Environmental Engineering Reference
In-Depth Information
32.2.7.1 Electrodialysis
This membrane technique uses electric current to remove perchlorate (Roquebert et al.,
2000). In this technology, a current is applied to perchlorate-contaminated water as it
passes through channels of alternating permeable membranes selective of anions and cat-
ions. The electric current dissociates perchlorate salts into cations and anions. Ammonium
perchlorate and potassium perchlorate are two common forms of perchlorate contamina-
tion. Perchlorate ions, being negatively charged (anion), accumulate at the cationic-selective
membrane and are eventually collected as concentrate or salty water. Similarly, positive
ions accumulate at the anionic-selective membrane. This method produces two types of
water—salty water and relatively deionized water. The deionized water is used, while the
salty water is disposed of or further treated by an appropriate method before disposal
(Urbansky and Schock, 1999).
The effectiveness of electrodialysis for perchlorate removal may be reduced because of
fouling. Furthermore, the concentrate resulting from this method may require large quan-
tities of water for further treatment before disposal (Urbansky and Schock, 1999), which
adds costs to the application of the process.
32.2.7.2 Reverse Osmosis
Reverse osmosis is another membrane technique used for perchlorate removal (USEPA
2005b; Urbansky, 1998). Osmosis can be deined as the movement of water molecules
from a region of lower solute concentration to a region of higher solute concentration
through a semipermeable membrane (Urbansky and Schock, 1999). In this case, the
solute is a perchlorate salt. A reverse osmosis system consists of a chamber in which
perchlorate-contaminated water is placed on one side of a semipermeable membrane
and freshwater is placed on the other side of the membrane. Pressure is applied at the
inlet to force water molecules against the concentration gradient from the contaminated
water into the freshwater section of the reverse osmosis system. This results in sep-
aration of perchlorate ions from contaminated water. Treated water can be used. The
water containing perchlorate and other contaminants is further treated before disposal
(USEPA, 2005).
The performance of a reverse osmosis membrane is affected by the presence of organic
matter and microbes that can foul and damage the membrane. The fouling can be further
enhanced by the presence of alkaline earth metals.
Reverse osmosis is normally suitable for point-of-use or small systems. Posttreatment
including application of sodium chloride or sodium bicarbonate is required to make water
palatable and prevent fouling of the distribution system (Urbansky, 1998).
32.2.8 Sorption Media
Various sorption media have been investigated to remove perchlorate from contaminated
water. These include modiied activated carbon, surface-modiied zeolites, and surface-
modiied ceramic porous media. All these approaches rely on high surface area resulting
from meso/nano porosity at the surfaces. The sorbent media are generally modiied by
a cationic surfactant providing a charged surface that can bond with the perchlorate ion
through ion bonding.
