Iron and pH Regulating the Photochemical Mineralization of Dissolved Organic Carbon

Yufei Gu; Anssi Lensu; Siiri Perämäki; Anne Ojala; Anssi V Vähätalo

doi:10.1021/acsomega.7b00453

Iron and pH Regulating the Photochemical Mineralization of Dissolved Organic Carbon

ACS Omega. 2017 May 9;2(5):1905-1914. doi: 10.1021/acsomega.7b00453. eCollection 2017 May 31.

Authors

Yufei Gu¹, Anssi Lensu¹, Siiri Perämäki¹, Anne Ojala^{2

3}, Anssi V Vähätalo¹

Affiliations

¹ Department of Biological and Environmental Science and Department of Chemistry, University of Jyväskylä, P.O. Box 35, FI-40014 Jyväskylä, Finland.
² Department of Forest Sciences, University of Helsinki, P.O. Box 27, FI-00014 Helsinki, Finland.
³ Department of Environmental Sciences, University of Helsinki, P.O. Box 65, FI-00014 Helsinki, Finland.

Abstract

Solar radiation mineralizes dissolved organic matter (DOM) to dissolved inorganic carbon through photochemical reactions (DIC photoproduction) that are influenced by iron (Fe) and pH. This study addressed as to what extent Fe contributes to the optical properties of the chromophoric DOM (CDOM) and DIC photoproduction at different pH values. We created the associations of Fe and DOM (Fe-DOM) that cover the range of loadings of Fe on DOM and pH values found in freshwaters. The introduced Fe enhanced the light absorption by CDOM independent of pH. Simulated solar irradiation decreased the light absorption by CDOM (i.e., caused photobleaching). Fe raised the rate of photobleaching and steepened the spectral slopes of CDOM in low pH but resisted the slope steepening in neutral to alkaline pH. The combination of a low pH (down to pH 4) and high Fe loading on DOM (up to 3.5 μmol mg DOM^-1) increased the DIC photoproduction rate and the apparent quantum yields for DIC photoproduction up to 7-fold compared to the corresponding experiments at pH >6 or without Fe. The action spectrum for DIC photoproduction shifted toward the visible spectrum range at low pH in the presence of Fe. Our results demonstrated that Fe can contribute to DIC photoproduction by up to 86% and produce DIC even at the visible spectrum range in acidic waters. However, the stimulatory effect of Fe is negligible at pH >7.