Reaction of organic phosphates with oxides: Effects on pH, the equation used to describe the reaction, and on desorption

Organic phosphates are an important component of soil phosphate reserves. We studied the sorption of three organic phosphates by goethite and by aluminium oxide and compared them with the sorption of inorganic phosphate (Pi). The organic phosphates were inositol hexa-phosphate (IHP), glycerol phosphate (GlyP), and glucose-6-phosphate (G6P). We used five different initial pH values and 10 different initial concentrations. We measured both sorption and desorption. We used an equation which we call the Sibbesen-Langmuir equation to successfully describe the observations over a 10,000 fold concentration range. Sorption increased the pH. The effects were largest at low pH and low initial concentration and were in the sequence Pi > IHP > G6P > GlyP. This sequence was a consequence of differences in ion size and mean charge. The different feedback effects of the different ions affected the shape of plots of sorption against concentration; similarity to the Langmuir shape was in the opposite sequence. Desorption curves always fell to the left of sorption curves; that is, there was always hysteresis. Part of this was an experimental artefact because desorption was measured at lower pH than sorption, but we provide evidence that this caused only part of the effect. The effects were greatest for Pi and least for IHP. We think that this hysteresis occurs because the adsorbed molecules diffuse into faults in the reacting medium. This would be more difficult for organic phosphate than for inorganic phosphate. Although hysteresis was smaller for organic phosphates, it was nevertheless substantial. We suggest that this is an important factor in their stability in soils.