Plastic pollution in aquatic systems has escalated due to extensive use and inadequate waste management, posing significant risks to ecological and human health. Nanoplastics—tiny fragments resulting from the environmental breakdown of larger plastics—interact with coexisting micropollutants such as antibiotics and natural substances like minerals and dissolved organic matter (DOM), influencing their aggregation behaviors and environmental fate. However, these interactions remain poorly understood under realistic aquatic conditions. This dissertation systematically explores the effects of these coexisting substances on the aggregation, sedimentation, and pollutant adsorption behaviors of various nanoplastics, offering mechanistic insights into their environmental processes and ecological implications. The first objective was to investigate the heteroaggregation of polystyrene nanoplastics (nPS) with hematite nanoparticles (nFe2O3) under varying levels of plastic pollution with and without DOM, and its effect on sulfadiazine (SDZ) adsorption. The relative abundance of nanoplastics to minerals significantly affected both heteroaggregation and SDZ adsorption. At pH 5.0, both heteroaggregation rates and SDZ adsorption initially increased, then declined with increasing relative abundance of nPS/nFe2O3. At low (≤ 0.1) and high (≥ 0.4) relative abundances, nPS and nFe2O3 aggregated slightly owing to electrostatic repulsion, while significant aggregation and SDZ adsorption occurred at a relative abundance of 0.2 through charge neutralization. At pH 6.0 and 8.0, both heteroaggregation rates and adsorption capacity progressively declined with increasing plastic pollution levels due to accumulating charge repulsion. The SDZ adsorption closely followed the heteroaggregation trends with electrostatic interactions, van der Waals forces, and hydrophobic forces contributing significantly. DOM further modulated aggregation through electrostatic interactions and steric hindrance, particularly under acidic conditions. The second objective aimed to elucidate the aggregation behaviors and mechanisms of four environmentally relevant nanoplastics (pristine and aged polystyrene, polyethylene, and polypropylene) in response to two antibiotics, ciprofloxacin (CIP) and sulfamethoxazole (SMX). At pH 5.0, both CIP and SMX significantly promoted nanoplastics aggregation, with CIP being more potent. CIP enhanced nanoplastics aggregation through charge shielding driven by electrostatic attraction, hydrogen bonding (HB), and charge-assisted HB (CAHB), whereas SMX promoted aggregation solely through molecular bridging involving HB and CAHB. At pH 7.0, only CIP facilitated aggregation, while neither antibiotic induced aggregation at pH 9.0. Aged polystyrene aggregated more readily than pristine polystyrene due to increased surface functional groups. Polyethylene and polypropylene showed weaker aggregation due to fewer surface functional groups. High OM levels (1.65 mg/L TOC) inhibited antibiotic-induced aggregation, whereas low OM levels (16.5 μg/L TOC) were more conducive. The third objective was to investigate the effects of CIP and SMX on the heteroaggregation and sedimentation of four nanoplastics with hematite nanoparticles under environmentally relevant pHs (5.0–9.0) and nanoplastics-to-mineral mass ratios (Rn/m = 0.2–1.0). Results showed that heteroaggregation was strongly dependent on pH and Rn/m, with the most pronounced aggregation occurring at pH 5.0 and low Rn/m (0.2). CIP enhanced heteroaggregation primarily via charge shielding and bridging effects, while SMX acted through charge shielding, bridging, and hydrophobic interactions, reflecting differences in speciation and surface affinity. Real-water experiments confirmed enhanced sedimentation linked to heteroaggregation, potentially reducing nanoplastic mobility but increasing benthic accumulation. Aged polystyrene exhibited higher aggregation and sedimentation potential due to surface oxidation, whereas polyethylene and polypropylene showed lower heteroaggregation potential and greater colloidal dispersibility.