Phosphate sorption to organic matter/ferrihydrite systems as affected by aging time

摘要

It's been hypothesized that organic matter (OM) can complex iron (Fe) from oxides minerals, which provide sorption sites for phosphate (PO_4) via the formation of Fe-PO_4-OM ternary complexes. To elucidate such mechanism, the coordination structure ofFe was examined, as well as PO_4 sorption capacity and crystallinity of ferrihydrite as it aged with OM. Stock suspensions containing 1 g ferrihydrite/kg and 0 to 3.1 g OM/kg were created at pH 6.8. PO_4 sorption capacities were measured as they have aged for 1, 7, 21, 40, and 55 days. After aging for 55 days, all systems had a 20% to 50% decrease of PO_4 sorption capacity. Extended X-ray absorption fine structure (EXAFS) analyses on selected aged peat/ferrihydrite samples showed no evidence of Fe(IU)-OM complexation. With the same amounts of Fe(LII) and peat, mixtures of Fe(III)-peat complexes and ferrihydrite provided higher PO_4 sorption capacity per unit Fe than ferrihydrite only. Indicated by HC1 extractable Fe, ferrihydrite crystallinity increased during aging. For ferrihydrite itself, crystallization was the main cause for its decreased PO_4 sorption capacity. It is very likely that the continuous competition between OM and PO_4 resulted in the decreased PO_4 sorption capacity as ferrihydriteaged with OM.