Retention of zero-valent iron colloids by sand columns: Application to chemical barrier formation

Chemical barriers are an emerging technology used for the in situ remediation of groundwater. They are placed in the subsurface environment perpendicular to groundwater how where they selectively remove targeted groundwater contaminants while permitting water and other nontargeted constituents to pass through freely. A novel approach to emplacing chemical barriers is by the injection of zero-valent iron (Fe-0) colloids into the subsurface. The objective of this study was to determine the effect of influent Fe-0 colloid concentration and injection rate on colloid retention by columns of sand. Suspensions of Fe-0 colloids (2 +/- 1 mu m) were injected into coarse-grain sand that simulated a simplified aquifer matrix. Influent colloid injection rate (P less than or equal to 0.01) and concentration (P less than or equal to 0.05) had a significant effect on colloid retention by the sand. Efficiency of the column to retain colloids decreased as the concentration of retained colloids increased. Colloids were uniformly distributed throughout 1-m long columns at concentrations >3 g kg(-1). Based on filtration theory, gravitational settling was clearly the primary mechanism controlling colloid retention; diffusion, electrostatic attraction, and interception were less important mechanisms. These results were rationalized as follows: the high density of the colloids (7.6 g cm(-3)) enhanced gravitational settling, the fast experimental how rates minimized diffusion, the weak surface charge of the colloids and sediments minimized electrostatic attraction, and the small size of the colloids relative to the sand particles (6;10 mu m) minimized interception.