Dissolved
silicate is an important background constituent of natural waters,
but there is little clarity regarding the effect of silicate on the
oxidizing capability of permanganate (Mn(VII)) and on its efficiency
for remediation applications. In the present study, we found that
dissolved silicate, metasilicate or disilicate (DS), could significantly
promote the oxidation of 2,4-dichlorophenol (2,4-DCP) by Mn(VII),
and the extent of the promoting effect was even more evident than
that of pyrophosphate (PP). The experiments showed that, unlike PP,
DS was not capable of coordinating with Mn(III) ions, and the promoting
effect of DS was not due to the oxidizing capability of complexed
Mn(III). Instead, DS ions, as a weak base, could combine with the
hydroxyl groups of MnO2 via hydrogen bonding to limit the
growth of colloidal MnO2 particles. The DS-stabilized colloidal
MnO2 particles, with hydrodynamic diameters less than 100
nm, could act as catalysts to enhance the oxidation of 2,4-DCP by
Mn(VII). The best promoting effect of DS on the performance of Mn(VII)
oxidant was achieved at the initial solution pH of 7, and the coexisting
bicarbonate ions further improved the oxidation of 2,4-DCP in the
Mn(VII)/DS system. Sand column experiments showed that the combined
use of Mn(VII) and DS additive could mitigate the problem of permeability
reduction of sand associated with the retention of MnO2 particles. This study not only deepens our understanding on the
role of dissolved silicate in a Mn(VII) oxidation process but also
provides an effective and green method to enhance the oxidizing capacity
of Mn(VII)-based treatment systems.
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