Experimental and modeling study of proton and copper binding properties onto fulvic acid fractions using spectroscopic techniques combined with two-dimensional correlation analysis

Tingting Li; Fanhao Song; Jin Zhang; Shijie Tian; Nannan Huang; Baoshan Xing; Yingchen Bai

doi:10.1016/j.envpol.2019.113465

Experimental and modeling study of proton and copper binding properties onto fulvic acid fractions using spectroscopic techniques combined with two-dimensional correlation analysis

Environ Pollut. 2020 Jan:256:113465. doi: 10.1016/j.envpol.2019.113465. Epub 2019 Oct 25.

Authors

Tingting Li¹, Fanhao Song², Jin Zhang¹, Shijie Tian¹, Nannan Huang¹, Baoshan Xing³, Yingchen Bai⁴

Affiliations

¹ State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing, 10012, China.
² State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing, 10012, China; College of Water Sciences, Beijing Normal University, Beijing, 100875, China.
³ Stockbridge School of Agriculture, University of Massachusetts, Amherst, MA, 01003, USA.
⁴ State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing, 10012, China. Electronic address: yingchenbai@163.com.

PMID: 31679870
DOI: 10.1016/j.envpol.2019.113465

Abstract

Fulvic acid (FA) significantly influences the bioavailability and fate of heavy metals in environments, while its acid-base characters and metal binding processes are still unclear. Here, spectroscopic techniques combined with multiple models (e.g., NICA-Donnan model) and two-dimensional correlation spectroscopy (2D COS) were applied to explore the proton and copper binding properties of FA sub-fractions (FA₃-FA₁₃). The charge densities, average contents of carboxylic and phenolic groups, average dissociation constants pKa₁ and pKa₂ of sub-fractions ranged 0-16 meq∙g∙C^-1, 5.03-9.58 meq∙g∙C^-1, 2.52-4.67 meq∙g∙C^-1, 4.15-4.33 and 8.52-9.72, respectively. FA sub-fractions had a relatively narrow distribution of carboxyl group and a broad distribution of phenolic group. FA sub-fractions also exhibited roughly two phenolic hydroxyl groups per every 1-3 phenyl rings. Differential absorbance spectra (DAS) derived Gaussian bands were associated to the inter-chromophore interactions, the changes of molecular conformations and functional groups with copper addition. Differential spectra slopes (DSlope_{275-295&325-375}) were more significant with higher copper concentration and copper amounts bonded to carboxylic groups. UV-Vis and fluorescence spectra with 2D heterospectral COS revealed the copper binding heterogeneities and sequential orders of chromophores and fluorophores, quantitatively confirming by the order of conditional stability constants (log K_Cu: 4.64-5.56). Salicylic-/polyhydroxyphenolic, hydroxyl and amino groups were strongly associated to the basic units for fluorophores. Sequential changes followed the order of humic-like→fulvic-like materials for FA₃/FA₅, humic-like→fulvic-like→tryptophan-like materials for FA₇, and humic-like→tryptophan-like→fulvic-like→tyrosine-like materials for FA₉/FA₁₃. Spectroscopic techniques combined with various models (especially for 2D COS) are beneficial to elucidate the binding heterogeneity and sensitivity for metal-organic matters at the functional group level.

Keywords: Asynchronous maps; Deconvolution; Spectral slope; Synchronous maps; Titration.

MeSH terms

Benzopyrans / analysis*
Copper / analysis*
Copper / chemistry
Humic Substances / analysis
Metals, Heavy
Models, Chemical*
Models, Theoretical
Protons
Spectrometry, Fluorescence

Substances

Benzopyrans
Humic Substances
Metals, Heavy
Protons
Copper
fulvic acid