Borohydride reduction of an aqueous iron salt in the
presence of a support material gives supported zerovalent
iron nanoparticles that are 10-30 nm in diameter.
The material is stable in air once it has dried and contains
22.6% iron by weight. The supported zero-valent iron
nanoparticles (“Ferragels”) rapidly separate and immobilize
Cr(VI) and Pb(II) from aqueous solution, reducing the
chromium to Cr(III) and the Pb to Pb(0) while oxidizing the
Fe to goethite (R-FeOOH). The kinetics of the reduction
reactions are complex and include an adsorption phase.
About 10% of the iron in the material appears to be located
at active surface sites. Once these sites have been
saturated, the reduction process continues but at a much
lower rate, which is likely limited by mass transfer.
Rates of remediation of Cr(VI) and Pb(II) are up to 30
times higher for Ferragels than for iron filings or iron powder
on a (Fe) molar basis. Over 2 months, reduction of Cr(VI)
was 4.8 times greater for Ferragels than for an equal weight
of commercial iron filings (21 times greater on the basis
of moles of iron present). The higher rates of reaction, and
greater number of moles of contaminant reduced overall,
suggest that Ferragels may be a suitable material for in
situ remediation.