Underestimation of phosphorus fraction change in the supernatant after phosphorus adsorption onto iron oxides and iron oxide-natural organic matter complexes

The phosphorus (P) fraction distribution and formation mechanism in the supematant after P adsorption onto iron oxides and iron oxide-humic acid (HA) complexes were analyzed using the ultrafiltration method in this study. With an initial P concentration of 20 mg/L (I = 0.01 mol/L and pH = 7), it was shown that the colloid (1 kDa-0.45 gm) component of P accounted for 10.6%, 11.6%, 6.5%, and 4.0% of remaining total P concentration in the supematant after P adsorption onto ferrihydrite (FH), goethite (GE), ferrihydrite-humic acid complex (FH-HA), goethite-humic acid complex (GE-HA), respectively. The <1 kDa component of P was still the predominant fraction in the supematant, and underestimated colloidal P accounted for 2.2%, 55.1%, 45.5%, and 38.7% of P adsorption onto the solid surface of FH, FH-HA, GE and GE-HA, respectively. Thus, the colloid P could not be neglected. Notably, it could be interpreted that Fe3+ hydrolysis from the adsorbents followed by the formation of colloidal hydrous ferric oxide aggregates was the main mechanism for the formation of the colloid P in the supematant. And colloidal adsorbent particles co-existing in the supernatant were another important reason for it. Additionally, dissolve organic matter dissolved from iron oxide-HA complexes could occupy large adsorption sites of colloidal iron causing less colloid P in the supematant. Ultimately, we believe that the findings can provide a new way to deeply interpret the geochemical cycling of P, even when considering other contaminants such as organic pollutants, heavy metal ions, and arsenate at the sediment/soil-water interface in the real environment. (C) 2016 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.