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

We have used cross flow filtration (CFF) to determine the pools of fine par ticulate (0.1-0.45 mum), colloidal (1,000 Dalton to 0.1 mum), and dissolved (<1,000 Dalton) metals in seven rivers that are all relatively rich in org anic matter but have differing pH, alkalinity, and ionic strength. The meta l content of <0.45-mum filtered river water primarily reflects a mixing of two metal pools with differing elemental compositions: a truly dissolved po ol and a colloidal pool. Fine particulate metals contribute <10% of the tot al metal load of <0.45-mum filtered water. Colloidal metals generally compr ise at least the following percentages of the metal load of <0.45-<mu>m fil tered water: >50% of Fe, Al, and trace metals; 30% Mn, 25% Ca and Mg; 15% N a and K; and a few percent Si. We believe that these figures are minima bec ause recovery experiments with the CFF apparatus suggest that whilst recove ries of pure metal salt solutions are close to 100%, only 70% of particle-r eactive metals are recovered from metal salt solutions spiked with humic ac id. We propose that the metal composition of riverine colloids is primarily controlled by element mobility during weathering. In organic-rich soils wi th low acid-neutralizing capacity, low pH solutions drive the dissolution o f aluminosilicates and oxides. Uptake of Al, Fe, and trace metals onto coll oidal organic matter helps to maintain low activities of dissolved metals a nd 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-<mu>m filtered water. In carbonate-rich soils, rapid neutralization of rainwater by carbonate minerals restricts the mobil ization of aluminosilicates and oxides. Compared to colloids from rivers dr aining silicate terrains, colloids in carbonate-rich systems have higher to tal loadings of metals, are enriched in alkali and alkaline earth metals, b ut are depleted in Al, Fe, and trace metals.