Removal of Perchlorate from drinking water by Polymeric Ligand Exchange

Perchlorate (ClO4-) is an anion contaminant in ground water and surface water when the salts ammonium, potassium, magnesium, or sodium perchlorate dissolved in water. Ammonium perchlorate is a main ingredient of solid rockets and missiles fuel. It has also been found in air bag inflators, manufacture of matches, analytical chemistry ingredients, nuclear reactors, electronic tubes, lubricating oils, leather tanning and finishing activities, fabrics and dye fixers, electroplating, aluminum refining, rubber manufacture, paint and enamel production. In the recent years, perchlorate has been widely detected at dangerous levels in drinking water in at least 22 states including Alabama (EPA 2003). Drinking water for more than 20 million Americans is contaminated with this toxic legacy of the Cold War (EWR, 2003a )

Investigations show that perchlorate poses a variety of health problem to human body, especially in children, newborns and pregnant women. It can cause both physical and mental retardation and has been linked to thyroid cancer (EPA 2003). Perchlorate interferes with iodide uptake into the thyroid gland. Because iodide is an essential component of thyroid hormones, perchlorate disrupts how the thyroid functions. EPA??s draft analysis of perchlorate toxicity is that perchlorate??s disruption of iodide uptake is the key event leading to changes in development or tumor formation (EPA 2003). Considering the potential health risk, a maximum contaminant level (MCL) of 1 ppb has been recommended by EPA. But because of the strong opposition from DoD, NASA, and DOE, the National Academy of Science (NAS) was asked to review the data and suggest a final verdict in mid-2004.

PERCHLORATE CHEMISTRY

The chemical characteristics of perchlorate make it very unusual. Perchlorate is a highly soluble inorganic anion (2.09 kg/L for NaClO4?C) (Logan, 2001) that adsorbs poorly to mineral surfaces and activated carbon and is not retarded during groundwater transport. In the solid state, the perchlorate anion has been determined by X-ray diffraction to have a nearly perfect tetrahedral geometry with the four oxygen atoms at the vertices and the chlorine atom at the center . In aqueous solution, the geometry is probably perfectly tetrahedral.
　

Perchlorate is present in water as ClO4- oxyanion. Although chlorine is at its highest oxidation state and the reduction potential is high, perchlorate is actually unusually stable in water due to the high activation energy barrier. Perchlorate is so persistent that it has been detected in milk of numerous Texas grocery stores at concentrations ranging from 1.7-6.4 ppb (Kirk et al, 2003). High concentrations of perchlorate were also detected in lettuce in southern California (Jacobs, 2003). Another characteristic of perchlorate is its high polarizability, witch makes perchlorate very difficult to be detected. Before 1997, only concentrations greater than 100ppb could be detected by Ion Chromatograph (IC), which explains partially why the toxic effects of perchlorate at lower concentrations were largely ignored of many decades. In March 1997, the California Department of Health Services (CDHS) developed a ground breaking method that lowered the detection limit to 4 ppb, which immediately triggered a nationwide interest in re-evaluation of the health and ecological risks associated with low concentration of perchlorate spearheaded by the US EPA. In 1998, the Dionex Cooperation invented the new AS-11 IC column, which further lowered the detection limit to < 1 ppb, which essentially eliminated the analytical barriers for regulating perchlorate to a very low level. Another unique and maybe most important property of perchlorate is that despite its high water solubility perchlorate is actually much more hydrophobic than most other inorganic anions such as sulfate, chloride and nitrate, namely, it is also very soluble in polar organic solvents (Urbansky,1998)

TREATMENT TECHNOLOGIES

In order to find efficient technologies to prevent perchlorate from sweeping into drinking water and other water resource, a lot of research about ion exchange, granular activated carbon (GAC), biological processes, membrane filtration, chemical reduction, electrochemical processes, phytoremediation have been done. Based on the results of past years, there currently are two promising technologies used to treat large volumes of water that contain perchlorate: ion exchange and anaerobic biological treatment(Logan, 2001; CWQ, 2003).

Under anaerobic conditions, Perchlorate can be biodegraded serving as an electron acceptor.  A number of microorganism have been identified to have capability to reduce perchlorate. Bacteria capable of perchlorate degradation appear to be widely distributed in nature at concentrations ranging from one to thousands of bacteria per gram of water, wastewater, sediment, and soil. Perchlorate is used as an electron acceptor by some bacteria for cellular respiration and is degraded completely to chloride ion. The bacteria that degrade perchlorate are diverse. Almost all of them fall within new species?? classifications based on a 16s rDNA classification scheme??a recombinant DNA methodology based on the 16s gene, which can be used to assess the phylogeny of bacteria. Most perchlorate-respiring microorganisms (PRMs) are capable of functioning under varying environmental conditions and use oxygen, nitrate, and chlorate (ClO3?C)??but not sulfate??as a terminal electron acceptor. Perchlorate can be successively degraded to chlorate and then chlorite (ClO2?C) by a novel chlorate reductase respiratory enzyme. A chlorate-respiring bacterium was the first isolate shown to be capable of benzene degradation, although only under denitrifying, and not chlorate-reducing, conditions (Logan,2001). A lot of biological reactors have been investigated for perchlorate removal. Most of these systems are attached growth reactors using either granular activated carbon(GAC) or sand, and are able to remove perchlorate to very low levels. A variety of donors including ethanol, methanol, acetate, hydrogen and cheese whey have been utilized in these reactors.

