Integration of flow injection analysis with hollow fiber membrane contactor for sulfite analysis in post-desulfurization seawater

Monitoring sulfite (SO 3 2− ) in post-desulfurization seawater is critical for ensuring operational safety and environmental compliance of coal-fired power plants. To address the analytical challenges posed by complex and variable matrices inherent in coastal seawater, we developed an automated flow injection analysis (FIA) system integrated with a hollow fiber membrane contactor (HFMC). This system accomplishes the conversion of SO 3 2− to SO 2 through acidification, followed by gaseous SO 2 diffusion across a porous membrane and absorption into a NaOH acceptor stream, with final detection via thionine-based kinetic spectrophotometry. Under optimized conditions, the system achieved a limit of detection of 0.70 μmol L –1 , a linear range of up to 100 μmol L –1 , and relative standard deviations below 6.9 %, with negligible interference from salinity and common ions in seawater. The accuracy was validated by strong correlation with a reference fluorescence method. A successful field application at a power plant further confirmed the system's reliability for continuous real-time analysis. These results demonstrate that the FIA-HFMC system is a reliable monitoring tool suitable for timely process control and the mitigation of discharge risks in industrial desulfurization operations.