Accompanying the booming of nanotechnology are the rising concerns on its potential environmental risks. The recent detection of engineered nanomaterials in natural streams, together with the recent discovery of ENPs' toxicity to a range of biological receptors, necessitates the investigation on their fate and transport. This dissertation research aimed to improve our understanding on one of the key ENP transport processes, agglomeration, and the crucial parameters that control this process.

ZnO, TiO2, CeO2, and citrate-coated Ag were used as model ENPs given their dominating production rate. The experimental techniques include a combination of material characterization, agglomeration, sedimentation and adsorption processes monitoring, and inter-particle interaction calculations. Our results showed that ENPs agglomeration is controlled by a range of external and internal parameters.

The external parameters investigated include ionic strength (IS), pH, natural organic matter (NOM) concentration, and the presence of clay particles. The agglomeration of ENPs was found to be sensitive to the indifferent electrolyte and exhibited DLVO-type two-regime pattern. The Critical Coagulation Concentration (CCC) increased significantly as the pH went further away from the point of zero charge. NOM substantially hindered the agglomeration of ENPs at high concentrations and facilitated attachment at low concentrations. We found that the patch-wise charge heterogeneity of clay minerals and its complex response to pH and IS variations produce rich interaction patterns between clay and ENPs. Clay particles were capable of destabilizing both positively and negatively charged nanoparticles under favorable solution chemistries.

The internal parameters examined include particle size, morphology, crystal structure, and the sintered structure. The agglomeration of 10 TiO2 with varying material properties was studied. CCCs of anatase spheres correlate well with particle size, which agrees with the DLVO prediction; while CCCs of rutile rods strongly depend upon the specific surface area, indicating it is the surface chemistry rather than the bulk properties that dominates rutile rods agglomeration. The unique sintered structured of metal oxide NPs formed during the high-temperature synthesis process were found to render NPs a rather weak inter-cluster attraction, which can be disrupted by common environmental stimuli such as light exposure or temperature variation.