A Fundamental Study on Soil-Based Carbon Dioxide Removal Using Amine-Functionalized Modified Red Mud

This study is positioned as a foundational, soil-based carbon capture and carbon dioxide removal (CDR) investigation, aiming to establish fundamental design principles for mineral- and soil-derived CO₂ sorbents rather than immediate industrial deployment. Soil and soil-like materials play a central role in long-term carbon sequestration strategies due to their abundance, stability, and compatibility with land-based CDR systems. In this context, red mud (RM), an industrial by-product of alumina production via the Bayer process, was selected as a representative mineral-rich, soil-analog material to explore its potential as a functional platform for CO₂ capture.

Globally, RM is generated at a scale of approximately 300 million tons per year, yet its reuse rate remains below 3%. Its disposal poses serious environmental concerns because of its high alkalinity, salinity, and fine particle size—characteristics that also resemble extreme or degraded soil conditions. From a soil-based CDR perspective, these properties make RM a valuable model system for investigating how mineral composition, pore structure, and surface chemistry influence CO₂ sorption behavior. Thus, this work focuses on transforming RM from an environmentally problematic residue into a functionalized, soil-like carbon capture medium.

To enable its application in soil-based carbon capture research, RM was structurally modified through acid digestion, alkali reprecipitation, and calcination. This treatment generated a mesoporous framework and increased the BET surface area from 17.47 to 140.05 m²/g, mimicking the hierarchical pore structures found in reactive mineral soils. The modified RM (ARM) therefore serves as a controlled mineral matrix for systematically studying the interaction between nitrogen-containing functional groups and CO₂.

Amine- and guanidine-functionalized sorbents were prepared by wet impregnation using polyethylenimine (PEI), triethylenetetramine (TETA), and 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD). Rather than focusing solely on maximizing adsorption capacity, this study emphasizes understanding how different nitrogen functionalities behave when immobilized on soil-like mineral surfaces. FT-IR spectroscopy, thermogravimetric analysis, and elemental analysis confirmed the successful anchoring of these functional groups, providing a reliable platform for mechanistic investigation.

CO₂ adsorption experiments were conducted in a fixed-bed reactor under mild conditions representative of ambient or near-surface environments relevant to soil-based CDR. Among the tested materials, ARM–TETA exhibited the highest CO₂ adsorption capacity (36.90 mg/g), highlighting the importance of molecular flexibility and amine accessibility within mesoporous mineral matrices.

Overall, this research serves as a baseline study for soil-based carbon capture, demonstrating how industrial mineral residues can be engineered into model systems for CDR research. The findings provide fundamental insights into pore–functionality relationships and support the broader development of scalable, land-compatible carbon capture materials derived from soil and mineral resources.