2.3.1. Effect of BFO Dosage
The photocatalytic activities of BFO were evaluated by the degradation of TC under visible light irradiation. The effect of BFO dosage within a range from 0.1 to 1.0 g/L was investigated. As shown in
Figure 3, when the BFO concentrations were 0.1, 0.2 and 0.5 g/L, the final residual TC concentrations were 45%, 42% and 31%. This revealed that the photo degradation efficiency increased within the concentration range from 0.1 to 0.5 g/L. However, as the BFO concentration kept going up to 1.0 g/L, the residual concentration was 42%, implying a decrease in photocatalytic efficiency with an increase in BFO concentration. To further quantify and express the change of TC removal with the variation of the BFO dosage, pseudo-first order kinetics was used to fit the photocatalytic results under different BFO dosages. This kinetics can be expressed as ln(
ct/
c0) =
kappt +
y, where
y is a constant,
t is the reaction time (min),
kapp is the apparent rate constant (min
−1) and
c0 and
ct are the TC concentrations (mg/L) at time of
t = 0 and
t =
t. The apparent rate constants (
kapp), shown in
Figure 3, were 6.7 × 10
−3, 7.1 × 10
−3, 9.7 × 10
−3, 7.1 × 10
−3 min
−1 for BFO dosages of 0.1, 0.2, 0.5, 1.0 g/L, respectively. All of the correlation coefficients
R2 were higher than 0.9, indicating that the pseudo first order kinetics model fit the experimental data well. The results also revealed that the photocatalytic activity was the highest at a BFO dosage of 0.5 g/L. The decline in the degradation of TC may be due to the growth of turbidity with BFO concentration increasing, which inhibited light penetration [
25]. Therefore, the optimal dosage for the following photocatalysis experiment was selected as 0.5 g/L.
Figure 3.
(a) Degradation and (b) removal kinetics of TC at different BFO dosages under visible light irradiation (initial TC concentration = 10.0 mg/L, pH = 3.0, time = 120 min).
Figure 3.
(a) Degradation and (b) removal kinetics of TC at different BFO dosages under visible light irradiation (initial TC concentration = 10.0 mg/L, pH = 3.0, time = 120 min).
2.3.2. Effect of Initial pH
The reaction mechanism of the variation of photocatalytic efficiency with the change of pH has been studied [
26,
27]. The reaction formula can be summarized as follows:
Equations (1)–(3) show that the ·OH radical formation under light excitation is caused by the positive holes reacting with H
2O and OH
− on the photocatalyst surface [
28,
29]. If H
+ ions are too high in concentration in the acidic condition, the excitation of H
2O and OH
− into ·OH radicals will be suppressed due to an excessive concentration of H
+ and a low concentration of OH
−. Furthermore, the reaction in Equation (5) proceeds inversely and, thus, is inhibited when the pH exceeds the pK
a. As a result, there will be less ·HO
2 radicals, which are lower in redox potential and oxidizing capacity in the reaction system. Furthermore, the lack of ·HO
2 radicals suppresses Equations (6)–(8), and these reactions will also inhibit oxidation, as they produce oxidizing substances lower in oxidizability. To summarize, the photocatalyst will show better oxidizing capacity at neutral pH or higher [
30,
31,
32,
33]. As can be seen in
Figure 4a,b, the residual concentrations of TC for pH values ranging from 3.0 to 6.0 were all around 35% at 120 min, and the apparent rate constants were 8.5 × 10
−3, 9.2 × 10
−3, 9.5 × 10
−3 and 9.7 × 10
−3 min
−1. This means that the photocatalysis efficiency remains unchanged within this pH range. However, when the pH value rose to 8.0, the final concentration of TC reduced to 22%, and the rate constant increased to 1.2 × 10
−2 min
−1. This phenomenon was seemingly in accordance with the theory mentioned that photocatalytic performance will be better at neutral pH or higher [
26,
27].
It was noteworthy that the correlation coefficient of degradation results when pH = 8.0 fit by pseudo first order kinetics was below 0.9, while that of the results at other pH values was above 0.9. It is rational to regard that some reaction other than photocatalysis makes the results inappropriate to be fitted by pseudo first order kinetics.
The results of a blank test conducted without photocatalyst are shown in
Figure 4c. TC will be degraded solely by irradiation of visible light, and the effect was pH relevant. The residual concentration of TC after 120 min was around 60% in a pH range from 3.0 to 6.0, while that at pH = 8.0 was 23%, which reflected that visible light photolysis could cause the degradation of TC [
34,
35,
36,
37,
38]. With the increase of pH, the adsorption spectrum of TC exhibits a red shift. Due to the shift of the adsorption spectrum to a visible light region with pH values rising, the number of photons adsorbed per unit time increased, which resulted in the higher photolysis efficiency at higher pH. The residual concentrations of the blank test were higher than those of the photocatalysis tests at pH values from 3.0 to 7.0. The initial concentrations for photocatalysis and the blank test were almost the same, because the adsorption test revealed that BFO had a low adsorption capacity. If other particles were added rather than the photocatalyst, the turbidity increase is supposed to weaken the photolysis. Therefore, this enhancement of degradation efficiency by adding BFO means that photocatalysis actually plays a part in the degradation. When the pH value was 8.0, the results of the two tests were almost the same. The degradation results above came from chromatography measurement. As can be seen from the results, the degradation efficiencies of both photolysis and photocatalysis on TC are pH dependent.
The actual effect of visible light photocatalysis degradation of TC under different pH conditions was hard to evaluate solely by using chromatography measurement as the standard. Therefore, TOC measurement was adopted in order to judge the change of photocatalysis efficiency. According to
Figure 4d, TOC concentration values after photolysis between the results from the experimental groups with different initial pH values showed almost no change. After 60 min of adsorption, the residual TC concentration at pH = 6.0 was the lowest. The isoelectric point of BFO is 6.0. Therefore, the repulsive force between the TC molecule and BFO is the lowest at pH = 6.0, which makes the adsorption capacity the highest at pH = 6.0. When it comes to the residual TOC concentration of TC after being degraded by photocatalysis at 120 min, the best result was also at pH = 6.0. The phenomenon of optimal adsorption and photocatalysis removal at the same pH = 6.0 instead of at higher pH may be because TC adsorption on the photocatalyst decreases with the pH increasing, which makes ·OH radicals entering the solution the rate determining step, which decreased the TC degradation at higher pH [
39]. The best degradation efficiency of TC was achieved at pH = 6.0 according to the TOC results. Many kinds of pollutants tend to be degraded by photolysis when measured by chromatography [
17,
40]. TOC measurement may be a good way to exclude the influence from visible light photolysis and to achieve the goal of evaluating the actual degradation efficiency of visible light photocatalysis, because it may be hard for visible light photolysis to bring about TOC reduction. The TOC measurement was adopted in all of the following degradation tests in this study.
Figure 4.
(a) Degradation and (b) removal kinetics of TC with BFO under visible light irradiation at different pH values; (c) Degradation of TC under visible light irradiation without BFO at different pH values; (d) Residual TOC after photolysis, adsorption and photolysis (initial TC concentration = 10.0 mg/L, initial BFO concentration = 0.5 g/L).
Figure 4.
(a) Degradation and (b) removal kinetics of TC with BFO under visible light irradiation at different pH values; (c) Degradation of TC under visible light irradiation without BFO at different pH values; (d) Residual TOC after photolysis, adsorption and photolysis (initial TC concentration = 10.0 mg/L, initial BFO concentration = 0.5 g/L).