level (assuming depletion e†ects are negligible), the key
parameters are the mass transfer coefficient, k , and the bulk
t
concentration of CP. For dilute CP solutions, where
K
[CP] > 1, we may conclude from the present studies that
CP
mass transport will be important when k \ k@ K . Under
t
MC CP
the conditions of the experiments in this paper, the latter
product has a value in the range 7 ] 10~4È2.2 ] 10~3 cm s~1.
Finally, it is informative to estimate the quantum yield of
the photomineralisation process. The quantum yield is Ñow-
rate dependent, attaining a maximum value when mass trans-
port is sufficiently high to be unimportant. In the high Ñow
rate limit, the data in Fig. 5 and 6(a) correspond to quantum
yields of 8.8 ] 10~4 and 6.4 ] 10~4, respectively. Increasing
the level of oxygen in solution increases the quantum yield to
ca. 1.9 ] 10~3.
Fig. 5 [Cl~] at the downstream detector electrode (…) as a function
of log (Ñow rate) for 5.0 ] 10~4 mol dm~3 4-chlorophenol solution
(aerated). The best Ðts are shown for the mixed-controlled model
(dashed line, with k@ \ 4.1 ] 10~10 mol cm~2 s~1) and the surface-
Conclusions
MC
limited model (dotted line with k@ \ 1.5 ] 10~10 mol cm~2 s~1).
SL
CFMED represents a powerful new approach for quantitat-
ively
investigating
heterogeneous
photomineralisation
mass transport in controlling the rate of photomineralisation
at lower substrate concentrations. Comparison of the analysed
data in Fig. 5 and 6(a) suggests that under aerated solution
conditions, the apparent rate constant is closely proportional
to the light intensity and the exponent, m, in eqn. (2) is e†ec-
tively unity. Similar behaviour was found in the stirred reactor
studies of Mills and Wang.5 The presence of higher levels of
kinetics. The initial studies reported in this paper show that,
for dilute (sub-millimolar) solutions, mass transport is a key
parameter in controlling the photomineralisation rate of CP.
Further studies, employing this methodology to examine a
wider range of reaction conditions and compare the kinetics
for di†erent chlorophenols are now underway. It is also likely
that there will be conditions where O transport to the cata-
lyst surface will play a role in the kinetics of the process. These
possible e†ects are also under investigation with CFMED.
2
O
speeds up the surface photomineralisation process (Fig.
2
6(b)) and the role of mass transport becomes clearer over the
whole range of Ñow rates considered.
It is useful to identify when mass transport will be impor-
tant generally compared to the surface kinetics of the photo-
mineralisation process. For a given light intensity and oxygen
Acknowledgements
We thank the Ministry of Science and Technology, Govern-
ment of Bangladesh, for a scholarship for SA.
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Fig. 6 [Cl~] at the downstream detector electrode (…) as a function
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for both aerated (a) and oxygenated (b) solution conditions. In each
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simulated with k@ \ 1.5 ] 10~10 mol cm~2 s~1 (a) and k@ \ 4.5
6
7
8
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MC
MC
] 10~10 (b)) and the surface limited model (dotted lines, simulated
with k@ \ 0.4 ] 10~10 mol cm2 s~1 (a) and k@ \ 0.4 ] 10~10 mol
SL
SL
cm2 s~1 (b)), the light intensity was 7.0 ] 1016 quanta cm~2 s~1.
5232
Phys. Chem. Chem. Phys., 1999, 1, 5229È5233