Cost-Effective Abatement of Tetrabromobisphenol A from Contaminated Water by a Visible-Light-Driven Photochemical System

Abstract

Micro-pollutants in water and wastewater pose significant risks to aquatic ecosystems due to their toxic and persistent nature. However, micro-pollutant abatement using conventional advanced oxidation processes requires high energy and chemical consumption. Therefore, a visible-light-driven photochemical system mediated by AgCl/AgBr composites (Vis-AgCl/AgBr system) was proposed to degrade tetrabromobisphenol A (TBBPA), a model micro-pollutant. The AgCl/AgBr composites, which were fabricated using a simple precipitation method, had a heterojunction structure (an interface formed between AgCl and AgBr). The AgCl/AgBr composites exhibited a narrow bandgap of 2.26 eV, which resulted in high catalytic activity under visible light. The Vis-AgCl/AgBr system efficiently degraded TBBPA in simulated and real water. The TBBPA degradation efficiency of the Vis-AgCl/AgBr system reached 99% within 30 min, which was 0.94–5.9 times higher than that by AgCl or AgBr alone. This efficient TBBPA degradation was attributable to reactive species produced in the Vis-AgCl/AgBr system, including photoelectrons (e⁻), holes (h⁺), hydroxyl radicals (•OH), and superoxide radicals (•O₂⁻). Reduction by e⁻ and oxidation by h⁺, •OH, and •O₂⁻ caused the destruction of TBBPA and the formation of bromide (Br⁻) and debrominated organic products. Debromination was anticipated to reduce the toxicity and persistency of TBBPA and increase its biodegradability. The findings of this study provide a cost-effective solution to the abatement of refractory emerging micro-pollutants in water and wastewater when sunlight can be used as a light source.

1. Introduction

Brominated flame retardants (BFRs) are a group of micro-pollutants of emerging concerns [1]. BFRs, such as tetrabromobisphenol A (TBBPA), polybrominated diphenyl ethers (PBDEs), and polybrominated biphenyl (PBB), are extensively utilized in manufacturing various commercial and industrial products [2]. The ubiquitous occurrences of BFRs in aqueous environments (e.g., sewage, rivers, lakes, and marine) at concentrations of a few ng/L–mg/L have been documented in the literature [3]. Most BFRs are recognized as persistent micro-pollutants due to their significant adverse effects on human health and aqueous ecosystems [4]. For instance, TBBPA causes acute toxicity, endocrine disrupting activity, immunotoxicity, neurotoxicity, nephrotoxicity, and hepatotoxicity in animals [5]. Therefore, the effective abatement of BFRs from contaminated water is necessary.

Conventional methods for BFR abatement include biological treatment, thermal decomposition, and advanced oxidation. Comamonas sp. isolated from anaerobic sludge degraded 86% of TBBPA within 10 days [6]. The anaerobic sludge process was observed to degrade 2,2-bis(bromomethyl)-1,3-propanediol completely within 58 days [7]. However, low removal efficiency is one major drawback of these biological processes [8]. Efficient degradation of BFRs (e.g., PBDEs and PBB) was also achieved via thermal decomposition under temperatures of 200–700 °C [9]. However, the extensive energy consumption by these methods often results in high treatment costs. Advanced oxidation processes (AOPs), which utilize reactive species, such as hydroxyl radicals (•OH) and sulfate radicals (•SO₄⁻), are capable of degrading refractory BFRs. A UV/persulfate system degraded 25 µM 2,2-bis(bromomethyl)-1,3-propanediol completely within 60 min [10]. Efficient degradation of TBBPA by the UV/Fenton process was observed as well [11]. However, AOPs require high inputs of chemicals (oxidants and activators) and energy, which limit the applications of AOPs. Therefore, the development of novel technologies is needed for the efficient and low-cost treatment of BFR-contaminated water.

