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Iron oxide minerals are ubiquitous in soils, sediments, and aquatic systems and influence the fate and availability of trace metals. Ferrihydrite is a common iron oxide of nanoparticulate size and poor crystallinity, serving as a thermodynamically unstable precursor to more crystalline phases. While aging induces such phase transformations, these are accelerated by the presence of dissolved Fe(II). However, the impact of trace metals on Fe(II)-catalyzed ferrihydrite phase transformations at ambient temperatures and the associated effects on trace metal speciation has seen limited study. In the present work, phase transformations of ferrihydrite that contains the trace metal zinc in its structure were investigated during aging at ambient temperature in the presence of two different Fe(II) concentrations at pH 7. X-ray diffraction reveals that low Fe(II) concentration (0.2 mM) generates hematite plus minor lepidocrocite, whereas high Fe(II) concentration (1.0 mM) promotes the production of a magnetite-lepidocrocite mixture. In both cases, a substantial fraction of ferrihydrite remains after 12 days. In contrast, Zn-free ferrihydrite forms primarily lepidocrocite and goethite in the presence of 0.2 mM Fe(II), with minor hematite and a trace of ferrihydrite remaining. For 1.0 mM Fe(II), magnetite, goethite, and lepidocrocite form when Zn is absent, leaving no residual ferrihydrite. Transformations of Zn-ferrihydrite produce a transient release of zinc to solution, but this is nearly quantitatively removed into the mineral products after 1 hour. Extended X-ray absorption fine structure spectroscopy suggests that zinc partitions into the newly formed phases, with a shift from tetrahedral to a mixture of tetrahedral and octahedral coordination in the 0.2 mM Fe(II) system and taking on a spinel-like local structure in the 1.0 mM Fe(II) reaction products. This work indicates that substituting elements in ferrihydrite may play a key role in promoting the formation of hematite in low temperature systems, such as soils or sediments. In addition, the retention of zinc in the products of ferrihydrite phase transformation shows that trace metal micronutrients and contaminants may not be mobilized under circumneutral conditions despite the formation of more crystalline iron oxides. Furthermore, mass balance requires that the abundance and isotopic composition of iron oxide-associated zinc, and possibly other trace metals, in the rock record may be retained during diagenetic phase transformations of ferrihydrite if catalyzed by dissolved Fe(II).

Trace metal are essential for microbially-mediated biogeochemical processes occurring in anoxic wetland soils and stream bed sediments, but low availability of these elements may inhibit anaerobic element cycling and transformations. Solid-phase speciation is likely a critical control on trace metal availability but has seen limited study in anoxic systems having concentrations similar to geological background levels, where metal limitations may be most prevalent. We have investigated trace metal concentrations and solid-phase speciation in three freshwater subsurface aquatic systems: marsh wetland soils, riparian wetland soils, and the sediments of a streambed. These systems displayed low solid-phase trace metal concentrations, generally at or below geological background levels, which generally followed the trend Zn > Cu ≈ Ni > Co and showed no correlation with major element compositions. All soils and sediments were dominated by quartz but varied in clay mineralogy as well as the organic matter, total sulfur, and total iron contents. X-ray absorption near-edge structure (XANES) spectroscopy shows that sulfur speciation in both wetlands is dominated by organic sulfur. Elemental sulfur and iron sulfides together made up <25% of the sulfur in the wetland soils, but the distribution between inorganic and organic forms was reversed in the stream sediments. Ferrous and ferric iron in clay minerals were common species identified by both XANES and extended X-ray absorption fine structure (EXAFS) spectroscopies at all sites. Iron(III) oxides were substantial components in all but the marsh wetland soils. Quantitative analysis of copper, nickel, and zinc XANES spectra revealed similar metal speciation across all sites. Copper speciation was dominated by sulfides, adsorbed species, and minor amounts of copper bound to organic matter; no metallic copper was detected. Nickel speciation also varied little and was dominated by nickel in clay mineral octahedral sheets and nickel sulfide, with adsorbed species also present. Zinc speciation was slightly more varied, with the marsh wetland soils and stream bed sediments containing adsorbed species, zinc associated with clay mineral structures, and zinc bound to reduced sulfur groups on organic matter, whereas the riparian wetland soils lacked clay-associated zinc but contained zinc sulfide. Trace metals bound to reduced sulfur occurred at every site, with a greater sulfur-bound fraction for copper. The fractional abundance of sulfur-bound species showed no relationship with soil or sediment total sulfur content, which varied by two orders of magnitude. More broadly, the observations in this study suggest that trace metal speciation in freshwater wetland soils and stream sediments is consistently dominated by a small set of recurring components which are distinct for each metal. This may represent a general geochemical phenomenon in anoxic soils and sediments containing trace metals at background concentrations (as low as 3 µg g-1) that was not predicted from systems that are contaminated with or naturally-enriched in copper, nickel, or zinc.

The solid-phase speciation of metals plays a crucial role in determining their bioavailability in aquatic ecosystems. The interactions of metals with metal (hydr)oxides, sulfides, clays, and organic matter govern their availability in subsurface environments. Our study combined solid−water partitioning experiments under anaerobic conditions (97% N 2 and 3% H 2 ) for 24 h and synchrotron-based spectroscopy to determine the factors controlling copper, nickel, and zinc availability in soils and sediments from three different natural aquatic systems. While soils and sediments from all three systems strongly bound added trace metals, there were substantial differences in trace metal partitioning trends between different sites, especially for Cu. There was no distinct correlation between trace metal partitioning and the total organic matter, iron, and sulfur contents of the samples. X-ray absorption spectroscopy indicated that the speciation of the freshly added metals taken up by the solids differs substantially from the speciation of the metals originally present in unamended samples. Cu sulfides dominated speciation at low loadings (1 μmol Cu/g solid), whereas complexation to thiol groups and formation of metallic Cu governed speciation at high loadings (10 μmol Cu/g solid). For Ni and Zn, adsorption to mineral surfaces and organic matter governed their speciation in materials from most sites at low (1 μmol Zn/g solid) and high (10 μmol Zn/g solid) loadings. Under the evaluated time scale (24 h), the background speciation of metals in natural aquatic systems is a poor predictor of the speciation and lability of metals introduced from anthropogenic or natural processes. Our findings imply that geochemical processes controlling trace metal speciation may vary considerably with metal loading in different natural systems.

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