Despite the large number of applications and the numerous advantages of surfactants in the industrial and economic fields, from an environmental point of view, they are considered as an important contaminant of the aquatic environment, also taking into account the high volumes of these substances that are spilled daily in this medium. Once used, the surfactants reach the treatment plants through urban and industrial wastewater and in certain cases are directly discharged into surface waters. During the treatment of wastewater, a high percentage of the surfactants present in the aquatic environment are eliminated by aerobic processes of biodegradation and adsorption in the particulate material, while the metabolites generated and the non-degraded surfactants are dispersed in the different environmental compartments. A growing problem is currently arising due to the mixing of waste, hospital and industrial water with significant surfactant load in wastewater treatment plants as well as surfactant mixtures of different nature. The concentrations of surfactants present in domestic wastewater can vary between 1–10 mg/L while they can reach levels of 300 mg/L in industrial wastewater (Siyal et al. 2020). Sewage treatment plants can lower the concentration of surfactants up to 1–3 mg/L but the surfactants are still present in the active sludge leading to significant environmental impacts (Bautista-Toledo et al. 2014).
The study of the environmental behavior of the main classes of nonionic and anionic surfactants has received increasing interest in recent years from downstream users and regulators. Consequently, the emphasis is being placed on considering biodegradation as a basic and determining parameter to minimize the use and release of substances that may persist in the environment, generating a great proliferation of regulatory efforts and industrial agreements.
The EU Safer Detergents Regulation (Regulation (EC) No 648/2004, 2004) which required standardized tests of primary and ultimate biodegradation for industrial and domestic uses and applications of formulations with surfactants, was fundamental to establish a harmonized performance in the global industry of surfactants.
Additionally, the widespread adoption of the Globally Harmonized System (Globally Harmonized System of Classification and Labelling of Chemicals (GHS), 2017) of classification and labelling has increased the demand for surfactants with favorable safety and environmental profiles. As biodegradation is the main mechanism for removing organic compounds, the knowledge of biodegradability of surfactants in combination with other additives is necessary to understand environmental behavior of this mixtures before designing a detergent formula.
At that respect, the special properties of the particles (1 nm to 1 µm) and the advantages that they offer in processes related to applications in catalysis, new materials or biomedicine, have led to their increasing use in consumer products such as detergents (Ma et al. 2008). Scientific interest in recent years has focused on silica nanoparticles (Slowing et al. 2010, Mamaeva et al. 2013), several detergent formulations and related formulations containing silica particles have been patented (Orlich et al. 2007). Actually nanoparticles are present in a large number of formulations and applications due to their physicochemical properties, low toxicity, stability and functionalization capacity with a range of polymers and molecules (Ríos et al. 2018a). Silica nanoparticles are frequently employed mixed with surfactants in oil recovery, nanofluids, immobilization of enzymes, removal of dyes, detergents or foam stabilizers (Maestro et al. 2014, Zhu et al. 2015, Patra et al. 2016, Plomaritis et al. 2019). Nanoparticles can contribute to the cleaning of surfaces both due to their abrasive effect and by improving the solubilisation.
In the same way as surfactants, particles of colloidal size can accumulate spontaneously in interfaces of the type liquid-gas or liquid-liquid acting as stabilizers of emulsions and foams (Eskandar et al. 2011). Simple algorithms have recently been used to estimate potential concentrations of NP from consumer products. However, the concentrations estimated by applying the abovementioned models are significantly lower than the results of many of the currently published studies (Tiede, 2009). A large variety of both organic (solid lipid nanoparticles) and inorganic materials (silica, clay, ferric oxide, titanium dioxide, alumina, carbon or latex) are used as stabilizers of oil emulsions in water or water in oil, finding numerous applications in the detergent market. When nanoparticles are used together with the surfactants, synergistic effects can be observed in the production of emulsions and the appearance of stable foams, so it is of great interest to study these interaction effects from an environmental point of view. The colloidal particles play a role similar to the surfactant molecules if they are adsorbed in a fluid-fluid interface. Just as the affinity of a surfactant for the aqueous or fatty phase is quantified in terms of its hydrophilic-lipophilic balance (HLB), for solid particles this tendency is described by its wettability, given by its contact angle (Paunov et al. 2002). Recently, Safounane et al. have reported an improved emulsifying capacity when using mixtures silica- nanoparticles and ionic surfactants as emulsion stabilizers with opposite charge (Safouane et al. 2007).
