Determination of adsorption characteristics of the nutrient orthophosphate to natural colloids by sedimentation field-flow fractionation

The surface adsorption density distributions (SADD) for P-32-labelled orthophosphate onto a suspended river colloid and a soil colloid were determined by the sedimentation field-flow fractionation (SdFFF) method. The surface adsorption density (SAD) was not constant with particle diameter as expected for homogeneous solids and were quite different for the two samples. The shape of particles was determined by examining separated fractions using scanning electron microscopy (SEM). A procedure was developed for correcting the SAD, which is normally calculated assuming constant spherical shape, for changes in morphology from spheres to plates or rods having different aspect ratios. It was shown that shape can have a large effect on the SADD plot. Changes in chemical composition across the size distribution were measured using on-line SdFFF-inductively coupled plasma-mass spectrometry. Significant trends in the Fe/Al and Mg/Al atomic ratios as a function of size were observed. These element distributions can help explain the trends in the SADD plots which could occur due to a variation in the hydrous iron oxide surface coating thickness or by changes in the mineralogy of the sample. The methodology gives very detailed adsorption data which yields insights into the nature of the pollutant-colloid binding in waters and soils.