Modified speciation scheme for inorganic phosphate in lake sediments

Abstract

Speciation schemes for phosphate in soils or sediments tend to yield results which are ‘conditional’ and subject to error. In the proposed modified scheme, secondary phosphate-cation interactions have been minimised by including a cation exchange resin in the extraction system; re-sorption errors have been reduced by using low (1:400) solid to liquid extractant ratios; and slow dissolution rates have been balanced by using longer extraction times (up to 62 h). Variables examined during scheme development included the effect of varying sample weights, extraction time, reagent type, exchanger type, exchanger to sample weight ratios and pH on the amount of phosphate released from a polluted lake sediment. It was found that the amount of inorganic phosphate released by chemical reagents (or by cation exchanger) varied markedly with system pH, being greatest in the pH 2 to 3 region, decreasing to near zero in the pH 4 to 6 zone, and then rising to an intermediate value at pH > 8. Monitoring of the Ca, Mg, Fe and Al released into the various solutions (or transferred into the exchanger phase) indicated that one cannot clearly distinguish between phosphate initially present in association with alkaline earth compounds or iron-rich matrix components. The advantages and limitations of this ‘differential solubility’ scheme have been considered and alternative approaches briefly reviewed.