Foam formation in a biotechnological process for the removal of hydrogen sulfide from gas streams

Foam formation in aqueous suspensions of biologically produced sulfur is studied in a foam generator at 30 °C, with the objective of describing trends and phenomena that govern foam formation in a biotechnological hydrogen sulfide removal process. Air is bubbled through a suspension and the development of the foam height in time is measured, showing essentially two types of foam, unstable foam of constant foam height and stable foam with a rapidly increasing foam height. The transition between these types of foam can occur when the local sulfur concentration near the surface of the liquid is higher than a critical concentration, so that a stable network structure can be formed. Sulfur particles are transported to the top of the liquid by flotation. Upon foam formation large aggregates of sulfur fall apart into smaller fractions. Especially the larger fraction of the sulfur particles is present in the foam, indicating that these particles have the right hydrophobicity to form a network structure. Furthermore, polysulfide anions were found to have antifoaming properties in biologically produced sulfur suspensions, either because of the changing of the surface properties of the biologically produced sulfur or because of the antifoaming properties of the hydrophobic elemental sulfur formed upon the chemical oxidation of polysulfide ions.