The Interaction of Two Emerging Pollutants, Radionuclides and Microplastics: In-Depth Thermodynamic Studies in Water, Seawater, and Wastewater

Microplastics and radionuclides pose significant challenges to the sustainable management of water systems. The interaction of uranium-232 and americium-241 with polyurethane (PU) and polylactic acid (PLA) microplastics has been investigated in aqueous laboratory and environ-mental solutions (e.g., seawater and wastewater) as a function of temperature in various pH (4, 7, 9). The temperature increase affects positively the binding of uranium-232 and americium-241. The highest adsorption efficiency for uranium and americium is observed at the neutral and al-kaline pH region, respectively. In environmental water samples (pH ~8) the adsorption efficiency decreases significantly due to the competitive adsorption of other metals present in natural wa-ters (e.g., Ca2+) as well as the stabilization of the actinides (particularly uranium) in solution (e.g., UO2(CO3)34-). The solution composition which governs both the actinide speciation, and the type of surface-active sites is strongly associated with the surface adsorption thermodynamics and de-termines the values of the associated parameters (ΔΗo and ΔSo). Generally, the values of ΔΗo and ΔSo are positive indicating an entropy-driven reaction. However, in the case of the U(VI) adsorp-tion by PLA in seawater samples both ΔΗo and ΔSo values become negative suggesting an enthal-py-driven binding mechanism associated with a decline in randomness at the surface upon ad-sorption.