Experimental and modeling studies of contaminant transport in capped sediments during gas bubble ebullition.

摘要

Fluxes of the three organics and eighteen metals from sediments were measured with diffusion chambers, and the effectiveness of a sand cap ascertained. The influence of sediment consolidation on contaminant flux and the mass transfer coefficient at the sediment-water interface was studied. A two-layer diffusion-advection model and consolidation model were coupled to elucidate the effect of sediment consolidation on chemical transport in the sediment-cap system. The model was tested and verified by the experimental data from microcosms, and then was used to predict phenanthrene flux under field conditions. The simulation results showed that consolidation could accelerate phenanthrene breakthrough and enhance the initial phenanthrene flux. At the same time, oxygen distribution in both cap and sediment were studied with microelectrodes.; A bubble column was employed to investigate sediment and phenanthrene release from sediment with methane injection. The experiments indicated that significant amounts of both solid particulate matter and phenanthrene could be released from a sediment bed by gas movement with the amount of release related to the volume of gas released. The effective mass transfer coefficient of gas bubble facilitated contaminant release was estimated under field conditions, being around three orders of magnitude smaller than that of bioturbation. A thin sand cap layer (2 cm) was found to dramatically reduce the amount of phenanthrene or particles released with the gas. X-ray computerized tomography was used to investigate the void space distribution in the sediment penetrated by gas bubbles. The results showed that gas bubble migration could redistribute the sediment void spaces and facilitate pore water circulation in the sediment. A model was developed for bubble-facilitated contaminant release from sediments. The model was first verified by experimental data and then was used to calculate the contaminant flux into the air under field conditions. It was also used to investigate the effects of total suspended soil, gas flux and sand cap on the contaminant flux into the air. Model sensitivity analysis suggested that the cap thickness and its organic carbon content be two important parameters for the effectiveness of a cap to control the contaminant flux into the atmosphere.