Introduction
Perchlorate is a known thyroid gland disruptor and its concentration is regulated in soil and groundwater. Its maximum contaminant limit (MCL) in the State of California is six micrograms per liter (µg/L). In January 2011, the Office of Environmental Health Hazard Assessment (OEHHA) proposed a one µg/L perchlorate public heath goal (PHG) for drinking water.
Rockets
In the United States, soil and groundwater at several properties has been impacted with perchlorate. Historical releases of perchlorate to the environment have resulted from the perchlorate manufacturing process, and the manufacture of perchlorate-containing products which includes matches, fireworks, flares and perchlorate containing waste associated with these products. Perchlorate was also used to manufacture and test ordnance, explosives, and solid rocket-fuel propulsion based systems.
Solid perchlorate is used as an oxidant since it decomposes exothermically to produce oxygen; however, perchlorate is very stable molecule in solution as a result of its chemical structure which consists of a chlorine molecule encased in four oxygen molecules. When perchlorate dissolves in water, it is stable, but generally will not be reduced in groundwater.
Remediation of perchlorate impacted soil may be limited excavation and disposal (dig and haul), and remediation of perchlorate impacted groundwater may be limited to pump-and-treat alternatives. Groundwater extraction systems are designed to hydraulically contain perchlorate plumes and require an arrangement and a design based on existing groundwater conditions. Once perchlorate impacted groundwater is extracted from the subsurface, a groundwater treatment system is required to remove perchlorate prior to disposal or reuse. Ion exchange systems and anoxic filters are considered proven technologies to treat water impacted with perchlorate.
Ion Exchange Systems
Ion exchange is a process in which ions of a species are displaced from an insoluble exchange material by ions of a different species in water. The removal of target ions is achieved through commercially available ion exchange resins. Anion exchange resins exchange target anions for chloride (Cl–) or hydroxide (OH–), and cation exchange resins exchange cations for sodium (Na+) or hydrogen (H+) ions depending on the functional groups. The functional group specifically used for removal of perchlorate is the quaternary amine functional group (-N(R)3+)
Ion exchange resins are comprised of small polymer beads with a large surface area per unit of volume where the ion exchange takes place. The resin is placed in vessels, and water is pumped through the resin until the resin’s ion exchange capacity is exhausted. To remove solids and restore the resin ion exchange capacity, the resin may be either replaced or backwashed with a prepared solution of salt, acid, or sodium hydroxide solution. If a backwash system is used, a backwash solution is prepared based on the resin specifications.
Ion exchange system selection, design, and specification will depend on water chemistry, flows, initial concentrations, and target concentrations. An initial assessment is required to determine if ion exchange is economically feasible based on water chemistry and treatment requirements.
Anoxic Filters
Biological treatment is used in a wide range of water and wastewater treatment applications for the removal of biodegradable constituents. The biological treatment process has been demonstrated to chemically reduce perchlorate, nitrates, hexavalent chromium in water through the use of anoxic filters which have been implemented as fluidized bed or packed bed anoxic digesters.
Perchlorate can be reduced biologically using microorganisms that use an electron donor (such as methanol, acetic acid, ammonia, or hydrogen) and a compound such as perchlorate as an electron acceptor. The electron donor and the electron acceptor are metabolized by the microorganisms to generate energy for growth and reproduction. The microbial metabolism will generate byproducts, of which the most common are carbon dioxide and water. In the biological reduction process used for the treatment of nitrate and perchlorate, the nitrates are reduced to nitrogen gas N2, and perchlorates are reduced to a chloride ion (Cl–).
The effective operation of the anoxic filter process requires adequate control of treatment system conditions (e.g., dissolved oxygen concentration, temperature, and nutrient availability). The adequate treatment conditions allow the reactor to select microorganisms responsible for the above-mentioned reactions.