Phosphorus immobilisation in sediment by using iron rich by-product as affected by water pH and sulphate concentrations

Iron (Fe) rich by-product from drinking water treatment plants can be added to rivers and lakes to immobilise phosphorus (P) in sediment and lower eutrophication risks. This study was set up to investigate the P immobilisation efficiency of an Fe rich by-product as affected by the pH and sulphate (SO4) concentration in the overlying water. Both factors are known to inhibit long-term P immobilisation under anoxic conditions. A static sediment-water incubation was conducted at varying buffered water pH values (6, 7 and 8) and different initial SO4 concentrations (0-170 mg SO4 L-1) with or without Fe rich by-product amendment to the sediment. In the unamended sediment, the P release to the overlying water was highest, and up to 6 mg P L-1, at lowest water pH due to higher reductive dissolution of Fe(III) oxyhydroxides. The Fe rich by-product amendment to the sediment largely reduced P release from sediment by factors 50-160 depending on pH, with slightly lowest immobilisation at highest pH 8, likely because of pH dependent P sorption. The total sulphur (S) concentrations in the overlying water reduced during incubation. The P release in unamended sediments increased from 2.7 mg L-1 to 4.2 mg L-1 with higher initial SO4 concentrations, suggesting sulphide formation during incubation and FeS precipitation that facilitates release of P. However, no such SO4 effects were found where Fe rich by-product was applied that lowered P release to <0.1 mg L-1 illustrating high stability of immobilised P in amended sediments. This study suggests that Fe rich by-product is efficient for P immobilisation but that loss of Fe in low pH water may lower its long-term effect.