Transport of adsorbing metal ions between stream water and sediment bed in a laboratory flume

Abstract

The transport of adsorbing metal ions (copper, zinc, calcium and magnesium) between the water column and the sand bed in a 5 meter long recirculating laboratory flume with bottom bedforms has been investigated. A non-adsorbing tracer, lithium, was used simultaneously to observe the exchange of water between bed and water column. The presence of bedforms and associated pumping increases the exchange rate by several orders of magnitude over molecular processes. The concentrations of initially added metal ions were monitored both in the circulating overlying water and in the pore-water of the sediment bed. The sand used for the bed was composed of over 99% silica, with geometric means of 500 [microns] and 195 [microns]. Before each run, the sand was acid-washed at pH 3.5 to provide reproducible experimental conditions. The chemical composition of the recirculating water was controlled and steady flow conditions were maintained in the experiments. Batch experiments were performed to investigate the chemical partitioning of the selected metal ions to the sand grain surfaces. The adsorption of zinc onto silica was modeled in detail and binding constants were determined. The observed adsorption of the metal ions in the flume experiments compared well with batch adsorption data. The transfer of metal ions into and out of a bed covered with stationary bedforms is dominated by advective pumping caused by pressure fluctuations over the bed. A residence-time model based on pressure-driven advective flow and linear equilibrium partitioning of the pollutant to the sediment was developed and describes the observed metal ion exchange between sediment and water column well. Increased partitioning of the metal ion onto the sediment leads to an increase of the amount of tracer stored within the sediment bed. Furthermore, the concentrations of metal ions released from the bed after passing of an initial pulse in the overlying water will be lower, but longer lasting for stronger partitioning, leading to tailing in the water column for long times. For a bed with moving bedforms, the main mechanism for mass exchange is the trapping and release of overlying water by the traveling bedform. The transport of metal ions can be approximately described for the initial phase of the experiment, but large deviations from the model occur for long times. The models do not require calibration since the parameters for transport into and out of the bed can be derived from flow conditions, sediment parameters, bedform dimensions and adsorption characteristics of the tracer on the sand. Criteria for the applicability of the models and appropriate scaling variables are identified. The experimental results are presented in nondimensional form.