Binding of Europium(III) and Uranium(VI) to Casein Micelles from the Milk of different Species

Environmental contamination with radioactive elements such as actinides poses a potential threat to ecosystems and human health. Actinides released into soil and water can be mobilized by binding to biologically relevant ligand systems and thus enter the food chain of animals and humans [1]. Examples of such ligand systems include the functional groups of proteins, like caseins from milk and dairy products. Caseins, which belong to the phosphoprotein family, are predominantly present in colloidal aggregates called micelles [2]. Casein micelles are animal species‐specific in size, structure, and composition. Initial studies have shown that free caseins from cow's milk can efficiently bind actinides such as uranium [3]. The purpose of our studies is to investigate the interaction of europium, as an analogue to relevant trivalent actinides, and the hexavalent uranium with micellar caseins from human, bovine and camels' milk. Casein micelles were isolated from raw milk from cow and camel as well as from human milk by ultracentrifugation (100 000xg, 20°C, 1 h). In batch adsorption experiments, casein suspensions with an adjusted protein content of 1 mg/mL were prepared and different concentration of Eu 3+ or UO 2 2+ (1 ‐ 500 mg/L) were added. The adsorption was performed at 6°C for 24 h in an overhead shaker. After that, the caseins were separated. The supernatants were analysed by ICP‐OES. Data obtained from the adsorption experiments were analysed using the Langmuir fit. For U(VI), a maximum adsorption capacity (qmax) of 3,00 times 10 ‐4 mol/g was determined for micelles from human milk, 1,21 times 10 ‐3 mol/g for micelles from cow milk and 7,35 times 10 ‐4 mol/g for micelles from camel milk. For Eu(lll), the calculated q max were 5,66 times 10 ‐5 mol/g, 5,49 times 10 ‐4 mol/g and 1,85 times 10 ‐4 mol/g for the micelles from human, cow and camel milk, respectively. Based on the results, the following trend for the maximum adsorption capacity among the animal species could be determined for both ions: human < camel < cow. However, it is notable that qmax is higher for U(VI) than for Europium in all animal species, whereas the sorption affinity, derived from the equilibrium constant K L , is higher for Europium than for Uranium in all animal species. To explain these results, the calcium and total phosphate content was also determined. The lowest levels of phosphate and calcium were measured in human micelles (0,312 mM Ca /g micelles and 0,130 mM P/g micelles). The highest content of total phosphate was found in micelles from cow's milk (0,628 mM/g micelles). It is assumed that the presence of phosphate especially the inorganic phosphate in the micellar structure is the key to the binding of f ‐elements.