Escherichia coli Sorption to Sand and Silt Loam Soil

Interactions between bacteria and soil particles influence bacteria retention and transport in the soil matrix and, consequently, bacterial contamination of water resources. The objective of this study was to quantify sorption and desorption of Escherichia coli to silt loam soil and sand by evaluating suspensions with a range of sorbent and bacteria concentrations. Three sorption isotherms were assessed. Soil or sand sorbent (1, 10, or 20 g) was added to 50 mL of E. coli solution (103, 105, 106, or 107 colony forming units per milliliter, cfu/mL). The bacterial concentration differences between the centrifuged supernatant of E. coli solutions with and without sorbent were used to determine the number of E. coli cells sorbed. The samples without sorbent were used to offset the effects of bacterial die-off, sedimentation, and sorption to surfaces other than that of the sorbent. Blank NaCl solution (0.15 M) was used to wash sorbed E. coli off the sorbent three times in desorption trails. Numbers of E. coli sorbed per gram of sorbent varied with the initial bacterial concentration and quantity of sorbent, and were greater for soil (1.2 x 104 to 9.5 x 108 cfu/g) than for sand (<0 to 3.6 x 107 cfu/g). Freundlich isotherms described observed sorption data well for both sorbents (model efficiency, E = 0.94 for soil and 0.95 for sand) with a greater log Kf (3.26) and a similar nf (1.23) for soil compared to sand (log Kf = 1.68 and nf = 1.22). Langmuir isotherms also described well both sand and soil sorbent data; model efficiency for soil improved upon removal of 20 g sorbent data, perhaps due to near-saturation sorption (99.7%) for these conditions. Model efficiency for the linear isotherm improved when only data within a limited concentration range (<107 cfu/mL) were used. The estimated linear partition coefficient for soil was an order of magnitude greater than that for sand (106 versus 14.5 mL/g). E. coli sorption to both soil and sand particles was reversible, but E. coli detachment from sand was nearly 100% of attached cells after one washing, whereas a total of less than 15% of cells were detached from soil after three washings. Differences in sorption and reversibility between sand and soil will lead to different patterns of retention and transport in the environment for those two media.