Surfactants estuaries

During the wastewater treatment, an oxidative conversion of the surfactant molecules leads predominantly to the formation of polar compounds. They display a particularly high solubility and mobility in the aqueous medium and, therefore, transportation over relatively long distances can occur if they are not further degraded, resulting in the wide dissemination of these pollutants in riverine systems and thus also to estuaries, coastal regions and ultimately the marine environment (see Chapters 6.2 and 6.3). In the latter, the final levels will mainly be influenced by dilution effects and physical removal by precipitation or adsorption [63] because of relatively low microbial activity in this ecosystem compared with fresh water environments [64]. 

Static die-away tests were performed by Potter et al. with an A9PEO7 24 mixture in water from a vertically well-mixed estuary in Florida [36]. Lag times of 0—12 days were observed, and after 4—24 days, primary degradation was complete. These rates are similar to those reported by Kvestak and Ahel [6]. It is likely that the microorganisms in these experiments were pre-acclimated to biodegrade nonionic surfactants, as a municipal sewage treatment plant discharge is present a few kilometres upstream. 

Dissolved concentrations of non-ionic surfactant in estuaries are lower than those found in rivers, with reported differences of around one order of magnitude [4,11,25]. However, in some cases local sources in the estuary are the cause of high surfactant levels [9,11]. Concentrations of octylphenol ethoxylates usually amount to levels about one order of magnitude below those of the NP derivatives, reflecting the production volumes of both classes of compounds. 

Surfactant loadings from estuaries to marine environments are in most cases very low. 

In stratified estuaries, surfactants tend to accumulate at the phase boundaries air-water and freshwater-saline water, leading to a complex vertical distribution pattern in the water column [5,6]. 

In conclusion, the processes dominating the environmental fate of non-ionic surfactants in estuaries are biodegradation in the water column, sorption/sedimentation and volatilisation, together with the dynamics of the estuary, in particular water residence times. 

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