Rare earth elements (REEs) are widely used in various high-tech industries such as permanent magnets and compact fluorescent and so on. However, REEs are discharged into the natural environment during the processes such as their mining, production, utilization, and recycling. Health hazards of REEs have been reported for workers in occupations related to these processes and for residents living near mines. Due to anthropogenic activities, REEs are ubiquitously distributed through river water to agricultural soil and coastal sediments in built-up areas in many countries, and therefore need to be considered as a pollutant. REEs concentrated in certain freshwater are in the nanomolar range and dissolved organic carbon is responsible for the bioaccumulation. Enriched REEs are strongly complexed with carbonate ions and fulvic acid. Speciation analysis is essential for assessing the mobility and toxicity of REEs, however, the data is limited and lacking. Europium (Eu) is used in a phosphor for desktops and other applications, and recycling is being attempted from its waste. Eu has been detected in workers at recycling facilities and toxicity has been reported in various living organisms. Complexation and speciation of Eu with carboxylic acids (CAs) such as succinic acid (Suc), fumaric acid (Fum), malic acid (Mal), α-ketoglutaric acid (Ket), oxaloacetic acid (Oxa), isocitric acid (Iso), and citric acid (Cit), and amino acids (AAs) such as alanine (Ala), phenylalanine (Phe), lysine (Lys), aspartic acid (Asp), serine (Ser), and cysteine (Cys) which are used in our daily life and industry were investigated.
After identifying stoichiometric recognition and a number of chemical species by ESI-MS and potentiometric titration, stability constants were determined by the potentiometric titration curves plugging to Hyperquad2008. From these determined stability constants, speciation diagrams were produced by HySS2009 in the condition under mimicking condition of river water containing carbonate, sulfate, and chloride ions and their Eu complexes including Eu hydroxide complexes.
The K1 values of Eu-CAs complexes were 3.5 of Suc, 3.8 of Fum, 4.2 of Mal, 4.2 of Ket, 4.4 of Oxa, 5.6 of Iso, and 6.7 of Cit, respectively. The K values of Eu-AAs complexes were 5.9 of K1 and 7.5 of K2 for Ala, 5.7 of K1 and 8.4 of K2 for Phe, 8.2 of K1 and 7.3 of K2 for Lys, 5.8 of K1 and 7.5 of K2 for Asp, 4.5 of K1 and 8.5 of K2 for Ser, 3.2 of K1 and 8.3 of K2 for Cys, respectively. A comparison of these obtained K values and a literature survey allowed the structure to be estimated. In the speciation diagrams, Eu-Cl complexes and solid phase did not form for all ligands. Eu and CAs were complexed as the main forms of deprotonated, protonated, and hydroxide species of M:L = 1:1 by binding to free ions or competing with sulfate or carbonate ions in the acidic range of pH depending on CAs. The maximum fraction of Eu-CA complexes was up to 0.4-82% depending on the ligand. Eu and AAs predominantly formed protonated species of M:L of 1:1 and 1:2, as the maximum fraction of Eu-AA complexes was in the range of 1-62%. For all ligands, carbonate species were dominant at pH 6.5-9.3, and Eu(OH)30 was the dominant species from pH 9.4.

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