Electrochemical Removal of Inorganic Pollutants

Additionally, electrochemical transformations may occur when the contaminants enter into the anode or the cathode, particularly chlorinated organic compounds, which are shown to undergo reductive dechlorination at the cathode and oxidative dechlorination at the anode. Such transformations should also be considered based on the redox conditions in the electrodes and the contaminant characteristics. 

Inorganic pollutants include (a) cationic heavy metals such as lead, cadmium, and nickel, (b) anionic metals and inorganics such as arsenic, chromium, selenium, nitrate and fluoride, and (c) radionuclides such as strontium and uranium. The geochemistry of these pollutants can widely vary and it depends on the specific pollutant type and soil/sediment properties. The speciation and transport of these pollutants also depend on the dynamic changes in the pH and redox potential of the soil that occurs under applied electric potential. The dominant transport process 

Numerous studies are reported on the electrokinetic removal of heavy metals from soils (Chapter 4). Many of these studies used ideal soils, often kaolinite, as a representative low-permeabiUty soil, which were spiked with a selected single cationic metal (such as lead and cadmium) in predetermined concentration. The spiked soil is loaded in a small-scale electrokinetic test setup and electric potential is applied. The transport and removal of the metal after specified test duration are determined. It is shown that cationic metals exist in soluble ionic form due to reduced pH near the anode regions and they are transported toward the cathode. However, when they reach near the cathode, they get sorbed or precipitated due to increased pH resulting from OH transport from the cathode. The actual removal from the soil is often negligible. 

Experiments with anionic metals (such as chromium and arsenic) showed behavior opposite to that of cationic metals. The anionic metals are found to exist in soluble ionic form near the cathode and are transported toward the anode. However, when they reach near the anode, they are adsorbed due to low-pH conditions existing due to electrolysis reactions. Nevertheless, the actual removal of anionic metals was found to be greater than that of cationic metals. The use of alkaline solution to increase soil pH near the anode is found to enhance the removal of anionic metals (Chapter 4). 

In addition to anionic metals, the problem of groundwater contamination with excess nitrate and fluoride is well recognized (Chapter 6). These anionic species are 

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