Rong, Qiuyu and Jones, Kevin and Zhang, Hao (2024) Development and applications of the diffusive gradients in thin-films (DGT) technique for the measurement of polycyclic aromatic hydrocarbons (PAHs) in waters and soils. PhD thesis, Lancaster University.
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Abstract
Polyaromatic hydrocarbons (PAHs) are a group of organic compounds, widely present in the environment, which are carcinogenic, teratogenic, mutagenic and bioaccumulative. They can exist in the environment for a long time and pose potential risks to humans and ecosystems, so they have attracted the attention of research scientists and regulators. It is therefore important to have sampling and analytical techniques to accurately and effectively detect PAHs, to understand their environmental sources, behaviour and fate, to provide a scientific basis for controlling their releases to the environment and to protect ecosystem and human health. The diffusive gradients in thin films (DGT) technique is an in-situ passive sampling technology, which has been used to measure heavy metals, nutrients and – more recently – some classes of organic pollutants in environmental media. In this thesis, a DGT sampler for PAHs was developed, tested, and used for studies of PAHs in waters and soils. A new sampler casing was designed using aluminum, to reduce adsorption during field deployment. The performance of three binding materials (MIP, HLB and XAD18) was compared in laboratory tests. The absorption capacity and uptake kinetics of the three materials were determined, and their performance under different pH, ionic strength and dissolved organic matter tested. The MIP-DGT sampler performed best and was not affected by environmental conditions across the range pH (3.9-8.1), ionic strength (0.01- 0.5 M) and dissolved organic matter concentration (<20 mg L -1 ). Field tests showed DGT can be used for time-integrated in situ measurement of PAHs in waters under a range of environmental conditions. DGT PAH samplers were deployed in the Nanjing section of the Yangtze River and urban rivers in summer and winter 2021. The concentrations of labile PAHs had marked seasonal differences under the influence of source inputs, river flow differences, and degradation/loss processes. The accumulation of PAHs in the Yangtze River upstream of Nanjing together with seasonal combustion sources, discharges from petrochemical enterprises and ship transportation are discussed as the main sources of PAHs in the studied area. Inputs from Nanjing urban rivers had little impact on the PAH mass loadings in the mainstream of the Yangtze River. The results of two different risk assessment methods showed that PAH concentrations are sufficient to be a potential risk to ecological systems during the winter and in some of the more contaminated industrial areas. This study highlighted how in situ DGT water sampling can be used to investigate the sources, fates and behavior of PAHs in aquatic environments. DGT can also be used to study trace organic contaminants in soil. DGT samplers were deployed in laboratory experiments investigating soil-compound interaction during aging. Six soils with different physicochemical properties were spiked with 4 PAH compounds (phenanthrene, anthracene, fluoranthene and pyrene) with final concentration of 10 mg kg-1 and allowed to age for 45 days. DGT sampling was conducted at time intervals, to investigate PAH adsorption/desorption behaviour and to derive kinetic information on their release from the soil solid phase. Soil physical and chemical properties such as pH, TOC, soil particle size and the properties of PAHs themselves all influenced compound behaviour during the aging process. The ability of most of the soils to resupply PAHs from the solid phase to the soil solution decreased with aging time. The DGT-induced fluxes in soils (DIFS) model was used to obtain information on PAH desorption kinetics from the soil solid phase to the soil solution phase. Desorption kinetics and the labile pool size both affected the ability of soils to re-supply PAHs. The labile pool size was affected by pH and TOC. DGT has been used to explore the relationship between soil and crop plant uptake of PAHs substances. An experiment was conducted, growing lettuce, radish and maize in six different soils, spiked initially with 10 mg kg-1 of 4 PAHs. DGT sampling was performed before planting and after harvesting, together with sampling of soil solution and a sequential extraction procedure. DGT measurements showed changes of the resupply of PAHs from the soil solid phase before and after crops planting. The presence of PAHs in the air affected the PAH content of aboveground plant parts. PAHs sampled by DGT, and in soil solution and in the sequential extraction procedure were compared to crop concentrations. None of the soil tests gave clear correlations to the plant concentrations. Possible reasons for this were discussed and recommendations were made for further studies. The thesis provides the foundation for future work on using DGT to further investigate PAHs (and other compounds) in the environment. Recommendations are made for applications of DGT to provide novel insights on the sources, fates and behaviour of PAHs in the environment.