**5. Environmental risks and toxicity of surfactants**

Surfactants are economically important and vital to our economy. They are a diverse group of chemicals, widely used by society and continue to be part of our daily life. However, as new surfactants are synthesized annually and surfactants production overall continue to rise, concerns about their impact on the environment and human health have been raised and studied [25–27]. Achieving high contaminant removal and mass transfer without causing any negative effects on the soil system are the primary considerations in the application of surfactants. Toxicity and biodegradability of surfactants are typically tested under different environmental conditions based on the intended application. Typically, most surfactants are not considered acutely toxic to organisms at concentrations typically encountered in the environment. Toxicity is measured in terms of effective concentration (EC50) or lethal concentration (LC50). EC50 represents the surfactant concentration (mg L�<sup>1</sup> ) that results in a 50% reduction in a microbial population or a biological community. LC50 refers to the concentration of a surfactant that causes the death of the microbial soil community or living organisms after 96 hours of exposure. Surfactants, including their metabolites, that have a toxic effect on a soil microbial community is referred to as xenobiotic surfactants. The harmful effects of xenobiotic surfactants occur through the rupture and penetration of the cellular membrane by interacting with lipids and proteins [28]. Nonetheless, the relationship between surfactants chemical structure, physicochemical parameters, biological activity and environmental impact is still ambiguous. Even less studied and understood are the comingling effects of multiple surfactants on the soil ecosystem. It can be hypothesized while a single surfactant may have minimal adverse impact on the environment. In the presence of other surfactants, it may have antagonistic effects in the soil and other terrestrial ecosystems.

In general, the two main challenges related to surfactant-enhanced soil remediation are their toxicity and biodegradability. Surfactants are considered to be

biodegradable if its molecular structure can be mineralized by the soil natural microbes through metabolic activities. On the other hand, toxicity reflects the adverse impact created by surfactants on the soil biota. Generally, the order of surfactants toxicity are biosurfactants < nonionic < anionic < cationic. Toxicity effects of surfactants may occur when a surfactant coats, sorbs onto soil particles and accumulate to toxic level. This leads to the formation of a hydrophobic layer around the soil aggregates which modifies the soil hydrophobicity. The effects are destruction of soil ability to absorb water, reduction of water infiltration into the soil. If surfactants accumulate in soils to toxic level around the plants rhizosphere, the phytotoxicity effects of the surfactant will lead to growth reduction and crops yield or death of vegetation. Most synthetic surfactants used in soil remediation are not readily biodegraded by the soil microbes and can result in toxic adverse effects on the soil ecosystem [29]. Ionic surfactants such as SDS and CATB are highly biodegradable, but exhibits high toxicity. In contrast, the nonionic surfactant Tween [30] and the biosurfactant Rhamnolipid [31] are highly biodegradable and has low toxic. Surfactants used in soil remediation and their degradation products may leach into the aquifer or enter other components of the terrestrial system. The endocrine system is a network of glands and organs that produce, store, and secrete hormones. If exposed to these substances, they would have the potential to disrupt the normal functioning of endocrine system in wildlife and human beings [32].
