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 µm), colloidal (1,000 Dalton to 0.1 µm), and dissolved (≪1,000 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 ≪0.45‐µm 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 ≪10% of the total metal load of ≪0.45‐µm filtered water. Colloidal metals generally comprise at least the following percentages of the metal load of ≪0.45‐µm filtered water: ≫50% 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 ≪0.45‐µm 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.