12 Chlorophenols Dechlorination Water Treatment Using …
281
as the main degradation products (Fig. 12.1d). Degradation of 2,4-DCP using Ni/Fe
resulted in the accumulation of minute concentrations of MCPs as the end product
(Fig. 12.1e).
The mass balance recoveries obtained for PCP and 2,3,4,6-TeCP with Ni/Fe
bimetallic system were within 93–96% of the amount of respective CP initially
added to the solution (Fig. 12.1). The remaining two CPs (2,4,6-TCP and 2,4-DCP)
demonstrated high mass balance recoveries (91–97%) during the initial reaction
period (Fig. 12.1). The 2,4,6-TCP and 2,4-DCP concentrations decreased over the
reaction time, and however, a noticeable increase in the corresponding dechlorination products was not observed resulting in a gradual decrease of mass balance of
these two CPs over time (Fig. 12.1). Despite the greater mass balance recoveries
demonstrated by 2,4,6-TCP and 2,4-DCP at the beginning of the reaction period,
these mass recoveries decreased considerably to between 70 and 80% during the
later stages of the reaction period (Fig. 12.1d, e). As such, it could be suggested that
CPs incorporation must have been a significant process of removal of 2,4,6-TCP and
2,4-DCP from solution when using Ni/Fe. The incomplete recovery of the parent CPs
with a lower degree of chlorination and/or their degradation products could possibly
be attributed to the formation of iron oxides with different characteristics, strong
incorporation of CP molecules with the oxides formed during the reaction and lack
of extraction/recovery of CPs and the degradation products during the extraction
process (these aspects are elaborated in the subsequent sections).
The CPs dechlorination reaction with Ni/Fe bimetal demonstrated pseudo-firstorder behaviour with C t = C 0 e
−kt , where C 0 is the initial CP concentration (µM), C t
is the CP concentrations at reaction time t (µM), and k is the first-order observed reaction rate constant (day
−1 ). The observed reaction rate constants were 0.0594 day
−1 ,
0.0275 day
−1 , 0.0180 day
−1 , and 0.0170 day
−1 for PCP, 2,3,4,6-TeCP, 2,4,6-TCP and
2,4-DCP, respectively. The trend of the observed rate constants (highest to lowest)
further supports the observed reactivity of the CPs with Ni/Fe (PCP > 2,3,4,6-TeCP >
2,4,6-TCP > ≈ 2,4-DCP). The behaviour and observed rate constants are consistent
with previous research reports where the batch experiment results proved first-order
behaviour when chlorinated phenols were degraded with bimetallic systems (Choi
et al. 2008; Kim and Carraway 2000; Ko et al. 2007; Liu et al. 2001). The general
trend of CP reaction rates with Ni/Fe bimetal (1) did not comply with the data presented by Ko et al. (2007) who reported increased reaction rates using Ni/Fe with the
decreased degree of chlorination and (2) agree with Patel and Suresh (2006) findings,
who reported a decrease in reaction rates with the decreased degree of chlorination
using Mg/Ag bimetal. This discrepancy between the reaction rates of this study and
previous studies could be due to the differences in, (1) experimental conditions, (2)
ZVI characteristics, (3) type/quantity of catalyst used, and (4) method of reaction
rates estimation. The surface area normalized dechlorination reaction rate constants
(K SA ) of CPs in water-Ni/Fe bimetal system was calculated based on the BET specific
surface area of Ni/Fe and found to be 0.02 × 10
−2 Lm
−2 day
−1 for PCP and 0.01 ×
10
−2 Lm
−2 day
−1 for 2,3,4,6-TeCP, 2,4,6-TCP and 2,4-DCP. It was not possible to
estimate the area of only the reactive sites on the Ni/Fe surface; the measured BET
specific surface area of Ni/Fe metal surface is an absolute measurement representing
281
as the main degradation products (Fig. 12.1d). Degradation of 2,4-DCP using Ni/Fe
resulted in the accumulation of minute concentrations of MCPs as the end product
(Fig. 12.1e).
The mass balance recoveries obtained for PCP and 2,3,4,6-TeCP with Ni/Fe
bimetallic system were within 93–96% of the amount of respective CP initially
added to the solution (Fig. 12.1). The remaining two CPs (2,4,6-TCP and 2,4-DCP)
demonstrated high mass balance recoveries (91–97%) during the initial reaction
period (Fig. 12.1). The 2,4,6-TCP and 2,4-DCP concentrations decreased over the
reaction time, and however, a noticeable increase in the corresponding dechlorination products was not observed resulting in a gradual decrease of mass balance of
these two CPs over time (Fig. 12.1). Despite the greater mass balance recoveries
demonstrated by 2,4,6-TCP and 2,4-DCP at the beginning of the reaction period,
these mass recoveries decreased considerably to between 70 and 80% during the
later stages of the reaction period (Fig. 12.1d, e). As such, it could be suggested that
CPs incorporation must have been a significant process of removal of 2,4,6-TCP and
2,4-DCP from solution when using Ni/Fe. The incomplete recovery of the parent CPs
with a lower degree of chlorination and/or their degradation products could possibly
be attributed to the formation of iron oxides with different characteristics, strong
incorporation of CP molecules with the oxides formed during the reaction and lack
of extraction/recovery of CPs and the degradation products during the extraction
process (these aspects are elaborated in the subsequent sections).
The CPs dechlorination reaction with Ni/Fe bimetal demonstrated pseudo-firstorder behaviour with C t = C 0 e
−kt , where C 0 is the initial CP concentration (µM), C t
is the CP concentrations at reaction time t (µM), and k is the first-order observed reaction rate constant (day
−1 ). The observed reaction rate constants were 0.0594 day
−1 ,
0.0275 day
−1 , 0.0180 day
−1 , and 0.0170 day
−1 for PCP, 2,3,4,6-TeCP, 2,4,6-TCP and
2,4-DCP, respectively. The trend of the observed rate constants (highest to lowest)
further supports the observed reactivity of the CPs with Ni/Fe (PCP > 2,3,4,6-TeCP >
2,4,6-TCP > ≈ 2,4-DCP). The behaviour and observed rate constants are consistent
with previous research reports where the batch experiment results proved first-order
behaviour when chlorinated phenols were degraded with bimetallic systems (Choi
et al. 2008; Kim and Carraway 2000; Ko et al. 2007; Liu et al. 2001). The general
trend of CP reaction rates with Ni/Fe bimetal (1) did not comply with the data presented by Ko et al. (2007) who reported increased reaction rates using Ni/Fe with the
decreased degree of chlorination and (2) agree with Patel and Suresh (2006) findings,
who reported a decrease in reaction rates with the decreased degree of chlorination
using Mg/Ag bimetal. This discrepancy between the reaction rates of this study and
previous studies could be due to the differences in, (1) experimental conditions, (2)
ZVI characteristics, (3) type/quantity of catalyst used, and (4) method of reaction
rates estimation. The surface area normalized dechlorination reaction rate constants
(K SA ) of CPs in water-Ni/Fe bimetal system was calculated based on the BET specific
surface area of Ni/Fe and found to be 0.02 × 10
−2 Lm
−2 day
−1 for PCP and 0.01 ×
10
−2 Lm
−2 day
−1 for 2,3,4,6-TeCP, 2,4,6-TCP and 2,4-DCP. It was not possible to
estimate the area of only the reactive sites on the Ni/Fe surface; the measured BET
specific surface area of Ni/Fe metal surface is an absolute measurement representing