Influence of metal loading and humic acid functional groups on the complexation behavior of trivalent lanthanides analyzed by CE-ICP-MS

•Free and complexed HA-Ln species are separated by CE-ICP-MS.•Weaker and stronger HA-binding sites for Ln-complexation can be detected.•Complexation by original and modified humic acid (HA) with blocked phenolic hydroxyl- and carboxyl-groups is compared.•Stronger HA-binding sites for Ln3+ can be assumed as chelating complexes.•Chelates consist of trivalent Ln and a combination of both OH- and COOH-groups.

The complexation behavior of Aldrich humic acid (AHA) and a modified humic acid (AHA-PB) with blocked phenolic hydroxyl groups for trivalent lanthanides (Ln) is compared, and their influence on the mobility of Ln(III) in an aquifer is analyzed. As speciation technique, capillary electrophoresis (CE) was hyphenated with inductively coupled plasma mass spectrometry (ICP-MS). For metal loading experiments 25 mg L−1 of AHA and different concentrations (cLn(Eu+Gd) = 100–6000 μg L−1) of Eu(III) and Gd(III) in 10 mM NaClO4 at pH 5 were applied. By CE-ICP-MS, three Ln-fractions, assumed to be uncomplexed, weakly and strongly AHA-complexed metal can be detected. For the used Ln/AHA-ratios conservative complex stability constants log βLnAHA decrease from 6.33 (100 μg L−1 Ln3+) to 4.31 (6000 μg L−1 Ln3+) with growing Ln-content. In order to verify the postulated weaker and stronger humic acid binding sites for trivalent Eu and Gd, a modified AHA with blocked functional groups was used. For these experiments 500 μg L−1 Eu and 25 mg L−1 AHA and AHA-PB in 10 mM NaClO4 at pH-values ranging from 3 to 10 have been applied. With AHA-PB, where 84% of the phenolic OH-groups and 40% of the COOH-groups were blocked, Eu complexation was significantly lower, especially at the strong binding sites. The log β-values decrease from 6.11 (pH 10) to 5.61 at pH 3 (AHA) and for AHA-PB from 6.01 (pH 7) to 3.94 at pH 3. As a potential consequence, particularly humic acids with a high amount of strong binding sites (e.g. phenolic OH- and COOH-groups) can be responsible for a higher metal mobility in the aquifer due to the formation of dissolved negatively charged metal-humate species.

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