Chelating agents in high temperature aqueous chemistry. 2. The thermal decomposition of some transition metal complexes of nitrilotriacetate (NTA)

MEINDERT B O O Y ~ A N D THOMAS WILSON SWADDLE^ Depcii'tt7'rnt of Cl~enzistiy, T17e Unil.ersiry qf' Cr i lgn~y, Calgarl), Alfr i . , Ccincidrr T2N IN4

Received November 10. 1976

MEIUDERT BOOY and THOMAS WILSON SWADDLE. Can. J . Chem. 55, 1770 (1977). The kinetics and mechanism of decon~position of NTA complexes of Fe"', Feu, Con, Ni", and

CLI" under hydrothermal conditions (425-573 K) have been examined. The relative rates at 573 K are Col 'NTA- < N T A 3 5 F e l ' N T A < NiLINTA- < Fe"'NTAo < H,NTAo < CLI"NTA- < H,NTAT. Aqueous CoI1NTA- and Fel'NTA-, like NTA3-, deconiposed at 573 K by decarhoxylation, precipitating Co(OH), and Fe,04 respectively: Nil'NTA- pre- cipitated Ni(OH), initially but s~rbsequently Ni metal. At 530 K , Fe"'NTAo solutions pre- cipitated FeU,(NTAj2.H,O, but at higher te~iiperatures Fe304 formed, the NTA ligand being reduced to HCHO and iminodiacetate (IDA) rather than decarboxylated. Similarly, Cul'NTA- gave IDA and HCHO at teinperatures as low as 425 K, forming first Cu' (Lbhich precipitated as CuCI in the presence of C I ) and then nietallic Cu. The applicability of NTA to corrosion con- trol in boilers and to "DCo renioval from \\ater-cooled nuclear reactors is briefly considered. The half-life of FeU'NTA in the hydrosphere is estimated at 80 years ( c j : 8 x 10"ears for free NTA), in the abscnce of photolysis or biodegradation.

M E I ~ D E K T BOOY et THOZ~AS W~LSON SWADDLE. Can. J . Cben~ . 55, 1770 (1977). On a determine la cinetique et lc mecanisme de decolnpoiition des coinplexes de NTA avec

Fe"', Fe", Col', Xi" et du CLI" dans des conditions hydrothermiques (425-573 K). Les vitesses relatives i 573 K soiit C o L I N T A i P\lTA3- 5 F c U N T A < NiLINTA- < Fc"'NTAo < I-I,NTAo < Cul'NTA- < H,NTA+. Cornrne le fait NTA3-, des solutions aqueuses de C o l l N T A et de F e l ' N T A a 573 K se decomposent par decarboxylation et provoquent res- pectivement la precipitation de Co(OH), et de Fe,O,; au debut de la reaction avec Nil'NTA-, il y a precipitation de Ni(OH)2 mais au fur et a mesure que la reaction progresse, il y a precipita- tion de nickel nietallique. A 530 K, des solutions de Fel"NTA- proboquent la precipitation de Fe'J,(NTA),~H,O mais a des temperatures plus elevees, il y a formation de Fe30, puisque le ligand NTA est reduit en HCHO et iminodiacetate plut6t que d'Etre decarboxyle. De la mime maniere, Cu1INTA- conduit a IDA et HCHO 5 des temperatures aussi basses que 425 K con- duisaiit a la formation en premier de Cu' (qui precipite sous forme de CuCl en presence de C1-) et ensuite de Cu SOLIS forme metallique. On a considere les possibilites d'appliquer le NTA pour contr6lerlacorrosion dans des bouiiloires et pour enlever du 'j°Co des reacteurs nucleaires refroi- dis a I'eau. On estime que le temps de demi-vie du Fel"NTA dans l'hydrospherc est d'environ SO ans (par opposition a 8 x 106 ans pour le NTA a I'etat libre) en ['absence de photolpse ou de biodegradation.