Due to the widespread use of nanoparticles in formulations in recent years, their release into the environment and wastewater is unavoidable (Huang et al. 2017), causing a toxic effect to biota and/or wastewater treatment processes. Because the increasing concern about the environmental impact of latest materials, the study of the toxicity, hazards, fate and environmental impact of nanoparticles is beginning (Liu et al. 2014; Skorochod et al. 2016 Ríos et al. 2018b). The environmental impact of individual surfactants has been widely studied, as they can show toxicity to several aquatic organisms, recalcitrant or harmful to wastewater treatment plants (Lechuga et al. 2016; F. Ríos et al. 2017). However, the interactions between nanoparticles and surfactants as well as the biodegradability in mixtures of surfactants have not been sufficiently studied until now. A recent paper by Bimová et al. 2021 summarizes possible toxic effects of nanomaterials on environment and living organisms as a consequence of its application in different technologies, environmental sectors, and medicine. However this work does not include any reference on the mixtures nanoparticle-surfactant. In our humble point of view, this reinforces the lack of knowledge on this particular field.
The predictability of joint effects of solutions containing surfactant and nanoparticles is of great interest for an adequate assessment of environmental risk due to the growing development of nanoproducts, nanomaterials and nanofluids.
The growing concern in recent years to design non-polluting detergents has led to the development and use of more environmentally friendly surfactants such as the ether carboxylic derivative surfactants and alkyl polyglucosides analyzed in this study. Surfactant ether carboxylic acid are used in cleaning and cosmetic products which are going to be in contact with the skin. These surfactants have the property of improving the foaming capacity of the surfactant formulations, decreasing the level of irritation (Jurado et al. 2011) when they are compared with other anionic surfactants. Alkyl polyglucosides have great advantages compared to other classes of surfactants. Their natural origin is the cause of their good physical and environmental properties. Moreover, alkyl polyglucosides present good compatibility and foam production, excellent cleaning efficiency, wettability and ocular and dermatological safety, and have been proved to be readily biodegradable in aerobic conditions (Jurado et al. 2002; Zgoła-Grześkowiak et al. 2008). All this makes them potential components in a variety of domestic and industrial applications (Pantelic and Cuckovic, 2014; Tasic-Kostov et al. 2014).
The OECD 301 series of readily biodegradable tests are considered a standard for screening purposes. (OECD, 1992). Ready biodegradability tests are conservative in nature and stringent enough to assume rapid and complete biodegradation of compounds in aquatic environments (OECD, 1992).
Biodegradability tests can produce variable results attributable to changes in inoculum, inoculum origin, and ratio, resulting in false negatives. (Lundgren et al. 2013). In this sense, "positive" results can be considered sufficient evidence of biodegradability and can generally substitute for negative results.
. Due to the high production volumes and the massive and dispersed use of surfactant-based formulations, this work has focused on biodegradation of anionic and nonionic surfactants and their relative risk profiles compared to mixtures of surfactants and surfactants-nanoparticles.
In this work, it has been studied the aerobic biodegradability of nanofluids, solutions containing silica nanoparticles in combination with an anionic surfactant (ether carboxylic acid), a non-ionic surfactant (alkyl polyglucoside), whose individual environmental impacts have been previously assessed (Jurado et al. 2013; Ríos et al. 2017), and mixtures of them. In addition, with the purpose of gain insight into the environmental behavior and other aspects related to interfacial phenomena and cleaning efficiency, the effect of nanoparticles on the surface, interfacial tension and Critical Micellar Concentration (CMC) of surfactants and mixtures have been measured.