Chemical speciation and fate of tripolyphosphate after application to a calcareous soil.

Adsorption and precipitation reactions often dictate the availability of phosphorus in soil environments. Tripolyphosphate (TPP) is considered a form of slow release P fertilizer in P limited soils, however, investigations of the chemical fate of TPP in soils are limited. It has been proposed that TPP rapidly hydrolyzes in the soil solution before adsorbing or precipitating with soil surfaces, but in model systems, TPP also adsorbs rapidly onto mineral surfaces. To study the adsorption behavior of TPP in calcareous soils, a short-term (48 h) TPP spike was performed under laboratory conditions. To determine the fate of TPP under field conditions, two different liquid TPP amendments were applied to a P limited subsurface field site via an in-ground injection system. Phosphorus speciation was assessed using X-ray absorption spectroscopy, total and labile extractable P, and X-ray diffraction. Adsorption of TPP to soil mineral surfaces was rapid (< 48 h) and persisted without fully hydrolyzing to ortho-P. Linear combination fitting of XAS data indicated that the distribution of adsorbed P was highest (~ 30-40%) throughout the site after the first TPP amendment application (high water volume and low TPP concentrations). In contrast, lower water volumes with more concentrated TPP resulted in lower relative fractions of adsorbed P (15-25%), but a significant increase in total P concentrations (~ 3000 mg P kg soil) and adsorbed P (60%) directly adjacent to the injection system. This demonstrates that TPP application increases the adsorbed P fraction of calcareous soils through rapid adsorption reactions with soil mineral surfaces.

Mechanisms of tripolyphosphate adsorption and hydrolysis on goethite.

Linear tripolyphosphates (TPP) are used extensively in liquid fertilizers though little is known about their chemical fate after sorption on mineral surfaces. The initial rapid adsorption of TPP to metal oxide surfaces has been observed, but it is not known whether this adsorption will catalyze or inhibit TPP hydrolysis. To determine the effects of adsorption upon TPP hydrolysis, batch reactors were set up with two surface loadings of TPP adsorbed on goethite (α-FeOOH) for up to 3months at pH 4.5, 6.5 and 8.5. Samples were analyzed for; adsorbed phosphorus (P), P speciation via both P K-edge XANES and FTIR vibrational spectroscopies. Additionally, it was learned through in-situ ATR-FTIR that drying of adsorbed TPP in the presence of Ca results in the formation of a Ca-trimetaphosphate type of surface complex. The rates of adsorbed TPP hydrolysis as measured with ex-situ FTIR were most rapid at pH 4.5 with 100% of the TPP fully hydrolyzed by 3months. Slower rates were observed at pH 6.5 and 8.5 through 3months. Compared to published rates of aqueous TPP hydrolysis, adsorption to mineral surfaces catalyzes TPP hydrolysis.