In situ transformations of bonechar and tri-poly phosphate amendments in phosphorus-limited subsurface soils

Nutrient availability is a key factor for the in situ bioremediation of petroleum hydrocarbon contaminated (PHC) soils. Phosphorus deficiencies in calcium rich PHC soils can often be challenging, as amendment P rapidly forms new mineral phases. In situ bioreactors were used to determine the chemical fate of several P amendments (ortho-P, tripolyphosphate, fishmeal and bonemeal biochar), which are currently under consideration to address P nutrient deficiencies during PHC remediation. Contaminated and uncontaminated soil from the study site were treated with the P amendments then suspended in onsite bioreactors (contaminated and uncontaminated). Amendment fate and transformation was determined utilizing a combination of bulk and micro-focused X-ray diffraction (XRD), P X-ray absorption near edge structure (XANES), and X-ray fluorescence imaging (XFI). The ortho-P amendment was found to rapidly precipitate as a Mg-brushite species, proposed as a surface precipitate on dolomite mineral surfaces. Tripolyphosphate increased the adsorbed P fraction through direct adsorption to mineral surfaces and displayed evidence of surface hydrolysis over time with subsequent formation of surface precipitated Mg-brushite. Both P rich biochars demonstrated a potential for dissolution and incorporation of Mg ions, resulting in increased fractions of Mg-brushite and newberyite on the edges of the biochar particles. Tripolyphosphate is an effective short-term P amendment; as there was an increased adsorbed P fraction without the formation of precipitate species until after hydrolysis. In contrast, biochars are potentially an effective long-term amendment, as significant time is required for mineral dissolution to occur and increase the P nutrient status of subsurface soils.