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

Abstract

The photogeneration of reactive species (RS) from dissolved organic matter (DOM) exhibits a great impact on the attenuation of pollutants in natural waters. However, the effect of metal ions on the photogeneration of excited triplet-state DOM (3DOM*), singlet oxygen (1O2), and hydroxyl radical (•OH) by effluent organic matter (EfOM), fulvic acid (FA), and humic acid (HA) is poorly understood. Here, we provided the first evidence that the quenching of 3DOM* was positively correlated with the complexation capacity of metal ions with DOM. Generally, the paramagnetic metal ions (Cr3+, Mn2+, Fe3+, and Cu2+) exhibited higher conditional stability constants (log K ML) with DOM and stronger inhibition for RS than the others (Mg2+, Ca2+, Al3+, and Zn2+). For DOM of different sources, the metal binding capacity increased in the order of EfOM < HA < FA and the humic substances were more susceptible to metal ions. The inhibition was attributed to both static and dynamic quenching of 3DOM* by metal ions. The dynamic quenching rate constants of metal ions for 3DOM* were estimated as ∼109 M–1 s–1, which was positively related to the corresponding log K ML. These findings highlight crucial links between metal–DOM complexation and 3DOM* quenching and, consequently, the inhibition of RS.

Relating natural organic matter conformation, metal complexation, and photophysics

“…This analysis does not make assumptions about a linear relationship between the fluorescence intensity change and the metalbound ligand concentration; rather, it estimates the contribution of unquenched fluorescence. 75,76 Although there are many different modifications of the Stern-Volmer analysis, 55,76 the modification The ability of metal ions, particularly Cu 2+ and Fe 3+ , to complex with NOM and the resulting complexation parameters have been reported previously, 18,21,24,28,33,35,45,48,67,68,[77][78][79][80][81][82][83] and qualitatively, our results fit reasonably well within the range of values reported for each metal ion. Supplementary Table S3 reports a range of logK ML values at similar pHs.…”

“…Further studies using complimentary techniques are needed to validate these findings, given the limitations of this study. It has been shown previously that NOM reactivity, via the formation of singlet oxygen, decreases in the presence of metals, 56,68,92 so the results of this study show that this decrease is not due to conformational changes affecting the ability of ground state oxygen to quench the excited triplet. We also showed that the analysis performed in this study, although very simplified, is applicable to both fluorescence quenching and enhancement interactions.…”

“…53 These changes in the conformation of NOM due to metal complexation have been suggested as the reason why the formation kinetics of photochemically produced reactive intermediates (PPRI) change in the presence of metal ions. 56,68,69 Fluorescence anisotropy has previously been used to look at how the conformation of NOM changes due to the addition of metals, as well as changes to the pH and ionic strength. 53,56,67,70 Fluorescence anisotropy measurements use polarized light to excite a fluorophore and interrogate the polarization state of emitted light, from which the relative speed of molecular rotation can be extracted.…”

Binding of Calcium and Magnesium Ions to Terrestrial Chromophoric Dissolved Organic Matter (CDOM): A Combination of Steady-State and Time-Resolved Fluorescence Study

“…Metal ions tend to bind to NOM, mainly through the phenolic and carboxylic functional groups [6]. The binding of metal ions to NOM not only altered the bioavailability of metal ions, but also affected the photophysical and photochemical properties of NOM [5,7,8].…”

New insights into mechanisms of sunlight-mediated high-temperature accelerated diurnal production-degradation of fluorescent DOM in lake waters

“…Conversely, autochthonous aquatic DOM includes extracellular polymeric substances (EPS), autochthonous humic-like substances (AHLS), protein-like substances (PLS), aromatic amino acids of various nature, etc (Shammi et al 2017a(Shammi et al , 2017b(Shammi et al , 2017cGuidi et al, 2016;Zhang et al, 2009;Mostofa et al 2013;Yamashita and Tanoue, 2003). These substances occur as major fractions, even if very diluted, in surface waters of lakes, estuaries and oceans 7 , thus they control many important biogeochemical functions and processes in aquatic systems, including cycling of C (Zark and Dittmar, 2018;Guidi et al, 2016;Amon and Benner, 1994), N (Yue et al, 2018;Liang et al 2019), P (Guidi et al, 2016;Carpenter et al, 1998;Parsons et al, 2017) and trace elements (Wan et al, 2019;Helms et al, 2013), as well as nutrient changes associated with diurnal sunlight-induced planktonic photosynthesis (Gao et al, 2010;Jung et al, 2013;Segschneider and Bendtsen., 2013).…”