Geochemical processes of phosphorus‑iron on sediment-water interface during discharge of groundwater to freshwater lakes: Kinetic and mechanistic insights

Abstract

Groundwater is widely recognized as a source of lake materials. When it discharges into lakes, phosphorus(P)‑iron(Fe) geochemical reactions occur due to environmental changes, affecting P discharge from groundwater. However, redox kinetics of Fe and associated P geochemical processes at the sediment-water interface are not fully understood. Taking Dongting Lake as an example, this study explored Fe and P geochemical processes at the sediment-water interface under groundwater discharge with high Fe and P concentrations. We incubated sediments from Dongting Lake under anoxic-oxic conditions with different initial aqueous P/Fe ratios and pH. Aqueous PO43−-P and Fe2+, and solid P and Fe phases in sediments were analyzed, and experimental data were further simulated using numerical reactive models. At the beginning of the experiment, aqueous P and Fe were adsorbed rapidly on sediments. Under anoxic conditions, the Fe reduction rate decreased with decreasing content of poorly crystalline ferric (oxyhydr)oxides, and the addition of aqueous P and Fe at neutral pH enhanced the reduction rate. The increased aqueous P was dominated by desorption caused by sediment Fe reduction and then fixed by gibbsite adsorption and hydroxyapatite precipitation. Under oxic conditions, Fe(II) oxidation under was pH- and (P:Fe)ini-independent, with a sharp rate decline. Furthermore, the final sediment Fe(II) content was higher than the initial content, indicating the formation of a low-oxidizability Fe(II) phase. The P dynamics were dominated by adsorption on the produced Fe-oxides. The numerical models also suggested that heterogeneity in natural sediments promotes hydroxyapatite formation at low pH, but restricts it at high pH. The findings reveal that although aqueous P concentration decreased during groundwater discharge to lakes, PO43−-P concentration remained much higher than that in natural lake water, increasing the risk of lake eutrophication. The paper provides references for further understanding of P loading from groundwater discharge into lakes.