Steady-state dissolution kinetics of mineral ferric phosphate in the presence of desferrioxamine-B and oxalate ligands at pH = 4–6 and T = 24 ± 0.6 °C

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 ≤ pH0 ≤ 6 by desferrioxamine-B (DFO-B) and oxalate (Ox) ligands. The composition of the influent solution was 10 mM NaClO4, 5 mM MES buffer. The influent solution was adjusted to 4 ≤ pH0 ≤ 6 by adding aliquots of HNO3 or NaOH stock solution. The initial concentrations of DFO-B and Ox, [DFO-B]0 and [Ox]0, ranged from 0 to 135 μM, 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–OxObs, was compared to the sum of dissolution rates by DFO-B (RDFO-B) or Ox (ROx), RDFO–OxSum (RDFO–OxSum = 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]0, and no evidence of surface masking became apparent. By contrast, dissolution rates by Ox (ROx) varied with [Ox]0 and pH0. The kinetics of dissolution by Ox was not explained by a first-order mineral dissolution behavior. Dissolution rates by DFO-B and Ox (RDFO–OxObs) 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–OxObs relative to RDFO-B. Observed (RDFO–OxObs) to calculated (RDFO–OxSum = RDFO‐B + ROx) ratio was found to be highest at [DFOB]0 = 50 μM and [Ox]0 = 49 μM. 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 22.5 μM DFO-B surpassed those for α-FeOOH or α-AlFeOOH by 20 μM DFO-B, namely, 37, and 11.6 and 3–5 μmol kg− 1 h− 1, respectively. ROx values for FePO4·2H2O by 49 mM Ox surpassed that for α-FeOOH by 70 μM Ox or α-AlFeOOH by 50 μM Ox, namely, i.e., 12, and 0.7 and 0.1 μmol 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 50 μM DFO-B and 49 μM Ox. The dissolution rates for FePO4·2H2O, α-FeOOH, and α-AlFeOOH correspond to 50, and 39–42 and 71–129 μmol 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.