Ind. Eng. Chem. Res. 2008, 47, 1277-1282
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GENERAL RESEARCH Pure Metal Chelate Solutions by Ion Exchange George M. St. George,*,† Chen-Chou Chiang,‡ and David A. Wilson† The Dow Chemical Company, 2301 Brazosport BouleVard, Freeport, Texas 77541, and Calgon Carbon Corporation, 500 Calgon Carbon DriVe, Pittsburgh, PennsylVania 15205
Aqueous solutions of the industrially important metal chelate ferric N-(2-hydroxyethyl)ethylenediamine-N,N′,N′triacetate (FeHEDTA) have been made by adsorbing an iron salt (nitrate or sulfate) on Dowex MSC-1 cationexchange resin (Na form), flushing out the sodium salt, and eluting with Na3HEDTA. The resulting eluate contained FeHEDTA with iron concentrations approaching 2% that were free from sodium salts. Achieving industrially useful iron concentrations (>4.5%) required countercurrent ion exchange using an ISEP system. Careful manipulation of pH was required to prevent plugging due to precipitated iron oxide/hydroxide. Introduction Metal chelate solutions are ubiquitous.1,2 For example, iron chelate solutions are used in photography,3 gas treating,4-11 and plant12 and human13,14 nutrition. Chelate solutions of calcium, manganese, zinc, and copper, among others, are also used as micronutrients.15 Chelate solutions of lanthanide metals are used in diagnostic imaging.16 Chelates of certain radioactive metals are used in medical therapies.17,18 Metal chelate solutions are generally prepared by one of the following three methods: (1) a metal is reacted with an acidic form of the chelant in aqueous medium to generate the chelate solution and hydrogen gas; (2) a metal oxide, hydroxide, or carbonate is reacted with an acidic form of the chelant in aqueous medium to generate the chelate solution; or (3) salts of the chelant and a metal are combined in aqueous medium to generate the chelate solution, which also contains a byproduct salt. Method 1 generates solutions of the desired metal chelate essentially uncontaminated by byproducts, but the pure metal and/or the acid form of the chelant might be expensive, and the hydrogen generated introduces a significant fire or explosion hazard. Method 2 also generates essentially pure chelate solutions, although the cost of the acidic chelant might be a concern and the oxide, hydroxide, or carbonate might not react readily with the chelant. Method 3 suffers from the coproduction of a salt. If a pure chelate solution is desired, the salt must be removed. In addition, many salts are corrosive (e.g., halides), reactive (nitrates and perchlorates), or of poor solubility (sulfates).19 Consider, for example, the production of a solution of the ferric chelate of N-(hydroxyethyl)ethylenediamine-N,N′,N′triacetic acid (HEDTA, CAS no. 150-39-0), a commercial chelate used both as a micronutrient15 and as a catalyst for the removal of hydrogen sulfide from gas streams22,23and geothermal condensate.24 Methods 1 and 2 are hampered by the difficulty in obtaining the HEDTA acid (H3HEDTA).21 Whereas the trisodium salt of HEDTA (Na3HEDTA) is a readily available, * To whom correspondence should be addressed. E-mail:
[email protected]. Tel.: 979-238-3114. Fax: 979-238-0414. † The Dow Chemical Company. ‡ Calgon Carbon Corporation.
