Enhanced radionuclide (U-232) adsorption by humic acid-coated microplastics

Polyethylene microplastics (PE-MPs) are among the most frequently found plastic pollutants, capable of adsorbing various contaminants such as radionuclides, raising potential environmental concerns as secondary pollutants. This study investigates the impact of natural organic matter (NOM), specifically humic acid (HA), on the adsorption of the U-232 radionuclide by HA-coated PE-MPs. The experiments were conducted in different pH regions (weak acidic, near neutral and weak alkaline), temperatures (25, 35, 45 °C) in deionized water solutions (DI), and in seawater samples. The surface coating of PE-MPs through HA enhances significantly their adsorption capacity for U-232, particularly under weakly acidic conditions. Batch adsorption experiments demonstrate that HA-coated MPs exhibit nearly 100 % adsorption efficiency at low pH, compared to only 25 % for non-treated PE-MPs. The adsorption process is found to be endothermic and entropy-driven, indicating that increasing temperature favors the adsorption capacity of the MPs. In seawater, the presence of competing ions decreases adsorption efficiency; however, HA-coated MPs still outperform their non-coated counterparts. These findings highlight the critical role of NOM coatings in enhancing the environmental stability and mobility of radionuclides and other pollutants by (PE-)MPs and underline the need to consider possible surface modifications in risk assessments related to MP polution.