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Chemical Osmosis in Compacted Dredging Sludge

Dep. of Geochemistry, Faculty of Earth Sciences, Utrecht Univ., P.O. Box 80021, NL–3508 TA Utrecht, the Netherlands

View larger version (19K): [in a new window]   Fig. 1. Locations of the dredging sludge depot De Slufter near the Port of Rotterdam, the Netherlands, and the locations of the sampling sites in the Beerkanaal (BK) and Beneden Merwede river (BMR).

View larger version (15K): [in a new window]   Fig. 2. The reflection coefficient, , according to the Fritz-Marine Membrane Model (FMMM); top as a function of the porosity at an average concentration of 0.1 mol L-1, and bottom as a function of the average concentration at a given porosity of 0.5. Curves were calculated using Eq. [4] for monomineral membranes made of kaolinite, illite or montmorillonite with values for the cation-exchange capacity (CEC) of respectively 15, 25, and 100 cmolc kg-1. For the frictional coefficient, Rwm, an initial value of 0.1 was taken which decreased linearly in the porosity range between 0 and 0.4. Above a porosity of 0.4 the value of Rwm was taken to be zero. Values of the other frictional coefficients are given in the text.

View larger version (23K): [in a new window]   Fig. 3. Schematic representation of the distribution of the ions in a normal (dotted lines) and truncated (solid lines) double layer. The position of the immobile and mobile water adjacent to the clay platelet for the truncated double layer according to the model by Bolt is also indicated.

View larger version (47K): [in a new window]   Fig. 4. Schematic drawing of the experimental instrument showing the cell in which the flexible wall permeameter is located and both water reservoirs. The close-up shows the location of the sample between the porous stones and the latex membrane within the cell.

View larger version (13K): [in a new window]   Fig. 5. Reflection coefficient, , as a function of the porosity for the three samples according to the FMMM. The curves were calculated using Eq. [4] with a value of s of 0.055 mol L-1. For the frictional coefficient, Rwm, an initial value of 0.1 was taken which decreased linearly in the porosity range between 0 and 0.4. Above a porosity of 0.4 the value of Rwm was taken to be zero.

View larger version (16K): [in a new window]   Fig. 6. The reflection coefficient, , of AWy as a function of the normalized mobile water layer, 0dl, according to Bolt. The curves were calculated using Eq. [8]. In the inset the curves for all the samples are shown between 5 0dl 10. As the value of 0 for the samples are of the same order of magnitude—0.02 for AWy, 0.04 for BK and 0.06 for BMR—the curves plot close to each other.

View larger version (10K): [in a new window]   Fig. 7. The development of a differential hydraulic pressure, P = Psalt – Pfresh, with time across the AWy bentonite as a result of chemical osmosis. The line is the best exponential fit from which the maximum value for P was obtained and used to calculate the experimental reflection coefficient, of the sample.

View larger version (14K): [in a new window]   Fig. 8. The increase in conductivity during the first 72 h as the result of diffusion of salt across the samples.

View larger version (11K): [in a new window]   Fig. 9. The development of a differential pressure, P = Psalt – Pfresh, with time across the BK dredging sludge as a result of chemical osmosis. The line is the best exponential fit from which the maximum value for P was obtained and used to calculate the experimental reflection coefficient, , of the sample.

View larger version (12K): [in a new window]   Fig. 10. The transport of water during the first 12 h for the Beerkanaal (BK) dredging sludge showing the flow of water from the fresh to the salt water reservoir.

View larger version (9K): [in a new window]   Fig. 11. Deterioration of the reflectance coefficient, for the Beerkanaal (BK) dredging sludge during the experiment. The values for were calculated using the water flux from the fresh to the salt water reservoir.