Effects Of Soil Components And Environmental Factors On The Abiotic Environmental Process Of Tetrabromobisphenol A In Soils And The Mechanisms

As the most entensively used brominated flame retardant in the world, tetrabromobisphenol A (TBBPA) is widely applied in plastics, printed circuit boards and kinds of electrical and electronic equipment, it can be released into various environmental matrices during processes such as treatment of e-wastes. Because TBBPA can pose a variety of toxic effects to organisms and especially it can potentially induce endocrine disrupting effects, TBBPA has gained growing attention. At present, studies on TBBPA mainly focus on the. its biological toxicity, distribution and degradation in the environment, exposure to human populations, while abiotic environmental process of TBBPA in soils has gained limited attention.In this study, we studied the effect of various soil components such as organic matter, ferric oxides and phyllosilicate minerals, and typical environmental factors such as pH, exogenous organic carbon, heavy metals and irrigation water on the sorption or desorption behavior of TBBPA in soils through batch adsorption experiments. More importantly, we examined the mineralization and bound residue formation of 14C-TBBPA with two kinds of soil humic substances (Elliott soil humic acid (HA) and Steinkreuz soil HA) through static degradation experiments. We investigated the effect of a typical active oxide in soils-manganese oxide on the mineralization and bound residue formation of TBBPA. The molecular distribution and chemical nature of the bound residues were characterized by gel permiation chromatography (GPC) and 13C nuclear magnetic resonance (13C-NMR). Effects of typical environmental factors such as pH and heavy metals on the formation of TBBPA bound residues in the presence of MnO2 and HA were also investigated. The following are main conclusions obtained:I Effect of typical soil components and environmental factors on the sorption behavior of TBBPA in soils.(1) As an ionizable organic contaminant under alkaline conditions, sorption of TBBPA by soils was greatly affected by pH. The sorption of TBBPA decreased with increasing pH.(2) Soil organic matter contributed most to the sorption of TBBPA in red soil and black soil, but the contribution of layer silicate minerals and iron oxides could not be neglected. Ferric oxides in soils could mask the sorption sites of phyllosilicate minerals for TBBPA leading to the decrease of TBBPA sorption in soils.(3) Effect of exogenous dissolved organic carbon on the sorption of TBBPA in red soil was limited only at neutral conditions when sewage sludge DOC and cow manure DOC enhanced the sorption while DOC from fulvic acid decreased the sorption.(4) Heavy metals (Cu, Pb, Cd) hardly affected the sorption of TBBPA in red soil.(5) Compared with alkaline black soil with stronger buffer capacity, the desorption of TBBPA from acidic red soil with weaker buffer capacity was apparently enhanced when irragated with alkaline river water due to eutrophication.In summary, soil solution pH and soil components are two most important factors controlling the mobility of TBBPA in soil environment.II Effect of δ-MnO2 on the bound residue formation of TBBPA in soil humic substances and the mechanisms.(1)δ-MnO2 can lead to the mineralization of TBBPA and overall such mineralization was almost not affected by HA. This may be due to the low mineralization of TBBPA induced by δ-MnO2 alone (<2%).(2) In the presence of δ-MnO2, under all the studied pH range (5-9), the boun residue formation of TBBPA or its transformation products in Elliott soil HA and Steinkreuz soil HA was greatly enhanced by 3 to 17 fold. The higher bound residue formation was found in Elliott soil HA with higher aromaticity (50%) than in Steinkreuz soil HA with lower aromaticity (7%), which may be because high aromaticity of HA can facilitate the stablization of radicals during the reaction.(3) GPC analysis showed that the molecular size distribution of TBBPA over Steinkreuz soil HA was homogenous while inhomogenous in Elliott soil HA. This may be due to the discrepancy between the molecular size distribution of aromaticity or active functional groups of the two HAs.(4) The 13C-TBBPA residues bound to 13C-depleted HA was characterized by 13C NMR and chemical treatment. Results showed that in the presence of δ-MnO2, the binding mainly occurred via covalent bonds, such as ester, ether or C-C bonds, between TBBPA or its products and HA, while the physically entrapping in HA contributed little to the formation of TBBPA bound residues.(5) The possible mechanisms on the formation of TBBPA bound residues induced by δ-MnO2 were:the phenoxy radicals and its resonance forms of TBBPA oxidized by δ-MnO2 were formed, then the phenoxy radicals underwent β scission leading to the formation of a phenyl radical with single ring and a carbocation intermediate with single ring. When HA was present, these radicals and carbocation intermediates interacted with HA via ways such as ester bonds, ether bonds and C-C bonds thus facilitating the formation of TBBPA bound residues in HA. Finally the mobility and bioavailability of TBBPA in soils were weakened.In summary, manganese oxides contributed significantly to the abiotic formation of TBBPA bound residues, and the mechanism may be Mn oxides stimulated the covalent binding between TBBPA and HA.Ⅲ Effect of δ-MnO2 on the bound residue formation of TBBPA in soil humic substances as affected by the coexistence of metals.Relatively high concentration of metals (Cu, Cd) had no significant effect on the formation of TBBPA bound residues with two HAs and the chemical bonding induced by δ-MnO2. This may be because concentrations of metals in HA were too low as a result of the strong adsorption interaction between metals and δ-MnO2 to directly affect the bound residue formation.In conclusion, abiotic environmental process of TBBPA in soils are mainly controlled by soil components and environmental factors. High content of soil organic matter, clay minerals and ferric oxides and low pH values facilitate the persistence of TBBPA in soils, while solubility and mobility of TBBPA under alkaline conditions greatly ascend. Active soil components such as Mn oxides with oxidability can significantly enhacne the interaction between TBBPA and soil humic substances leading to the stimulation of TBBPA bound residue formation via facilitating the formation of covalent bonds between TBBPA and humic substances. Thus to some extent, Mn oxides may "strengthen" the sorption of TBBPA in soils and apparently weaken the mobility and bioavailability of TBBPA in soil environment. And high aromaticity of HA may facilitate the bound residue formation. pH and coexistence of heavy metal cations have no obvious effect on the formation of TBBPA bound residues in humic substances.