The in situ formation of reactive species via photocatalytic processes could be an emerging alternative to the treatment of BFR-contaminated water. Catalysts based on titanium dioxide (TiO₂) have been widely used for UV photocatalytic processes. The UV irradiation of TiO₂ catalysts was found to generate •OH and to remove 99% of TBBPA within 60 min [12,13]. A graphene–TiO₂ composite was also demonstrated to be an efficient catalyst for the degradation of TBBPA [14]. However, UV light-driven photocatalytic processes are still energy-intensive. Graphene–BiFeO₃ composites fabricated by the sol–gel method showed reasonable reactivity under visible light and were able to degrade 67% of TBBPA within 180 min [15]. Moreover, silver-based photocatalysts (e.g., Ag₃PO₄, AgI, and Ag₂S) have a much lower bandgap (1.0–2.8 eV) than TiO₂-based photocatalysts (up to 3.21 eV) and are highly reactive under visible light [16]. Silver/silver halides (Ag⁰/AgX) have been found to have high reactivity under both UV and visible light [17,18]. However, the stability and the light-utilizing efficiencies of these silver-based photocatalysts are often low. Furthermore, the fabrication of Ag⁰/AgX catalysts involves complicated processes. Composites containing multiple AgX components, which can be easily prepared using a precipitation method, may be attractive photocatalysts with good reactivity and stability. The effectiveness and mechanisms of BFR degradation by multiple-component AgX-based composites deserve further investigation.

Herein, this study aimed to investigate the degradation of BFRs by a photocatalytic system mediated by AgCl/AgBr composites under visible light (Vis-AgCl/AgBr). AgCl/AgBr composites were fabricated using a precipitation method. TBBPA, which is a widely used BFR and ubiquitous in aqueous environments [19,20], was selected as a model pollutant. The efficiency of TBBPA degradation by the Vis-AgCl/AgBr system was evaluated. The potential mechanisms of reactive species formation and TBBPA degradation were also elucidated. This study may provide a low-cost and efficient alternative for the abatement of refractory emerging micro-pollutants from contaminated water.

2. Materials and Methods

2.1. Reagents and Catalyst Preparation

The reagent-grade chemicals used in this study were purchased from Sigma-Aldrich (St. Louis, MO, USA). All solutions were prepared by dissolving corresponding chemicals in deionized water (0.055 μS/cm) generated by a NANOpure Diamond ultrapure water system (Barnstead, NH, USA). The AgCl/AgBr composites were synthesized via a precipitation method using a silver-ammonia complex as the silver source. A total of 0.1 mol/L of ammonia solution was dropwise added into 30 mL AgNO₃ solution (0.6 wt%) and continuously mixed by a magnetic stirrer at 200 rpm until all of the precipitate was dissolved. Subsequently, a 1.5 mL solution containing 0.45 mol/L NaCl and 0.45 mol/L KBr was dropwise added to allow the formation of AgCl-AgBr co-precipitates. The molar ratio of AgCl to AgBr was approximately 1:1. The obtained co-precipitates were separated through centrifugation and rinsed with ethanol and deionized water three times. The co-precipitates were dried at 105 °C for 24 h and then stored in the dark before use. A real water sample was collected from the Pearl River (Guangzhou, China) and filtered through a 0.45 μm membrane before use. The concentrations of dissolved organic carbon and bicarbonate were 3.45 mg C/L and 2.18 mM, respectively.

