Fluorinated surfactants are used in manifold industrial applications and consumer goods due to their outstanding and unique physical-chemical properties. However, they are also under discussion due to their persistence, possible toxicity to humans as well as bioaccumulation of long-chain representatives. Fluorinated surfactants are used amongst others in electroplating manufacturing. Nowadays, 6:2 fluorotelomeric sulfonate (6:2 FTSA) is applied in many companies as a substitute for perfluorooctanoic sulfonate (PFOS), which formerly was almost exclusively used for that purpose. So far, satisfying methods for the treatment of fluorosurfactant-containing electroplating wastewater are still missing.
For the trace-analysis of perfluorinated alkylated acids (PFAA) as potential transformation products of 6:2 FTSA in chromium-containing matrix a solid phase extraction (SPE) was established. Therefor a hydrophilic lipophilic balanced SPE material was used and recovery rates of the different PFAA ranged from 75 to 120 %. Applying this SPE-method, perfluorinated carboxylic acids (PFCA) with chain length of C5, C6 and C7 were found in wastewater of an electroplating company. Experiments in a laboratory scale confirmed the electrolytic degradation of 6:2 FTSA to PFCA with a chain length ≤ C7. However, the amount of those transformation products was relatively low (< 0.1 % within 900 minutes of electrolysis).
In this work a new treatment method to remove fluorosurfactants from aqueous solutions has been established, especially from electroplating wastewater. The elimination technique is based on the generation of gas bubbles in solution by passing gas through fritted glass, which leads to an enrichment and scavenging of fluorosurfactants by rising gas bubbles and subsequent transport of the gas bubbles to the water surface. Finally the bubbles collapse and release an aerosol which is enriched with fluorosurfactants. These processes are also the reason for an elimination of fluorosurfactants during electrolysis when aerosol-formation takes place due to gas evolution.
Fluorinated surfactants are powerful in their surface activity. Therefore this method, which is based on surface effects, is selective for this class of compounds. This stands in contrast to other treatment options like adsorption or membrane processes.
By experiments that used aqueous solutions with defined matrix an elimination rate of > 99 % within 60 minutes could be achieved for 6:2 FTSA. The 6:2 FTSA concentration decreased exponentially with a half-life of approximately 2 minutes under optimum conditions. When using fritted glass with a low pore diameter a faster elimination was observed. In respect to the gas flow rates no significant influence on the surfactant elimination was found between 3 and 30 ml·min-1·cm-2. However, if the surfactant content exceeds the characteristic concentration resulting in the formation of a foam layer, the elimination is hindered. Additionally, a high ionic strength is advantageous to achieve a reproducible, effective and fast elimination. In this respect, this method differs from other techniques like adsorption and membrane treatments, which usually suffer from a decrease in effectiveness in presence of high amounts of salts. Additionally, by applying the established method it is possible to reduce the fluorosurfactant concentration to < 0.3 μg·l-1. Other surfactants like PFOS and perfluorooctanoate (PFOA) could also be removed from aqueous solutions with elimination rates of 99.9 and 99.8 % within 60 minutes. The concentration of perfluorobutanoic sulfonate (PFBS) could only be decreased by 70 % due to its lower surface activity.
The investigated method was further applied successfully to different electroplating wastewater samples, whereas no sample pre-treatment was carried out. By collection of the released aerosols and drawing a corresponding mass balance, it was shown that the surfactants, which were removed from the solution, were contained in the aerosol phase. When comparing the surfactant concentrations in the released aerosols with those previously in solution, enrichment factors between 50 and 2000 were determined. Thus, there is the possibility of achieving a closed loop for the surfactants, especially due to the fact that only the surfactants but no other matrix compo¬nents are enriched by the described procedure.
In respect to the aerosol formation process, visual examination by a high-speed camera confirmed the release of jet drops as a result of the gas bubble collapse at the solution surface.