Obtaining acetic acid from bacteria and CO2: the case of Photo2Fuel

One of the chemicals that is being obtained in the Photo2Fuel project, apart from biomethane, is acetic acid. Discover more.



One of the chemicals that is being obtained in the Photo2Fuel project, apart from biomethane, is acetic acid. The recovery of this compound is being studied by the Spanish research institution TECNALIA. In this article we will discover more about the process and the team behind it.

What is acetic acid?

Acetic acid is an organic compound with a global demand of approximately 15 million tons per year widely used in printing, food and pharmaceutical industries and, also, for the manufacture of cellulose acetate (the one used in photographic films, for example) and different types of plastics. Most of the chemical processes used in petrochemical plants produce acetic acid and other types of carboxylic acids. The acids are not pure and cannot be recovered easily, making the recovery of acetic acid complex, less efficient, and expensive. In order to run the processes smoothly, these substances need to be removed from the solution. That's why it's important to recover acetic acid from water-based solutions like chemical waste or fermentation liquids, and to make sure that the separation processes used are sustainable.

Acetic acid in the Photo2Fuel project

The specific aim of Photo2Fuel is to convert CO2 into acetic acid and methane by means of artificial photosynthesis using a hybrid system of non-photosynthetic bacteria and organic photosensitisers, with sunlight as the only energy source. In Photo2Fuel, we are studying how to recover acetic acid in the purest form possible from the media in which this photosynthesis process takes place. The acetic acid is obtained in a very low concentration in a solar reactor, in an aqueous medium where bacteria convert CO2 into the acid with the aid of polymer dots (photosensitisers). The team in TECNALIA (Spain), one of the Photo2Fuel partners, is in charge of this challenging research. 

TECNALIA is the leading private and independent research and technology organisation in Spain and one of the largest in Europe, employing around 1,472 people. TECNALIA collaborates with companies and institutions to improve their competitiveness, and people's quality of life and achieve sustainable growth. Their main areas of action are fully aligned with the Sustainable Development Goals: smart manufacturing, digital transformation, energy transition, sustainable mobility, personalised health and urban ecosystem. The team in TECNALIA working in Photo2Fuel is composed of Dra. Laura Sánchez, Dra. Estefania Estalayo, Dr. Tomás Roncal and Dra. Yolanda Belaustegui of the Energy, Climate and Urban Transition Unit. TECNALIA’s role in the Photo2fuel project is in the development of methodologies for the separation of acetic acid employing ion exchange resins, liquid-liquid extraction using ionic liquids and electrodialysis separation technologies.

Recovering the purest acetic acid: challenge accepted!

As we have mentioned before, the recovery and purification of acetic acid is often very costly, especially when its concentration is extremely low, as expected in this project. Photo2Fuel will study 3 technologies in order to achieve an efficient separation of the acetic acid produced in the project: ion exchange resins, liquid-liquid extraction using ionic liquids and electrodialysis.

I) Ion Exchange resins

Using ion exchange resins, the acetic acid can be extracted from the aqueous medium. But the acetic acid-containing medium also contains a mixture of inorganic salts that could interfere with the purification of the acid. Therefore, a pre-treatment of the medium, called demineralization, involving Ca-precipitation and a mixed-bed ion exchange resin has been applied to remove such salts to facilitate subsequent purification of acetic acid through the different technologies proposed. Following demineralization, both strong and weak anion exchange resins are to be applied to recover acetic acid from the medium.

Ion exchange resins for acetic acid

II) Liquid-liquid extraction using ionic liquids (ILs)

Ionic liquids are salts usually composed of organic cations and organic or inorganic anions. Many of them are liquid at room temperature and their vapor pressure is very low being, therefore, non-volatile. The specific objective of the liquid-liquid extraction studies in Photo2Fuel using ionic liquids is to separate and concentrate the acetic acid from the Moorella Thermoacetica culture used for the artificial photosynthesis process, avoiding the use of hazardous solvents and increasing the efficiency and selectivity of the process. In this case, the acetic acid aqueous media is put in contact with the ionic liquid containing phase forming two phases (one aqueous and another one organic). During the contact of the two phases, a complex is formed between the acetic acid and the ionic liquid and this migrates to the organic phase which has a smaller volume than the aqueous one in order to concentrate the acetic acid. Subsequently, a back-extraction step is carried out recovering the extracted acetic acid and regenerating the ionic liquid. Preliminary results using this method have demonstrated that the selected ionic liquid could achieve acetic acid extraction yields up to 90% and distribution coefficients up to 36, depending on the conditions.

Liquid-liquid extraction for acetic acid

III) Electrodialysis (ED)

The ED is an electrochemical separation process using an electrical potential difference as a driving force to separate ionic species through ion exchange membranes. In electro-membrane processes, organic acids are separated from water and other impurities based on the electro-migration of ions through ion exchange membranes. The preliminary results using electrodialysis showed that it is possible to remove 33% of acetic acid from Thermoacetica culture with 10% current efficiency. The low current efficiency is due to the acetic acid-containing medium also containing a mixture of inorganic salts which interfere with the purification of the acid, so it is necessary to find an ion exchange membrane selective to carboxylic acids.

Electrodialysis for acetic acid

Next steps in the recovery of acetic acid

In the next year of the Photo2Fuel project, the three separation technologies will be tested to reach the most promising, sustainable solutions at an economic and environmental level for the recovery of acetic acid.  
The aim of this task of the project is not only to recover as much acetic acid as possible (>90%) in the purest form (>95%) from the extremely diluted medium, but also to increase its final concentration by, at least, two orders of magnitude.


Tomás Roncal, Estefanía Estalayo, Laura Sánchez-Cupido, Yolanda Belaustegui (TECNALIA), Mara Mennella (KNEIA)

A poster from TECNALIA on acetic acid separation is also available on Zenodo.  

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