Abstract:
The dechlorination of carbon tetrachloride (CT) by Fe(II)
associated with goethite in the presence of transition
metal ions was investigated. X-ray photoelectron spectroscopy
(XPS) and X-ray powder diffraction (XRPD) were used to
characterize the chemical states and crystal phases of
transition metals on solid phases, respectively. CT was
dechlorinated to chloroform (CF) by 3 mM Fe(II) in 10 mM
goethite (25.6 m2 L-1) suspensions. The dechlorination
followed pseudo-first-order kinetics, and a rate constant
(kobs) of 0.036 h-1 was observed. Transition metal ions have
different effects on CT dechlorination. The addition of
Ni(II), Co(II), and Zn(II) lowered the kobs for CT dechlorination,
whereas the amendment of 0.5 mM Cu(II) into the Fe(II)-
Fe(III) system significantly enhanced the efficiency and
the rate of CT dechlorination. The kobs for CT dechlorination
with 0.5 mM Cu(II) was 1.175 h-1, which was 33 times
greater than that without Cu(II). Also, the dechlorination
of CT by surface-bound iron species is pH-dependent, and
the rate constants increased from 0.008 h-1 at pH 4.0 to
1.175 h-1 at pH 7.0. When the solution contained Cu(II) and
Fe(II) without goethite, a reddish-yellow precipitate was
formed, and the concentration of Fe(II) decreased with the
increase in Cu(II) concentration. XPS and XRPD analyses
suggested the possible presence of Cu2O and ferrihydrite
in the precipitate. Small amounts of aqueous Cu(I) were also
detected, reflecting the fact that Cu(II) was reduced to Cu-
(I) by Fe(II). A linear relationship between kobs for CT
dechlorination and the concentration of Cu(II) was observed
when the amended Cu(II) concentration was lower than
0.5 mM. Moreover, the kobs for CT dechlorination was
dependent on the Fe(II) concentration in the 0.5 mM Cu(II)-
amended goethite system and followed a Langmuir-
Hinshelwood relationship. These results clearly indicate
that Fe(II) serves as the bulk reductant to reduce both CT
and Cu(II). The resulting Cu(I) can further act as a
catalyst to enhance the dechlorination rate of chlorinated
hydrocarbons in iron-reducing environments.
