Universität zu Köln

Lux, Constantin ORCID: 0000-0002-0860-8542, Ufer, Kristian ORCID: 0009-0004-8236-5157 and Mansfeldt, Tim ORCID: 0000-0002-7557-6827 (2025). Dissolved trivalent manganese in forest soils – interactions of natural organic ligands with manganese oxides. Environmental Science: Processes & Impacts, 27 (9). pp. 2875-2893. Royal Society of Chemistry. ISSN 2050-7887

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Abstract

The presence of dissolved trivalent manganese (Mn 3+ ) in soils has been neglected largely due to its rapid disproportionation. However, natural organic ligands (NOLs) complex and stabilize Mn 3+ by the formation of stable Mn 3+ –NOL complexes. Our objectives were (i) to investigate the influence of NOLs on the dissolution of synthetic Mn oxides, (ii) to perform the speciation analysis of the resulting dissolved total Mn (Mn T ) pool, and (iii) to elucidate the principles governing abiotic formation of Mn 3+ –NOL complexes. NOLs were obtained by extraction (0.001 M CaCl 2 , 24 h) from a terrestrial forest floor Oe horizon (moder-like raw humus). In batch operations, NOLs reacted with either birnessite (containing Mn IV and minor Mn III ) or manganite (containing solely Mn III ). The interaction between NOLs and Mn (hydr)oxides was investigated as a function of time (1–168 h, 7 steps), and pH (3–7, 5 steps). Mn speciation analysis was performed using a spectrophotometric protocol based on kinetic modeling. Results show that the dissolution of the Mn oxides increased with decreasing pH and increasing time. Mean proportions of Mn 3+ –NOL complexes relative to the Mn T pool ranged from 0 to 87 ± 18% (birnessite), and from 0 to 69 ± 14% (manganite). A pH-dependent formation of Mn 3+ –NOL complexes was observed, highlighting pH as the critical parameter. Complex stability decreased with decreasing pH, while an influence of time was only assumed for strongly acidic conditions. Overall, Mn 3+ –NOL complexes appeared to be metastable at pH 3–5 (birnessite) and below pH 7 (manganite). In addition, the formation of Mn 3+ –NOL complexes was influenced by the individual properties of the Mn oxides as they were differing in their average oxidation state, point of zero charge, specific surface area and morphology and structure. These properties influence the formation mechanisms of Mn 3+ –NOL complexes and, consequently, the Mn speciation. For example, they affect NOL adsorption rates and capacities, as well as the transformation and degradation of NOLs. We suggest (i) ligand-promoted non-reductive dissolution, (ii) ligand-promoted reductive dissolution, (iii) H + -promoted dissolution, and (iv) ligand exchange as the four possible abiotic dissolution mechanisms for Mn release and Mn 3+ –NOL complex formation. Following dissolution, either Mn 3+ –NOL complexes were released, or released Mn 2+ and Mn 3+ may be complexed by additional NOLs with and without oxidation. We demonstrate that Mn 3+ –NOL complexes are important, previously underestimated, constituents of the Mn T pool in forest floor solutions and propose that they are a non-negligible component in terrestrial environments.

Item Type:	Article
Creators:	Creators Email ORCID ORCID Put Code Lux, Constantin UNSPECIFIED https://orcid.org/0000-0002-0860-8542 UNSPECIFIED Ufer, Kristian UNSPECIFIED https://orcid.org/0009-0004-8236-5157 UNSPECIFIED Mansfeldt, Tim UNSPECIFIED https://orcid.org/0000-0002-7557-6827 UNSPECIFIED
URN:	urn:nbn:de:hbz:38-811555
Identification Number:	10.1039/D5EM00388A
Journal or Publication Title:	Environmental Science: Processes & Impacts
Volume:	27
Number:	9
Page Range:	pp. 2875-2893
Number of Pages:	19
Date:	1 September 2025
Publisher:	Royal Society of Chemistry
ISSN:	2050-7887
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Faculty of Mathematics and Natural Sciences > Department of Geosciences > Geographisches Institut
Subjects:	Earth sciences Geography and travel
['eprint_fieldname_oa_funders' not defined]:	Publikationsfonds UzK
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/81155