# Enhanced Electrokinetic Remediation for the Removal of Heavy Metals from Contaminated Soils
## 3 Results
### Electrokinetic Treatment  
The removal of heavy metals from the soil specimen was analyzed in a set of electrokinetic tests using EDTA and citric acid as facilitating agents to enhance the solubilization of the metals and their removal in the processing fluid in the electrode chambers. The results of the tests with EDTA and citric acid were compared with an unenhanced electrokinetic test using deionized water as processing fluid in anode and cathode (test 1). Test 2 used 0.1 M citric acid as catholyte and deionized water as anolyte. Test 3 used 0.1 M EDTA as catholyte and 0.1 M NaOH as anolyte. Finally, an additional electrokinetic test (test 4) was run with 0.5 M citric acid as processing fluid in anode and cathode with the objective to enhance the solubilization and removal of metals from soil (Table 3). Figure 3 shows the pH in the electrode chambers and the pH profile in the soil for tests 1, 2, 3 and 4. The unenhanced electrokinetic test with deionized water as processing fluid (test 1) showed the typical pH profile induced by the electrolysis of water upon the electrodes [9]. The pH decreases in the anode to pH 2 and increases in the cathode to pH higher than 12. The pH of the soil in test 1 (the blue line in Figure 3) decreases in the fraction of soil close to the anode and increases in the fraction of soil close to the cathode. The profile of soil pH has an enormous influence on the speciation and mobilization of the contaminating metals in soil (Figure 4). Cd, Cu and Zn were mobilized in sections 1-3,  
Figure 2: Fraction of extracted metals from the soil with mineral acids, organic acids and water (All the compounds were tested at 0.1 M). where the soil pH was lower than pH = 4 (the initial pH of soil). These metals were accumulated in sections 4-6, where the pH conditions induced the precipitation of metals in the soil forming the corresponding metallic hydroxide, especially in sections 5 and 6. Cobalt was mobilized in section 1-4 and mainly accumulated in section 6 because the immobilization of cobalt as Co(OH)\({}_{2}\) takes place at pH? 7. Chromium and lead were not mobilized or removed in the unenhanced electrokinetic test. Chromium is an anionic metal, and it was not expected to be mobilized in very acid conditions. In fact, a slight mobilization of Cr was observed in section 6, where the pH was alkaline. Lead is a metal that can be retained in the soil attached to the organic matter, and it is difficult to mobilize [25]. The alkaline environment in section 6 may have favored the dissolution of organic matter and a slight mobilization of Pb (Figure 4). Overall, the unenhanced electrokinetic test was not effective in the removal of metals from soil. Four metal species were redistributed in the soil (Cd, Co, Cu and Zn), and two species (Cr and Pb) showed no significant mobilization. No significant metal removal was observed, as presented in Table 4. Test 2 and 3 were designed to evaluate the efficiency of citric acid and EDTA to mobilize and remove the metals from soil. Figure 4: Unenhanced electrokinetic test of soil contaminated with metals using deionized water as processing fluid in anode and cathode chambers. C is the metal concentration in soil at the end of the test, and Co is the initial metal concentration in soil. Figure 3: pH in soil and processing fluid in the electrode chambers for the electrokinetic tests with deionized water (test 1), 0.1 M citric acid (test 2), 0.1 M EDTA (test 3) and 0.5 M citric acid (test 4). Test 2 used citric acid as a facilitating agent in the cathode. The citrate ion penetrated the soil specimen by electromigration, mobilizing the metals that could be stabilized in solution forming complexes or chelates with citrate. The presence of citric acid in the cathode avoided the formation of an alkaline environment. The pH in the cathode at the end of the test was pH = 5 that was the result of the partial neutralization of the 0.1 M citric acid solution (the natural pH of the citric acid solution was 2) with the electrochemical consumption of H\({}^{+}\) on the cathode.