Ozonation is a critical step to further remove trace organic chemicals (TrOCs) in wastewater. TrOCs are generally not completely mineralized during ozonation, but transformed to smaller and more polar ozonation products (OPs), which might show elevated persistence and mobility in water cycle due to their high polarity. While many oxygen-rich OPs are biodegraded by the post-treatment of ozonated wastewater, some OPs (e.g., <i>N</i>-oxides) can remain stable. Therefore, the characterization and persistence assessment of OPs have received considerable attention. Most previous studies on OPs rely on the reversed-phase liquid chromatography-high resolution mass spectrometry (RPLC-HRMS), which is, however, not suited for highly polar analytes. In this study, we applied the supercritical fluid chromatography (SFC) coupled to HRMS to trace OPs during pilot-scale ozonation and subsequent post-treatments (three different dual-media filters) of municipal wastewater treatment plant effluent. SFC has a unique mobile phase comprising the supercritical CO<sub>2</sub> (non-polar) and polar modifiers (e.g., methanol), thus allows the separation of analytes with varied polarity. We conducted the suspect and non-target screenings using SFC-HRMS, in comparison to the generic RPLC-HRMS, to explore if SFC-HRMS could provide additional information on OPs. While comparable results were obtained from these two techniques during suspect screenings for known OPs, 23 unknown OPs were only observed by SFC-HRMS via non-targeted screening. Several SFC-only OPs were proposed as the derivatives of methoxymethylmelamines, phenolic sulfonates, and metformin; the latter was confirmed by laboratory-scale ozonation experiments. Only 20% of the SFC-only OPs were removed by subsequent dual-media filters in steady-states, revealing the non-biodegradability of most SFC-only OPs. Fresh granular activated carbon showed better removal efficiency due to the additional adsorption capacity. Nevertheless, six (out of 23) of the SFC-only OPs escaped through all post-treatments, raising concerns on their occurrence in drinking water sources impacted by the ozonated wastewater discharge.

Tracing ozonation products in municipal wastewater using supercritical fluid chromatography-high resolution mass spectrometry

ACS Fall 2021

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.