Cobalt-mediated activation of peroxymonosulfate
(PMS) has been
widely investigated for the oxidation of organic pollutants. Herein,
we employ cobalt-doped Black TiO2 nanotubes (Co-Black TNT)
for the efficient, stable, and reusable activator of PMS for the degradation
of organic pollutants. Co-Black TNTs induce the activation of PMS
by itself and stabilized oxygen vacancies that enhance the bonding
with PMS and provide catalytic active sites for PMS activation. A
relatively high electronic conductivity associated with the coexistence
of Ti4+ and Ti3+ in Co-Black TNT enables an
efficient electron transfer between PMS and the catalyst. As a result,
Co-Black TNT is an effective catalyst for PMS activation, leading
to the degradation of selected organic pollutants when compared to
other TNTs (TNT, Co-TNT, and Black TNT) and other Co-based materials
(Co3O4, Co-TiO2, CoFe2O4, and Co3O4/rGO). The observed
organic compound degradation kinetics are retarded in the presence
of methanol and natural organic matter as sulfate radical scavengers.
These results demonstrate that sulfate radical is the primary oxidant
generated via PMS activation on Co-Black TNT. The strong interaction
between Co and TiO2 through Co–O–Ti bonds
and rapid redox cycle of Co2+/Co3+ in Co-Black
TNT prevents cobalt leaching and enhances catalyst stability over
a wide pH range and repetitive uses of the catalyst. Electrode-supported
Co-Black TNT facilitates the recovery of the catalyst from the treated
water.
Copper phosphide
(Cu
x
P) was synthesized
and tested for its reactivity for generating H2O2 through spontaneous reduction of dioxygen under ambient aqueous
condition. The in situ generated H2O2 was subsequently
decomposed to generate OH radicals, which enabled the degradation
of organic compounds in water. The oxygen reduction reaction proceeded
along with the concurrent oxidation of phosphide to phosphate, then
copper ions and phosphate ions were dissolved out during the reaction.
The reactivity of Cu
x
P was gradually reduced
during 10 cycles with consuming 8.7 mg of Cu
x
P for the successive removal of 17 μmol 4-chlorophenol.
CoP which was compared as a control sample under the same experimental
condition also produced H2O2 through activating
dioxygen but did not degrade organic compounds at all. The electrochemical
analysis for the electron transfers on Cu
x
P and CoP showed that the number of electrons transferred to O2 is 3 and 2, respectively, which explains why OH radical is
generated on Cu
x
P, not on CoP. The Cu+ species generated on the Cu
x
P
surface can participate in Fenton-like reaction with in situ generated
H2O2. Cu
x
P is proposed
as a solid reagent that can activate dioxygen to generate reactive
oxygen species in ambient aqueous condition, which is more facile
to handle and store than liquid/gas reagents (e.g., H2O2, Cl2, O3).
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