Parallel Factor Analysis of Excitation–Emission Matrix Fluorescence Spectra of Water Soluble Soil Organic Matter as Basis for the Determination of Conditional Metal Binding Parameters

Abstract

Organic matter-metal complexes in soil solution and aquatic systems are involved in important environmental and ecological processes such as plant nutrient availability and the solubilization and transport of metals. Our work presented here extends the use of fluorescence spectrometry for determining conditional stability constants for such complexes. We combine the use of excitation-emission matrix (EEM) fluorescence spectrometry and parallel factor analysis (PARAFAC) to determine the stability constants of the chemically meaningful components modeled by PARAFAC. Water-soluble organic matter (WSOM) from O-horizon soils of deciduous and coniferous forest stands were extracted and titrated at pH = 4.7 with iron(lll) (Fe) and aluminum (Al) which are important metals in acid soil systems. The EEM spectra were then recorded and PARAFAC analysis showed that the WSOM contained three humic-substance-like components. Fe titration led to fluorescence quenching of the three components, while Al titration enhanced fluorescence for two components and quenched one of the components. The average Ryan-Weber stability constants at pH 4.7 ranged from log K of 4.28 to 4.91 for Fe and 4.84 to 5.96 for Al. The conditional stability constants were similar for Fe binding for deciduous and coniferous stand-derived WSOM, while they were stronger for Al binding with coniferous stand-derived WSOM. This difference in binding strengths for Al may affect the chemical behavior of Al in soil and aquatic systems. Determining the individual binding parameters of organic matter components with metals represents a significant advance over current approaches that utilize fluorescence quenching at a single excitation-emission wavelength pair to characterize organic matter-metal interactions.

Study of the pH influence on the optical properties of dissolved organic matter using fluorescence excitation–emission matrix and parallel factor analysis

“…The approach of PARAFAC modeling of the EEMs followed the procedure described in detail by Stedmon and Bro [25] and Ohno et al [28]; only a brief description will be provided here. Essentially, PARAFAC examines a consistent series of EEM landscapes that form a so-called three-way array of size I Â J Â K, where J is the number of emission wavelengths, K is the number of excitation wavelengths, and I is the number of samples, which corresponds to the fluorescence EEM spectra of the DOM samples measured at various pH values in this study.…”

“…An alternative, multiway methods, such as parallel factor (PARAFAC) analysis of the EEM sample pool, can be used to resolve mixtures of fluorescent compounds into their individual constituents by relying on the visual identification of peaks and the ratios of fluorescence in different regions of the spectrum, thereby reducing the interference problem and allowing a more accurate quantification [25][26][27]. Therefore, EEM in combination with PARAFAC has been used to investigate the oxidation and metal complexation processes of DOM [28][29][30][31].…”

Effects of storm events on mobilisation and in-stream processing of dissolved organic matter (DOM) in a Welsh peatland catchment

“…For absorbance and sUVa, Fe corrected values were included in the revised paper, based on a published equation (Weishaar et al 2003). The fluorescence estimate was based on a similar regression, but it was more crude as we had to produce the equation ourselves based on the data in the paper by Ohno et al (2008). Hence, we did not feel it would be appropriate to present these data, but the effect was much the same as that for absorbance, and again did not alter the interpretation.…”

The Role of Aluminum-Organo Complexes in Soil Organic Matter Dynamics

“…Multidimensional fluorescence spectroscopy has been previously demonstrated to provide better characterization of metal binding to organic compounds than traditional quenching at a single excitation-emission wavelength. Aluminum complexation with GA varied the fluorescence fingerprinting and therefore provided information on the types of metal-organic complexes, which depended on Al(III) concentration in solution (Ohno et al 2008). Spectral variations confirmed the formation of metal-organo complexes (Figure 4) but furthermore suggested the formation of at least two different types of complexes (Figure 4b).…”