electron transfer at low and neutral pH. Thereby, pH value has a greater impact on
TCE reduction at low pH. For carbon tetrachloride dechlorination, Jiao et al. (2009)
have shown by an electrochemical analysis that the lower the pH value, the faster the
dechlorination reaction proceeds.
Later, Luo and Farrell (2013) have studied pH effects on PCE and TCE adsorption energies on iron by a molecular mechanics simulation. A decrease in pH is
characterized by a rise in the fractional atomic hydrogen surface coverage, which
leads to a decrease of potential energies of TCE and PCE complexes with iron. As
this phenomenon is more important for TCE complexes, the authors propose that the
complexation equilibrium constant of TCE is increased, resulting in the improvement of TCE dechlorination rate.
Values of reduction constant rates of four COCs at different pH values (ranging
from 1.7 to 13) are shown in Table 6.13. For TCE, a decrease in pH from 8 to 1.7
results in a rate constant six times greater. It is however important to note that the use
of buffers can affect the reactivity of ZVI as the nature of the buffer can affect the
corrosion rate (He et al. 2018a; Qin et al. 2018).
Treatment in acidic conditions is also a benefit as iron corrosion by-products are
more soluble in water in these conditions (Beverskog and Puigdomenech 1996),
which can prevent the formation of passive layer at iron surface (see Pourbaix
diagram in Fig. 6.1b). Therefore, pH affects also the structural evolution of the
particles (Tang et al. 2017b).
Table 6.13 Influence of initial pH on constant rate of reaction with ZVI
Reactant
Pollutant pH
k
References
ZVI
CT
k ¼ À0.018 pH + 0.20
(5.5 < pH < 10.0)
Matheson and Tratnyek
(1994)
ZVI
(0.077 m² g
À1
)
TCE
1.7
0.59 L h
À1 m
À2
Chen et al. (2001)
3.8
0.044 L h
À1 m
À2
4.9
0.050 L h
À1 m
À2
6.1
0.035 L h
À1 m
À2
7.1
0.017 L h
À1 m
À2
8.0
0.009 L h
À1 m
À2
ZVI (0.1582
m² g
À1
)
γ-HCH
4.67 0.0798Æ 0.0054 min
À1
Wang et al. (2009c)
6.73 0.0125 Æ 0.0004 min
À1
8.30 0.00039 Æ0.00004 min
À1
nZVI (40.3
m² g
À1
)
HCB
3.2
0.12 h
À1
Shih et al. (2011b)
5.1
0.088 h
À1
6.8
0.073 h
À1
9.2
0.052 h
À1
13.0 0.052 h
À1
log k ¼ À0.058 pH – 0.755
(3.2 < pH < 9.3)
330
R. Rodrigues et al.
TCE reduction at low pH. For carbon tetrachloride dechlorination, Jiao et al. (2009)
have shown by an electrochemical analysis that the lower the pH value, the faster the
dechlorination reaction proceeds.
Later, Luo and Farrell (2013) have studied pH effects on PCE and TCE adsorption energies on iron by a molecular mechanics simulation. A decrease in pH is
characterized by a rise in the fractional atomic hydrogen surface coverage, which
leads to a decrease of potential energies of TCE and PCE complexes with iron. As
this phenomenon is more important for TCE complexes, the authors propose that the
complexation equilibrium constant of TCE is increased, resulting in the improvement of TCE dechlorination rate.
Values of reduction constant rates of four COCs at different pH values (ranging
from 1.7 to 13) are shown in Table 6.13. For TCE, a decrease in pH from 8 to 1.7
results in a rate constant six times greater. It is however important to note that the use
of buffers can affect the reactivity of ZVI as the nature of the buffer can affect the
corrosion rate (He et al. 2018a; Qin et al. 2018).
Treatment in acidic conditions is also a benefit as iron corrosion by-products are
more soluble in water in these conditions (Beverskog and Puigdomenech 1996),
which can prevent the formation of passive layer at iron surface (see Pourbaix
diagram in Fig. 6.1b). Therefore, pH affects also the structural evolution of the
particles (Tang et al. 2017b).
Table 6.13 Influence of initial pH on constant rate of reaction with ZVI
Reactant
Pollutant pH
k
References
ZVI
CT
k ¼ À0.018 pH + 0.20
(5.5 < pH < 10.0)
Matheson and Tratnyek
(1994)
ZVI
(0.077 m² g
À1
)
TCE
1.7
0.59 L h
À1 m
À2
Chen et al. (2001)
3.8
0.044 L h
À1 m
À2
4.9
0.050 L h
À1 m
À2
6.1
0.035 L h
À1 m
À2
7.1
0.017 L h
À1 m
À2
8.0
0.009 L h
À1 m
À2
ZVI (0.1582
m² g
À1
)
γ-HCH
4.67 0.0798Æ 0.0054 min
À1
Wang et al. (2009c)
6.73 0.0125 Æ 0.0004 min
À1
8.30 0.00039 Æ0.00004 min
À1
nZVI (40.3
m² g
À1
)
HCB
3.2
0.12 h
À1
Shih et al. (2011b)
5.1
0.088 h
À1
6.8
0.073 h
À1
9.2
0.052 h
À1
13.0 0.052 h
À1
log k ¼ À0.058 pH – 0.755
(3.2 < pH < 9.3)
330
R. Rodrigues et al.