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题名: 水环境中天然有机质异质性及其对纳米材料 转化与毒性的影响
作者: 沈墨海
学位类别: 博士
答辩日期: 2015-05
授予单位: 中国科学院研究生院
授予地点: 北京
导师: 刘景富
关键词: 天然有机质,分子量分级,纳米材料,物理化学转化,毒性,Natural organic matter, Molecular weight fractions, Nano material, Physicochemical transformation, Toxicity
其他题名: Heterogeneity of natural organic matter and its impacts on the transformation and toxicity of nanomaterials in aqueous environment
学位专业: 环境科学
中文摘要:      随着人们对纳米材料(Nanomaterials, NMs)的特性认识愈加广泛深入,越来越多的NMs被应用到工业和日常消费品中,其中碳纳米材料(Carbon nanomaterials, CNMs)和纳米银(AgNPs)是使用最多的几种NMs。随着使用量和种类逐年增长,它们会不可避免地释放到环境当中,成为潜在的污染物。因此,NMs在水环境中迁移转化、归趋及毒性,已经成为近十年来环境健康和安全领域重要的研究课题。
    NMs的环境行为和效应除了受颗粒本身性质影响之外,还受多种环境因素的影响。其中,天然有机质(Natural organic matter, NOM)是水环境介质的重要组成部分。由于具有复杂的化学组成和丰富的官能团,而且其典型浓度范围通常比NMs浓度范围高若干个数量级,NOM极易与释放到水体中的NMs发生反应,从而改变NMs的性质和迁移转化等环境行为。但是,由于NOM是一类由分子量(Molecular weight, MW)跨度极大的聚合物构成复杂体系,其官能团构成、表面电荷、物理结构、化学组成和光学性质等随MW变化极大。因而,研究NOM异质性对NMs的环境行为和归趋的影响,对评估其生物有效性和毒性并评价其环境风险具有十分重要的意义。此外,环境中共存的不同价态金属阳离子也可能对NOM和NMs的物理化学性质,进而对NMs在水环境中的迁移转化行为产生影响。本文针对水环境NOM分子量异质性及其对纳米富勒烯(nC60)和AgNPs的物理化学转化及毒性的影响展开了一系列研究,主要包括以下几个部分:
    首先,我们利用超滤离心法将未分级的萨旺尼河NOM(Pristine-SRNOM)按MW分级,得到<3 kD、3-10 kD、10-30 kD、30-100 kD和>100 kD五个组分(Mf-SRNOMs),发现其官能团、生色团丰度和光谱性质等随MW变化显著;采用0.125 - 4.0 mmol L-1 Ca(ClO4)2和0.25 - 8.0 mmol L-1 NaClO4对Pristine-和Mf-SRNOMs进行处理,并考察Ca2+/Na+离子存在下,Pristine-和Mf-SRNOMs的光谱性质的变化。结果发现,Ca2+/Na+离子对Pristine-和Mf-SRNOMs的紫外光区吸光度、荧光量子产率等有不同程度的影响。其中,高浓度Ca2+对>100 kD和30-100 kD等高MW组分的影响最大,具体表现为:促使这些MW组分的紫外光区吸光度显著降低、荧光量子产率明显提高。本部分研究说明,NOM的物理化学性质不仅随MW变化而显著改变,而且受环境常见金属阳离子的影响,且不同MW组分受影响程度变化显著。
    然后,我们研究了4 mg C L-1的Pristine-和Mf-SRNOMs在光照条件下产生活性氧自由基(Reactive oxygen species, ROS)的过程,并系统考察了Ca2+、Na+存在下,对Mf-SRNOMs生成ROS过程的影响。结果显示,光照条件下Pristine- 和Mf-SRNOMs生成的单线态氧(1O2)、羟基自由基(.OH)的稳态浓度([1O2]SS和[.OH]SS)与随MW增大而升高的吸光度和芳香性有良好的正相关。其中,这主要是由于高MW组分具有更高的吸光度和芳香烃含量。然而,Pristine-和Mf-SRNOMs(除<3 kD组分之外)的1O2量子产率(Φ1O2)基本随MW降低而升高、与芳香性呈负相关,说明低MW组分具有更高的光敏性和生成1O2的能力。Ca2+和Na+(阴离子ClO4-在光照条件下稳定)对Mf-SRNOMs中[1O2]SS和[.OH]SS的影响表现为(1)对各组Mf-SRNOM中的[1O2]SS和[.OH]SS都具有削弱作用但程度不同;(2)相较于Na+,Ca2+对同一Mf-SRNOM中的[1O2]SS和[.OH]SS具有更强的削弱作用;(3)对高MW组分Mf-SRNOMs中的[1O2]SS和[.OH]SS削弱程度更大。我们的研究表明,地表水NOM中ROS的生成与依赖于其MW变化的光谱性质显著相关,而且二价或多价金属阳离子会显著降低高MW组分NOM中ROS稳态浓度。因此,在研究由ROS介导的NOM光降解环境污染物的过程时,需要考虑环境介质相关要素的影响。
