THESIS
2014
xvi, 102 pages : illustrations ; 30 cm
Abstract
Halonitromethanes (HNMs) and haloacetonitriles (HANs) are emerging contaminants
posing potential threats to human health even at low concentrations because they have been
found a few orders of magnitudes more toxic than regulated carbonaceous DBPs. The controls
of HNMs, HANs, and their major precursors cannot be achieved via conventional treatment
processes for drinking water supply. Thus, their removals by direct and hydrogen peroxide
(H
2O
2)-assisted UV degradation were investigated.
Preliminary studies revealed that the direct UV photolysis was effective in removing
HNMs, but the H
2O
2-assisted UV degradation was needed in removing HANs. The removal
rates of HNMs followed first-order degradation kinetics and those of HANs followed pseudo
first-order degradation kinetics.
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Halonitromethanes (HNMs) and haloacetonitriles (HANs) are emerging contaminants
posing potential threats to human health even at low concentrations because they have been
found a few orders of magnitudes more toxic than regulated carbonaceous DBPs. The controls
of HNMs, HANs, and their major precursors cannot be achieved via conventional treatment
processes for drinking water supply. Thus, their removals by direct and hydrogen peroxide
(H
2O
2)-assisted UV degradation were investigated.
Preliminary studies revealed that the direct UV photolysis was effective in removing
HNMs, but the H
2O
2-assisted UV degradation was needed in removing HANs. The removal
rates of HNMs followed first-order degradation kinetics and those of HANs followed pseudo
first-order degradation kinetics.
The degradation rates of trichloronitromethane (TCNM) were low at all pHs tested, while
those of bromonitromethane (BNM), dichloronitromethane (DCNM) and
dibromonitromethane (DBNM) increased with increasing pH. The increasing degradation
rates of BNM, DCNM, and DBNM were correlated with their pH-dependent molar
absorptivities and determined by the concentrations of their deprotonated fractions. Half
removals of deprotonated HNMs could be achieved at neutral to alkaline pH with UV dose
similar to that of UV disinfection processes. A homolytic cleavage pathway was proposed for
the protonated BNM, DCNM, and DBNM and TCNM. A heterolytic cleavage pathway was
proposed for deprotonated mono- and di-HNMs.
The H
2O
2-assisted UV degradation rates of monochloroacetonitrile (MCAN),
dichloroacetonitrile (DCAN), and trichloroacetonitrile (TCAN) are determined by the
H
2O
2-assisted hydrolysis rates and the hydroxyl radical oxidation rates of them. The H
2O
2-assisted hydrolysis rates of chlorinated acetonitriles (CANs) increase with increasing
degrees of halogenation of CANs and hydroperoxide ion concentrations in the solution. The
hydroxyl radical oxidation rates of CANs generally decrease with increasing degrees of
halogenation of CANs. The H
2O
2-assisted hydrolysis was considered to dominate the
degradation of TCAN at tested pH and DCAN at pH 7.5 and H
2O
2 concentrations of 5 mM
and 10 mM over the hydroxyl radical oxidation. Complete hydrolysis of TCAN at pH 7.5 was
achieved within 20 minutes at 1 mM H
2O
2, but produced another toxic DBPs TCAcAm.
The H
2O
2-assisted hydrolysis and the hydroxyl radical oxidation pathways of CANs were
proposed.
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