Enhanced electrochemical remediation

Electrochemical remediation is also referred as electrokinetics, electrokinetic remediation, electroremediation, electroreclamation, and other such terms in the published literature. It should be noted here that when water alone is used at the electrodes, the process is known as unenhanced electrochemical remediation. However, when enhancement strategies (such as using conditioning solutions and ion exchange membranes at the electrodes) are used, then the process is known as enhanced electrochemical remediation. 

The chelating agents used by researchers to enhance electrochemical remediation include EDTA, citric acid, oxalic acid, ammonia, iodide/iodine, potassium iodide solution, sodium chloride solution, l-hydroxyethane-l,l-diphosphonic acid (HEDPA), HPCD, and so on. EDTA is the most frequently used chelating agent in electrochemical remediation. The complexation chemistry of EDTA is briefly outlined as an illustration. 

Giannis A, Gidarakos E. (2005). Washing enhanced electrochemical remediation for removal cadmium from real contaminated Journal of Hazardous Materials 123 165-175. 

Although excellent removal efficiencies can be achieved at bench scale by the use of different enhanced electrochemical remediation strategies, several practical problems arise in using them at actual field sites. These problems include high cost... 

Yeimg AT, Gu YY (2011) A review on techniques to enhance electrochemical remediation of contaminated soils. J Hazard Mater 195 11-29... 

Gu YY, Yeung AT (2012) Use of citric acid industrial wastewater to enhance electrochemical remediation of cadmium contaminated natural clay. Geocongress 3995-4004... 

The migration of pore fluid, ions, charged particles, colloids, and bacteria can be utilized to remove contaminants from polluted soil and/or to inject enhancement agents, nutrients, and so on to facilitate various remediation processes. The geochemical processes can be used to provide the necessary environmental conditions to control the direction of electroosmotic flow and to solubilize contaminants in the soil, so as to enhance the efficiency of the electrochemical remediation processes. 

The recognition of soil-fluid-chemical systems as active electrochemical systems is very important in the development of a better understanding of the physics and chemistry involved in electrochemical remediation of soils. It laid the foundation to support the use of established principles in electrochemistry to explain the phenomena observed during electrochemical remediation of soils, and to explore the use of different enhancement techniques to improve the efficiency of the technology. 

The formation of complexes modifies the metal species dissolved in pore fluid and those sorbed on soil particle surfaces interactively. The formation of complexes reduces the concentration of free metal ions in the dissolved phase. As a result, concentration-dependent properties of metals are altered. The effects include (a) modification of solubility, toxicity, and possibility biostimulatory properties of metals (b) modification of surface properties of solids and (c) sorption of metals from solution (Snoeyink and Jenkins, 1980). As a result, chelating agents can enhance the electrochemical remediation of subsurface contamination. 

The effect of ammonia as a complexing agent to enhance the efficiency of electrochemical remediation was evaluated by Vereda-Alonso et al. (2007). Laboratory-scale experiments on copper-spiked kaolin were carried out to explore if metal precipitation could be prohibited at neutral and alkaline pH. Ammonia was added at the anode compartment and migrated into the soil as ammonium by electromigration. Tvo set of experiments were performed (a) using ammonia solution as anolyte and sodium nitrate as catholyte, and (b) combining the ammonia enhancement with the addition of acetic add into the cathode (weak-acid enhancement). The results indicate that metal recovery with only ammonia was less than 40%, while the combination of ammonia and add enhancements achieved a copper recovery of more than 90%. 

The geochemical processes discussed in previous sections are interrelated and interdependent. If they are properly understood and utilized, they can enhance the efficiency of electrochemical remediation. Examples are given to demonstrate the importance of these interactions of geochemical processes in electrochemical remediation of contaminated fine-grained soils. 

In addition to transformation by corrodable metals (such as Fe° and Zn°), bimetallic combinations of a catalytic metal with a corrodable metal (such as Pd/Fe or Ni/Fe) have also been shown to transform a variety of contaminants. In most cases, rates of transformation by bimetallic combinations have been significantly faster than those observed for iron metal alone [26,96,135-139]. Not only have faster transformation rates been observed with bimetallic combinations, but, in some cases, transformation of highly recalcitrant compounds, such as polychlorinated biphenyls (PCBs), chlorinated phenols, and DDT has been achieved [24,140,141]. The mechanism responsible for the enhanced reactivity with bimetallic combinations is still unclear however, it has been suggested that electrochemical effects, catalytic hydrogenation, or intercalation of H2 may be responsible. A likely limitation to the full-scale application of bimetallic combinations to groundwater remediation is deactivation of the catalytic surface either by poisoning (e.g., by sulfide) or by formation of thick oxide films [136,142,143]. 

Hanna, K., Chiron, S. and Oturan, M. A. (2005) Coupling enhanced water solubilization with cyclodextrin to indirect electrochemical treatment for pentachlorophenol contaminated soil remediation. Water Res. 39, 2763-2773. 

For some combinations of heavy metals, it is also necessary to use enhancement solutions to ensure the simultaneous removal of all pollutants (Ottosen et ai, 2003). Especially, the presence of As in the soil necessitate alternative solutions to the acidic front since As generally has low mobility under acidic conditions, whereas As is more mobile under alkaline conditions, where most heavy metals are not mobile (Le Hecho, TelUer, and Astruc, 1998 Ottosen et aL, 2000). Le Hecho, Tellier, and Astruc (1998) conducted laboratory experiments with spiked soils, where the pollutants were As and Cr. Successful remediation was obtained in the developing alkaline front combined with the injection of sodium hypochlorite. As was mobile in the alkaline environment, and Cr(III) was oxidized to Cr(VI) by hypochlorite and mobilized in the alkaline environment. In loamy sand polluted with Cu and As from wood preservation. As and Cu were mobile simultaneously after the addition of NH3 to the soil (Ottosen et a/., 2000). As was mobile due to the alkaline environment and Cu formed charged tetra-ammine complexes. For the simultaneous mobilization and electrochemical removal of Cu, Cr, and As, ammonium citrate has shown to be successful (Ottosen et al, 2003). 

Tiehm A, Augenstein T, Ilieva D, Schell H, Weidlich C, Mangold K-M (2010) Bio-electro-remediation electrokinetic transport of nitrate in a flow-through system for enhanced toluene biodegradation. J Appl Electrochem 40 1263-1268... 

Despite the relatively good results found in literature for soil contaminated with TCE, the removal and elimination of COCs requires enhanced electrokinetic technologies. They comprise the use of solubilizing agents such as cosolvents, surfactants, or cyclodextrins. The other possible alternative for the removal of COCs from soil implies the combination of electrokinetic with other remediation techniques such as chemical and electrochemical oxidation/reduction, permeable reactive barriers, electrolytic barriers, and electric heating. 

Gonzini O, Plaza A, Di Pahna C, Lobo MC (2010) Electrokinetic remediation of gasoil contaminated soil enhanced by rhamnolipid. J Appl Electrochem 40 1239-1248... 

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