Citric Acid Promoted Dissolution of Phosphate in Ferrihydrite-Boehmite Mixtures as Affected by Its Solid State Speciation.

Phosphorus is an essential plant macronutrient and also a potential water pollutant. Most soils are P deficient because phosphate minerals are sparingly soluble. Citric acid is released by plant roots and microbes in response to P deficiency and is hypothesized to be the main mechanism for solubilising P and ameliorating Al toxicity. Our goal is to investigate citric acid promoted P dissolution in ferrihydrite-boehmite mixtures using an integrated wet chemical, spectroscopic (P-K x-ray absorption near edge structure-XANES), and microscopic (transmission electron microscopy) approach.  P dissolution and release in acid soils is mainly controlled by P sorption to Fe and Al-oxyhydroxide minerals. Ferrihydrite and boehmite are poorly crystalline analogs of Fe- and Al-oxyhydroxides respectively in soils. The current understanding of organic acid mediated phosphate dissolution is limited by the lack of a molecular scale characterization of its solid state speciation. Furthermore, phosphate dissolution has not been studied in context of its uptake without the complicating influence of metabolism as in microbial cells or plants. We hypothesize that citric acid promoted phosphate dissolution in ferrihydrite-boehmite mixtures is controlled by its quantitative distribution between ferrihydrite and boehmite. Previously, we showed using P K-XANES spectroscopy that phosphate distributed itself between ferrihydrite and boehmite essentially in proportion to its maximum sorption capacity on each mineral. We will present results from experiments characterizing citric acid promoted phosphate dissolution in ferrihydrite, boehmite, and ferrihydrite-boehmite mixtures as a function of pH (4, 6, and 9) and time in the presence of a functional phosphate sink to simulate phosphate dissolution under natural conditions and to understand the role of mineral interactive effects on P dissolution in acid soils. A high affinity transporter for yeast cells reconstituted into proteoliposomes will function as the phosphate sink to prevent re-sorption and re-distribution of phosphate, post dissolution, in ferrihydrite-boehmite mixtures.