Sunlight Photochemistry of Ferric Nitrilotriacetate Complexes Trevor Trott, Robbie W. Henwood, and Cooper H. Langford1 Department of Chemistry, Carleton University, Ottawa, Canada K1S 5B6
Photochemical degradation under natural water conditions of the acidic solution monomeric and the basic solution monomeric and dimeric complexes formed between Fe3+ and nitrilotriacetic acid (nta) is described. The degradation products are C02, CH20, and iminodiacetic acid (ida). The efficiency declines somewhat with increasing pH but quantum yields above approximately 0.01 may be expected in all natural waters. The evidence for such photodegradation serving as a mechanism to limit accumulation of nta in natural waters is discussed. B
fate of aminopolycarboxylate compounds (which all potential detergent builders and are important components of industrial wastes) in natural waters must be the fate of their metal complexes since complex formation constants (Ringbom, 1963) with all common metal ions are so large. The largest formation constants for nitrilotriacetic acid (nta) are those for formation of ferric complexes. Equilibrium calculations on a model of Lake Ontario (Childs, 1971) conclude that the fraction of nta bound in ferric complexes varies between 6 and 36% of the total nta in the water for total nta concentrations from 10-7 to 10_5M. These calculations probably underestimate the ferric nta species because they limit Fe to that found in the absence of nta and intentionally ignore solubilization of ferric oxide by
The
are
NTA.
We have undertaken a study of the photochemistry of ferric nta complexes using irradiation characteristic of sunlight. In initial experiments, solutions were prepared containing 0.010M Fe3+ and 0.010M nta (to provide convenient concentrations of ferric nta species). pH values were adjusted from 4 to 8 to cover the freshwater range indicated by Mason (1952) and to ensure examination of all of the hydrolytic forms of ferric nta (Childs, 1971; Gustafson and Martel, 1963). The solutions were enclosed in borosilicate glass flasks and irradiated outdoors in varying light conditions. In all experiments, precipitation of iron was complete in less than 14 days, indicating extensive degradation of the solubilizing agent, nta. No reaction was observed in dark controls. Examination of the stoichiometry of the photodegradation was developed following the hint provided by the known photodecarboxylation of carboxylic acids coordinated to Fe(III) (Balzani and Carassiti, 1970), and the observation of Hall and Lambert (1968) that ethylenediaminetetraacetate (edta) complexes of Fe(III) release C02 on irradiation. Solutions irradiated in a Rayonnet photoreactor equipped with 3500A fluorescence-coated mercury lamps to greater than 10% loss of nta were examined for products. Aldehydes were detected by SchifF’s test, phenylenediamine-H202 reagent, and finally a carbazole test. Taken together, these
1
indicate formaldehyde (Feigl, 1954). Solid derivatives of the aldehyde could be isolated. A 2,4-dinitrophenylhydrazone melting at 163.5-165.0°C and a p-nitrophenylhydrazone melting at 155-165°C confirm the identification of formaldehyde. An amine product was sought. A ninhydrin test was positive indicating an a or ß amino acid or a primary or secondary amine. This requires that nta has lost at least one —CH2COOH claw. By acidification of the solution, a white solid could be isolated which formed amine derivatives with phenyl urea and p-toluenesulfonyl chloride. (Melting points were ca. 195° and 65-65.5°C, respectively.) These derivatives correspond to those prepared from an authentic sample of iminodiacetic acid (ida) (mp, 195° and 65-67 °C). A quantitative determination of the ida yield was achieved using ninhydrin and the procedure of Moore and Stein (1948). A solution which had decreased in nta concentration by 0.0015M (according to the titrations described below) was found to have an amine concentration of 0.0016M. More than 90% of the nta nitrogen appears in an amine product (presumably all ida). Equation 1 is proposed to describe the photodecomposition of nta. [In the equation, the low pH form of Fc(nta) is indicated. Hydrolyzed species appear at higher pH.] 2[Fem(NTA)]
[Fen(NTA)]" + [Fen(iDA)] + CH2G + C02 [Fen(iDA)]
[Fc1ii(ida)]
(1)
The second step occurs in the presence of air and interferes with attempts to determine stoichiometry of the first step with respect to iron. Irradiations were carried out to confirm stoichiometry with respect to iron. Solutions were deoxygenated by bubbling through nitrogen for 20 min then sealing the flasks with serum caps. After suitable times of irradiation, samples were withdrawn with a syringe and analyzed for Fe2+ by the o-phenanthroline method (Hatchard and Parker, 1956) and formaldehyde by spectrophotometric determination of the 2,4-dinitrophenylhydrazone derivative dissolved in acetone. The ratio of Fe2+/CH20 is close to 2 after short irradiation times in the Rayonnet reactor (1.93 after 10 min, 2.20 after 20 min) but rises at longer times. (The puzzle was resolved by observing a secondary reaction, an acidified solution of formaldehyde and Fe3+ irradiated at 3500A showed reduction of Fe3+ and oxidation of CH20.) The stoichiometry of Equation 1 is confirmed by study of the Fe2+/CH2G product ratio. There remains the problem of the efficiency of the photoWe believe that the photochemical degradation of nta. reaction originates from a charge transfer excited state of the complex. Efficiency is not a simple matter over the range of pH found in natural waters because there is not a single
Table I. Values of the Quantum Yield-Related Function Y (Defined in the Text) as a Function of pH 7.00 7.65 6.10 4.25 4.73 pH 0.018 0.039 0.089 0.047 Y 0.078
To whom correspondence should be addressed. Volume 6, Number 4, April 1972
