Active Barriers to Reduce Phosphorus Release from Sediments: Effectiveness of Three Forms of CaCO3

Abstract

Three forms of calcite—crushed limestone and two forms of precipitated calcite—were tested in laboratory bio-reactors as possible active barriers to prevent phosphorus release from sediments, and therefore reduce the risks due to algal blooms (or eutrophication). The two precipitated calcite materials proved to be quite effective at reducing the release of phosphorus from Lake Carramar sediments under anaerobic conditions. Over a 20-day period, a 2% (1.1 kg (CaCO3) m–2 layer of SoCal (a commercial product from Germany) reduced the amount of phosphorus released by almost 100 times over that occurring with no barrier. The Australian product (ESCal 2%), while not as effective as the SoCal, still reduced the phosphorus released by around 15 times that with no barrier. Limestone was ineffective in preventing the release of phosphorus. Mean phosphorus flux rates under anaerobic conditions were: control 66, SoCal 0.8, and ESCal 2.9 µmol (P) m–2 d–1. There is considerable scope to further optimize the conditions under which the ESCal calcite is formed to produce a product with smaller particle size and higher surface area than that tested here. Additionally, there is potential to precipitate the ESCal in situ and thus achieve even greater cost savings. Preliminary cost estimates are that it should be possible to dispense calcium hydroxide directly into the water column at around $A 200 per tonne, and then use a CO2 bubbler system to precipitate CaCO3 directly in the water column.