Fluorescence Studies of Metal-Humic Complexes with the Use of Lanthanide Ion Probe Spectroscopy

Abstract

The acidic functional groups of humic materials are an abundant source of metal binding sites in the natural environment. Studies of metal binding to humics are of great environmental interest because the biological and physicochemical properties of metals are often changed dramatically as a result of complexation with humics. In order to understand how these heterogeneous organic macromolecules bind metals with such a large range of binding energies, lanthanide ion probe spectroscopy (LIPS) has been used to study changes in the fluorescence lifetime of the europium probe metal as it binds to these substances. A method developed by Horrocks and Sudnick for the determination of the number of water molecules bound to Eu3+ was used to calculate the coordination number of humic-bound Eu3+ from the fluorescence data. The peak shift of the Eu3+ hypersensitive emission band (616 nm) was used to calculate the change in charge of the complex. Equations based on Horrocks and Sudnick's method were also developed to calculate the distribution of metal associated with the different types of binding sites on humic substances by computer modeling of the fluorescence lifetime data.

Europium(III) complexed by HPSEC size-fractions of a vertisol humic acid: Small differences evidenced by time-resolved luminescence spectroscopy

“…This technique has been used to study the Ln(III)/HS interactions, and the rationale was to either determine interaction constants [37,39,40], or gather information on the Ln/An(III) chemical environment [26,[41][42][43][44][45][46][47][48]. The observed luminescence corresponds to the 5 D 0 → 7 F 0 transition (electric and magnetic dipole forbidden, maximum around 580 nm), the 5 D 0 → 7 F 1 transition (magnetic dipole, maximum around 593 nm), and the 5 D 0 → 7 F 2 'hypersensitive' transition (electric dipole, maximum around 615 nm).…”

Systematic change in relative stabilities of REE-humic complexes at various metal loading levels

“…On the other hand, a study using the fluorescence lifetime of Eu 3+ reported that Eu 3+ -HS complexation (mixture of SRHA and SRFA) at pH 3.5 and at a lower REE loading level (10 -2 -10 -7 M Eu 3+ , 30 mg/dm 3 HS) is dominated by tetradentate complexes (Thomason et al, 1996). According to the study, a decrease in the REE loading level leads to an increase in the denticity of HS to Eu 3+ .…”

Lanthanide–humic substances complexation. I. Experimental evidence for a lanthanide contraction effect

“…Poly-dentate metal coordination is therefore possible within one SRFA molecule or among SRFA molecules. A study using lanthanide ion probe spectroscopy (LIPS) found that Eu 3+ complexation by Suwannee river DOM (mixture of HA and FA) at pH 3.5 is dominated by tetra-dentate complexes at low metal to ligands ratios (100 nM Eu 3+ , 30 mg L À1 DOM) (Thomason et al, 1996). The LIPS study also indicated that an increase in metal/DOM ratio resulted in progressively less chelated complexes, with a gradual succession from 4 to 3 to 2 to 1 carboxyl groups bound to Eu 3+ .…”