2.2. Experimental Procedures

Photocatalytic experiments were performed using a reactor setup equipped with a magnetic stirrer at an ambient temperature of 26 °C. Six LED lamps (AC220V, 14 W, 950 lm) were used as light sources to simulate the visible light from the sun. A total of 50 mg of catalysts was added to a 100 mL glass flask with a 50 mL solution containing 5.0 mg/L TBBPA. The dosage of the catalysts was 1.0 g/L to ensure the efficient degradation of TBBPA. The solution pH was adjusted to the desired value with 0.1 M NaOH or HCl. The mixture was stirred for 30 min in the dark to achieve adsorption–desorption equilibrium and then irradiated under visible light to initiate the photocatalytic reactions. Various scavengers, including benzoquinone (200 mg/L), isopropanol (0.1%), and EDTA (1000 mg/L), were added into the reactor to reveal the contributions of reactive species to TBBPA degradation. Supplementary tests were conducted to evaluate the stability of the catalysts by repeatedly reusing the AgCl/AgBr composites. The efficiency of the Vis-AgCl/AgBr system was also evaluated by spiking 5.0 mg/L TBBPA into the real water sample. Samples were periodically collected and quenched with methanol to terminate radical reactions. The samples were filtered through a 0.22 μm membrane before measurement. In addition, the solid samples were also collected and freeze-dried for the surface characterizations.

2.3. Analytical Methods

TBBPA was measured via an HPLC system (Waters, E2695) equipped with a reversed-phase C18 column (4.6 mm × 150 mm, 5 μm) and a UV–Vis detector at 222 nm. The eluent was a mixture of methanol and deionized water (85:15, v/v) at a flow rate of 0.8 mL/min. Halide ions were analyzed on an ion chromatograph (100, Dionex, Sunnyvale, CA, USA) equipped with a conductivity detector and an IonPac AS9-HC analytical column. X-ray diffraction (XRD) analysis was performed with an X-ray diffraction spectrometer (D8 ADVANCE, Bruker, Karlsruhe, Germany) with Cu Kα radiation (λ = 1.5406 Å) over the 2θ range of 10–80°. The morphologies and surface elemental compositions of the catalysts were obtained using a scanning electron microscope (SEM, Ultra 55, Zeiss, Oberkochen, Germany) equipped with an energy-dispersive X-ray spectrometer (EDX, ThermoNoran VANTAG-ESI). XPS analysis was performed using an X-ray photoelectron spectrometer (Thermo Scientific K-Alpha+, ThermoFisher, Waltham, MA, USA). The UV–vis diffuse reflectance spectroscopy (DRS) of the solid samples was measured by a UV–vis spectrophotometer (U-3010, Hitachi, Tokyo, Japan) using a BaSO₄ pellet as a blank.

2.4. Statistical Analysis

The results are expressed as the mean ± standard deviation of the two duplicate experiments. Statistical significance (p < 0.05) was evaluated by one-way analysis of variance (ANOVA) using Origin 9.0 software.

3. Results and Discussion

3.1. Catalyst Characterizations

The physical and chemical properties of the catalysts were characterized using SEM-EDS, XRD, XPS, and UV-vis DRS. As shown in Figure 1a, the AgCl/AgBr composites were in spherical particle form with particle sizes of 1.0–3.5 μm. The results of EDS elemental mapping show that Cl, Br, and Ag elements were evenly distributed on the particles (Figure 1b–e), suggesting the close combination of AgCl and AgBr. The XRD patterns (Figure 2a) confirmed the coexistence of orthorhombic AgCl (JCPDS 31-1238) and AgBr (JCPDS 06-0438) phases in the composites. The AgCl crystals had planes of (111), (200), (220), (222), and (400), and the AgBr crystals had planes of (200), (220), (222), and (400). The individual diffraction peaks of AgCl and AgBr were not observed in the composites due to the close proximity of the crystals. The XPS spectra further confirm the presence of Cl, Br, and Ag elements on the surface of the AgCl/AgBr composites (Figure 2b). It is worth noticing that the two peaks of Ag⁺ were identified at 373.7 eV (3d_3/2) and 367.6 eV (3d_5/2), respectively [21], while the peak of Ag⁰ was not observed (Figure 2c). These results suggest that all Ag was in the form of Ag⁺. Ag⁺ may play an important role in AgCl/AgBr photocatalysts [17,21]. Ag⁺ can mediate the separation and transfer of photoelectrons and inhibit the re-combination of e⁻ and h⁺, and improve photocatalyst stability via the formation of a Ag⁰/AgCl/AgBr heterojunction. Moreover, Ag⁺ may serve as a key active site for the degradation of organic pollutants.

