Complexation and reduction of iron by phenolic substances: Implications for transport of dissolved Fe from peatlands to aquatic ecosystems and global iron cycling

Abstract

Iron (Fe) is among the limiting nutrients in marine environments and mainly originates from terrigenous sources. Dissolved organic carbon (DOC), an important natural organic ligand, plays a key role in the dissolution and transport of terrestrial iron. Peat-derived DOC contains a large number of phenolics, and a systematic study of the geochemical interaction between DOC and Fe will contribute to the understanding of the global carbon and iron cycles. Phenolics, including phenolic acids, are secondary metabolites from plants and fungi. These compounds possess an aromatic ring bearing one or more hydroxyl groups and their structures may range from that of a simple phenolic molecule to that of a complex high-molecular weight polymer. Despite the extensive literature describing the biological effects of phenolic acids, the effect of phenolics on the iron cycle in peatlands is not known very well. Zoige peatland, the world's largest plateau peat wetland, was studied through field investigation and laboratory simulated experiments. The content of phenolics, filterable Fe and some physicochemical parameters in the rivers of the area were investigated. The waters are generally alkaline, with an average pH of 8.04, and aqueous Fe(III) and Fe(II) concentrations are higher than most of the common rivers in the world. The results of our experiments suggested that the high iron solubility was closely related to the phenolic content of the DOC. In particular, phenolics with catechol or galloy moieties forming complexes with iron and significantly impede the oxidation of complexed Fe(II). In addition, some phenolic substances are able to reduce ferric iron to ferrous iron. The combination of complexation and reduction in Fe by phenolics effectively increased the proportion of ferrous iron to total iron and slowed the oxidation and precipitation in the waters, maintaining a high content of “soluble iron” in the peatland. This mechanism has great implications for the effective transport of soluble iron from peatlands to aquatic ecosystems and the global coupled iron and carbon cycles.