     在考察了Pristine-SRNOM基于MW差异的光谱性质和光化学过程的异质性之后,我们进一步研究了Pristine-和Mf-SRNOMs对典型纳米材料nC60在一价(Na+)和二价(Ca2+和Mg2+)电解质中团聚行为的影响;分析了在不同种类和浓度的电解质存在下, nC60的初始团聚动速率(k)、附着系数(a)和电解质临界絮凝浓度(CCC)与Mf-SRNOM的MW、结构和官能团等特性的相关性。研究发现无论在一价或二价的低浓度电解质溶液中,随着Mf-SRNOMs分子量的增大,nC60分散稳定性显著提高。在高浓度电解质溶液中,Mf-SRNOMs对nC60团聚行为的影响随着电解质价态变化而显示出差异性,即在高浓度Na+溶液中,nC60的稳定性与Mf-SRNOMs的MW仍呈现正相关;但在高浓度Ca2+和Mg2+存在下,大多数Mf-SRNOMs可增强nC60的团聚;其中>100 kD和30-100 kD等高MW组分对nC60团聚的增强作用尤为显著。各部分结果综合显示,高分子量的Mf-SRNOMs对nC60在一价和二价电解质中的团聚行为影响显著不同。本研究表明:Mf-SRNOMs随MW升高而增大的空间位阻效应是抑制nC60在电解质溶液中团聚的首要因素。但是Mf-SRNOMs与二价金属阳离子之间存在络合及桥连作用且随MW升高而增强。因而,MW越高的Mf-SRNOM组分在高浓度二价电解质存在下,与, nC60产生的共团聚现象越明显。
    本文最后一部分深入研究了AgNPs在水环境中物理化学转化的高度动态性可能对大型蚤(Daphnia magna)急性毒性评价带来的影响。尽管已有研究表明AgNPs对D. magna等水生生物具有急性毒性,但不同形态银在AgNPs急性毒性中的贡献仍不明确,光照下NOM还原Ag+生成AgNPs的过程对D. magna急性毒性的影响尚待深入研究。本研究发现,AgNPs在传统急性毒性暴露基质(OECD、EPA推荐基质)中会发生不可控、持续性的物理化学转化,导致实验测得的AgNPs对D. magna的急性毒性(半数致死浓度,LC50)偏离真实值,无法识别不同形态银对AgNPs急性毒性的贡献。为此,我们发展了一种能够降低AgNPs在大型蚤急性毒性暴露过程中发生物理和化学转化的新基质:, 0.1 mmol L-1 NaNO3(pH 7.8 ± 0.2 由NaOH调节)体系,对D. magna进行8-h暴露。实验选用7种具有不同表面包裹剂和粒径的AgNPs对D. magna急性毒性进行考察,计算了8-h LC50表观总银浓度并测定了8-h LC50条件下的不同银形态浓度。结果表明,自由溶解态银离子是AgNPs中对D. magna产生急性毒性的最本质银形态;粒径、包裹剂等仅是影响AgNPs的急性毒性的表观因素,它们最终通过影响自由溶解态银离子的浓度而影响AgNPs的毒性。利用新型暴露基质进一步评价了光照条件下NOM还原Ag+生成AgNPs过程对D. magna的急性毒性变化,发现随着AgNPs的生成,总银含量表示的对D. magna的急性毒性逐渐降低,但总银离子(其中自由溶解态银离子含量>93%)对D. magna的LC50值稳定不变,说明NOM参与的Ag+-AgNPs这一环境动态体系对D. magna的急性毒性来自自由溶解态银离子。
英文摘要:     Due to their unique physicochemical properties, nanomaterials (NMs), such as fullerenes (nC60) and silver nanoparticles (AgNPs), are widely used in industry and consumer goods. With the increase of usage and production, NMs are inevitably released into the environment and become potential contaminants. Therefore, extensive and growing interests in the field of environmental health and safety have been focused on the transport, transform, fate and toxicity of NMs.
