Adsorption of phosphate in anoxic marine sediments1

The adsorption of dissolved phosphate in anoxic marine sediments has been investigated in samples of mud from two locations in Long Island Sound kept under strictly anoxic conditions. Kinetic experiments showed that desorption is more rapid than adsorption, but that both reach equilibrium within 24 h. The process of adsorption of phosphate can be described by a simple linear isotherm: the dimensionless in situ adsorption coefficient ( K ) determined from four separate runs on mud from the FOAM site was 1.7 ± 0.2; for sediment from the NWC site it was 1.9. Diffusion measurements provide independent experimental confirmation of this value for K. These values of K are much lower than those in oxic sediments because the iron oxyhydroxides which are important in the adsorption of phosphate in oxic sediments are converted to iron sulfides in anoxic muds.