Application of isothermal calorimetry to the study of phosphorus sorption onto soils in a flow-through system

The degree, mechanisms, and kinetics of phosphorus (P) sorption onto soils can have a significant influence on leaching losses of P from soil. The objectives of this study were to measure the impact of retention time (RT) on P sorption in a flow-through system intended to simulate downward movement of a P solution through two different riparian soils, and determine if isothermal titration calorimetry (ITC) can provide useful information reflective of flow-through results. Topsoil from two riparian/alluvial soils was sampled and characterized for P concentrations and parameters related to P sorption. Flow-through P sorption experiments were conducted in order to examine the effect of RT and inflow P concentration on P sorption; this was compared to results of ITC experiments where the heat of reaction was measured with the addition of P to soils. Results of ITC experiments were reflective of both soil characterization and flow-through sorption in that the Barren Fork soil sorbed less P, but at a faster rate, compared to Clear Creek. Based on thermograms, the dominant P sorption reaction was ligand exchange onto Al/Fe oxides/hydroxides, with a lesser degree of precipitation. Phosphorus removal for both soils was limited by physical nonequilibrium instead of chemical nonequilibrium (sorption kinetics). For the Clear Creek soil, the ITC analysis illustrated that P sorption was limited more by physical diffusion of P through micropores to sorption sites rather than chemical kinetics. For the Barren Fork soil, results indicated that the limiting process was pore scale transport from bulk flow to sorption sites, which was a function of flow rate. The calorimetry approach presented can help provide soil-specific information on the risk of P inputs to leaching (degree of P sorption) under different conditions (flow rate or RT), and potential for desorption (P sorption mechanisms).