Dissolution of silica and the development of concentration profiles in freshwater sediments

The dissolution of silica and diffusion of reactive dissolved Si in the por ewaters of river sediments are investigated using sediments of different ph ysical and chemical properties. Three sediments are considered: (a) from se ctioned cores taken from a river-bed, (b) fine organic-rich surface sedimen t(<5 cm depth) installed in a fluvarium channel and, (c) coarse river sedim ent of low organic matter content also installed in a fluvarium channel. Di ssolution rates of silica are measured at 10 degrees C using batches of sus pended material. The derived dissolution rate constants show large differen ces between the sediments. The river bed-sediment cores had vertical concen tration profiles of dissolved Si that are consistent with the diffusion and dissolution of biogenic silica. Experiments in a fluvarium channel enabled Si fluxes to be calculated from a mass-balance of the overlying solution. The results are consistent with the attainment of a steady-state concentrat ion profile of dissolved Si in the sediment. There are no discernible effec ts of water velocity over the sediment between 5 and 11 cm s(-1). However, at 20 cm s(-1), the flux increases as a result of either entrainment of fin e particles at the surface or advective effects in the surface sediment. A fluvarium experiment with the fine sediment (<125 mu m) over 61 days, produ ced a concentration profile with the highest concentration of 1025 mu mol d m(-3) at a depth of 4-5 cm in the sediment. A FORTRAN program is used to mo del the results of the increase in dissolved Si in the overlying water and development of a concentration profile in the porewater. This leads to a se diment diffusion coefficient of 1.21 x 10(-9) m(2) s(-1) at 8.8 degrees C a t the beginning of the experiment and rate constant k =13.1 x 10(-7) s(-1) at pH = 7.82 and average temperature of 7.6 degrees C for the entire experi ment. Fluxes measured at the sediment-surface interface and calculated assu ming steady-state profiles had developed are typically 0.01-0.04 mu mol m(- 2) (of river bed) s(-1). The approach enables the efflux of dissolved Si fr om bottom-sediments to be estimated from dissolution rates measured using s uspensions of bed-sediment. (C) 2000 Elsevier Science Ltd. All rights reser ved.