A Comparative Study of the Application of Fluorescence Excitation-Emission Matrices Combined with Parallel Factor Analysis and Nonnegative Matrix Factorization in the Analysis of Zn Complexation by Humic Acids

The main aim of this study was the application of excitation-emission fluorescence matrices (EEMs) combined with two decomposition methods: parallel factor analysis (PARAFAC) and nonnegative matrix factorization (NMF) to study the interaction mechanisms between humic acids (HAs) and Zn(II) over a wide concentration range (0–50 mg·dm−3). The influence of HA properties on Zn(II) complexation was also investigated. Stability constants, quenching degree and complexation capacity were estimated for binding sites found in raw EEM, EEM-PARAFAC and EEM-NMF data using mathematical models. A combination of EEM fluorescence analysis with one of the proposed decomposition methods enabled separation of overlapping binding sites and yielded more accurate calculations of the binding parameters. PARAFAC and NMF processing allowed finding binding sites invisible in a few raw EEM datasets as well as finding totally new maxima attributed to structures of the lowest humification. Decomposed data showed an increase in Zn complexation with an increase in humification, aromaticity and molecular weight of HAs. EEM-PARAFAC analysis also revealed that the most stable compounds were formed by structures containing the highest amounts of nitrogen. The content of oxygen-functional groups did not influence the binding parameters, mainly due to fact of higher competition of metal cation with protons. EEM spectra coupled with NMF and especially PARAFAC processing gave more adequate assessments of interactions as compared to raw EEM data and should be especially recommended for modeling of complexation processes where the fluorescence intensities (FI) changes are weak or where the processes are interfered with by the presence of other fluorophores

Interactions of Zn(II) Ions with Humic Acids Isolated from Various Type of Soils. Effect of pH, Zn Concentrations and Humic Acids Chemical Properties

<div><p>The main aim of this study was the analysis of the interaction between humic acids (HAs) from different soils and Zn(II) ions at wide concentration ranges and at two different pHs, 5 and 7, by using fluorescence and FTIR spectroscopy, as well as potentiometric measurements. The presence of a few areas of HAs structures responsible for Zn(II) complexing was revealed. Complexation at α-sites (low humified structures of low-molecular weight and aromatic polycondensation) and β-sites (weakly humified structures) was stronger at pH 7 than 5. This trend was not observed for γ-sites (structures with linearly-condensed aromatic rings, unsaturated bonds and large molecular weight). The amount of metal complexed at pH5 and 7 by α and γ-structures increased with a decrease in humification and aromaticity of HAs, contrary to β-areas where complexation increased with increasing content of carboxylic groups. The stability of complexes was higher at pH 7 and was the highest for γ-structures. At pH 5, stability decreased with C/N increase for α-areas and -COOH content increase for β-sites; stability increased with humification decrease for γ-structures. The stability of complexes at α and β-areas at pH 7 decreased with a drop in HAs humification. FTIR spectra at pH 5 revealed that the most-humified HAs tended to cause bidentate bridging coordination, while in the case of the least-humified HAs, Zn caused bidentate bridging coordination at low Zn additions and bidentate chelation at the highest Zn concentrations. Low Zn doses at pH 7 caused formation of unidentate complexes while higher Zn doses caused bidentate bridging. Such processes were noticed for HAs characterized by high oxidation degree and high oxygen functional group content; where these were low, HAs displayed bidentate bridging or even bidentate chelation. To summarize, the above studies have showed significant impact of Zn concentration, pH and some properties of HAs on complexation reactions of humic acids with zinc.</p></div

FTIR spectra of HA1 and HA4 at pH 5 and 7 with increasing Zn(II) concentrations (mg dm^-3).

<p>FTIR spectra of HA1 and HA4 at pH 5 and 7 with increasing Zn(II) concentrations (mg dm^-3).</p

Frequencies of the characteristic FTIR bands.

<p>Frequencies of the characteristic FTIR bands.</p

Parameters generated by fitting experimental, fluorescence data of α, β and γ sites to the model of Ryan and Weber [26], i.e., the correlation coefficients of predicted vs. measured fluorescence intensity (R), the stability constants of HA-Zn complexes (logK), and the complexing capacities (C_L) of HAs.

<p>Parameters generated by fitting experimental, fluorescence data of α, β and γ sites to the model of Ryan and Weber [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0153626#pone.0153626.ref026" target="_blank">26</a>], i.e., the correlation coefficients of predicted vs. measured fluorescence intensity (R), the stability constants of HA-Zn complexes (logK), and the complexing capacities (C_L) of HAs.</p

Drop of pH in solutions of the HAs from the values 5 (A) and 7 (B) as function of increasing Zn(II) concentration.

<p>Drop of pH in solutions of the HAs from the values 5 (A) and 7 (B) as function of increasing Zn(II) concentration.</p

Physicochemical description of the soil samples.

<p>Physicochemical description of the soil samples.</p

Chemical properties of isolated humic acids.

<p>Chemical properties of isolated humic acids.</p

Modes of metal binding by carboxylate ligands: A—ionic or uncoordinated forms, B—unidentate complexes, C—bidentate chelates, D—bidentate bridging coordination.

<p>Modes of metal binding by carboxylate ligands: A—ionic or uncoordinated forms, B—unidentate complexes, C—bidentate chelates, D—bidentate bridging coordination.</p

Percentage changes of (ΔCOO-) in HA-Zn complexes at pH 5 and 7 in relation to ΔCOONa in ionic form.

<p>Percentage changes of (ΔCOO-) in HA-Zn complexes at pH 5 and 7 in relation to ΔCOONa in ionic form.</p