Abstract
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.
Highlights
Humic acids (HAs)—principal component of humic substances, belong to complex and heterogeneous mixtures of polydispersed dark brown to black organic substances
On the basis of the above issues, the main aim of this paper is to study interaction mechanisms of Zn ions with HAs isolated from different soils, focusing on the influence of pH and Zn ion concentration, as well as various chemical properties of HAs such as the content of functional groups, electron donor atoms, aromaticity and degree of humification on interactions with Zn ions
The Cation exchange capacity (CEC) changed in a similar way as the Corg., which can indicate a dominant role of organic colloids
Summary
Humic acids (HAs)—principal component of humic substances, belong to complex and heterogeneous mixtures of polydispersed dark brown to black organic substances. A large sorption capacity in the range of 300–1400 meq 100 g-1 makes HAs one of the most charged substances among naturally occurring polyelectrolytes (mineral parts of the soil have from 2 to 30 times lower sorption capacity) Because of this feature, HAs demonstrate a great impact on soil-buffering capacity [1,2,3] and especially the form, bioavailability and retention abilities of metals in soils [4,5,6,7]. High reactivity of HAs has an impact on transport and degradation of both natural and anthropogenic organic compounds Such molecules as lignin, its transformation products, polysaccharides, proteins, lipids, nucleic acids remain in continuous equilibrium with fraction of humic substances during mineralization and humification processes. The principal differences in HA structure result from a variable atom composition, e.g., C, O, H and N, which form structures of diverse configurations with various molecular weights, aromatic rings (hydroxyphenol derivatives), heterocycled and condensed rings with heteroatoms, side aliphatic chains and functional groups
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