While biological processes have been widely used for municipal wastewater treatment for its low cost and good environmental friendliness, treating contaminated drinking water using biological processes is rather scarce primarily due to 1) bacteria activities may generate additional odor, taste and other undesirable by-products, 2) the organic contents and nutrient in drinking are usually too low to warrant efficient degradation rate, and 3) reproduction of bacteria creates an additional disinfection demand.

Ion exchange is the most commonly used physical chemical removal technology to handle perchlorate problem. Many anion exchange resins are commercially available to reduce perchlorate to safe level in drinking water. And a number of full-scale treatment facilities have been built up to treat perchlorate-contaminated drinking water and groundwater in California and other states. In contrast with biological processes, ion exchange process in general possess the following advantages: 1) it is rather simple (fixed-bed has been the typical process configuration as shown in Figure*), 2) its maintence and operation are rather straightforward and can be automated, 3) no harmful by-product will be produced, 4) ion exchange resin can be reused for many cycles after regeneration, 5) it can be easily inserted in an existing treatment process. However, this technology confronts the following obstacles: 1) although satisfactory sorption capacity was observed with some commercial resins, regenerability of resins have been found extremely poor, making it cost prohibitive, 2) associated with the poor regenerability, large volumes of process waste residuals (spent regenerant) are produced and need to be further disposed of, and 3) no perchlorate specific sorbent has been developed, i.e., the sorption of perchlorate is severely retarded by other anions such as sulfate, nitrate and dissolved natural organic matter (NOM) in water. Consequently, the key breakthrough in applying ion exchange technology calls for a novel ion exchange resin that has high selectivity for perchlorate and high and efficient regenerability.

POLYMERIC LIGAND EXCHANGE

The ligand exchange technology was firstly conceived and developed by Fred Helfferich in 1961 (Walton, 1995). It combines two fields of chemistry, namely, ion exchange and coordination chemistry, in order to accomplish a task that neither could do alone. The advantage of ligand exchange over other chromatographic techniques is that complex formation is, as a rule, a much stronger interaction than ordinary physical sorption or ion exchange. Because of the strong tendency of the ligand to form complexes with the metal in the resin, almost the full ligand-exchange capacity is utilized even if the ligand concentration in the external solution is very low. And the complexes of various even rather similar ligands with a properly chosen metal ion usually differ considerably in their strengths. Accordingly, high selectivities can be attained (Helfferich, 1962).

During the past years, Zhao and SenGupta made an intensive investigation on a new class of polymeric ligand exchanges for highly effective removal of various oxyanions including phosphate, chromate and arsenate from water (Zhao et al,1995). They also patented a PLE-based treatment process, which has been employed at a number of sites worldwide. As shown in Figure2, the new anion exchanger is a polymeric ligand exchanger which consists of the followings: first, a cross-linked polystyrene-divinylbenzene matrix; second, covalently attached chelating functional groups containing nitrogen donor atoms; and third, a Lewis acid cation (e.g., Cu2+) coordinated to the chelating functional group in a manner that its positive charges are not neutralized. Consequently, the performance of a PLE will be affected by properties and chemistry of the metal-housing polymer (e. g., porosity, pore size distribution, hydrophobicity, and surface area), the reactivity of the chelating functionality and the reactivity of the immobilized metal. Unlike conventional standard ion exchange resins, the PLE employs the loaded metal as its functionality. Since the metal can interact with the target ligand (e. g., perchlorate) through concurrent Lewis acid-base interaction (i. e., surface complexation) and electrostatic interactions (ion pairing) and since such metal-ligand interaction can be readily manipulated, the PLE offers unparalleled advantages over standard resins with respect to both selectivity enhancement and regenerability.

The PLEs showed unusually high selectivity and sorption capacity toward oxyanionic contaminants such as arsenate, chromate, phosphate, and trichlorophenol even in the presence of high concentrations of common anions such as sulfate, nitrate, and dissolved NOM (Zhao et al,1995; Zhao and SenGupta, 1998; 2000). Note that both electrostatic interaction and ion-pairing (IP), and Lewis acid-base (LAB) are concurrently operative. The PLE??s high selectivity for the target ligands results from the synergy of various concurrent specific interactions between the target ligands and metal functional groups including the metal-ligand complexation or Lewis acid-base interaction, electrostatic (cation-anion) interactions, and hydrophobic interactions for hydrophobic contaminants such as trichlorophenol and perchlorate.

Another property of PLEs is that they have high regenerability, which will make this technology cost-efficient and practical. Unlike other commercial ion exchange resins, the PLE??s reactions with perchlorate can be well manipulated simply by controlling the pH of solution: very high uptake of perchlorate can be achieved at pH below 8.5, however, when pH is above 10 nearly all capacity will be lost due to the competition from the hydroxyl anions.