Screening redox stability of iron rich by-products for effective phosphate immobilisation in freshwater sediments

Iron (Fe) rich by-products can be added to lake or river sediments to immobilise phosphate (PO₄) and lower eutrophication risks. These Fe materials differ in mineralogy and specific surface area, hence differing in PO₄ sorption capacity and stability under reducing conditions. This study was set up to identify key properties of these amendments in their capacity to immobilise PO₄ in sediments. Eleven Fe rich by-products, collected from drinking water treatment plants and acid mine drainage, were characterised. The PO₄ adsorption to these by-products was first determined under aerobic conditions and the solid-liquid distribution coefficient K_D for PO₄ correlated strongly to oxalate extractable Fe content. A static sediment-water incubation test was subsequently used to evaluate the redox stability of these by-products. The reductive processes gradually released Fe to solution and more Fe was release from the amended than from the control sediments. The total Fe release to solution was positively related to ascorbate reducible Fe fractions in the by-products, suggesting that such fractions indicate potential loss of P retention capacity on the long term. The final PO₄ concentration in the overlying water was 5.6 mg P L^-1 in the control and was successfully lowered by factor 30-420 depending on the by-product. The factor by which solution PO₄ was reduced in Fe treatments increased with increasing K_D determined under aerobic conditions. This study suggests that efficient by-products to trap P in sediments are characterised by a high oxalate Fe content and a low reducible Fe fraction.