Soil persistence of the anionic surfactant sodium lauryl ether sulphate in two different foaming agents used for a mechanized tunnelling

The anionic surfactant sodium lauryl ether sulphate (SLES) is the main component of most commercial products used for soil conditioning in the excavation industry, in particular as lubricants for mechanized tunnelling. Its use during the excavation processes can result in either the subsequent possible re-use of the huge amount of soil debris as by products (e.g. land covering) or its discharge as waste. Currently, there are neither SLES soil threshold limits in European legislation, nor comprehensive studies on the environmental risk for soil ecosystems in these exposure scenarios. Although SLES is generally reported to be biodegradable in standard tests, data on its biodegradation in environmental studies are quite scarce. Consequently, assessing SLES biodegradation rates in field conditions is crucial for evaluating if in residual concentrations it can or not be a potential hazard for terrestrial and water organisms. In this context, a set of microcosms was set up using soil samples from a construction site. Soil samples were treated separately with one of two commercial products (P1 and P2) in absence/presence of lime at the concentrations used for the mechanized drill. Lime treatment is used because chemically transforms unstable soils into usable materials, making it possible their appropriate use for land covering. At selected times (0, 7, 14, 21, 28 d) soil samples were collected for assessing SLES concentration in the different conditions. Moreover, microbiological analysis were performed in order to assess microbial abundance, cell viability, dehydrogenase activity and microbial structure. The SLES degradation rates were higher when SLES was present in the P1 (DT50=11 d) foaming agent than in the P2 (DT50= 17d) one. Lime addition slowed down SLES halving in P1 (DT50= 31 d), while inhibited its degradation in P2. In line with these results, the bacterial community was differently affected by the foaming agent P1 or P2 both in presence and absence of the lime. The overall results show SLES as a biodegradable compound and that the different formulation of each specific foaming agents (P1 or P2) can affect its persistence.