Results of Laboratory Studies of
Phosphate Interactions with Calcium Carbonate

Our laboratory investigations of phosphate interactions with calcium carbonate at present consist of the following four areas of studies:

The results of phosphate adsorption as a function of time 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 has higher adsorptive capacity compared to that of calcite when the same amount of solid is used. Our results suggest that synthetic solids have a stronger affinity toward phosphate when compared with other natural minerals. Aragonite offers more active surface and more active adsorptive sites compared to calcite.

The desorption of phosphate from aragonite surface is also a fast process. There is a steady increase of phosphate concentration from 1 to 10 minutes. After 20 minutes the phosphate concentration in solution reaches a relatively stable value. These results show that calcium carbonate can act as a fast scavenger for phosphate. Phosphate quickly reaches equilibrium with respect to the carbonate phase. The resuspended carbonate sediment may act as a source of phosphate to the water column. The fast desorption kinetic may suggest that the interaction of phosphate on a pure carbonate surface is largely electrostatic in nature.

From pH 8.7 to pH 7.4, the adsorption of phosphate decreases. The experimental design was originally based on the assumption that phosphate adsorption obeys the conventional anion adsorption, which typically exhibits a reverse S-type pH curve: as pH increases, the adsorption decreases. The phosphate adsorption onto aragonite surface defies such generalization and makes the experiment a desorption experiment. Due to the high buffering capacity of the carbonate solid, our pH experiment was restricted to neutral pH range. The mechanism of such pH dependence of phosphate adsorption on carbonate surface is not clear at this moment and may involve the change of phosphate species as well as the interaction of phosphate species with major ions such as Ca and Mg.

The adsorption of phosphate onto carbonate surfaces as a function of phosphate concentration is being studied. Combined with the kinetic data and other experimental results, the interaction mechanism of phosphate with carbonate will be elucidated. Adsorption onto calcium carbonate under natural pH shows a stepwise isothermal curve. This type of curve shows that the functional groups on aragonite surfaces are not the same and have different energy levels when interacting with phosphate. In the concentration range of our experiment we have not observed saturation (maximum adsorption concentration on the surface).

Future works of laboratory investigation of phosphate interaction with calcium carbonate include the effect of important environmental factors such as temperature and salinity on the adsorption. The effects of natural organic matters and iron oxide on the surface properties will be investigated using natural suspended materials and sediments collected during cruises.