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 reme diation of groundwater. They are placed in the subsurface environment perpendicular to groundwater how where they selectively remove targete d groundwater contaminants while permitting water and other nontargete d constituents to pass through freely. A novel approach to emplacing c hemical barriers is by the injection of zero-valent iron (Fe-0) colloi ds into the subsurface. The objective of this study was to determine t he 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 sim plified aquifer matrix. Influent colloid injection rate (P less than o r 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 t he column to retain colloids decreased as the concentration of retaine d colloids increased. Colloids were uniformly distributed throughout 1 -m long columns at concentrations >3 g kg(-1). Based on filtration the ory, gravitational settling was clearly the primary mechanism controll ing colloid retention; diffusion, electrostatic attraction, and interc eption 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 diff usion, 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.