Carboxyl-richness controls organic carbon preservation during coprecipitation with iron (oxyhydr)oxides in the natural environment

Lisa Curti,Burkhard Kaulich,Caroline L Peacock,Oliver W Moore,Ke-Qing Xiao,Clare Woulds,Peyman Babakhani,Majid Kazemian,Ben J Fisher,Andrew W Bray

doi:10.1038/s43247-021-00301-9

Lisa Curti, Burkhard Kaulich + Show 8 more

Open Access

https://doi.org/10.1038/s43247-021-00301-9

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Abstract

The coprecipitation of organic carbon with iron minerals is important for its preservation in soils and sediments, but the mechanisms for carbon-iron interactions and thus the controls on organic carbon cycling are far from understood. Here we coprecipitate carboxylic acids with iron (oxyhydr)oxide ferrihydrite and use near-edge X-ray absorption fine structure spectroscopy and wet chemical treatments to determine the relationship between sequestration mechanism and organic carbon stability against its release and chemical oxidative remineralisation. We show that organic carbon sequestration, stabilisation and persistence increase with an increasing number of carboxyl functional groups. We suggest that carboxyl-richness provides an important control on organic carbon preservation in the natural environment. Our work offers a mechanistic basis for understanding the stability and persistence of organic carbon in soils and sediments, which might be used to develop an overarching relationship between organic functional group-richness, mineral interactions and organic carbon preservation in the Earth system.

Highlights

The coprecipitation of organic carbon with iron minerals is important for its preservation in soils and sediments, but the mechanisms for carbon-iron interactions and the controls on organic carbon cycling are far from understood
To represent carboxyl-rich organic carbon (OC), we chose a selection of saturated aliphatic mono, di- and tri-carboxylic acids containing only COOH carboxyl and CH methyl groups (Fig. 1). These acids are directly relevant to soil and sediment porewaters where they and other low molecular weight organic acids are prevalent[31,32]. They serve as model organic compounds for understanding the role of carboxyl groups in the sequestration of more complex dissolved NOM33, which is recently shown to comprise a number of universal molecular structures containing predominantly carboxyl and hydroxyl groups[34]
We investigate the role of carboxyl groups in the sequestration, stabilisation and persistence of OC during its coprecipitation with ferrihydrite

Summary

Introduction

The coprecipitation of organic carbon with iron minerals is important for its preservation in soils and sediments, but the mechanisms for carbon-iron interactions and the controls on organic carbon cycling are far from understood. Despite the importance of OC–iron (oxyhydr)oxide phases, the mechanisms responsible for OC sequestration, stabilisation against remineralisation and persistence with iron (oxyhydr)oxides are poorly understood[1]. OC is often present in the porewaters from which iron (oxyhydr) oxides precipitate, and the coprecipitation of OC with these minerals is a common process for OC–iron (oxyhydr)oxide formation[11,12,13,14] that is recently shown to be quantitatively important for OC persistence on a global scale[2,15]. An incorporation mechanism must locate OC inside mineral pore spaces for example, where it can become adsorbed and/or is otherwise physically inaccessible to remineralisation[23]

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