Influence of EDTA on the Electrochemical Removal of Mercury (II) in Soil

The electrokinetic treatment (EKT) is based on the generation of an electric field from the imposition of a direct current or potential using electrodes (anode and cathode), which are placed in pit excavated soil. For the EKT is required moistened with an electrolyte to improve the driving electric field. The EKT has shown ability to remove organic compounds, but more efficiently for metals such as Cr, Cd, Hg, Pb, Mn, Zn, and As with removal efficiencies close to 100 % when pollution is caused by a single metal. In field applications, the results are variable, depending on the type of soil, the type of pollutant and technological use. Studies have been conducted on the recovery of metals with sequestering agents, EDTA being the most efficient by the synthesis of strong complexes with metals like mercury in major proportion close to the anode by the negative charge of EDTA^2-, which was moved by electric field to positive anode rounding the mercury ion: Hg²⁺.

Polarization curves were done in order to determine energy requirement, the highest removal of mercury in soil and clay was applying 1 V (close to 60 % removal). EKT was carried out at different time of treatment: 1, 3, 7, 9, 11 and 13 h, using 0.1 M HCl and 0.1 M EDTA quantifying mercury removal, pH, and conductivity. Results indicated that the highest mercury removal efficiency was obtained in bentonite with  66 % in a period of 13 h using HCl (cathode efficiency); while 70 % was removed in 9 h in the presence of EDTA (anode efficiency). On the other hand, the higher efficiency for soil in the presence of EDTA was 70.7% with pH 4 and 11.3 mS cm^-1 close to the anode by the Hg²⁺-EDTA^2- synthesized complex, and only 29.3 % was removed in the presence of HCl with constant pH 4 and a conductivity of 11 mS cm^-1 by the absent of some complex.