Samarium (Sm3+) and copper (Cu2+) complexation property with dissolved organic matter

This study investigates the speciation of Samarium (III) in the presence of natural organic matter, standard humic and fulvic acids from the Suwannee River. Fluorescence quenching and Time-Resolved Laser Fluorescence Spectroscopy (TRLFS) were employed to analyze various samples, using logarithmic additions of Sm³⁺ and Cu²⁺. The 1 L:1 M and 1 L:1 M:H models were applied to account for pH effects, yielding pH-independent complexant parameters K_Sm and K_Cu as well as $C_{L_{\mathrm{Sm}}}\mathrm{and}{C}_{L_{\mathrm{Cu}}}$that revealed specific binding sites. A notably higher affinity was observed for humic acids with Sm³⁺ compared to Cu²⁺. The logarithms of the complexation constant for the Sm³⁺ were 5.4, 5.2 and 4.7 for SRNOM, SRHA and SRFA, respectively. Fluorescence quenching analyses identified two distinct fluorescent components related to dissolved organic matter, showing different affinities for Sm³⁺ and Cu²⁺ that varied with pH. Humic acids exhibited the highest complexation capacity for Sm³⁺, attributed to their structural properties, with complexation capacities of 4.77 10⁻⁴, 0.82 and 4.46 10⁻³ mol of Sm per g of SRNOM, SRHA and SRFA, respectively. Temporal deconvolution of fluorescence responses revealed a tri-exponential decay with three lifetimes (τ₁, τ₂ and τ₃) remaining relatively constant during Sm³⁺ or Cu²⁺ titrations, confirming that the fluorescence quenching is primarily a static mechanism. Speciation modeling demonstrated a shift from humic-associated complexes in freshwater to carbonate complexes in seawater as salinity increases. These findings highlight the importance of accurately characterizing DOM complexation properties to understand the environmental behavior of trace metals, particularly in response to salinity and carbon concentration gradients in estuarine systems.