Application of ferrihydrite and calcite as composite sediment capping materials in a eutrophic lake
School of Engineering
Ferrihydrite (Fh) and calcite were used as reactive capping layers for a eutrophic lake. The effectiveness of Fh and/or calcite capping for demobilizing phosphorus (P) in the sediment from the overlying water was tested, and the responses of carbon (C), nitrogen (N), and iron (Fe) to different capping treatments were analyzed as well.
Materials and methods
Twelve sediment cores were collected randomly using polycarbonate tubes from Ornamental Lake in the Royal Botanic Gardens of Melbourne, Australia. Four groups with three replicates were separated randomly and labeled as the control (no capping), calcite (capped with 1-cm-deep calcite), Fh (capped with 1-cm-deep synthesized Fh powder), and calcite + Fh (CaFh, capped with 0.5-cm-deep Fh powder first and then with 0.5-cm-deep calcite). Gentle bubbling was employed to prevent anoxic conditions in the sediments and stimulate vascular plant ventilation during a 15-day static incubation. The overlying water sample mixed by a small magnetic stirrer was withdrawn 1, 3, 7, 11, and 15 days after capping for the analyses of filterable reactive phosphorus (FRP), filterable N, and Fe species.
Results and discussion
The results showed that CaFh had a greater effect on the FRP removal in the overlying water than Fh alone. No significant Fe release was observed for Fh and CaFh compared to the control and calcite. The introduction of Fh increased the release of ammonium and total N. After incubation, no significant differences were observed among total P, total N, and organic carbon in the upper sediments, except for total Fe.
Composite capping with Fh and calcite could be used as an effective, low-cost natural method to block the release of P from the sediment without a significant Fe pollution risk. The potential environmental risks need further attention.