Influence of aqueous chloride and bromide ions on bisphenol A degradation efficiency with zinc oxide nanoparticles

Zinc oxide (ZnO) nanoparticles (NPs) have been widely investigated for applications in photocatalytic degradation of organic pollutants in wastewater. Despite the advantages of robust ZnO material, its photocatalytic activity is greatly affected by environmental factors. Halogen ions are commonly found in wastewater, which directly affect the pollutant aggregation and sedimentation, therefore it is necessary to discuss their effect on the photocatalytic degradation. The current study assesses the halogen ions effect on the photocatalytic degradation of bisphenol A (BPA) using different dosage of sodium chloride (NaCl) and sodium bromide (NaBr). The microstructural characterization of ZnO NPs was conducted by transmission electron microscopy and hydrodynamic size was analyzed through dynamic light scattering. The effective BPA degradation with ZnO NPs was observed and pseudo-first-order kinetics was calculated. The increase of ZnO NPs dosage from 10 to 100 mg L− 1 enhanced the degradation rate constant of BPA up to 0.089 min− 1 (14.8 folds). In order to evaluate the role halogen ions to degrade BPA, NaBr and NaCl were used. The degradation rate was reduced to 0.0034 min− 1 after the addition of NaBr due to the increase in hydrodynamic particle size, thereby restricting the light adsorption capacity. Noteworthy, upon addition of NaCl up to 500 mM concentration, only a slight decrease on BPA degradation rate was observed. Therefore, this study unveils the role of chloride ions as an effective medium for BPA degradation by ZnO NPs, without aggregation, and provides a novel platform for the treatment of organic pollutants in saline water.