[Traduit par le journal]

Inkoduction The possibility of using chelating agents, in

particular nitrilotriacetate (NTA), in the control of corrosion processes in boilers ( 1 , 2) and of cobalt-60 transport in ~vater-cooletl nuclear reactors (3) prompted our study (4) of the kinetics of decomposition of aqueous NTA over a wide range of temperature and pH. That study pro- vided incidentally an upper limit to the environ- mental half-life of free NTA reaching the hydro- sphere through its use as a builder in detergents;

such NTA may present a biological hazard through mobilization of heavy metals. or through decompos~tion to secondary anlines and thence to carcinogenic N-nitrosa~~iines (5-9). The kinetic data provided by the study (4) would not, honever, be directly applicable to conditions in boilers. nuclear reactors, or the hydrosphere if the decoinposition of NTA \+ere significantly accelerated or retarded by chelation of the metal ions which ~ o u l d inevitably be present.

Accordingly, the present article describes a setnl-quantltatlre study of the effect of chelatlon

'Present add1 ess Uni\er\ity Chernlcal Laborator], The Unl\ersltl of Kent, Canterbury, Englalld CT2 of lrol'("). lroll(l'l?. cobalt(ll)l and 7NH copper(l1) upon the kinetlcs and n~echan~sm of

2To xhonl correspoildence should be addressed thermal degradat~on of aqueous NTA. The

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BOOY A N D SWADDLE 2 1771

photochemical decomposition of FeHINTA (10, 11) has been shown to proceed by deacetylation (loss of -CH,COO-) rather than by decar- boxylation (loss of CO,). and the photochemical degradations of c u H N T A - (12) and FeT"- EDTA- ( 1 3, 14) have also been examined. The thermal decompositlon of aqueous NTA com- plexes, however, seems to ha le been little studied. although extenshe research In the U.S.S.R. has shown that EDTA complexes of Inany metal ions have half lives of several hours in \\ater a t 473 K. tlie relative kinetic stabilities being Na,CaEDTA - Na,MgEDTA > Na- FeH'EDTA > Na,H,EDTA > Na,CuEDTA (15, 16), and it has been found (2) that hydro- thernlal decompositlon of iron chelates in bo~lers can produce corrosion-resistant coatings of magnetite on steel surfaces.

Experimental Experimental procedures Lverc in general those

described previously (4). Unless othernise stated, analyt- ical grade hydrated transition metal chlorides vere used, and sho\\n to be of acceptable purity by com- plexonietric analysis using EDTA. Iron \\a5 determined by titration \\it11 K 2 C r 2 O 7 after reduction to iron(I1) ~ v i t h SnCI, (17). Metal chelates were prepared according to R i a b a l e e (18). Reaction niixtnres were made up by ~ ' e i g h i n g the reagents into the PTFE-lined autoclaves, a n d adding deoxygenated deionized \\ater and closing the vessels under dinitrogen.

F o r analysis of the chelating agents after reaction, the transition lnetal ions were removed by addition of sodium hydroxide solution and filtration n i th a sintered glass funnel. Copper(l1) mas removed by reduction n i t h a small ekcess of hydrazine and removal of the yello\\ product o n a sintered glass filter, but the buirer action of the excess hydrazine limited the use of this preliniinary procedure to the determinations of di- and tribasic acids using zinc(I1) (I), \\here the endpoints of the titrations a re at p H 6.0.

Results I . Nitr.ilofi.iucetntoiro,l (111)

This colnplex decomposed readily above 530 K with reduction of iron(1II) to iro11(11), yielding the solid FeH,(NTA),.H,O (see below) a t 530 K, and magnetite at higher temperatures. The rate of reaction was tlie sanie whether the starting material was FeT"NTA,H,O or an equi- inolar mixture of FeCI, \vith Na,NTA, i .e . , it was unaffected by the presence of Na' or CI-. The proton magnetic resonance spectrum of a solution, initially 0.3 MI in each of FeC1, and Na,NTA, showed after 2 h at 563 K that irni~lodiacetate (IDA, formed by deacetylation) and decarboxylation products (notably IV-

methyliminodiacetate, MTDA) were present in small but approximately equal amounts. This in- dicated that deacetylation mas of comparable importance to decarboxylation (in contrast to the decomposition of Na,NTA, etc. (4)), a conclu- sion rvhich \vas verified by the identification of formaldehyde as a major product, and that IDA and MIDA decomposed at rates sinlilar to Fe"'NTA and therefore did not accumulate as reactioii products.

