274
B. Gunawardana et al.
The application of bimetals synthesized with the combination of another secondary metal such as Ni, Pd, or Pt [comprise of a high reduction potential than ZVI;
ZVI, −0.44 V; Ni, −0.257 V; Pd, 0.915 V; or Pt, 1.188 V; Arning and Minteer (2007)]
on the ZVI surface has been tested to mitigate the limitations to the use of ZVI to
remove chlorinated organic compounds (Choi et al. 2008; Gunawardana et al. 2019;
Kim and Carraway 2000; Ko et al. 2007; Shih et al. 2011; Wang et al. 2008; Zhang
et al. 2006). Enhanced hydrodechlorination of chlorinated phenols was reported by
bimetals (Choi et al. 2008; Ko et al. 2007; Liu et al. 2001; Xu et al. 2012; Zhou et al.
2010) and aliphatic compounds (Feng and Lim 2005; Schrick et al. 2002). On the
other hand, some studies did not observe any PCP degradation when using unmodified ZVI (Hou et al. 2009; Morales et al. 2002), while Pd/Fe, Pd/Mg (Morales et al.
2002), and Ni/Fe (Cheng et al. 2010; Zhang et al. 2006) partially dechlorinated PCP
with build up of lower degree CPs and phenol. The nanoscale bimetals (e.g. Pd/Fe)
showed high reactivity and transformed 4-CP, 24-DCP, and 246-TCP to phenol (Zhou
et al. 2010). In contrast, Kim and Carraway (2000) reported a significant reduction
in PCP dechlorination when using bimetals such as Pd/Fe, Pt/Fe, Ni/Fe, Cu/Fe compared to unmodified ZVI. The increased reactivity of bimetals could be attributed
to the (1) secondary metal acting as a catalyst and enhancing the CP hydrogenation
reaction and (2) formation of galvanic cells and enhances the electron transfer process (Tian et al. 2009). The cost of Pd or Pt is high, thus limiting the field application
potential. Hence, Ni as an effective and economical alternative secondary metal is
preferable for actual field application of bimetals as the reactive medium in PRB
systems (Kim and Carraway 2000).
During the reactions, dechlorination is the preferred mechanism. However, other
than the dechlorination process, incorporation of CP molecules with the oxides during
their evolution throughout the reaction claims a crucial role in the CPs removal from
water by bimetal/water systems (Gunawardana et al. 2018, 2019; Noubactep 2008).
Interestingly, CPs incorporation, that is sorption, co-precipitation, and/or physical
entrapment, with the iron oxide phases occurs concurrently with the dechlorination
(Gunawardana et al. 2019). The incorporated CP molecules with the oxide layers
limit the availability of CPs in the aqueous phase in direct contact with the bimetals
for dechlorination, thus such incorporated CPs cannot be further degraded. Further,
the incorporation process could be influenced by the physical–chemical properties of
CPs and affect the efficiency of bimetals for the CPs dechlorination process. Increased
amounts of ligand sorption on to iron oxides have been observed with the increase in
pKa values of the ligand compounds (Song et al. 2008). Thus, it can be hypothesized
that the affinity for incorporation of CPs with the oxide phases will increase with the
decrease in the degree of chlorination of CPs; pKa of 2,4-DCP, 2,4,6-TCP, 2,3,4,6
TeCP, and PCP are 7.68, 5.97–7.42, 5.22–5.62, 4.60–5.25, respectively (Olaniran
and Igbinosa 2011; Shiu et al. 1994). In addition, the changes in the morphology of
the ZVI or bimetal surface due to the formation of various iron oxide phases over
time under different conditions can change the surface properties/reactivity of iron
and dramatically affect the CP removal processes from solution (Gunawardana et al.
2018, 2019).
B. Gunawardana et al.
The application of bimetals synthesized with the combination of another secondary metal such as Ni, Pd, or Pt [comprise of a high reduction potential than ZVI;
ZVI, −0.44 V; Ni, −0.257 V; Pd, 0.915 V; or Pt, 1.188 V; Arning and Minteer (2007)]
on the ZVI surface has been tested to mitigate the limitations to the use of ZVI to
remove chlorinated organic compounds (Choi et al. 2008; Gunawardana et al. 2019;
Kim and Carraway 2000; Ko et al. 2007; Shih et al. 2011; Wang et al. 2008; Zhang
et al. 2006). Enhanced hydrodechlorination of chlorinated phenols was reported by
bimetals (Choi et al. 2008; Ko et al. 2007; Liu et al. 2001; Xu et al. 2012; Zhou et al.
2010) and aliphatic compounds (Feng and Lim 2005; Schrick et al. 2002). On the
other hand, some studies did not observe any PCP degradation when using unmodified ZVI (Hou et al. 2009; Morales et al. 2002), while Pd/Fe, Pd/Mg (Morales et al.
2002), and Ni/Fe (Cheng et al. 2010; Zhang et al. 2006) partially dechlorinated PCP
with build up of lower degree CPs and phenol. The nanoscale bimetals (e.g. Pd/Fe)
showed high reactivity and transformed 4-CP, 24-DCP, and 246-TCP to phenol (Zhou
et al. 2010). In contrast, Kim and Carraway (2000) reported a significant reduction
in PCP dechlorination when using bimetals such as Pd/Fe, Pt/Fe, Ni/Fe, Cu/Fe compared to unmodified ZVI. The increased reactivity of bimetals could be attributed
to the (1) secondary metal acting as a catalyst and enhancing the CP hydrogenation
reaction and (2) formation of galvanic cells and enhances the electron transfer process (Tian et al. 2009). The cost of Pd or Pt is high, thus limiting the field application
potential. Hence, Ni as an effective and economical alternative secondary metal is
preferable for actual field application of bimetals as the reactive medium in PRB
systems (Kim and Carraway 2000).
During the reactions, dechlorination is the preferred mechanism. However, other
than the dechlorination process, incorporation of CP molecules with the oxides during
their evolution throughout the reaction claims a crucial role in the CPs removal from
water by bimetal/water systems (Gunawardana et al. 2018, 2019; Noubactep 2008).
Interestingly, CPs incorporation, that is sorption, co-precipitation, and/or physical
entrapment, with the iron oxide phases occurs concurrently with the dechlorination
(Gunawardana et al. 2019). The incorporated CP molecules with the oxide layers
limit the availability of CPs in the aqueous phase in direct contact with the bimetals
for dechlorination, thus such incorporated CPs cannot be further degraded. Further,
the incorporation process could be influenced by the physical–chemical properties of
CPs and affect the efficiency of bimetals for the CPs dechlorination process. Increased
amounts of ligand sorption on to iron oxides have been observed with the increase in
pKa values of the ligand compounds (Song et al. 2008). Thus, it can be hypothesized
that the affinity for incorporation of CPs with the oxide phases will increase with the
decrease in the degree of chlorination of CPs; pKa of 2,4-DCP, 2,4,6-TCP, 2,3,4,6
TeCP, and PCP are 7.68, 5.97–7.42, 5.22–5.62, 4.60–5.25, respectively (Olaniran
and Igbinosa 2011; Shiu et al. 1994). In addition, the changes in the morphology of
the ZVI or bimetal surface due to the formation of various iron oxide phases over
time under different conditions can change the surface properties/reactivity of iron
and dramatically affect the CP removal processes from solution (Gunawardana et al.
2018, 2019).