Ferrihydrite mineral transformations in the presence of Fe(II) and organic ligands
- Soil Chemistry Group, Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, Switzerland (laurel.thomas@usys.ethz.ch)
In soils and sediments, poorly-crystalline, short-range order (SRO) iron minerals constitute one of the most abundant and reactive components. With high surface areas, SRO minerals like ferrihydrite (Fe10O14(OH)2+mH2O) influence the biogeochemical cycling of trace elements and nutrients, particularly in redox dynamic environments. While under oxic conditions SRO iron mineral adsorption capacity is high, in the absence of O2, FeIII acts as an electron acceptor during microbial respiration. Electron transfer induces transformations in pure iron minerals, impacting the release and re-distribution of SRO-associated trace elements and nutrients.
In nature, however, pure SRO iron minerals rarely form. Rather, the ubiquitous presence of natural organic matter (OM) in soils and sediments promotes the formation mineral-organic associations. Coprecipitation of ferrihydrite with OM decreases particle size and alters the mineral susceptibility towards microbial reduction. Thus, under reducing conditions, an increased rate and extent of mineral transformation could be expected for OM-associated ferrihydrite. However, in the presence of abiotic reductants, mineral transformation rates and extents in OM-associated ferrihydrite are markedly inhibited when compared to that of a pure ferrihydrite. Using polygalacturonic acid (PGA) as a proxy for acid carbohydrate fraction found in exopolymeric substances, we reacted ferrihydrite-PGA coprecipitates of varying C:Fe molar ratios (0-2.5) with ferrous Fe (Fe(II), 0.5-5.0 mM) at neutral pH for up to 5 weeks. Through a combination of XRD and 57Fe Mössbauer spectroscopy, we showed that at all Fe(II) concentrations, the kinetics and extent of mineral transformation decreased with increasing C content of the coprecipitates. Similarly, ferrihydrite-OM coprecipitates comprising PGA, citric acid (CA), or galacturonic acid (GA) of similar C:Fe molar ratios (~0.6) also showed inhibited mineral transformations compared to a pure ferrihydrite, whereby the extent of inhibition of mineral transformations followed the order GA>>CA>PGA. In addition, electron microscopy imaging showed that the crystal morphology of the secondary mineral phases varied with the varying chemical structure of the coprecipitating organic ligands. Despite this, applications of stable Fe isotope tracers revealed that all OM-associated ferrihydrite actively partook in iron atom exchange, suggesting that the presence of OM inhibited crystal growth of more crystalline phases, therefore again leading to SRO phases during iron atom exchange. Collectively, the stabilization of high surface-area ferrihydrite under reducing conditions via recrystallization has implications for the release and re-distribution of ferrihydrite-associated trace elements and nutrients in redox-dynamic environments.
How to cite: ThomasArrigo, L. K. and Kretzschmar, R.: Ferrihydrite mineral transformations in the presence of Fe(II) and organic ligands, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21278, https://doi.org/10.5194/egusphere-egu2020-21278, 2020
Comments on the display
AC: Author Comment | CC: Community Comment | Report abuse
Hi Laurel, I like this work a lot. For NOM inhibition of ferrihydrite transformation, you mentioned in your paper that it inhibits the Ostwald ripening or oriented aggregation, which I agree. From that perspective, do you think if we have OMs with a smaller molecular weight but no carboxylic group could also inhibit Fh transformation (only steric hindrance)?
BTW, sorry I missed your question in the chat session, I used OMs (PPHA, SRNOM) bought from IHSS and used their offered information.
Hi Zhe. Thanks for your question!
Our study included galacturonic acid (GA) and citric acid (CA) coprecipitates. Both these ligands have similarly low MW (194 and 192 Da for GA and CA, respectively), but where CA has 3 carboxyl groups, GA has only 1. We assume that the ligands are most likely complexed to the Fh surface through the carboxyl groups. Therefore, all Fh-associated GA has no extra free carboxyl groups, whereas Fh-associated CA has ≤2. With CA, we see formation of crystalline mineral phases (albeit inhibited compared to the OM-free Fh), but with GA, we see no transformation. So yes, our work shows that OM with low Mw and no carboxyl groups inhibit Fh transformation.