Extent and Mechanism of Interaction between Phosphate and Citrate in a Calcareous Soil

In subsurface systems that have been contaminated with petroleum hydrocarbons (PHCs), the degradation by microbes is limited by nutrient availability, primarily by bioavailable phosphorus (P). Low-molecular-weight organic acids (LMWOA), particularly citrate, have been proposed as a remediation amendment to enhance P mobility, but the effect of citrate on P retention by calcium-rich soils is poorly understood. In this study, the sorption of P in the absence and presence of citrate on calcareous cold soils was investigated under controlled laboratory conditions using a combination of laboratory-based adsorption and desorption studies and synchrotron-based spectroscopic techniques. The results indicate that calcium exerts a strong control on P sorption by forming calcium phosphate minerals, thus limiting the availability of P for transport or microbial uptake. Treatments that reduce reserve and exchangeable Ca increase the relative proportion of adsorbed P. When citric acid is also added to these calcareous soils, their buffering capacity largely inhibits any desorption of P or changes in P speciation until citrate levels exceed P additions by more than 10 to 1. By increasing citrate to P ratios to between 10:1 and 50:1, it is possible to overcome the soil's buffering capacity, decrease pH, and enhance P desorption via competition with citric acid. This suggests that one can enhance P transport in this soil system, but only when the citrate to P ratio is properly adjusted for the site's Ca levels.