Title:
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Characterisation of electrochemically activated solutions for use in environmental remediation
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This thesis aims to investigate the production of electrochemically activated solution
via electrolysis of diluted NaCI solution using a commercial electrochemical system
(STEL device), characterise the activated solution, and evaluate the potential
application of this solution in the treatment of water containing organic compounds.
The research shows that the electrolytic cell in the STEL device consists of a titanium
based metal oxide (Ir02, Sn02, and Ti02) coated anode, a tubular titanium cathode, and
a tubular ceramic diaphragm that separates the anode and cathode. A model
electrolytic cell using STEL anode material was designed and constructed for
investigating reaction mechanisms occurring at the electrode and identifying the
oxidising species generated at the anode during the electrolysis of NaCI solution.
Experimental results show that chlorine and oxygen were two main oxidants contained
in the anolyte, indicating that the generation of chlorine and oxygen are involved in the
electrode reactions. The evolutions of chlorine and oxygen were found to be achieved
via the formation of a series of adsorbed intermediates such as OHad, Oad on the anode
surface during electrolysis. Evidences for the occurrence of the intermediates were
obtained by several electrochemical observations. It is suggested that the evolution
reaction of chlorine involves a mechanism in which an intermediate of OClad is formed
instead of Clad.
The adsorbed intermediates may also be released from the anode surface to form
chlorine free radicals and hydroxyl radicals when the electrolysis is carried out at
higher potentials. This was investigated using the electro-oxidation of salicylic acid
(SA). It was found that in a buffer solution containing chloride ions, the oxidation
processes of SA were dependent on the potential applied on the anode. At + l.5V, the
reaction product was 2,4,6-trichlorophenol, indicating that chlorine free radicals were
generated. At +2.5V, the obtained products included 2,5-dichloro-l,4-benzoquinone,
indicating that a hydroxyl group is introduced to the benzene ring of SA. These results
suggest chlorine free radicals were generated at + l.5 V while hydroxyl radicals were
generated at +2.5V.
The operating conditions of the STEL device were optimised with respect to the redox
potential, pH, and chlorine concentration of the generated ECAS. Anolyte produced
from the STEL system, using a 100/0 NaCI solution under optimised electrolysis
conditions, is an acidic solution (PH 2.2) containing a mixture of oxidants with a redox
potential of 1190 m V and an available free chlorine concentration of 280 mgIL. The
catholyte solution is a reductive basic solution with a pH of 12.6 and a redox potential
of -950m V. The anolyte solution can maintain the oxidation ability up to 6 days when
it is stored in an air tight container. However, it will lose its oxidation ability in 30 min
if is purged by nitrogen or in 60 min if is stirred in the atmosphere.
The anolyte solution can degrade trace amounts of aromatic compounds in aqueous
solution. The degradation products are dependent on the concentration and chemical
nature of the reagents. Usually, 10-4M SA can be converted to ring opened
compounds, while 10·3M SA can only be converted to quinonic compounds. A
subsequent dechlorination step is required when using anolyte for treating aqueous
solution containing organic compounds as the by products include chlorinated
compounds.
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