The AgCl/AgBr composites were also analyzed using the UV–vis diffuse reflectance spectrum to evaluate the optical properties. As shown in Figure 2d, the AgCl/AgBr composites can absorb light with a wavelength of 200–800 nm. These results suggest that AgCl/AgBr composites may have good visible light catalytic activity. Based on the Kubelka–Munk theory [22], the energy gap (E_g) of the AgCl/AgBr composites was calculated to be 2.26 eV, while those of AgCl and AgBr were 2.88 eV and 2.49 eV, respectively. These results imply that the formed heterojunction decreased the E_g of the AgCl/AgBr composites. These findings confirm the successful fabrication of AgCl/AgBr composites with potential visible-light catalytic activity by the simple precipitation method.

3.2. Degradation of TBBPA by Vis-AgCl/AgBr System

The efficiency of TBBPA degradation by the Vis-AgCl/AgBr system was investigated, and the results are presented in Figure 3. As shown in Figure 3a, visible light irradiation alone exhibited negligible TBBPA removal within 40 min (less than 5%). Over 99% of TBBPA (5.0 mg/L) was removed at a AgCl/AgBr dosage of 1.0 g/L within 30 min. For comparison, the removal efficiencies with the presence of AgCl alone or AgBr alone after 30 min of reaction were only 14.3% and 51.7%, respectively, which were 85.6% and 47.8% lower than that with the presence of AgCl/AgBr. These observations demonstrate the outstanding catalytic activity of AgCl/AgBr composites under visible light. Compared to AgCl and AgBr, the better performance of the AgCl/AgBr composites may be due to the constructed heterojunction, which decreased E_g (2.26 eV). Reactive species, such as photoelectrons (e⁻), holes (h⁺), superoxide radicals (•O₂⁻), and •OH, may be generated from the Vis-AgCl/AgBr system [23,24].

The effects of operational parameters on the efficiency of TBBPA degradation were evaluated. Figure 3b shows that increasing the dosage of AgCl/AgBr composites from 0.25 to 1.0 g/L promoted TBBPA degradation, and the efficiency within 45 min increased from 54.3% to 99.5%. In addition, the light intensity was also found to obviously influence TBBPA degradation (Figure 3c). The removal efficiency within 45 min increased from 76.7% to 99.5% when the light intensity increased from 1900 lm to 5700 lm. These results suggest that the formation of reactive species from the Vis-AgCl/AgBr system can be elevated at a high AgCl/AgBr dosage and light intensity.

3.3. Reuse of AgCl/AgBr Composites for TBBPA Degradation

The AgCl/AgBr composites were reused for TBBPA degradation to examine the stability of the catalysts. Figure 4 shows that the TBBPA removal efficiency in the first use cycle reached 99% within 30 min, and it slightly decreased to 83.5% in the seventh use cycle. The findings demonstrate the good stability and reusability of the AgCl/AgBr composites. In real practice, AgCl/AgBr composites with micron particle sizes (1.0–3.5 μm) can be easily separated from the treated water through sedimentation or microfiltration, and then repeatedly used for the TBBPA removal to reduce the chemical costs of water treatment. Compared to conventional AOPs (e.g., Fenton and UV/persulfate processes), the reuse of AgCl/AgBr composites without the addition of chemical oxidants and activators will reduce chemical consumption significantly. Moreover, sunlight can be directly employed as a light source to drive photocatalytic reactions, which could minimize energy consumption. Therefore, the Vis-AgCl/AgBr system is expected to be an efficient and low-cost approach for the removal of refractory emerging micro-pollutants (e.g., BFRs, endocrine-disrupting chemicals, and pharmaceutical and personal care products) from industrial wastewater and contaminated water bodies [25].

