High-efficient removal of tetrabromobisphenol A in aqueous by dielectric barrier discharge: Performance and degradation pathways

Highlights

• Tetrabromobisphenol A removal in aqueous was studied in a DBD plasma system.

• Effects of some operational conditions on TBBPA removal were evaluated.

• The roles of chemically oxidizing species in TBBPA removal were identified.

• The possible decomposition pathways of TBBPA in this system were proposed.

• Residual toxicity after TBBPA decomposition was evaluated using T.E.S.T.

Abstract

Brominated flame retardants are widely used in fire protection area, but also bring great threats to ecological environment and human health. In this study, the potential of brominated flame retardant removal in wastewater by dielectric barrier discharge (DBD) plasma was investigated, with Tetrabromobisphenol A (TBBPA) as a model pollutant. The experimental results showed that TBBPA could be effectively removed by the DBD plasma oxidation. Almost all of the TBBPA in wastewater could be successfully decomposed within 15 min’s oxidation treatment, and the decomposition process fitted well with the pseudo-first-order kinetic model. Relatively lower TBBPA initial concentration favored its decomposition; and the effect of solution pH value on the oxidation process was negligible in the selected pH range. Electron paramagnetic resonance analysis showed that O₂^–, OH, and ¹O₂ were produced in the DBD plasma process; and O₂^– was the main reactive species for TBBPA decomposition, OH and ¹O₂ also played important roles. The molecular structure of TBBPA was effectively destroyed, and some byproducts including bisphenol A, and dibromophenol were generated. The possible decomposition pathways for TBBPA degradation were proposed. Furthermore, the acute toxicity and bioaccumulation factor of intermediate byproducts were alleviated via the analysis of Toxicity Estimation Software Tool.

Introduction

Brominated flame retardants, including polybrominated diphenyl ethers, hexabromocyclododecane, polybrominated biphenyls, and tetrabromobisphenol A (TBBPA), have been widely utilized in plastics, textiles, electronic circuitry and other materials to prevent fires [1]. These flame retardants are inevitably discharged into wastewater, leading to bioaccumulation due to the property of persistence [2]. Previous study reported that the flame retardants were difficult to be eliminated in a sewage treatment plant, especially for TBBPA [3]. Even worse, the toxicity of TBBPA was the highest, compared with other brominated retardants [4]. Kuiper et al [5] reported that adult fishes exposed to 3–6 μM TBBPA during 30 days were all lethal. As persistent organic pollutants with bioaccumulation properties and potential toxicities, brominated flame retardants pose adverse risks to ecological environment and human health [6], and thus it is urgent to remove them from wastewater.

Advanced oxidation processes have been widely used as effective methods for treatment of refractory organics in wastewater, which have the high performance for the degradation of organic pollutants [7], [8], [9], [10], [11]. Fenton oxidation [8], electrochemical oxidation [9], ozonation [10], photocatalysis [11], ferrate oxidation [12], persulfate oxidation [13], and chlorination [14] have been used for brominated flame retardant removal in wastewater in the past decades. Although Fenton oxidation could eliminate organic pollutants in wastewater, it was strongly affected by solution pH value and iron sludge also needed to be further treated [15]. Electrochemical oxidation could effectively remove organic pollutants in wastewater, but the process mainly depended on electrode materials [16]. Ozone alone could not effectively remove organic pollutants; it was more commonly used in combination with other oxidation technologies [17]. For photocatalysis, visible light absorption capacity and secondary pollution from the catalysts should be taken into consideration [18], [19], [20]. Fe or Cl⁻ residual in the Ferrate oxidation and chlorination process would bring secondary pollution. And exogenous materials or energy were needed to activate the persulfate [21].

Recently, plasma-based oxidation technologies have attracted more and more attention in term of environment protection, because they could remove various organic pollutants and had excellent performance on the degradation of organic pollutants. Several kinds of plasmas including dielectric barrier discharge (DBD), corona discharge, and glow discharge were all used for organic pollutants removal [22], [23], [24], [25]. Among these methods, DBD as an eco-friendly and economical non-thermal plasma technology was considered to be one of the most promising technology for the removal of organic pollutants in wastewater [26], [27], [28], [29]. Many kinds of oxidative species such as O₃, O, H₂O₂, and OH were produced in the DBD plasma process, and various physical effects such as ultraviolet irradiation and shockwaves were also generated, which all promoted organic pollutant removal. Tichonovas et al [30] reported that DBD plasma avoided the consumption of additional chemicals and did not produce lots of dangerous byproducts during the treatment of organic pollutants. In our newly study, another type of DBD plasma system was developed to remove plasticizer in water [23]. In this system, the DBD plasma was triggered along a quartz glass tube, and the chemically oxidative species generated in the plasma process were rapidly injected into the aqueous via gas flow, and thus improved their mass transfer and utilization in aqueous. Hence, it is supposed that this type DBD might effectively remove TBBPA in wastewater. However, little has been reported on the removal performance and possible mechanisms of TBBPA by the DBD plasma.

In this study, the potential of brominated flame retardant removal using the DBD plasma was investigated. TBBPA was selected as the target flame retardant. The removal performance of TBBPA by the DBD plasma at different conditions was first evaluated. Then, the reactive species that played important roles in the reaction process were identified. Subsequently, the internal mechanism of DBD in TBBPA degradation was explored. Finally, the potential toxicity after TBBPA decomposition was evaluated via Toxicity Estimation Software Tool.