Rate, leach

Diffusion and Mass Transfer During Leaching. Rates of extraction from individual particles are difficult to assess because it is impossible to define the shapes of the pores or channels through which mass transfer (qv) has to take place. However, the nature of the diffusional process in a porous soHd could be illustrated by considering the diffusion of solute through a pore. This is described mathematically by the diffusion equation, the solutions of which indicate that the concentration in the pore would be expected to decrease according to an exponential decay function. 

Sulfur Polymer Cement. SPC has been proven effective in reducing leach rates of reactive heavy metals to the extent that some wastes can be managed solely as low level waste (LLW). When SPC is combined with mercury and lead oxides (both toxic metals), it interacts chemically to form mercury sulfide, HgS, and lead sulfide, PbS, both of which are insoluble in water. A dried sulfur residue from petroleum refining that contained 600-ppm vanadium (a carcinogen) was chemically modified using dicyclopentadiene and oligomer of cyclopentadiene and used to make SC (58). This material was examined by the California Department of Health Services (Cal EPA) and the leachable level of vanadium had been reduced to 8.3 ppm, well below the soluble threshold limit concentration of 24 ppm (59). 

Usually, however, it is not feasible to establi a stage or overall efficiency or a leaching rate index (e.g., overall coefficient) without testing small-scale models of likely apparatus. In fact, the results of such tests may have to be scaled up empirically, without explicit evaluation of rate or quasi-equilibrium indices. 

It was found that the dissolution rate of the material depends both on its surface area and on its crystalline size, but the importance of the crystalline size seems to be greater. The empiric equation describing the above dependence of the leaching rate on these two parameters is as follows (130) ... 

The most economical route Is probably to use screening studies to determine the dominant fate processes and then study only those In detail. In this paper we review some simple screening techniques that can be used to quantify volatilization and leaching rates at the soil/air Interface. Volatilization and leaching rates are then compared with estimates of transformation rates to determine the compound s overall fate and Identify the process requiring further study If a more exact fate assessment Is required. 

The critical metal leaching rate depends on critical dissolved metal concentrations, which can be derived in different ways, for instance as a critical limit for soil solution (see above). 

The amount of biocides needed in an antifouling paint will be determined by actual tests performed in real life. A rule of thumb says that the minimum level of protection with copper is a leaching rate of 10 pg/(cm2 day) (WHOI, 1952). The amount of cuprous oxide can be up to 40 wt % of an antifouling paint and it is normally assisted by booster biocides (5 to 10 wt %) to provide full protection against both animals and algae. The amount of biocides will be based on practical performance and, at the same time, be restricted to given legislation. 

Leaching rate measurements Any method with a detection limit < 6 ppb Any method with a detection limit < 10 ppb... 

Chemical reaction and diffusion determine the leaching rate... 

Miller has shown that TBTO will prevent fouling attachment at leaching rates as low as 1.25 yg/cm2/day (18). It is thus reasonable to assume that fouling commences when the rate of release falls below 0.5 yg Sn/cm2/day. Based on this, the effective dif-fusivities are calculated, using Crank s rate equation. The calculated effective diffusivities are then substituted in the integral form of Crank s equation to estimate the amount of Sn lost. 

Thus, ultrasound can improve the leaching rate over the conventional method and the effects of ultrasound are summarised in Fig. 4.4b. 

N-management practices to attain environmental sustainability are expected to have both low nitrate leaching potential and low nitrate leaching rate. 

The nitrate leaching rate can be described by the N-concentration in the leaching water and the amount of leakage water. Table 4-7 shows research results indicating nitrate leaching rates with respect to a per hectare and a per output unit scale. 

Table 4-7 Nitrate leaching rates from organic farming relative to conventional...

Reference scale Compared with conventional farming the nitrate leaching rates in organic farming systems are lower similar higher Author... 

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