The rates of disappearance of NTA were first order in [FeHTNTA], and first order rate coeffi- cients k were estimated ( t 2 0 z ) for solutions initially 0.1 nz in FeHTNTA and ionic strength 2.0 171 (with NaC1) from the expression

[1 1 kt - In ([NTA], [NTA],)

where [NTA], represents the total NTA con- centration at time t . Values of 10" k (sC1) of 0.22, 0.60. 0.94, 1.1, and 1.4 were obtained at temperatures of 503. 540, 554. 562. and 573 K respectively, and are represented to kvithin the experimental uncertainty by the enthalpy AH':' and entropy of activation given i n Table 1 .

When Fel"NTA was prepared in solution at 503 K using Fe(NO,), rather than FeCl,, or ~vhen NaNO, \\.as added as such in 1.5:l molar ratio to FeH'NTA, the rate of disappear- ance of NTA increased about tenfold, the solu- tion of products being bright yellow (due to iroii(ll1)) and a red. non-ferromagnetic precipi- tate (presumably x-Fe20,) being formed. This occurred even though NaNO, had no effect on NTA in the absence of iron(Il1): ~bhicll indicates that the role of nitrate in pron~oting the decom- position of FeH'NTA consisted in maintaining the iron in the trivalent state.

In an equimolar mixture of FeCI, and H,NTA, decomposition was almost complete in 4.5 h at 503 K, and almost all the iron was re- duced to iron(l1). Thus, hydrogen ion facilitates the redox (deacetylation) decomposition path- way, the decomposition being only about one- third co~nplete for F ~ " ' N T A alone under these conditions.

2. iVitif,.ilofr.iacetatofe~~nt~lI/) 1017

On heating a t 503 K: a solution 0.1 nl in FeCI, and H,NTA deposited the white solid Fe,(NTA),.H,O, which was identified by titra- tion and by its infrared spectrunl (lS), whereas a si~nilar solution of FeCl, and Na,NTA pre- cipitated a small amount of a black solid. The

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1772 CAN. J . C H E M . VOL. 5 5 . 1977

TABLE 1. Pseudo-first order rate coefficients and enthalpies and entropies of activation for the decornposi- tion of aqueous transition metal NTA complexes"

Experiluental k (s-I) temperature A H * AS* -

Species range (K) (kJ n ~ o l - ' ) ~ (J K-' mol-I)b At 473 K At 573 K

alnitial total [VTA] = 0.1 i n ; ionic ~ t r e n g t h maintained at h T A , bur otheruise nor coiitrolled.

uncertainties are standard errors of regression coeficients. Ckxtr i ip~lated val~le. OAt 566 K ; from )ields of F e 3 0 d . *Approximate value based o n assulnption of first order kine JSolurions of V a C u h T A . rRefcreiice 4.

black ferromagnetic solid product obtained at higher temperatures n-as identified by X-ray dif- fraction as magnetite, and was seen under the scanning electron microscope to be in the form of good octahedra up to about 50 pm across. The rate of disappearance of NTA was greater when [Na,NTA] : [FeCI,] was 2 : 1 rather than 1 : 1.

For a v ~ r i e t y of initial [Fel'NTA-1, the frac- tional extent of loss of NTA over a given time interval n.as the same, at a given temperature, so that the decomposition \\.as first order in [Fel'NTA-1. Unfortunately, a meaningful rep- resentatio~l of the time-dependence of the con- ceiltratioi~ of NTA surkiving in solution in terms of the integrated first order rate equation could not be obtained, since the main slow phase of the disappearance of NTA from the solution was preceded by an apparent accelerated initial stage (probably due to the deposition of a s~nal l amount of Fe,(NTA),.H,O, as noted above) and followed by a second accelerated phase. The latter probably originated in heterogeneous catalysis of the decomposition reaction by the precipitated magnetite: as it was established separately that aqueous Na,NTA decomposed to the extent of 35y0 in 2 h at 573 K under N, in the presence of added magnetite, but only 9;$ in its absence. This erect \vas not observed in the decomposition reactions of Fel"NTA, which were relatively rapid and qualitatively different from those of FeNNTA-.

