Characteristics and environmental fate of the anionic surfactant sodium lauryl ether sulphate (SLES) used as the main component in foaming agents for mechanized tunnelling☆
Graphical abstract
Introduction
Anionic surfactants (ANS) are a heterogeneous group of chemicals that owing to their amphipathic characteristics are currently used in a wide range of commercial products, including those used in many technological applications and research. They consist of a water soluble polar head group (which can be charged or uncharged) and hydrophobic non-polar hydrocarbon tails (Ying, 2006) and are designed to have mainly cleaning or solubilisation properties (Swisher, 1987).
Anionic surfactants are constituted by a predominantly linear aliphatic hydrocarbon chain (length between C8 and C18) with a polar sulphate or sulfonate group, neutralized with a counter ion (e.g. Na+, K+, NH4+, or alkanolamine cation) (Cserhàti et al., 2002, Wibbertmann et al., 2011). Many ANS are substances constituted by several homologues, in which the composition and length of both the hydrocarbon and the ethoxy tails can differ; owing to the variability of their molecular composition, ANS can be considered Unknown or Variable composition, Complex reaction products or Biological materials (UVCB) as defined by the European Chemical Agency (ECHA).
Over the past 50 years anionic surfactants have been extensively used as detergents and cleaning products due to the shift from the use of powder domestic detergents to synthetic liquid ones (Scott and Jones, 2000). In any case, commercial ANS are available in different forms, such as powders, granules, needles, pastes and solutions (Könnecker et al., 2011). ANS concentration varies between 3% and 20% in household cleaning and personal care products, such as laundry and liquid dishwashing detergents, shampoos, hair conditioners and liquid soap. They are also successfully employed in pharmaceutical, agricultural and pesticide formulations and the oil recovery, pulp and paper and excavation industries (Ying, 2006, Cserhàti et al., 2002, Van Ginkel, 1996, Lara-Martin et al., 2008; Baderna et al., 2015). It has been estimated that ANS are about 60% of worldwide surfactant production (Holberg et al., 2002). Given that they can be produced easily and cheaply, it is forecast that worldwide surfactant sales will grow between 2015 and 2019 at a Compound Annual Growth Rate (CAGR) of 6.0% (Jackson et al., 2016). Although wastewater treatment plants have a high removal efficiency (95–99%) owing their diffuse use and high consumption rates, surfactants can be found in water ecosystems at quite low concentrations, ranging from few μg/L to mg/L (Lara-Martin et al., 2008, Corada-Fernandez et al., 2011). Soil can be contaminated through biosolids used as fertilizer or soil irrigation using reclaimed water at concentrations from 120 to 1180 μg/kg (Corada-Fernández et al., 2015).
The ANS category includes several classes of compounds. The most common are alkylbenzene sulfonate (ABS) which can be linear (LABS) or branched (BABS), alkyl sulphate (AS) and alkyl ether sulphate (AES) (Ying, 2006, Van Ginkel, 1996), (Fig. 1). As many as twenty-one anionic surfactants are considered “high production volume chemicals (HPV)” in many Countries (Könnecker et al., 2011) and this implies a potential impact of these chemicals on the environment.
Particular interest has recently been focused on the environmental fate of AS and AES, used as soil conditioning agents in the excavation industry. Over the last years, the excavation industry has grown significantly, with a radical switch from “drilling and blasting” to tunnelling using a tunnel boring machine (TBM) (Milligan, 2000, Baderna et al., 2015).
Moreover, in hydraulic fracturing (fracking), where a mixture of fluids is pumped into recovery wells under high pressure to fracture low permeability formations to increase gas and oil production, water is mixed with 0.5–2.0% volume of selected chemicals (Stringfellow et al., 2014). A mixture of several chemicals including anionic surfactants like sodium lauryl sulphate SLS (Waxman et al., 2011) and sodium lauryl ether sulphate, SLES (Milligan, 2000), is added to optimize the fracturing process. A review on the overall chemicals (approximately 1000 chemicals) used in fracking has been published recently (Waxman et al., 2011). However, the environmental concern raised by fracking is limited to the countries producing oil and gas through this process. On the contrary, there is an increasing global interest on the environmental fate of the high amount of soil foaming agents containing AES, and particularly SLES, used in mechanized tunnelling with TBMs to build railways and road tunnels (Cserhàti et al., 2002, Baderna et al., 2015, Ivanković and Hrenović, 2010, Ground Water Protection Council, 2009). The presence of high concentrations of anionic surfactants can pose a possible risk for the environment, influencing the subsequent re-use of tons of soil debris as by-products. There are currently neither SLES soil threshold limits in European legislation, nor comprehensive studies on its ecotoxicological effects on the soil ecosystem. Moreover, data on SLES biodegradation in field studies and environmental risk evaluations examining soil exposure scenarios during the excavation process are not available so far. It is of crucial importance to assess if SLES occurrence and persistence in soil debris can or not pose a potential risk for the terrestrial and water compartments and human health.
In this context, the present review critically reports the data available and the gaps in knowledge regarding SLES biodegradation and its ecotoxicological effects (acute and chronic ecotoxicity), with a focus on the terrestrial compartment. Moreover, it describes the main analytical tools currently available for detecting anionic surfactants in soils and solid matrices.
Section snippets
Soil foaming agents in mechanized tunnelling
Recent decades have been characterized by a fast worldwide growth in underground construction in the form of new infrastructures such as pipelines and communication cables as well as road and railway tunnels. The utilization of underground space offers a new strategy to urban planning (ISST, 1998), including a huge development of the mechanized tunnelling industry by the use of TBM. There are two main types of TBM, hard rock and soft ground (Shahriar et al., 2008), with the latter subdivided
Sodium lauryl ether sulphate: main use, physico-chemical properties and hazard classification
Sodium lauryl ether sulphate or laureth sulphate (SLES) is a sodium salt produced through the ethoxylation of the AS sodium lauryl sulphate (SLS) (Cserhàti et al., 2002). This generates an ethoxylate, which is transformed into a half ester of sulphuric acid and neutralized to achieve SLES. SLES and SLS are among the most common anionic surfactants found in a wide range of commercial products for different purposes. SLS, a member of the alcohol sulphate family, is made through the sulphation of
Concluding remarks and future perspectives
The anionic surfactant sodium lauryl ether sulphate is the main component of most foaming agents used worldwide in soil conditioning during mechanized excavation. Foaming agents are typically used in tunnelling excavation between 0.1 and 3 L/m3 of soil. Because the SLES percentage in these commercial products ranges from 10% to 50%, its expected environmental concentrations are from 40 to 500 mg/kg in the treated soil. Considering the values of the effective concentrations for many aquatic
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This paper has been recommended for acceptance by Charles Wong.