Carbon sequestration could help to neutralise red sludge

The red sludge that escaped a containment pond in Hungary last week could be made less toxic with the help of carbon sequestration.

This is the claim of an Indiana University Bloomington geologist who has a patent pending on the technique.

The bauxite residue now covers 40km2 south of the Danube River and has caused the deaths of eight Hungarians and injured at least 150.

While human deaths in the wake of the disaster may have been strictly a result of the containment failure, injuries have mostly been attributed to the chemical properties of the sludge, whose high pH (between 11 and 13) can quickly damage and kill living cells. Bauxite residue is between 10,000 and 1,000,000 times more basic than pure water, which has a pH of around 7.

‘We propose that one way to reduce the pH of bauxite residue is to mix it with another kind of industrial waste – oil-field brine, which is a byproduct of oil and gas production – and then carbon dioxide,’ said IU Bloomington geologist Chen Zhu, who submitted a US Department of Energy patent application in 2007 describing the technique.

The water-based brine provides the medium for carbon dioxide to dissolve. Once dissolved, the carbon dioxide can chemically react with water to form carbonic acid. The carbonic acid counteracts some of the red mud’s alkalinity and what’s left – the negatively charged carbonate – can serve as a partner for positively charged metal ions, such as iron, calcium and magnesium.

Bauxite residue, more commonly referred to as red sludge, is the waste created by industries that produce aluminum. At present, the residues simply accrue in containment ponds. Worldwide, there are in excess of 200 million tons in these ponds.

‘Companies don’t voluntarily spend money to neutralise waste unless someone tells them to do it, sadly,’ Zhu said. ‘Our technique could be quite expensive. When you have a disaster like the one we’re seeing in Hungary, though, I think perhaps companies and governments may think twice about what ’too expensive’ means.’

Zhu is working with the American aluminum producer Alcoa and the Department of Energy to perfect the technology.