Agricultural systems face challenges related to toxic heavy metal pollution, which increasingly leads to food insecurity and adverse impacts on human health. Traditional heavy metals remediation technologies are often not readily applicable on farms due to potential negative effects on agricultural crops. Based on a hypothesis that nanoparticles can be engineered to selectively interact with toxic heavy metals in polluted irrigation water sources and farm soils while not having any serious adverse effects on food crops, this dissertation focused on creating knowledge on the fundamentals of interactions between nanoscale zerovalent iron (NZVI) and metallic contaminants in water and farm soils. Modifications of NZVI were also synthesized to explore the possibilities of improving performance, which could decrease the amount of materials needed to achieve the same treatment goal. In Chapter 1, the use of engineered nanomaterials (ENMs) as tools for treatment and remediation was reviewed. The chapter also discusses the different pathways and mechanisms via which ENMs are used for different classes of target contaminants, including heavy metals. Chapter 2 examines the synergistic removal of copper (Cu2+) and phosphate (P) from industrial wastewater using pristine and sulfidated NZVI (SNZVI), which is aimed at repurposing this treated water for agricultural irrigation. This chapter established that the dissolved iron (Fe2+) from the reactions between the ENMs and Cu2+ serves as a reactant for P removal from water. This interconnecting role of Fe2+ drives the removal of both reactants based on Le Chatelier's principle. An artificial neural network (ANN) was used to model the co-removal processes and identify key treatment factors. The capacity of NZVI and SNZVI to immobilize arsenic (As) in an agricultural soil and prevent its bioavailability to lettuce (Lactuca sativa) was studied in Chapter 3. Different spectroscopic and spectrometric tools were employed to establish the mechanisms of interactions between the ENMs and arsenic in soil. That chapter also reveals that remobilization of previously immobilized As occurs in the presence of both NZVI and SNZVI. Chapter 4 details the role of silica-coating on the efficacy and safety of NZVI (to form NZVI@SiO2) for As immobilization in an agricultural soil. Silica-coating improved nanoparticle stability and provided additional sites for As immobilization. Details of As immobilization were revealed through sequential extraction processes and X-ray-based characterization of soils amended with the ENMs. Chapter 5 summarizes the whole dissertation and provides the scenarios where NZVI-based nanoparticles can be applied in the agricultural system. Overall, this dissertation contributes valuable mechanistic insights into using NZVI-based nanoparticles for water and agricultural soil treatment, addressing challenges posed by heavy metal/loid pollution. These insights are critical for tailoring NZVI modifications to meet specific remediation needs.

Arsenic (As) is widely distributed in soil in the United States and around the world, highly toxic, and not efficiently remediated by existing traditional methods. Nanoscale zero-valent iron (nZVI)-based remediation is an emerging technology that has successfully been used to immobilize metals and metalloids due to nZVI’s high surface area and strong reducing ability. However, immobilization efficiency of nZVI decreases over time due to its oxidation under natural conditions. Modification of nZVI via sulfidation, to produce sulfide-modified nZVI (SnZVI), has been shown to overcome some of the shortcomings of nZVI. The main objective of this study was to compare the immobilization performance of nZVI and SnZVI for As in soil and determine if the toxicity of nZVI is influenced by sulfidation. The immobilization efficiency of nZVI and SnZVI (0.3, 1, and 5 wt.%) for As (50 ppm) in soil was monitored for 112 days under aerobic and anoxic conditions. The aerated soil also contained earthworms (Eisenia fetida) to evaluate the toxic impacts of the nanoparticles-based remediation. In general, SnZVI had a slower reaction kinetics, leading to lower As immobilization efficiency than nZVI after 7 days at 0.3 and 1% group. However, after 112 days, sulfidation was found to inhibit the corrosion of nanoparticle under aerobic conditions and the immobilization efficiency was similar to that of nZVI at all nanoparticle dosage concentrations. Both nanoparticles were toxic to E. fetida, but SnZVI was remarkably less toxic than nZVI. This study demonstrates the potential of SnZVI for effective and sustainable remediation.

