Effects of inorganic ions on the photocatalytic degradation of carbamazepine

Carbamazepine (CBZ) is a typical class of pharmaceuticals and personal care products (PPCPs), and is a serious threat to the environment and human health. Photocatalytic degradation is an ef ﬁ cient technology to remove CBZ. However, the present work focused mainly on the improvement of photocatalytic degradation performance. The information about the effects of inorganic ions on the photocatalytic degradation activity of environmental pollutants is still scarce. This study systematically investigated the effects of inorganic ions on the photocatalytic degradation of CBZ in view of the practical applications. The addition of inorganic anions showed a negative effect on photocatalytic degradation of CBZ with the order of inhibition effects of HCO 3 (cid:1) > Cl (cid:1) > NO 3 (cid:1) . This was due to the quenching effects of hydroxyl radicals or holes, which decreased the photocatalytic degradation of CBZ. The presence of Al 3 þ could adsorb on the surface of a photocatalyst to shield the active site, resulting in the decreased CBZ degradation, while coexistence of Ca 2 þ signi ﬁ cantly promoted the photocatalytic degradation of CBZ owing to the enhanced CBZ adsorption. Mg 2 þ showed concentration and time-dependent effects (suppression or promotion) on the photocatalytic degradation of CBZ.


INTRODUCTION
Pharmaceuticals and personal care products (PPCPs), as emerging contaminants, are the subject of growing concern.
Carbamazepine (CBZ) is a class of typical PPCPs which is mainly used to treat epilepsy, anti-central neuralgia, and prevent or treat manic depression. Its wide use has caused massive releases of CBZ into the environment (Yonetani et al. ; Ali et al. ). The presence of CBZ not only causes environmental pollution (Paz et al. ), but also poses a serious threat to human health. Therefore, an efficient technology must be developed to remove CBZ from the environment.
A variety of techniques have been applied to remove CBZ, such as adsorption, photocatalysis, and ozone oxidation (Hübner et al. ). Among them, photocatalytic technology can remove CBZ effectively. Our group has previously synthesized hierarchical BiOCl microspheres, and first demonstrated their excellent photocatalytic activity for CBZ degradation (Gao et al. a, b). To broaden the light absorption of BiOCl, we fabricated a visible-light-driven BiOCl photocatalyst via an ethylene glycol mediated solvothermal method (Gao et al. ).
Our findings indicate that the BiOCl photocatalysis is a promising candidate for the efficient and cost-effective removal of recalcitrant pharmaceutical contaminants.
Furthermore, the effectiveness of BiOCl photocatalysis in removing recalcitrant pharmaceutical contaminants has also been demonstrated by another group (Zeng et al. ).
Unfortunately, photocatalytic technology is still limited in terms of practical applications due to the complicated effects of environmental coexistence (Rincon & Pulgarin  Batch experiments were performed to examine the effects of common inorganic anions (HCO 3 À , Cl À , and NO 3 À ) and cations (Al 3þ , Ca 2þ , and Mg 2þ ) on the photocatalytic degradation efficiency and degradation kinetics of CBZ.
Furthermore, the underlying mechanism was discussed, based on the experimental data.

Methods
A stock solution of CBZ (500 mg·L À1 ) was prepared and was diluted with deionized water to 2.5 mg·L À1 .
Batch experiments were carried out on a photocatalytic reactor irradiated by a 350-W xenon lamp. The beaker containing the reaction solution was placed in a thermostatic water bath and stirred by a magnetic stirrer.
The vertical distance between the light source and the reactor was 20 cm. Typically, 0.04 g of photocatalyst and 1 mL of the investigated ions solution were added to the 50 mL CBZ solution. Then the solution was placed in the dark and stirred by a magnetic stirrer to achieve adsorption. After adsorption equilibrium, the light source was turned on, and the solution was placed in an irradiation state to undergo photocatalytic degradation reaction.
The effects of coexisting anions of HCO 3 À , Cl À , and NO 3 À on the photocatalytic degradation of CBZ were examined by adding various concentrations of NaHCO 3 , NaCl, and NaNO 3 , respectively. The influences of inorganic cations of Al 3þ , Ca 2þ , and Mg 2þ were investigated by adding AlCl 3 , CaCl 2 , and MgCl 2 , respectively.

Photocatalytic efficiency
Upon irradiation, the concentration of CBZ solution was analyzed at intervals by a Varian Cary-50 UV-vis spectrophotometer. The photocatalytic efficiency was calculated by the following formula (Equation (1)): where C 0 and C are the CBZ concentration prior to irradiation and after irradiation in 5 min intervals, respectively.

Photocatalytic kinetic
The photocatalytic degradation kinetic of CBZ was recorded as a function of irradiation time. The data were fitted to a second-order rate model (Equation (2)): where k is the pseudo-second order rate constant, C 0 is the initial concentration of CBZ, C 1 is the concentration of CBZ after adsorption equilibrium, and C is the concentration of CBZ after irradiation in 5 min intervals.

Specifications of BiOCl photocatalyst
BiOCl photocatalyst with high purity was used in the present study. The BiOCl photocatalyst shows compact hierarchical structures. Calculated by Scherrer equation, the crystalline size of BiOCl is 14.3 nm. Moreover, hydroxyl groups appeared on the surface of BiOCl with highly exposed facets of (110) lattice plane.

Effect of inorganic anions
Actual water samples contain a large amount of various inorganic ions, thus the process of research on CBZ photocatalytic degradation in deionized water is very different from the actual situation (Ravikumar et al. ). Therefore, we studied the effects on photocatalytic degradation of CBZ by adding three different anions in environmentally relevant concentrations to the photocatalytic reaction system.

