Retardation of phosphate release from freshwater benthic sediments by application of ocher pellets with calcium nitrate

This article presents an in situ treatment method for retardation of phosphate release from freshwater benthic sediments. The method is based on the addition of ocher pellets into benthic sediments. The pellets consist of ocher and calcium nitrate (OCN pellet). The OCN pellet slowly releases calcium and nitrate, together with ocher, into the sediment-water interface, where all three components play an important role in reducing phosphate release from sediments. Each component was applied to sediments to examine the individual effects in this study. Bench scale experiments were performed with the following treatments: (1) ocher, (2) calcium, (3) nitrate, (4) calcium nitrate, (5) calcium nitrate and ocher, and 6) no treatment. The sediment without any treatment released a phosphate flux of 10.82 mg/m²/d. The sediment with ocher, calcium, and nitrate treatments, released phosphate fluxes of 8.41, 6.41, and 0.95 mg/m²/d, respectively. This can be attributed to the fact that nitrate, acting as an electron acceptor of organic matter degradation, prevents the dissolution of phosphate bound to iron oxides. In addition, calcium and ocher can bind the dissolved phosphate. An increase of pH together with nitrate reduction helps the calcium bind to phosphate. Ocher also can act as a vehicle to bring the calcium nitrate from the water surface to the bottom. The proposed method has two major advantages compared to previous methods: (1) effective reduction of phosphate release due to the synergism of calcium, nitrate, and ocher; and (2) case of use in natural systems, in such manner that the OCN pellets can be simply scattered over the water surface.