Decoupling between sediment immobilization and modeled ecological risk of emerging metals in coastal mariculture...

Emerging metals (Ti, Sr, Tl, Be, Al, Li, Ag, and Bi), increasingly introduced from high-tech industries into coastal mariculture sediments, have poorly constrained environmental behavior and ecological risks. Here, we demonstrate that sediment immobilization does not mitigate ecological risk, revealing a fundamental decoupling between geochemical stabilization and ecological response. Most metals are strongly sequestered in sediments via adsorption onto clay minerals, sulfide precipitation, and incorporation into organic-rich matrices, governed primarily by sediment geochemistry and redox dynamics rather than water-column conditions. However, a small but persistent operationally defined acid-extractable fraction remains in fine-grained, organic-rich sediments and areas with relatively lower oxygen conditions, potentially contributing to conservative exposure estimates. By integrating species sensitivity distribution (SSD), probabilistic risk assessment (PRA), and the inclusion-exclusion principle (IEP) within the SPI framework, we show that the joint modeled ecological risk probability reached 46.72% under the primary 100% transfer scenario, exceeding the mild-risk alert threshold of 25% and approaching, but not exceeding, the high-risk alert threshold of 50%, despite strong bulk sediment immobilization. Be, Sr, and Tl dominate individual risks, while localized enrichment of Ag and Bi further amplifies mixture-level effects. These results demonstrate that modeled ecological risk was more closely associated with operationally defined acid-extractable fractions than with total sediment inventories, challenging conventional assumptions that sediment immobilization ensures environmental safety. This study establishes a mechanistic framework linking sediment geochemistry, residual bioavailability, and mixture toxicity, providing new insight into the environmental behavior of emerging metals and advancing risk assessment strategies for aquaculture-impacted coastal ecosystems.