Concept and determination of exchangeable phosphate in aquatic sediments

Exchangeable phosphate represents a reservoir of bioavailable phosphorus, since it can be rapidly released into a water body when the soluble phosphate concentration decreases. In the absence of a clear definition we first propose to precisely define exchangeable phosphate with reference to phosphate released in extreme conditions of solid dilution. A survey of the literature indicates that a variety of methods have been developed to provide its determination. The theoretical approach behind the corresponding release experiments is presented to support an evaluation of these methods with respect to the definition given. It appears that most are not based on the rigorous application of thermodynamic principles. Therefore, we have presented an infinite dilution extrapolation (IDE) approach, both rigorous and simple, enabling reliable comparison to be made. The method is based on extraction in natural water or a soluble substitute. The effect of pH was studied. Experimental conditions for use of the described method have been developed and various side applications are shown such as comparison of the extracting power of extractants. Results of application to estuarine sediments are briefly presented.