Sulphurous compounds are process chemicals in industries (such as Kraft-pulping and are present in crude oil used in petroleum refining. Sulphurous compounds can accumulate in the process streams increasing chemical and water consumption. Some are toxic and odorous and cause corrosion. Therefore, the excess sulphur needs to be removed from the process streams.
Sulphur oxidizing bacteria (SOB) from soda lakes produce elemental sulphur from soluble sulphurous compounds under extreme industrial conditions of high pH and salinity. SOB biologically transform reduced sulphurous compounds (e.g., hydrogen sulphide, polysulphide and thiosulphate) to elemental sulphur that can be separated from solutions. Harsh process stream conditions (high pH, salinity) are tolerated by SOB from alkaline and salt-saturated environments such as soda lakes.
“These high pH and salty environment loving bacteria represent interesting options for developing circular economy in process industries,” Reka Hajdu-Rahkama explains.
Biotransformation into elemental sulphur and sulphate
In her doctoral dissertation Hajdu-Rahkama used two bacterial strains, Thioalkalivibrio versutus and T. denitrificans. Those make their living by conversions of inorganic sulphur compounds especially present in pulping streams.
In her research, Réka Hajdu-Rahkama investigated the potential of biological sulphur recovery by haloalkaliphilic SOB from haloalkaline solutions for possible application in industrial process streams and wastewaters.
The laboratory studies showed thiosulphate biotransformation to elemental sulphur by both extremophiles at high thiosulphate concentrations.
“Efficient thiosulphate biotransformation to elemental sulphur and sulphate at high loading rates was obtained in a continuous laboratory-scale bioreactor. The process and bioreactor design can be further optimised to increase elemental sulphur recovery,” Hajdu-Rahkama says.
High tolerance towards wastewaters and organic compounds
Sulphur oxidizing bacteria build up all their cells from carbon dioxide but cannot use organic compounds as we do. On the opposite, SOB may be sensitive to organics present in industrial wastewaters.
“The results were very promising as T. versutus tolerated the concentrated wastewaters and their organic constituents and thus showing potential for sulphur recovery from these wastewaters,” she concludes.
Biologically produced elemental sulphur can serve e.g., as raw material for fertilizer or sulphuric acid production. Biological sulphur is hydrophilic, meaning that it is easy to use by microorganisms and plants.
Réka Hajdu-Rahkama is originally from Hungary and has lived in Finland since 2010. She is currently working as a researcher at the Faculty of Engineering and Natural Sciences at Tampere University, focusing on nutrient recovery and reduction of off-flavour compounds in recirculating aquaculture systems with (bio)electrochemical approaches.
The doctoral dissertation of Réka Hajdu-Rahkama in the field of Bio- and Circular Economy titled Potential of Biological Sulphur Recovery Under Haloalkaline Conditions will be publicly examined in the Faculty of Engineering and Natural Sciences at Tampere University in auditorium Pieni Sali 2 in the Festia building (address: Korkeakoulunkatu 8, Tampere, Finland) at 12:00 on Friday, 9 December 2022. The Opponent will be Professor David Gabriel from the Autonomous University of Barcelona, Spain. The Custos will be Professor Jaakko Puhakka from the Faculty of Engineering and Natural Sciences at Tampere University.
Photo: Jaakko Rahkama