黑碳纳米颗粒在天然水体中的凝聚动力学机制及模型构建

Black carbon nanoparticles (nBC) produced from incomplete combustion eventually enter natural water via atmospheric deposition and travel along with surface runoff, possessing severe adverse impacts on aquatic ecosystems and the global carbon cycle. The aggregation processes of nBC in aquatic environments dictate their particle size and environmental fate and transport. Although the aggregation behavior of BC under atmospheric conditions has received great attention, the aggregation kinetics of BC in water bodies remained poorly characterized. Therefore, this project is designed to investigate the aggregation kinetics of diesel exhaust nBC in aqueous phase under the influence of three key environmental factors (i.e., UV-irradiation, natural macromolecules, and natural colloids). Using time-resolved dynamic light scattering and laser diffraction techniques as well as model simulations, the individual and collective effects of these three factors on the colloidal stability and aggregation kinetics of nBC in various solution chemistries will be examined. The homoaggregation kinetics of aged nBC under UV-irradiation will be studied, in the presence or absence of natural macromolecules. The heteroaggregation kinetics between nBC and natural colloids and also the collective effects of macromolecules on heteroaggregation will be investigated. We will construct kinetic models for describing the homo- and hetero-aggregation rates of nBC in aquatic environments. This study will quantitatively elucidate the microscopic mechanisms governing the aggregation kinetics of nBC based on their inter-particle interactions in natural water, thereby providing a fundamental understanding on the environmental behavior of BC in natural aquatic systems.

不完全燃烧产生大量的黑碳纳米颗粒，沉降到天然水体并随之迁移，严重危害水体生态与环境，并影响全球碳循环。黑碳纳米颗粒在水体中的凝聚过程决定其尺寸、迁移规律与环境效应。虽然空气中的黑碳凝聚行为已引起广泛关注，但其在水体中的凝聚行为及其动力学机制尚未引起足够重视。本项目拟以柴油机尾气中的黑碳纳米颗粒为对象，采用时间分辨动态光散射法、激光衍射法及模型拟合等手段，系统研究在光照老化、天然高分子物质及天然胶体这三个关键环境因子的单独及复合作用下，黑碳纳米颗粒在系列背景电解质中的凝聚速率和胶体稳定性；揭示经光照老化处理的黑碳纳米颗粒间的同型凝聚动力学，及其与天然高分子的定量关系，阐明黑碳纳米颗粒与天然胶体间的异型凝聚动力学及天然高分子的复合影响，构建描述水中黑碳的同型及异型凝聚速率的动力学模型。从而，定量阐明天然水体中黑碳颗粒间作用的凝聚动力学微观机制，为全面理解黑碳在天然水体中的环境行为提供科学依据。

不完全燃烧产生大量黑碳纳米颗粒（SNPs），进入水体威胁生态环境并影响碳循环。本项目以柴油机尾气SNPs为研究对象，采用时间分辨-动态光散射法，从天然高分子、光照老化与其它颗粒三个方面，研究其对SNPs凝聚动力学影响机制并构建预测模型。研究内容1开展4种天然高分子（富里酸FA、胡敏酸HA、海藻酸钠ALG、牛血清蛋白BSA）对SNPs凝聚行为的影响作用。结果表明增强盐浓度抑制静电斥力而引起SNPs凝聚，CaCl¬2比NaCl具有更强电荷中和能力，符合Derjaguin-Landau-Verwey-Overbeek（DLVO）理论。SNPs凝聚受大分子稳定作用排序为BSA>HA>ALG>FA。大分子通过吸附到SNPs表面产生位阻效应是增强胶体稳定性的主要机制。BSA吸附在SNPs表面具有更紧凑的球形结构而引起长距离位阻斥力，使SNPs凝聚速率受到10-100倍抑制。高CaCl2浓度下，ALG通过钙离子桥联作用使SNPs凝聚受到~10倍促进。研究内容2开展紫外光老化对SNPs在水中凝聚行为影响研究，结果表明受光照1天后SNPs比新鲜SNPs在NaCl和CaCl2溶液中更易凝聚，临界聚沉浓度（CCC）分别下降72%和40%。SNPs光照大于3天后未展现显著凝聚速率差异。光照增强SNPs凝聚作用在低盐浓度以及pH>4更显著。大分子存在时，光照SNPs因位阻效应被抑制凝聚的排序为BSA>HA>ALG>FA，而ALG通过钙离子桥联作用使光照老化SNPs失稳。拟合的Hameker常数从新鲜SNPs的7.8 × 10-20 J提升到老化SNPs的1.2 × 10-19 J，揭示光照过程的去羧基化会弱化静电斥力和增强范德华引力，从而促进凝聚。研究内容3开展SNPs与纳米塑料（PSNPs）的异型凝聚动力学研究，结果表明异型凝聚过程符合DLVO理论，CCCPSNPs＞CCCPSNPs-SNPs＞CCCSNPs；在NaCl溶液不同颗粒浓度比下，两种颗粒受到强静电斥力而异型凝聚速率相似；在CaCl2中随SNPs浓度增加，CCCPSNPs-SNPs受桥连作用显著减小。酸性条件下颗粒表面负电荷减少而利于PSNPs-SNPs异型凝聚。SNPs浓度升高会促进PSNPs凝聚，因而可能在海水中发生异型凝聚。以上研究成果可为理解水环境中SNPs凝聚动力学机制提供依据，为预测其迁移、归趋与暴露风险提供模型。