commercial chelant, its reaction with ferric salts (method 3) is complicated by the ferric salts available. The commercial chelate is prepared from ferric nitrate and Na3HEDTA. Ferric chloride would cause corrosion problems, and ferric sulfate would cause processing complications, because of the precipitation of sodium sulfate as the decahydrate “Glauber’s salt” (Na2SO4‚10H2O). The commercial FeHEDTA solution, then, contains 3 mol of sodium nitrate for each mole of active chelate.25,26 The sodium nitrate is at best an inert diluent and could be, under the wrong conditions, a potential hazard. Solutions of ferric nitrate also require special handling, as they must be kept warm to avoid crystallization and NOx vapors emanating from them must be scrubbed. The production of purified ferric chelates, including ferric HEDTA, using ion exchange in batch mode was demonstrated years ago. Wymore27 contacted a ferric salt solution with an ion-exchange resin (in Na form). Elution with water removed the salt byproduct. The iron-loaded resin was then contacted with a solution of Na3HEDTA. Elution with water removed the ferric HEDTA and returned the resin to the sodium form. A drawback to this method is its batchwise operation, which requires several time-consuming steps and large amounts of resin and water. Furthermore, the ferric HEDTA eluate is of relatively low concentration (ca. 1-2% Fe by weight) and requires energyintensive concentration (evaporation) to achieve a product with a more desirable iron concentration. We present an improved process whereby, using a series of ion-exchange resin columns to effect a countercurrent of resin vs eluents, metal chelate solutions of desirable concentration that are essentially free from salt byproducts can be obtained continuously. Additionally, we demonstrate that this process allows the replacement of expensive and reactive ferric nitrate with the much less expensive and reactive ferric sulfate in the production of FeHEDTA. Experimental Section Raw Materials. Na3HEDTA was provided as a 50% (w/w) solution by The Dow Chemical Company. H3HEDTA was purchased from Aldrich and TCI America. Solutions of HEDTA below pH 13.5 were produced by adding H3HEDTA to Na3HEDTA solutions and diluting with water. Ferric nitrate was
10.1021/ie061565s CCC: $40.75 © 2008 American Chemical Society Published on Web 01/17/2008
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Figure 1. FeHEDTA pilot-plant flow diagram.
provided by Blue Grass Chemical Specialties as a solution containing 11.8% iron by weight and stabilized with excess nitric acid. Ferric sulfate solution used in laboratory-scale experiments was produced by dissolving reagent-grade ferric sulfate hydrate [77% as Fe2(SO4)3] in water, resulting in a solution that was 11.4% Fe by weight. For the larger-scale (continuous) experiments, ferric sulfate was provided by Kemiron Corp. as solutions containing 12% iron by weight. (Solutions stabilized with 2% sulfuric acid and acid-free solutions were provided.) Dowex MSC-1 macroporous ion-exchange resin (sodium form) was provided by The Dow Chemical Company. Analytical Methods. pH measurements were performed using an Orion model 250A meter fitted with an Orion combination electrode calibrated at pH 7 and pH 10. Nitrate concentrations in eluate solutions were determined using EM Quant Nitrate Test papers. Ferric ion concentrations were determined by reaction with iodide in acid and titration of the resulting iodine with 0.1 N thiosulfate. Ferrous ion was determined by potentiometric titration with 0.1 N ceric sulfate. The concentration of HEDTA in iron-free solutions was determined by titration with 0.5 N calcium chloride, with ammonium oxalate as the end-point indicator. Excess HEDTA (relative to iron) in iron chelate solutions was determined by first oxidizing all the iron to Fe(III) using ammonium persulfate and then titrating with 0.1 N ferric chloride with salicylic acid as the end-point indicator. (Excess HEDTA is reported as weight percentage of H3HEDTA.) Single-Column Experiments. (Except where noted, all operations were performed at ambient temperature.) A 1.5-in.diameter glass column was packed with Dowex MSC-1 resin (Na form) and saturated with water. Fe(NO3)3 solution was diluted with water to about 2.5% Fe by weight and adsorbed onto the column. The column was then flushed with water until the concentration of nitrate in the eluate was less than 50 ppm, as determined by test strips. Na3HEDTA solution (1.05 mol/ mol of Fe) was diluted with water to about 15% by weight and then added. The column was eluted with water at a rate of ca.
