OCCURRENCE AND USES OF PERCHLORATE
Perchlorates are salts derived from Perchloric Acid [HClO4]. The prefix “per” signifies “complete” or “extreme”, specifically denoting an element in its highest oxidation state, the element in this case being chlorine.
Perchlorates occur naturally but, since they are readily soluble in water, an accumulation of perchlorates only occurs in arid areas with little or no rainfall, such as the Atacama desert of South America, which contains large amounts of nitrates, interspersed with perchlorates in the milligrams per kilogram range. “Chile Saltpeter” was widely used as a fertilizer throughout the world, thus perchlorates were also distributed along with the nitrates. The Haber-Bosch process obviated the need for Chile Saltpeter by using atmospheric Nitrogen to produce ammonia, which is then oxidized to nitrate. With the advent of ‘synthetic’ nitrate fertilizers, which contain no perchlorates, the Chile Saltpeter source of perchlorates nearly vanished. In 2006 a mechanism for the formation of perchlorates was proposed which is particularly apropos to the discovery of perchlorate at the Mars Phoenix Lander site. It was shown that soils with high concentrations of natural salts could have some of their chloride converted to perchlorate in the presence of sunlight and/or ultraviolet light. The mechanism was reproduced in the laboratory using chloride-rich soils from Death Valley, California.
Industrially, perchlorates are either produced by the electrolysis of chloride salts or by the neutralization of perchloric acid. Potassium or ammonium perchlorates [KClO4, NH4ClO4] are used extensively as oxidizers in rocket fuels and in explosives, and are also used in airbags and pyrotechnics. Lithium perchlorate [LiClO4], which decomposes exothermically to release oxygen is used in “oxygen candles” on spacecraft, in submarines and other esoteric situations where a reliable backup or supplementary oxygen supply is needed.
Perchlorate impacts human health by interfering with iodide uptake in the thyroid gland. In adults, the thyroid gland helps regulate the metabolism by releasing hormones. In children, the thyroid helps in proper physical development. The National Academy of Sciences (NAS) found that perchlorate only affects the thyroid gland. It is not stored in the body, it is not metabolized, and any effects of perchlorate on the thyroid gland are fully reversible once exposure stops. perchlorate has been used as a medication to treat hyperthyroidism for many decades. At very high doses, 70 – 300 milligrams, the administration of potassium perchlorate was considered the standard of care in the United States and remains the approved pharmacological intervention in many countries for the treatment of hyperthyroidism.
The United States Environmental Protection Agency (EPA) has been working with states, federal agencies, tribes, water suppliers and the private sector for several years to address perchlorate. In 1998, EPA released an Interim Assesment Guide, which was then peer-reviewed in 1999. The external review draft of the revised document “Perchlorate Environmental Contamination: Toxicological Review and Risk Characterization” (November 2002) responds to the recommendations emanating from the peer review.
The National Research Council of the National Academies published its technical review of the Health Implications of Perchlorate Ingestion in January 2005. From this review, EPA has established an official reference dose of 0.0007 mg/kg/day for perchlorate. For a 154 lb individual, this would represent 49 millligrams of perchlorate per day. A reference dose is a scientific estimate of a daily exposure level that is not expected to cause adverse health effects in humans. EPA divided the reference dose by the standard interspecies uncertainty factor of 10, and then calculated a “drinking water equivalent level” of 24.5 micrograms per liter (parts per billion). Thus, 25 ppb was set by the EPA as the recommended drinking water standard.
On October 10, 2008 EPA issued its Preliminary Regulatory Determination for perchlorate (Federal Register / Vol. 73 No. 198 Page 60274 Para IV) as follows:
In making preliminary regulatory determinations, EPA uses the criteria mandated by the 1996 SDWA Amendments. EPA has found that perchlorate, at sufficiently high doses, may have an adverse effect on the health of persons, and that perchlorate is found in a small percentage of public water supply systems. However, EPA has determined that regulation of perchlorate in drinking water systems does not present a meaningful opportunity to reduce health risk for persons served by public water systems. This section describes how EPA has evaluated these three (3) criteria in light of the data presented in Section III to make a preliminary regulatory determination for perchlorate.
A. May Perchlorate Have an Adverse Effect on the Health of Persons?
Yes. Perchlorate interacts with the sodium iodide symporter, reducing iodine uptake into the thyroid gland and, at sufficiently high doses, the amount of T4 produced and available for release into circulation. Sustained changes in thyroid hormone secretion can result in hypothyroidism. Thyroid hormones stimulate diverse metabolic activities in most tissues, and individuals suffering from hypothyroidism experience a general slowing of metabolism of a number of organ systems. In adults, these effects are reversed once normal hormone levels are restored (NRC, 2005).
In fetuses, infants, and young children, thyroid hormones are critical for normal growth and development. Irreversible changes, particularly in the brain, are associated with hormone insufficiencies during development in humans (Chan and Kilby, 2000 and Glinoer, 2007). Disruption of Iodide uptake presents particular risks for fetuses and infants (Glinoer, 2007 and Delange, 2004). Because the fetus depends on an adequate supply of maternal thyroid hormone for its central nervous system development during the first trimester of pregnancy, Iodide uptake inhibition from perchlorate exposure has been identified as a concern in connection with increasing the risk of neurodevelopmental impairment in fetuses of high-risk mothers (NRC, 2005). Poor iodide uptake and subsequent impairment of thyroid function in pregnant and lactating women have been linked to delayed development and decreased learning capability in infants and children with fetal and neonatal exposure (NRC, 2005).
