This work focuses on investigating the sorption behaviour of the free fatty acid (FFA) (Palmitic Acid) onto varied materials. Understanding sorption behaviour on different materials is essential in helping to design materials that will act as filters in the water/sewage system to help prevent the formation of FOG (fats oil grease) along the sewer network which cause blockages. Palmitic, oleic and linoleic acid are the three major free fatty acids that tend to be found present in FOG deposits. FOG deposits tend to form along the pipeline cross-section above the water level causing blockages (He, et al., 2013). Removing the fatbergs (large deposits formed due to build-up of FOG materials, wet wipes, sanitary products etc) results in high costs (Stein & Miller, 2019). In FOG solids, palmitic acid has been identified as the most prominent free fatty acid (Iasmin, et al., 2016).

The Quartz crystal microbalance with dissipation (QCMD) is an acoustic technique where a change in resonance frequency directly correlates with the interaction of the adsorbate on the adsorbent i.e., the sensor surface and thus useful in understanding sorption behaviour. Typically, when a mass adsorbs onto the surface of the electrode sensor, it causes stress on the sensor surface which is interpreted as a negative shift in frequency while the dissipation energy increases (Biolin Scientifc, 2020). The frequency and dissipation shift can also give indication of whether the adlayer is a thin rigid film or a viscoelastic material of which appropriate models are applied. For example, Saubery model might be applied for rigid film while Kelvin-Voight model applied for more viscous materials (Anderson, et al., 2007) (Lapidot, et al., 2016). This work investigated how palmitic acid solutions at different concentrations (25mg/ml, 50mg/ml, 75mg/ml, 100mg/ml, 125mg/ml, 150mg/ml) interacted with different surfaces (silicon dioxide, hydrophobic polystyrene and aluminium dioxide). Figure 3 shows palmitic acid, a saturated fatty acid that is found in many household goods and foods. For example, it is found in cocoa butter and in many meats and products containing dairy. The stock solutions were prepared by dissolving palmitic acid (white solid) in absolute ethanol (99%). A water bath was used to maintain the temperature at room temperature. Results showed that palmitic acid adsorbed more onto polystyrene at concentrations above 90mg/ml. Below that the adsorption varied between the 3 materials as shown on Figure 1.

Xray photoelectron spectroscopy is a powerful surface analysis technique that can be used to detect the surface groups on the first 10nm of the outermost surface (Heide, 2012). For this reason, XPS was used to identify the surface groups of the different materials before they were used for detection of adsorption/desorption events on QCMD. Following QCMD experiments, the sensors were then taken for further analysis using K-alpha XPS system and Avantage software to verify if acid groups could be found on the surface of the sensors to corroborate the results from the QCMD experiments. The spot size for analysis was set to 400µm. The number of scans ranged from 10 to 90 depending on the number of counts of each element on the survey spectra. Full Width at Half Maximum (FWHM) was set to 1.70eV for Al2_P. This parameter was varied on the Avantage software depending on the element being peak fitted. The Lorentzian/Gaussian (L/G) mix did not exceed 30% for all samples.It was found that the acid group -COOH was found on the surface of the sensors. The results from XPS showed that palmitic acid had adsorbed more onto hydrophobic polystyrene especially at higher concentrations. This was shown by looking at the atomic percentages of each element from the survey spectra. The elements of interest were carbon, oxygen, silicon dioxide and aluminium oxide. After identifying the elements present, peak fitting was done to identify the peaks under each element. Depending on the binding energy, the chemical states were determined. The results showed that COOH peak with binding energy of 288eV was found on the C1s spectra for silica as shown in Figure 2. This is consistent with the results obtained from QCMD where a negative shift was observed with time showing that palmitic acid was adsorbing onto the surface.

The results showed that palmitic acid adsorbed onto the different sensors by varying amounts with the highest sorption observed in hydrophobic polystyrene above concentration of 90mg/ml. The results showed that adsorption of palmitic acid increased with increasing concentration for all three surfaces. This was determined observing frequency and dissipation shifts on the QCMD. XPS was able to detect presence of the COOH group which is characteristic for fatty acids thereby supporting the events observed on QCMD.

References

Anderson, H. et al., 2007. Quartz crystal microbalance sensor design I. Experimental study of sensor response and performance. Sensors and Actuators B, Volume 123, pp. 27-34.

Biolin Scientifc, 2020. Measurements QCMD. [Online]
Available at: https://www.biolinscientific.com/measurements/qcm-d
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Heide, P. V. D., 2012. X-ray photoelectron spectroscopy : an introduction to principles and practices. 1st ed. Hoboken: John Wiley & Sons, Inc.

He, X. et al., 2013. Mechanisms of Fat, Oil and Grease (FOG) deposit formation in sewer lines. Water Research, Volume 47, pp. 4 4 5 1 - 4 4 5 9.

Iasmin, M., Dean, L. O. & Ducoste, J. J., 2016. Quantifying fat, oil, and grease deposit formation kinetics. Water Research, Volume 88, pp. 786-795.

Lapidot, T., Campbell, K. S. & Heng, J., 2016. Model for Interpreting Surface Crystallization Using Quartz Crystal Microbalance: Theory and Experiments. Analytical Chemistry, Volume 88, p. 4886−4893.

Stein, L. & Miller, D., 2019. Fatbergs can cost millions to remove each year. Here's how to stop them. [Online]
Available at: https://www.abc.net.au/news/2019-01-10/fatberg-how-to-remove-and-stop-th...
[Accessed 17 March 2023].