An electrically regenerated separation process has been developed for
removing unwanted ions
from aqueous waste streams as a minimally polluting, energy-efficient,
and potentially cost-effective alternative to ion exchange, reverse osmosis, electrodialysis,
and evaporation. Ground
water containing various anions and cations is passed through a stack
of carbon aerogel electrodes,
each having a very high specific surface area (400−1100
m2 g-1) and exceptionally low
electrical
resistivity (≤40 mΩ·cm). After polarization of the
stack, impurity ions are removed from the
electrolyte by the imposed electric field and adsorbed on the electrode
surfaces. Field tests have
shown that hexavalent chromium in the form of
HCrO4
-/CrO4
2-/Cr2O7
2-
can be selectively
removed from contaminated ground water with a 530 ppm total dissolved
solids (TDS) background.
The concentration of Cr(VI) can be lowered from 35 to 2 ppb,
well below the acceptable level for
the regulatory surface water discharge limit of 11 ppb. The
mechanism for Cr(VI) separation
involves chemisorption on the carbon aerogel anode, a process that can
be reversed by cathodic
polarization. Cr(VI) removal is not based upon simple
double-layer charging.