Nanoparticles (NPs), < 100 nm in diameter, make up the smallest component of solid material. This small size often causes increased reactivity in soil/water environments, which is true for both natural NPs, such as very fine clay particles, and for manufactured nanoparticles, such as silver nanoparticles (AgNPs). As the importance of these particles is more widely recognized, and as manufactured nanoparticles, especially AgNPs, are increasing in production, it is essential to consider their effect on terrestrial and aquatic environments. The studies presented in this dissertation show that both the physicochemical characteristics of the NPs (e.g., particle size, surface coating, elemental composition), as well as soil-water interfacial chemistry (e.g., ionic strength, ligand concentration, pH), are instrumental in predicting environmental fate and reactivity.
Ligand type and concentration were especially important in NP reactivity and bioavailability. Using the hard/soft acid/base concept, the effect of phosphate ligand (hard base) on Fe/Al (hard acid) oxyhydroxide natural NPs was investigated in Chapters 2 and 3. Adding phosphate to soil NPs and reference nano-minerals (Fe-(oxyhydr)oxides and kaolinite) caused coagulation or dispersion, changing the particle size of the NPs, as well as affecting the amount of phosphate in its bioavailable (i.e., dissolved) form. A review of the literature in Chapters 1 and 3 revealed that changes in the soil conditions, and therefore, soil colloids/NPs (e.g., increasing organic matter via amendments), also has a direct impact on the soil NP-facilitated phosphate transport processes.
Silver, a soft acid, reacts readily with thiol functional groups, soft bases, in humic substances prevalent in soil environments. The effect of soil constituents on AgNP reactivity and phase transformation was investigated in Chapters 4 and 5. The presence of solid surfaces facilitated sorption and phase transformation in all AgNPs studied over the course of 30 days, especially when compared to the same AgNPs aged in aqueous environments in the absence of soil. When the bioavailability of Ag (as ionic and NPs), a known antimicrobial agent, was assessed via denitrification experiments in Chapter 5, the AgNPs exhibited much less toxicity than expected, perhaps due to their strong sorption onto soil particles, as observed in the adsorption isotherm experiments conducted in Chapter 4.
A more in-depth study of Ag(I) and AgNPs, and their interactions with cysteine, an amino acid with a thiol functional group, at the goethite-water interface in Chapter 6 revealed that, while cysteine enhanced the sorption of Ag(I) on goethite surfaces by forming inner-sphere ternary surface complexes, AgNP sorption to goethite was largely unaffected by cysteine. The behavior suggests hydrophobic interactions of AgNPs on goethite surfaces, revealing the effects of a soft ligand on Ag are via species specific (Ag(I) or AgNPs) mineral interactions, and are important in predicting AgNP fate in soil systems.
This dissertation provides a novel viewpoint of natural and manufactured NP interactions in soil environments. These interactions are dictated by both particle-specific characteristics and environmental conditions. When environmental conditions, especially the presence of reactive ligands (based on the hard/soft acid/base theory), are altered by anthropogenic or indigenous means, the reactivity of certain NPs changes dramatically, impacting the bioavailability of contaminants such as phosphate, Ag(I), or AgNPs.