Humic Substances - Excited States, Quenching by Metal Ions, and Photosensitized Degradation of Chlorophenols

Abstract

Alteration of photochemical properties of humic substances (HS) by the addition of metals abundant in natural waters has been investigated. Excited HS states have been characterized by fluorescence spectra and by laser kinetic spectroscopy. The effect of Cu2+, Fe2+, and Mn2+ has been explained in terms of static quenching of the HS excited states. In agreement with this, the metal ions were found to inhibit HS sensitized photooxidative degradation of chlorophenols.

Mechanistic Insight into the Effect of Metal Ions on Photogeneration of Reactive Species from Dissolved Organic Matter

“…As shown in Scheme 1, the mechanism of triplet quenching initially involves the formation of encounter complexes between 3 DOM* and metal ions, likely followed by electron transfer and energy transfer within the complex or radiationless transitions to the ground state. 22,30,31 The metal ions with strong DOM binding capacity have a high affinity to the excited triplet states, thereby leading to a strong quenching effect. 29,30 According to the dynamic quenching rate constants of 3 DOM* by metal ions, the theoretical R 1 O 2 values in the presence of metal ions were corrected with eq 7.…”

“…20,21 These metal ions, especially the transition metal ions, readily tend to form complexes with DOM, thereby affecting its photoreactivity. 22 Previous studies showed that the paramagnetic metal ions markedly inhibit the photodegradation of organophosphorus pesticides, chlorophenols, and β-blockers sensitized by HS. 22−24 Accordingly, the co-existent metal ions exhibit a significant impact on the DOM-sensitized photodegradation of organic pollutants.…”

“…DOM is a polydisperse mixture of different types of functional groups, mainly including carboxyl, phenolic, alcoholic, and ketonic oxygen-containing moieties, which can act as binding sites for cations . The metal ions, including Mg 2+ , Ca 2+ , Al 3+ , Cr 3+ , Mn 2+ , Fe 3+ , Cu 2+ , and Zn 2+ , are ubiquitous in both natural waters and effluents of WWTPs. , These metal ions, especially the transition metal ions, readily tend to form complexes with DOM, thereby affecting its photoreactivity . Previous studies showed that the paramagnetic metal ions markedly inhibit the photodegradation of organophosphorus pesticides, chlorophenols, and β-blockers sensitized by HS. − Accordingly, the co-existent metal ions exhibit a significant impact on the DOM-sensitized photodegradation of organic pollutants.…”

“…The metal ions, including Mg 2+ , Ca 2+ , Al 3+ , Cr 3+ , Mn 2+ , Fe 3+ , Cu 2+ , and Zn 2+ , are ubiquitous in both natural waters and effluents of WWTPs. , These metal ions, especially the transition metal ions, readily tend to form complexes with DOM, thereby affecting its photoreactivity . Previous studies showed that the paramagnetic metal ions markedly inhibit the photodegradation of organophosphorus pesticides, chlorophenols, and β-blockers sensitized by HS. − Accordingly, the co-existent metal ions exhibit a significant impact on the DOM-sensitized photodegradation of organic pollutants. It is thus essential to investigate the effect of metal ions on the photoproduction of 3 DOM*, 1 O 2 , and • OH in both EfOM and HS systems.…”

Fulvic acid-mediated phototransformation of mecoprop. A pH-dependent reaction

“…A lot of compounds chosen among the most pertinent class of pollutants have been photolyzed in conditions relevant to those prevailing in surface waters. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] In many cases, humicmediated phototransformation significantly contributed to the abiotic transformation pathway.…”