On the other hand, the yield of Fe,O,, deter- mined by dissolving the black precipitate in con- centrated HCI and analyzing it for total iron

-21916 8 . 5 ~ 1 . 4 ~ 1 . 6 ~ 1 0 - " "

- 1 . 6 ~ 1 0 - ~ ' 5 x

- 6 9 k 7 1 . 1 ~ 1 0 - ~ 1 . 8 x 10-I' - 1 6 6 ~ 8 9 . 0 ~ 1 .1 x l 0 - j - 1 0 9 1 10 3 . 1 ~ 1 0 - ~ 5 . 3 x

+ I l k 3 2 7 . 5 ~ 9 . 6 x lo- ' '

2.0 in \kith h-aCl for FeU'NTA, F e U h T A - , and uncomplexrd

tics.

(17), indicated that the dissolved iron content declined smoothly in accordance with the integrated first order rate equation, with rete coefficient k = (1.6 + 0.2) x lo- ' s-I at 566 K. This rate of decomposition is of the same order of magnitude as that reported by Castle and Thornpson (19) for the decomposition of iron(I1) hydroxide in aqueous suspensions at 573 K ac- cording to the Schikorr reaction

The 'H nInr spectrum of solutions, initially 0.3 m in each of Na,NTA and FeCI,, decom- posed at 571 K for 2 h, showed the presence of decarboxylation products such as MIDA in small amounts: but 110 deacetylation products such as IDA could be detected except when the reaction had been carried out under air (when iron(ll1) was formed). These observations con- firmed that deacetylation in non-acidic solutions is associated with oxidation of the ligand by the metal ion.

The addition of HCl or NaOH to solutions of F ~ " N T A produced modest increases in the rate of disappearance of NTA; thus, at 571 K for solutions initially 0.1 m in FeC12 and Na,- NTA; 20% NTA was lost after 2 h, as compared with 35% with addition of an equimolar amount of HC1 and 42Pb with MaOH.

3. Nitrilotriutetatocobalt~~te(II) Iorz The initial decomposition of Co(I1)-NTA

complexes showed the same behaviour as that of Fe(l1)-NTA complexes. At 503 K, CoCI, + H3NTA gave Co3NTA,.H,0. Small amounts of

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BOOY A N D SWADDLE. 2 1773

brown solid, most likely oxidized Co(OH),, were formed upon heating solutions of CoC1, + Na,NTA at temperatures above 500 K. After 44 h at 573 K only 10% of the initial amount of cobalt was precipitated. In each case: direct com- plexo~netric titration showed that the extent of decomposition of NTA was equal to the extent of formation of Co(OH),. The ' H nmr spectrum after reaction for 8 h at 573 K showed the presence of NTA but only very small amounts of decarboxylation products and IDA: the latter most likely due to traces of dioxygen. The pre- cipitation of Co(I1) by hydroxide in the analyt- ical procedure (4) was incomplete. and reliable analyses for chelating agents could not be ob- tained following attempts to remove cobalt(I1) by this method. However, all other evidence showed that the Co(1l)NTA- complex was kinetically the most stable of the complexes studied.

4. Nitr.ilotr.iacctatot7iekeIate(II) Iotz When solutions containing equimolar amounts

of Na,NTA and NiC1, were heated at 573 K under N,, Ni(OH), was precipitated initially, but Lvas subsequently reduced to metallic nickel. After 44 h? over 80% of the nickel had been pre- cipitated as the -metal, and the solution was almost colorless. Co~nplexometric titrations showed that the NTA content of the solutions was only slightly in excess of the remaining nickel(I1) at any time, that is, that the reduction of nickel(I1) proceeded at essentially the same rate as the decomposition of NTA, and the two reactions are probably causally connected. The hydrotherlnal reduction of ~iickel(ll) to the metal by the chelating agents etkylenedi- amine and dieihylenetriamine at about 570 K has been observed in our laboratory by J. C. Arnold.

5. Nitr.ilotr.iacetatocuprate(II) Ion Of the divalent transition metal ions, cop-

per(I1) has the highest stability constant for the formation of an NTA complex, and this strong interaction manifests itself (for example) in the dissolution of copper tubing by aqueous NTA at pH - 1 I at room ten~perature, giving NaCu- NTA.H,Q (20). Thus, although copper(I1) often acts as an oxidant, e.g. of aliphatic amines (211, copper(1) colnplexes usually disproportionate to copper nletal and their copper(11) analogues be- cause of the high thermodynalnic stability of the latter (22).