3.4. TBBPA Degradation in Real Water

The efficiency of TBBPA degradation in the real water sample by the Vis-AgCl/AgBr system was evaluated. As shown in Figure 5, the degradation of TBBPA was retarded in the real water sample. About 37.5% of 5 mg/L TBBPA was degraded within 30 min of the reactions, which was 61.6% lower than that in deionized water. This poor performance may be due to natural organic matter (3.45 mg C/L) and bicarbonate (2.18 mM) present in the real water sample. Natural organic matter and bicarbonate are known as radical scavengers and can compete for reactive species with TBBPA [26,27]. After 120 min of reactions, 99.1% of TBBPA was degraded. These results demonstrate that the complete removal of TBBPA can be achieved in a prolonged time. The reusability of the AgCl/AgBr composites should be evaluated using real water/wastewater samples when the Vis-AgCl/AgBr system is applied in practice.

3.5. Contributions of Reactive Species to TBBPA Degradation and Debromination

Various reactive species generated in the Vis-AgCl/AgBr system can play important roles in the degradation of TBBPA. EDTA (1000 mg/L), isopropanol (0.1%), and benzoquinone (200 mg/L) were used as the quenchers of h⁺, •OH, and •O₂⁻, respectively. As shown in Figure 6, the efficiency of TBBPA degradation with the addition of EDTA only reached 64.5% within 30 min, which was 34.8% lower than that of the control without any scavengers. Moreover, the addition of isopropanol and benzoquinone decreased the efficiency of TBBPA degradation by 54.4% and 90%, respectively. These observations imply that h⁺, •OH, and •O₂⁻ all contributed to the degradation of TBBPA. h⁺ is capable of oxidizing organic pollutants directly and also reacts with water to form •OH [28]. •OH is a well-known reactive oxygen species with a redox potential of 2.8 V and can attack organic pollutants via hydrogen abstraction, electron transfer, and OH addition [29]. Although •O₂⁻ has a lower redox potential (-0.33 V), it can be converted into stronger reactive species (e.g., •OH) and oxidize organic pollutants [30]. h⁺, •OH, and •O₂⁻ can thus degrade TBBPA via oxidation and result in the formation of bromide (Br⁻) and oxidation products [10]. In addition, e⁻ with low redox potential (−2.34 V, versus NHE at pH 7.0) is able to induce the reductive transformation of organic pollutants [31,32]. Reductive debromination driven by e⁻ was found to be an important pathway for the photochemical degradation of PBDEs in the presence of carboxylate [33]. The contributions of reductive debromination by e⁻ should thus be significant to the degradation of TBBPA in the Vis-AgCl/AgBr system.

Dehalogenation is crucial to the detoxification of halogenated compounds. The debromination of TBBPA by the Vis-AgCl/AgBr system was evaluated. As shown in Figure 7, about 33.6 μM bromide (Br⁻) was produced when 99.1% of 5.0 mg/L TBBPA was degraded within 30 min of the reactions. The corresponding debromination rate was calculated to be 92.0%, which was very close to the TBBPA degradation rate (p > 0.05). These results confirm that the destruction of TBBPA by the reactive species led to the complete debromination of TBBPA. Therefore, the Vis-AgCl/AgBr system could be a promising alternative to reduce the toxicity and adverse impacts of BFRs, because halogenated groups are critical contributors to the toxicity and persistent nature of halogenated compounds [34,35].

