On the nonlinear relationship between k(obs) and reductant mass loading in iron batch systems

Area of Science:

• Environmental Science
• Green Chemistry
• Materials Science

Background:

• Granular iron is widely used for dehalogenation.
• A first-order relationship between reaction rate and iron loading is often assumed but not verified.
• Understanding this relationship is crucial for optimizing remediation processes.

Purpose of the Study:

• To investigate the relationship between reaction rate and iron mass loading for polyhalogenated alkane reduction.
• To determine if a linear relationship exists and under what conditions.
• To explore factors influencing the observed reaction kinetics.

Main Methods:

• Batch experiments were conducted with varying granular iron mass loadings (rhom).
• Polyhalogenated alkanes were used as target contaminants.
• Pseudo-first-order rate constants (k(obs)) and surface-area-normalized rate constants (k(SA)) were calculated.
• System pH and aqueous iron(II) generation were monitored.

Main Results:

• A linear relationship between k(obs) and rhom was observed only under mass-transport-limited conditions.
• Most alkyl polyhalides exhibited a nonlinear relationship, with k(SA) decreasing as rhom increased.
• Decreasing pH led to a reaction order approaching unity, suggesting a link to passive oxide layer thickness.
• Strong correlations were found between k(obs) and the concentration of aqueous iron(II) and reduced protons/water.

Conclusions:

• The commonly assumed first-order relationship between reaction rate and granular iron loading is not universally applicable.
• Nonlinear kinetics are prevalent and influenced by factors like iron loading and pH.
• The reduction of polyhalogenated alkanes may be linked to proton/water reduction sites on the granular iron surface.