Lux Constantin, Ufer Kristian, Mansfeldt Tim
Institute of Geography, Faculty of Mathematics and Natural Sciences, Department of Geosciences, University of Cologne, Albertus-Magnus-Platz, D-50923 Cologne, Germany.
Federal Institute for Geosciences and Natural Resources, Stilleweg 2, D-30655 Hannover, Germany.
Environ Sci Process Impacts. 2025 Aug 22. doi: 10.1039/d5em00388a.
The presence of dissolved trivalent manganese (Mn) in soils has been neglected largely due to its rapid disproportionation. However, natural organic ligands (NOLs) complex and stabilize Mn by the formation of stable Mn-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) pool, and (iii) to elucidate the principles governing abiotic formation of Mn-NOL complexes. NOLs were obtained by extraction (0.001 M CaCl, 24 h) from a terrestrial forest floor Oe horizon (moder-like raw humus). In batch operations, NOLs reacted with either birnessite (containing Mn and minor Mn) or manganite (containing solely Mn). 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-NOL complexes relative to the Mn pool ranged from 0 to 87 ± 18% (birnessite), and from 0 to 69 ± 14% (manganite). A pH-dependent formation of Mn-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-NOL complexes appeared to be metastable at pH 3-5 (birnessite) and below pH 7 (manganite). In addition, the formation of Mn-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-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-NOL complex formation. Following dissolution, either Mn-NOL complexes were released, or released Mn and Mn may be complexed by additional NOLs with and without oxidation. We demonstrate that Mn-NOL complexes are important, previously underestimated, constituents of the Mn pool in forest floor solutions and propose that they are a non-negligible component in terrestrial environments.
土壤中溶解态三价锰（Mn）的存在很大程度上因快速歧化反应而被忽视。然而，天然有机配体（NOLs）通过形成稳定的Mn-NOL络合物来络合并稳定锰。我们的目标是：（i）研究NOLs对合成锰氧化物溶解的影响；（ii）对所得溶解态总锰（Mn）库进行形态分析；（iii）阐明控制Mn-NOL络合物非生物形成的原理。通过从陆地森林地表Oe层（类似中度的原始腐殖质）中用0.001 M氯化钙萃取24小时获得NOLs。在分批操作中，NOLs与水钠锰矿（含锰和少量锰）或锰矿（仅含锰）反应。研究了NOLs与Mn（氢）氧化物之间的相互作用随时间（1 - 168小时，7个时间点）和pH值（3 - 7，5个pH值点）的变化。使用基于动力学模型的分光光度法进行Mn形态分析。结果表明，锰氧化物的溶解随pH值降低和时间增加而增加。相对于锰库，Mn-NOL络合物的平均比例在水钠锰矿中为0至87 ± 18%，在锰矿中为0至69 ± 14%。观察到Mn-NOL络合物的形成与pH值有关，突出了pH值作为关键参数。络合物稳定性随pH值降低而降低，而时间的影响仅在强酸性条件下才被认为存在。总体而言，Mn-NOL络合物在pH值为3 - 5（水钠锰矿）和pH值低于7（锰矿）时似乎是亚稳的。此外，Mn-NOL络合物的形成受锰氧化物的个体性质影响，因为它们在平均氧化态、零电荷点、比表面积以及形态和结构方面存在差异。这些性质影响Mn-NOL络合物的形成机制，进而影响锰的形态。例如，它们影响NOL的吸附速率和容量，以及NOL的转化和降解。我们提出（i）配体促进的非还原溶解、（ii）配体促进的还原溶解、（iii）H促进的溶解和（iv）配体交换作为锰释放和Mn-NOL络合物形成的四种可能的非生物溶解机制。溶解后，要么释放出Mn-NOL络合物，要么释放出的锰和锰可能在有或无氧化的情况下被额外的NOLs络合。我们证明Mn-NOL络合物是森林地表溶液中锰库的重要组成部分，此前被低估，并提出它们是陆地环境中不可忽视的成分。
