Assessment of biodegradation of the anionic surfactant sodium lauryl ether sulphate used in two foaming agents for mechanized tunnelling excavation

The anionic surfactant sodium lauryl ether sulphate (SLES) is the main component in most foaming agents used for mechanized tunneling excavation. The process produces huge amounts of soil debris that can have a potential impact on ecosystems. The lack of accurate information about SLES persistence in excavated soil has aroused increasing concern about how it is recycled. The objective of this study was to assess SLES biodegradability in two commercial foaming agents (P1 and P2). Microcosm experiments were performed with two different soils collected from a tunnel construction site and conditioned with P1 or P2 (85.0 or 83.0 mg kg -1 of SLES, respectively). At selected times soil samples were collected for assessing the SLES residual concentration using Pressured Liquid Extraction followed by methylene blue active substance analysis (MBAS). Simultaneously, soil microbial abundance (DAPI counts), viability (Live/Dead method), activity (dehydrogenase analysis) and phylogenetic structure (Fluorescent In Situ Hybridization) were evaluated. SLES halved faster in the silty-clay soil (6 d) than in the gravel in a clay-silty-sand matrix (8-9 days). At day 28 it was degraded in both soils. Its biodegradation was ascribed to the significant increase in Gamma-Proteobacteria. At this time, the spoil material can be considered as a by-product.