Tidal fluctuations induce accumulation and transformation of seawater Cr(Ⅵ) in coastal sediments

Tidal fluctuations play a critical role in regulating the transport and fate of contaminants in coastal environments. This study explored the dynamic redistribution of chromium (Cr) from seawater to sediment under tidal influence, as well as the accumulation and transformation of Cr in sediment through laboratory experiments and numerical simulations. After 35 tidal cycles, Cr concentrations in seawater declined rapidly and stabilized at approximately 27 % of the initial level. Notably, Cr migrated into sediment, ultimately accumulating in the bottom layer. Colloidal particles (350-800 nm) composed of clay minerals served as the primary transport vectors for Cr within sediment. During tidal fluctuations, 94.5 %-98.2 % of Cr(VI) in sediment was reduced to Cr(III), predominantly mediated by Fe(II) in the top sediment and by sulfur-reducing bacteria in the bottom layers. Consistent with experimental findings, numerical reactive transport modeling demonstrated that Cr(III) initially peaked in the middle sediment layer before stabilizing in the bottom layer, whereas Cr(VI) remained confined to the top layer. These findings elucidate tide-induced mobilization and natural reduction mechanisms governing Cr-contaminated seawater infiltration into sediment, offering novel insights into the fate of Cr discharged from coastal wastewater sources within seawater-sediment systems.