    Besides their inherent particle characteristics, many environmental factors have impacts on the environmental behaviors and effects of NMs. Natural organic matter (NOM) is one of the most important environmental media in aquatic system. With diverse chemical composition and functional groups, and typical concentration range of several magnitudes higher than concentrations of NMs released into the environment, NOM always interacts readily with various NMs and therefore has significantly influences on their transport, transformation, fate and toxicity in the aquatic environment. However, as NOM is comprised of heterogeneous polymers with a wide range of molecular weight (MW), the physical structure and chemical components, functional groups, surface charges would vary significantly for fractions of different MW range. Besides, the matel ions with different valents would change the physicochemical properties of NOM and NMs significantly. Thus, it is important to understand the combined influences of these environmentally relevant factors on the transport and transformation of NMs in aqueous environment. This dissertation focuses on the MW-dependent heterogeneity of NOM and their potential impacts on the physicochemical transformation of the widely used nC60 and AgNPs. The following parts are included in the dissertation:
    First, using ultrafiltration a MW based fractionation was performed to separate pristine Suwannee River NOM (Pristine-SRNOM) into fractions of >100 kD, 30-100 kD, 10-30 kD, 3-10 kD and <3 kD (collectively referred as Mf-SRNOMs). It was observed that functional groups, chromophores and optical properties were significantly changed with the variation of MW. The presence of cations (Ca2+ and Na+) could reduce the UV-vis absorbance and elevate the fluorescence of Mf-SRNOMs, among which high concentrations of Ca2+ had the most significant influences on the >100 kD and 30-100 kD fractions. Our findings suggest that the spectroscopic properties not only correlated with MW of NOM samples, but also significantly influenced by the divalent cations.
    On the basis of the knowledge of the MW-depented optical properties of Mf-SRNOMs and their variations in the presence of metal ions, we futher studied the photogeneration of reactive oxygen species (ROS) by Pristine- and Mf-SRNOMs under simulated sunlight irradiation, and systematically studied the impacts of the presence of metal ions (Ca2+ and Na+). Results showed that steady state concentrations of singlet oxygen ([1O2]SS) and hydroxyl radicals ([.OH]SS) of Pristine- and Mf-SRNOMs strongly correlated with the MW-dependent spectroscopic properties. The greater absorbance, more abundant aromatic chromophore in higher MW of Mf-SRNOM contributed to higher [1O2]SS and [.OH]SS. Whereas, the quantum yield of 1O2 by Mf-SRNOMs generally increased with the MW decreased (excepting <3 kD fraction), suggesting the photosensitizing ability of Mf-SRNOMs were generally higher in lower fractions. We further studied the impacts of metal ions (Ca2+ and Na+ as perchlorate) at freshwater level on ROS photogenaration from Pristine- and Mf-SRNOMs, and found that (1) Ca2+ and Na+ could weaken the [1O2]SS and [.OH]SS in Mf-SRNOM solutions to different extents; (2) Ca2+ weakened the [1O2]SS and [.OH]SS more effectively than Na+ in any Mf-SRNOM; (3) greater reductions of [1O2]SS and [.OH]SS occurred in higher Mf-SRNOM. Our findings suggest that the photogeneration of ROS correlated with MW-dependent spectroscopic properties of NOM samples, and the relatively stronger reductions of steady state concentration of ROS in higher MW NOM in the presence of Ca2+ should be considered when determining the fate of inorganic/organic contaminants in photo-mediated processes.
