One of the larger commercial electrokinetic remediations was carried out in a Dutch Royal Air Force base in 1992 with 2600 m3 of soil contaminated with cadmium and other heavy metals [70]. This operated a hybrid vertical-horizontal system consisting of large tubular cathodes and short vertical anodes (Figure 18.5e). The tubular anodes were separated at 1.5-m intervals between the cathodes. The electrokinetics reduced the cadmium from 7300 to 47 ppm in 580 days.

These one-dimensional arrangements usually have the advantage of being the simplest and most cost-effective configuration. The frequent existence of areas of inactivity of electric fields or dead zones between electrodes of the same polarity are disadvantages. For this reason, the efficiency of the operations carried out with one-dimensional systems depends especially on spacing between electrodes of the same polarity (anode-anode or cathode-cathode) determining the number of electrodes required. If spacing between same-polarity electrodes is decreased, dead area inside the interest dominiom is minimized, improving the efficiency. Naturally, this also results in an increase of the cost of the equipment.

If two-dimensional systems are employed, the performance is very similar, although when two-dimensional configurations are employed, usually the principal objective is the generation of a radial or axial-symmetrical flow from the peripheral electrodes to the central electrode. If the selected contaminants to be removed are positive ions of heavy metals, the peripheral electrodes will be anodes and the center will be occupied by the cathode favoring the apparition of high concentrations of cations around the central zone and their easier and faster removal. If anions are to be removed, the polarity of electrodes reverses this position.

The peripheral electrodes can be ordered according to different geometrical distributions: hexagonal, square, and triangular, or, ideally, in any way close to a circle (Figure 18.6). The number

FIGURE 18.6 Two-dimensional configurations for electrokinetic remediation at field scale.

FIGURE 18.6 Two-dimensional configurations for electrokinetic remediation at field scale.

of outer electrodes corresponding to an inner electrode depends not only on the geometrical configuration but also on additional technical decisions.

Thus, the central electrode is surrounded by six peripheral electrodes in the hexagonal distribution (Figure 18.6a) and in the triangular by three (Figure 18.6b). However, if a square distribution is used, the center can be surrounded by four (Figure 18.6c), eight (Figure 18.6d), or even more electrodes with opposite charge to the central electrode. Also, these two-dimensional configurations generate the corresponding dead zones, without electrical activity. Obviously, the extension and situation of these areas strongly depend on the selected geometrical configuration and the number of electrodes in the system; however, in general terms, these dead zones in two-dimensional systems are smaller than in one-dimensional arrangements.

Electrical field spatial distribution indicates [71] that dead area in all the cells has a shape of a curvilinear triangle whose base is the distance between same-polarity electrodes (Figure 18.7). The height of this triangular area depends on processing time, electrode spacing, and alignment. The height of this triangle is expected to be larger in the case of one-dimensional compared to two-dimensional configurations due to the electrode alignment. An approximate practical method for comparing the efficiency between configurations assumed that this height is half the length of the triangle base for one-dimensional and a quarter of the length of the base for two-dimensional applications [71]. Figure 18.7 shows approximate distributions of the resulting inactive spots for selected configurations.

Another important difference between one- and two-dimensional configurations is that the current density is constant between the different polarity electrodes in the former and increases from external to center electrodes in the latter. Also, as another geometrical consequence, the soil volume extension swept for the desired acid or basic front proceeding from the outer electrodes is favored with respect to this in the one-dimensional installations.

In both types of configurations, the highest is the total number of electrodes in the installation, the highest is the cost, the more cost of immobilized capital and less time to do the cleanup. This produces two partial costs going in opposite directions and, therefore, an optimization of them is recommended to reach a minimum total final cost.

18.8.3 Number of Electrodes

A comparison of the number of electrodes required per unit surface area for one-dimensional, two-dimensional, hexagonal, and square configurations has been published [71]. In the one-dimensional configuration, three cases were provided in which the spacing among same-polarity electrodes equals: (1) spacing among opposite-polarity electrodes; (2) one-half of opposite-polarity spacing; and (3) only one-third of opposite-polarity spacing. Then, the number of electrodes is calculated based on unit of surface area. Considering a unit cell, the number of electrodes per unit area is

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