Development and application of a passive sampling device for synchronously determining freely dissolved concentrations profiles of inorganic and organic contaminants across the sediment-water interface

水体沉积物是水生态系统中物质循环和能量流动的重要区域。随着生活污 水、工业废水、农田径流等点面源污染输入，以及大气干湿沉降进入水环境中 的污染物，除小部分溶解在水体外，大部分吸附在悬浮颗粒物上，在重力沉降 等作用下最终储存在沉积物中，使沉积物成为各种化学品的归宿地和蓄积库。 当水体外源输入受到控制时，沉积物中的污染物又会通过再悬浮、分子扩散等 作用扩散进入上覆水中，从而对水中生物造成危害。在这些释放途径中，只有 分子扩散涉及到自由溶解态物质，它是两相中化学品逸度最直接的体现。研究 自由溶解态污染物在水体沉积物-水界面的迁移与扩散通量，对揭示多因子耦合 作用下的污染物迁移规律，以及评估污染物对沉积物、上覆水的潜在风险，具 有重要的理论和现实意义。 当前，关于有机污染物在沉积物-水界面的扩散通量研究，还存在主动采样 样品损耗、垂向样品分辨率不高以及计算结果误差较大等方法学上的问题，本 研究基于被动采样原理，分别以渗析小瓶和低密度聚乙烯（LDPE）膜为吸附相， 研制了同步测定沉积物-水界面无机/有机污染物的被动采样装置，开展关键参 数测定和野外原位应用，并验证被动采样装置测定沉积物孔隙水中自由溶解态 污染物浓度的可行性。主要研究结论如下： （1）设计并制作了含有无机/有机被动采样单元的沉积物-水界面被动采样 装置，开展了应用参数的测定 基于被动采样原理，设计并制作了由支撑框架、无机/有机污染物被动采样 单元组成的沉积物-水界面被动采样装置。室内静态模拟下磷酸盐和重金属等无 机污染物在 68 h能完全达到平衡， PAHs及 SPAHs类疏水性有机物约在 30 d 能达到平衡。基于动力学吸附测定的 16种 PAHs和 12种不同类别取代多环芳 烃（SPAHs）在 LDPE相与水相之间的分配系数（KLDPE）。验证了使用的 LDPE 膜满足 PAHs的“零汇”（Zero sink）假设条件。 （2）探究了典型浅水湖泊太湖沉积物 PAHs沉积过程，明确了流域发展 会显著影响沉积物 PAHs负荷 考察了太湖北部湖湾 16种优控 PAHs时空分布特征，并测定柱状沉积物上 覆水和孔隙水中自由溶解态 PAHs浓度，评估其毒性风险。北部湖湾柱状沉积 物中 PAHs含量、沉积通量以及 BaP毒性当量表现出一致性的变化趋势，从 20 世纪 80年代迅速增加并在 2000年以后稳步下降。流域社会经济发展会显著影 响水体沉积物中 PAHs负荷，近十几年来 PAHs沉积通量的下降也显示太湖流 域控制沉积物 PAHs取得了初步成效。北部湖湾沉积物是自由溶解态 PAHs的 “源”，但上覆水和孔隙水中 PAHs没有或仅有低的生态风险。 （3）开展了被动采样装置在典型浅水湖泊巢湖的原位应用，同步获取了 高分辨率沉积物-水界面无机（1 cm）/有机（2 cm）污染物自由溶解态浓度剖 面，识别了沉积物的“源/汇”作用 被动采样装置的吸附相（LDPE）上 PAHs和 SPAHs的吸附量随着萃取时 间的增大而增大，并呈现良好的线性关系（R2 > 0.90，p < 0.05），实验得到的 PAHs和 SPAHs采样速率常数（R）范围为 0.000030（NaP）~0.000409（AQ） cm3 s-1。在典型浅水湖泊巢湖开展原位应用，获取了高分辨率的无机/有机污染 物自由溶解态浓度剖面，并计算了界面扩散通量，低环类 PAHs及 SPAHs具有 较高扩散通量，表现为沉积物孔隙水向上覆水释放，呈现出“源”的作用，但 沉积物对高环类目物却呈现出“汇”的作用。沉积物对部分有机污染物（如 3,6- 二甲基菲（3,6-DMP）、1-硝基芘（1-NP）等）会进行“源-汇”作用的切换。 研制的沉积物-水被动采样装置能获取目标物在沉积物 -水体界面两侧高分辨的 无机（1 cm）/有机（2 cm）污染物的浓度分布，为研究沉积物-水界面污染物 的迁移转化行为提供了良好的技术手段。Sediments are important for materials cycling and energy flow in aquatic ecosystems. Pollutants were discharged into aquatic environment from several ways, such as the discharge of sewage or industrial wastewater, input of runoff, wet and/or dry deposition. Only a small part of the pollutants were in dissolved fractions, and most of them were bounded on the suspended particles and eventually sink into aquatic sediments. However, when the exogenous input sources of pollutants are controlled, the pollutants in the sediments would be diffused into overlying water through resuspension and molecular diffusion, which can cause harm to the aquatic organisms. Among these ways, only molecular diffusion mainly involves the freely dissolved fractions, which is often recognized as a direct reflection of the chemical fugacity difference for two specific phases. Therefore, the direction and magnitude of the diffusive fluxes of pollutants across the sediment-water interface are helpful to explore the fate of pollutants in the geochemical cycle and provide better information for predicting the natural recovery time of sediment pollution and environmental management. In this thesis, based on the passive sampling theory, a novel passive sampling device was designed to synchronously measure free dissolved profiles of inorganic and organic pollutants across the sediment-water interface. Several parameters were investigated in the lab and this passive sampler was carried out to get an application on situ in the shallow lake. The main conclusions were as follows: (1) A novel passive sampling device for synchronously measuring inorganic and organic pollutants profiles across the sediment-water interface was designed and several parameters were investigated. The adsorption kinetics of the passive sampling showed that the heavy metals and phosphates in the passive sampling units could get an equilibrium status within 68 hours, polycyclic aromatic hydrocarbons (PAHs) and substituted