13: Zero-Valent Iron and Nanoparticles Flashcards
Describe iron filings and explain why they serve as a good substrate for PRBs.
Iron filings are cuttings and shavings from iron and steel metals operations. The smaller the size the greater the surface area and the more reactive. Granular and stable so excellent porous fill material. Cheap and nontoxic. Very good at reducing reactions particularly with chlorinated solvents. Can remain reactive for long periods of time.
Explain what types of pollutants are best treated by zero-valent iron.
Toxic metals and chlorinated hydrocarbons are best treated with ZVI. Toxic metals react with ZVI to lower their oxidation state and sorb to the iron surface resulting in decreased toxicity and mobility. Chlorinated hydrocarbons degrade into ethene as chlorine atoms are released.
Explain how iron filing particle size affects performance in PRBs.
Particle size impacts the hydraulic conductivity and surface area of iron filings. A balance must be maintained between these two properties because a low hydraulic conductivity will result in plugging, while a lower surface area will result in less effective treatment.
Describe how zero-valent iron reductive dehalogenation is different than biological reductive dehalogenation.
ZVI reductive dehalogenation is a chemical reaction that is much more rapid and complete than biological reductive dehalogenation. Biological reductive dehalogenation requires multiple TEA organisms to break down the chlorinated organics, but ZVI does not.
Explain the role of corrosion in zero-valent iron in remediation.
Corrosion occurs when an oxyhydroxide layer rapidly forms on the exterior surface of the iron fillings, which prevents the contaminant from actually coming into contact with the iron. In addition to potentially reducing surface area and reaction rates, it can also cause clogging.
List and explain the most common causes of zero-valent iron wall failure.
Design flaws including improper hydraulic characterization of a site are the most common causes of failure.
Other causes include loss of reactivity and loss of permeability
Explain how to determine the thickness for a ZVI PRB.
The thickness (b) of a ZVI PRB is determined by the following formula: b = V* tw
Where:
b = thickness,
V = Velocity in the flow direction,
tw = residence time
Describe the four mechanisms that ZVI uses to remediate contaminated groundwater.
Reductive dechlorination: Mechanism for chlorinated organic contaminants. Inorganic chloride ions are released
Reductive precipitation:
Surface adsorption:
Co-Precipitation:
Describe advantages and disadvantages of ZVI PRBs.
Advantages: No pumping required since the system is passive. This significantly reduces remediation costs. Also, ZVI is used up very slowly and giving it an extended design life a
Disadvantages: The placement of the ZVI PRB requires substantial excavation. Storing and/or treating the excavation spoils may be a challenge if it is contaminated.
Describe three advantages to nano-ZVI compared to iron filings.
1) Nano-ZVI havs a high surface area to mass ratio. The high surface area leads to higher reaction rates compared to iron filings.
2) Near complete use of the iron atoms since most iron atoms are located near the surface of the nanoparticles.
3) Nanoparticles can be suspended in a slurry and injected into an aquifer rather than contained in a PRB. Allows for a more versatile treatment method with more options.
Describe bimetallic nano-ZVI and the advantages and disadvantages of it.
Advantages
Disadvantages:
Explain why a directly injected nano-ZVI solution needs the addition of a stabilizer and a surfactant.
ZVI will react with background constituents in water, and will create precipitates that can clog the PRB.
Stabilizer and surfactants emulsify the nano-ZVI to increase remediation efficiency
