Leachates diffusion processes

Leakage through a synthetic liner is controlled by Fick s first law, which applies to the process of liquid diffusion through the liner membrane. The diffusion process is similar to flow governed by Darcy s law except that it is driven by concentration gradients and not by hydraulic head. Diffusion rates in membranes are very low in comparison with hydraulic flow rates even in clays. In synthetic liners, therefore, the factor that most influences liner performance is penetrations. Synthetic liners may have imperfect seams or pinholes, which can greatly increase the amount of leachate that leaks out of the landfill. 

Chemicals can pass through soil liners by molecular diffusion as well as by advective transport. One can study the molecular diffusion of chemicals in the soil by compacting soil at the bottom of an impermeable beaker and ponding waste liquid or leachate on top of the soil. At the start of the experiment, the concentration c is equal to c0 in the waste liquid. The soil is clean. Even though no water flows into the soil by advection, chemicals move into the soil by the process of molecular diffusion. Eventually, the concentration of the waste liquid and the soil will be one and the same (see Figure 26.12). 

If the immersion medium is strongly alkaline, the anions will remain in solution, but if it is weakly alkaline or neutral, dissociation of a polysulfide will occur as it departs the concrete, and sulfur will precipitate either on the concrete surface or in the surrounding liquid. In either case the concrete should be leached of sulfur, but the process is much slower with neutral media, possibly because diffusion of leachate and polysulfide is slowed as sulfur precipitates near the surface. 

A perfectly installed, faultless eomposite liner is extremely impermeable to the large number of oiganie and inoiganie substances in the leachate. However, the permeability calculated by Eq. 7.35 only applies to the described simple boundary conditions. In the field, however, more complex conditions emerge which promote diffusive migration on the one hand but also obstmct it on the other the partition coefficient of a substance in the multiple-substance leachate mixture only approximately corresponds to that in the pure aqueous solution. In particular, the partition coefficients can be different at the interfaces above and beneath the geomembrane. Depositions develop on the geomembrane which affect the sorption processes. The pore space of the mineral liner is only partially saturated. The... 

Using different percolation rates and different percolation times the behavionr of molybdate in the soil system as a result of the changing composition of the soil pore water by ash leachate can be quantified. The retention capacity of the soil imder these conditions can be calculated. By measuring the behaviour of Na-22 tmder the same circumstances the dispersion of the system can be characterized. Finally it allows for kinetic processes to be modelled. The intention to store waste materials above groundwater levels and the measures taken to prevent percolate production emphasize the need for information at extremely low percolation rates, where diffusion becomes more important. 

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