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ferric complex. At higher pH, Fe(NTA) forms an oxo-bridged dimer (Gustafson and Martel, 1963) so that efficiency may be expected to vary with pH. The efficiency was measured under irradiation with a ge mercury lamp, TP 109, which has strong emission at 3660, 4100, 4400, 5750, and 6500Á. The of reaction was the amount of nta decomposed. measure This was determined by titration of nta remaining after a period of irradiation by one of two methods. In the first, activity of iron in the solution was minimized by adjusting the pH to 9.4 and addition of an equal volume of 0.01M sodium acetate after which nta could be titrated with Cu(II) to a murexide end point. In the second, the Bi(III) complex of nta was formed after acidification of the solution to a pH of 1.00 and determined spectrophotometrically at 2430Á (Karadakov and Vankava, 1970). The best measure of efficiency is quantum yield but that is difficult to determine in the case of irradiation with a wide wavelength range source. As an approximation, we compare yields to the yields for the standard chemical actinometer system, ferrioxalate (Hatchard and Parker, 1956) and define a function related to quantum yield, Y, equal to the ratio of moles nta decomposed to the moles of Fe(II) produced from ferrioxalate multiplied by the quantum yield for reduction of ferrioxalate at 3660A. Values appear in Table I. The observed decline in Y with increasing pH may be associated with facile relaxation of the charge transfer states of the Fe(NTA) system in the dimer form where the spin forbidden character of radiationless transitions to the ligand field states is relaxed. We intend to discuss this issue elsewhere. The photochemical mode of nta degradation should be of significance in limiting the accumulation of nta (and most
probably related aminopolycarboxylates) in natural waters if it can be shown that the actinic light is available. Whether photosynthesis can occur in natural waters is essentially the same question since the actinic wavelengths are very close. The euphotic zone may vary from a depth of less than a meter in an unusually turbid river to over 100 meters in clear ocean water. Duntley’s review of near uv penetration (Duntley, 1963) suggests that the reaction described here would be important to depths of at least 10 meters in typical situations.
Literature Cited Balzani, V., Carassiti, V., “Photochemistry of Coordination Compounds," p 172, Academic Press, New York, NY, 1970. Childs, C. W., 14th Conference, International Association for Great Lakes Research, Toronto, Canada, April 19-21,1971. Duntley, S. Q., J. Opt. Soc. Amer., 53, 214 (1963). Feigl, F., “Spot Tests,” Vol. 2, “Organic Applications,” p 240, Elsevier, New York, NY, 1954. Gustafson, R. L., Martel, A. E., J. Phys. Chem., 67, 576 (1963).
Hall, L. H., Lambert, J., J. Amer. Chem. Soc., 90, 2036 (1968). Hatchard, C. G., Parker, A. C., Proc. Roy. Soc. (London), 235, 518 (1956).
Karadakov, B. P., Vankava, L. L., Tañíala, 17, 878 (1970). Mason, B., “Principles of Geochemistry,” p 158, Wiley, New York, NY, 1952. Moore, S., Stein, W. H., J. Biol. Chem., 176, 367 (1948). Ringbom, A. J., “Complexation in Analytical Chemistry," p 351, Interscience, New York, NY, 1963. Received for review May 24, 1971. Accepted October 21, 1971. We thank the National Research Council of Canada and the National Advisory Council for Water Resources Research for support. C. H. L. acknowledges an Alfred P. Sloan Fellowship for 1968-70.
CORRESPONDENCE
Uranium Concentrations in Surface Air Sir: The article “Uranium Concentrations in Surface Air at Rural and Urban Localities within New York State” (es&t, August 1971, p 700) states on p 703: “These levels are much less than the 90 and 300 ng U/m3 levels found near gaseous diffusion plants at Paducah, KY, and Piketon, OH, respectively.” These values are compared with values found in various parts of New York State ranging from 0.1 to 1.47 ng U/m3. The statement would lead a reader to conclude that the diffusion plants emit large mass quantities of uranium to the atmosphere; and, indeed, such a conclusion would be reached if one assumed that the radioactivity levels at the Piketon plant were 5% of the radioactivity concentration standards and that the source was normal assay uranium. However, neither assumption is valid. The official reports from the Piketon plant state that the average radioactivity level is “less than 5 %” of the applicable standards. A more significant error lies in the assumption that uranium discharged from the Piketon plant is of normal assay; and it is obvious that this assumption was made by the authors. The article reports all measurements in mass concentration units (ng U/m3), whereas 368
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all the data reported by the Piketon plant are given in radioactivity concentration units. To convert from one set of units to the other set, the uranium isotope assay must be known. If one assumes that the uranium is of normal assay and that the radioactivity ievel is 5% of the applicable standard, he will reach the same conclusion reached by the authors—i.e., 300 ng U/m3. However, the Piketon plant is an enrichment plant, and the concentrations of radioactive isotopes of uranium, other than U-238—i.e., the enrichment, may be as great as 94%. The specific activity of the enriched uranium may be 200 times as great as that of normal assay uranium. Obviously, then, the calculated mass concentrations, 300 ng U/m3, is incorrect, perhaps by a factor of 200—two orders of magnitude. I feel that this should be pointed out to your readers to correct any possible misinterpretation. Ben Kalmon
Industrial Hygiene & Health Physics Goodyear Atomic Corp. Piketon, OH 45661
