Sediment capping in eutrophic lakes – efficiency of undisturbed calcite barriers to immobilize phosphorus

The application of a calcite barrier was investigated to prevent P release from eutrophic lake sediments. For this, different calcite materials varying in grain size, specific surface area (SSA) and roughness of the surface were applied. Rohrbach calcite (RB) is a crushed Jurassic limestone with an SSA of 4.3 m2 g−1 and a microporosity of 7%. Merck calcite (M2066) is an analytical reagent with an SSA of 1 m2 g−1 and a microporosity of 17%. The industrially manufactured calcites U1 and U3 have higher SSAs of 20 and 70 m2 g−1 and a microporosity of 20% and 9%, respectively. Short-term batch experiments and long-term sediment incubation experiments showed that the calcite saturation of the water (SICaCO3) predetermines the P fixation mechanisms. In waters supersaturated with respect to calcite (SICaCO3 > 0), adsorption and simultaneous co-precipitation with calcite are the predominant processes for P fixation. In addition, precipitation of Ca–P compounds occurs on the surface of calcite seed crystals due to the decrease in the interfacial nucleation energy. The capacity of P fixation was greatly influenced by the physical properties of calcite grains. An increase in the SSA from 1 (M2066) and 4 m2 g−1 (RB) to 67 m2 g−1 (U3) improves the efficiency of P removal from about <5% (RB, M2066) to 90% (U3). The grain size affects the P fixation especially in waters close to equilibrium and of weak supersaturation, where small calcite grains dissolve and initiate supersaturation with respect to Ca–P compounds. Under these conditions SRP concentrations decrease by 30% (M2066), 60% (RB) and about 95% (U3) relative to their initial concentrations. At SICaCO3 < 0, calcite dissolution causes an increase in the dissolved Ca concentration and initiates Ca–P precipitation lowering the SRP concentrations by about 65% (M2066, RB) and about 100% (U3). This process is significantly enhanced by the surface roughness of the calcite grains (M2066) due to numerous micropores of <2 nm in diameter. The results showed that calcite barriers could be optimized individually in accordance to the hydrochemical conditions in lakes to increase the efficiency of P retention in sediments. An application of a 1 cm thick RB barrier resulted in an 80% reduction of the P flux from the sediment for at least 2–3 months, whereas a quartz sand barrier of 2–4.5 cm thickness containing 2 wt% of highly active calcite such as U3 and U1 quantitatively prevents release of P from eutrophic lake sediments for at least 7 and 10 months, respectively.