Red mud is an industrial solid waste rarely utilized and often disposed of in landfills, resulting in resource waste and environmental pollution. However, due to its high pH and abundance of iron and aluminum oxides and hydroxides, red mud has excellent adsorption properties which can effectively remove heavy metals through ion exchange, adsorption, and precipitation. Therefore, red mud is a valuable resource rather than a waste byproduct. In recent years, red mud has been increasingly studied for its potential in wastewater treatment and soil improvement. Red mud can effectively reduce the migration and impact of heavy metals in soils and water bodies. This paper reviews the research results from using red mud to mitigate cadmium pollution in water bodies and soils, discusses the environmental risks of red mud, and proposes key research directions for the future management of red mud in cadmium-contaminated environments.

Cadmium (Cd) contamination in agricultural soils has emerged as a significant concern, particularly due to its potential impact on plant-based food. Soil pH reductions can exacerbate Cd mobility, leading to excessive accumulation in crops. While liming has been demonstrated as an effective method to mitigate Cd accumulation in rice grains in acid soils of southern China, its efficacy in remediating acid soils in northern China remains unclear. In this study, a multi-year field experiment was conducted on farmland impacted by zinc ore smelting at coordinates of 33.92° N 112.46° E to investigate the use of limestone for controlling Cd accumulation in wheat and maize grains. The results indicated that applying 7.5 t ha⁻¹ of limestone significantly raised the soil pH from 4.5 to 6.8 as anticipated. Different rates of limestone application (2.25, 4.45, and 7.50 t ha⁻¹) reduced Cd bioavailability in the soil by 20–54%, and Cd accumulation in wheat grains by 5–38% and maize grains by 21–63%, without yield penalty. The remediation effects were sustained for at least 27 months, highlighting limestone as a promising ameliorant for smelting-affected farmland in northern China.

Feammox, one of the potential pathways for nitrogen loss in the environment, plays an essential role in nitrogen cycling and provides new ideas for the biological denitrification of wastewater. However, the Feammox reaction has low nitrogen removal efficiency and stagnates due to insufficient Fe(III) sources. It strongly depends on an Fe(III) source supply, significantly limiting its development. In this study, a synergistic nitrogen removal system using Feammox and Nitrate-Dependent Fe(II) Oxidation (NDFO) driven by NO₃⁻-N was constructed within an organic carbon environment. It uses the synergy between Feammox and NDFO to improve nitrogen removal. The removal efficiency of NH₄⁺-N reaches over 70% in stages III-V, with a maximum removal efficiency of 89.4%. NH₄⁺-N oxidation and Fe(III) reduction are positively coupled in the Feammox reaction. The Fe(II)/Fe(III) cycle process driven by Feammox and NDFO improves the utilization of the iron source, thus guaranteeing the sustainability of the NH₄⁺-N oxidation reaction. In addition, the organic carbon environment also enriched NDFO bacteria (Thermomonas and Acinetobacter) and increased the reaction rate of NDFO, which enhanced the transformation of Fe(II). We improved the nitrogen removal efficiency of Feammox and provided a new approach for nitrogen removal in wastewater treatment.

Red mud is a highly alkaline solid waste with an annual emission of over 200 million tons, which requires large-scale utilization methods. Soil Cd remediation is a global concern, due to its high toxicity and strong mobility. Given red mud’s potential for soil Cd remediation, this study reviews its basic characteristics, the mechanisms of soil Cd immobilization by red mud, and the use of red mud-based passivators for agricultural soil Cd remediation. In general, red mud regulates soil pH, thus increasing the soil’s Cd adsorption capacity; provides abundant surface active sites for adsorption and complexation with soil Cd; introduces cations to immobilize Cd via ion exchange; and enriches Cd-resistant microbe species to reduce soil Cd toxicity. Furthermore, the potential environmental risks and suggestions on red mud application are discussed. Further research should focus on improving the remediation effectiveness of red mud on cadmium-contaminated agricultural soil, demonstrating its long-term efficacy and economic costs, and proposing practical technical models and standards for application.

Background and aims: Plants encounter various abiotic stress during their growth, making it essential to understand their responses to such challenges. Though function of OsHMA3 in cadmium (Cd) sequestration into rice root vacuoles has been well characterized, little is known about how different OsHMA3 alleles influence plant responses to high Cd exposure. Methods: In this study, we utilized rice cultivars carrying a functional OsHMA3 allele (9311) and a loss-of-function OsHMA3 allele (W4) to investigate their differences in Cd detoxification responses. Morphological, physiological and subcellular changes, as well as Cd species were analyzed. Results: The results indicated that Cd was more extensively transported to shoots in W4 than 9311 as expected. However, W4 exhibited a more pronounced growth inhibition with reductions in shoot and root length, more severe leaf chlorosis with chlorophyll a, b, and carotenoid levels decreased by 31%, 43%, and 45% in W4, respectively, while no significant reductions were observed in 9311. Enhanced cell ultrastructural damage, including membranolytic changes, nuclear membrane lysis, and damage to the nucleus, was noted in W4, while, relatively normal cell structure in 9311. The antioxidant enzyme systems exhibited greater responses in W4 than those in 9311. Additionally, lower levels of nitrate/nitrite, chloride, and aminophenol Cd forms were found in W4 roots, while higher concentrations of these were observed in shoots, indicating distinct Cd detoxicity response between W4 and 9311. Conclusion: This study underscores the significance of the OsHMA3 gene in Cd detoxification responses, offering valuable insights into the mechanisms of plant Cd detoxification.