THE ROLE OF ADSORPTION IN SEDIMENT-WATER EXCHANGE OF PHOSPHATE IN NORTH-SEA CONTINENTAL-MARGIN SEDIMENTS

The effect of adsorption on the sediment-water exchange of PO4 was inv estigated in sediments from four different types of sedimentary enviro nments in the southern and eastern North Sea in August 1991 and Februa ry 1992. Nonlinear adsorption isotherms for oxidized sediment from eig ht stations indicate that North Sea sediments differ widely in their c apacity to adsorb PO4. A good correlation between the value of the ads orption coefficient and NH4-oxalate-extractable Fe was observed. A com bination of the adsorption data with porewater PO4 profiles, solid pha se results, and measured and calculated rates of sediment-water exchan ge of PO4 for 15 stations in both August and February indicates that a dsorption plays an important role in controlling sediment-water exchan ge of PO4 during at least a part of the year in three of the four Nort h Sea environments. At most stations, PO4 adsorption constrains the fl ux of PO4 to the overlying water. At one station in the depositional e nvironment of the Skagerrak, however desorption is responsible for the maintenance of a flux of PO4 to the overlying water. A one-dimensiona l reaction-diffusion model, describing porewater PO4 and solid phase P profiles, was developed and applied to results for two stations. The model results show that both enhanced retention and enhanced release o f PO4 can be adequately described when simultaneous equilibrium and fi rst-order kinetic reversible adsorptive reactions are assumed.