How Red Mud Can Help Lock Up Nickel in Soil

Soil pollution is a big deal, and nickel is one of those tricky metals that can cause problems. Scientists have been looking into how to manage nickel better in the soil. One interesting solution? Bauxite residue, also known as "red mud. " This stuff is a leftover from aluminum production, and there's a lot of it—over 4 billion tons by 2022. Red mud has some cool properties. It can change the chemistry of soil, making it less likely for nickel to move around and cause trouble. In a recent study, researchers mixed red mud with different types of soil to see how it affects nickel. They found that adding 20% red mud to soil can make a big difference. First off, red mud makes the soil more alkaline, which means it raises the pH. For alkaline soils, the pH went up to over 9, and for acidic soil, it stayed below 8. 5. This change in pH can make it harder for nickel to move around in the soil. The study also looked at how much nickel the soil could hold. They tested different amounts of nickel and found that red mud really boosted the soil's ability to hold onto nickel. For alkaline soils, the increase was up to 24. 5%, for neutral soils it was 71. 8%, and for acidic soils, it was a whopping 204%. That's a big deal because it shows that red mud can help lock up nickel, making it less available to plants and water. But how exactly does red mud do this? The researchers used a method called sequential extraction to figure out where the nickel was ending up in the soil. They found that in alkaline soils, about half of the nickel was stuck in the residual fraction—the part that's really hard to get out. As they added more nickel, this fraction decreased, and the exchangeable fraction increased, which means more nickel could be easily released. In acidic soil, over 50% of the nickel was in the exchangeable fraction, which is not great because it means the nickel can easily move around. But when red mud was added, over 80% of the nickel was locked up in the residual and oxidizable fractions. This suggests that red mud is really good at immobilizing nickel. The researchers also 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 and Ni-Al layered double hydroxides, which are hard to dissolve. It also adsorbs onto the negatively charged surface of cancrinite, a mineral found in red mud. So, what does all this mean? It means that red mud could be a useful tool for managing nickel pollution in soil. By adding red mud, we can make the soil less likely to release nickel, which is good for the environment and for plants. But there are still questions to answer. How much red mud should we use? What are the long-term effects? And what about other metals? More research is needed, but the results so far are promising.