Introduction
World-wide contamination of phenolic compounds in aquatic environmental has attracted increasing attentions in the research society. Bisphenol A (BPA) used as basic monomer in the synthesis of brominated flame retardants, epoxy resins and polycarbonates due to which it became the highest produced chemical worldwide [1]. However, their characteristics, environmental distribution and adverse effect on human health remain lessknown compared to conventional pollutants with the worldwide demand dramatically increasing [2]. BPA is one of the endocrine disruptors due to its genotoxic and estrogenic activity, which damage the reproduction and brain development [3]. Owning to the discharge of wastewater and improper accumulation of solid waste, BPA has been ubiquitous in the environment [4]. In some rivers, BPA concentration could even reach up to 8 μg L − 1 [5], and the report from Barboza et al. [6] also revealed the present of BPA and its analogs in muscle and liver of fish. In China, up to 144 ng mL − 1 of bisphenol analogues was detected in serum samples from pregnant women [7]. Since even low concentration of BPA causes hazardous health, the development of treatment techniques with high efficiency is required.
Treatment of contaminated wastewater has received increasing attention. Several techniques such as chemical oxidation [8], sono-photo-Fenton oxidation [9], UV-assisted chemical oxidation [10], electrochemical degradation [11], zerovalent metal-based reduction [12], biodegradation [13], and adsorption methods [14] were studied. Chemical oxidation method has been widely used for wastewater treatment including Fenton process [15] and ozone treatment [16] process, which could effectively oxidize the organic contaminants in the wastewater. Nanoscale zero valent iron (NZVI) has appropriate redox potential to reduce organic contaminant [17,18,19] under anaerobic conditions but cannot totally mineralize the contaminant due to its losing reducing activity and quick oxidation when exposed in the air. Previous studies reported that BPA is biodegradable by bacterial strains [20], while it takes at least several days to remove BPA and produces byproducts. Activated carbon is commonly used for adsorption of compounds in air or aquatic system and the adsorption capacity depends on the carbon used and the characteristic of wastewater [21,22]. Physical adsorption is cost effective treatment procedure [23] but the contaminated adsorbent requires further treatment and proper disposal. Various studies suggest that chemical reduction, advanced oxidation and electrochemical processes are desired and effective treatment procedure for BPA, which could be activated by using expensive and noble metal catalysts [24]. Park et al. have demonstrated improved photocatalytic degradation of BPA in the presence of silver nanoparticles (NPs) immobilized with polyvinyl alcohol and activated by low hydrogen peroxide (H 2 O 2 ) concentration [25]. The quick dissolution of silver NPs was also reported due to the presence highly reactive H 2 O 2 in the solution. NZVI has been exploited in last 4 decades for wastewater treatment but agglomeration due to intrinsic magnetic properties and quick aerial oxidation of NZVI remain the major challenges [26].
The natural phenomenon involves the direct photocatalytic degradation of organic chemicals in the environment. The photocatalysis is the most convenient method for the treatment of wastewater due to its facile procedure, inexpensive operational cost, high degradation efficacy facilitated by reactive oxygen species generated during the reaction. In this regard, zinc oxide (ZnO) NPs, an environmentally benign material has been used for photocatalytic degradation of organic pollutants from wastewater. ZnO NPs have attained the huge acceptance for environmental applications because of their remarkable optical properties, electron affinity, high redox potential, environmentally benign, and relatively low cost [27,28]. The commercial or chemically modified ZnO NPs have demonstrated substantial differences in their morphology, shape and size, and photoelectric properties which show important role in the optical properties and photoactivity of ZnO NPs. However, previous studies showed that the effect of environmental factors such as pH, inorganic ions [29,30,31] and organic compounds are important and needed to be considered in the ZnO NPs photocatalytic degradation process. Wang 4 − ions restricted the BPA degradation to 10% [32]. It was observed that the adsorption of the dyes onto the surface of ZnO NPs is strongly influenced by the pH of the solution due to the surface charge of both ZnO NPs and contaminants [33]. The effect of inorganic ions on ZnO photocatalytic degradation has been widely reported, yet whether the addition of inorganic ions can increase or decrease degradation efficiency is still unsure in different conditions. Lee et al. [34]  and SO 3 − could be advantageous to enhance the photocatalytic degradation of organic contaminants in the presence of ZnO NPs. However, some inorganic ions such as Cl − , NO 3 − and CO 3 2− tend to retard the photocatalytic degradation of ZnO NPs. The anions have been known to be OH radical and photo-hole scavengers, strong reactive species for BPA degradation. In addition, the cations including Na + , K + , and Ca + slightly inhibit the BPA degradation by reducing the electrical double layer to enhance the ZnO NPs aggregation [35]. Therefore, it is noteworthy to evaluate the performance of ZnO-based photocatalysts in the presence of inorganic ions in the wastewater during the photocatalytic treatment process.
The current study demonstrates the effect of inorganic ions on the photocatalytic degradation of BPA using ZnO NPs under UV light irradiation. The effect of ZnO NPs dosages on photocatalytic degradation of BPA was investigated to optimize the optimum catalyst dose. The experiments under the influence of different inorganic ion concentrations (NaCl and NaBr) were performed and the change in hydrodynamic particle sizes were also monitored. The relationship among inorganic ion concentration, ZnO particle size and reaction kinetics is discussed.

Materials
The commercial ZnO NPs powder (Uniregion Biotech) with particle size of 20 nm was used in the experiments. Bisphenol A (CAS No. 80-05-7), sodium chloride and sodium bromide in this research were obtained from Acros Organics. All solutions were prepared with ultrapure water and used without further purification.

Characterization of ZnO NPs analysis
The crystal patterns of the ZnO NPs were measured by Bruker D8 Advanced X-ray diffractometer (XRD) with Cu-Kα radiation (Ni filtered, λ = 1.5405 Å, at voltage 40 kV and current 40 mA). The morphology of ZnO NPs was analyzed by transmission electron microscopy (TEM). NPs suspension was prepared using desired ZnO NPs concentrations and ultrasonicated for 10 min at room temperature. DELTA ultrasonic cleaning machine, DC150H/150 W was used for homogenous suspension.
The hydrodynamic particle size of ZnO NPs and their distributions were measured by a dynamic light scattering (DLS) instrument (Zetasizer nano ZS, Malvern nano series V6.0). UV-Vis spectra was used to record the light absorption property of ZnO NPs under the wavelength between 200 to 800 nm. The sedimentation of those large agglomerates of ZnO NPs was also monitored with UV-Vis spectra at 365 nm in 90 min. High performance liquid chromatography (HPLC, Agilent 1200 series) equipped with C-18 column, auto-injector, and variable wavelength detector was used to analyze the concentration of BPA in the experiments.