3.6. Proposed Mechanisms of TBBPA Degradation by the Vis-AgCl/AgBr System

Based on the results obtained above, the possible mechanisms of TBBPA degradation by the Vis-AgCl/AgBr system in contaminated water can be proposed. As shown in Figure 8, visible-light irradiation induces the production of e⁻-h⁺ pairs on the AgCl/AgBr composites (Equation (1)). e⁻ reacts with Ag⁺ to form elemental silver (Ag⁰) (Equation (2)), which can improve photocatalyst stability via the formation of a Ag⁰/AgCl/AgBr heterojunction [17,21]. The XPS spectra of the AgCl/AgBr composites after repeated use confirm the presence of Ag⁰ (Figure 9). The Ag⁰/AgCl/AgBr heterojunction hinders the recombination of e⁻-h⁺ pairs. Subsequently, h⁺ reacts with water or OH⁻ to form •OH (Equations (3) and (4)). On the other hand, the interactions of e⁻ with dissolved oxygen (O₂) in solution result in the formation of •O₂⁻ (Equation (5)), which can be further converted into hydrogen peroxide (H₂O₂) and •OH (Equations (6) and (7)). The reactive species generated in the Vis-AgCl/AgBr system degrade TBBPA via both reduction and oxidation pathways. The photo-induced e⁻ with reducing capacity reacts with TBBPA through reductive debromination, which leads to the formation of Br⁻ and reduction products (Equation (8)). In addition, the oxidizing species (i.e., h⁺, •OH, and •O₂⁻) degrade TBBPA and produce Br⁻ and oxidation products (e.g., CO₂ and H₂O) (Equations (9) and (10)). Organic intermediates containing hydroxyl and carboxyl groups may be generated. Although the Vis-AgCl/AgBr system may not completely mineralize TBBPA, debromination is the key step in reducing the toxicity and persistency of TBBPA. Sunlight can be used as a green light source to drive the Vis-AgCl/AgBr system in contaminated water, such as rivers and lakes. Organic reduction and oxidation products can be easily degraded by microorganisms in water bodies.

$$\mathrm{AgCl}/\mathrm{AgBr}\stackrel{hv}{\to }{\mathrm{h}}^{+}+{\mathrm{e}}^{-}$$

(1)

$${\mathrm{Ag}}^{+}+{\mathrm{e}}^{-}\to {\mathrm{Ag}}^0$$

(2)

$${\mathrm{OH}}^{-}+{\mathrm{h}}^{+}\to •\mathrm{OH}$$

(3)

$${\mathrm{H}}_2{\mathrm{O}+\mathrm{h}}^{+}\to •{\mathrm{OH}+\mathrm{H}}^{+}$$

(4)

$${\mathrm{e}}^{-}+{\mathrm{O}}_2\to •{{\mathrm{O}}_2}^{-}$$

(5)

$$•{{\mathrm{O}}_2}^{-}+{\mathrm{e}}^{-}+{2\mathrm{H}}^{+}\to {\mathrm{H}}_2{\mathrm{O}}_2$$

(6)

$${\mathrm{H}}_2{\mathrm{O}}_2+{\mathrm{e}}^{-}+{\mathrm{H}}^{+}\to •\mathrm{OH}+{\mathrm{H}}_2\mathrm{O}$$

(7)

$${4\mathrm{e}}^{-}+\mathrm{TBBPA}\to {\mathrm{Reduction}\mathrm{products}+4\mathrm{Br}}^{-}$$

(8)

$$•\mathrm{OH}+\mathrm{TBBPA}\to {\mathrm{Oxidation}\mathrm{products}+4\mathrm{Br}}^{-}$$

(9)

$$•{{\mathrm{O}}_2}^{-}+\mathrm{TBBPA}\to {\mathrm{Oxidation}\mathrm{products}+4\mathrm{Br}}^{-}$$

(10)

4. Conclusions

The results of this study demonstrate that the Vis-AgCl/AgBr system driven by visible light degraded TBBPA in simulated and real water more efficiently than AgCl alone or AgBr alone. The AgCl/AgBr composites had good stability and could be reused for at least seven cycles. Reactive species (i.e., e⁻, h⁺, •OH, and •O₂⁻) significantly contributed to the degradation of TBBPA via reduction and oxidation, which resulted in the formation of Br⁻ and debrominated organic products with lower toxicity and higher biodegradability. The Vis-AgCl/AgBr system driven by visible light and sunlight can be a low-cost and efficient approach for the abatement of refractory emerging micro-pollutants in contaminated water, such as BFRs, endocrine-disrupting chemicals, and pharmaceutical and personal care products.