CHEMISTRY OF RIVERINE AND ESTUARINE SUSPENDED PARTICLES FROM THE OUSE-TRENT SYSTEM, UK

Three samples of suspended particulate matter (SPM), two riverine and one estuarine, were characterised by bulk chemical analysis, electron microscopy, chemical extraction, electrophoresis and acid-base titrati on. The river samples contained both mineral and biological material a nd were rich in organic matter (R. Ouse sample 24%, R. Trent sample 54 %), while the estuarine sample consisted mostly of clay minerals, with only 9% of organic matter. The river particles had higher contents of base-extractable carbon and phosphate and acid-extractable trace meta ls than the estuarine sample. The electrophoretic mobilities of all th ree samples were negative, but acid-base titrations revealed differenc es in their proton-dissociating characteristics, the R. Ouse sample ha ving approximate to 2 meg g(-1) of pH-dependent charge titrating betwe en pH 4 and 9, approximately twice the content of the R. Trent sample, and four times that of the estuarine sample. However, the estuarine s ample had a relatively large fixed (negative) charge (approximate to-0 .7 meq g(-1)). Centrifugation-depletion experiments showed that for al l three samples, phosphate adsorbed most strongly at pH 6-7, the parti cles releasing phosphate to solution at both low and high pH values. T race metal adsorption experiments, carried out by centrifugation-deple tion and, in the case of Cu, with an ion-specific electrode, showed th at adsorption increases with pH, except in some cases at pH > 7, where solution complexation reverses the trend. A simple metal-proton excha nge model provided an approximate description of the observations, and allowed comparison amongst metals and particle samples. The three sam ples adsorbed trace amounts of metals with the same selectivity: Cs < Sr < Co similar or equal to Ni similar or equal to Cd < Zn < Cu < Pb < Eu. The strengths of adsorption of a given metal by the three samples varied over an order of magnitude, increasing in the order Ouse estua ry < R. Trent less than or equal to R. Ouse. These results, and other published data, show that natural SPM varies appreciably in its (surfa ce) chemical properties. The possibilities of accounting for the varia bility in terms of the properties of constituent particles are discuss ed.