STATE PERCHLORATE REGULATIONS
The State of California, on October 18, 2007 set a Maximum Contaminant Level (MCL) of 6.0 micrograms per liter µg / L (PPB).
The State of Massachusetts, in May 2004 set a Maximum Contaminant Level (MCL) of 2.0 mirograms per liter, µg / L (PPB).
Perchlorate Removal Processes
The following perchlorate removal processes are listed neither in order of preference, nor of viability. Some of the processes, though feasible, are still in the research and development stage. Nevertheless, they will be discussed as some may develop into viable alternative treatment options.
- Ion Exchange• Reverse Osmosis
- Packed Bed Bioreactor• Iron / Batch Reactors
- Membrane / Biofilm reactor• Preloaded Granular Activated Carbon
Ion Exchange Perchlorate Removal
Several manufacturers produce ANSI Standard 61 ion exchange resins capable of removing perchlorate to detection limit levels. Some resins are highly selective for the removal of perchlorate, others, to a lesser extent. In many applications, Nitrate removal is an added benefit. Other anions, such as sulfates, will also compete for resin exchange capacity. Some resins have a high capacity for perchlorate removal. Considering the low concentrations of perchlorate the resin is expected to remove, a selective resin may well be able to treat up to one (1) million gallons of water per cubic foot of resin. Resins are typically regenerated with a strong acid, such as sulfuric acid.
If the raw water contains contaminants that can foul the resin (such as iron, manganese, or particulate matter) Pureflow strongly recommends protecting the [expensive] ion exchange resin from fouling by installing a permanent media filter upstream of the ion exchange system.
Reverse Osmosis (RO)
RO is a membrane process primarily designed for desalting saline or brackish waters by the application of hydrostatic pressure to overcome (and reverse) osmotic pressure, driving the water molecules through a semi-permeable membrane designed to allow passage of water, but not of dissolved contaminants. The process requires expensive and fragile membrane stacks of either cellulose acetate or thin-film composite. Cellulose acetate membranes may require a working pressure of 400 psi, or higher, and are subject to fouling by iron / manganese, biological attack and hydrolysis. They will also allow salt passage to double after a service life of about 3 years. The more expensive thin film composites are capable of the same, or greater, flux rate, but at half the applied pressure. Both membrane types require considerable pretreatment to prevent scaling, plugging and colloidal or biological fouling.
Since the recovery of product water, as a percentage of feedwater, is a function of applied hydrostatic pressure, the process tends to be quite energy-intensive. Most RO plants are designed for 75-80% recovery, i.e., up to 25% of the flow must be “wasted” as a concentrated, possibly hazardous, waste. Some RO membranes can recover approximately 90% of the processed water at the initial start-up of the system, but the recovery rate can be significantly reduced by fouling of the membrane as noted above. RO is capable of removing perchlorate to detection limit levels, but it must be remembered the process is not perchlorate-specific or selective, thus will remove, virtually, all dissolved solids. Process operations and maintenance costs, as well as operator-intensity tend to rule out RO for all but very small volume treatment systems.
Biological Perchlorate Removal
Both, Packed-bed or Membrane-Biofilm biological reactors rely on the presence of a flora of anaerobic microorganisms to reduce Perchlorate and other oxidized species, such as nitrates and sulfates. Microorganisms require nutrients, such as a carbon source, nitrogen, and phosphorus. Carbon is generally supplied by the addition of small amounts of ethanol or an acetate. Nitrogen and phosphorus are generally applied as salts. It follows that dosing of nutrients, and the maintenance of the bacteriological colony is operator–intensive, as the carbon source feed must be exactly stoichiometric to prevent formation of disinfection byproducts downstream. It is reported that in fixed bed reactors, 80% perchlorate removal can be achieved with an empty bed contact time of approximately five (5) minutes.
Perchlorate Removal with Iron in Batch Reactors, and GAC Preloaded with Organic Cation Monomers or Polymers
Little information on the efficiency of these treatment options is listed in literature. It would appear that both unit processes are still in the research and development stage and are not yet ready for industry applications.
As with all water treatment applications, the more that is known about the quality of the water to be treated, and the possible variability of that quality, the more successful will be the design of the treatment system. As a minimum (prior to pilot testing), an accurate general mineral analysis of the water should be performed. Consideration should also be given to the possible seasonal variability of the stated water composition.
Since, as stated above, ion exchange resins are not entirely perchlorate selective, the composition of the water will dictate not only perchlorate removal efficiency, but also strongly affect resin exchange capacity. A pilot filter test must be performed prior to final design. The pilot system must verify removal of perchlorate throughout the process run cycle, as well as determine any pretreatment requirements. The pilot test report must include the cost of operations, labor, media replacement and regenerant disposal. Local regenerant disposal requirements should be researched to ascertain whether the spent acid will be deemed a hazardous waste and how legal disposal will be achieved.
Although opinions appear to differ about the possible health threats posed by perchlorate, the California MCL has been set at 6.0 PPB, the Massachusetts MCL has been set at 2.0 PPB, and the EPA may promulgate a national standard in the future. The MCL will largely drive and dictate the installation of perchlorate removal systems. Of the unit processes listed, ion exchange is the most applicable and the one generally used. Reverse osmosis is equally capable of perchlorate removal, but may only be financially viable in very low flow applications, or where total dissolved solids (TDS) must also be reduced.