Photocatalytic degradation experiments
The typical photocatalytic experiments were conducted in the laboratory. A photochemical reactor was equipped with 4 × 8-W UV lamps at the irradiation wavelength of 365 nm. Experimental solutions in 30 mL tubes were placed in a rotor inside the photochemical reactor to irradiate them equally. In degradation experiments, 25 mL solution that contained different ZnO NPs photocatalyst dosages (10-100 mg L − 1 ) and 5 ppm BPA was prepared to conduct the experiments. After 30-min dark adsorption experiment, the solutions were illuminated under the light at the irradiation wavelength of 365 nm. All the experiments were performed in 100 mg L − 1 ZnO NPs dose, 5 ppm BPA conc., and neutral pH at room temperature until unless mentioned. During the experiment, the aliquot of 2 mL was collected, filtered through 0.22-μm filter and the concentration of BPA was analyzed by HPLC through auto-injector at selected intervals. In addition, Coumarin has been employed to detect ·OH radicals during the photocatalytic degradation of BPA. The fluorescence probe compound (coumarin) was used which upon reaction with ·OH radicals generated byproduct (7-hydroxycoumarin), measured by photoluminescence, as described in the literature [36]. In brief, 0.1 mM coumarin solution was taken as blank and desired amount of ZnO NPs was dispersed in the dark under continuous stirring. The aqueous solution was then exposed to UV light and aliquots were withdrawn at specific time intervals and filtered immediately. The obtained solution was then analyzed by fluorescence spectrophotometer (Hitachi, F-7000 FL spectrophotometer).

Chloride and bromide ions effect
NaCl and NaBr were used as inorganic ions under specific dosage of ZnO NPs to evaluate BPA degradation in the following experiments. Different chloride or bromide ion concentrations (0 to 500 mM) were used for BPA degradation process. Before performing the experiments, the inorganic ions and ZnO NPs were added to the solution containing 5 ppm BPA, stirred for 1 min and kept in the dark for 30 min to attain adsorption and desorption equilibrium before the photocatalytic degradation. The reaction solution was later exposed to UV light to observe the photocatalytic efficiency of ZnO NPs. The aliquot of 2 mL was collected, filtered through 0.22-μm filter and analyzed by HPLC through auto-injector at desired intervals. DLS and pH analysis were also carried out after every batch of experiment.
3 Results and discussion

Morphological characterization of ZnO NPs
The crystal patterns of ZnO NPs were analyzed by XRD as displayed in Fig. 1a Fig.  1b demonstrates the clear particle distribution and also the formation of small clusters. However, due to the high surface potentials of ZnO NPs, they aggregate when store for long time and need further dispersion before every experiment. ZnO NPs aggregate to form big agglomerates and the particle sizes can approach microscale, when suspended in DI water. ZnO NPs aggregation was further analyzed by DLS (Fig. 1c), depicts the average particle size of ZnO NPs in water is about 250 nm after sonication. Many studies applied ultrasonication to suspend NPs in aqueous solution before experiments [37]. Therefore, ZnO NPs suspensions used in experiments was allowed for 30 min ultrasonication in advance to properly disperse NPs. The full-scale UV-Vsible spectra of ZnO NPs were obtained as depicted in Fig. 1d to confirm the light absorption range (365 nm), which is corresponding to a broad band gap (3.39 eV) in Choi et al.'s report [38].

