The fate of tetrathionate during the development of a biofilm in biogenic sulfuric acid attack on different cementitious materials.

The biodeterioration of cement-based materials in sewer environments occurs because of the production of sulfuric acid from the biochemical oxidation of HS by sulfur-oxidizing bacteria (SOB). In the perspective of determining the possible reaction pathways for the sulfur cycle in such conditions, hydrated cementitious binders were exposed to an accelerated laboratory test (BAC test) to reproduce a biochemical attack similar to the one occurring in the sewer networks. Tetrathionate was used as a reduced sulfur source to naturally develop sulfur-oxidizing activities on the surfaces of materials. The transformation of tetrathionate was investigated on materials made from different binders: Portland cement, calcium aluminate cement, calcium sulfoaluminate cement and alkali-activated slag. The pH and the concentration of the different sulfur species were monitored in the leached solutions during 3 months of exposure. The results showed that the formation of different polythionates was independent of the nature of the material. The main parameter controlling the phenomena was the evolution of the pH of the leached solutions. Moreover, tetrathionate disproportionation was detected with the formation of more reduced forms of sulfur compounds (pentathionate, hexathionate and elemental sulfur) along with thiosulfate and sulfate. The experimental findings allowed numerical models to be developed to estimate the amount of sulfur compounds as a function of the pH evolution. In addition, biomass samples were collected from the exposed surface and from the deteriorated layers to identify the microbial populations. No clear influence of the cementitious materials on the selected populations was detected, confirming the previous results concerning the impact of the materials on the selected reaction pathways for tetrathionate transformation.