Application of isothermal calorimetry to 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 sites (Barren Fork and Clear Creek) was sampled and characterized for P concentrations and parameters related to P sorption. Flow-through P sorption experiments were conducted 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). 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).