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Palladium/magnesium bimetallic systems for dechlorination of polychlorinated biphenyls.

机译:钯/镁双金属系统,用于多氯联苯的脱氯。

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摘要

Polychlorinated biphenyls (PCBs), a family of 209 congeners, are recalcitrant environment pollutants and suspected carcinogens prevalent in nation's surface waters, sediments and superfund sites. The threat of PCBs, current state and critical aspects of in-situ reclamation technologies that warrant research focus has been discussed. Toward this end, an effective system has been devised for PCB dechlorination by modifying magnesium (Mg) through the deposition of very small amounts of palladium (Pd). Every Pd/Mg bimetallic particle functioned as numerous nanoscale batteries generating electrons that eventually led to PCB dechlorination. Through our study, we demonstrate Mg to be a promising substrate in Pd-based bimetallic systems for PCB dechlorination. It is the first instance of relating the unique corrosion properties of Mg to distinctive advantages it offers in synthesis, storage and application of bimetallic particles in environmental systems, aspects which have been onerous in Fe-based systems. Also, Mg at a high oxidation potential of 2.37V provides a greater thermodynamic force for dechlorination. These advantages of Mg, coupled with its natural abundance, low density and cost and environmentally friendly nature further strengthen its candidature. The Pd/Mg systems were enhanced using novel techniques of nano-synthesis to obtain tailor-made bimetallic particles with small Pd islands and maximized reduction potential at drastically reduced Pd requirements.;A better understanding of the unexplored Pd/Mg systems was sought by studying their behavior in presence of naturally abundant anions such as sulfate, chloride, nitrate, hydroxide and carbonate and organic solvents that may accompany PCB contamination. While there have been studies on some of these parameters influencing corrosion of pure Mg, literature on Mg galvanically coupled with Pd remains limited. Insights on effect of initial PCB concentration and system pH on dechlorination rates were also provided.;For PCBs undergoing catalytic hydrodechlorination (HDCl), conflicting accounts occur on the reactivity of substituted chlorines and the ensuing dechlorination pathways. In order to understand these relationships, intermediates and dechlorination pathways of carefully selected 17 congeners were investigated. The preferential site of electrophilic attack and its mechanistic aspects were interpreted in terms of steric, inductive and resonance stabilization. The trends for electrophilic substitution were consistently p- > m- > o-positions indicating that more toxic 'coplanar' PCBs were preferentially reduced. Dechlorination rates and pathways were influenced both by inductive effect of Cl that governs the stability of the intermediates and by steric effects primarily effecting the adsorption step. In the absence of steric effects, dechlorination occurred preferentially from the less substituted phenyl ring.;Finally, we evaluated the ability of Pd/Mg to dechlorinate (i) PCBs spiked in clean clays and sediments and (ii) historically contaminated aged sediments (Waukegan Harbor, WHS). While (i) Pd/Mg completely dechlorinated multi-component commercial PCB mixtures, and (ii) active components in extraction media did not inhibit dechlorination, sulfide in WHS poisoned Pd, compromising Pd/Mg reactivity. We discuss major factors limiting Pd/Mg performance in sediments, proposing possible solutions. Recommendations to improve Mg based systems were made focusing on their nanotechnological aspects including synthesis techniques, challenges in nanoscaling Mg and potential applications in broader environmental spheres.
机译:多氯联苯(PCB)是一个由209个同源物组成的家族,是顽固的环境污染物和可疑的致癌物,普遍存在于该国的地表水,沉积物和超级基金所在地。已经讨论了多氯联苯的威胁,当前状态和需要重点研究的现场回收技术的关键方面。为此,已经设计了一种有效的系统,用于通过沉积极少量的钯(Pd)来修饰镁(Mg)来进行PCB脱氯。每个Pd / Mg双金属颗粒都充当大量纳米级电池,产生电子,最终导致PCB脱氯。通过我们的研究,我们证明了镁是用于PCB脱氯的基于Pd的双金属系统中有希望的底物。这是将Mg的独特腐蚀性能与其在环境系统中双金属颗粒的合成,存储和应用中提供的独特优势联系起来的第一个实例,而在基于Fe的系统中,这是很繁重的。同样,Mg在2.37V的高氧化电位下为脱氯提供了更大的热力学力。镁的这些优点,加上其天然的丰富度,低密度和低成本以及对环境友好的性质,进一步增强了其候选资格。使用新型的纳米合成技术增强了Pd / Mg体系,以获得具有小Pd岛的定制双金属颗粒,并在大幅降低Pd需求量的情况下最大程度地降低了还原潜力;通过研究寻求对未探索的Pd / Mg体系有更好的了解它们在自然丰富的阴离子(例如硫酸根,氯离子,硝酸根,氢氧根和碳酸根)和有机溶剂(可能伴随PCB污染)存在下的行为。虽然已经研究了一些影响纯Mg腐蚀的参数,但有关与Pd电耦合的Mg的文献仍然有限。还提供了有关初始PCB浓度和系统pH值对脱氯速率的影响的见解。对于经历催化加氢脱氯(HDCl)的PCB,在取代氯的反应性和随之而来的脱氯途径上发生了矛盾的解释。为了理解这些关系,对精心挑选的17个同类物的中间体和脱氯途径进行了研究。亲电攻击的优先位及其机制方面的解释空间,感应和共振稳定。亲电取代的趋势始终是p-> m-> o-位,表明优先降低了毒性更大的“共面” PCB。脱氯速率和途径都受控制中间体稳定性的Cl的感应作用和主要影响吸附步骤的空间作用的影响。在没有空间效应的情况下,脱氯优先发生在取代度较低的苯环上。最后,我们评估了Pd / Mg脱氯的能力(i)加标在干净的粘土和沉积物中的PCBs和(ii)历史上受污染的老化沉积物(Waukegan WHS海港)。 (i)Pd / Mg将多组分市售PCB混合物完全脱氯,以及(ii)萃取介质中的活性成分不能抑制脱氯,而WHS中的硫化物会毒化Pd,从而损害Pd / Mg的反应性。我们讨论了限制沉积物中Pd / Mg性能的主要因素,并提出了可能的解决方案。提出了改进基于镁的系统的建议,重点是其纳米技术方面,包括合成技术,纳米氧化镁的挑战以及在更广泛的环境领域中的潜在应用。

著录项

  • 作者

    Agarwal, Shirish.;

  • 作者单位

    University of Cincinnati.;

  • 授予单位 University of Cincinnati.;
  • 学科 Engineering Environmental.
  • 学位 Ph.D.
  • 年度 2009
  • 页码 245 p.
  • 总页数 245
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类 环境污染及其防治;
  • 关键词

  • 入库时间 2022-08-17 11:37:37

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