     After systematically studying the MW-depented physicochemical heterogeneity of Mf-SRNOMs, we futher studied the impacts of Mf-SRNOMs on the aggregation of nC60 in mono- and di-valent electrolyte solutions. We found that, in low concentration treatments of either NaCl or CaCl2/MgCl2, nC60 was stabilized by Mf-SRNOMs and the stabilization efficiency of Mf-SRNOMs was positively correlated with the MW. Whereas, in high concentration treatments of the three electrolyte, impacts of Mf-SRNOMs on aggregation of nC60 varied with the valent of electrolytes. In high concentration treatments of Na+, the stability of nC60 was still positively correlated with the MW of Mf-SRNOMs. However, due to the preferential complex and cation-bridging between Ca2+/Mg2+ and carboxyl/hydroxyl functional groups in NOM, most Mf-SRNOMs engaged in bridging nC60 aggregates, especially the high MW Mf-SRNOM (>100 kD and 30-100 kD). Our results indicate that the MW-dependant steric stabilization effects provided by Mf-SRNOMs was the primary mechanism for stabilizing nC60 in monovalent electrolyte. Whereas, the high MW Mf-SRNOMs could play a reversed role in promoting the attachment of nC60, especially in long term aggregations and at high concentrations of divalent cations, due to the preferential complex and cation-bridging between Ca2+/Mg2+ and high MW Mf-SRNOMs resulting in the formation of more condensed aggregates and engaging in bridging nC60 aggregates.
    In the last part, we focused on the effect of highly dynamics of AgNPs in the aqueous environment on the evaluation of their acute toxicity on the Daphnia magna. It was well-accepted that AgNPs is acute toxic towards aquatic organisms like D. magna. However, the contribution of different Ag species to the acute toxicity of AgNP suspension is still not well explained. Moreover, the acute toxicity of AgNPs in a typical process of environmental importance, i.e. the sunlight-induced reduction of Ag+ into AgNPs by NOM, is urgently needed to study. In the present study, we found that in the traditional acute toxicity exposure media recommended by Economic Co-operation and Development (OECD) and Environmental Protection Agency (EPA), the various ions in media could result in the consistent and uncontrollable physicochemical transformations of AgNPs, and challenge the accurate speciation of AgNP/Ag+ and evaluation on their acute toxicity towards D. magna. To overcome these problems, we developed a new medium for studying the acute toxicity of AgNPs to D. magna, i.e. the 0.1 mmol L-1 NaNO3 medium, and further quantified the median lethal concentrations (LC50) towards D. magna as various Ag speciation. The results showed that free Ag+ was the intrinsic and ultimate factor that governs the acute toxicity of AgNPs, while size and surface coating are apparent factors that influence the toxicity through affecting the free Ag+ concentration of AgNPs. The developed medium was further applied in predicting the toxicity changes of the sunlight-induced reduction of Ag+ into AgNPs by Pristine-SRNOM. The results showed that the acute toxicity of apparent total Ag concentration towards D. magna decreased as the transformation of Ag+ into AgNPs. However, the LC50 values presented as total Ag+ concentration, in which >93% was free Ag+, were consistent. Our study demonstrated that the acute toxicity of dynamic system of AgNPs towards D. magna originated from free Ag+.
内容类型: 学位论文
URI标识: http://ir.rcees.ac.cn/handle/311016/34366
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Recommended Citation:
沈墨海. 水环境中天然有机质异质性及其对纳米材料 转化与毒性的影响[D]. 北京. 中国科学院研究生院. 2015.
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