Estuarine chemical reactivity at the particle-water interface
A systematic study of the microstructures of particulate material from the Tamar Estuary using a BET nitrogen adsorption technique has been carried out. The results showed that suspended material had a higher BET surface area (approximately 20m²/g ) than the associated sediments (approximately 13m²/g) . Also the BET surface areas (8-20m²/g) of suspended material collected during axial transects (S = 0-30°/oo) of the Tamar Estuary indicated relatively higher BET surface areas in the turbidity maximum zone as compared to material from up or down estuary. The BET surface areas were inversely related to the carbon content of the particles and although the role of Fe and Mn coatings was examined no definitive relationship to BET surface area was evident. Analysis of nitrogen adsorption-desorption hysteresis loops indicated the pores to be of the parallel plate or slit type in the size range <2-50 nm. The shape and dimensions of these pores would accomodate the penetration of metal ions, like Zn and Cu, into the pore spaces in the particle matrix. A method was designed to enable the analysis of natural Zn and Cu concentrations in small volumes extracted from a reactor on a timescale suitable for kinetic analysis. Dissolved Cu and Zn adsorption-desorption experiments were carried out under controlled conditions using Tamar suspended solids as the adsorbents. The uptake and release profiles were interpreted in terms of a two stage reaction mechanism which involved both surface adsorption and solid state diffusion into the pores. Rate constants were derived from a kinetic analysis to enable evaluation of the chemical timescales of the sorption reactions. When compared to field data of Zn distributions in the Tamar Estuary the time constants allowed a reasoned explanation of the observations. They also pointed to a strong coupling between the physical and chemical timescales within estuaries. This work has indicated an association between trace metal sorption reaction rates and estuarine particle microstructure. These rate constants are of value in the refinement of hydrodynamic models and this study has implications for the availability of trace metals for biological or chemical remobilisation.