| Hexabromocyclododecanes(HBCDs) are the most widely used additive brominated cyclic alkane flame retardants, mainly added to the building decoration materials, texile and electronic products such as expandable polystyrene board and polystyrene foam board. Due to its environmental persistence, bioaccumulation and toxicity, HBCDs have been classified as persistent organic pollutants(POPs) by Convention of Stockholm, and their pollution and control in the environment gain global attention. HBCDs easily accumulate in soil and water sediment due to their strong hydrophobicity and are harmful to human health through the food chain. However, information on the natural attenuation process of HBCDs such as biodegradation and abiotic transformation in soil and sediment is limited. Iron sulfide(Fe S) as an important natural reductant produced by sulfate reducing bacteria(SRB) widely distribute in soil, sediment, groundwater and coastal waters. A large number of studies have shown that Fe S has the potential capability of reductively transforming halogenated POPs and affect their environmental fate in the surface of soil/sediment. However, research on the reductive debromination of HBCDs by Fe S is unavailable.In this study, two different types of sulfate reducing bacteria were chosen to biosynthesize two kinds of Fe S minerals. The surface characteristics of two Fe S were analyzed by a series of surface characterization methods(i.e., BET, SEM, XRD and XPS). The reductive debromination kinetics, pathways and mechanism of HBCDs and the effect of the different environmental factors(p H, ion concentration, concentration of Fe S, aging, temperature) on the rate of HBCDs transformation by two kinds of Fe S were studied in the batch reaction system. In addition, the synthesis of Fe S/Fe oxides by SRB using iron oxides(magnetite, goethite and hematite) as the iron source and their ability of transforming HBCDs was also explored. The main conclusions are as follows:(1) The specific surface area, iron/sulfur atomic ratio and relative content of Fe S on the surface of the Fe S mineral synthesize by the acidophilic SRB(Desulfosporosinus sp.) using Fe2+ as Fe source, designed as Z-Fe S, were 13.35 m2g-1, 0.91 and 63.6%, respectively. And the specific surface area, iron/sulfur atomic ratio and relative content of Fe S on the surface of the Fe S mineral synthesize by the neutrophilic SRB(Desulfomicrobium baculatum), designed as Z-Fe S, were 7.64 m2g-1, 0.84 and 77.2%, respectively. And the SEM and XRD results show that the main components of the S-Fe S and Z-Fe S are both amorphous Fe S and poorly crystallized mackinawite. Thus there was no significantly difference in the specific surface area, iron/sulfur atomic ratio and relative content between the two types of Fe S.(2) Both of the two kinds of Fe S could completely transform HBCDs within 12 hours when the concentration of Fe S was 1.2 g L-1. The transformation kinetics were well described by pseudo-first-order reaction models and the rate constants increased with the initial Fe S concentration. As the concentration of Fe S increased from 0.3 g L-1 to 1.2 g L-1, the reaction rate constants of HBCDs by S-Fe S and Z-Fe S increased from 0.0049 h-1 to 0.3194 and 0.3868 h-1, respectively. All of three kinds of reductive debromination intermediate products of HBCDs weredetected in two kinds of Fe S reaction system(by GC-MS analysis), they were tetrabromocyclododecane(C12H18Br4), dibromocyclododecane(C12H18Br2) and cyclododecatriene(C12H18), respectively, and the accumulation of cyclododecatriene continually increased with the reaction time. It suggested that HBCDs might undergo sequential dibromoelimination to form tetrabromocyclododecane, dibromocyclododecane, and cyclododecatriene in both of S-Fe S and Z-Fe S reation system. Both of two kinds of Fe S can transform all the three major isomers of HBCDs, and their debromination rate are ranked as β-HBCD > γ-HBCD > α-HBCD. The reductive debromination efficiency of HBCDs is affected by different environmental factors. For example, the reductive debromination rate of HBCDs by Fe S increased as an increase in the p H(4, 6, 8) and temperature(15, 30 and 45 ℃), while aging process and ion concentration can inhibit Fe S reductive debromination rate of HBCDs. However, the effect of different Fe S on the reductive debromination rate of HBCDs under various environmental conditions was insignificant. The XPS analysis showed that no obvious change in the amount of Fe(II) on the surface of Fe S was observed after reaction, while the amount of S(-II) decreased and oxidized to S22- and S0, indicating that S(-II) may play an important role in the reductive debromination of HBCDs. In conclude, both of S-Fe S and Z-Fe S can reductively debrominate HBCDs, and their ability and mechanism of reductive debrominating HBCDs were insignificantly different. This may due to that they had similar amounts of active Fe S component on the surface and similar characteristics.(3) The ability of two kinds of sulfate reducing bacteria utilize iron oxides as the sole iron source was observed with the naked eye when the black Fe S was formed. The results showed that the acidophilic sulfate-reducing bacteria can only use hematite and magnetite as the sole source of iron to synthese Fe S whereas neutrophil sulfate-reducing bacteria can use hematite goethite and magnetite as the sole source of iron to synthese Fe S. It indicated that two bacteria played different role in the Fe S formation in the natural environment. The XPS analysis confirmed that the presence of Fe S on iron oxide surface, while it was not detected by SEM and XRD analysis, which may be caused by the few amount of Fe S on the surface of iron oxide. When the sulfate-reducing bacteria using iron oxide as the sole source of iron, sulfate utilization was very low with only 2.8%- 13.3%. However, the utilization rate increased(by 5.8 to 27.5 times) after Fe2+ was added. This could explain that few amount of Fe S formed on the surface of iron oxide was due to that the iron in mineral crystal lattice was hard to be utilized by SRB. Because of the few amount of the formation of Fe S on surface of iron oxide, no obvious debromination of HBCDs occurred after six months in the Fe S/Fe oxides reaction systems.The results of this study will help to deeply understand the abiotic transformation of HBCDs by Fe S and the influence of different environmental factors on the transformation of HBCDs in natural environment such as soils and sediments. It provided the theoretical basis for risk assessment, pollution control and remediation of HBCDs-contaminated sites with rich Fe S. |
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