In-situ technologies for groundwater treatment: the case of arsenic - In-Situ Remediation of Arsenic-Contaminated Sites - page 24

Figure 1.7. Diagram of EK based on electromigration (adapted from USEPA, 1997).

When the conductivity of the electrolyte reaches about 20 S m − 1 , the electrolytes are pumped
to a treatment plant. Depending on the type of contaminants, they can be treated by:

• NaOH to precipitate metal hydroxides, which are then removed by a press filter. The amount

of cake produced depends not only on the concentration of heavy metals but also on the
concentrations of iron, alkaline and alkaline earth metal ions, carbonates, hydrogencarbonates,
magnesium, etc. As a rule, between 0.05 and 0.1% of the volume of the treated soil is collected
as a residue;

• Ion exchange;

• Another adsorbent (in the case of As, activated alumina, iron oxides and hydroxides, etc.).

In alkaline medium, As species are poorly adsorbed, although As(V) is more easily adsorbed
than As(III). The alkaline conditions favor the electromigration of As, although this is very slow
(Virkutyte et al ., 2002). To improve the process of electromigration, sodium hypochlorite is added
to the cathode compartment. In experiments made by Hécho et al . (1998), hypochlorite was added
directly to the cathode compartment and As began to be collected in the anolyte after 15 days.
After the remediation, the rate increased rapidly: after 20 days, 60% of As had been eliminated.
However, following this, the elimination rate decreased again. Complete removal took 41 days,
when the analysis showed that the residual concentration of As in the soil was less than 1 ppm.
Another alternative is the addition of complexing agents such as EDTA, which compete
favorably with the soil for metal capture. Usually, a combination of three types of reagents
is added: chelating agents, acidic solutions and cationic surfactants. The results presented by