Abstract

Dissolved organic matter (DOM) in soils and sediments plays a fundamental role in regulating organic carbon (OC) cycling and long-term storage, critically influencing global climate. Ferrihydrite, a ubiquitous iron oxyhydroxide, is a key mineral for DOM adsorption due to its high surface area and reactivity. However, its properties are often modified by naturally occurring aluminum (Al) substitution. This article bridges insights from a comprehensive field-based investigation of Al and chromium (Cr) partitioning in natural sediments (Eswaramoorthi et al., 2021, and subsequent work) with cutting-edge nanoscale and molecular evidence for DOM adsorption by Al-bearing ferrihydrite (Li et al., 2025). We demonstrate how field observations of Al's strong association with organic matter and preferential adsorption onto iron oxides provide crucial environmental validation for the mechanistic findings of Al-substituted ferrihydrite's enhanced DOM binding capacity. While acknowledging nuances in distinguishing bulk complexation from surface adsorption, this comparative analysis reveals powerful synergies, offering a more holistic understanding of OC immobilization and metal biogeochemistry. The implications extend to improving predictive models for carbon sequestration, metal mobility, and interpreting paleoenvironmental records under changing environmental conditions.

1. Introduction

The dynamic interaction between dissolved organic matter (DOM), characterized by its complex small molecule compounds, and mineral surfaces in soils and sediments is a cornerstone of organic carbon (OC) immobilization. This process significantly impacts the global carbon cycle and climate change (Gong et al., 2024; Li et al., 2025). Among the various minerals, ferrihydrite, a poorly ordered iron oxyhydroxide, stands out due to its exceptionally high specific surface area and strong adsorption affinity for DOM, making it a crucial mineral for OC storage (Li et al., 2025).

However, ferrihydrite in natural environments is rarely pure. It frequently incorporates other elements, most notably aluminum (Al), into its structure. These Al substitutions can profoundly alter the structural, textural, and reactive properties of ferrihydrite, thereby influencing its interactions with DOM. Understanding these Al-mediated interactions is critical for accurate predictions of OC cycling and metal mobility in diverse natural environments, yet a comprehensive understanding that bridges field observations with molecular-level mechanisms remains a significant challenge.

This article aims to address this gap by drawing parallels between our previous field-based research on Al and Cr partitioning in natural sediments (Eswaramoorthi et al., 2021, and subsequent in-depth discussions) and a recent laboratory study by Li et al. (2025) investigating the nanoscale and molecular evidence for adsorptive fractionation of DOM at the interfaces of Al-bearing ferrihydrite and water. By comparing the large-scale observational patterns with fine-scale mechanistic insights, we seek to reveal crucial synergies and discuss the broader implications for understanding OC cycling and associated metal biogeochemistry.

2. Al and Cr Partitioning in Natural Sediments

Insights from Pulicat Lake Our earlier work on core samples collected from Pulicat Lake, a brackish water lagoon on the east coast of India, utilized sequential extraction and cross-correlation analysis to unravel the partitioning and early diagenetic remobilization of labile Al and Cr (Eswaramoorthi et al., 2021). The findings provided a quantitative understanding of the distribution of these metals among various geochemical phases.

On average, our analyses revealed that a substantial 61% of labile Al in the mobile phase was associated with organic matter, underscoring the critical role of organic matter as a primary host for mobile Al in these sediments. Fe-Mn oxides played a secondary, albeit significant, role, with approximately 28% of labile Al remaining adsorbed onto their surfaces.

A more refined cross-correlation analysis within the Fe-Mn oxide phase yielded a crucial distinction: Al showed a significant positive correlation with Fe-oxides, indicating that Al preferentially remains adsorbed onto Fe-oxides, while Cr primarily associated with Mn-oxides. This direct field observation highlights the specific importance of iron minerals in Al dynamics within sedimentary environments.

Furthermore, our study demonstrated an intricate interdependence between organic-bound metals. Cross-correlation analysis revealed that organic-bound Fe and Mn largely control the down-core variations of organic-bound Al. This is exemplified by observations such as the lower level of organic-bound Al (27 μg/g) in depth intervals where organic-bound Fe (25 μg/g) and Mn (2 μg/g) were similarly low (Periakali et al., 1999). This mechanism suggests that the surface adsorption of organic molecules onto Fe-Mn oxides can transform these oxides into Fe- and Mn-organic complexes, leading to an increase in organically bound Fe and Mn, which, in turn, influences organic-bound Al. Consequently, the degradation of organic matter and the subsequent release of Fe and Mn from their organic complexes might also trigger the release of associated organic-bound Al, impacting its mobility.

