Effects of Citrate on the Rates and Mechanisms of Phosphate Adsorption and Desorption on a Calcareous Soil

Adsorption at the soil-water interface is a fundamental chemical process that often controls the retention of compounds in soils. The rates of adsorption therefore have a vital role in the fate and transport of phosphate in the soil environment. In this study, the effects of a range of organic acid (citrate) levels and soil Ca levels on phosphate (PO43-) adsorption and desorption were investigated in detail through a combination of laboratory (miscible displacement kinetics) and synchrotron-based X-ray absorption spectroscopy. A pseudo-second-order kinetics model was applied to the data that provided good fits for the entire datasets (both adsorption and desorption) with a single set of equations. In general, it was observed that citrate directly competed for adsorption sites in soils with reduced levels of Ca but in soils high in labile Ca, citrate was not competitive in desorbing P. The most likely explanation for this is that citrate instead chelates with Ca to form soluble complexes when an excess of Ca is present.