Effect of nitrate ions (NO 3 À )
The effect of NO 3 À on photocatalytic degradation of CBZ is provided in Figure 1. The added concentration of NO 3 À was 10, 20, 30, 40, and 50 mg·L À1 , respectively. The presence of NO 3 À slightly decreased the degradation efficiency of CBZ.
The inhibition effects of NO 3 À can be explained by the fact that NO 3 À could act as a radical scavenger, which reacts with positive holes (h þ ) and hydroxyl radical (·OH) (Equations (3) and (4)  [BiOCl] ¼ 0.8 g·L À1 ).
The pseudo-second order model was applied to evaluate the CBZ degradation process in the presence of NO 3 À . As shown in Figure 1(b), all the CBZ degradation followed the pseudosecond order kinetic. The degradation rate constant of k decreased from 0.0860 to 0.0586 mg À1 ·L·min À1 at the NO 3 À concentration of 50 mg·L À1 . The half-life of CBZ during photocatalysis in the presence of inorganic anions is presented in Table 1. The half-life of CBZ during photocatalysis was longest at the NO 3 À concentration of 20 mg·L À1 .
Effect of chloride ions (Cl À ) The effects of Cl À on the photocatalytic degradation of CBZ were studied with various concentrations of 100, 200, 300, 400, and 500 mg·L À1 , respectively. As shown in Figure 2, it is interesting to find that almost all concentrations of Cl À improve the photocatalytic degradation of CBZ in the first 15 min irradiation. However, inhibited photocatalytic degradation of CBZ was observed in all concentrations of Cl À during the subsequent photocatalytic process. It was reported that the Cl À could react with h þ and ·OH to form chlorine radicals (Cl · , Equations (5)- (7) OH þ Cl À ! Cl þ OH À  [BiOCl] ¼ 0.8 g·L À1 ).

Effect of bicarbonate ions (HCO 3 À )
HCO 3 À is an important substance in water. Here, we investigated the effect of the addition of HCO 3 À on photocatalytic degradation of CBZ. The added concentrations were 50, 100, 150, and 200 mg·L À1 , respectively. As can be seen from Figure 3, HCO 3 À had a significant negative effect on degradation of CBZ. After 60 min of adsorption process, the concentration of CBZ was determined to be increased compared to that prior to adsorption. This phenomenon was particularly remarkable at the concentration of 100 mg·L À1 of HCO 3 À . This was due to the varied pH in the solution in the presence of HCO 3 À , which changed the dissociation of CBZ. When the added concentration of HCO 3 À was 200 mg·L À1 , the degradation rate was decreased to 18%.
As shown in Figure 3( (8) and (9)). Therefore, the degradation of contaminants will be inhibited in the . Therefore, the effect of the addition of HCO 3 À on photocatalytic degradation is related to specific conditions such as the reaction system, the type of contaminant, the type of catalyst, and so on.
Based on the above analysis, it can be seen that the effects of the three anions on photocatalytic degradation of CBZ are different, and the order of inhibition of ions on the reaction is HCO 3 À > Cl À > NO 3 À .

Effect of inorganic cations
There are a variety of cations in natural waters. Herein, the effects of three typical cations on photocatalytic degradation of CBZ were investigated.

Effect of aluminum ions (Al 3þ )
It can be seen from Figure 4 that Al 3þ has a negative effect on the photocatalytic degradation of CBZ. The added concentrations of Al 3þ were 10, 20, 30, and 40 mg·L À1 , respectively. With the concentrations of Al 3þ increasing from 10 mg·L À1 to 40 mg·L À1 , the inhibition on the photocatalytic degradation of CBZ was also enhanced. When the added concentration of Al 3þ was 40 mg·L À1 , the photocatalytic degradation efficiency was reduced about 16% compared to the solution without Al 3þ . The degradation kinetics of CBZ was quantitatively calculated according to the pseudo-second order model, as shown in Figure 4(b). The k value of CBZ degradation decreased to 0.0329 mg À1 ·L·min À1 at the concentration of 40 mg·L À1 , confirming the inhibited degradation kinetics of CBZ in the presence of Al 3þ .

Effect of calcium ions (Ca 2þ )
The effect of Ca 2þ on photocatalytic degradation of CBZ is shown in Figure 5. [BiOCl] ¼ 0.8 g·L À1 ). more negative than the valence band potential (oxidation reaction) or more positive than the conduction band potential (reduction reaction). Therefore, the effect of cations on the photocatalytic degradation of pollutants is strongly dependent on the redox potential of the metal.  15, and 20 mg·L À1 . As a whole, the influence of all the concentrations of Mg 2þ was not obvious with longer experimental durations of 30 min. As shown in Figure 6(

CONCLUSIONS
The effect of various ions on the photocatalytic degradation of CBZ was investigated in the presence of the BiOCl photocatalyst. The addition of inorganic anions showed a negative effect on photocatalytic degradation of CBZ with the order of inhibition effects of HCO 3 À > Cl À > NO 3 À , which was due to the quenching effects of hydroxyl radicals or holes.
The coexistence of inorganic cations influences the photocatalytic degradation of CBZ based on a different mechanism: cations could adsorb on the surface of the photocatalyst, thereby affecting the photocatalytic degradation of CBZ.
Al 3þ , Ca 2þ , and Mg 2þ exhibited suppression, promotion, and concentration-dependent effects on the photocatalytic degradation of CBZ. Considering the varied effect of inorganic ions on CBZ degradation, complicated effects of environmental coexistence must be considered in the practical application of photocatalytic technology.