The effect of surface coatings on the association of orthophosphate with natural colloids.

A new method has been utilised for the characterisation of natural particle surface coatings. The method involves the use of sedimentation field-flow fractionation (SdFFF), radiolabelling and inductively coupled plasma-high resolution mass spectrometry (ICP-HR MS) techniques to study the effect of colloidal surface coatings on the adsorptive behaviour of orthophosphate. Colloidal river sediment and soil samples were chemically treated in an attempt to selectively remove metal hydroxyoxides and natural organic matter. The samples were then radiolabelled with 33PO4(3-) and analysed by SdFFF to determine the surface adsorption density (SAD) of orthophosphate as a function of particle size. The SdFFF unit was directly coupled to an ICP-HR MS to determine the chemical composition of the colloidal samples as a function of particle size. Element concentration/UV detector signal and element atomic molar ratios were plotted against particle size, and the trends used in the interpretation of SAD distribution (SADD) changes for the samples were studied. In general, non-constant trends in the orthophosphate SADDs were found, except for the river sediment treated with hydroxylamine hydrochloride. The results indicated that, in the soil sample studied, the Mn oxide coating was a dominant factor in determining phosphorus adsorption. This method could also be applicable to other industrial or similar samples.