Although the risks of engineered nanomaterials (ENMs) to human and the environment are poorly understood, over 1800 consumer products currently on the market contain at least one ENM and the list continues to grow. The use of many of these consumer products will lead to direct release of ENMs into natural aquatic systems and several studies have demonstrated that ENMs can have adverse ecological effects. It is therefore important to understand the fate of ENMs in aquatic systems in order to predict and/or investigate their environmental implications. Microorganisms are abundant in natural waters into which they also release exudates such as soluble extracellular polymeric substance (sEPS). However, studies on how microorganisms and sEPS influence the fate and transformations of ENMs in natural waters are rare. In this doctoral research, sEPS were extracted from freshwater and marine phytoplankton (an important class of microorganisms) and their effects on the environmental fate of a variety of ENMs—including carbon nanotubes, copper and its oxides, iron, and titania nanomaterials—were studied. Interactions between algae, sEPS and ENMs were investigated via electron microscopy with energy dispersive X-ray spectroscopy, electrophoretic and dynamic light scattering, UV-Vis spectroscopy, X-ray diffraction, and infra-red spectroscopy. Effects of sEPS were investigated by studying changes in physicochemical properties, aggregation and sedimentation kinetics, and transformation of ENMs. The major components of sEPS were polysaccharides and proteins; and the ratio of both components varied with species and age of organism, as well as the nutrient composition of growth media. sEPS contained charged functional groups, such as carboxyl, phosphoryl, carbonyl, hydroxyl, amide, etc. Evidences for direct interactions (physisorption and chemisorption) between algae/sEPS and ENMs, mediated by the various functional groups on proteins and nucleic acids or phospholipids were found. Adsorption of sEPS to ENMs changed the size and surface charge of the particles. In addition, the attachment efficiencies of ENMs decreased in the presence of sEPS due to electrosteric stabilization. Impacts of algae and sEPS were also found on ENMs’ dissolution, transformation, and toxicity. This study demonstrated that the presence of phytoplankton and EPS in natural waters will have unique and important impact on the fate, transport, transformation, and effects of ENMs.

Microplastics interact with per- and polyfluoroalkyl substances (PFAS), potentially acting as vectors for these contaminants. However, little is known about how residential sources, physicochemical properties, and water chemistry influence these interactions and the subsequent fate of microplastics and PFAS. Elucidating the role of residential PFAS inputs, sorption mechanisms, and microplastics aging is critical to understand microplastic-mediated PFAS proliferation. This work aimed to address these knowledge gaps by investigating PFAS in wastewater sources, sorption to microplastics, and effect of aging (sodium sulfide treatment and natural organic matter loading) on adsorption of PFAS onto microplastics. Raw wastewater sampling at three sites revealed complex PFAS profiles, with 9 compounds detected - 5 carboxylates, 2 sulfonates, and 2 precursors. Long-chain PFAS including perfluorododecanoic acid and perfluoropentanesulfonic acid dominated, contrasting with the lower levels of legacy PFAS like PFOS. Fluorotelomer precursors, especially 6:2 FTS, were frequently detected and abundant. PFAS showed distinct variation from heavy metals, although PFOA uniquely correlated with iron. Controlled experiments found PFAS rapidly sorb to microplastics within hours, following thermodynamically favorable and spontaneous partitioning. Perfluoroalkyl sulfonic acids preferentially adsorbed over carboxylates, attributed to hydrophobicity. Water chemistry parameters like pH, salinity, and natural organic matter impacted sorption. Na2S treatment enhanced adsorption capacity by 28-95%, while NOM loading increased it by 16-42%, highlighting the impact of aging on PFAS uptake. This work advances understanding of residential PFAS inputs, microplastic interactions, and effects of MPs aging and aqueous matrices on PFAS adsorption. The results can inform source control and treatment strategies targeting these ubiquitous contaminants.