The Effects of Chelant Agent and Pulse Current on Enhancing Electrokinetic Remediation of Zn/Pb Contaminated Soil

Document Type : Original Research

Authors
F. Diba 1
A. Goodarzi 2
S. sobhan ardakani 1
M. cheraghi 1
B. Lorestani 1
1 Department of Environment, College of Basic Sciences, Hamedan Branch, Islamic Azad University, Hamedan, Iran
2 Faculty of Engineering, Hamedan Branch, Islamic Azad University, Hamedan, Iran
10.22034/23.5.6
Abstract
Electrokinetic (EK) remediation is a very effective option for the soil decontamination; however, its efficiency depends on several factors. In the present study, the ability of Ethylene-diaminete-traacetic-acid (EDTA) and pulse current to improve this method for treating fine-grained soils containing heavy metals (HMs) was investigated. In so doing, first, the studied sample (mainly kaolinite) was mixed with a concentration of 5000 mg/kg zinc (Zn) and lead (Pb) and then subjected to an electrokinetic test with voltage gradient of 2 V(DC)/cm2 (in the form of continuous current and pulse) under 7, 14 and 28 days. The pulse current used was ON for 30 minutes and OFF for 10 minutes. In this process, different concentrations of EDTA (including a concentration of 0.1 M and 0.2 M) were also added to the anode and cathode reservoirs, separately and simultaneously. The obtained results showed that in the conditions of continuous current and without EDTA addtion (the common EK method), the EK removal efficiency, especially for lead, was not noticeable. According to the changes in the microstructure of soil sample and its electrical conductivity (EC) between the anode and the cathode electrodes, the reason can be ascribed to the decrease in current density due to precipitation of pollutants in the soil matrix and decresing the HMs transportation in the cathode side, as clearly confirmed by the XRD patterns and EC tests. In this case, increasing the test time from 7 to 28 days (despite more energy consumption) mainly caused the change of the pollution position in the around of anode side and the HMs removal in the cathode side is not enhanced, indicating that there is a limited effect (about 20%) on the total efficiency of the EK tests. It was found that the addition of EDTA only in the form of catholyte solution, even with the equivalent concentration of soil pollution, has a low effect on improving the electrokinetic response. On the other hand, the presence of the chelating agent in both reservoirs of the EK device, especially by applying the pulse current (with a frequency equal to 36 cycles/day) accelerates the treating process of EK remediation. In fact, as the results of macro-structural tests, scanning electron microscope (SEM) images and X-ray diffraction (XRD) analyses indicated, such improvement can be attributed to two major changes in soil-pollutant interaction. First of all, the presented EK method, by developing the penetration of the acid front towards the cathode side and limiting the polarization ability of clay particles, causes the formation of flocculation and reduces the soil ability to keep pollutants. Also, this system greatly reduces the contribution of insoluble phases through the processes of redissolution and formation of the stable complexes as well as generates a disturbance in the initial formation of metal precipitation due to the reduction in the hydrolysis reaction of the cathode part. Synergy of these changes has an prominent role in accelerating the EK mechanisms; so that compared to the conventional EK model, while reducing energy consumption by 25%, it can also increase the removal efficiency by nearly 2.6 times.

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Modares Civil Engineering journal
Volume 23, Issue 5
November and December 2023
Pages 69-84