Role of colloids and fine particles in the transport of metals in rivers draining carbonate and silicate terrains

Abstract

We have used cross flow filtration (CFF) to determine the pools of fine particulate (0.1-0.45 m), colloidal (1,000 Dalton to 0.1 m), and dissolved (Ͻ1,000 Dalton) metals in seven rivers that are all relatively rich in organic matter but have differing pH, alkalinity, and ionic strength. The metal content of Ͻ0.45-m filtered river water primarily reflects a mixing of two metal pools with differing elemental compositions: a truly dissolved pool and a colloidal pool. Fine particulate metals contribute Ͻ10% of the total metal load of Ͻ0.45-m filtered water. Colloidal metals generally comprise at least the following percentages of the metal load of Ͻ0.45-m filtered water: Ͼ50% of Fe, Al, and trace metals; 30% Mn, 25% Ca and Mg; 15% Na and K; and a few percent Si. We believe that these figures are minima because recovery experiments with the CFF apparatus suggest that whilst recoveries of pure metal salt solutions are close to 100%, only 70% of particle-reactive metals are recovered from metal salt solutions spiked with humic acid. We propose that the metal composition of riverine colloids is primarily controlled by element mobility during weathering. In organic-rich soils with low acid-neutralizing capacity, low pH solutions drive the dissolution of aluminosilicates and oxides. Uptake of Al, Fe, and trace metals onto colloidal organic matter helps to maintain low activities of dissolved metals and enhances mineral dissolution. Colloids derived from these soils are thus enriched in Al, Fe, Mn, and trace metals, resulting in high concentrations of these elements in Ͻ0.45-m filtered water. In carbonate-rich soils, rapid neutralization of rainwater by carbonate minerals restricts the mobilization of aluminosilicates and oxides. Compared to colloids from rivers draining silicate terrains, colloids in carbonate-rich systems have higher total loadings of metals, are enriched in alkali and alkaline earth metals, but are depleted in Al, Fe, and trace metals.Quantifying weathering processes and the subsequent transport of elements to the ocean is a fundamental aim of geochemistry. Although the abundance of dissolved elements has been commonly and pragmatically quantified by filtering water at 0.45 m, it is well known that sub-0.1-m colloidal phases, such as organic matter and oxides, interact strongly with dissolved metals (for reviews see Buffle 1988;Stumm 1992Stumm , 1993Gustafsson and Gschwend 1997). Several previous studies suggest that colloids can be important metal carriers in freshwater (e.g., Giesy and Briese 1977;Reuter and Perdue 1977;Hoffmann et al. 1981;Salbu et al. 1985;Tanizaki et al. 1992;Dai and Martin 1995;Ross and Sherrell 1999), and it is likely that the loss of metals from estuarine water is related to the coagulation of colloids (Sholkovitz 1976;Boyle et al. 1977).This paper reports the results of a study aimed at determining the importance of colloids as metal carriers in a series of rivers of widely varying pH and ionic strength. Cross flow filtration (CFF) is developed as a method of fraction-1...

Heavy metal contamination in the Arieş river catchment, western Romania: Implications for development of the Roşia Montană gold deposit

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