How Red Mud Can Help Lock Up Nickel in Soil

The Nickel Problem

Soil pollution is a significant environmental concern, and nickel is one of the metals that can cause considerable issues. Scientists have been exploring ways to manage nickel better in the soil. One promising solution? Bauxite residue, commonly known as "red mud."

What is Red Mud?

Red mud is a byproduct of aluminum production. By 2022, over 4 billion tons of this material had accumulated. Interestingly, red mud possesses unique properties that can alter soil chemistry, making it less susceptible to nickel mobility.

The Study

Researchers conducted a study to investigate the effects of mixing red mud with different types of soil. They discovered that adding 20% red mud to soil can significantly impact nickel behavior.

Alkalinity and pH Changes

• Alkaline Soils: pH increased to over 9.
• Acidic Soils: pH remained below 8.5.

This change in pH makes it more difficult for nickel to move around in the soil.

Nickel Retention

The study also examined how much nickel the soil could retain. The results were impressive:

• Alkaline Soils: Increase of up to 24.5%.
• Neutral Soils: Increase of 71.8%.
• Acidic Soils: Increase of 204%.

This indicates that red mud can help immobilize nickel, reducing its availability to plants and water.

Mechanisms of Immobilization

Researchers used sequential extraction to determine where the nickel was ending up in the soil.

Alkaline Soils

• About half of the nickel was found in the residual fraction (the hardest to extract).
• As more nickel was added, the residual fraction decreased, and the exchangeable fraction increased, indicating that more nickel could be easily released.

Acidic Soils

• Over 50% of the nickel was in the exchangeable fraction, which is concerning as it means the nickel can easily move around.
• However, when red mud was added, over 80% of the nickel was locked up in the residual and oxidizable fractions, suggesting that red mud is highly effective at immobilizing nickel.

Chemical Changes

The researchers used FTIR analysis to understand the chemical changes happening in the red mud when nickel is added. They found that nickel likely forms compounds like:

• Ni(OH)2
• Ni-Al layered double hydroxides (hard to dissolve).
• Adsorption onto the negatively charged surface of cancrinite, a mineral found in red mud.

Conclusion

Red mud shows great potential for managing nickel pollution in soil. By adding red mud, we can reduce the likelihood of nickel release, benefiting both the environment and plants.

Future Research

While the results are promising, several questions remain:

• How much red mud should be used?
• What are the long-term effects?
• How does it affect other metals?

More research is needed, but the current findings are encouraging.