Phosphorus Speciation and Amendment Fate in Calcareous Petroleum Hydrocarbon Contaminated Soils

Abstract

Adsorption and precipitation reactions often limit phosphorus (P) availability in soil environments. The Federated Co-operatives Ltd. (FCL) Meadow Lake SK bulk and residential fueling station is an in situ bioremediation research site with a P nutrient deficiency resulting from petroleum hydrocarbon contamination. Two potential P sources, tripolyphosphate (TPP) and P-rich biochars, were studied to deliver P at this site. Tripolyphosphate (TPP) is a form of slow release P fertilizer that must undergo hydrolysis to be bioavailable; the mechanisms of TPP hydrolysis in soils remain controversial. It has been proposed that TPP rapidly hydrolyzes in the soil solution, but in model systems, TPP has been shown to rapidly adsorb to mineral surfaces. Using molecular-scale X-ray absorption spectroscopic techniques it was found that TPP rapidly adsorbs to soil mineral surfaces (<48 hrs) and potentially remains as an adsorbed P surface complex for up to a year in an alkaline/calcareous soil environment. To better understand how adsorption reactions with mineral surfaces influences polyphosphate hydrolysis, TPP was adsorbed to a goethite (α-FeO(OH)) mineral surface at pH 4.5, 6.5, and 8.5 and solid-state speciation was performed as a function of time from 48 to ~2000 hrs using Attenuated Reflectance Fourier Transform Infrared (ATR-FTIR) spectroscopy. The hydrolysis rates of adsorbed TPP demonstrated that adsorption to mineral surfaces catalyzes the hydrolysis of TPP. Specifically, at alkaline pH (8.5) where aqueous hydrolysis is known to completely stall, it was found that ~40% of adsorbed TPP was hydrolyzed to ortho-P during the study. This provides evidence that mineral surfaces provide a catalytic effect for the hydrolysis of adsorbed TPP. Phosphorus rich biochars tend to be composed of the sparingly soluble mineral hydroxyapatite, however, evidence was found of mineral dissolution and re-precipitation of soluble calcium phosphate mineral species. Transformation of P to more soluble mineral species resulted in increased fractions of labile extractable P. The results of this study indicate that TPP is an effective short-term amendment that increases the adsorbed P fraction. Biochars, however, may be an effective long-term amendment due to slow mineral dissolution that over-time results in increases to the labile P fraction.