Immobilization of P by oxidation of Fe(II) ions leading to nanoparticle formation and aggregation

Abstract

Ferrous iron was oxidized at pH 6.0 in the presence of dissolved oxygen and increasing concentrations of phosphate. The resulting precipitates were characterized by TEM, SEM, XRD, IR spectroscopy, Mössbauer spectroscopy, EXAFS spectroscopy, and chemical analyses. The kinetics and the stoichiometry of oxidation were also determined. Chemical analyses revealed that all the P introduced was immobilized up to an introduced P/Fe molar ratio of 0.6–0.7. In the presence of excess phosphate, the maximum P/Fe ratio of the precipitates was found to be equal to about 0.86. Incorporation of phosphate hindered the sorption of dissolved carbonates, but favored the immobilization of monovalent cations such as Na or K. The number of OH− ions per Fe atom introduced during the reaction decreased from 2 in the absence of P to about 1.5±0.1 in the presence of excess phosphate. In all cases, no residual Fe(II) could be detected. In the absence of phosphate, the samples were composed of poorly crystallized ferrihydrite, lepidocrocite and goethite nanoparticles. Even just a small amount of phosphate (P/Fe=0.02) was sufficient to effectively restrict the formation of goethite. In contrast, the formation of lepidocrocite was detected by XRD for P/Fe ratios as high as 0.1. At higher P/Fe ratios, only non-crystalline particles were detected. For 0.1<P/Fe<0.5, the characteristic size of all particles was smaller than 10nm. For P/Fe>0.5, a new category of particles with characteristic length scales larger than 10nm appeared and became prominent as P/Fe increased. The transition was accompanied by a change in color of the suspension, from dark red to light yellow. For an introduced P/Fe ratio larger than 1, only the larger particles remained. As the introduced P/Fe ratio increased further, the incorporated P/Fe ratio increased only slightly. In contrast the size of the particles increased significantly, reaching a size larger than 50nm in the presence of a large excess of PO43-. The kinetics of oxidation and hydrolysis were shown to obey a typical autocatalytic process in the presence as well as in the absence of PO43-.