1-2 mL/min. (“Slow” elution experiments involved an elution rate of ca. 0.5 mL/min.) The eluate was collected from the first sign of red color, indicative of the FeHEDTA complex, until the end of the color. Fractions were analyzed for Fe and HEDTA content. Experiments involving Fe2(SO4)3 were performed in essentially the same manner, but the sulfate concentration in the first flush was not measured. Continuous Ion-Exchange Process (ISEP Process). A 20port, pilot-scale ISEP unit (C-920) from Calgon Carbon Corporation (CCC) was used to demonstrate the feasibility of producing salt-free ferric HEDTA from ferric sulfate and Na3HEDTA using a continuous ion-exchange process. The heart of this unit is a single multiport rotating distributor containing 20 inlet and outlet ports. The distributor rotates at a specified rate using only one moving part. The stationary portion (Figures 1and 2, section A) of the distributor manages and directs all incoming and outgoing fluid streams into the appropriate zones and ports of the rotating portion of the distributor. The rotating portion (Figures 1 and 2, section B) of the distributor draws process fluid streams from the stationary component and feeds the columns containing ion-exchange resin. The columns (Figures 1 and 2, section B) are mounted on a turntable that rotates along with the distributor. During a 360° rotation, each resin column is passed through several zones/steps in an entire ion-exchange cycle. This cycle usually consists of loading, wash, regeneration, and rinse steps. Additional steps might be provided depending on the process complexity. This ISEP unit was equipped with 20 25.4 mm × 1000 mm long columns. The columns included polytetrafluoroethylene (PTFE) end caps and sintered glass top and bottom screens with PTFE-encapsulated silicone O-ring seals. Most of the tubing in the system was 1/8in.-o.d. perfluoroalkoxy resin. Valves and fittings were made from PTFE, polypropylene, or Hastalloy. Pumps were variablespeed piston type with wetted parts of stainless steel, ceramic, sintered carbon, and PTFE. Flow rates were computer-controlled using feedback from digital scales. Dowex MSC-1 resin was
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Figure 2. FeHEDTA pilot-plant flow diagram. Table 1. Preliminary Single-Column Experiments
expt
scale
2.5% Fe soln (g)
1 2 3
small small large
127 127 317.5
H2O flush (g)
15% HEDTA soln (g)
eluate soln (g)
eluate Fe concn (wt %)
Fe recovery (%)
HEDTA recovery (%)
HEDTA/Fe (mol/mol) in eluate
200 200 700
137.5 137.5 293.7
405.6 355.8 752.0
0.753 0.803 0.955
96 90 90
97 91 99
1.05 1.06 1.14
used in this pilot demonstration. It is a macroporous strongacid resin with a mean particle size of ∼500 µm. The process configuration is illustrated in Figures 1 and 2. In both figures, the configuration can be grouped into several zones. A brief description of each zone and its functionality is given below. Wash zone (Figure 1, ports 1-3; Figure 2, ports 1-3): The wash zone was used to wash out the Fe feed material. It consisted of three ports in series. The wash water was fed to port 1 directly to ensure a clean wash out. Loading zone (Figure 1, ports 4-7; Figure 2, ports 4-7 and 9): This zone was used to load Fe3+ cations onto the resin. It consisted of four or five ports in series. This arrangement was used to saturate the resin with the Fe3+ cations in the feed material. Entrainment reject (ER) zone 1 (Figure 1, ports 8 and 9; Figure 2, none): This zone was where part of outlet stream was fed back into the ISEP unit to remove the water entrapped between resin beads. Rinse zone (Figure 1, ports 10-13; Figure 2, ports 10-13): In this zone, rinse water was used to wash out regenerant in the columns and to prepare the resin for the loading operation. Regeneration zone (Figure 1, ports 14-17; Figure 2, ports 14-17): This zone was used to produce the major product, FeHEDTA, and to regenerate the resin. Entrainment reject (ER) zone 2 (Figure 1, ports 18-20; Figure 2, ports 18 and 19): This zone was where part of product
