So far, alkaline industrial wastewater from steel or cement production has been neutralized with acids and then discharged into rivers unused. But this wastewater can be used to bind the greenhouse gas carbon dioxide from air or exhaust gases. Researchers have developed a process with which this can be done easily and without requiring a lot of energy. As they have calculated, millions of tons of CO2 could be neutralized globally.
Climate protection has not yet been enough to stop the continued rise in carbon dioxide emissions and thus global warming. This is why technical solutions such as Carbon Dioxide Removal (CDR) are increasingly coming into focus. The aim of these methods is to separate the greenhouse gas from the air or exhaust gases and then bind it – either in usable chemicals or in permanent storage.
Industrial wastewater with untapped potential
Researchers led by Helmuth Thomas from the Helmholtz Center Hereon in Geesthacht are now proposing a new approach to binding the greenhouse gas CO2. The focus of their method is on alkaline industrial wastewater: “This wastewater occurs in large quantities – in cement or steel production, for example,” explains Thomas. Environmental regulations stipulate that this wastewater may only be discharged into rivers if it has previously been brought to largely neutral pH values. “So far, strong mineral acids such as sulfuric acid and hydrochloric acid have been used for this,” writes the team.
But this means that the potential of this wastewater for binding CO2 remains unused, as Thomas and his colleagues explain. It is known from the oceans that dissolved carbon compounds are released in the form of carbonate ions (CO32-) or hydrogen carbonate (HCO3–) act as a buffer: They are formed when CO2 dissolved in water reacts with OH ions under alkaline conditions. Exactly this reaction could also be used for carbon removal in alkaline industrial wastewater. “Our process is basically based on a reaction that we still know from chemistry lessons – the neutralization of an alkali by an acid,” says Thomas.
CO2 and lye become hydrogen carbonate
The heart of the system, dubbed “Wastewater Alkalinity Preservation” (WAP), is a mixing chamber into which wastewater, CO2 and river water are introduced. “This key component is designed to promote gas-liquid contact under controlled hydraulic conditions,” the researchers explain. “The CO2 neutralizes the wastewater by binding the excess hydroxide ions. This lowers the pH value to a level that meets environmental requirements,” write Thomas and his team. The CO2 is converted into hydrogen carbonate and carbonate ions.
The researchers used a model calculation to determine the potential of these methods. “This wastewater treatment could bind 4 to 10 kilograms of CO2 per cubic meter of wastewater for highly alkaline wastewater such as from aluminum processing or certain cement processes,” report Thomas and his colleagues. A medium-sized factory with such wastewater could bind around 1.3 kilotons of CO2 per year. With less alkaline wastewater it would be correspondingly less. “Globally, our calculations suggest a total potential for this CDR process of 11 to 32 million tons of CO2 per year,” said the researchers. In her opinion, this approach is worthwhile – also because the energy consumption of the systems is low and the necessary technology is already available.
Source: Helmholtz Center Hereon; Specialist article: Environmental Science & Technology Letters, doi: 10.1021/acs.estlett.6c00081