Suppression of phosphate release from coastal sediments using granulated coal ash

Abstract

In order to evaluate suppression of phosphate release from sea bottom sediments using granulated coal ash (GCA), a byproduct from coal thermal electric power stations, field experiments were carried out at oyster culture sites where the sediments are rich in organic matter. Two types of GCA with the diameter of <40 mm were applied on separate bottom sediment plots of 50 m × 75 m with ca. 20 cm depth each. The first GCA type is composed of pulverized coal combustion ash (ExpA) and another consists of pressurized fluidized bed combustion ash (ExpB). Sediment pH was increased up to ca. 8.5 in both experimental plots due to hydrolysis of CaO. Phosphate concentration in the interstitial water of the sediment was significantly decreased in ExpA due to adsorption onto the GCA. A numerical model to quantify chemical and biological processes in the sediment was applied to estimate how phosphorus cycle was changed due to the application of GCA. The model output showed that release rate of phosphate from the sediment at ExpB was suppressed to the level of about 1 order lower (400 μmol m−3 d−1) than that in the control site (4800 μmol m−3 d−1), whereas decomposition of detrital phosphorus in the experimental site was estimated to have been enhanced 4.8 times compared to the control site. Considering both laboratory and simulated field experiments carried out previously, it was proven in the present study that GCA can effectively adsorb phosphate from interstitial water in organically enriched sediments and suppress the release of phosphate from the sediments to the upper water column in the field experiments.