Topical Area: Marine Chemistry

Frank J. Millero, University of Miami/RSMAS, Miami, FL; Jia-Zhong Zhang, NOAA/AOML/OCD, Miami, FL

Phosphate, as a limiting nutrient, is critical to the onset and sustainment of phytoplankton blooms in Florida Bay. The adsorption of phosphate on suspended biogenic calcium carbonates (aragonite and calcite) is believed to be an important mechanism for removal of phosphate in the natural waters. Understanding the transportation and transformation of phosphate in the environment is important to the water quality of Florida Bay.

Laboratory experiments on phosphate adsorption show that the adsorption of phosphate on calcite and aragonite is a fast process. Generally it takes about 5 minutes for the adsorbed phosphate concentration to reach a constant value. Aragonite offers more active surface and more active adsorptive sites than calcite. The capacity of adsorption on the aragonite is about 20 times higher than calcite based on moles of phosphate adsorpted on per gram of particle. Adsorption isotherms have been constructed based on the equilibrium experiments of phosphate adsorption on aragonite at pH of 8.0 in seawater over salinity range of 5-35 and temperature range of 5 - 45 ^oC at initial phosphate concentrations over a range of 2 to 60 m M. Both Langmuir and Freundlich adsorption models have been used to fit the experimental data. The results indicated the existence of different binding sites with a wide spectrum of binding energies on the calcium carbonate surface. At low adsorption densities, surface sites with highest energy are occupied first. For both aragonite and calcite, the adsorption capacity increased as the salinity of seawater decreased. The effect of salinity on the adsorption might be related to the competition of phosphate between particle surface and magnesium in seawater. These results show that calcium carbonate can act as a fast scavenger for phosphate in the water. Future works will be focused on the natural suspended matter collected in Florida Bay. The composition of suspended matter, adsorption and desorption characteristic and the effect of natural organic matters and iron oxide on the surface properties will be investigated using the suspended matter and sediments from Florida Bay.