The interaction of two emerging pollutants, radionuclides and microplastics: In-depth thermodynamic studies in water, seawater, and wastewater

Abstract

Microplastics and radionuclides create considerable obstacles for the sustainable management of aquatic systems. The interactions between uranium-232 and americium-241 with two microplastics, specifically polyurethane (PU) and polylactic acid (PLA), have been explored in aqueous matrixes mimicking seawater and wastewater. The tests were performed at different pH levels (4, 7, 9) and the remediation efficiency was evaluated predominately as a function of temperature. An increase in temperature positively influences the sorption of uranium and americium. The highest removal performance for both uranium and americium was observed in the neutral (log(K_d)_PLA=1.6 and log(K_d)_PU=1.7 for U; log(K_d)_PLA=1.7 and log(K_d)_PU=1.6 for Am) and alkaline (log(K_d)_PLA=1.4 and log(K_d)_PU=1.3 for U; log(K_d)_PLA=1.8 and log(K_d)_PU=1.9 for Am) pH ranges, respectively. In seawater mimicking water samples (pH ∼8), the adsorption efficiency declines significantly (log(K_d)_PLA=1.1 and log(K_d)_PU=1.2 for U; log(K_d)_PLA=1.3 and log(K_d)_PU=1.4 for Am) due to competitive interactions arisen from other metals/ions found in natural waters (e.g., Ca²⁺) and the stabilization of actinides (particularly uranium) in solution (e.g., UO₂(CO₃)₃^4-). The composition of the water matrixes, which govern both actinide speciation and the types of surface-active sites, is strongly linked to the thermodynamics of surface adsorption and determines the values of the associated parameters (ΔΗ° and ΔS°). Generally, the ΔS° and ΔΗ° values were found positive, indicating that the interactions are entropy-driven. However, in the case sorption of U(VI) in seawater samples by PLA, both ΔS° and ΔΗ° became negative, denoting enthalpy-driven binding mechanisms associated with a decrease in randomness on the MP’s surface upon adsorption.