(FeHEDTA) stream was fed back into the ISEP unit to remove the water entrapped between resin beads. Acid wash zone (Figure 2, port 9): This zone was used to control the pH value in the loading section. Base wash zone (Figure 2, port 20): This zone was used to control the pH value in the regeneration section. Results and Discussion As described in the Introduction, metal chelates have manifold uses, from the commodity ferric ammonium EDTA in photographic processes to the specialty gadolinium DTPA in diagnostic imaging. No matter the scale, the challenge is to produce the desired chelate in a cost-effective, safe manner. We have examined the production of ferric HEDTA, a chelate of some commercial importance,15,22-24 whereby the product would be free from byproduct NaNO3. Wymore described a batchwise method of using ion exchange to obtain FeHEDTA in pure form, although the solutions made by his method are relatively dilute.27 We improved the method by using the ion-exchange resin in column chromatography. Our earliest experiments showed the feasibility of the method. As described in the Experimental Section, ferric nitrate was added to the top of a column containing a strong-acid cationexchange resin (Dowex MSC-1) in its sodium form. Elution with water removed sodium nitrate, leaving the resin saturated with ferric ion. Elution with a solution of Na3HEDTA removed
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Table 2. Base Case Ion-exchange Run with Fe(NO3)3 and Na3HEDTA fraction
weight (g)
Fe concn (wt %)
mmol Fe
mmol HEDTA
HEDTA/Fe
forerun 1 2 3 4 5 flush
148.8 92.0 104.4 105.8 104.1 102.7 690.9
0.026 0.324 1.00 1.76 1.84 1.02 0.083
0.68 5.33 18.7 33.4 34.2 18.8 10.3
0.68 5.33 23.0 38.8 37.7 20.0 10.9
1.00 1.00 1.23 1.16 1.10 1.06 1.06
Table 3. Results of Other (Single-Pass) Ion-Exchange Runs
run
chelant pH
T (°C)
elution rate
1 2 3 4 5 6 7
12 12a 8 8 8 12 8
25 25 25 25 50b 25 25
normal normal normal slow slow slow normal
resin
Fe recovery (%)
HEDTA recovery (%)
max Fe concn (wt %)
MSC-1 MSC-1 MSC-1 MSC-1 MSC-1 MSC-1 SST-60
93 80 90 97 74 93 59
100 94 91.6 100 90 100 94
1.81 1.10 1.39 1.79 1.28 1.82 1.20
a Chelant solution added in two increments. b Maintained using heating tape around the column.
Table 4. Single-Pass Ion-Exchange Runs Using Ferric Sulfate Fe Fe HEDTA max Fe MSC-1 chelant elution breakthrough recovery recovery concn run (g) pH rate (%) (%) (%) (wt %) 1 2 3 4 5 6a
160 206 260 260 260 260 *
8 8 8 13 9 9
normal normal normal slow slow slow
28 17 7 16 12 11
73 77 83 77 82 88
93 91 92 95 100 100
1.10 1.28 1.25 1.47 1.58 1.28
Used Fe2(SO4)3 solution supplied by Kemiron.
the iron as FeHEDTA, returning the resin to the sodium form. Most of the experiments were performed at a convenient laboratory scale, with ca. 159 g of the resin, 57.0 mmol of Fe solution, and 59.8 mmol of HEDTA solution. A few experiments were conducted on a larger scale (2.6×) to determine possible scale-up effects. Excellent recovery of both iron and HEDTA was possible, although the amount of water required to remove all of the chelate resulted in low overall concentrations of FeHEDTA. These preliminary experiments are summarized in Table 1. Given the variability in Fe and HEDTA recoveries, there do not seem to be any significant scale-up effects. More “industrial” solutions were obtained by rotary evaporation. Specifically, 300 g of the product from experiment 2 was evaporated to 50 g, so that the resulting solution (pH 3.3) contained 4.85% Fe and 1.24% excess HEDTA. Similarly, 300 g of the product from experiment 3 was evaporated to 59.5 g, and its pH adjusted to 5.54 with a few drops of 28% NH3
