『Abstract
　Ferric phosphate (FePO4・2H2O) is one of the most common secondary phosphate minerals in the environment. Nevertheless, few studies address the biological dissolution mechanism(s) of FePO4・2H2O. This paper reports steady-state dissolution rates of synthetic FePO4・2H2O at 4≦pHo≦6 by deferrioxamine-B (DFO-B) and oxalate (Ox) ligands. The composition of the influent solution was 10 mM NaClO4, 5mM MES buffer. The influent solution was adjusted to 4≦pHo≦6 by adding aliquots of HNO3 or NaOH stock solution. The initial concentrations of DFO-B and Ox, [DFO-B]o and [Ox]o, ranged from 0 to 135μM, and 0 to 345 μM. Geochemical thermodynamic equilibrium modeling was conducted using MINEQL⁺ (Schecher and McAvoy, 1998). Speciation calculations were based on thermodynamic formation constants at 298.17 K, K298 (infinite dilution reference state). Ligand-promoted dissolution rates were determined after steady-state values. Iron concentrations in the effluent solution were quantified (t＞500 h). Typical effluent-flow rate was maintained at 0.10±0.01 mL min^-1. The measured dissolution rate of FePO4・2H2O by DFO-B and Ox, RDFO-B-Ox^Obs, was compared to the sum of dissolution rates by DFO-B (RDFO-B) or Ox (ROx), RDFO-Ox^Sum (RDFO-Ox^Sum = RDFO-B + ROx). results were analyzed using the t student test. Obtained data values with p≦0.05 (^*) and ≦0.01 (^**) were considered to differ statistically from control experiments. Dissolution rates by DFO-B (RDFO-B) increased with [DFOB]o, and no evidence of surface masking became apparent. By contrast, dissolution rates by Ox (ROx) varied with [Ox]o and pHo. The kinetics of dissolution by Ox was not explained by a first-order mineral dissolution behavior. Dissolution rates by CFO-B and Ox (RDFO-Ox^Obs) surpassed RDFO-B or ROx, and increased with proton activity. Reacting FePO4・2H2O with DFO-B and high amounts of Ox resulted in higher values for RDFO-Ox^Obs relative to RDFO-B. Observed (RDFO-Ox^Obs) to calculated (RDFO-Ox^Sum = RDFO-B + ROx) ratio was found to be highest at [DFOB]o=50μM and [Ox]o=49μM. Increases in the proton activity favors the dissolution of FePO4・2H2O by DFO-B and Ox, explained because the sequestration of Fe(III) at the surface vicinity in the form of adsorbed Fe(III)-oxalate complexes. A direct comparison between the dissolution behavior of FePO4・2H2O by DFO-B and Ox against those for goethite (α-FeOOH) and Al goethite (AlFeOOH) was conducted. The dissolution behavior was found to be a function of the mineral structure. RDFO-B values for FePO4・2H2O by 22.5μM DFO-B surpassed those for α-FeOOH or α-AlFeOOH by 20μM DFO-B, namely, 37, and 11.6 and 3-5μmol kg^-1 h^-1, respectively. ROx values for FePO4・2H2O by 49 mM Ox surpassed that for α-FeOOH by 70μM Ox or α-AlFeOOH by 50μM Ox, namely, i.e., 12, and 0.7 and 0.1μmol kg^-1 h^-1. The latter results agree with the idea of the inhibition of Fe release in goethite because its sequestration in the form of adsorbed Fe(III) oxalate complexes. In contrast, a different scenario holds true for dissolution by 50μM DFO-B and 49μM Ox. The dissolution rates for FePO4・2H2O, α-FeOOH, and α-AlFeOOH correspond to 50, and 39-42 and 71-129μmol kg^-1 h^-1, respectively. The high extent of iron release from Al goethite is best explained because high-energy surface sites formed after Al substitution in goethite.

Keywords: Siderophore; Microbial dissolution; Phosphorus; Ligand competition』

1. Introduction
2. Materials and methods
　2.1. Materials
　2.2. Synthesis of crystalline iron(III) phosphate (FePO4・2H2O)
　2.3. Ligand-promoted dissolution kinetics
3. Results and discussion
　3.1. DFO-B, oxalate, and proton promoted-dissolution kinetics
　　3.1.1. Iron release by H⁺, DFO-B and Ox
　　3.1.2. Iron release by DFO-B and Ox
　3.2. DFO-B and Ox-promoted dissolution of strengite and other minerals
4. Conclusions
Acknowledgments
References