Effect of Mixed Coal Fly Ash (CFA) and Oyster Shells (OS) on Benthic Nutrient Removal in Eutrophic Coastal Sediments:...

Valorization of mariculture and industrial solid wastes of oyster shell (OS) and coal fly ash (CFA) into granulated sediment amendments offers a cost-effective and environmentally sustainable alternative to conventional landfill disposal. This study evaluates the geochemical effects of granulated OS-CFA at three mixing ratios (1:1, 1:3, and 3:1) on nutrient dynamics in polluted coastal sediments through a 120-day microcosm experiment. Thermally treated OS-CFA enhances pollutant adsorption by increasing porosity and active surface sites, effectively suppressing nutrient release from sediment into overlying water. Hydrolysis of CaO, the main component in OS and CFA, releases Ca²⁺ and OH^- ions, which increase sediment pH and promote ion exchange and precipitation. The amendment of OS-CFA in sediment achieved PO₄-P removal efficiencies of 41.7% for OS:CFA1:3, 50% for OS:CFA3:1, and 66.7% for OS:CFA1:1, with PO₄-P concentrations 1.8-3.5 times lower than the control by Day 120, likely due to the formation of calcium phosphate and insoluble phosphate precipitates. The formation of calcium silicate hydrate (C-S-H) further suppresses SiO₂-Si concentrations in all OS-CFA treatments. High sediment pH suggests that OS-CFA can neutralize acidic coastal sediments, with increased NH₃-N concentrations due to the release of ammonia salts from CFA under alkaline conditions. These results show that granulated OS-CFA is a promising material for coastal sediment remediation and eutrophication mitigation, supporting circular economy principles.