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Role of Organic Acids in Phosphate Mobilization from Iron Oxide

Soil & Crop Sciences Dep., 2474-TAMU, Texas A&M University, College Station, TX 77843-2474

View larger version (16K): [in a new window]   Fig. 1. Relationship between citrate adsorbed, P released, and Fe dissolved over time for citrate-induced P desorption from ferrihydrite at pH 4.0, 0.013 M citrate, and 7.14 g ferrihydrite L–1. The initial P/citrate/P-adsorption maximum ratio was 0.25:1:1. Each data point represents the mean of three replications, with error bars representing standard error.

View larger version (28K): [in a new window]   Fig. 2. Influence of pH on organic-acid induced P release from ferrihydrite (A) and goethite (B). Experimental conditions: 7.14-g oxide L–1, 1/4 of P-adsorption maximum, 0.0357 M organic acid, and 28-min reaction at room temperature. Each bar represents the mean of three or more replications, with error bars representing standard error. Different letters represent significant differences by Tukey's at = 0.05.

View larger version (35K): [in a new window]   Fig. 3. Influence of pH on organic-acid induced Fe dissolution from ferrihydrite (A) and goethite (B). Experimental conditions: 7.14 g oxide L–1, 1/4 of P-adsorption maximum, 0.0357 M organic acid, and 28-min reaction at room temperature. Each bar represents the mean of three or more replications, with error bars representing standard error. Different letters represent significant differences by Tukey's at = 0.05.

View larger version (21K): [in a new window]   Fig. 4. Influence of pH on concentrations of Fe-containing species and total Fe in equilibrium with ferrihydrite (A) and goethite (B), at 0.0357 M citrate. All calculations were made using MINTEQA2, at 0.05 ionic strength.

View larger version (34K): [in a new window]   Fig. 5. Influence of pH and organic ligand on concentration of total dissolved Fe in equilibrium with ferrihydrite (A) and goethite (B). These theoretical values were calculated using MINTEQA2, with 0.0357 M ligand and 0.05 ionic strength.

View larger version (15K): [in a new window]   Fig. 6. Experimentally determined Fe dissolution from ferrihydrite versus the formation constants (log KFe) of the FeLn-1 complexes (FeLn-1). Formation constants were obtained from the NIST Database 46, and experimental data were obtained from reactions of 7.14-g ferrihydrite L–1 (1/4 maximum P-adsorption sites filled) with 0.0357 M organic ligand for 28 min. Each data point represents the mean of three replications, with error bars representing standard error. Each point represents one of the six organic ligands, according to log KFe as presented in Table 2.