Adsorption-desorption kinetics of phosphate between sediment and water in coastal areas

Abstract

In this article the kinetics of adsorption-desorption processes of phosphate on sediments is studied according to the Langmuir theory. The theoretical developments are described and applied to the laboratory experiments which rely on desorption of adsorbed phosphate from coastal and estuarine sediments.Desorption (kd) and adsorption (ka and kaQ°) rate constants were determined using sediments from various areas with a wide range of grain size characteristics. Accurate determination of kd is of primary importance since desorption may directly enhance algal growth. At a temperature of 20 °C, values of kd range from 0.057 h−1 to 0.128 h−1. The results show that the use of ka, deduced from the Langmuir coefficient b (b = ka/kd), is not recommended since this coefficient is affected by poor precision. Alternatively, adsorption rates can use the more reliable combined constant kaQ°. Values of kaQo range from 0.013 to 0.054 L g−1 h−1 (20 °C). The sediment grain size has only moderate influence on the rate constants. The relationship of these rate constants with temperature was studied in the range 1–30 °C, encountered in most marine areas. The exponential law with temperature is determined and an increase factor of 1.6 is found for each 10 °C increase. This work has strong implications for environmental modelling to encompass the sediment phase as a contributor to the cycling of essential nutrients in estuarine and coastal systems.