High-field asymmetric ion mobility spectrometry (FAIMS) study for on-site monitoring of aromatic volatile organic compounds in groundwater and their degradation in the presence of ferrous iron and carbonate

Possibilities and limitations of remediation of VOC-contaminated groundwater and its analytical monitoring are the focus of this dissertation. Focal points are the development of a novel, continuous sample introduction system in combination with APPI-FAIMS for on-site monitoring of contamination, site-specific assessment analyses and introduction of a possible additional remediation step by oxygen implementation (e.g. with a permeable membrane) in iron- and carbonate-rich groundwater environments.

For this purpose, the possibility of using a low-cost, stand-alone, continuous real-time FAIMS system for on-site monitoring of groundwater was determined. The effects of high humidity and humidity variations on the signal in the analysis of aromatic VOCs were investigated for two different ion sources (APPI, APCI). To reduce the influence of humidity, a novel gas-water separation unit coupled to the FAIMS was implemented. Signal-specific values were determined from the maximum signal intensity at a given compensation voltage (CVmax)and a given dispersion field strength (DF) for selected aromatic VOCs, including BTEX from water. Successful calibration of the aromatic VOCs as a single analyte was performed with the gas-water separation APPI-FAIMS from spiked water samples. The limit of detection (LOD) of benzene, toluene, ethylbenzene, o-xylene and indane in water was 0.1, 0.9, 1.1, 0.3, 0.8 mg L-1 at 0% DF and 0.1, 0.9, 1, 0.4, 0.7 mg L-1 at 36% DF, respectively.

The effectiveness of the sample introduction system (i.e. the gas-water separation unit) and the repeatability of the APPI-FAIMS signal were confirmed by monitoring of aromatic VOCs in water/groundwater samples under laboratory conditions. Similarly, an aromatic VOCs sum-signal was used to monitor contamination intensity at a groundwater contamination site. The sum-signal was enabled due to the low humidity provided by the gas-water separation unit. This allowed effective charge transfer and minimized influence of humidity on the signal in the analysis of aromatic VOCs. The results were validated with HS-GC/MS.

The geochemical composition of the remediation site was investigated, including the iron concentration and the concentration of BTEX pollutants in the groundwater. In addition to BTEX, other groundwater pollutants such as indane were also determined.

Furthermore, a membrane-supported oxygen injection (OxyTech) was used as a non-standard remediation method and its effect on the redox conditions in the remediation well was investigated. In order to determine the short-term effects of the oxygen addition to anoxic groundwater on the contaminants, the previously described gas-water separation APPI-FAIMS method was applied for on-site aromatic VOC monitoring. The results were confirmed with HS-GC/MS. In addition, the general properties and oxygen permittivity of OxyTech (silicone) membrane were determined. Further aspects involving ferrous iron oxidation followed by subsequent, induced OH production and aromatic VOCs oxidation were carried out. This included the chemical potential involved in oxygenation in anoxic groundwater containing ferrous ions and carbonate/bicarbonate ions in high concentration. By using the coumarin detection method the formation of the reactive OH radicals during the oxidation of the iron ions was confirmed. The possible oxidation products from the aromatic volatile organic groundwater contaminants were detected and identified after liquid-liquid extraction and direct analysis by GC/MS.

Additionally, the oxidation capabilities of the system were confirmed on other water contaminants. Ibuprofen as a non-volatile organic water contaminant was also oxidized in presence of oxygen and ferrous ion in carbonate/bicarbonate rich water. In addition, the presence of one or more carboxyl groups was shown to play a special role in Fe-complex coordination and the formation of OH radicals, as found when trisodium citrate dihydrate was used instead of sodium bicarbonate.


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