Use of hydrous iron(III) oxide in a concentration step for the determination of trace amounts of organophosphorus compounds in aqueous solutions
Adsorption onto hydrous iron oxide (HIO) was compared as a function of pH for a variety of organophosphorus compounds (OPs), including phosphate esters of ethanolamine, hydroxyamino acids and sugars, phosphonates with methyl and aminoethyl substituents, and nucleotides. The percentage adsorption versus pH curves could be classified into four types according to an empirical rule, viz., that the adsorptivity of OPs depended primarily on the number of unsubstituted P–O moieties in the tetrahedral structure around the P atom of the compound. The rule predicted that a large group of OPs containing more than three unsubstituted P–O moieties should be collected quantitatively by HIO from waters of pH 5.0–6.5. The OPs collected by adsorption onto HIO did not show appreciable degradation during storage for at least 2 weeks. In addition, they could be released from the HIO by using pentane-2,4-dione [acetylacetone (Hacac)] so that they entered a small-volume awueous phase which was derived from the HIO by the following reaction: Fe2O3·nH2O·(OPs)+ 6Hacac → 2Fe(acac)3+(n+ 3)H2O +(OPs). The whole procedure, involving adsorption of OPs onto HIO from a 1 l water sample, separation of the HIO from water by filtration and release of the OPs from the HIO into a 2.5 ml aqueous phase, realized a 400-fold concentration with efficiencies ranging from 45%(for adenosine-5′-triphosphate) to 92%(for 2-aminoethylphosphonate).