Key Takeaways & Executive Findings
- •• • At 250 °C under micro-oxidation, red mud-coal product pH dropped from 10.29 to 8.47, meeting the alkaline regulation target (5.5–9.0) for in-situ remediation, enabling growth of alkali-tolerant plants. • • Organic matter content reached 12.98% (SOC 7.53%) at 250 °C, classified as 'high' by soil organic matter grading standards, providing a stable organic-inorganic complex for soil microecology. • • Water-stable aggregates of 0.250–2.000 mm increased from 10% to 28% at 250 °C, with capillary porosity of 22%, enhancing soil structure and water retention. • • The process leverages endogenous humic acid generation via coal oxidation, avoiding high-cost external organic amendments and achieving synergistic dealkalization and aggregation in a single step.
Abstract
Red mud, a highly alkaline industrial solid waste from alumina production, poses severe environmental risks due to its high alkalinity, low organic matter content, and poor aggregation, which critically impede its soil reconstruction and ecological utilization. This study proposes a novel approach of low-temperature micro-oxidation to drive the synergistic soil reconstruction of red mud and coal. By constructing a low-temperature micro-oxidation atmosphere, the soil properties of the reaction products were investigated, the decomposition and reconstruction of alkaline minerals and alkali release were analyzed, and the oxidation of carbon-based minerals and organic matter transformation in coal were examined. The mechanism of the synergistic reaction between red mud and coal under low-temperature micro-oxidation was elucidated. Results showed that at 250 °C under micro-oxidation, the pH of the product decreased to 8.47, organic matter content increased to 12.98%, and the proportion of aggregates >0.250 mm increased. Alkaline minerals such as cancrinite and grossular in red mud underwent decomposition and reconstruction in the low-temperature hydrothermal environment, releasing substantial free alkali. The condensed aromatic rings of carbon-based minerals in coal were oxidized by free radicals, leading to ring-opening and bond cleavage, producing small-molecule organic acids and macromolecular humic acids. The continuous oxidation of carbon-based minerals in coal generated acids, which neutralized the alkali released from red mud, driving sustained dealkalization. The inorganic particles of red mud flocculated with macromolecular humic acids, forming micro-aggregates and significantly improving soil properties. This research provides technical support for the rapid ecological utilization of red mud at industrial scale.
1. Introduction
Red mud, a byproduct of alumina extraction, is discharged globally at over 200 million tonnes annually, with cumulative stockpiles exceeding 4 billion tonnes. Its high alkalinity (pH 10.5–12) and poor physical structure render it inhospitable for vegetation, while leaching of caustic soda threatens groundwater and soil ecosystems. Conventional dealkalization methods—acid neutralization, carbonation, electrochemical treatment, and biological approaches—suffer from high salt residues, low efficiency, prohibitive costs, or lengthy processing times. Similarly, attempts to improve organic matter and aggregation via external amendments like sugar industry byproducts require large dosages and incur high disposal costs, limiting scalability.
This study introduces a low-temperature micro-oxidation strategy that couples red mud with coal, a carbonaceous material. Under controlled micro-oxidation at 250 °C, coal's carbon-based minerals oxidize to generate organic acids, including humic acids, which neutralize the alkali released from red mud's alkaline minerals. This synergistic reaction not only reduces pH to 8.47 but also enriches organic matter to 12.98% and enhances aggregate formation, addressing the core bottlenecks of alkalinity, organic matter deficiency, and poor aggregation in a single process. The approach offers a cost-effective, industrially scalable pathway for red mud ecological rehabilitation.
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XIE Erman, ZENG Wei, LI Zhesong, SONG Haoran, TIAN Senlin, HU Xuewei, JU (2026). Low-Temperature Micro-Oxidation-Driven Synergistic Dealkalization and Soil Reconstruction of Red Mud with Coal. Chinese Journal of Environmental Engineering. https://doi.org/10.12030/j.cjee.202506053
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Frequently Asked Questions
What is the optimal temperature range for the synergistic reaction, and how does temperature affect pH and organic matter content?
The study tested temperatures from 100 to 300 °C. At 250 °C, the product pH reached 8.47, the lowest among tested conditions, and organic matter content peaked at 12.98%. Temperatures above 250 °C may cause excessive oxidation or decomposition of organic matter, while lower temperatures are insufficient for complete mineral decomposition and organic acid generation.
How does the process achieve sustained dealkalization beyond the initial neutralization?
The continuous oxidation of coal's carbon-based minerals produces organic acids over time, which neutralize the free alkali released from red mud's alkaline minerals. This acid generation disrupts the dissolution equilibrium of alkaline minerals, driving further alkali release and enabling deep dealkalization.
What is the role of water in the reaction system, and is the process applicable to dry conditions?
Water is essential for the hydrothermal environment that facilitates mineral decomposition and organic acid formation. The experiments used a solid-to-liquid ratio of 1:1. Dry conditions would likely hinder the reaction kinetics and mass transfer, making the process less effective.
What are the potential industrial scalability challenges, and how does the energy input compare to conventional methods?
Industrial scale-up would require reactors capable of maintaining micro-oxidative conditions at 250 °C with controlled oxygen supply. The energy input is moderate compared to high-temperature thermal treatments, and the use of coal as a low-cost feedstock offsets operational costs. However, handling large volumes of slurry and ensuring uniform heating and gas distribution are engineering challenges.
What is the long-term stability of the reconstructed soil, particularly regarding pH rebound and organic matter degradation?
The study indicates that the organic-inorganic complexes formed are stable due to intimate contact and bonding. However, long-term field studies are needed to assess pH rebound from residual alkaline minerals and the rate of organic matter mineralization under natural conditions.
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