IONIC STRENGTH AND PH DEPENDENCE OF BACTERIA ADSORPTION ONTO OXIDE MINERAL SURFACES

Understanding the initial adsorption or deposition of bacteria to mineral surfaces is fundamental to developing useful models for bacteria transport through aquifers, and for understanding the role of bacteria in contaminant transport. An attempt to gain a more rigorous understanding of the energetics of bacteria adsorption to oxide mineral surfaces was conducted using column experiments interpreted in terms of surface speciation models. Suspensions of Bacillus subtilis (a gram-positive aerobic species) under varying pH and ionic strength conditions were pumped through columns packed with 0.45 mm diameter corundum (Al₂O₃) grains. Bacteria concentrations in the effluent suspension were measured by UV-Vis spectrometry, and bacteria adsorption was measured by integrating breakthrough curves. Conditions in the experimental pH range of 5 to 7, typical for near-surface ground water, are favorable to bacteria adsorption due to the oppositely-charged surfaces of the corundum (positive) and bacteria (negative).

Results indicate that adsorption is essentially Langmuirian in the pH 5 to 7 range. At pH 5.0 and an ionic strength of 0.01, masses of bacteria adsorbed approach a calculated monolayer density of 0.05 g/m² (BET surface area) with increasing concentration of bacteria in suspension. At the same ionic strength and suspension concentrations, masses adsorbed ranged from 0.002 to 0.012 g/m² at pH 6.1, and adsorption was negligible at pH 7.0. At an ionic strength of 0.1, adsorption increased dramatically at pH 7.0 and slightly at pH 6.1 and 5.0. This behavior strongly suggests that the apparent Gibbs free energy of adsorption changes in response to pH and ionic strength due to changing electrostatic attraction between the bacteria and mineral surfaces. The positive surface charge of corundum increases 4.5-fold between pH 7 and 5 at an ionic strength of 0.01, and increases 2-fold at pH 7 between ionic strengths of 0.01 and 0.1. The negative surface charge of the bacteria decreases by 20% between pH 7 and 5, with relatively little ionic strength dependence. Changes in surface speciation appear to result in dramatic increases in electrostatic attraction and hence adsorption with decreasing pH, and significant increases with increasing ionic strength over the range of experimental conditions.