The role and fate of organic carbon during aging of ferrihydrite

The persistence of organic carbon (OC) in natural environments is widely attributed to mineral protec-tion, especially by iron (Fe) (oxyhydr)oxides. The effect of OC binding strength on the aging of Fe (oxy-hydr)oxides and the mobility and fate of OC during aging however, is unknown. Here we investigate how OC binding strength controls the aging of ferrihydrite (Fh) and subsequent retention or release of the associated OC. We focus on carboxyl-rich OC coprecipitated with Fh and track the physiochemical properties and OC stability as a function of carboxyl-richness over time. In agreement with previous work we find that during carboxyl-rich OC coprecipitation with Fh, OC is adsorbed to the Fh particle surfaces and that increasing carboxyl-richness results in an increasing number of carboxylate-Fe bonds between the OC and the mineral particles and thus increasing OC binding strength. We show that OC substantially retards the aging of Fe (oxyhydr)oxide from Fh to more crystalline Fe minerals and that this retardation increases with increasing OC binding strength. We also show that the total amount of OC decreases dur-ing aging and that the proportion of the remaining OC that is non-desorbable with 0.1 M NaOH decreases during aging for OC with relatively low binding strength but increases during aging for OC with relatively high binding strength. Our results therefore indicate that OC with higher binding strength coprecipitated with Fh becomes proportionally more stable with the solid phase and thus less mobile during aging in natural environments. We suggest that our work might offer a deeper mechanistic insight into the pro-cesses responsible for OC persistence with minerals and thus the long-term preservation of OC in natural environments.(c) 2022 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/).

Automated summary

This study investigates the impact of organic carbon (OC) binding strength on the aging of ferrihydrite and the fate of associated OC. It is found that OC substantially retards the aging process and that OC with higher binding strength becomes more stable with the solid phase during aging. The results provide insights into the mechanisms behind OC persistence with minerals and its long-term preservation in natural environments.