The role of adsorption in sediment-water exchange of phosphate in North Sea continental margin sediments

Abstract

The effect of adsorption on the sediment-water exchange of PO4 was investigated in sediments from four different types of sedimentary environments in the southern and eastern North Sea in August 1991 and February 1992. Nonlinear adsorption isotherms for oxidized sediment from eight stations indicate that North Sea sediments differ widely in their capacity to adsorb PO4. A good correlation between the value of the adsorption coefficient and NH4-oxalate-extractable Fe was observed. A combination of the adsorption data with porewater PO4 profiles, solid phase results, and measured and calculated rates of sediment-water exchange of PO4 for 15 stations in both August and February indicates that adsorption plays an important role in controlling sediment-water exchange of PO4 during at least a part of the year in three of the four North Sea environments. At most stations, PO4 adsorption constrains the flux of PO4 to the overlying water. At one station in the depositional environment of the Skagerrak, however, desorption is responsible for the maintenance of a flux of PO4 to the overlying water. A one-dimensional 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 of PO4 can be adequately described when simultaneous equilibrium and first-order kinetic reversible adsorptive reactions are assumed.