Desorption of myo-inositol hexakisphosphate and phosphate from goethite by different reagents

Inositol phosphates are abundant organic phosphates found widely in the environment. The sorption and desorption of organic phosphate (Po) are important processes in controlling the mobility, bioavailability and fate of phosphorus (P) in soil and sediment. The desorption characteristics of myo-inositol hexakisphosphate (IHP) and inorganic phosphate (Pi) from goethite were studied by pre-sorption of IHP or Pi followed by desorption by KCl, H2O, and citrate. Batch experiments and in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy were used to investigate the desorption of IHP/Pi. The desorption percentage of IHP/Pi by citrate was much higher than that by H2O/KCl. The desorption of P by citrate was mainly achieved through ligand exchange, and the desorption increased with decreasing pH. Desorption by H2O was slightly greater than that by 0.02 M KCl because the electrostatic repulsion between the P molecules is larger in H2O. Due to the higher affinity of IHP for goethite than that of Pi, the maximum desorption of IHP was lower than that of Pi. Desorption curves (desorption concentration in solution vs. sorption density) of IHP or Pi on goethite by KCl or H2O was well fitted by an exponential equation, while those by citrate were well fitted by a linear equation. The desorption amounts of P in the first cycle account for more than 58% of the total desorption followed by substantial decreases in the second and third cycles. There was a re-sorption of Pi from solution in the late stage of desorption by KCl and H2O, resulting in a sharp decrease in desorption. Re-sorption of IHP did not occur, which is probably due to its poor diffusion into goethite. The initial desorption rate of Pi with KCl and H2O decreased with increasing pre-sorption time, whereas that of IHP was opposite. This study indicates that strong sorption on and weak desorption of IHP from iron (hydr)oxides may explain the accumulation of IHP in soils.