title: Changes in optical properties caused by UV-irradiation of aquatic humic substances from the amazon river basin: seasonal variability evaluation.
abstract: Aquatic humic substances (AHS) isolated from two characteristic seasons of the Negro river, winter and summer corresponding to floody and dry periods, were structurally characterized by 13C nuclear magnetic ressonance. Subsequently, AHS aqueous solutions were irradiated with a polychromatic lamp (290-475 nm) and monitored by its total organic carbon (TOC) content, ultraviolet-visible (UV-vis) absorbance, fluorescence, and Fourier transformed infrared spectroscopy (FTIR). As a result, a photobleaching up to 80% after irradiation of 48 h was observed. Conformational rearrangements and formation of low molecular complexity structures were formed during the irradiation, as deduced from the pH decrement and the fluorescence shifting to lower wavelengths. Additionally a significant mineralization with the formation of CO2, CO, and inorganic carbon compounds was registered, as assumed by TOC losses of up to 70%. The differences in photodegradation between samples expressed by photobleaching efficiency were enhanced in the summer sample and related to its elevated aromatic content. Aromatic structures are assumed to have high autosensitization capacity effects mediated by the free radical generation from quinone and phenolic moieties.
authors.affiliation: Embrapa Agricultural Instrumentation, P.O. Box. 741, 13560-970, São Carlos-SP, Brazil.
authors.forename: Ursula Fabiola
authors.initials: UF
authors.lastname: Rodríguez-Zúñiga
authors.forename: Débora Marcondes Bastos Pereira
authors.initials: DM
authors.lastname: Milori
authors.forename: Wilson Tadeu Lopes
authors.initials: WT
authors.lastname: da Silva
authors.forename: Ladislau
authors.initials: L
authors.lastname: Martin-Neto
authors.forename: Luciana Camargo
authors.initials: LC
authors.lastname: Oliveira
authors.forename: Julio Cesar
authors.initials: JC
authors.lastname: Rocha
mesh_terms: D002244:Carbon / Q000032:analysis; D006812:Humic Substances / Q000528:radiation effects*; D055095:Optics and Photonics; D045483:Rivers; D012621:Seasons; D017550:Spectroscopy, Fourier Transform Infrared; D014466:Ultraviolet Rays; D014874:Water Pollutants, Chemical / Q000528:radiation effects*
chemical_list: D006812:Humic Substances; D014874:Water Pollutants, Chemical; D002244:Carbon