In lake liming operations in Sweden, acidified lakes are reclaimed by neutralization with calcite powder. The amount added is intended to neutralize the water column as well as to delay the reacidification. The reacidification of limed lakes is dependent on the dilution of the dissolved calcium carbonate with time and, for a limited period of time, the dissolution of calcite from the lake sediments. Calcite on the lake bottom will, in addition to being covered by sedimentation, become inactivated by precipitates of humus and clay minerals clogging the calcite surfaces. A model has been developed to calculate the reacidification of a limed lake which includes the following mechanisms: (1) the dissolution of calcite and a subsequent neutralization of acid water, (2) owing to the increase inpH value, occurrence of precipitation of humus and dissolved metals onto the calcite surface and inhibition of the dissolution of calcite (3) reversible sorbtion of calcium from the water column by sediments not covered with calcite, and (4) diffusive transport through a boundary bottom layer to the water column. In a first approach the lake was modeled as a continuously stirred tank. The equations were derived from a mass balance and the dissolution kinetics for calcite to describe the long-term development ofpH, alkalinity, and calcium concentration in the lake. The differential equations describing the mechanisms were solved with the help of a computer code. The model accurately describes the reacidification and the mass balances observed in several limed lakes.
Subject classification (UKÄ)
- Chemical Engineering