Photocatalytic degradation of BPA
The photocatalytic degradation of 5 ppm BPA was performed by varying the dosage of ZnO NPs from 10 to 100 mg L − 1 under 365 nm UV light (Fig. 2a). The degradation rate constants (k) were determined by a pseudo-first-order rate kinetics [39,40]: where C is the concentration of BPA after time t, C 0 is the initial concentration of BPA, t is the reaction time, and k is the pseudo-first-order rate kinetics. The photocatalytic degradation rate can be properly described by pseudo-first-order kinetics under different ZnO NPs dosages. Enhanced rate of BPA was observed when ZnO NPs dose increased from 10 to 100 mg L − 1 (0.006 to 0.089 min − 1 ). Previous studies also indicated that photocatalytic degradation rates of NPs on organic contaminants can be described by pseudo-first-order kinetics [41,42]. The enhancement of the degradation rate of BPA was observed with the increase of ZnO NPs dosage from 10 mg to 100 mg L − 1 , owning to the increase in the concentration of catalyst and adsorption sites on ZnO NPs surface. The result of full scan also showed that the fraction of UV light absorbed by the catalyst increased as the concentration of ZnO NPs increased in a suspension (Fig. 2b). Therefore, 100 mg L − 1 ZnO NPs catalyst dose was used during entire photocatalytic experiments. Previous studies indicate that OH radicals and positive holes play the major role in ZnO photocatalytic degradation process [43,44]. Owing to the increased yield of OH radicals as the dosage of ZnO NPs increased, the proportion of reactive oxygen species that attack BPA and its intermediates increases. There was no significant adsorption was observed when the dosage of ZnO NPs was 20, 50 and 100 mg L − 1 , while the removal of 7% BPA by 100 mg L − 1 ZnO NPs was observed in the first 30 min under dark condition. Considering of economic efficiency, the following experiments were performed with the dosage of 100 mg L − 1 ZnO NPs. The linear relationship between ZnO NPs concentration (10-100 mg L − 1 ) and k obs (pseudo-first-order rate constant) can be observed. Figure 2c shows the rate of BPA enhances as catalyst dose increased from 10 mg L − 1 (0.006 min − 1 ) to 100 mg L − 1 (0.089 min − 1 ), suggesting the degradation of BPA is catalyst dose dependent. The reusability and stability of ZnO NPs have been analyzed by performing 3-consecutive BPA degradation cycles (Fig. 2d). 100 mg L − 1 of ZnO NPs dispersed in the BPA (5 ppm) solutions under similar reaction conditions for reusability test. The sample aliquots were collected at selected time intervals during entire experiment. The initial BPA concentration was maintained to 5 ppm after each photocatalytic cycle. Slight decrease in the photocatalytic rate was observed after 3 repeated cycles, suggest that ZnO NPs can be utilized for BPA degradation repeatedly more than one cycle. The ·OH radicals generated during BPA degradation were measured by PL probe method using coumarin. 100 mg ZnO NPs were dispersed in 0.1 mM coumarin solution and stirred under dark for 30 min before UV light illumination. The photogenerated 7-hydroxycoumarin compound were then collected and analyzed measuring PL signals. The excitation wavelength was set at 332 nm and scan range was selected between 380 to 600 nm with the scanning speed of 240 nm min − 1 . Figure 3a shows the different PL curves of the solution obtained at different time period. The PL signal intensity increases as time increases, suggesting the formation of ·OH radicals in the solution. The change in fluorescence intensity (%ΔF) respective to time was analyzed and shown in Fig. 3b. the linear curve of fluorescence intensity change suggesting that ZnO NPs exposure under UV light generates ·OH radicals, which are responsible for BPA degradation. Our previous study demonstrates the enhanced photocatalytic BPA mineralization by metal-free graphitic carbon nitride was achieved due to the formation of ·OH radical, and byproducts were identified by high-resolution electrospray ionization mass spectrometry analysis [45].

The effect of chloride ions on photocatalytic degradation of BPA
The effect of different NaCl concentrations on the photocatalytic degradation process of BPA in the presence of ZnO NPs was evaluated. Figure 4a shows the increase of NaCl concentration resulting in slower BPA removal; the degradation rate constant was decreased around 50% by 500 mM NaCl. The BPA rate constants were calculated 0.089, 0.038, 0.035 and 0.032 min − 1 for 0, 10, 100 and 500 mM NaCl, respectively ( Fig. 4b and Table 1). The previous studies reported that in the presence of inorganic ions, the compression of the electrostatic double layer of NPs occurs, causing particle aggregation, decrease of surface area and therefore the decline of the photocatalytic degradation activity [46,47]. The effect of NaCl on ZnO NPs was investigated and observed the absorbance change over time. The average hydrodynamic particle size of ZnO NPs after degradation experiments were determined by UV-Visible spectra and DLS. Furthermore, ZnO NPs sedimentation was also observed in the presence of various NaCl concentrations. The absorbance decreased up to 32% when NaCl concentration increased to 500 mM NaCl concentration after 90 min sedimentation time (Fig. 5a).
On the other hand, the average hydrodynamic particle sizes were 1240, 1421, 1330 and 1265 nm for 0, 10, 100, and 500 mM addition of NaCl, respectively ( Fig. 5b and Table 1). The decreasing trend of the absorbance of ZnO NPs indicated the settlement of ZnO NPs with the increasing of NaCl concentration. However, there was no significant enhancement in the aggregation behavior showed in DLS result when the addition of NaCl concentration increased up to 500 mM, which was consistent with the result of absorbance change and the slightly suppression on degradation rate constants. The dual effect of NaCl on photocatalytic degradation was also observed by Liu et al. [48]. Chloride ions could enhance or suppress the degradation of Acid Orange 7 in different systems [49]. The higher NaCl concentration (> 50 mM) is beneficial to increase the amount of reactive chlorine species, while the dye photocatalytic degradation rate decreased with low NaCl concentration (< 10 mM), signifying that the concentration impacts the effect of NaCl on the photocatalytic degradation [50].

