# Enhanced Electrokinetic Remediation for the Removal of Heavy Metals from Contaminated Soils
## 3 Results  
### Extraction of Metals from the Soil  
Figure 2 shows the metal extraction fraction when 2 g of soil was in contact for 24 h with 100 mL of 0.1 M solution of the corresponding extracting agent. In general, Cd, Co, Cu and Zn were easier dissolved than Pb and Cr, with both mineral acid and organic acids. The mineral acid solutions tended to show slightly better extraction ratios than organic acids. This is due to the lower pH solution of the mineral acids that extracted the metals from the surface of the soil particles by the direct attack of the H\({}^{+}\) ions, stabilizing the metal in solution at pH below 1. Conversely, the organic acids showed a solubilization ratio based on their capacity to form soluble complexes and chelates in solution [24]. A closer analysis of the extraction results in Figure 2 revealed that the metal solubilization ratio depended on the chemistry of each pair of metal-acid. Hydrochloric acid and nitric acid showed 70-80% extraction for all the metals, but Cr, because this metal was present in the soil as Cr(VI) and the acidification was not able to desorb the anionic Cr species. Phosphoric and sulfuric acid also showed a high extraction ratio for Cd, Co, Cu and Zn, but very low extraction for lead because of the formation of insoluble salts between Pb and sulfate (PbSO\({}_{4}\), Ksp = 1.6 \(\times\) 10\({}^{-8}\)) or phosphate (Pb\({}_{3}\)(PO\({}_{4}\))\({}_{2}\), Ksp = 9.9 \(\times\) 10\({}^{-55}\)). The organic acids showed a 60-70% extraction ratio for Cd, Co, Cu and Zn. The lead was extracted with EDTA, oxalic acid and tartaric acid, whereas acetic and citric acids showed negligible removal due to the tendency of lead to adsorb to the soil particles and organic matter [25]. Cr was extracted with oxalic acid due to the low acidity of the solution (pH = 1.40) and the reducing capacity of oxalic that transformed Cr(VI) into Cr(III) and formed stable complexes in solution [26]. Figure 1: Experimental setup. (1) Soil specimen, soil sections are represented as S1–S6, (2) filter paper and porous stone, (3) electrodes (anode and cathode), (4) electrode chambers, (5) processing fluid vessel (anolyte and catholyte), (6) gas release valves, (7) recirculating pumps, (8) DC power supply. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Test** & **Anolyte** & **Soil** & **Catholyte** & **Potential Drop** & **Time** \\ \hline Test 1 & Deionized water & 320 g & DI water & 20 V & 30 days \\ Test 2 & Deionized water & 320 g & 0.1 M citric acid & 20 V & 30 days \\ Test 3 & 0.1 M NaOH & 320 g & 0.1 M EDTA & 20 V & 30 days \\ Test 4 & 0.5 M Citric acid & 320 g & 0.5 M citric acid & 20 V & 65 days \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Experimental conditions of the electrokinetic tests. The information in Figure 2 can be used for the selection of facilitating agents for the electrokinetic remediation of the contaminated soil. Mineral acids were very effective in the mobilization of the metals, but their activity was always associated with a very acidic pH, below 1. In the remediation of soils, such low pH will affect the physicochemical and biological properties of soil to an unacceptable level, and further treatment would be necessary to restore the natural properties of soil. The organic acids seem to be more appropriate for soil remediation projects because they were able to extract the contaminated metals in less harsh conditions than mineral acids. Moreover, the organic acids tested in Figure 2 are biodegradable and can be removed from the soil for natural biotic degradation while enhancing the microbial activity of the soil [27]. Considering the results in Figure 2 and the available information in the literature, EDTA and citric acid were selected as appropriate facilitating agents for the electrokinetic remediation of soil [14,28]. EDTA is a complexing agent for all the contaminating metals in the soil specimen [24]. EDTA can be degraded by the soil microflora at the end of the electrokinetic treatment [29].