Competition between bacteria and phosphate for adsorption sites on gibbsite: An in-situ ATR-FTIR spectroscopic and macroscopic study.

Sorption and desorption of phosphate (P) on Fe and Al (hydr)oxides may be affected by bacteria in soils because their ubiquitous and strong interactions. The role of Bacillus subtilis and Pseudomonas fluorescens in adsorption of P on gibbsite (γ-AlOOH) was systematically investigated under a wide range of conditions by combining in-situ attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectroscopy with batch macroscopic experiments. In-situ ATR-FTIR observations of the ternary systems (bacteria, P, and gibbsite) showed simultaneous desorption of P from, and adhesion of the bacteria to, gibbsite, indicating a competition between the two for surface sites. Batch desorption experiments showed that bacteria could mobilize the P from gibbsite into solution, and macroscopic adsorption data showed that the amount of P adsorbed on the bacteria-gibbsite complex was less than that on gibbsite alone over durations from 0h to 26h, concentrations of P from 0.1mM to 2.0mM, pH from 5 to 8, and ionic strength from 0M to 0.5M, suggesting that bacteria inhibit the adsorption of P on gibbsite. The degree of inhibition increased with the number of bacteria in the system and was significantly but non-linearly correlated with the decline in the positive charge on gibbsite induced by the bacteria. Therefore, competition for suitable sites on the surface of gibbsite between P and the bacteria and reduction in the positive charge on the surface of gibbsite induced by bacteria are proposed as two important mechanisms that inhibit P adsorption. These findings highlight the role of bacteria in regulating the availability of P to plants and its mobility in natural environments.