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题名: 水环境中天然有机质与碳纳米管对汞和甲基汞的吸附及转化的影响
作者: 张 丹1
学位类别: 博士
答辩日期: 2017-05
授予单位: 中国科学院大学
授予地点: 北京
导师: 刘 景 富
关键词: 汞,甲基汞,天然有机质,分子量,多壁碳纳米管 ; Hg2+, MeHg, Natural organic matter, Molecular weight, Multi-walled carbon nanotubes
其他题名: Effects of natural organic matter and carbon nanotubes on the sorption and transformation of mercury and methylmercury in aqueous environment
学位专业: 环境科学
中文摘要: 汞作为一种全球性污染物,由于它具有长距离迁移性和生物富集性等特点, 受到人们的广泛关注。汞的化合物均有毒性,其中甲基汞是毒性最强的汞化合物。 汞在水环境中的迁移和转化受很多环境因素影响。水环境中普遍存在的天然有机 质(Natural organic matter, NOM)和随着纳米技术的快速发展而进入环境的碳纳 米管等人工纳米材料,因对汞和甲基汞具有很强的吸附作用,可影响水环境中汞 的迁移转化过程和生物效应。 NOM是环境介质的重要组成部分,它具有复杂的组成和丰富的官能团,极 易与环境中的汞和甲基汞发生络合等化学反应,从而影响其在环境中的转化、归 趋和毒性。NOM的分子量跨度极大,不同分子量 NOM的结构、物理化学性质 等存在明显差异性。因而,研究不同分子量 NOM对环境中汞和甲基汞的环境行 为和归趋的影响具有十分重要的环境意义。此外,随着纳米技术的发展,纳米材 料的应用日益广泛。例如,碳纳米管等纳米材料因其良好的吸附性能可被作为吸 附剂用于重金属污染的废水处理中。与 NOM结构相类似,经修饰处理后的碳纳 米管也含有丰富羟基、氨基、羧基等官能团,这些官能团同样能与溶液中的汞和 甲基汞发生络合等反应,从而影响水环境中汞和甲基汞的迁移和转化。 本文主要研究水环境中汞、甲基汞在不同分子量 NOM中的分布和不同官能 团修饰碳纳米管上吸附特征及其不同分子量 NOM对甲基汞光降解的影响,包括 以下几部分内容: 首先,我们以国际腐殖酸协会推荐的苏维尼河 NOM为模型,研究了汞和甲 基汞在不同分子量 NOM中的分布特征,同时考察了含巯基小分子半胱氨酸和 pH 值对其分布的影响。我们采用光还原蒸汽发生,高效液相尺寸排阻色谱与电感耦 合等离子体质谱联用技术,利用同位素示踪手段,研究了汞和甲基汞在不同分子 量 NOM中的分布特征。结果表明,当汞和甲基汞同时分布于 NOM中时,汞与 NOM结合物的分子量普遍高于甲基汞与 NOM结合物的分子量,说明汞倾向于 分布在更高分子量 NOM中,而甲基汞更多的分布在低分子量 NOM中。汞和甲 基汞在 NOM中的分布与 pH有很强的相关性。在不同 pH条件下,在相同 NOM 组分中,Hg2+与 NOM结合物分子量,随 pH值增加而先降低后增加。在不同 pH 条件下,在> 10 kD组分中,甲基汞与 NOM结合物分子量随 pH增加而增加。我 们的研究表明,半胱氨酸可与 NOM竞争结合汞和甲基汞,优先结合形成更稳定 的物质。因此,汞和甲基汞在不同分子量 NOM中的分布受含巯基小分子物质和 pH影响,研究汞和甲基汞在实际环境水体中的分布特征要考虑水体成分及 pH 影响。 其次,利用同位素稀释技术考察了不同分子量 NOM与其他环境相关因素对甲基汞光降解的影响及其机制。结果发现,甲基汞在去离子水中不能被降解,这 就排除了甲基汞直接光降解的途径;NOM或者 Fe3+的单独存在均可诱发甲基汞 光降解;但是,共存的 NOM(2 mg C L-1)可以显著地抑制 Fe3+介导的降解,降 解速率从 0.74±0.01 d-1降到 0.37±0.03 d-1。这些结果表明,NOM在甲基汞光降 解中有复杂的作用。进一步的研究发现,在不同分子量 NOM存在下,甲基汞表 现出不同的光降解速率。高浓度NOM引起的光屏蔽效应可以抑制甲基汞光降解, 特别是对于高分子量 NOM,这种屏蔽效应尤其显著,矫正前后降解速率分别为 0.09±0.007 d-1和 0.29±0.02 d-1。在不同分子量 NOM存在下,甲基汞光降解还 受到光照波长的影响,其中紫外光部分在甲基汞降解中贡献率大于 83%。此外, 在不同分子量 NOM存在下,pH对甲基汞光降解影响比较复杂。pH为 5.0到 7.0 没有明显影响,pH为 8.0时降解速率降低,pH为 9.0时降解速率增加。低浓度 氯离子存在有利于甲基汞降解,高浓度氯离子对降解产生抑制作用。随着硝酸根 离子浓度增加,甲基汞的降解速率先增加后降低,这主要是由于硝酸根离子产生 羟基自由基的促进作用和光屏蔽效应引起的抑制效应。本研究有助于理解表层水 中 NOM和其他环境因素在甲基汞光降解中所起的关键作用。 最后,以不同官能团修饰的碳纳米管为例,考察了纳米材料对水环境中汞和 甲基汞的吸附行为。分别研究了羟基、氨基、羧基修饰的多壁碳纳米管 (MWCNTs)对水中汞和甲基汞的吸附行为,并考察了初始 pH值、NOM、氯 离子、吸附剂投加量对吸附行为的影响。研究发现,在各种 pH下,氨基修饰的 MWCNTs对汞和甲基汞有很强的吸附作用,去除效率达到 92%以上。NOM对吸 附的影响比较复杂,这主要是由于 MWCNTs可以吸附 NOM,同时也可以吸附 汞和甲基汞,MWCNTs与 NOM形成的复合物也吸附汞和甲基汞。氯离子对吸 附有很强的抑制作用,这是因为在溶液中生成了汞和甲基汞的稳定的氯化物。吸 附剂投加量的增加能增加汞和甲基汞在 MWCNTs上的吸附效率。我们使用 Langmuir,Frendlich,Dubinin-Redushkevich和 Temkin四种热力学模型描述汞和 甲基汞在 MWCNTs上的吸附行为。结果表明,汞在 MWCNTs上的吸附属于不 均匀的化学吸附;甲基汞在 MWCNTs上的吸附属于均匀的化学吸附。拟二级动 力学模型能更好地表达汞和甲基汞在 MWCNTs上的吸附过程。我们的研究发现, 不同官能团修饰的 MWCNTs对水中汞和甲基汞有高效的吸附,但吸附效率受初 始 pH值、NOM、氯离子、吸附剂投加量的影响。本研究结果对评估 MWCNTs 进入环境后对汞和甲基汞的环境行为的潜在影响具有参考价值。
英文摘要: Because of its characteristics of long distance migration and bioaccumulation, mercury as a global pollutant has been attracting widespread attention. Although all the mercury compounds are toxic, methylmercury (MeHg) is the most toxic one. The transfer and transformation of mercury in the water environment is affected by many environmental factors. It is well known that natural organic matter (NOM) is prevalent in the water environment, and the engineered nanomaterials such as carbon nanotubes is released into the environment with the rapid development of nanotechnology. As these substances show strong sorption to Hg2+ and