Investigating the properties of various minerals to determine their potential application as chromatographic resins for the speciation of phosphate in natural waters

Abstract

Since the 1960's, much research has focused upon the speciation of phosphate in natural water. To date however, no technique has been developed where-by the concentration of the major chemical classes can be determined; speciation is performed most commonly on the basis of size (gel permeation chromatography) or reactivity (molybdenum blue colorimetry, combined with acid digestion). Speciation is made all the more difficult because phosphate species exist in such low concentrations that preconcentration is a necessary precursory step.It is the authors opinion, that one of the most promising approaches to the problem of phosphate speciation, is the use of certain minerals, to concentrate and separate the various chemical classes chromatographically. When a regular anion exchange resin is employed, phosphate only competes well as orthophosphate (when it has a high negative charge). In the case of minerals however, the type of bonding relates more to the Lewis basicity of the anion compared to the mineral. Phosphate is known to be one of the strongest 'hard Lewis bases', with an acute affinity for minerals whose metals are 'hard Lewis acids'. If the correct mineral is chosen therefore, phosphate ought to compete well for sites on the mineral, even when it has a lower charge or is present in much lower concentrations than other anions. The first mineral which was investigated was hydroxyapatite; which has been used in biochemistry for separating nucleic acids. It is widely accepted that the hydroxyapatite immobilise the nucleic acid via the phosphodiester groups situated along the length of the polymer. This mineral contains calcium, which is a hard Lewis acid. The problem with hydroxyapatite, is that it also contains orthophosphate which would be introduced into the system if the mineral dissolved to any degree. Experiments were therefore performed to test the solubility of hydroxyapatite and it was found to be too soluble for use in phosphate speciation. ........