Porewater velocity influence on zinc sorption by a clay-bearing sand
Natural clay materials can limit the subsurface mobility of dissolved heavy metals by providing sites for cation sorption and exchange. The objective of this research is to determine the influence of porewater velocity on these attenuation mechanisms. Zinc was selected to evaluate the reactive properties of a clay-bearing sand (4% bentonite clay, 96% silica sand) under different flow conditions. Columns containing the clay-bearing sand were used to generate solute breakthrough curves (BTCs) for zinc solutions at porewater velocities of 1.4 x 10-5, 6.1 x 10-4, 8.3 x 10-4, and 4.8 x 10-3 cm s-1. Zinc influent concentrations of 2.1 x 10-4 M and 2.1 x 10-5 M were employed in column and batch experiments. Chloride is used as a conservative solute to determine the mechanical flow properties of the clay-bearing sand. Zinc BTCs indicate a non-equilibrium process based upon rate-limited diffusive mass transfer between bulk solution and non-advective pore space, where numerous sorption-exchange sites may exist. Models for similar systems which incorporate the 'local equilibrium assumption' and neglect diffusive mass transfer limitations may underestimate contaminant mobility at higher flow rates and over-estimate the contaminant removal efficacy of pump-and-treat remediation in field applications.
Heijn, EC; Peirce, JJ; Sperry, JM
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