Interactions Between Micro- and Nano plastics and Engineered Nanoparticles in Wastewater and Receiving Environments: Implications for Treatment Performance, Transport Pathways, and Ecotoxicity

Abstract

Microplastics (MPs), nanoplastics (NPs), and engineered nanoparticles (ENPs) are increasingly recognized as interconnected contaminants in wastewater treatment systems and receiving environments. This review examines their occurrence, interactions, treatment performance, transport pathways, persistence, and ecotoxicological implications, with emphasis on evidence from primary studies. The review considered research addressing wastewater, sludge, effluent, receiving waters, particle interactions, aggregation, treatment processes, and biological effects, with attention to analytical approaches capable of improving particle identification and characterization. Evidence from wastewater treatment plants shows that treatment systems can remove substantial proportions of MPs and related particles, but removal commonly represents redistribution rather than destruction. Sludge functions as a major retention compartment, while fine particles and fragmented materials can remain in treated effluent and reach receiving waters. Particle behaviour is strongly influenced by ionic strength, pH, salinity, dissolved organic matter, temperature, surface functional groups, particle aging, and interactions with mineral phases. These factors regulate heteroaggregation, colloidal stability, deposition, and transport across freshwater, brackish, and marine environments. Treatment related interactions can also alter sludge properties, microbial communities, extracellular polymeric substances, and membrane operation without necessarily causing immediate deterioration in effluent quality. After discharge, particle mixtures may form transport hotspots in rivers and sediments and can influence microbial processes, antibiotic resistance patterns, oxidative stress, developmental responses, and food web functioning. Evidence from aquatic organisms further indicates that nanoscale particles and mixed exposures may produce stronger or distinct biological effects because of their high surface area, mobility, and carrier potential for engineered nanoparticles and associated contaminants. Overall, the literature indicates that wastewater treatment should be understood as a particle transformation and redistribution system rather than a complete removal barrier. Future research should prioritize realistic mixed particle exposures, size resolved assessments, wastewater relevant chemistries, improved analytical methods, and integrated evaluation of water and sludge pathways to strengthen environmental risk assessment and treatment strategies.