The degradation and transformation of TBBPA-derived NER in soil

Abstract

Tetrabromobisphenol A (TBBPA) is a kind of widely used brominated flame retardants. After entering to the soil, considerable TBBPA and its metabolites can form non-extractable residues (NER) under the flooded condition (F-NER) and oxic condition (O-NER). However, little is known about the stability and environmental behavior of these NERs in soil after long-term incubation. In this study, two 14C-TBBPA-NERs was used to study: (1) The fate of F-NER in oxic and oxic-flooded-oxic soil including their releasing, mineralization, degradation and the distribution of 14C in soil matrix in the presence or absence of rice root exudate; (2) the biotransformation different between the F-NER and O-NER in oxic-flooded-oxic soil in the presence or absence of rice root exudate. During 231 d incubation, slow biotransformation of F-NER was observed in oxic soil. More than 10% of F-NER was mineralized accompanied by the releasing of ester and ether-linked NER, while the extractable TBBPA and its metabolites kept quite low during the incubation (1%−6%). In addition, the appearance of rice root exudate inhabited the mineralization of F-NER in oxic soil at the end of the experiment. In oxic-flooded-oxic incubation, the mineralization of F-NER is consistent with oxic condition, while the more extractable radioactivity was observed in soil extracts during the flooded incubation. We proposed that higher extractable 14C may attribute to the released dibrominated products and BPA tended to accumulated in flooded soil, and these extractable residues removed fast after soil turns into oxic condition. Meanwhile, O-NER was more stable than F-NER in soil presented as lower mineralization and releasing of TBBPA and its metabolites during the incubation, and further analysis showed that ester and ether-linked NER also decreased slower in O-NER treatment. No obvious extractable 14C accumulated in soil extract during the flooded incubation. O-NER was more persistent in soil after long-term incubation than F-NER may attribute to the metabolism of TBBPA is different in oxic and flooded conditions. We proposed that during the formation of F-NER in soil, anaerobic metabolites such as debrominated products and BPA were bounding to the soil matrix with TBBPA, and after releasing, these metabolites can be removed fast during the aerobic incubation. While O-NER mainly contained TBBPA and its aerobic metabolites such as MeO-TBBPA and MeO-triBBPA, and these unpolar products were more persistent in environment. Therefore, our results demonstrated that the biotransformation of two NERs was different in soil, which was closely related to the metabolism of TBBPA. In addition, both redox conditions changing and the addition of root exudate can affect the bioavailability of F-NER especially in long-term incubation.