The reduction of 4-chloronitrobenzene by Fe(II)-Fe(III) oxide systems - correlations with reduction potential and inhibition by silicate

• Published in: Journal of hazardous materials 2016-12, Vol.320, p.143-149
• Main Authors: Jones, Adele M., Kinsela, Andrew S., Collins, Richard N., Waite, T. David
• Format: Article
• Language: English
• Subjects: Contaminant degradation; Contaminants; Coprecipitation; Electrode potentials; Mediated electrochemistry; Nanostructure; Phases; Redox potential; Reduction; Reduction kinetics; Silicates; Sorption
• ISSN: 0304-3894; 1873-3336
• DOI: 10.1016/j.jhazmat.2016.08.031

[Display omitted] ⿢The impact of silica on Fe(II)-Fe(III) oxide reduction potentials is unknown.⿢Reduction potentials determined using mediated open circuit potentiometry.⿢Reduction kinetics and measured reduction potentials generally well correlated.⿢Kinetics slower than predicted from reduction potential for Si-doped ferrihydrite.⿢Contaminant degradation in high silicate groundwaters may be less effective. Recent studies have demonstrated that the rate at which Fe(II)-Fe(III) oxyhydroxide systems catalyze the reduction of reducible contaminants, such as 4-chloronitrobenzene, is well correlated to their thermodynamic reduction potential. Here we confirm this effect in the presence of Fe(III) oxyhydroxide phases not previously assessed, namely ferrihydrite and nano-goethite, as well as Fe(III) oxyhydroxide phases previously examined. In addition, silicate is found to decrease the extent of Fe(II) sorption to the Fe(III) oxyhydroxide surface, increasing the reduction potential of the Fe(II)-Fe(III) oxyhydroxide suspension and, accordingly, decreasing the rate of 4-chloronitrobenzene reduction. A linear relationship between the reduction potential of the Fe(II)-Fe(III) oxyhydroxide suspensions and the reduction rate of 4-chloronitrobenzene (normalized to surface area and concentration of sorbed Fe(II)) was obtained in the presence and absence of silicate. However, when ferrihydrite was doped with Si (through co-precipitation) the reduction of 4-chloronitrobenzene was much slower than predicted from its reduction potential. The results obtained have significant implications to the likely effectiveness of naturally occurring contaminant degradation processes involving Fe(II) and Fe(III) oxyhydroxides in groundwater environments containing high concentrations of silicate, or other species which compete with Fe(II) for sorption sites.