It was therefore not surprising that aqueous CuI1NTA- decon~posed readily at 425 K to de- posit essentially pure copper metal; the color of the solution changed from blue to green to yellow as the reaction proceeded. Solutions of CuCI,, similarly treated, showed no change other than the precipitation of a small quantity of a basic copper(I1) chloride. When the solu- tions of Cul'NTA- were made up from solid NaCuNTA.H,O (20), the hydrothermally pro- duced copper metal took the form of a fine powder, whereas solutions made from equi- molar amounts of Na,NTA and CuCI, yielded initially solid CuCl and subsequently large particles of copper metal. The presence of C1- also slowed the reduction to copper metal significantly; at 433 K: the ainouiits of copper produced in 5 h in the presence of 0.0, 0.2, and 1.8 771 NaCl were 56: 52: and 217> respectively. In a highly acidic solution (initially 0.1 nz in each of H,NTA and CuCI,: pH,,, 1.2), how- ever, the solid product was CuCl (20%) and about 80% of the copper(I1) remained in solution after 2 h at 473 IS: in which time the decomposi- tion of NTA was about 50% complete; in this case, therefore, the latter reaction proceeded mainly by a pathway 170f involving reduction of copper(I1).

An excess of copper(11) over coinplexing agents in the products was also observed for mixtures of CuC1, and Na,H,EDTA(pH,,, = 2: and also when pH,,, = 6 with NaOH). CuCl was the main product but some Cu metal was also present. Non-oxidative pathways were ap- parently predominant and the decomposition of C~"EDTA'- was faster than the decomposition of Cu(1I)NTA under the same conditions. Iminodiacetatocopper(I1j gave CuCl and Cu metal in a ratio 2: 1 . The rates of decomposition at 433 K decreased in the order EDTA >> IDA > NTA.

The decomposition of the organic part of the complexes gave fornlaldehyde, and the mass spectrum of the gaseous products showed CO, and some CO. We also found that at 473 K aqueous CuCI, was reduced to CuCl by form- aldehyde (23). 'H nmr spectra indicated IDA as the major product and MIDA as a minor product. Only in the case of H,Cu(NTA), at pH,,, - 2.5 were significant amounts of amino- rnonocarboxylic acids (mainly sarcosine) and nlethyla~ilines (mainly dimethylamine) detected.

The aniount of aqueous NTA that was

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I I I

Na3 NTA t Cu CI2 1

1 2 0 1 2 3 TIME ( h o u r s )

FIG. 1. The deco~iiposition of Cu1INTA- at 473 K . [Cu2+], = [NTA3-], = 0.1 111. C, [Cu(O)],, copper rnetal formed; A, [L],; total amount of chelating agent; 0, [L], - [Cu(II)L],, uilcornplexed chelating agent; 0, [Cu(II)], = [L], - ([L]? - [Cu(II)L],); V, [Cu(I)], = 1.0 - [Cu(O)], - [Cu(II)],.

oxidized by an initially equimolar amo~in t of solid CuCl in 2 h at 473 K was about half that oxidized by the same molar ainount of cop- per(l1). The decomposition of 2CuC1 + Na,- NTA progressed someivhat further than for CuCI, + Na,NTA after 5 h at 433 K and no CuCl was detected among the products. but for 4CuCl + Na,NTA considerable amounts of CuCl were still present after the reaction.

In the decoinposition of aqueous NaCuNTA, or of CuC1, + Na,NTA, complete reduction of copper(I1) to the metal resulted in 75% loss of NTA. the remaining 2 5 7 of the reductio~l being accounted for by the further oxidation of the products JDA. HCHO, etc. This is consistent tvith the stoichiornetry

which also a c c o u ~ ~ t s for the observed initial fall in pH: eventually, however, the p H again rose because of the incursion of non-oxidative deg- radation of the aminoacids (e .g. , decarboxyla- tioii). The kinetics of the decompositions were followed at 433, 453, and 473 K (470 K in the last case for CuC1, + Na,NTA) with respect to various reactants and products, and are il- lustrated by Fig. l . The ainount of Cu(1I) re- maining in solution was calculated from the dif- ference between the total amount of com- plexing agent (determii~ed after precipitation of copper ions with hydrazi~ie by potentiometric

titration after addition of Zn(NO,),) and the ainount of free ligand (from direct titration of the reaction mixture with CuC1, and nlurexide as indicator). The disappearance of Cu(1I) followed closely the fortnation of copper metal. Formally, [CuiI)] can be expressed as the amount of copper not accounted for by copper metal and Cu(LI), and Fig. 1 shows clearly the initial build-up and subsequent decay of copper(1) ex- pected of it as an intermediate in a series of coilsecutive reactions.

