2. Materials and methods
Fig. 1 is the schematic diagram of the electrolysis cell system
used in this study. The system is made of high-strength
polypropylene (PP). As can be seen, it is a divided electrolysis
cell comprising an anolytic half-cell and a catholytic half-cell
separated by a cation-exchange membrane. Each of the half-cell
has a capacity of 1 L, containing NaCl as the anolyte and NaOH as
the catholyte. Note that during electrolysis in an undivided cell, Cl2
and H2 are produced in the area near the anode and cathode,
respectively. Mixing of these two gases will lead to thermal
reaction which may cause explosion if inappropriately handled.
Moreover, in an undivided electrolysis cell, disproportionation
reaction will occur between the Cl2 produced near the anode and
the OH present in the cell [12]:
Cl2 þ 2OH ! H2O þ OCl þ Cl; (1)
thus undermining the efficiency of Cl2 generation. Hence, the
cation-exchange membrane used in the proposed electrolysis cell
system serves to prevent direct mixing of Cl2 and H2. In addition,
the membrane also enables the Na+ ions not participating in Cl2
generation to pass into the catholytic half-cell for charge balancing
and for reacting with OH to produce NaOH.
The electrolysis operation was conducted using a titanium
anode composed of iridium (Ir) and ruthenium (Ru) as well as a
geometry grid to enhance its activation. During electrolysis,
gaseous ClO2 generated in the anolytic half-cell was drawn out
by a Venturi injector and then dissolved in the water inside the
injector to form ClO2 solution. The concentration and purity of the
ClO2 solution thus obtained were analyzed according to the
method proposed by Aieta et al. [13]. Batch electrolysis was
conducted in all experiments and the measurements taken were
the instantaneous concentrations of ClO2 solution flowing out from
the Venturi injector. In addition, all experiments were performed
in triplicate with the mean measurement used for analysis. All
measurements of concentration and purity showed deviations of
less than 5%.
Table 1 lists the experimental parameters used in this study
to determine the appropriate operation conditions for ClO2
generation by membrane electrolysis. Except for the experiments
on the effect of initial pH and temperature on ClO2
generation, all other experiments were conducted at an initial
pH and initialtemperature of anolyte of 7 and 20 8C, respectively.
According to the Alternative Disinfectants and Oxidants Guidance
Manual [14], ClO2 can be produced through acidification of
NaClO2, as expressed in the following reactions:
2NaClO2 þ Cl2ðgÞ ¼ 2ClO2ðgÞ þ 2NaCl (2)
2NaClO2 þ HOCl ¼ 2ClO2ðgÞ þ NaCl þ NaOH (3)
5NaClO2 þ 4HCl ¼ 4ClO2ðgÞ þ 5NaCl þ 2H2O (4)
Hence, this study also explored using anolyte made up of NaCl
and NaClO2 in different ratios to determine the most appropriate
mix proportions under the experimental conditions. The maximum
levels of 10% and 6% for NaCl and NaClO2, respectively were
set according to the limits of their current capacities when they are
used alone as anolyte. In the experiment on composition of
anolyte, the respective maximum levels of NaCl and NaClO2 added
were determined by whether the mix proportions would attain the
maximum load current of the electrolysis cell system.