(commercial FeHEDTA solutions are adjusted with NH3). The resulting solution contained 4.71% Fe and 1.11% excess HEDTA. Several experiments were performed with a view toward maximizing the iron concentration in a “heart cut” of the eluate. As a base case, a run was performed as in the large-scale run above. A small amount of Fe breakthrough was observed as the nitrate was being flushed from the column. After addition of the Na3HEDTA solution, a forerun (approximately 150 mL) was collected until color was observed in the eluate. Thereafter, 50-mL fractions were taken until the eluate became faint pink. A final flush (ca. 700 mL) removed the remaining iron. At the end of the elution, a rust-colored band, presumably containing iron oxides or hydroxides, was observed at the top of the column. Analyses of the fractions are reported in Table 2. The recovery of Fe was 121.4 mmol (84.6%); the recovery of HEDTA was 136.4 mmol (91.6%). Several runs were performed at the smaller scale, with varying temperature, pH, and resin. Iron recovery, HEDTA recovery, and maximum iron concentration (most concentrated cut during the run) are recorded in Table 3. As can be seen from Tables 2 and 3, iron concentrations approaching 2% were obtained from the middle fractions. This is still considerably lower than the Fe concentration in commercial FeHEDTA (4.5-5%), but it suggested that a desired chelate concentration could be obtainable, if the conditions could be adjusted properly. The best results with regard to the Fe recovery and the highest heart-cut iron concentrations were obtained when the chelant pH was 8 and the elution was carried out very slowly (ca. 0.5 mL/min; run 4). On the other hand, comparable heart-cut concentrations were obtained when the chelant pH was 12 and/ or the elution was more rapid (1-2 mL/min; runs 1 and 6). Other variables had deleterious effects. Warming the column resulted in the more highly concentrated fractions coming off earlier rather than later but did not give more highly concentrated fractions than the runs performed at ambient temperature (run 5). Adding the chelant to the column in two increments reduced both the Fe recovery and the maximum concentration (run 2). Use of Purolite SST-60 resin in lieu of Dowex MSC-1 resulted in significantly lower iron recovery, implying that the former resin binds iron too tightly, competing with HEDTA for the iron (run 7). Elution with a partially acidified (pH 8) chelant solution also reduced the amount of precipitate at the top of the column. In fact, minimization of the precipitate proved important in the continuous, multicolumn runs, where plugging was a concern. Another aspect of using ion exchange to produce chelate solutions is the flexibility in the choice of reagents. Whereas ferric nitrate is used to produce FeHEDTA solutions commercially, it would be preferable to use ferric sulfate as the iron source, because of its substantially lower cost and ease of handling, not to mention the fact that disposal of a sodium sulfate byproduct stream should be easier than disposal of
Table 5. Phase 2: Impacts of Various Parameters on FeHEDTA Product Qualities conditions
results
run
wash water (mL/min)
Fe2(SO4)3 (mL/min)
rinse water (mL/min)
Na3HEDTA (mL/min)
ER2 (mL/min)
Fe content in product (%)
Fe recovery (%)
HEDTA/Fe ratio in product
1 2 3 4
25 25 25 25
4.7 5 5 5
25 25 22.5 22.5
7.8 7.8 7.8 7.8
17.5 17.5 17.5 16.25
3.7 4.5 5.4 5.2
94.3 93.6 93.1 92.3
1.7 1.16 1.06 1.08
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sodium nitrate. A few runs were performed using ferric sulfate solution (11.4% Fe) in place of ferric nitrate. Data from these experiments are reported in Table 4. In general, more resin was used to hold the iron; still, some iron “breakthrough” was observed during the water elution stage prior to the addition of chelant. As seen in Table 4, this resulted in lower iron concentrations than in the ferric nitrate runs, but it was still possible to achieve iron concentrations close to 1.5% readily. It was, of course, possible to evaporate water from the resulting chelate solutions to achieve the 4.5% iron concentrations of the commercial product. A less energy-intensive solution was desired, however, and so we turned to continuous, countercurrent ion exchange. A continuous, countercurrent ion-exchange process, compared to a batch process, has the advantage of continuous operation and the potential of producing more concentrated product. Plugging, however, is always an operational concern, especially with a system in which precipitation is known to occur. Therefore, the first series of runs was performed to demonstrate the feasibility of continuous production of FeHEDTA from ferric sulfate and Na3HEDTA using CCC’s ISEP unit. Initially, the