Environ. Sci. Techno/. 1995, 29, 3070-3071
Comment on "Nickel Adsorption to Hydrous Ferric Oxide in the Presence of EDTA: Effects of Component Addition Sequence"
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SIR: Bryce and co-workers ( 1 )discuss some characteristics of the nickel/EDTA/hydrous ferric oxide (HFO) system in their paper. Recently, some research has focused on the reactions of EDTA and metal-EDTA species (2-6). EDTA is of special interest regarding the mobility of heavy metals and especiallyradionuclides in soils and aquifers. Reactions at oxide surfaces and metal exchange in solution were found to be the major processes determining the speciation of EDTA. However, speciation of EDTAis often poorly defined in systems containing oxides that can be dissolved by EDTA. We believe that Bryce and co-workers ( 1 )did not pay enough attention to this point. We would therefore like to make the following remarks: Dissolution of iron oxides by umcomplexed EDTA has been studied for several crystalline iron oxides, mainly akaganeite Q3-FeOOH) (81, and goethite (a-FeOOH) lepidocrocite (y-FeOOH) (9). Bondietti et al. (9),for example, found a linear relationship between the adsorbed EDTA and the dissolution rate, yielding fast dissolution at low pH. However, these crystalline oxides were shown to be dissolving slower than HFO by several ligands (10, 11). Aggett et al. (12) found that freshly prepared HFO can be dissolved by EDTA within 10 min at pH 7 . Likewise, Borggaard (11)reported fast dissolution rates. Szecsody et al. (3)found a dissolution rate of 1.2 x lo-" mol of Fe m-L M h - ' at pH 4.5 (for 2.36 x 10-j M HFO and 8.51 x EDTA). This would give an FeEDTA concentration of 3 x M after 1 h under the conditions used by Bryce et al. ( 1 ) . The assumption of the authors that the formation of FeEDTA is insignificant under the described experimental conditions is therefore wrong. The adsorption isotherm of uncomplexed EDTA shown in Figure 3 is most likely the one of the FeEDTA and not the one of uncomplexed EDTA. Dissolution of HFO by metal-EDTA species has also been considered. Szecsodyet al. (3)have shown that this process takes place at low pH, where equilibrium is on the side of FeEDTA. In a system with NiEDTA in contact with HFO (log Kso -38.61, FeEDTA is the most prevalent species in equilibrium condition at pH below 5 , whereas NiEDTA is stable above pH 6 (Figure1). Dissolution of HFO by NiEDTA must be considered therefore only at low pH. Bryce and co-workers ( 1 ) used a system with preequilibration of nickel to the surface prior to the addition of EDTA. It can be seen in Figure 5 that NiEDTA is only slowly formed. They explain this by slow complexation of the adsorbed nickel to the uncomplexed EDTA. However, even in this system, fast formation of FeEDTA occurs. Bryce et al. (1) have stated that nickel and EDTA do not adsorb to the same surface sites. Therefore, adsorbed nickel should not change the dissolution kinetics. The formation of FeEDTA can change the kinetics of the ligand exchange reactions in the system to a great extent. FeEDTAin contrast to CaEDTA was shown to have very slow exchange kinetics with other metals (13). The half-life of the exchange of FeEDTA against zinc in river water was found to be about 20 days at pH 8. The rate-limiting step is the dissociation
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PH FIGURE 1. Speciation of l O V M EDTA in the system EDTA/nickel/ HFO (log Kso -38.6).
of the FeEDTA complex. In the system with the preequilibration of EDTA to the surface prior to the addition of nickel, dissolution also occurred, and the EDTA was present as FeEDTA. After the addition of nickel, again the slow exchange reaction of FeEDTA has to be considered. It is to be expected that, after a long equilibrium time, all EDTA will be present as NiEDTA at pH > 6. This could be verified by conducting experiments with equilibrium times longer than 50 h. For both systems (ligand-first addition and metal-first addition),the adsorption and desorption behavior of both EDTA and nickel is the same as can be seen from Figures 5 and 6. This can be explained by (a) fast absorption of nickel in both systems to strong surface sites; (b) fast adsorption of EDTA in the nickel-first addition system to weak surface sites; (c) fast adsorption of FeEDTA (which was formed during the preequilibration) in the ligand-first adsorption system to weak surface sites; (d)fast formation of FeEDTA in the nickel-first addition system and fast readsorption of the FeEDTA to weak surface sites; (e) slow exchange of dissolved FeEDTA with dissolved nickel: and (f) slow exchange of dissolved FeEDTAwith adsorbed nickel. Evaluation of the speciation in a system with EDTA and iron oxides should therefore always include the measurement of dissolved and adsorbed FeEDTA. In the system investigated by Bryce and co-workers ( I ) , this formation of FeEDTA is the main process influencing the behavior of the system. If only adsorption should be studied, a crystalline oxide like goethite would be better surface as for EDTA and several investigated by Nowack and Sigg (5) metal-EDTA complexes.
Literature Cited (1) Bryce, A. L.; Kornicker, LV. A.; Elzerrnan, A. LV.; Clark, S. B. Environ. Sci. Technol. 1994, 28, 2353-2359. (2)Jardine, P. M.; Jacobs, G. K.; O'Dell, 7. D. Soil Sci. SOC.Am. 1. 1993, 57, 954-962. (3) Szecsody, J. E.; Zachara, J. M.; Bruckhart, P. L. Etutron. Sci. Technol. 1994, 28, 1706-1716. (4) Gin~in,D. C.; Gassrnann, P. L.; Bolton, €1. Soil Sci. Soc. A n . 1. 1993, 57, 47-57.
(5) Nowack, B.; Sigg, L. 1. Colloid Interface Sci., in press. (6) Kent, D. B.; Davis, J. A,;Anderson, L. D.; Rea, B. A. In WarerRock Interaction; Kharaka, Y. K., Maest, A. s., Eds.; A. A. Balkema: Rotterdam, 1992. (7) Rueda, E. H.; Grassi, R. L.; Blesa, M. A. J. Colloid Interface Sci. 1985, 106, 243-246. ( 8 ) Rubio. J,; Matijevic, E. 1. Colloid Interface Sci. 1979, 68.408-
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(9) Bondietti, G.; Sinniger, J.; Stumrn, W. Colloids Surf: 1993, 79, 157- 167. (10) Deng, Y.; Stumm, W. Appl. Geochem. 1994, 9, 23-36. (11) Borggaard, 0. K. Clays Clay Miner. 1991, 39, 324-328. (12) Aggett, J.; Roberts, L. S. Environ. Sci. Technol. 1986, 20, 183186. (13) Xue, H.; Sigg, L.; Kari, F. G. Environ. Sci. Technol. 1995, 29, 59-68.
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Bemd Nowack* and Laura Swiss Federal Institute for Environmental Science and
Technology and Swiss Federal Institute of Technology CH-8600 Dubendorf, Switzerland ES950064N
VOL. 29, NO. 12, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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