Abstract
In this thesis, a combination of laboratory experimentation under well
defined conditions coupled with a kinetic modelling approach is used to
verify the existence and respective kinetic rates of previously unconfirmed
or postulated mechanisms that drive and limit dark Fenton (Fe(II)/H2O2) -
mediated As(III) oxidation at pH 3 and 8 and dark Cu(II) - H2O2 - mediated
As(III) oxidation at pH 8.
Dark Fenton - mediated oxidation of As(III) at pH 3 is first examined and
the effects of the variation in the concentration of reactants (As(III), Fe(II)
and H2O2), oxygen, phosphate and organics (2 - propanol, formate, and
citrate) are reported and analysed. The kinetic models developed for these
systems show high applicability to full scale water treatment application
and key mechanistic findings include the significance of the cycling of Fe(II)
/ Fe(III) via HO2
•/O2
•−, the effects of As(IV) termination reactions in the
absence of oxygen and the retarding effects of phosphate due to the
postulated formation of a Fe(III) - phosphate complex (at a derived rate
constant of 2.2 x 106 M-1s-1, that also appears to have negligible kinetic
activity in terms of reduction to Fe(II) by HO2
•/O2
•−). The work also
demonstrates the significance of the free radical by products of formate
and citrate oxidation by •OH (HCOO•/CO2
•− and 3HGA•2−).
The examination of the dark Cu(II) - H2O2 - mediated oxidation of As(III) at
pH 8 with variation in the concentration of reactants (As(III), Fe(II) and
H2O2), carbonate and organics (2 - propanol, formate and citrate)
demonstrated for the first time the high applicability of this system to the
pre - oxidation of As(III) in water treatment and mechanistically that •OH
and CO3
•− are the dominant As(III) oxidants in this system. The As(III)
oxidant CO3
•−, is suggested to be generated by the interaction of •OH and
O2
•− with the carbonate matrix, at the respective rate constants of 4.9 x 107
M-1s-1 and 5.5 x 106 M-1s-1.
Examination of the dark Fenton - mediated oxidation of As(III) at pH 8 and
the effects of variation in the concentration of reactants (As(III), Fe(II) and
H2O2), carbonate, organics (2 - propanol, formate and citrate) and Cu(II)
demonstrates the varied potential mechanistic pathways in relation to the
generation of As(III) oxidants from the Fenton reaction, Fe(II) + H2O2 such
as Fe(IV) and CO3
•− and the previously dismissed •OH, due to the
presence of Fe(II) - citrate complexes. This work also demonstrates and
models the enhancement of As(III) oxidation in the presence of an
additional transitional metal ion, Cu(II).