Contribution of ferric iron to light absorption by chromophoric dissolved organic matter

Abstract

This study experimentally determined the contribution of ferric iron (Fe(III)) associated with humic substances (HS) to light absorption by chromophoric dissolved organic matter (CDOM). The associations between Fe(III) and HS (HS‐Fe) were generated by mixing HS standards with Fe(III) in acidic conditions and adjusting pH to 8. HS‐associated Fe was separated from total Fe by filtering (0.7 µm and 0.2 µm filters) and by removing the free Fe ions by cation exchange chromatography. The maximum Fe‐binding capacities (at pH 8) of Suwannee River humic acid, Suwannee River fulvic acid, and Pony Lake (Antarctica) fulvic acid were 13.0, 13.5, and 7.64 µmol Fe [mg C]−1, respectively, indicating that wetland‐derived HS had a higher Fe‐binding capacity than plankton‐derived HS. HS‐associated Fe increased the absorption coefficient of CDOM by up to several fold in the visible range of the spectrum and reduced the spectral slope coefficient of CDOM. The Fe‐induced increase in light absorption was spectrally similar among different HS examined. The Fe‐specific absorption coefficient spectrum for HS‐associated Fe (aλ,Fe*) was calculated from the Fe‐induced increase in light absorption by normalizing it with the concentration of Fe in the HS pool. The aλ,Fe* was adopted in estimation of the contribution of HS‐associated Fe to light absorption by CDOM in 13 circum‐neutral natural waters collected from a spring, 10 major rivers, a lake, and a coastal area. HS‐associated Fe was calculated to be responsible for from 0.6% (Mississippi River) to 56.4% (Löytynlähde spring) of light absorption by CDOM at a wavelength of 410 nm. This study shows that HS‐associated Fe can be an important component in light absorption by CDOM and also influence the spectral slope coefficient of CDOM.

Photochemical Mineralization of Terrigenous DOC to Dissolved Inorganic Carbon in Ocean

“…High φ 330 s correlate positively with CDOM, the specific UV absorption coefficient and the aromaticity of tDOC , which are typically highest for tDOC imported freshly from land to inland waters without earlier exposure to solar radiation [Salonen and Vähätalo, 1994;Xiao et al, 2013Xiao et al, , 2015.…”

“…Aliquots of the same river water samples have been examined earlier (Tables S2, S3) to show that the chromophores of iron can contribute to the tCDOM (several percent of tCDOM in the Congo and Amazon Rivers for example) [Xiao et al, 2013], and that dissolved black carbon (DBC) contributes about 10% to tDOC in all rivers [Jaffé et al, 2013]. Photomineralization leads to the preferential removal of the 12 C-isotope from tDOC [Lalonde et al, 2014] and, together with biodegradation, to a decrease in the contribution of DBC to tDOC to a level comparable to the concentrations measured in the deep ocean [Riedel et al, 2016].…”

Revisiting the disappearance of terrestrial dissolved organic matter in the ocean: a <i>δ</i><sup>13</sup>C study

“…The samples were not refrigerated during transportation (about one week in most cases), but they were stored at 4 • C in the dark upon arrival in Helsinki. The same water samples used here have been also examined for the concentration of dissolved black carbon and the contribution of iron to CDOM Xiao et al, 2013). Additional samples from the St. Lawrence River were collected at the same time as the polyethylene container, filtered onboard (pre-combusted GF/F filters, 0.7 µm nominal pore size), acidified with ultrapure HCl to a pH < 2, and stored in pre-combusted glass vials.…”

Changes in dissolved organic matter quality in a peatland and forest headwater stream as a function of seasonality and hydrologic conditions

“…UV-VIS spectra of all samples were recorded with a Lambda 25 (Perkin Elmer, USA) in the range of 200-800 nm at 0.5 nm resolution. A possible iron interference was excluded as the maximum iron concentration of 500 µg L −1 (and an iron to carbon ratio of about 0.01) was well be-low published critical concentration levels (see Weishaar et al, 2003;Xiao et al, 2013;Poulin et al, 2014). For subsequent fluorescence spectroscopy, samples were diluted to absorption < 0.3 at 254 nm to reduce inner-filter effects.…”