However, a key nuance emerged from our cross-correlation analysis: while Al is adsorbed onto Fe-oxides and significantly associated with organic matter, our data indicated that bulk Fe- and Mn-oxides are not important in the *direct complexation* of Cr and Al with organic matter (Figure 4e,g,h in Eswaramoorthi et al., 2021). This suggests a more indirect or surface-mediated pathway for the association of metals with organic matter rather than direct bulk oxide-organic complex formation.

3. Molecular Insights into DOM-Ferrihydrite Interactions (Li et al., 2025)

Complementing large-scale field observations, Li et al. (2025) employed advanced spectroscopic techniques to investigate the nanoscale and molecular-level processes governing the adsorptive fractionation of dissolved organic matter (DOM) at the interfaces of Al-bearing ferrihydrite and water. Their study provides crucial mechanistic insights into how Al substitution within the ferrihydrite structure influences its interaction with DOM.

The central finding of Li et al. (2025) is that Al substitution significantly enhances the specific surface area and binding capacity of ferrihydrite, leading to increased immobilization of DOM. This implies that the presence of Al within the ferrihydrite matrix fundamentally alters the mineral's surface chemistry, making it a more effective adsorbent for organic molecules. Their work delves into the specific molecular conformations and binding energies involved in these interactions, contributing to a more precise understanding of DOM sequestration mechanisms.

4. Synergies: Reinforcing and Explaining Field Observations

The combined insights from our field-based observations and the mechanistic study by Li et al. (2025) reveal powerful synergies, allowing for a more comprehensive understanding of Al-DOM-mineral interactions.

Environmental Validation of Al-Fe Oxide Connection: Our field data from Pulicat Lake, unequivocally demonstrating that Al remains mainly adsorbed onto Fe-oxides in natural sediments, provides robust environmental validation for the premise of Li et al.'s work. Their choice of Al-bearing ferrihydrite as a model system for investigating DOM interactions is directly supported by our observations of Al's strong affinity for iron oxides in real-world environments. This bridging establishes that the processes they investigate are indeed critically important in natural settings.

Mechanistic Basis for Field Observations: Li et al.'s findings offer the "how" for many of the "what" and "where" observations from our study. The enhanced DOM adsorption capacity of Al-substituted ferrihydrite, as demonstrated by Li et al., provides a molecular mechanism explaining *why* organic-bound Al is so prevalent in our sediment samples and *why* Fe-oxides are significant hosts for Al. The increased binding capacity of ferrihydrite due to Al substitution directly contributes to the immobilization of DOM, which, in turn, influences the observed partitioning of Al. This mechanism helps to explain how DOM, often already carrying Al, can become strongly associated with iron oxide surfaces.

Bridging Adsorption and Association: Our work highlights that Al is both adsorbed onto Fe-oxides and significantly associated with organic matter. Li et al.'s focus on adsorptive fractionationby Al-bearing ferrihydrite offers a plausible pathway for how these two forms of Al association (adsorbed to mineral surfaces and associated with organic matter) are interconnected. The adsorption of DOM (which itself may carry Al or facilitate Al mobility) onto Al-bearing ferrihydrite surfaces represents a key initial step in their co-localization and subsequent long-term immobilization.

5. Nuances and Apparent "Contradictions": A Deeper Dive

While largely synergistic, a nuanced interpretation is required when comparing the scale and definitions of "complexation" and "adsorption" between the two studies.

The "Complexation" vs. "Adsorption" Distinction: Our cross-correlation analysis suggested that "bulk Fe- and Mn-oxides are not important in the direct complexation of Cr and Al with organic matter." In contrast, Li et al. (2025) focus on the adsorptive fractionation of DOM onto Al-bearing ferrihydrite surfaces. This is not a direct contradiction but rather a difference in the specific type of interaction being probed and the scale of observation.

