! COLLOIDS A Colloids and Surfaces SURFACES ELSEV|ER A: Physicochemical and Engineering Aspects 120119971183 198 Chemistry of riverine and estuarine suspended particles from the Ouse-Trent system, UK J.R. Ferreira ~, A.J. Lawlor, J.M. Bates, K.J. Clarke, E. Tipping * Institute o[ Freshwater Ecology, Ambleside. Cumhria. 1,A22 OLP. UK Received 2 October 1995: accepted 24 April 1996 Abstract Three samples of suspended particulate matter (SPMt, two riverine and one estuarine, were characterised by bulk chemical analysis, electron microscopy, chemical extraction, electrophoresis and acid base titration. The river samples contained both mineral and biological material and were rich in organic matter (R. Ouse sample 24%, R. Trent sample 54%1, 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 metals than the estuarine sample. The electrophoretic mobilities of all three samples were negative, but acid base titrations revealed differences in their proton-dissociating characteristics, the R. Ouse sample having ~ 2 meq g ~ of pH-dependent charge titrating between pH 4 and 9, approximately twice the content of the R. Trent sample, and four times that of the estuarme sample. However, the estuarine sample had a relatively large fixed (negative) charge ( ~ -0.7 meq g t l. Centrifugation depletion experiments showed that for all three samples, phosphate adsorbed most strongly at pH 6 7, the particles releasing phosphate to solution at both low and high pH values. Trace metal adsorption experiments, carried ou! by centrifugation-depletion and, in the case of Cu, with an ion-specitic electrode, showed that adsorption increases v~ith pH, except in some cases at pH > 7, where solution complexation reverses the trend. A simple metal proton exchange model provided an approximate description of the observations, and allowed comparison amongst metals and particle samples. The three samples adsorbed trace amounts of metals with the same selectivity: Cs < Sr < Co-Ni-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 estuary < R. Trent ~<R. Ouse. These results, and other published data, show that natural SPM varies appreciably in its (surface) chemical properties. The possibilities of accounting for the variability in terms of the properties of constituent particles are discussed. Keywords: Adsorption; Metals; Phosphate; Surface chemistry; Suspended particles 1. Introduction Suspended particulate matter (SPM) in rivers and estuaries is considered to exert a major influ- ence on the behaviour and bioavailability of solutes, especially metals and phosphate [1-4]. * Corresponding author. 1 O11 leave from: lnstituto de Pesca and CENA/USP, Sao Paulo, Brazil. The interactions depend on the types and amounts of particle present, pH, concentrations of the solutes in question, chemical interactions in the solution phase, and ionic strength. Research per- formed during the last two decades has provided considerable insight into the processes and mecha- nisms of environmental surface chemistry (see e.g. Ref. [5]), allowing the qualitative or semi- quantitative interpretation of field observations. Perhaps the biggest obstacle to more quantitative 0927-7757:97/$17.00 Copyright ~" 1997 Elsevier Science B.V. All rights reserved PII S0927-7757( 96)03721- 1