Adsorption of Pseudomonas putida on soil particle size fractions: effects of solution chemistry and organic matter

Attachment of bacteria on soil particles is ubiquitous and governs the transformation of nutrients and degradation of pollutants in soil and associated environments. The nature on the binding of bacteria by soil particles has remained unclear. The objectives of the present study were to investigate the adsorption of Pseudomonas putida on particle size fractions from an Ultisol as influenced by solution chemistry and organic matter. An Ultisol was collected from a forest land. One part of the soil was oxidized by H2O2 to remove organic matter. The other part was without such oxidization. Each part of the soil was separated into four size classes: coarse sand (200-2,000 mu m), fine sand (20-200 mu m), silt (2-20 mu m), and clay (< 2 mu m). The corresponding organic matter-left fractions (OM-left) and organic matter-removed (OM-removed) fractions were obtained. Meanwhile, P. putida was grown in beef extract peptone medium at 28A degrees C to the stationary growth phase. Cells were harvested by centrifugation, washed in deionized water, and resuspended in 10 mM acetate buffer (pH 5.5). Batch experiments were carried out to analyze equilibrium adsorption of bacteria and the effects of pH and electrolyte concentrations on bacterial adsorption. The adsorption isotherms of P. putida on the size fractions conformed to the Langmuir equation. The maximum amount of P. putida adsorbed by clay fraction was 4.3 and 62.3 times as great as that by silt and sand fractions, respectively. The number of P. putida attached to OM-removed fractions was significantly larger than that to OM-left fractions. P. putida adsorption on OM-left fractions with increasing pH from 4.0 to 9.0 was reduced by 44.0-78.8%. At the same time, further decreases (7.5-21.1%) were observed in the adsorption for OM-removed ones. Mg2+ was much more effective than Na+ in enhancing P. putida attachment. Na+ and Mg2+ ions more strongly promoted P. putida adsorption on OM-left fractions than on OM-removed fractions. Clay fraction presented the largest adsorption capacity for bacteria, followed by soil silt and sand fractions. As compared with silt and sand fractions, it is likely that the greater amounts of bacteria adsorbed by clay fractions were attributed to their higher content of clay minerals and iron oxides. Soil organic matter plays a suppressive role in the interfacial processes occurring during the initial bacterial attachment.