Comparison of Rectangular and Cylindrical-Flux Reactors in Electrochemical Remediation of Mercury Polluted Soil

Mercury removal from soil is one of the most important approaches in the environmental field. In this way, electroremediation have shown a good alternative to remove it from soil. Electrochemical remediation 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.

Two configuration reactors laboratory scale were tested in order to analyze the effect of current, flux, soil type, pH and conductivity. Both reactors include two titanium electrodes, separated 7 cm, and placed face to face in a 1D configuration. First reactor is a rectangular cell, and second reactor is a cylindrical-flux cell. In both cases same amount of soil was placed. Different potential values (5, 10, 15, 20 and 25 V) were tested in order to obtain the better conditions of electroremediation. All the experiments were done at room temperature and EDTA [0.1 M] was used as a complexing agent to improve electrochemical treatment.

Results indicated that in the rectangular reactor without flux, mercury moves toward the anode (+), by the presence of EDTA that forms a strong complex in a negative charge that promotes the movement of the complex ([EDTA-Hg]^2-) to the electrode by the current application; pH and conductivity vary slightly in this experiments. The best condition is to apply a potential of 20 V, for a period of 1 h. At the end of the treatment passivation was observed in the electrodes at 20 and 25 V applied.

In the case of cylindrical reactor, flux of 2.5 and 6.5 mL·min were tested, this flux was imposed in a cathode-anode direction because of previous results. In this way, for the case of soil 1, a potential of 1 V shows same effectiveness as 25 V; but in the case of soil 2, 20 V are needed to remove most mercury amount. It depends on soil composition as organic matter content. Lower flux improves the current distribution. In this analysis pH and conductivity does not vary as in the case of rectangular cell.

Cylindrical-flux configuration improves the distribution of current and the efficiency of mercury removal. In this case no damage was observed in the electrodes, and pH and conductivity follows as initial values.