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Ferrous and ferric ion generation during iron electrocoagulation.
Divagar Lakshmanan1, Dennis A Clifford, Gautam Samanta
1Department of Civil and Environmental Engineering, University of Houston, Houston, Texas, 77204-4003, USA.
Environmental Science & Technology
|June 24, 2009
Summary
Iron electrocoagulation (EC) primarily generates ferrous iron (Fe2+), not ferric iron (Fe3+). Incomplete oxidation of Fe2+ at lower pH and dissolved oxygen can lead to secondary contamination during water treatment.
Area of Science:
- Environmental Science
- Water Treatment Technologies
- Inorganic Chemistry
Background:
- Iron electrocoagulation (EC) is utilized for contaminant removal, but its efficacy is hampered by inconsistent results.
- Contradictory literature exists regarding the iron species (ferrous vs. ferric) generated during EC using iron anodes.
- Understanding iron speciation is crucial for optimizing EC processes for water purification.
Purpose of the Study:
- To definitively identify the iron species generated at the anode during iron electrocoagulation.
- To investigate the factors influencing the oxidation of generated ferrous ions to ferric ions.
- To assess the implications of iron speciation on contaminant removal efficiency and potential for secondary contamination.
Main Methods:
- Electrochemical experiments using iron anodes to generate iron species.
- Varying parameters such as current, dissolved oxygen (DO) levels, and pH (6.5-8.5).
- Analysis of iron species (Fe2+ and Fe3+) and their oxidation products (e.g., Fe(OH)3(s)/FeOOH(s)).
Main Results:
- Experimental results confirmed the generation of ferrous iron (Fe2+) at the iron anode, not ferric iron (Fe3+).
- Theoretical current efficiency was achieved based on Fe2+ production, independent of current, DO, or pH.
- Fe2+ oxidation to Fe3+ was enhanced by increasing pH and DO; complete oxidation occurred at pH 8.5, while incomplete oxidation was observed at pH 6.5-7.5.
- Compared to chemical coagulation, EC's transient pH increase accelerated Fe2+ oxidation.
Conclusions:
- Iron electrocoagulation predominantly produces Fe2+, requiring subsequent oxidation for effective contaminant removal.
- Inadequate oxidation of Fe2+ in the pH 6.5-7.5 range, especially under low DO conditions, can lead to soluble iron passing through filtration.
- This incomplete oxidation poses a risk of secondary contamination and reduced overall contaminant removal efficiency in water treatment.
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