MeHg, they can affect the migration, transformation, and biological effects of Hg2+ and MeHg in aqueous environment. As one of the most important component in aquatic system, NOM has diverse chemical compositions and functional groups, which make it readily react with Hg2+ and MeHg, and therefore can greatly affect the transformation, fate and toxicity of Hg in the aquatic environment. Because NOM has a wide range of molecular weight (MW), the structure and physicochemical properties would vary significantly for fractions of different MW range. Therefore, it is of great significance to investigate the effects of NOM with different MW on the environmental behavior and fate of Hg2+ and MeHg in aqueous environment. Besides, nanomaterials (NMs) has been widely used with the development of nanotechnology. For example, carbon nanotubes (CNTs) can be used as adsorbents for toxic heavy metal removal in wastewater treatment due to their good adsorption performance. Like the NOM, the modified CNTs have many hydroxyl groups, amino groups, carboxyl groups and other functional groups. These surface functional groups can lead to the reaction with Hg2+ and MeHg such as complexation. Thus, CNTs can also play an important role in the migration and transformation of Hg2+ and MeHg in water environment. This dissertation focuses on the effects of NOM with different MW on the distribution of Hg2+ and MeHg, and photodegradation of MeHg, as well as on the sorption of Hg2+ and MeHg on CNTs modified by different functional groups. It consists of the following three parts. Firstly, the distribution of Hg2+ and MeHg in Suwannee River NOM with various MW was studied by applying combination techniques of photo-induced chemical vapor generation, high performance size exclusion chromatography-inductively coupled plasma mass spectrometry hyphenated system and isotope tracer, and the impacts of cysteine and pH on this distribution were investigated. The results showed that both Hg2+ and MeHg can combine with NOM, and the MW of Hg2+-NOM conjugates was generally higher than that of MeHg-NOM conjugates, indicating that Hg2+ tended to distribute in NOM with higher MW, while MeHg in NOM with lower MW. There was a high correlation between pH and the distribution of Hg2+ and MeHg. At different pHs, the MW of Hg2+-NOM conjugates in NOM decreased first and then increased with increasing pH. At different pHs, the MW of MeHg-NOM conjugates increased with increasing pH in the > 10 kD fraction. Cysteine can greatly affect the binding of Hg2+ and MeHg with NOM because it can preferentially bind to Hg2+ and MeHg to form more stable substances. Our findings suggest that the distribution of Hg2+ and MeHg in NOM with various MW was influenced by pH and small molecules containing sulfydryl groups. Therefore, the distribution of Hg2+ and MeHg in real environment needs to consider the water constituents and pH. Secondly, we studied the role and mechanism of NOM with different MW and other environmental relevant factors in the photodegradation of MeHg with isotope dilution technique. We found that MeHg could not be photodegraded in deionized water, while could be photodegraded in the presence of NOM or Fe3+, suggesting that MeHg was not subjected to direct photodegradation. When NOM(2 mg C L-1)and Fe3+ co-existed, the photodegradation of MeHg was obviously inhibited compared to in the presence of Fe3+ alone, and the degradation