72 73 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 1 / 2 2 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 1 / 2 2 Organic carbon preservation linked to carbon chemistry and sediment mineralogy Related publication: Curti, L., Moore, O.W., Babakhani, P., Xiao, K.-Q.,Woulds, C., Bray, A.W., Fisher, B. J., Kazemian, M., Kaulich, B., & Peacock, C. L. Carboxyl-richness controls organic carbon preservation during coprecipitationwith iron (oxyhydr)oxides in the natural environment. Communications Earth& Environment 2 , 229 (2021). DOI: 10.1038/s43247-021-00301-9 Publication keywords: Organic carbon; Iron (oxyhydr)oxides; Coprecipitation; Soils; Sediments; Preservation O rganic carbon preservation in soils and sediments helps control atmospheric carbon dioxide concentrations. Previous studies suggest that organic carbon can be preserved by becoming associated with soil and sediment minerals. With a better understanding of the carbon-mineral interactions, itmay be possible to predict spatial and temporal patterns of carbonpreservation thatmay help explain the evolution of climate over Earth’s history and be used to helpmitigate current climate change. Researchers from the University of Leeds investigated the mechanisms responsible for organic carbon preservation in soils and sediments at a micro scale. They used the Diamond’s Scanning X-ray Microscopy Beamline (I08), one of the only beamlines that can spatially and chemically analyse organic carbon at micro scale resolution. Their results showed that the binding of organic carbon to minerals in soils and sediments is strongly controlled by the chemistry of the carbon and the type of minerals available. Organic carbon that is rich in carboxyl functional groups bindsmore strongly andmore stably to ironminerals thanmolecules that are poor in carboxyl groups. This implies that carboxyl-richness provides an important control of organic carbon preservation in natural environments. The findings suggest that increasing the carboxyl-richness of the carbon and the quantity of iron minerals in soils could improve organic carbon storage and help store organic carbon in soils where it is locked away from the atmosphere. The preservation of organic carbon in soils and sediments is an important control on atmospheric carbon dioxide and oxygen 1 . Despite decades of research however, the controls on carbon preservation are still poorly understood 2 . The factors controlling the preservation of carbon are complex, but in order for carbon to become preserved, it must escape microbial remineralisation which oxidises it back into carbon dioxide 3 . One leading hypothesis for carbon preservation asserts that carbon bound with soil and sediment minerals is less accessible to microbes and thus protected from microbial attack 4 . Work to date identifies that carbon that is rich in carboxyl functional groups might bind very strongly to iron (oxyhydr)oxide minerals and be protected from microbial remineralisation 5 . Whether and to what extent the carboxyl-richness of carbon can control carbon binding and stability however, is unknown. This work takes a direct mechanistic approach to test whether the carboxyl- richness of carbon can control carbon binding, stability and hence persistence with iron (oxyhydr)oxide minerals. Macro scale binding and stability experiments are combined with micro scale synchrotron spectroscopy analyses to determine the affinity of carboxyl-rich carbon for iron (oxyhydr)oxide minerals and the stability of bound carbon against chemical remineralisation. Carboxyl-rich carbon is represented by simple carbon compounds that are found prevalently in soils and sediments and serve as model compounds for more complex forms of carbon. Iron (oxyhydr) oxide minerals are represented by ferrihydrite which is found ubiquitously in soils and sediments and is commonly associated with carbon. Affinity, stability and persistence are investigated with respect to the number of carboxyl groups ( n ) at constant number of total carbon atoms ( m ), and the number of total carbon atoms ( m ) at constant number of carboxyl groups ( n ) (Fig. 1, comparing across (sequentially increasing n ) or down columns (sequentially increasing m )). The corresponding coprecipitates are denoted Fh_acid n / m _C:Fe, where Fh is ferrihydrite, acid indicates the first three letters of the compound name, followed by n carboxyl groups and m total carbon atoms, and C:Fe is the molar C:Fe ratio of the solid phase. To investigate the affinity of the acids with ferrihydrite, each acid was coprecipitated with ferrihydrite at a sequentially increasing initial carbon and fixed initial iron concentration. The data show that as the number of carboxyl groups increases, the molar amount of carbon bound with the ferrihydrite increases. Thus carboxyl-richness likely provides an important control on carbon affinity with iron (oxyhydr) oxides in soils and sediments. To