Proceedings of the European Summer School of Photovoltaics 4 – 7 ...

Fig. 2. Changes of absorbance at 254 nm of HA solution under UV
and ASL irradiation at the presence of TiO 2
. Concentration of photocatalyst:
100 mg l -1 , concentration of HA: 40 mg l -1
Fig. 1. The scheme of the quartz flat bottomed photoreactor
ture was stabilized at 25 ±0.5°C. Together with the sample used
in a photocatalytic experiment an identical reference sample was
always prepared and kept in dark during the same period of time.
After the experiment three portions of both suspension and reference
samples were taken, subjected to 25 min of centrifugation
at 15000 rpm to separate TiO 2
and then analyzed for the concentration
of humic acid. In that way the influence of HA adsorption
on the TiO 2
surface on the uptake of the contaminant from the
solution was eliminated. The absorption of the supernatant at the
wavelength of 254 nm was measured using Specord 40 (Analytic
Jena) single beam spectrophotometer. COD values were determined
according to Standard Metod PN-85/C-04578.02.
The photooxidation experiment included a sets of tests with
different objectives. First two sets of experiments was carried in
order to determine the degree of HA degradation under UV and
ASL irradiation in the absence and in presence of photocatalyst
(initial HA concentration in that series of experiments was always
40 mg l -1 ). The details of these experiments were given in our
previous work [9]. Then the measurements of the efficiency of the
photocatalytic process for simultaneous application of chemical
oxidants (Na 2
S 2
O 8
, H 2
O 2
) and titanium dioxide were performed.
Results
Decomposition of HA during UV/ASL irradiation and in UV
(ASL)/TiO 2
system
The effect of ASL or UV illumination on humic acid degradation
was described in detail in our previous work [9]. We observed no
degradation under visible light and a very low degradation rate
under UV as after 9 hr irradiation with the UV lamp only 5.3% decrease
of the HA concentration (as measured by the UV absorbance
at 254 nm (A 254
) was observed. Since the ultraviolet light alone
wasn’t enough to decompose humic acid, the effect of simultaneous
action of irradiation and application of photocatalyst – TiO 2
was determined. In the experiment the suspension consisting of
40 mg l -1 of HA sodium salt and 100 mg l -1 of TiO 2
was exposed
to the ASL and UV radiation. The results are shown in Fig. 2. and
compared with the blank experiment.
After 20 min of UV irradiation 15% decrease of A 254
was observed,
whereas one hour UV irradiation of HA in TiO 2
suspension
resulted in nearly 50% drop of HA concentration (Fig. 2) [9]. The
correlation between the results of absorbance, A[254 nm], and
COD determination during 5 – hours of UV irradiation process is
illustrated in Fig. 3.
As it can be seen, the normalized COD and A[254 nm] are
well correlated, which substantiate the selected spectroscopic
detection method. However, for low concentrations of humic acid
COD was still measurable, whereas decrease of absorbance was
almost 100%. As it was explained in our previous work [9], HA is
116
Fig. 3. Changes of normalized absorbance at 254 nm and COD values
of humic acid solution under UV irradiation
composed of at least two components, one can adsorb easily at
the TiO 2
surface, the other one does not adsorb at all or adsorbs
to a very limited extent. Moreover, the photodegradation reaction
of such complicated compounds as HS does not probably occur
in one stage, so intermediate species formed during degradation
may not absorb UV light in the range near 254 nm. Therefore,
it seems that non-zero COD value indicate exactly on these parts
of humic substances, which do not degrade in the first step and
consequently their decay is not detected by UV spectrometry.
Decomposition of HA by simultaneous application of TiO 2
and sodium persulphate
First the suspensions consisting of HA sodium salt (40 mg l -1 )
and different dosages of sodium persulphate (1×10 -3 mol l -1 ,
5×10 -4 mol l -1 , 2×10 -4 mol l -1 , 1×10 -4 mol l -1 , 5×10 -5 mol l -1 ) were UV
and ASL irradiated for 60 min. Then same procedure was repeated
for the suspension containing additionally 100 mg l -1 of TiO 2
.
The results of these experiments are showed in Fig. 4.
Fig. 4. The effectiveness of the oxidation of HA by simultaneous
action of UV or ASL radiation, TiO 2
and sodium persulphate. Irradiation
time: 60 min, concentration of HA: 40 mg l -1 , concentration of
TiO 2
: 100 mg l -1 ,data for experiment without Na 2
S 2
O 8
– 40 mg l -1 HA +
100 mg l -1 TiO 2
in dark
Elektronika 6/2012