Abstract
The anionic surfactant sodium lauryl ether sulfate (SLES) is the main component of most commercial foaming agents (FAs) used in the excavation of highway and railway tunnels with Earth pressure balance-tunnel boring machines (EPB-TBMs). Several hundreds of millions of tons of spoil material, consisting of soil mixed with FAs, are produced worldwide, raising the issue of their handling and safe disposal. Reducing waste production and reusing by-products are the primary objectives of the “circular economy,” and in this context, the biodegradation of SLES becomes a key question in reclaiming excavated soils, especially at construction sites where SLES degradation on the spot is not possible because of lack of space for temporary spoil material storage. The aim of the present work was to apply a bacterial consortium (BC) of SLES degraders to spoil material excavated with an EPB-TBM and coming from a real construction site. For this purpose, the BC capability to accelerate SLES degradation was tested. Preliminary BC growth, degradation tests, and ecotoxicological evaluations were performed on a selected FA. Subsequently, a bioaugmentation experiment was conducted; and the microbial abundance, viability, and SLES concentrations in spoil material were evaluated over the experimental time (0.5, 3, 6, 24, 48, and 144 h). Moreover, the corresponding aqueous elutriates were extracted from all the soil samples and analyzed for SLES concentration and ecotoxicological evaluations with the bacterium Aliivibrio fischeri. The preliminary experiments showed the BC capability to grow under 14 different concentrations of the FA. The maximum BC growth rates and degradation efficiency (100%) were achieved with initial SLES concentrations of 125, 250, and 500 mg/L. The subsequent bioaugmentation of the spoil material with BC significantly (sixfold) improved the degradation time of SLES (DT50 1 day) compared with natural attenuation (DT50 6 days). In line with this result, neither SLES residues nor toxicity was recorded in the soil extracts showing the spoil material as a by-product promptly usable. The bioaugmentation with BC can be a very useful for cleaning spoil material produced in underground construction where its temporary storage (for SLES natural biodegradation) is not possible.
Highlights
The anionic surfactant sodium lauryl ether sulfate (SLES) is the main component in most foaming agents (FAs) used in the excavation of highway and railway tunnels with Earth pressure balance-tunnel boring machines (EPB-TBMs) (Barra Caracciolo et al, 2017)
A positive correlation between SLES concentrations and a toxic effect on the bacterium was found in a 2-year monitoring of the spoil material from a highway construction site, making it possible to establish 2 mg/L as a “threshold value” of the toxic effect in the elutriates produced from spoil material (Mariani et al, 2020)
The aim of the present work was to test if adding the SLES pre-grown microbial culture (BC) to spoil material from a tunnel construction site enhanced the removal of the anionic surfactant residues, with potentially significant economic and environmental benefits
Summary
The anionic surfactant sodium lauryl ether sulfate (SLES) is the main component in most foaming agents (FAs) used in the excavation of highway and railway tunnels with Earth pressure balance-tunnel boring machines (EPB-TBMs) (Barra Caracciolo et al, 2017). Surfactants, including SLES, can be toxic for aquatic ecosystems if present at concentrations higher than the critical micellar one (Barra Caracciolo et al, 2017; Mariani et al, 2020; Rolando et al, 2020). The presence of SLES in spoil material can influence its recycling as a by-product; if the residual concentration of SLES is high, there can be toxic effects. A positive correlation between SLES concentrations and a toxic effect (inhibition of bioluminescence) on the bacterium was found in a 2-year monitoring of the spoil material from a highway construction site, making it possible to establish 2 mg/L as a “threshold value” of the toxic effect in the elutriates produced from spoil material (Mariani et al, 2020)
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