rate reduced from 0.74±0.01 d-1 to 0.37±0.03 d-1. These results illustrated that NOM had dual roles in MeHg photodegradation. Further studies observed that the photodegradation rates of MeHg varied with NOM of different MW. High concentration of NOM could retard the photodegradation of MeHg due to light attenuation effect. This was more significant for NOM with higher MW, as the degradation rate before and after correction was 0.09 ± 0.007 d-1 and 0.29 ± 0.02 d-1, respectively. Moreover, sunlight wavelength could affect MeHg photodegradation in the presence of NOM with different MW, and the contribution of ultraviolet light was greater than 83%. Beyond that, pH had complicated influences on the MeHg photodegradation induced by NOM of various MW, as the photodegradation rate was unaffected at pH 5.0 and pH 7.0, while decreased and increased at pH 8.0 and pH 9.0, respectively. The presence of low concentration of chloride ions promoted MeHg photodegradation, while high concentration inhibited. The photodegradation rate of MeHg in the presence of nitrate ion increased and then decreased with the increase of nitrate concentration, this was mainly related to the facilitation effect of production of hydroxyl radical and the inhibitory effect of light shielding caused by nitrate ion. This study is helpful for better understanding the critical roles of NOM and other environmental factors in MeHg photodegradation in surface water. Finally, multi-walled carbon nanotubes (MWCNTs) were modified with different hydroxyl, amine and carboxyl groups, and then was used to study on the sorption behavior of Hg2+ and MeHg to NMs in aqueous environment. It was found that amine-modifed MWCNTs had a strong adsoprtion capacity to Hg2+ and MeHg at various pH, and the removal efficiency reached up to 92%. NOM had complex effects on the adsorption of Hg2+ and MeHg to MWCNTs, and this was mainly because that MWCNTs could adsorb NOM, and the formed MWCNTs-NOM complexes were also able to adsorb Hg2+ and MeHg. Chloride ions inhibited the adsorption of Hg2+ and MeHg to MWCNTs due to the reaction between Chloride ions and Hg2+ or MeHg, which would result in the formation of stable chlorides. The adsorption efficiency of Hg2+ and MeHg increased with the increase of the content of adsorbent. Langmuir, Freundlich, Dubinin-Radushkevich and Temkin thermodynamic models were used to describe the adsorption behavior of Hg2+ and MeHg on MWCNTs. The results showed that the adsorption of Hg2+ on MWCNTs was inhomogeneous chemisorption, and MeHg on MWCNTs was homogeneous chemisorption. The pseudo-second-order kinetic model could better illustrate the adsorption process of Hg2+ and MeHg on MWCNTs. Our study suggests that the modified MWCNTs with different functional groups can efficiently adsorb Hg2+ and MeHg in aqueous environment, and the sorption efficiency is influenced by the initial pH, NOM, the content of chloride ions and adsorbent. This study can be referred to the assessment of the potential impact of MWCNTs on the environmental behavior of Hg2+ and MeHg.
内容类型: 学位论文
URI标识: http://ir.rcees.ac.cn/handle/311016/38742
Appears in Collections:环境化学与生态毒理学国家重点实验室_学位论文

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作者单位: 1.中国科学院生态环境研究中心

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张 丹. 水环境中天然有机质与碳纳米管对汞和甲基汞的吸附及转化的影响[D]. 北京. 中国科学院大学. 2017.
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