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 m(2) g(-1) and a microporosity of 7%. Merck calcite (M2066) is an analytical reagent with an SSA of 1 m(2) g(-1) and a microporosity of 17%. The industrially manufactured calcites U1 and U3 have higher SSAs of 20 and 70 m(2) 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 m(2) g(-1) (RB) to 67 m(2) 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. (C) 2004 Elsevier Ltd. All rights reserved.