Cd migration in water-level fluctuation zones of Three Gorges reservoir: Interactions of periphytic biofilms and tryptophan.

Periphytic biofilms (PBs) and dissolved organic matter (DOM) are key factors affecting the migration of Cd at the "water-sediment" interface. However, the specific effects of PBs and protein-like components of DOM on Cd migration within the "water-biofilm-sediment" system remain poorly understood. This study simulates the dissolution and re-immobilization of Cd at the "water-biofilm-sediment" interface in the water-level fluctuation zone (WLFZ) of the Three Gorges Reservoir (TGR), aiming to clarify the role of PBs in Cd stability. The results indicate that the concentration of dissolved Cd increased by 0.96-fold (R2 = 0.992) with each doubling of L-tryptophan (L-Trp) concentration, as the functional groups of L-Trp (COOH and NH) formed complexes with Fe-Mn oxide-bound Cd in the sediment. In the presence of PBs and at an L-Trp concentration of 150 mg·L-1, PBs utilized L-Trp, resulting in a 16.7 % reduction in dissolved organic carbon (DOC) (p < 0.05) and an increase in protein content. Additionally, PBs contributed to a 35.12 % reduction in the peak concentration of dissolved Cd, thereby stabilizing the final Cd levels. Cd enrichment within PBs, facilitated by functional groups such as >CC<, OH, >CO, and CO, led to a significant increase in Cd content (0.43 mg·kg-1, p < 0.05). These findings suggest that L-Trp degradation and Cd enrichment by PBs act synergistically to promote the re-immobilization of dissolved Cd. This study offers a novel perspective on Cd migration in WLFZs and provides insights that may be applicable to other aquatic environments where PBs are present.