Phosphorus sorption by sediments in a subtropical constructed wetland receiving stormwater runoff

This study investigated the potential of using a mixture of fishpond bund material, completely decomposed granite and river sand as substrate in a constructed wetland for phosphorus removal. Core samples were collected from the newly constructed Hong Kong Wetland Park (HKWP) receiving influent stormwater from a nearby new town, and batch incubation experiments were conducted to determine the P sorption characteristics of sediments. The HKWP sediments adsorbed the majority of available P in the initial 20 min of incubation, with a first-order rate constant of 1.01-2.11 h(-1). Sediments in the reedbeds and freshwater marshes possessed a great capacity for P adsorption with the high Langmuir sorption maxima (478-858 mg kg(-1)) and Freundlich adsorption constants (417-672 L kg(-1)) obtained, attributable to the high amorphous iron and aluminium concentrations compared to other constructed wetlands. Moreover, sediment equilibrium P concentrations were generally low (4.6-23.6 mu g L-1), facilitating a net P adsorption by sediments under moderate P loadings. Yet, the amount of P adsorbed by the HKWP sediments was limited by the low ambient porewater P concentrations and there was even a risk of P desorption when sediments in the freshwater marshes were resuspended into the water column. While substrates in the HKWP demonstrated a great potential for P adsorption, consideration should also be given to P loadings in influent water to fully utilize the P sorption capacity of sediments and enhance the P removal efficiency of constructed wetlands. (C) 2008 Elsevier B.V. All rights reserved.