Discussion: Our study examines bulk sediment phases and the ultimate state of metal-organic association (complexation), which might reflect the long-term history of interactions, including formation of stable organic complexes. Li et al., however, are investigating the immediate, molecular-level surface adsorption processes. Adsorption is a primary physical-chemical interaction at the mineral-water interface. This adsorption can then lead to the formation of surface complexes or facilitate the incorporation of metals (like Al, either from the ferrihydrite structure or from solution) into organic matter structures upon subsequent transformation or degradation. Therefore, while bulk oxides may not be the primary complexing agents for pre-existing metal-organic complexes, the Al-substituted oxide surface is demonstrably crucial for adsorbing organic matter, thereby controlling the initial fate and potential immobilization of both DOM and associated Al. The "organic-bound Fe and Mn largely control organic-bound Al" observation from our work could be interpreted as showing that the presence and stateof organic matter, which can itself be profoundly influenced by surface interactions with Fe/Mn oxides, ultimately dictates the fate of Al.

6. Implications for the Findings of Li et al. (2025)

Our field-based findings offer several critical implications for interpreting and extending the molecular insights provided by Li et al. (2025):

Enhanced Environmental Relevance and Validation: Our work provides compelling field evidence that Al-bearing iron oxides are indeed a critical component in Al and DOM cycling in natural sedimentary environments. This robustly validates the importance and environmental significance of the highly controlled laboratory investigation conducted by Li et al.

Context for DOM Fate and Stability: Li et al.'s work illuminates how DOM is initially immobilized. Our findings, particularly the observation that organic-bound Al variations are controlled by organic-bound Fe and Mn, and that organic matter degradation can lead to Al release, provide crucial context for the long-term stability and dynamic remobilization of the DOM immobilized by Al-ferrihydrite. This suggests that the stability of these adsorbed DOM fractions (as explored by Li et al.) is intimately linked to the broader redox and degradation state of the organic matter and the Fe/Mn biogeochemical cycles in natural environments.

Guiding Future Mechanistic Studies: Our detailed correlations (e.g., organic-bound Al with organic-bound Fe) suggest compelling avenues for future laboratory studies building upon Li et al.'s work. For instance, future research could explore how the presence of specific organic-bound Fe/Mn species might further influence the adsorption, fractionation, and ultimate fate of DOM by Al-bearing ferrihydrite, mimicking more complex natural conditions.

7. Consequences for OC Cycling and Metal Biogeochemistry

Integrating the insights from our field observations and Li et al.'s mechanistic study leads to a more comprehensive understanding with significant consequences for biogeochemical cycling:

Improved Predictive Models: By bridging the gap between large-scale field observations and molecular-level mechanistic understanding, we can develop more accurate and robust conceptual and numerical models for organic carbon sequestration and release in diverse sedimentary environments. These models can now specifically account for the critical role of Al substitution in iron minerals in controlling DOM-mineral interactions.

Nuanced Understanding of Metal Mobility: The strong and intricate interplay between Al, Fe-oxides, and organic matter (as demonstrated by both studies) is vital for predicting Al mobility and bioavailability in various environmental compartments, including soils, sediments, and aquatic systems. This understanding has important implications for water quality, nutrient cycling, and ecosystem health.

Intertwined Biogeochemical Cycles: This comparative analysis underscores that the global carbon, iron, and aluminum cycles are not independent but are deeply intertwined. Mineral surfaces, particularly those of Al-substituted ferrihydrite, act as critical hubs for these complex interactions, fundamentally influencing the fate and transport of both organic carbon and associated metals.

Diagenetic Implications: The understanding of how DOM is adsorbed and subsequently released due to organic matter degradation, significantly influenced by Al-bearing minerals, is crucial for interpreting paleoenvironmental records. Variations in Al substitution in paleo-ferrihydrites could serve as proxies for past DOM sequestration efficiency and redox conditions.

8. Future Research Directions

To further bridge the gap between field observations and molecular-level understanding, future research should consider:

• Investigating the long-term stability and reversibility of Al-substituted ferrihydrite-DOM complexes under varying redox conditions and diagenetic pressures that mimic natural sedimentary environments.
• Exploring how different fractions of DOM (as potentially "fractionated" by Al-bearing ferrihydrite in Li et al.'s work) behave during subsequent degradation and how this influences Al mobility.
• Developing integrated models that combine the molecular-scale understanding of surface interactions (e.g., from Li et al.) with the bulk partitioning and correlative relationships observed in natural systems (e.g., from our work).
• Examining the combined effects of Al and other common metal or non-metal substitutions (e.g., Si, Mn) on ferrihydrite properties and their collective influence on DOM interactions.

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