Effect of surface coatings, grain size, and ionic strength on the maximum attainable coverage of bacteria on sand surfaces

The injection of bacteria in the subsurface has been identified as a potent ial method for in situ cleanup of contaminated aquifers. For high bacterial loadings, the presence of previously deposited bacteria can result in decr eased deposition rates-a phenomenon known as blocking. Miscible displacemen t experiments were performed on short sand columns ( similar to 5 cm) to de termine how bacterial deposition on positively charged metal-oxyhydroxide-c oated sands is affected by the presence of previously deposited bacteria. A pproximately 8 pore volumes of a radiolabeled bacterial suspension at a con centration of similar to 1 X 10(9) cells ml(-1) were introduced into the co lumns followed by a 2-pore-volume flush of cell-free buffer. It was found t hat the presence of Al- and Fe-coated sand increased both deposition rates and maximum fractional surface coverage of bacteria on the sediment surface s. The effect of grain size on maximum bacterial retention capacity, howeve r, was not significant. Decreasing ionic strength from 10(-1) to 10(-2) M K Cl resulted in noticeable decreases in sticking efficiency (a) and maximum surface coverage (Om) for clean silica sand-results consistent with DLVO th eory. In columns containing positively charged Al- and Fe-coated sands, how ever, changes in alpha and theta (max) due to decreasing ionic strength wer e minimal. These findings demonstrate the importance of geochemical control s on the maximum bacterial retention capacity of sands. (C) 2001 Elsevier S cience B.V. All rights reserved.