Environmental monitoring and risk assessment of secondary nanowaste in nano-enabled water treatment systems.

Nano-enabled technologies are now widely used in water and wastewater treatment. They show strong ability to remove heavy metals, dyes, pathogens, pharmaceuticals, and other emerging pollutants. However, nanomaterials may not be fully recoverable and have the potential for breakdown, leaching, accumulation in sludge, or release into the environment. This creates a new form of pollution that occurs after treatment rather than before it, widely known as secondary nanowaste. However, most studies highlight high removal efficiency and material performance of these nano-enabled technologies, while post-treatment scenarios garner much less attention and remain severely underdiscussed. Therefore, this review introduces the concept of post-treatment pollution in nano-enabled water systems and focuses on their environmental monitoring aspects including release, transformation, or buildup of nanomaterials and nano-derived residues during operation, regeneration, disposal, or reuse stages. The review covers research from the last 10 years and examines major nano-enabled treatment approaches, including nano adsorbents, membranes, catalytic systems, and magnetic materials, and also discusses key pathways of secondary nanowaste formation such as sludge accumulation, membrane aging, catalyst loss, and incomplete recovery. In addition, the review critically discusses the environmental fate of this secondary nanowaste across water, soil, and sediment systems, and its potential impacts on microbes, aquatic organisms, and drinking water safety are reviewed. An in-depth discussion about the strategies and safe-by-design approaches utilized for the mitigation of secondary nanowaste have also been provided. Gaps in long-term toxicity data and exposure assessment are highlighted, along with a forward-looking approach for secondary nanowaste treatment possibilities. The review aims to support the development of risk-informed monitoring strategies and sustainable design principles for the responsible deployment of nano-enabled water treatment technologies.