Measuring Surface Chemical Properties of Soil Using Flow Calorimetry 1

Abstract

Flow calorimetry, which is ideally suited for measuring reactions oc curring at the liquid/solid interface, has been used to study the surface chemistry of many types of solids, but little use of it has been made in the study of surface reactions of soils. The purpose of this study was to demonstrate the application of flow calorimetry to the study of two fun damental soil chemical processes, namely cation exchange and phosphate sorption. Surface horizon samples of a Typic Acrorthox and a Typic Tropohumult from Puerto Rico, a strong acid cation exchange resin (Dowex 50W-8), and an amorphous Al(OH) 3 were used. Heats for K/Ca exchange on the Dowex resin and the Oxisol, and K/Na exchange on the Ultisol, were consistent with literature values that were obtained using conventional batch calorimetry or derived from the temperature depen dence of the exchange constant. Although peak areas associated with a given pair of exchange reactions were equal, peak shapes were generally not equivalent, indicating differences in the rate at which the two reac tions occurred. For example, Ca displacing exchangeable K occurred more rapidly than the reverse reaction on the Dowex resin. The reaction of phosphate with the Ultisol and amorphous Al(OH) 3 was exothermic. Exposure of the soil to several cycles of phosphate was sufficient to sat urate the sorption sites, as evidenced by the loss of a detectable heat sig nal. However, phosphate reactive sites were regenerated by flushing the column with a salt solution at pH 10. Precipitation of Al-phosphate was shown to be endothermic, confirming that precipitation was not the pri mary mechanism for phosphate sorption in this study.

Application of Isothermal Calorimetry to Phosphorus Sorption onto Soils in a Flow-through System

“…Isothermal titration calorimetry data can be used to somewhat distinguish between these mechanisms. For example, exothermic (producing heat) reactions regarding P sorption in neutral and acid soils indicate ligand exchange mechanisms onto Al and Fe minerals and kaolinite (Appel et al, 2013;Penn and Zhang, 2010;Rhue et al, 2002;Harvey and Rhue, 2008). On the other hand, endothermic (absorbing heat) reactions indicate precipitation of Al and Fe phosphates (Penn and Warren, 2009;Rhue et al, 2002).…”

“…For example, Aharoni and Sparks (1991) describe sorbate transport processes in a solid-liquid system with several scenarios: transport of sorbate in the soil solution, transport across a liquid film at the solid-liquid interface, transport in a liquid-filled micropore, and diffusion of a sorbate at the soil surface or into a micropore. The "slow burn" (Rhue et al, 2002) displayed by the Clear Creek soil has been observed in other studies (Kabengi et al, 2006b;Harvey and Rhue, 2008). Since both soils were dominated by ligand exchange chemical mechanisms which are extremely fast (Sposito, 1994) explanation for the long "slow burn" for P sorption in the Clear Creek soil is that after the initial P sorption reactions occurred on the surfaces of minerals, further P sorption occurred only after the slow physical transport and diffusion of P into micropores (micropore scale physical nonequilibrium).…”

“…The quantity and patterns of heat measured can provide information on the degree of reaction, type of reaction, and kinetics (Harvey and Rhue, 2008;Rhue et al, 2002;and Kabengi et al, 2006a). For example, Penn and Warren (2009) were able to distinguish between P sorbed by ligand exchange vs. surface precipitation in titration of kaolinite with a P solution.…”

Mechanisms of lead, copper, and zinc retention by phosphate rock

“…Heats (DH ) of Pb, Cu, and Zn sorption onto PR were determined using flow calorimetry Rhue et al, 2002). Amounts of 120 G 0.5 mg of PR (0.25e0.50 mm) were placed in the column of the flow calorimeter.…”

“…This helps to better un derstand the binding strength of a particular reactive species onto the solid surface. Flow calorimetry has been used to study the surface chemistry of solids Rhue et al, 2002), but no study has been conducted on the surface reaction of metals with phosphate minerals.…”