# Enhanced Electrokinetic Remediation for the Removal of Heavy Metals from Contaminated Soils
## 3 Results
### Electrokinetic Treatment
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Test—Catholyte** & **Fractions** & **Cd** & **Cr** & **Co** & **Cu** & **Pb** & **Zn** \\ \hline \multirow{4}{*}{Test 1—Deionized water} & Soil (\%) & 98.1\% & 97.4\% & 99.9\% & 97.4\% & 96.8\% & 98.5\% \\  & Anode (\%) & 0.0\% & 0.5\% & 0.1\% & 0.0\% & 0.0\% & 0.0\% \\  & Cathode (\%) & 0.0\% & 0.0\% & 0.0\% & 0.0\% & 0.0\% \\  & Error (\%) & 1.9\% & 2.1\% & 0.1\% & 2.6\% & 3.2\% & 1.5\% \\ \hline \multirow{4}{*}{Test 2—0.1 M citric acid} & Soil (\%) & 96.1\% & 95.4\% & 92.7\% & 93.5\% & 97.8\% & 86.7\% \\  & Anode (\%) & 0.1\% & 0.1\% & 0.1\% & 0.0\% & 0.0\% & 0.1\% \\  & Cathode (\%) & 0.2\% & 0.0\% & 4.5\% & 2.1\% & 0.0\% & 9.3\% \\  & Error (\%) & 3.7\% & 4.5\% & 2.7\% & 4.3\% & 2.1\% & 3.9\% \\ \hline \multirow{4}{*}{Test 3—0.1 M EDTA} & Soil (\%) & 99.7\% & 99.8\% & 98.3\% & 94.1\% & 99.4\% & 99.2\% \\  & Anode (\%) & 0.0\% & 0.0\% & 0.1\% & 0.0\% & 0.0\% & 0.1\% \\  & Cathode (\%) & 0.6\% & 0.1\% & 0.9\% & 0.2\% & 0.3\% & 0.9\% \\  & Error (\%) & \(-\)0.3\% & 0.1\% & 0.7\% & 5.6\% & 0.3\% & \(-\)0.1\% \\ \hline \multirow{4}{*}{Test 4—0.5 M citric acid} & Soil (\%) & 21.3\% & 88.2\% & 21.4\% & 27.5\% & 90.2\% & 26.7\% \\  & Anode (\%) & 1.2\% & 2.3\% & 0.3\% & 0.0\% & 0.0\% & 0.1\% \\ \cline{1-1}  & Cathode (\%) & 78.3\% & 6.3\% & 76.9\% & 71.4\% & 8.2\% & 73.9\% \\ \cline{1-1}  & Error (\%) & \(-\)0.8\% & 3.2\% & 1.4\% & 1.1\% & 1.6\% & \(-\)0.7\% \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Mass balance of the metals in the soil at the end of the electrokinetic tests. confirm that Pb can be dissolved with citric acid, whereas it was not mobilized in test 1 with deionized water. Finally, chromium was mobilized on the cathode side by the combined effect of the pH and the presence of citrate. Cr was accumulated in soil sections 2-4. As a conclusion, citrate appeared to be an active compound for the mobilization of all the metals, including Cr and Pb, that showed major resistance to the mobilization in the unenhanced electrokinetic test 1. However, there was minor net removal of metals from soil in the electrode solutions. Only 9% of Zn, 4.5% of Co and 2% of Cu were found in the cathode chamber (Table 4). These results suggested that the treatment time and test conditions (pH, concentration of citric acid, etc.) were not enough for the effective removal of metals from contaminated soil. Test 3 was run with EDTA 0.1 M as catholyte. The electrokinetic phenomena would transport EDTA into the soil specimen by electromigration. EDTA forms complexes and chelates with all the metals [24] in the soil specimen. The analysis of the Zn\({}^{2+}\) speciation with EDTA in an aqueous system is shown in Figure 7. EDTA formed three different complexes with EDTA: [ZnL]\({}^{2-}\), [ZnHL]\({}^{-}\), [ZnOHL]\({}^{3-}\). The relative abundance of each complex depends on the pH. EDTA is very effective in complexing Zn in the studied pH range. The relative abundance of the cationic species  
Figure 5: Electrokinetic test of soil contaminated with metals using 0.1 M citric acid as catholyte and deionized water as anolyte. C is the metal concentration in soil at the end of the test, and Co is the initial metal concentration in soil. Figure 6: Speciation of zinc and zinc complexes formation in an aqueous system with 0.1 M citrate and 0.1 M Zn\({}^{2+}\). (L represents the ligand citrate). Zn\({}^{2+}\) was always below 0.1% (<10\({}^{-3}\) in Figure 7) at any pH. That is the reason the cationic Zn\({}^{2+}\) does not appear in Figure 7. The speciation analysis informs that EDTA formed a very stable negative complex [ZnL]\({}^{2-}\) with zinc in all the pH intervals (pH = 3.5-10). The study did not include pH below 3.5 because EDTA is not stable in solution in acidic pH. In conclusion, the use of EDTA requires neutral or slightly alkaline pH to favor the solubility of EDTA and the stability of the metal complexes. Hence, in test 3, the anolyte solution was 0.1 M NaOH to neutralize the acid environment generated by the electrolysis of water in the anode.