Understanding the surface chemistry of long term environmentally transformed nanomaterials for regulatory assessment

Abstract

Environmental conditions significantly influence nanomaterial behaviour and physico-chemical transformations. Surface coatings initially enhance stability but degrade under environmental conditions, exposing nanomaterial cores and increasing transformation rates. These environmental transformations will impact nanomaterial bioavailability and ecotoxicity, with smaller particles and ionic species posing heightened toxicity to aquatic organisms, while larger agglomerates settle into sediments, posing risks to benthic ecosystems. The chronic environmental transformations of silver and titanium dioxide nanomaterials with different surface coatings over four years in diverse synthetic aquatic environments designed to mimic natural conditions are investigated. These conditions include a high-hardness salt only OECD-test medium (HH combo), and two synthetic waters representing low-alkalinity (class I) and high-alkalinity (class V) conditions, reflecting regional chemistries of areas such as Norway, the Alps, southern UK, and parts of southern Europe. A suite of analytical techniques, including dynamic light scattering, single particle inductively coupled plasma mass spectrometry, transmission electron microscopy, and X-ray photoelectron spectroscopy, were employed to assess morphological and physicochemical transformations of the nanomaterials over time. This study underscores the importance of incorporating nanomaterial ageing and environmental transformation studies into regulatory frameworks to capture the dynamic lifecycle behaviours of nanomaterials and their evolving physico-chemical characteristics and consequent toxicity. Recommendations include mandating environmental testing under diverse conditions to simulate real-world scenarios, assessing coating stability, and adopting tiered risk assessment approaches. These findings provide crucial data for regulators to develop guidelines that reflect the dynamic nature of nanomaterials, enhancing our understanding of their long-term environmental persistence, mobility, and ecological impacts.