水泥窑协同处置是固体废物处置的重要方式，该处置方式的环境安全性一直受到关注。水泥窑协同处置含溴物质，尤其是处置含溴代阻燃剂（十溴二苯醚BDE209）的土壤时，会产生溴代二噁英/呋喃（PBDD/Fs），存在潜在的环境风险。水泥窑预热区具备生成PBDD/Fs的温度、前驱体等必要条件，是水泥窑系统中最重要的二噁英产生源，开展BDE209分解生成2,3,7,8-PBDD/Fs的机理研究，对控制其生成和释放具有重要意义。

本文围绕揭示水泥窑预热区BDE209分解生成2,3,7,8-PBDD/Fs机理这一关键科学问题，通过实际工况调研、实验室模拟、量子化学计算等研究手段，研究了水泥窑预热区温度、氧气含量等多因素作用下2,3,7,8-PBDD/Fs的生成及影响规律，分析了各合成路径的热力学和动力学特征，对关键中间体进行了实验验证，探究了水泥窑预热区BDE209生成2,3,7,8-PBDD/Fs的反应路径。取得主要结果如下：

（1）BDE209热分解的特征产物是高溴代多溴二苯醚（6-9溴的PBDEs）、高溴代2,3,7,8-PBDD/Fs（7-8溴）以及HBr和Br₂。HBr、Br₂产生量最大，平均值为7.08、18.82 μg/g，PBDEs产生量平均值为0.71μg/g，PBDD/Fs产生量平均值为0.05 μg/g。特征产物间的相关性分析显示，2,3,7,8-PBDD/Fs与PBDEs相关性强（r=0.42），表明2,3,7,8-PBDD/Fs更可能来源于以PBDEs为前驱体的合成反应。

（2）响应曲面实验的方差分析表明，温度（p=0.0201）、氧气含量（p<0.0001）、BDE209浓度（p=0.0034）和氧化铁含量（p=0.0018）均显著影响2,3,7,8-PBDD/Fs的生成，是影响其生成的关键因素。温度与氧气含量的交互作用显著影响2,3,7,8-PBDD/Fs的生成（p<0.0001），是关键交互作用因素。在温度、氧气含量与2,3,7,8-PBDD/Fs产生量的三维曲面中，温度为675℃、氧气含量为10%时，PBDD/Fs产生量呈现局部最高值。

（3）采用密度泛函理论方法（DFT），计算了温度-氧气关键因素作用下11种优势PBDEs生成2,3,7,8-PBDD/Fs的反应路径。PBDEs的一个邻位C-Br键断裂形成苯基自由基类物种，然后再环化，是生成2,3,7,8-PBDFs的重要步骤，该步骤相对于PBDEs同时脱除两个邻位Br然后再环化，反应速率更快。苯基自由基类物种，氧化成过氧苯基自由基类（无垒放热反应）或苯氧自由基类物种是生成2,3,7,8-PBDDs重要步骤。由于生成PBDDs的基元反应较多，生成PBDFs的速率常数较大，因此2,3,7,8-PBDFs产生量高于2,3,7,8-PBDDs。

通过电子自旋共振波谱（ESR）检测，发现了持久性自由基物种（苯基自由基类、过氧苯基自由基和苯氧类自由基类等）的信号（g=2.0051），与反应路径的理论计算结果一致，据此推断2,3,7,8-PBDD/Fs可能是PBDEs通过上述路径形成。

（4）模拟实验表明，以高温、高氧含量为特征的水泥窑预分解炉上部区域，2,3,7,8-PBDD/Fs生成有三个主要反应过程。首先，PBDEs发生邻位C-Br断裂后产生的苯基自由基类物种，进行氧化反应，是生成2,3,7,8-PBDD的典型反应过程。其次，氧气与PBDEs邻位溴的置换反应是该区域2,3,7,8-PBDDs偏高的可能原因。再其次，高溴代的2,3,7,8-PBDD/Fs同类物，脱溴生成低溴代同类物，也是该高温区域的重要反应。实验发现PBDDs/PBDFs=1.02，同时低溴代2,3,7,8-PBDD/Fs同类物增加，验证了上述主要反应路径。

（5）相较于水泥窑预分解炉上部，预热器区域温度较低，生成2,3,7,8-PBDD/Fs的优势路径为：PBDEs脱除邻位溴然后环化。模拟实验发现，在预热器区域，随着温度下降（模拟一级到五级旋风筒），PBDD/Fs产生量增加，其中一级是五级的8.9倍。

Cement kiln co-processing is an important method of solid waste disposal, and the environmental safety of this disposal method has always been concerned. Brominated dioxins/furans (PBDDs/Fs) are generated when cement kilns co-processing of brominated substances, especially when disposing of soils containing brominated flame retardants (decabromodiphenyl ether BDE209), posing a potential environmental risk. The preheating zone of the cement kiln has the necessary conditions of temperature and precursors for the generation of PBDD/Fs, which is the most important source of dioxins in the cement kiln system. The study of the mechanism of BDE209 decomposition to generate 2,3,7,8-PBDD/Fs is important to control its generation and release.