- - The formation of~copper metal followed first order kinetics in [Cul'NTA-] to 80% conversion with an induction time of about 0.4 h. The rate for this process was faster for XaCuNTA than for CuCI, + Na,NTA, presulnablp because solid CuCl was formed as an intermediate in the latter case and its rate of disappearance was of the same order of magnitude as that of its formation. The rates of formation of copper metal \yere used to provide approximate first order rate coefficients k (f 15%) for the de- composition of Cul 'NTA-. For NaCuNTA, loq k (s-') was 0.7, 2.6, and 10 at 433, 453, and 473 K ; for Na,NTA + CuCl,, 10" was 0.5, 2.5. and 6 at 433, 453, and 470 K.

Discussion

Of the transition metal NTA complexes con- sidered here, only Col'NTA- decomposed by simple decarboxylation of the ligand and de- position of Co(OH),. without net reduction or oxidation of the central metal atom. This com- plex s h o ~ e d remarkable thermal stability, being

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BOOY A K D SWADDLE. 2 1775

less reactive than NTA"- itself by an order of inagnitude (Table 1). Thus, in the absence of complicating redox processes, coordination stabilizes the NTA3- ligand.

Nitrilotriacetatoferrate(II) decomposed ex- clusively by decarboxylation, as did the cobalt(T1) coniplex, but the product Fe(OH),, being un- stable in aqueous systems near 570 K, decorn- posed to magnetite (19), which appeared to catalyze further decomposition of the NTA to a moderate extent. At low pH, Fe(OH), forma- tion would be suppressed, and indeed we found the solid product formed under acidic conditions to be Fe,(NTA),.H,O, ~vhicli presunlably re- sulted from reaction of FeNTA- with the Fe2+(aq) released as the ligand decomposed.

Iron(I1I) oxidized the NTA ligand with the formation of IDA. formaldehyde, and CO,, whereupon the iron(11) product yielded mag- netite as above. The oxidation is rapid relative to the decarboxyiation of N T A 3 : etc., because the very favorable enthalpy of activation out- weighs the strikingly negative entropy of activa- tion in the temperature range of interest.

The decompositioii of Cul%TA- clearly pro- ceeded via a copper(1) intermediate, which in the presence of chloride yielded solid CuCl but otherwise disproportionated

Solid CuCl would act as a buffer for Cu(1) in solution. As noted above, the disproportionation of copper(1) is favored by interaction with ligands such as TVTA which strongly stabilize copper(I1). The stability constant of C u 1 ' 1 ~ A 0 is much smaller than that of Cu1'NTA- (by a factcr of at least 200, at 298 K (24)). so that the IDA formed by oxidation of NTA not only can- not compete effectively with the remaining NTA for coordination to copper(11) but is also less conducive than IVTA to reaction 4 even when coordinated. Consequently. IDA accumulated as the chief oxidative degradation product, while the products of lower niolecular weight (sarcosine and dimethylarnine) were formed by decarboxylation rather than further oxidation of IDA. The oxidation of the NTA ligand in CuNTA- and in FeNTAO probably proceeded

through' successive one-electron transfers, the first being rate determining, and indeed a mechanism of this type has been proposed by Carey and Langford (14) for the photochemical degradation of FcHIEDTA-.

The product distributions in the oxidations of NTA by metal ions were in accordance with the schemes proposed by Langford and co-workers for the photolysis of FeH'NTA (10) and Cult- NTA- (12), if it is recognized that their experi- ments were carried out under air and reoxidation of the reduced metal ions could occur (and; in particular, no copper(1) was isolated).

