The importance of surface area in metal sorption by oxides and organic matter in a heterogeneous natural sediment

This study provides empirical validation of current trace metal sorption theory in a small urban river. We demonstrate that trace metal complexation reactions occur predominantly at the suspended particulate surface involving surface layers of Fe oxides and organic matter. Associated surface areas of these geochemical fractions were calculated where possible, using the total surface area (TSA) of the suspended particulate matter pool (SPM) in conjunction with estimates of suspended iculate Fe and Mn oxides (SPOX) and organic matter (SPOM) concentrations. Iron and Mn oxides concentrations were estimated using an extraction scheme. For two samples where no SPOM or Mn oxides were present, estimates of Fe oxides associated surface area were determined which compared favourably to literature estimates, providing further evidence for acceptance selectivity of extraction schemes. The utility of literature estimates of surface areas for single component sediments in heterogeneous sediments was also assessed. In mixed sediment samples, exposed surface areas of discrete phases are probably reduced due to mixed layering effects of the coatings, and the use of constants to estimate the surface areas of individual fractions does not work, since the relationship between the concentration of a given sedimentary fraction and its exposed surface area is no longer predictable.