Clayey Sediment Compaction-Driven Structural Changes of Pyrite Trigger Iodine Enrichment in Groundwater

Clayey sediments in aquitards are considered primary sources of endogenous iodine in groundwater of coastal areas. Aquitard compaction was assumed to elevate iodine concentrations in groundwater beneath. Nonetheless, the mechanisms governing iodine migration during compaction remain poorly understood. Field-simulated compaction experiments with pristine clayey sediments from the Pearl River Delta were conducted to elucidate iodine mobilization dynamics during aquitard compaction. The results reveal that a substantial amount of authigenic pyrite occurred in the clayey sediments and carried approximately 30% of total iodine. With intensifying sediment compaction, iodine was continuously released primarily as I^-, resulting in high iodine concentrations (>100 μg/L) in the expelled porewater. Combined sequential chemical extraction, micro-X-ray fluorescence (μ-XRF), and scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) analyses suggest that, as compaction pressure increased, progressive pyrite fragmentation and formation of surface Fe oxide minerals (hematite) led to a marked decrease of pyrite-bound iodine and IO₃^- reduction. Concurrently, dynamic transformation of iodine species among IO₃^-, I^-, and organic iodine occurred in the porewater, with I^- being the dominant species finally. In summary, structural changes of iodine-bound pyrite play a critical role in iodine transformation and mobilization within compacting aquitards, exerting overlooked implications for the genesis of high-iodine groundwater in coastal areas.