Cadmium adsorption by sediment in a turbulence tank

The mechanism of sediment motion that affects cadmium (Cd) adsorption on sediment particles was studied in a turbulence tank in the presence (and absence) of bed mud. The experimental results were verified by a mathematical model for heavy metal transport–transformation developed for the turbulence tank. The mathematical model includes the equations of water flow, sediment motion, heavy metal transport–transformation, heavy-metal reaction kinematics and equations for prescribing the initial conditions and boundary conditions for the experiment. The model conforms the transport–transformation of heavy-metal pollutants in surface waters to following the law of convective–diffusive of common tracers and the characteristics of fate and transport of sediment motion. Variations of dissolved Cd concentrations and suspended particulate Cd concentrations with time and in the water column were measured and computed. The experimental measurements correspond with the computed results. Both the experimental measurements and computed results show that it takes about 6 h to reach equilibrium condition for cadmium adsorption by sediment particles. This is different from the result obtained from experiments conducted in continuously stirred tank reactors (or batch reactors) in which adsorption equilibrium can be achieved in about 20 min. Determination of the model parameters for sediment adsorption–desorption of metals in the tank is discussed. The experimental and computed results obtained in this study are useful to solve practical engineering problems in surface waters.