configuration in Figure 1 was used. Shortly after the startup of the pilot run, solid precipitation was observed in several locations, and the system was forced to be shut down because of the high pressure buildup in the system. Apparently, the precipitation caused by the pH swing in the process, even though it is only a nuisance in the batch process, has a detrimental effect in the continuous process. To overcome this problem, the following modifications were made (Figure 2): (1) The wash water was acidified, and an acid wash zone was added in the ER1 zone with H2SO4 solution to maintain the pH in the loading section at ∼2. (2) A base wash zone was added in ER2 with NaOH solution to maintain the pH in the regeneration section at ∼11.0. This new configuration was operated for >120 h continuously without any precipitation and without any operating problems. From the regeneration zone, an FeHEDTA product stream that contained ∼3.7% iron was obtained (Table 5, run 1). Several more runs were performed to maximize the iron concentration in the FeHEDTA solution, and the results are reported in Table 5. During all of these operations, again, no precipitation was observed, and the operation ran smoothly, without any problems. As Table 5 shows, even in the first run, the iron concentration reached 3.7%, with an iron recovery of 94%. Both of these values are far better than those obtained from the batch process, although the iron concentration was still lower than the commercial target of 4.5-5%. After the ferric sulfate feed rate was increased by ∼6% to 5.0 mL/min (run 2), the iron concentration jumped to 4.5%. Upon further reduction of the rinse water rate by 10% (runs 3 and 4), the iron concentration reached 5.2-5.4% with an iron recovery of 92-93%. This iron concentration is already higher than the 4.5% iron concentration of the commercial product, making any further evaporation operation unnecessary. Therefore, it is feasible to produce FeHEDTA product with a commercially desirable Fe concentration (>4.5%) in CCC’s continuous ion-exchange unit (ISEP), using ferric sulfate and Na3HEDTA as the raw materials. Summary and Conclusions We have shown that it is possible to achieve pure chelate solutions with concentrations desirable for commercial use by countercurrent ion exchange. Although ferric HEDTA solutions were specifically described, it is likely that solutions of any
(soluble) metal chelate can be made by this route. This approach is particularly advantageous whenever the acid form of the chelant is expensive or unreactive and/or the metal, metal oxide, metal hydroxide, or metal carbonate is difficult to obtain or unreactive. It is also advantageous when an inexpensive metal salt that might otherwise create processing problems [e.g., Fe2(SO4)3] can replace a more expensive one [e.g., Fe(NO3)3]. Whereas Dowex MSC-1 resin was quite suitable for this system, other chelates might be produced more effectively with other resins. Maintenance of proper pH throughout the system is required for smooth operation. Acknowledgment We thank Gene Shull, Kemiron Corporation (now Kemira), for many helpful discussions. Literature Cited (1) Howard, W. L.; Wilson, D. A. Chelating Agents. In Kirk-Othmer Encyclopedia of Chemical Technology, 4th ed.; Kroschwitz, J. I., Ed.; John Wiley and Sons: New York, 1993; Vol. 5, pp 764-795. (2) Hart, J. R. Ethylenediaminetetraacetic Acid and Related Chelating Agents. In Ullmann’s Encyclopedia of Industrial Chemistry, 5th ed.; Gerhartz, W., Ed.; VCH: Weinheim, Germany, 1987; Vol. A10, pp 95100. (3) Kapecki, J.; Rodgers, J. Color Photography. In Kirk-Othmer Encyclopedia of Chemical Technology, 4th ed.; Kroschwitz, J. I., Ed.; John Wiley and Sons: New York, 1993; Vol. 6, pp 965-1002. (4) Asanuma, H.; Takemura, A.; Toshima, N.; Hirai, H. Adsorption of Nitrogen Monoxide by the Chelate Resin-Immobilized Iron(II) Complex and Its Application for Simultaneous Removal of Nitrogen Monoxide and Sulfur Dioxide. Ind. Eng. Chem. Res. 1990, 29, 2267. (5) Diaz, Z. H2S Removal from gas streams. U.S. Patent 4,518,576, 1985. (6) Blytas, G. C. Process for the removal of H2S and adjustment of the