Inuence of Aqueous Chloride and Bromide Anions on the Reactivity of ZnO on Bisphenol a Degradation

Zinc oxide (ZnO) nanoparticles 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 inuence the pollutant aggregation and sedimentation, therefore it is necessary to discuss their effect on the photodegradation. The current study assesses the halogen ions effect on the photocatalytic degradation of BPA using different dosage of sodium chloride (NaCl) and sodium bromide (NaBr). The microstructural characterization of ZnO was conducted by transmission electron microscopy and hydrodynamic size was analyzed through dynamic light scattering. Degradation reactions of BPA with ZnO nanoparticles followed pseudo-rst-order kinetics. The increase of ZnO dosage from 0.01 g/L to 0.1 g/L enhanced the degradation rate constant of BPA up to 0.089 min -1 (14.8 folds). In order to evaluate the the role halogen ions to degrade BPA, NaBr and NaCl were used. The degradation rate was reduced up to 26 folds (0.0034 min -1 ) after the addition of NaBr, which was attributed to the increase in hydrodynamic particle size leading, thereby restricting the light adsorption capacity. Noteworthy, upon addition of NaCl from 10 mM to 500 mM concentration, there was only a slight decrease (2.4 folds, 0.037 min -1 ) on the degradation rate of BPA. Therefore, this study unveils the role of chloride ions as an effective medium for BPA degradation by ZnO nanoparticles, 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 ame retardants, epoxy resins and polycarbonates due to which it became the highest produced chemical worldwide (Liu et al. 2019). However, their characteristics, environmental distribution and adverse effect on human health remain less-known compared to conventional pollutants, the worldwide demand is dramatically increasing (Yang et al. 2018). BPA is one of the endocrine disruptors due to its genotoxic and estrogenic activity, which damage the reproduction and brain development physical adsorption is cost effective treatment procedure but generate huge wastage, which requires further treatment. 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 (Mercante et al. 2021;Park et al. 2017). Silver nanoparticle activation by hydrogen peroxide system was used for photocatalytic degradation of BPA by Park et al. (Park et al. 2017). However, enhanced BPA removal was reported but presence of H 2 O 2 can quickly dissolute silver nanoparticles in the solution, therefore PVA was further immobilized to stabilize the nanoparticle. Moreover, nZVI has been exploited in last 4 decades for waste water treatment but agglomeration due to intrinsic magnetic properties and quick arial oxidation of NZVI remain the major challenges (Krajangpan et al. 2008).
The natural phenomenon involves the direct photodegradation of organic chemicals in the environment. Halladja et al., reported slow direct photolysis of uometuron, a herbicide in the aqueous media due to photogenerated OH radicles, photolysis was further enhanced by 2.5 fold after fulvic acid addition to the reaction (Halladja et al. 2007). The addition of photocatalysts in uence the degradation e cacy as well as complete mineralization of organic pollutants. In this regard, ZnO environmentally benign material has been used for photocatalytic oxidation of pollutants. ZnO is attractive to be used in photodegradation since its non-toxicity, great optical properties, and relatively low cost However, some inorganic ions such as Cl -, NO 3 and CO 3 2will tend to retard the photodegradation of ZnO. It is obvious that electrolytes existing in wastewater will pose a challenge for photocatalyst treatment. Therefore, an understanding performance of ZnO photocatalyst in existence of electrolytes is crucial for assessing the feasibility of ZnO photodegradation in the practical application.
The current study demonstrates the effect of electrolytes on the photodegradation of BPA, one of environmental endocrine disruptor using ZnO nanoparticles under UV light irradiation. The effect of ZnO dosages on photocatalytic degradation of BPA was investigated to optimize the optimum catalyst dose.
The experiments under the in uence of different electrolyte concentrations (NaCl and NaBr) were performed and the change in hydrodynamic particle sizes were also monitored. The relationship among electrolyte concentration, ZnO particle size and reaction kinetics is discussed.

2.1Materials
The commercial powder zinc oxide (ZnO, Uniregion Biotech) with particle size of 20 nm was used in the experiments. Bisphenol A (BPA, 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.

Characterization of ZnO analysis
The morphology of ZnO nanoparticles was analyzed by transmission electron microscopy (TEM).
Nanoparticle suspensions were prepared by adjusting to appropriate concentrations at room temperature under ultrasonication for 10 min. In aqueous systems, particle size of photocatalysts 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 nanoparticle under the wavelength between 200 nm to 800 nm. The sedimentation of those large agglomerates of ZnO NPs was also monitored with UV-vis spectra at 365 nm in 90 minutes. High performance liquid chromatography ((HPLC, Agilent 1200 series) equipped with C-18 column, auto-injector, and variable wavelength detector (VWD) was used to analyze the concentration of BPA in the experiments.

