Red Remediation

On Monday, Oct. 4, 2010, a storage reservoir belonging to a Hungarian aluminum refinery, the Ajkai Timfoldgyar plant, burst releasing a 6–8ft wave of “red toxic sludge” with the volume of 35 million cubic feet roaring behind it. The sludge, enough to fill 440 Olympic swimming pools, killed nine people, injured more than 120 people, and contaminated plant and wildlife over a 16-square-mile area.

Hearing the term “red toxic sludge” conjured up images from those bad Toxic Avenger movies from the 1980s. I took a personal interest in this story because I have friends and family members living in Hungary, although not near the devastated towns of Kolontar and Devecser.

The red toxic sludge is a byproduct of the alumina extraction process. During aluminum (Al) production, alumina (Al2O3) is first extracted from bauxite under pressure using sodium hydroxide at 150-200°C (Bayer Process). The alumina is then precipitated out of this soda lime solution, and the resulting “red mud” is decanted into settling ponds. The sludge that flowed through village houses and backyards in Hungary from the burst reservoir was this red mud, which had a reported pH of 13.0! Analysis of red mud from another aluminum refinery suggests the color comes from a high iron oxide content ([Fe2O3]≈ 35%). In terms of toxicity, the Hungarian Academy of Science had sampled the red muck the day after the reservoir breach and reported the heavy metal content to be considerably below toxic levels.

So, the most immediate danger to humans, animals and the environment became exposure to the caustic sludge. A high-pH solution can produce second to third degree burns on the skin similar to burns created from exposure to a concentrated acid or high heat. Any woman who has ever used a depilatory cream knows how slippery and soapy the skin feels after using one of these high-alkaline creams to “dissolve” hair off of the skin surface. If left on the skin too long, a person can experience irritation with a burning sensation. Anyone who has ever accidentally splashed a 1M NaOH solution on themselves in the lab can also relate. It takes an enormous amount of water rinsing to be rid of this feeling.

Neutralizing the enormous volume of alkaline byproduct in the Hungarian accident required multiple approaches. The Hungarian response teams immediately started dumping gypsum and fertilizer into the rivers and creeks. Roadways, buildings, humans and animals were washed with vinegar and copious amounts of water. As for the settled red sludge, reports indicated that all contaminated top soil would need to be hauled to specialized landfills since it could not support vegetation.

My curiosity latched onto fertilizers used as neutralizing agents. None of the news sources disclosed which fertilizers had been used in the Hungarian disaster. However, a 2010 paper by Chen et. al, studying pH on ammonia (NH3) volatilization using di-ammonium phosphate(DAP) fertilizer to neutralize bauxite-processing residue sand (red mud) gave me some insight.

According to this paper, Australia is the largest producer of bauxite residue, where for every ton of alumina processed, 2 tons of bauxite residue is produced. Alcoa World Alumina, a key producer in Australia, applies 2.7 metric tons of DAP per hectare (2.4 acres) on the outer edges of their residue disposal areas in hopes of “rehabilitating” the soil to reestablish vegetation. The authors determined that the higher the pH of the residue, the more ammonia escaped from the soil after applying DAP fertilizer. This meant less nitrogen was available in the soil to support plant growth. This is one explanation as to why the top soil in the Hungarian red mud accident needs to be completely removed if there is to be any sustainable plant growth. And it’s not surprising that it is too costly to continue adding DAP to bring the pH down to where native plant species can take hold. So the search continues for low cost methods to remediate the red mud in situ or find alternative uses for it.

Here is the chemical equation showing neutralization of NaOH in red mud with DAP fertilizer to satisfy my curiosity and I hope yours:

(NH4)2PO4 + 2NaOH            →            Na2PO4 + 2NH3 + 2H2O

Commenting on the neutralization efforts, Dr. David Dzombak, a professor of civil and environmental engineering at Carnegie Mellon University, described the Hungarian red mud disaster: “It’s alkaline material mixed in with soil, and there are a limited number of things you can do to address that. It’s a crude problem and the solution is pretty straightforward.” Although the solution is simple, the consequences are not.

Further Reading

1.) Chen, C.R., et al. (2010) Behaviour and dynamics of di-ammonium phosphate in bauxite processing residue sand in Western Australia–I. NH3 volatilisation and residual nitrogen availability. Environ Sci Pollut Res Int. Jun;17(5):1098-109. Epub 2009 Nov 25.

2.) Clay Dillow (Oct. 13, 2010) How to Clean Up Hungary’s 16-Square Mile Toxic Mess, POPSCI,

3.) Oster, J. D. and Frenkel, H. (1980). The Chemistry of the Reclamation of Sodic Soils with Gypsum and Lime. Soil Sci Soc Am J 44 (1): 41–45.

4.) Geza Molnar (Oct. 7, 2010) Hungary Toxic Sludge Spill Reaches Danube, AFP

5.) George Jahn (Oct. 7, 2010) Toxic Red Sludge Reaches the Danube River, El Paso Times,

6.) Stuart Burns (Oct. 7, 2010) Hungarian Alumina Refinery Disaster – What Exactly is This Red Mud?, MetalMiner,

7.) Remy Melina (Oct. 8, 2010) Why is the Toxic Hungarian Sludge Red?, Life’s Little Mysteries,

8.) Duncan Kennedy (Oct. 12, 2010) Hungary Emergency Toxic Sludge Dam ‘Almost Complete’, BBC,

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Maria Perr


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