The effect of sodium bromide on photocatalytic degradation of BPA
Experiments of different NaBr concentration on the photocatalytic degradation performances of ZnO on BPA was carried out and shown in Fig. 6a. Upon addition of NaBr from 10 to 500 mM concentration, the degradation of BPA was found to be suppressing, suggesting a reaction constraining effect. This inhibition could be due to the competition between BPA molecules and bromide ions on the active sites of the catalysts. The photogenerated rate constants were 0.089, 0.0076, 0.0053 and 0.0038 min − 1 for 0, 10, 100 and 500 mM NaBr addition, respectively ( Fig. 6b and Table 1). The suppression effect of NaBr on BPA degradation was noticeably seen, which was in the following sequence 10 > 100 > 500 mM. In order to observe the hydrodynamic changes of the ZnO NPS, which may affect the surface area and active sites, the DLS study was conducted. The results clearly showed the increase of the average hydrodynamic size of ZnO NPs after the addition of NaBr. The sedimentation curve of solution indicates the aggregation of ZnO NPs, which could be the reason for the suppression of BPA degradation reaction (Fig. 7a). Further, the average hydrodynamic size of ZnO NPs was found to be 1240 nm for 0 mM NaBr, 1935 nm for 10 mM NaBr, 2399 nm for 100 mM NaBr and 2115 nm for 500 mM NaBr ( Fig. 7b and Table 1). These results show that the BPA photocatalytic degradation depends on the ion concentration. The presence of NaCl and NaBr also causes the salting-out effect, resulting in the insolubility of BPA, hence decrease the photocatalytic degradation [51]. Garcia-Espinoza et al. [52] demonstrated the striking effect of halide ions for dye degradation due to halide radical formation. This study of the inorganic ion effect (usually negative) over BPA photocatalytic degradation is found to be supporting the degradation process (in the presence of NaCl), by ZnO NPs, hence providing a novel platform for better understanding of the inorganic ion-based wastewater treatment.

Conclusions
The photocatalytic degradation kinetic rate constant of BPA increased with the increase of ZnO NPs dosages from 10 to 100 mg L − 1 , with insignificant adsorption during 30 min dark experiment. An obvious aggregation was observed after aqueous dispersion of ZnO NPs, leading to formation of large particles, contrary to the commercial 20 nm particle size. The effect of halide ions during photocatalytic degradation of BPA was investigated using NaCl and NaBr salt solutions. NaBr salt solution restricts the BPA degradation due to aggregation and limited light adsorption capacity. The average hydrodynamic particle size of ZnO NPs increased around 139%, leading to reduced rate of BPA up to 90% compared to the one without NaBr. However, NaCl could slightly suppress the degradation rate of BPA around 50% by 500 mM NaCl. The sedimentation curve reveals no significant change on the particle size and light absorbance efficiency, suggesting the relation between the rate constant and the average hydrodynamic particle size. Suppressed rate of BPA could be anticipated due to competition between NaCl salt and BPA molecules. This study reveals the impact of NaCl and NaBr salt solution on BPA removal and provides an essential information to facilitate the remediation of inorganic ion-included wastewater in the future.