Photodegradation experiments
The typical photocatalytic experiments were conducted in the laboratory. A photochemical reactor was equipped with four 8 W UV lamps at the irradiation wavelength of 365 nm. Experimental solutions in tubes were placed in a rotor inside the photochemical reactor to irradiate them equally. In degradation experiments, a solution that contained different ZnO NPs photocatalyst dosages and 5 ppm BPA was prepared to conduct the experiments. After 30-minutes dark adsorption experiment, the solutions were illuminated under the light at the irradiation wavelength of 365 nm. During the experiment, the aliquot of 2 mL was collected, ltered through 0.22-micron lter and the concentration of BPA was analyzed by HPLC through auto-injector at selected intervals.

Chloride and bromide ions effect
NaCl and NaBr were used as electrolyte under speci c dosage of ZnO to evaluate BPA degradation in the following experiments. The concentration of chloride or bromide ranged from 0 mg/L to 500 mg/L.
Before performing the experiments, the electrolyte and ZnO was added to the solution containing 5 ppm BPA, stirred for 1 minute and kept in the dark for 30 minutes to attain adsorption and desorption equilibrium before the photodegradation. The aliquot of 2 mL was collected, ltered through 0.22-micron lter and analyzed by HPLC through auto-injector at desired intervals. DLS and pH analysis were also carried out after every batch of experiment.

Morphological characterization of ZnO nanoparticles
The ZnO nanoparticles (ZnO NPs) were analyzed by TEM. Fig. 1a shows that the particle sizes of ZnO NPs are homogenously distributed in the range of 35 nm. However, due to the high surface potentials of nanoparticles, ZnO NPs aggregate in storage and need dispersion before the usage in experiments. Once ZnO NPs enter water, the nanoparticles aggregate to form big agglomerates and the particle sizes can even approach microscale. DLS result in Fig. 1b

Photodegradation of BPA
The photocatalytic degradation of 5 ppm BPA was performed by varying the dosage of ZnO from 0.01 to 0.1 g/L under 365 nm UV light (Fig. 2a). The degradation rate constants (k) were determined by a pseudorst-order rate kinetics (Sahu et al. 2021): 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-rst-order rate kinetics.
The photocatalytic degradation rate can be properly described by pseudo-rst-order kinetics under different ZnO dosages. Enhanced rate of BPA was observed when ZnO NPs dose increased from 10 mg L -1 to 100 mg L -1 (0.006 min -1 to 0.089 min -1 ). Previous studies also indicated that photocatalytic degradation rates of nanoparticles on organic contaminants can be described by pseudo-rst-order kinetics ( BPA was evaluated (Fig. 3). Fig. 3a shows the increase of NaCl concentration in the reaction resulting in slower BPA removal, except that 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 mM, 10 mM, 100 mM and 500 mM NaCl, respectively (Fig. 3b). The Previous studies reported that in the presence of electrolytes, the compression of the electrostatic double layer of nanoparticles occurs, causing particle aggregation, decrease of surface area and therefore the decline of the photocatalytic degradation activity ZnO NPs after degradation experiments were determined by UV-visible spectra and DLS, respectively. It was observed that the absorbance of ZnO NPs without NaCl decreased 17%. The absorbance decreased up to 32% with 500 mM NaCl concentration after 90 minutes sedimentation time (Fig. 4a). On the other hand, the average hydrodynamic particle sizes were 1240, 1421, 1330 and 1265 nm for 0 mM, 10 mM, 100 mM and 500 mM addition of NaCl, respectively (Fig. 4b).

The effect of sodium bromide on photodegradation of BPA
Experiments of different NaBr concentration on the photocatalytic degradation performances of ZnO on BPA was carried out and shown in Fig. 5a. Upon addition of NaBr from 10 mM to 500 mM concentration in the reaction, 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 rate constants were 0.089, 0.0076, 0.0053 and 0.0038 min -1 for 0 mM, 10 mM, 100 mM and 500 mM addition of NaBr, respectively (Fig. 5b). The suppression effect of NaBr on BPA degradation was noticeably seen, which was in the following sequence 10 mM>100mM>500mM with almost similar rate constant. 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. 6a). Further, the average hydrodynamic size of ZnO NPs was found to be 1004 nm for 0 mM NaBr, 1935 nm for 10 mM NaBr, 2399 nm for 100 mM NaBr and 1715 nm for 500 mM NaBr (Fig. 6b). These results show that the BPA photodegradation depends on the ion concentration

Conclusions
The photodegradation kinetic rate constant of ZnO nanoparticle powder on BPA increased with the ZnO dosage from 0.01 to 0.1 g/L, while there was no signi cant adsorption during rst 30 minutes dark experiment. The obvious aggregation was observed after aqueous dispersion of ZnO NPs, leading to formation of large particles, contrary to the commercial 20 nm particle size. Role of halide ions was investigated using NaCl and NaBr salt solutions. NaBr salt solution restrict the BPA degradation due to increased hydrodynamic particle size 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 signi cant change on the particle size and light absorbance e ciency, 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 electrolyte-included wastewater in the future.  Tables   Table 1. Effect of NaCl and NaBr dosages (mM), observed rate constant (min -1 ), DLS particle size (nm), and the nal pH after certain loading.      (a) Sedimentation curve after loading different NaCl concentration, (b) effect of NaCl on DLS particle size distribution and rate observed.