Catalytic effects of photogenerated Fe(II) on the ligand-controlled dissolution of Iron(hydr)oxides by EDTA and DFOB

• Published in: Chemosphere (Oxford) Vol. 263; p. 128188
• Main Authors: Biswakarma, Jagannath, Kang, Kyounglim, Schenkeveld, Walter D.C., Kraemer, Stephan M., Hering, Janet G., Hug, Stephan J.
• Format: Journal Article
• Language: English
• Published: England Elsevier Ltd 01.01.2021
• Subjects: Bioavailability; Carbonatebuffered; Catalysis; Dissolution; Edetic Acid; Ferric Compounds; Ferrous Compounds; Iron; Ligands; Oxidation-Reduction; Oxides; Photochemistry; Solubility; UV-A illumination; Iron; Bioavailability; Carbonatebuffered; Dissolution; Photochemistry; UV-A illumination
• ISSN: 0045-6535; 1879-1298
• DOI: 10.1016/j.chemosphere.2020.128188

Low bioavailability of iron due to poor solubility of iron(hydr)oxides limits the growth of microorganisms and plants in soils and aquatic environments. Previous studies described accelerated dissolution of iron(hydr)oxides under continuous illumination, but did not distinguish between photoreductive dissolution and non-reductive processes in which photogenerated Fe(II) catalyzes ligand-controlled dissolution. Here we show that short illuminations (5–15 min) accelerate the dissolution of iron(hydr)oxides by ligands during subsequent dark periods under anoxic conditions. Suspensions of lepidocrocite (Lp) and goethite (Gt) (1.13 mM) with 50 μM EDTA or DFOB were illuminated with UV-A light of comparable intensity to sunlight (pH 7.0, bicarbonate-CO2 buffered solutions). During illumination, the rate of Fe(II) production was highest with Gt-EDTA; followed by Lp-EDTA > Lp-DFOB > Lp > Gt-DFOB > Gt. Under anoxic conditions, photochemically produced Fe(II) increased dissolution rates during subsequent dark periods by factors of 10–40 and dissolved Fe(III) reached 50 μM with DFOB and EDTA. Under oxic conditions, dissolution rates increased by factors of 3–5 only during illumination. With DFOB dissolved Fe(III) reached 35 μM after 10 h of illumination, while with EDTA it peaked at 15 μM and then decreased to below 2 μM. The observations are explained and discussed based on a kinetic model. The results suggest that in anoxic bottom water of ponds and lakes, or in microenvironments of algal blooms, short illuminations can dramatically increase the bioavailability of iron by Fe(II)-catalyzed ligand-controlled dissolution. In oxic environments, photostable ligands such as DFOB can maintain Fe(III) in solution during extended illumination. [Display omitted] •Photogenerated Fe(II) accelerates ligand-controlled dissolution of Fe(III)(hydr)oxides.•Dissolution with DFOB or EDTA is accelerated up to 40-fold under anoxic conditions.•Short illuminations (5–15 min) lead to continued dissolution in the dark.•Even short exposures to sunlight might lead to increased bioavailability of iron.