Abstract
Drinking‐water treatment residuals (WTR) control excessive soluble phosphorus (P) concentrations in soils and appear to do so for a long time. Knowledge of the solid phases controlling the solubility of WTR‐immobilized P could enable the prediction of the long‐term stability of WTR‐immobilized P. We hypothesized that stable aluminum (Al)– and iron (Fe)–phosphate minerals control the solubility of WTR‐immobilized P, thus ensuring that WTR‐immobilized P remains stable in the long term. We utilized both artificially (laboratory) and naturally (field) aged WTR‐amended samples to test our hypothesis and the MINTEQA2 geochemical model to perform chemical equilibrium calculations. The field‐aged WTR‐amended soil (Granby loamy sand and Granby sandy loam) samples were obtained from two sites where an alum‐based WTR (Al‐WTR) had been applied 7.5 years ago. Artificial aging of fresh (freshly amended) soil (Immokalee fine sand) samples was achieved through incubation at elevated temperatures (46 or 70 °C) for 4.5 years and repeated wetting and drying for 2 years. Similar solid phases were predicted to control P solubility in both artificially and field‐aged samples. Without WTR amendment, P solubility in the P‐impacted soil was controlled by soluble magnesium‐phosphate (Mg‐P) minerals (farringtonite, newberyite), and calcium‐phosphate (Ca‐P) minerals (brushite and monetite). In the presence of WTR amendment, soil solutions of both field‐ and artificially aged samples were supersaturated with respect to hydroxyapatite, β‐tricalcium phosphate, and octacalcium phosphate and undersaturated with respect to brushite, monetite, farringtonite, and newberyite. No solid phase was predicted to control the solubility of WTR‐immobilized P. Thus, we reject our hypothesis that stable Al‐ and Fe‐phosphate mineral control the solubility of WTR‐immobilized P and conclude that sorption–desorption reactions determine P retention/release (P solubility) in WTR‐treated samples rather than precipitation–dissolution reactions.
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