polycyclic aromatic hydrocarbons (SPAHs) would need about 30 days to get this status. The coefficients of partitioning between low-density polyethylene (LDPE) film (50 μm thickness) and water (KLDPE) for 16 priority PAHs were determined, and the values of KLDPE for 12 SPAHs were also first reported in this study. Besides, the selected sorbent (LDPE) was proven to meet the hypothesis of ―zero sink‖ for 16 priority PAHs. (2) Sedimentary records of 16 priority PAHs in sediment cores collected from the northern bays of Taihu Lake were analyzed to evaluate the influence of anthropogenic impacts on the sedimentary PAHs. Here, a recirculated device used for rapidly extracting organic contaminants in the sediments was introduced in the present study. Organic pollutants in the sediment can be extracted quickly and efficiently in a gaseous form, which was suitable for batch processing for sediment samples. High levels and ecological risk of PAHs were distributed in the northern bays, especially in Zhushan Bay. The concentrations, accumulation flux and total toxic benzo[a]pyrene (BaP) equivalent (TEQcarc) of PAHs in sediment cores from the northern bays began to dramatically increase in the 1980s, and subsequently decreased after the 2000s. The decrease of PAHs flux may be attributed to the switch from coal to cleaner energy and good effectiveness of taking mandatory measures to combat water pollution, indicating that anthropogenic activities remarkably affected the load of sedimentary PAHs in the Lake. The freely dissolved PAHs were present in higher levels in porewater than in overlying water; such a concentration gradient implies a potential flux of PAHs from porewater to overlying water. Taking the chronic toxicity values as reference, the hazard index (HI) values of all overlying water and porewater samples from the northern bay of Taihu Lake do not excess 1, indicating no or low ecological risk in water from the northern bay of Taihu Lake. (3) The passive sampler was used for acquiring freely dissolved concentrations profiles of typical pollutants on situ in the shallow lake, the diffusive fluxes across the sediment-water interface of these pollutants were calculated and the role of “source” or “sink” were also identified. In laboratory testing, the amounts of PAHs and SPAHs in LDPE increased linearly with increasing extraction time (R2 > 0.90, p﹤0.05). The sampling rates of these targets ranged from 0.000030 (Naphthalene, NaP) to 0.000409 (Anthraquinone, AQ) cm3 s-1. Field deployment of this passive sampling device in a typical shallow lake (Chaohu Lake) was able to obtain profiles of freely dissolved PAHs and SPAHs in the sediment porewater. The changes of abundances for typical PAHs and SPAHs porewater were in accordance with the variation in sediment core that obtained by an equilibrium partitioning approach, indicating that the results from this passive sampler devices could reconstructed the profiles of the target pollutants across the sediment-water interface. Besides, the field deployment yielded low-ring PAHs and SPAHs have higher diffusive fluxes. Moreover, diffusive flux of some typical organic pollutants (such as 3,6-Dimethylphenanthrene (3,6-DMP), 1-Nitropyrene (1-NP)) showed that the sediments may switch between ―source‖ and ―sink‖ during the passive sampling time, while this process could not be found by using the the method of benthic chamber or active sampling. Thus, the passive sampler in this study showed good reliability and it could be used to obtain the freely dissolved concentrations profiles for inorganic (1 cm resolution) and organic pollutants (2 cm resolution) with in high resolution across the sediment-water interface, which provide an effective method for getting better information about the environmental fates of pollutants in aquatic ecosystems