      This paper reveals the key scientific problem of the mechanism of the decomposition of 2,3,7,8-PBDD/Fs generation from the decomposition of BDE209 in the preheating zone of cement kilns. Through actual working conditions, laboratory simulations, quantum chemical calculations, and other research methods, this study investigates the formation and influence of 2,3,7,8-PBDD/Fs in the preheating zone of cement kilns under the effect of multiple factors such as temperature and oxygen content, analyzes the thermodynamic and kinetic characteristics of each synthesis pathway, conducts experimental verification of key intermediates, and explores the reaction pathway of BDE209 in the preheating zone of cement kilns to form 2,3,7,8-PBDD/Fs in the preheating zone of cement kiln. The main conclusions are as follows:

(1) The characteristic products of thermal decomposition of BDE209 were high brominated polybrominated diphenyl ethers (PBDEs with 6-9 bromine), high brominated 2,3,7,8-PBDD/Fs (7-8 bromine), and HBr and Br₂. HBr and Br₂ were produced in the largest amounts with mean values of 7.08 and 18.82 μg/g (reaction substance), PBDEs were produced in mean values of 0.71 μg/g, and PBDD/Fs were produced in mean values of 0.05 μg/g. The correlation analysis between the characteristic products showed that 2,3,7,8-PBDD/Fs correlated strongly with PBDEs (r=0.42), indicating that 2,3,7,8-PBDD/Fs were more likely to originate from the synthesis reaction using PBDEs as precursors.

(2) The analysis of variance in response surface experiments showed that temperature (p=0.0201), oxygen content (p <0.0001), BDE209 concentration (p=0.0034), and iron oxide content (p=0.0018) significantly affected the production of 2,3,7,8-PBDD/Fs and were the key factors affecting their generation. The interaction between temperature and oxygen content significantly affected the production of 2,3,7,8-PBDD/Fs (p<0.0001), which is a key interaction factor. In the three-dimensional surface of temperature, oxygen content, and 2,3,7,8-PBDD/Fs production,  the PBDD/Fs production showed the highest local value at a temperature of 675°C and 10% oxygen content.

(3) Based on density functional theory (DFT), the reaction pathways for the generation of 2,3,7,8-PBDD/Fs from 11 dominant PBDEs under the effect of temperature-oxygen critical factors were calculated. The cleavage of an ortho-position C-Br bond of PBDEs to form phenyl radical species, and then cyclization was an important step for the formation of 2,3,7,8-PBDFs. This step results in a faster reaction rate compared to the simultaneous removal of two ortho-position Br and then cyclization of PBDEs. The oxidation of phenyl radical species to peroxide radical species (no-barrier exothermic reaction) or phenoxy radical species was an important step for the formation of 2,3,7,8-PBDDs. Because there were many elementary reactions for the formation of PBDDs, the rate constant for the formation of PBDFs was relatively high, and therefore 2,3,7,8-PBDFs were produced in higher amounts than 2,3,7,8-PBDDs.

The signals of persistent radical species (phenyl radical class, peroxide radicals, and phenoxy radical class, etc.) were detected by electron spin resonance spectroscopy (ESR) (g=2.0051), which was consistent with the theoretical calculation of the reaction pathway. It was inferred that 2,3,7,8-PBDD/Fs may be formed by PBDEs through the above pathways.

(4) Simulation experiments showed that there were three main reaction processes for 2,3,7,8-PBDD/Fs generation in the upper region of the cement kiln pre-calciner characterized by high temperature and high oxygen content. First, the phenyl radical species generated after the ortho-position C-Br cleavage of PBDEs and then were oxidated, which is a typical reaction process for the formation of 2,3,7,8-PBDD. Second, the substitution reaction between oxygen and ortho-position bromine of PBDEs is the possible reason for the high 2,3,7,8-PBDDs in this region. Third, the 2,3,7,8-PBDD/Fs congeners with more Br, debromination to produce low-brominated congeners, is also an important reaction in this high-temperature region. The experiment found that PBDDs/PBDFs=1.02, and the number of low-brominated 2,3,7,8-PBDD/Fs congeners increased, which verified the main reaction pathways mentioned above.

(5) Compared to the upper part of the cement kiln pre-calciner, the temperature in the preheater area is lower, and the advantageous pathway for generating 2,3,7,8-PBDD/Fs is the removal of ortho bromine by PBDEs followed by cyclization. The simulation experiment revealed that in the preheating area, the PBDD/Fs generation increased with decreasing temperature (simulated from one to five cyclone stages), where the first stage was 8.9 times higher than the fifth stage.