Two-stage Reduction/Subsequent Oxidation Treatment Of Brominated Flame Retardants In Aqueous Solutions

Brominated flame retardants (BFRs) have been used as important additives in synthetic polymers, such as plastics, textiles, electronics/electrical, transportation packaging, construction materials and other fire prevention materials. Due to leaks, spills, and releases from industrial sourses, they inevitably comtaminate the enviroment. A "two-stage reduction/subsequent oxidation" (T-SRO) process was employed to remove BFRs by the combination of first Fe-Ag debromination and succeeding Fenton-like decomposition.The Fe-Ag bimetallic nanoparticles with core-shell structure were successfully synthesized by the liquid phase reduction method, depositing of Ag onto nanoscale Fe surface. The characterization results revealed that the displacement plating produced a non-uniform overlayer of Ag additive on iron; the as-synthesized bimetallic nanoparticles were spherical with diameters of 20-100 nm aggregated in the form of chains and a surface area of 72 m2 g-1. The three characteristic peaks in XRD spectrum appeared at 44.66 (main peak),64.34 and 38.06°correspond to cubic a-Fe (110,200) and Ag (111) diffraction peaks, respectively.Batch studies demonstrated that the tetrabromobisphenol A (TBBPA,2 mg L-1) solution was completely degraded in 20 min over Fe-Ag nanoparticles under ultrasound (US). It indicated that both the deposition of Ag and US played important roles in the reduction of TBBPA. The effects of Fe-Ag bimetallic nanoparticles loading, initial TBBPA concentration, pH of the solution, Ag loading and temperature on the reduction efficiency of TBBPA were investigated. Reaction conditions were determined selectively as metal addition of 0.4 g L-1, Ag loading of 1 wt.%, initial TBBPA concentration of 2 mg L-1, pH=6.0±0.5, T=30℃.The enhanced debromination of TBBPA, decabromodiphenyl ether (BDE-209) and 2,2',4,4'-tetrabromodiphenyl ether (BDE-47) with Fe-Ag nanoparticles under microwave (MW) radiation was studied. The rates of debromination revealed that MW irradiation could accelerate the reductive degradation obviously. Attempts were made to compare degradation efficiency under microwave and conventional heating condition, which demonstrated that the thermal effect of MW radiation is the main factor to promote the debromination of TBBPA or PBDEs. The effects of Fe-Ag dosage and MW energy level on the degradation efficiency were also investigated. Moreover, the number of bromines could have an effect on the stability against reduction of PBDEs. Major reduction products of TBBPA and PBDEs were identified by LC-MS/MS and GC-MS, respectively. TBBPA was transformed to tri-BBPA, di-BBPA, mono-BBPA and BPA; di-to nona-brominated congeners were formed during BDE-209 reduction; and DE to tri-BDEs were observed in BDE-47 reduction. The degradation possibly proceeds through stepwise debromination from n-bromo- to (n-1)-bromo-BPA/DE, with bromine being sequentially substituted by hydrogen.It can be seen that the partial or complete debrominated products still pose a threat to the environment and need to be treated further. In this study, the feasibility and effectiveness of the removal of BFRs by T-SRO process were investigated. TBBPA was transformed to bisphenol A (BPA) completely by Fe-Ag/US and then BPA was 99.2% oxidized by the homogeneous Fe-Ag/H2O2/US system. Meanwhile, the T-SRO process resulted in a efficient debromination of BDE-47 and a 99.2% decrease in diphenyl ether (DPE) concentration. However, TBBPA and BDE-47 were difficult to be degraded by Fenton-like processes alone. Fenton-like reactions could make full use of the remaining Fe-Ag nanoparticles after reduction stage. LC-MS/MS and GC/MS were employed to monitor the main intermediates and final products during BPA/DPE oxidation. On the basis of these analysis, reactions with·OH radical were identified as the major pathways in the Fenton-like system.In addition, luminescent bacteria test showed that the acute toxicity of the original solution (before reduction) was evidently lower than that of Fe-Ag/US reduction-treated solution, but no toxicity was detected after the Fenton-like oxidation processes. Evidence for the significance of a T-SRO treatment to decompose BDE-47 was presented.