title: Transport of environmental contaminants in saturated porous media
Creators: name: Lu, Taotao
Publisher: name: University of Bayreuth
Year: 2022
Subjects: subject: Contaminants
subject: Transport
subject: Saturated porous media
subject: 550

description: With the development of industrial and agricultural activities, groundwater pollution is becoming a more serious concern. The presence of contaminants in groundwater is known to significantly increase the occurrence of diseases, which constitutes a significant threat to physical health. Water is a substance necessary to the human body, as well as a natural carrier of contaminants. Thus, contaminants can transport in porous media with groundwater flow. Understanding the migration patterns of contaminants in groundwater aquifers can provide valuable information for solving groundwater pollution problems. In addition, although contaminants can dissolve or suspend in water and migrate with the water flow, they are more likely to bind strongly to porous media, allowing most contaminants to be retained in the porous media. Therefore, it is crucial to elucidate the interactions between environmental contaminants and porous media in order to reduce the potential risks of these contaminants to physical health. In this dissertation, typical and new emerging environmental contaminants, namely heavy metals, petroleum colloids, and nanoplastics, were chosen as study subjects. Column experiments were conducted to investigate the migration patterns of these contaminants in saturated porous media (quartz sand, sandy soil, iron oxyhydroxide coated sand, clay minerals) under different hydrochemical conditions (low-molecular-weight organic acids, pH, ionic strength, etc.). Meanwhile, batch experiments and numerical simulations were also carried out to comprehensively understand the interaction mechanisms between the pollutants and porous media. In total, 4 studies were addressed in the present dissertation. In study 1, the effects of low-molecular-weight organic acids (LMWOAs) and competing cations (Pb2+) on the transport of heavy metal (Cd2+) in saturated pure quartz sand under different pHs were discussed. At pH 5, LMWOAs inhibited the transport of Cd2+, and inhibition capacity decreased in the following sequence citric acid &gt; tartaric acid &gt; acetic acid, which depends on their molecular structures (i.e., amount and type of functional groups) and complexing strength with Cd2+. Contrastingly, at pH 7, LMWOAs promoted the transport of Cd2+ due to the formation of stable aqueous non-adsorbing Cd-organic acid complexes. Pb2+ promoted Cd2+ transport due to its stronger complex affinity to citric acid. Therefore, the effects of solution chemistry play an important role in the transport of heavy metal ions. In study 2, the transport of petroleum colloids in quartz sand and sandy soil was studied. More petroleum colloids were retained with increasing ionic strength (IS) as well as decreasing pH. This observation can be explained by the increase in hydrodynamic diameter of petroleum colloids under these conditions, resulting in apparent physical straining. On the other hand, the electrostatic repulsion between colloids and grain surfaces reduces. In addition, greater mobility of petroleum colloids was found in the sand rather than in soil. This can be ascribed to the unique physicochemical properties of soil, like containing a lot of metal oxides and clay minerals. Hence, the solution chemistry and the porous media characteristics greatly influence the migration of petroleum colloids. In study 3, the migration of polystyrene nanoplastics (PS-NPs) in iron oxyhydroxide coated quartz sand under different water chemistry conditions was investigated. The effect of iron oxyhydroxide coated sand on PS-NPs transport was greater than that in pure quartz sand under the same conditions. For most cases (pH &lt; 9), the iron coating was positively charged, while PS-NPs were negatively charged. Thus, electrostatic attraction is the main reason for the weakened PS-NPs transport. In addition, solution chemistry changed the surface charge of PS-NPs and uncoated quartz sand, resulting in changes in the interaction energy between them. Charge heterogeneity of porous media is crucial for PS-NPs transport. In study 4, the effects of porous media properties on PS-NPs transport were further investigated based on the findings of study 3. Different clay minerals were used to understand their influence. Compared with quartz sand, clay minerals have a relatively high surface charge, which results in PS-NPs being more easily deposited onto clay minerals. Thus, both kaolinite and illite inhibited the transport of PS-NPs. Except for the permanent negative charges, some variable charges on the edge of the clay mineral are sensitive to the water chemistry. The positive edge sites of kaolinite and negative edge sites of illite under experimental conditions (pH 5.9) led to a stronger inhibition on PS-NPs transport in kaolinite than illite. It can be seen that the transport of environmental contaminants is closely related to the properties of contaminant and porous media, as well as water chemistry conditions. In order to obtain a more precise understanding, all these factors need to be taken into account when evaluating contaminant transport in groundwater aquifers