Comparing the Heterogeneity of Copper-Binding Characteristics for Two Different-Sized Soil Humic Acid Fractions Using Fluorescence Quenching Combined with 2D-COS

Jin Hur,Bo-Mi Lee

doi:10.1100/2011/640598

Abstract

Heterogeneous distributions of copper-binding characteristics were compared for two ultrafiltered size fractions of a soil HA using fluorescence quenching combined with two-dimensional correlation spectroscopy (2D-COS). The apparent shapes of the original synchronous fluorescence spectra and the extent of the fluorescence quenching upon the addition of copper were similar for the two fractions. The stability constants calculated at their highest peaks were not significantly different. However, the 2D-COS results revealed that the fluorescence quenching behaviors were strongly affected by the associated wavelengths and the fraction's size. The spectral change preferentially occurred in the wavelength order of 467 nm → 451 nm → 357 nm for the 1–10 K fraction and of 376 nm → 464 nm for the >100 K fraction. The extent of the binding affinities exactly followed the sequential orders interpreted from the 2D-COS, and they exhibited the distinctive ranges of the logarithmic values from 5.86 to 4.91 and from 6.48 to 5.95 for the 1–10 K and the >100 K fractions, respectively. Our studies demonstrated that fluorescence quenching combined with 2D-COS could be successfully utilized to give insight into the chemical heterogeneity associated with metal-binding sites within the relatively homogeneous HA size fractions.

Highlights

The accumulation and the mobility of heavy metals in soil environments are strongly affected by the presence of soil organic matter (SOM), in which humic substance (HS) is a major component [1]
The 1–10 K fraction of the soil HA exhibited a slightly higher specific UV absorbance (SUVA) value than the >100 K fraction (Table 1), suggesting the smaller size fraction contains more aromatic carbon content at the same Dissolved Organic Carbon (DOC) concentration the difference was not so pronounced compared to those reported in other studies using different size fractions of terrestrial HS [18, 26]
Other studies indicate that PLF characteristics are not likely to be prominent for soil environments where microbial transformation of terrigenous organic matters is dominant the feature could be associated with the presence of protein and/or amino acid bound HA and/or tannin-like substances [21]

Summary

Introduction

The accumulation and the mobility of heavy metals in soil environments are strongly affected by the presence of soil organic matter (SOM), in which humic substance (HS) is a major component [1]. The ability of HS to bind metal ions is largely attributed to oxygenated reactive groups attached to aliphatic and/or aromatic structures in HS [2]. Carboxylic and phenolic groups in HS operate as weak and strong binding sites for metals, respectively [3]. Previous studies have shown that carboxylic content and aromatic structures of HS were closely correlated with metal-binding capacities at a neutral pH condition [4, 5]. Condensed aromatic structures in HS structures are known to favor to form highly stable complexes with metal ions by rendering a phthalic acidlike and/or a salicyclic acidlike binding sites [1, 6]. Characterization of metal-HS interactions is viewed as an essential element for assessing the remediation of metal-contaminated soils as well as for predicting the fate of the inorganic pollutants in natural environments

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