零价铝协同过硫酸盐降解溴代阻燃剂HBCD的效能与机理

Hexabromocyclododecane (HBCD) is a widely used additive brominated flame retardant (BFR) and has therefore been widely detected in biota and other environmental matrices. Due to its bioaccumulation, long range transport, environmental persistence, and potential toxicity, HBCD was listed as one of the persistent organic pollutants (POPs) at the sixth meeting of the Stockholm Convention in 2013. Up till now, knowledge about the degradability of HBCD is very limited, and a fast and effective treatment technology for HBCD is urgently required. Comparing to its difficultly being oxidized, HBCD is theoretically easier to be reduced because of the substitution of Br atom. As an excellent electron donator, zero-valent aluminum (ZVA) [E0(Al3+/Al0) = −1.66 V] can provide a very strong thermodynamic driving force for the charge transfer, but the potential application of ZVA in the field of contaminant removal has not been explored until now. Therefore, in this proposal, we would develop a mild and effective removal technique for aqueous HBCD based on the simple combination of commercial Al powder with the common oxidants, persulfate or peroxymonosulfate. It is a sequential reduction/oxidation strategy (i.e. ZVA debromination followed by activated persulfate/peroxymonosulfate oxidation). Firstly, ZVA is used as a reductant to debrominate for HBCD. Then, the debrominated products could be rapidly oxidized by the generated sulfate radicals (SO4•-) [E0(SO4•-/SO42-) = +2.5 ~ +3.1 V] where ZVA is used as an activator. In this proposal, researches would focus on: 1) exploring the surface corrosion chemistry of ZVA with the presence of HBCD, so as to get the ZVA activation method and reaction parameters for HBCD degradation; 2) studying the reductive degradation mechanism of HBCD by ZVA; 3) obtaining the initial radical mechanism and the performance of ZVA-activated persulfate/peroxymonosulfate oxidation for HBCD degradation; and 4) revealing the synergistic effect of reduction and subsequent oxidation. Based on these researches, the reductive and oxidative reaction mechanism of HBCD degradation by the combined ZVA and persulfate/peroxymonosulfate system would be deeply understood, which could provide a theoretical and technological guide for the removal of the newly-emerging contaminant HBCD.

溴代阻燃剂六溴环十二烷（HBCD）在2013年被增列入《斯德哥尔摩公约》POPs黑名单，目前国内外仍缺乏有效的降解方法。受溴取代影响，HBCD具有接受电子还原降解的趋势。零价铝（ZVA，E0 = −1.66 V）是优良的电子供体，但在污染物去除领域的潜力尚未被充分开发。本项目通过ZVA的表面活化，先采用ZVA将HBCD还原脱溴，然后利用ZVA催化过硫酸盐产生强氧化性的硫酸根自由基（SO4•-）将还原中间产物氧化降解，建立一种两段式高效去除水中HBCD的新方法。研究内容包括：1）探讨HBCD存在时ZVA的表面腐蚀反应过程，获得适用于HBCD还原降解的ZVA活化方法和反应条件；2）考察HBCD的ZVA还原降解机理；3）阐明ZVA催化过硫酸盐的自由基机理及其对HBCD的氧化性能；4）揭示ZVA还原与SO4•-氧化的协同机制。本研究可为水中新兴污染物HBCD的去除提供理论和技术参考。

具有极强还原性能的零价铝（Zero-Valent Aluminum，ZVAl）在污染物去除领域具有巨大的潜力。但是，ZVAl表面极易被氧化形成的致密氧化层从而阻碍其还原性能的发挥，这成为该技术发展的限制因素。为此，本项目首先和重点研究了ZVAl氧化层破坏方法与表面腐蚀机制。研究发现，采用碳酸盐缓冲体系能有效且无诱导期地对mZVAl（微米级）表面腐蚀并实现污染物的降解。目前ZVAl在近中性（pH 4 ~ 9）的还原能力尚被怀疑，我们发现nZVAl（纳米级）在广泛pH范围（pH 2~12）包括中性条件下（pH 7）维持低氧环境时，也可以实现污染物的高效还原去除，更加惰性的mZVAl（100 μm）甚至也能在低氧时实现表面活化。在反应中，核壳结构的ZVAl颗粒的铝内核被腐蚀，生成Al（氢）氧化物，其表面的氧化膜并没有被直接去除，而是生成一个更为粗糙的膜。在水介质中，O2、H2O和污染物存在明显的竞争关系，可以捕获ZVAl释放的电子。致密的氧化膜被水化作用破坏。当污染物竞争电子的机会增强，即使在近中性条件下，有效地还原反应仍可发生。机械化学球磨是一种快速简单有效破坏ZVAl表面致密氧化膜的好方法，对比mZVAl体系，采用NaCl辅助机械球磨活化后的mZVAl可以在广泛pH范围（3~11）内提高污染物的还原去除速率，初始pH 7时可提高近300倍。接着，在上述ZVAl表面活化和还原性能挖掘的基础上，探讨了过硫酸盐（PS）存在下nZVAl表面腐蚀机理和电子迁移过程。PS/nZVAl体系无诱导期，二者共存时具有强烈的协同作用，PS促进ZVAl的表面腐蚀从而释放更多的电子，而ZVAl能够有效活化PS分解为强氧化性SO4•-和•OH自由基，从而实现污染物的氧化降解和矿化。最后，研究发现促溶剂/水相体系中的六溴环十二烷（HBCD）可以被ZVAl有效还原降解（可达100%），实现完全脱溴，但需经历较长的诱导期（发生水化作用破坏表面氧化膜）。而该体系中的HBCD并不能被nZVAl/PS氧化降解。ZVAl联合PS的先还原后氧化良好的协同性能值得进一步探索。总之，基于ZVAl的还原或氧化技术正在发展成为一种新兴的水处理技术，本项目研究成果可为其提供重要的理论和技术支持。