Effects of humic acid on adhesion of Bacillus subtilis to phyllosilicates and goethite

• HA inhibits bacterial adhesion to montmorillonite & goethite.
• Decreased bacterial adhesion on minerals is ascribed to various interactions.
• HA strongly alters surface charge and aggregation of kaolinite and goethite.
• Enhanced attachment of bacteria to kaolinite was due to HA aggregation.

Bacterial attachment is a physicochemical interfacial process probably affected by the presence of humic acid (HA) in natural environments. However, the effect of HA coating on bacterial adhesion to soil minerals remains unknown, despite many studies focusing on cell attachment to pure or iron oxide-coated quartz in the presence of HA. The influence of HA on Bacillus subtilis adhesion to kaolinite, montmorillonite, and goethite under neutral pH and 1 mM ionic strength was examined using batch experiments coupled with Fourier-transform infrared spectroscopy (FTIR), isothermal titration calorimetry, and Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory. Results showed a reduction in cell adhesion on goethite and montmorillonite with increasing HA content from 0% to 2%, indicating an inhibitory effect of HA coating on the interaction. However, a promotive effect of HA was observed for kaolinite, which disagreed with previous findings based on cell–quartz systems. In addition, the presence of HA reduced the exothermic adhesion enthalpy for kaolinite and goethite, but increased that for montmorillonite. For goethite, the HA coating depressed its positive charge and increased its aggregation, thereby leading to a weaker electrostatic attraction and lesser available surface area, which probably contributed to the observed decreasing adhesion. For montmorillonite, the reduced attachment was likely caused by the increase in electrostatic repulsion that overcompensated for the appeared chemical interactions as suggested by FTIR analysis. The enhanced cell adhesion on kaolinite, which was unexpected from DLVO theory based on its surface charge change, is likely ascribed to the HA-induced decrease in its aggregation. Additionally, the effect of HA vanishes at HA contents higher than 2%, likely due to a saturation effect. Overall, the influence of HA on bacterial attachment is likely a combined result of multiple factors, including not only electrostatic forces and chemical interactions, but also mineral aggregation.