H2 /CO ratio in gaseous streams containing hydrogen sulfide, carbon monoxide, and hydrogen. U.S. Patent 4,536,381, 1985. (7) Bedell, S. A. Stabilized chelating agents for removing hydrogen sulfide. U.S. Patent 4,891,205, 1990. (8) Bedell, S. A. H2S abatement with stabilized chelates in geothermal drilling. U.S. Patent 5,096,691, 1992. (9) Bedell, S. A.; Myers, J. D. Removal of hydrogen sulfide from fluid streams. U.S. Patent 5,338,778, 1994. (10) Primack, H. S.; Reedy, D. E.; Kin, F. R. Method of stabilizing chelated polyvalent metal solutions. U.S. Patent 4,455,287, 1984. (11) McManus, D.; Kin, F. R. Hydrogen sulfide removal. U.S. Patent 4,622,212, 1986. (12) Chen, Y.; Barak, P. Iron Nutrition of Plants in Calcareous Soils. AdV. Agron. 1982, 35, 217. (13) Allen, L. H. Advantages and Limitations of Iron Amino Acid Chelates as Iron Fortificants. Nutr. ReV. 2002, 60, S18. (14) Heimbach, J.; Rieth, S.; Mohamedshah, F.; Slesinski, R.; SamuelFernando, P.; Sheehan, T.; Dickmann, R.; Borzelleca, J. Safety Assessment of Iron EDTA [Sodium Iron (Fe3+) Ethylenediaminetetraacetic Acid]: Summary of Toxicological, Fortification and Exposure Data. Food Chem. Toxicol. 2000, 38, 99. (15) Keys to Chelation; The Dow Chemical Co.: Freeport, TX, 2000. (16) Lauffer, R. B. Paramagnetic Metal Complexes as Water Proton Relaxation Agents for NMR Imaging: Theory and Design. Chem. ReV. 1987, 87, 901. (17) Goodwin, D. A.; Meares, C. F. Bifunctional Chelates for Radiopharmaceutical Labeling. In Radiopharmaceuticals: Structure-ActiVity Relationships; Spencer, R. P. Ed.; Grune & Stratton, Inc.: New York, 1981; pp 281-306. (18) Simon, J.; Wilson, D. A.; Volkert, W. A.; Troutner, D. E.; Goeckeler, W. F. Organic amine phosphonic acid complexes for the treatment of calcific tumors. U.S. Patent 4,898,724, 1990. (19) A reviewer has kindly pointed out a fourth alternative, i.e., the reaction of a metal salt with the acid form of the chelant, and has provided a literature preparation of FeHEDTA using ferric perchlorate and H3HEDTA.20 Although an excellent method for the production of small quantities of crystalline FeHEDTA, this approach would be unsuitable at an industrial scale for a few reasons. H3HEDTA is available from specialty houses, but it is not available in commercial quantities because of its difficult preparation compared to, say, H4EDTA.21 Furthermore, the use of a perchlorate would be unwise at large scale, especially given that the byproduct
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is perchloric acid. Although it is possible that other ferric salts would work as well as the perchlorate, there would still be the matter of disposal of the acid byproduct. Finally, the separation of the FeHEDTA by crystallization required the use of acetone, which, although recyclable, involves flammability and toxicity issues that the ion-exchange method avoids. (20) Meier, R.; Bedell, S. A.; Henkel, G. Structure of the Sevencoordinate Aqua-(N-hydroxyethyl)ethylenediaminetracetato Iron(III) Complex and Solution Coexistence Between Six- and Seven-coordinate Species. Inorg. Chim. Acta 2002, 337, 337. (21) Wilson, D. A.; Schmidt, R. W. Process for the preparation of carboxylic acid. U.S. Patent 4,212,994, 1980. (22) McManus, D.; Martell, A. E. The Evolution, Chemistry and Applications of Chelated Iron Hydrogen Sulfide Removal and Oxidation Processes. J. Mol. Catal. A 1997, 117, 289. (23) Chen, D.; Martell, A. E.; McManus, D. Studies on the Mechanism of Chelate Degradation in Iron-based, Liquid Redox H2S Removal Processes. Can. J. Chem. 1995, 73, 264.
(24) Sanopoulos, D.; Karabelas, A. H2S Abatement in Geothermal Plants: Evaluation of Process Alternatives. Energy Sources 1997, 19, 63. (25) Material Safety Data Sheet for Dissolvine H-FE-4.5 (MSDS #16075409), Akzo Nobel Functional Chemicals LLC. (26) Material Safety Data Sheet for VERSENOL Iron Chelate (MSDS #001538), The Dow Chemical Co. (27) Wymore, C. E. Process for preparation of metal chelates of aminopolycarboxylic compounds. U.S. Patent 3,172,898, 1965.
ReceiVed for reView December 6, 2006 ReVised manuscript receiVed November 9, 2007 Accepted November 13, 2007 IE061565S
