In-situ ATR-FTIR spectroscopic investigation of desorption of phosphate from haematite by bacteria

Phosphate (P) fixed on Fe- and Al-(hydr)oxides can be released into solution by coexisting anions or molecules that have strong reactivity with the mineral surfaces. Bacteria have been shown to adhere strongly on to the (hydr)oxides through physical or chemical forces, or both. It is still unknown, however, whether bacteria can desorb P from oxides. We examined the desorption of P from haematite (α-Fe2O3) by Bacillus subtilis and Pseudomonas fluorescens through the combined use of in-situ attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectroscopy and macroscopic batch experiments. The ATR-FTIR data of ternary bacteria–P–haematite systems indicated a release of P that was concurrent with the attachment of bacteria on to the haematite surface, giving direct and in-situ evidence for the displacement of P by bacteria. The P-desorbing ability of bacteria was quantified by batch desorption experiments, and was further supported by the inhibitory role of bacteria in P adsorption on to haematite under varying concentrations of P and amounts of bacteria. In-situ ATR-FTIR investigations and Derjaguin–Landau–Verwey–Overbeek (DLVO) prediction suggested that bacteria might compete for sorption sites with P through their electrostatic and chemical interactions (coordination of bacterial phosphate and carboxyl groups on the haematite surface) with the haematite surface. In addition, the bacteria reduced greatly the positive charge of haematite, and the reduction correlated non-linearly with the decline in P adsorption. Therefore, the P-mobilizing ability of bacteria is probably attributed to the competition between P and bacterial surface groups and reduction in the positive charge on haematite by bacteria. These findings elucidate a potential role for bacteria in mobilizing P that is chemically adsorbed on Fe-oxides, which enhances the availability of P to plants and its mobility in natural environments. Highlights Whether bacteria can desorb phosphate from oxides remained unknown. Bacteria enhanced desorption of phosphate from haematite and inhibited its adsorption by haematite. Electrostatic and chemical interactions might help bacteria compete with phosphate. Reduction in positive charges might also contribute to phosphate-mobilizing ability of bacteria.