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Title: Causes and implications of colloid and microorganism retention hysteresis

item Bradford, Scott
item KIM, HYUNJUNG - Chonbuk National University

Submitted to: Journal of Contaminant Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/27/2012
Publication Date: 7/20/2012
Citation: Bradford, S.A., Kim, H. 2012. Causes and implications of colloid and microorganism retention hysteresis. Journal of Contaminant Hydrology. 138-139:83-92.

Interpretive Summary: Release of disease causing microorganisms into groundwater poses a risk to human health through consumption and use of contaminated water. The objective of this research was to better understand the causes and implications of microorganism release in soils and aquifers with changes in solution chemistry. Results indicate that the amount of microorganisms that are retained in aquifer sand is a function of the solution chemistry history because of the presence of small-scale differences in chemical properties on sand grains. Released of microorganisms from the sand depended on the size of the microorganism relative to these chemical variations on the sand. This information will be of interest to scientists and engineers concerned with predicting the fate of colloids and microorganisms in soils and aquifers.

Technical Abstract: Experiments were designed to better understand the causes and implications of colloid and microorganism retention hysteresis with transients in solution ionic strength (IS). Saturated packed column experiments were conducted using two sizes of carboxyl modified latex (CML) microspheres (0.1 and 1.1 µm) and microorganisms (coliphage fX174 and E. coli D21 g) under various transient solution chemistry conditions, and 360 µm Ottawa sand that was subject to different levels of cleaning, namely: a salt cleaning procedure that removed clay particles, and a salt+acid cleaning procedure that removed clay and metal oxides. Comparison of results from the salt and salt+acid treated sand indicated that chemical heterogeneity was a major contributor to colloid retention hysteresis. The influence of this heterogeneity increased with IS (decreasing Debye length) and decreased with colloid/microbe size. The adhesive interaction was largely irreversible for smaller sized 0.1 µm CML colloids and fX174, whereas it was reversible for larger 1.1 µm CML colloids and E. coli D21 g. These observations suggested that the heterogeneity created a local minimum in the interaction energy that was similar or smaller than the size of the 0.1 µm CML and fX174. The effect of this heterogeneity on retention in an otherwise repulsive region depended on the zone of electrostatic interaction that was a function of the colloid/microbe size and the Debye length. Thus, larger 1.1 µm CML colloids and E. coli D21 g experienced a reversible secondary minimum type interaction of higher magnitude but less variability than the 0.1 µm CML and fX174. However, retention hysteresis still occurred for colloids on the salt+acid treated sand and this indicates that other factors (e.g., differences in the solid phase mass to low velocity regions, and additional chemical heterogeneity on the sand and colloids) may also be contributing to the retention hysteresis.