investigate the stability of the acids with ferrihydrite against release and chemical remineralisation, coprecipitates were exposed to solutions typically used to either release or oxidise carbon from iron (oxyhydr)oxides. The data show that as the number of carboxyl groups increases, themolar amount of carbon released or oxidised from ferrihydrite decreases. Thus carboxyl-richness likely provides an important control on carbon stability with iron (oxyhydr) oxides in soils and sediments. To determine the mechanisms responsible for the increased affinity and stability of carboxyl-rich carbon with ferrihydrite, Scanning Transmission X-ray Microscopy (STXM) coupled with Near Edge X-ray Absorption Fine Structure (NEXAFS) Spectroscopy was used on I08. The spectra show that the carboxyl peak for all the coprecipitates is reduced in amplitude and broadened compared to their respective standards (Fig. 2). This indicates that carboxyl-rich carbon binds to ferrihydrite via a carboxyl group ligand exchange adsorption mechanism, occurring between the hydroxyl part of a carboxyl group and the hydroxyl part of a ferrihydrite adsorption site. The spectra also show that as the number of carboxyl groups increases in the coprecipitates, the carboxyl peak is increasingly shifted to lower energy compared to the respective standards (Fig. 2). This indicates that as the number of carboxyl groups increases in the coprecipitates, the number of carboxyl ligand exchange bonds between each adsorbing acid molecule and the ferrihydrite particles increases (Fig. 2). Thus carboxyl-rich carbon binds to ferrihydrite via a multi-carboxyl ligand exchange adsorption mechanism. Overall this work indicates that the increased affinity and stability of carboxyl-rich carbon with iron (oxyhydr)oxides results from a multi- carboxyl ligand exchange adsorption mechanism, in which carboxyl-rich carbon forms more bonds with ferrihydrite particles and is subsequently more difficult to release and oxidise. In soils and sediments carboxyl-rich carbon and iron (oxyhydr) oxides are prevalent and their association likely imparts a strong control on carbon persistence in natural environments and thus an important control on carbon cycling and atmospheric carbon dioxide and oxygen. References: 1. Berner, R. A. The phanerozoic carbon cycle, (2004). DOI: 10.1093/ oso/9780195173338.001.0001 2. Arndt, S. et al . Quantifying the degradation of organic matter in marine sediments: A review and synthesis. Earth- Science Reviews 123 , 53–86 (2013). DOI : 10.1016/j.earscirev.2013.02.008 3. Hedges, J. I. et al. Sedimentary organic matter preservation: an assessment and speculative synthesis. Marine Chemistry 49 , 81–115 (1995). DOI: 10.1016/0304- 4203(95)00008-F 4. Keil, R. G. et al. Sorptive preservation of labile organic matter in marine sediments. Nature 370 , 549–552 (1994). DOI: 10.1038/370549a0 5. Kaiser, K. et al. The role of DOM sorption to mineral surfaces in the preservation of organic matter in soils. Organic Geochemistry 31 , 711–725 (2000). DOI: 10.1016/S0146- 6380(00)00046-2 Funding acknowledgement: This research project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (Grant agreement No. 725613 MinOrg). Royal SocietyWolfson Research Merit Award (WRM/ FT/170005) is gratefully acknowledged. Diamond Light Source provided access to Beamline I08 (STFC grant numbers SP21323, SP20839 and MG23049) that contributed to the results presented here. Corresponding author: Prof. Caroline Peacock, University of Leeds,
[email protected] Imaging andMicroscopy Group Beamline I08 Figure 1: Carboxylic acids coprecipitated with ferrihydrite. Mono-, di- and tri-carboxylic acids, including systematic name, common name in parentheses, coprecipitate nomenclature and data symbol; number carboxyl functional groups (nCOOH); number total carbon atoms (mTotC); Molecular Weight (MW); and pK a of carboxyl functional groups. Influence of number of carboxyl groups (n) at constant number of total carbon atoms (m) on carbon affinity and stability investigated by comparing acids across columns (sequentially increasing n). Influence of number of total carbon atoms (m) at constant number of carboxyl groups (n) on carbon affinity and stability investigated by comparing acids down columns (sequentially increasing m). Figure 2: Carbon 1s NEXAFS spectra for carboxylic acids coprecipitated with ferrihydrite. Spectra for carboxylic acid standards (dotted lines) and carboxylic acid ferrihydrite coprecipitate samples (solid lines) plotted as energy (eV) vs. normalised absorbance (arbitrary units). Green, blue and purple colour codes depict mono-, di- and tri-carboxylic data, respectively. The coprecipitate nomenclature is given in Fig. 1. Spectra are stacked with an arbitrary offset for clarity. Carboxyl carbon peak positions for the unreacted mono-, di- and tri-carboxylic acid standards are shown with vertical dashed green, blue and purple lines, respectively .