Photochemical Reactivity of Humic Substances in an Aquatic System Revealed by Excitation-Emission Matrix Fluorescence

Abstract

The photochemical reactivity of humic substances plays a critical role in the global carbon cycle, and influences the toxicity, mobility, and bioavailability of contaminants by altering their molecular structure and the mineralization of organic carbon to CO2. Here, we examined the simulated irradiation process of Chinese standard fulvic acid (FA) and humic acid (HA) by using excitation-emission matrix fluorescence combined with fluorescence regional integration (FRI), parallel factor (PARAFAC) analysis, and kinetic models. Humic-like and fulvic-like materials were the main materials (constituting more than 90%) of both FA and HA, according to the FRI analysis. Four components were identified by the PARAFAC analysis: fulvic-like components composed of both carboxylic-like and phenolic-like chromophores (C1), terrestrial humic-like components primarily composed of carboxylic-like chromophores (C2), microbial humic-like overwhelming composed of phenolic-like fluorophores (C3), and protein-like components (C4). After irradiation for 72 h, the maximum fluorescence intensity (Fmax) of C1 and C2 of FA was reduced to 36.01–58.34%, while the Fmax of C3 of both FA and HA also decreased to 0–9.63%. By contrast, for HA, the Fmax of its C1 and C2 increased to 236.18–294.77% when irradiated for 72 h due to greater aromaticity and photorefractive tendencies. The first-order kinetic model (R2 = 0.908–0.990) fitted better than zero-order kinetic model (R2 = 0–0.754) for the C1, C2, and C3, of both FA and HA, during their photochemical reactivity. The photodegradation rate constant (k1) of C1 had values (0.105 for FA; 0.154 for HA) that surpassed those of C2 (0.059 for FA, 0.079 for HA) and C3 (0.079 for both FA and HA) based on the first-order kinetic model. The half-life times of C1, C2, and C3 ranged from 6.61–11.77 h to 4.50–8.81 h for FA and HA, respectively. Combining an excitation-emission matrix with FRI and PARAFAC analyses is a powerful approach for elucidating changes to humic substances during their irradiation, which is helpful for predicting the environmental toxicity of contaminants in natural ecosystems.

Second-Order Scattering Quenching in Fluorescence Spectra of Natural Humates as a Tracer of Formation Stable Supramolecular System for the Delivery of Poorly Soluble Antiviral Drugs on the Example of Mangiferin and Favipiravir

“…However, to apply this approach to the studied transport system based on the humic complex, it is necessary to characterize the fluorescent properties of the solvent itself and its aqueous dilutions, as well as pure water used. It is known that humic substances, as compounds with a mostly polyphenolic structure, are capable of fluorescing upon excitation in the range from 380 to 480 nm [51][52][53]. The intensity of the emission band in the spectrum in the case of humates depends on the pH of the medium: most phenols fluoresce in a neutral or acidic medium, while in an alkaline medium, deprotonation of phenolic hydroxyls leads to the formation of nonfluorescent phenolate ions.…”

Study on anaerobic fermentation of waste activated sludge to produce volatile fatty acids by thermal–rhamnolipid treatment

“…Based on the excitation and emission wavelength ranges, the fluorescence spectrum can be divided into two regions: region I (Ex: 250–550 nm; Em: 290–380 nm) and region II (Ex: 250–550 nm; Em: 380–602 nm). Region I mainly represents soluble microbial metabolites, while region II represents humic acids 24‐26 . The components present in region I include proteins, coenzymes, small‐molecule organic acids, pigments, etc 27 .…”

“…Region I mainly represents soluble microbial metabolites, while region II represents humic acids. [24][25][26] The components present in region I include proteins, coenzymes, small-molecule organic acids, pigments, etc. 27 Region II mainly includes humic acid, polycyclic aromatic hydrocarbons and fulvic acid in the visible region, as well as other organic substances with high molecular weight and high degree of aromatization.…”

Study on the effect of alkali on the production of volatile fatty acids from rhamnolipid‐alkali‐thermal pretreated waste activated sludge

“…The scanning range of Ex is 250–550 nm, the scanning range of Em is 300–600 nm, and the scanning speed is 1200 nm/min. The data were analyzed by FRI (Fluorescence Regional Integration) and PARAFAC (Parallel factor analysis) 23 …”

“…The data were analyzed by FRI (Fluorescence Regional Integration) and PARAFAC (Parallel factor analysis). 23…”