The studies described here and elsewhere (4) demonstrate that the high hydrothermal stabili- ties of NTA3-, FeIINTA-, and especially Col'NTA- augur well for the application of NTA as a decontaminating reagent for the re- moval of cobalt-60 ferrite deposits in the heat transfer systems of v~ater-cooled nuclear reactors under operating conditions of relatively high temperature and p H in conjunction with re- dacing conditions. Furthermore, the slow deposi- tion of magnetite, copper, and nickel from NTA complexes in solution may lead to the formation of cohesive protective coatings in place of mobile particulate col-I-osion products. On the other hand, heterolytic catalysis of NTA decomposition on magnetite surfaces was sug- gested by some of our observations, and radi- olysis of aqueous NTA has not been considered. Complete recovery of dissolved 60C01'NTA- from solution may also prove to be technically difficult. It is perhaps more likely that NTA will find application in corrosion control in conven- tional steam-raising.

Finally, the activation parameters of Table 1 predict that, while NTA itself is expected to have a thermal half-life of some 8 million years in the hydrosphere (assumed 288 K: ph ' 7) (4). coniplexing with the ubiquitous iron(IT1) will reduce its environmental half-life to about 80 years as a consequence of the low AH* for de- composition of FeHINTA. The correspondi~lg half-life for Cul 'NTA- is 3000 years. By con- trast, photochemical degradation of FeH1NTA or biodegradation of the free ligand should. when available, consume NTA pollutants in a matter of weeks (5-14).

Ackimswledgements We thank the National Research Council of

Canada, the University of Calgary, and the

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1776 C A N . 1. CHEM. 1'OL. 5 5 . 1977

Izaak Walton Killam Fund for Advanced Studies for scholarships (to M.B.), and Atomic Energy of Canada, Ltd., for general finailcia! support of this project through Whiteshell Nuclear Research Establishment.

1. J . R . METC.ILF. Boiler chelant treatment: an update. 31st Ann. Intern. Water Conf.. Pittsburgh. Penn. (1970).

2. T. C . MARGULOVA. 0. I. MARTINOVA, Y u . P. S A M O I L O ~ . and P. I>. MEDVEDEV. Teploenergetika. 65 ( 1974).

3. M. TOAILINSON. Chem. Can. 21 (October 1974). 4. M. BOOY and T. W. S W A D D L ~ . Can. J . Chem. This

issue. 5. P. S . T H A Y E R and C . J . KEIVSLER. C . R. C. Cnt . Rev.

Environ. Sci. 3. 375 (1973). 6. H . MOTTOL \ . Toxicol. Enbiron. Chern. Rev. 2. 99

( 1974). 7. A. E. MARTELL. Pure Apol. Chem. 44.81 (1975) . . 8. S. S. EPSTEIN. Intern. J. Environ. Stud. 2. 291 (1972):

3, 13 (1973). 9. A . H . P I C K A V ~ R . Soil Biol. Biochem. 8, 13 (1976).

10. T. TROTT. R. W. HEN\VOOD, and C. H. LANGFORD. Envil-on. Sci. Technol. 6 . 367 (1972).

11. R . J . STOLZBERG and D. N. HUME.. Environ. Sci. Technol. 9.654 (1975).

C. H . LANGFORD. M. WINCHAAI. and V. S. S ~ S T R I . Environ. Sci. Technol. 7.820 (1973). H. B. L O C K H A K ~ and R. V. BLAKELEY. Environ. Sci. Technol. 9. 1035 (1975). J. H. CAREY and C. H . LAXGFORD. Can. J . Chem. 51, 3655 (1973). Yu . E . LEBEDEV. Trans. Moscow Energy Inst. 126, 40 (1972). N. I . K U Z ' M E N K O and E. M. YAKIMETS. Trudy U1.al'sk Politekhn. Inst. 190, 76 (1970): Energetik, 14 (1974). A. I . VOGEL. Quantitative inorganic analysis. 3rd ed. Longmans, London. 1961. F. J . M. R ~ J A B ~ L E E . Spectrochim. Acta. A30. 891 (1974). J. E. C.ASTLE and R. G. THOMPSON. J . Appl. Chem. 17. 177 (1967). S. H. WHITLOW. Inorg. Chem. 12,2286 (1973). T . A. LANE and J. T. YOKE. Inorg. Chem. 15. 484 (1976). J . BJERRUM and E . J . NIELSEN. Acta Chem. Scand. 2. 297 ( 1948). J . J. BYERLEY and W. K . TEO. Can. J . Chem. 47,3355 (1969). A. E. MARTELL and R. M. SMITH. Critical stabilit). constants. Vol. 1 . Amino acids. Plenum Press, K e u York, N Y . 1974.

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