Contrasting nickel mobilization pathways driven by manganese redox cycling and dissolved organic matter complexation...

Nickel (Ni) remobilization in eutrophic lake sediments is controlled by interactions between iron-manganese (Fe-Mn) redox processes and dissolved organic matter (DOM), but the mechanisms under different ecological regimes remain unclear. Here, we employed an in situ high-resolution approach integrating HR-Peeper sampling and multi-analytical techniques to simultaneously characterize Ni, Fe, Mn, and DOM in sediment pore water, and applied it to algal- (MLW) and macrophyte-dominated (DTH) zones of Taihu Lake over one year. Dissolved Ni concentrations in overlying water exhibited a sharp, synchronous peak during March-April 2021, exceeding the WHO drinking-water limit (70 μg L^-1) by up to 1.4 times. In MLW, this Ni pulse was associated with humic-like DOM-mediated Mn(IV) reduction, evidenced by concurrent increases in Ni, Mn, and humic-like DOM, with Mn identified as the dominant predictor in random forest analysis. Partial least squares path modeling (PLS-PM) further indicated that DOM acted as an electron shuttle, accelerating Mn-oxide reduction (λ = 0.971, p < 0.001) and releasing adsorbed Ni. In contrast, Ni mobilization at the DTH site was primarily governed by DOM complexation. Visual MINTEQ simulations showed that more than 55% of dissolved Ni occurred as Ni-DOM complexes. Consistently, fluorescence titration demonstrated strong coordination between Ni and both protein-like and humic-like DOM components (log K_m > 3.682; r > 0.939). Fourier transform infrared spectroscopy combined with two-dimensional correlation analysis further confirmed that Ni primarily interacted with aromatic C-H, alkene CC, aliphatic -COOH, alcoholic C-O, and aliphatic C-H functional groups. These findings provide new mechanistic insight into redox-DOM coupling and its control on Ni cycling in algal- and macrophyte-dominated sediments.