Fe
III oxides
and oxyhydroxides claim for a higher bandgap between the valence and conduction
bands (e.g. lepidocrocite 2.06 eV; goethite 2.10 eV; maghemite 2.03 eV; and wustite
2.30 eV) (Cornell and Schwertmann 2003). These oxides result in ZVI surface passivation and hinder the ability of electrons and contaminant molecules transfer process
through the interfaces of solid and liquid (Farrell et al. 2000). ZVI surface passivation
has been observed when non-conducting oxides (haematite, 2.20 eV; lepidocrocite,
wusite) form on the ZVI surface which seems to hinder the electron transfer process
from the Fe
0 to CP molecules (Gunawardana et al. 2019). Therefore, the application of ZVI in PRBs for CP dechlorination is limited by various factors such as (1)
decrease in degradation rates during the reaction period because of reduction in ZVI
surface reactivity as a result of passive oxide phases formation, and (2) build up of
degradation products during the reaction which may be competitors simultaneously
present in the aqueous medium competing as co-contaminants for the same reactive
sites and electrons for dechlorination (Gunawardana et al. 2018).