Adsorption of Phosphate and Tartrate on Hydroxy-Aluminum–Oxalate Precipitates

Sorption of phosphate in the presence of low molecular mass organic ligands on hydroxy-Al–organics coprecipitates may be the rule and not the exception in rhizospheric soils. We studied the competitive sorption of phosphate and tartrate on hydroxy-Al–oxalate precipitates obtained by coprecipitating Al and oxalate at pH 7.0 and initial oxalate/Al molar ratios of 0, 0.1, 0.2, and 0.5. The noncrystalline precipitates showed different chemical and physicochemical properties. Oxalate was not homogeneously distributed throughout the solids, and the greater its content in the samples, the higher the surface area, reactivity, and solubility of the solids. Oxalate was released from each complex mainly at pH <5.0 and >7.0 and more in the presence than in the absence of phosphate or tartrate. More phosphate than tartrate was sorbed on the samples, but tartrate appeared to be more specific than phosphate in replacing oxalate. The quantity of phosphate and tartrate sorbed on the organomineral complexes, containing greater amounts of oxalate, was little affected by pH (4.0–9.0), probably due to the release of oxalate from the surfaces of the complexes by increasing the pH. When mixtures of equimolar amounts of the two ligands were added at pH 4.0 to 9.0, more phosphate than tartrate was sorbed on the organomineral solids. The sorbed tartrate/sorbed phosphate molar ratio was always <1.0, but usually increased by decreasing pH and increasing the content of oxalate in the precipitates. At pH ≤5.0 the presence of high tartrate concentrations reduced the phosphate sorption, releasing Al and Al-bounded oxalate. Only at pH >5.0 could the decrease in sorption of phosphate be attributed mainly to competition in sorption between the organic and inorganic ligands for different sorption sites.