Laboratory microcosm evidence for denitrification-driven iodine mobilization in coastal groundwater systems: Implications for salinity and redox control

Iodine is essential, yet excessive iodine in groundwater threatens water quality and public health. Denitrification, a widely distributed biogeochemical process in groundwater systems, plays a key role in regulating iodine behavior. We investigated denitrification driven iodine mobilization in high-iodine groundwater from Tianjin’s Binhai area through laboratory simulations integrating redox conditions, salinity gradients, and bioaugmentation. The results showed that salinity and redox conditions reshaped the structure and metabolic activity of denitrifying communities. Low total dissolved solids (TDS) and more reducing conditions promoted the increased relative abundance of denitrification-related genera, including Stutzerimonas , Hydrogenophaga , and Pseudomonas . Compared with uninoculated controls, the bioaugmented treatments exhibited stronger iodide (I - ) accumulation. Under low TDS (500 mg/L) and high nitrate (NO 3 - , 100 mg/L) conditions, the denitrification rate constant reached its maximum (23.54 × 10 -6 s -1 ), and I - increased most markedly from 18.61 to 700.91 μg/L. Denitrification genes (e.g., narG and nirS ) showed a moderate positive correlation with I - concentrations (r = 0.570, p < 0.05). In the bioaugmented treatments, the contribution of denitrification to I - accumulation was stage dependent: under more oxidising conditions, iodine mobilisation from the solid phase to the aqueous phase was associated with enhanced alkalinity and competitive adsorption by bicarbonate (HCO 3 - ) (r = 0.620, p < 0.001); under more reducing conditions, I - accumulation synchronised with ferrous iron (Fe 2+ ) accumulation, indicating a stronger linkage to iron reduction processes (r = 0.638, p < 0.001).This study reveals a salinity dependent coupling between denitrification and iodine mobilization, providing a framework for managing iodine mobility in coastal aquifers. Schematic diagram of biogeochemical processes for denitrification-driven iodine mobilization and accumulation. (Dashed arrows represent decomposition/degradation, solid black arrows indicate facilitation, and wide blue arrows denote biological processes. Numbers ①-⑥ indicate the main processes.① microbial denitrification, NO 3 - reduction that consumes electron donors; ② elevated HCO 3 - promotes iodine mobilization and accumulation via carbonate dissolution and competitive occupation of sorption sites; ③ Fe 2+ abiotically reduces NO 3 - and is consumed or oxidized, destabilizing Fe associated sorption phases and promoting iodine desorption; ④ redox transition with decreasing Eh and increasing pH favors iodine speciation shift and mobilization; ⑤ microbial reduction of Fe 2+ hydrous oxide, FeOOH, produces dissolved Fe 2+ ; ⑥ Fe 2+ and reducing DOM abiotically reduce IO 3 - to I - , leading to iodide accumulation.) • Bioaugmentation significantly enhances denitrification efficiency and iodide enrichment. • Denitrification gene abundances correlate positively with iodide concentrations. • Denitrification mediates iodide release via carbonate dissolution, competitive adsorption, and abiotic reduction of iodate by bacterial metabolites. • A critical salinity-dependent coupling between denitrification and iodine mobilization is established.