SORPTION OF PHOSPHATE BY SEDIMENTS AS A RESULT OF ENHANCED EXTERNAL LOADING

In artificial test ditches, originally poor in nutrients, the effects of enhanced external loading with phosphorus were studied. An importan t term in the mass balance of phosphorus is retention by sediment. Par ameters concerning the uptake of phosphorus by the sandy sediment of a ditch have been measured or were obtained from curve-fitting and were used in a mathematical model to describe diffusion into the sediment and subsequent sorption by soil particles. On a time scale of hours up take of phosphorus from the overlying water by intact sediment cores c ould be simulated well with a simple diffusion-adsorption model. Mixin g of the overlying water resulted in an enhanced uptake rate caused by an increased effective diffusion coefficient in the top layer of the sediment. Laboratory experiments revealed that after a fast initial ad sorption, a slow uptake process followed that continued for a period o f at least several months. This slow sorption can immobilize a substan tial part of the phosphorus added. It may physically be described as a n intraparticular diffusion process, in which the adsorbed phosphate p enetrates into metaloxides, probably present as sand grain coating, an d thereby reaches sorption sites not immediately accessible otherwise. The total sorption capacity of the soil particles is ca. 3.3 times th e maximum instantaneous surficial adsorption capacity.