Chapter 4 Figures
5.3 Results and Discussion
5.3.2 EDTA
Figures 5.11 and 5.12 show the concentration of solids and solution species versus time for dissolution of MnOOH by EDT A at two different pHs. In both cases only total Mn is shown as no Mn(III)EDTA complexes were detected in solution.
In these two experiments both excess of EDT A and pH were varied. Although this does not allow a clear interpretation of the effect of either variable it allows insight into both with fewer experiments. In both cases the dissolution is rapid and complete.
The reaction is much faster at lower pH and higher EDT A excess than at higher pH and lower EDT A excess. Rate constants were found using a first order rate expression to fit the solids concentration and the expression in equation 5.1 to fit total
Mn. The first order rate constant for pH 8.0 is 6.83
±
0.05 x 10-5 s-1, and the rate constant at pH 7.0 is 1.8±
0.8 x 10-4 s-1. These results are consistent with both the solution phase EDT A experiments and the pyrophosphate dissolution experiments.The rates of both Mn(IIl)EDTA reduction and dissolution of MnOOH have been found to be faster at lower pH. Mn(lll)EDTA reduction has also been found to be faster at higher EDTA concentration. Most likely; however, the pH is the dominant effect since EDTA is in such large excess that the surface is probably saturated in both cases.
Because the fit rate constant is independent of solids as explained in section 5. 3 .1.2, if the ligand has no effect on the rate constant then the change in rate constant can be considered wholly the result of pH changes. This would occur if the surface sites are saturated with ligand, which is likely. Considering this the case, the dependence of the rate constant on [I-I+] is approximately 0.4 order. This is very close to the result observed for pyrophosphate and is most likely the result of acid base chemistry of the surface.
It is interesting that the dissolution in the presence of EDT A is an entirely . reductive process, unlike that with the other ligands. This is not entirely surprising as the reduction of the Mn(III)EDT A complexes was found to be quite rapid in solution.
Yet here, there was no evidence of even a transient Mn(III) solution species. This could be because of the inability of the EDTA to bind more than 2 coordination positions at the surface, whereas the solution species would require 5 to 6 coordination positions to be occupied. Therefore on the surface the EDT A has at least four coordination positions free which could bind other redox active species and thus facilitate further electron transfer. This would allow the reduction to proceed even more quickly than in solution where there are no free coordination positions.
The reduction process could either be a surface binding of EDT A followed by reduction or it could be an outer sphere reduction reaction. The following candidate reactions are proposed (where not all species are balanced with respect to charge):
> Mn(III)OH + EDTA 4-
2 > Mn(III)OH + EDT A 4-
> Mn2 (IIl)EDTA3-
➔ Mn2+ + EDTA3- • + OH-
➔ > Mn2 (Ill)EDT A 3- + 2OH-
➔ > Mn(II)Mn(III)EDTA 3- •
> Mn(Il)Mn(III)EDTA3- • ➔ > Mn2(II)EDTA3-:
[5.16]
[5.17]
[5.18]
[5.19]
[5.20]
The EDTA radicals would further react, giving off CO2 and then reacting with oxygen to produce formaldehyde. The mechanism given by reactions 5.17 - 5.20 would be favored, given what is known about EDTA adsorption onto surfaces and reduction of solids(30-32). Although the solution reaction is outer sphere, it is the electron transfer away from the EDT A that is the rate limiting outer sphere reaction in solution. In solution because all 4 oxygen plus both nitrogens are coordinated to the metal any electron transfer must either be between the single Mn atom and the EDT A or it must be outer sphere. On the surface this limitation does not exist. EDTA is known to form bi and tetra nuclear complexes with iron(33,34). This should be possible on manganese¥ well. Xyla et. al. found that there were an average of 5 OH- sites per nm2 on y-MnOOH, close enough allow multinuclear binding. Therefore a second inner sphere electron transfer can take place thus eliminating the need for an outersphere transfer. Therefore the reduction is most likely occurring through surface binding. The pH data would also support reactions 5 .17 - 5 .20 as the mechanism. This proposed mechanism is shown in more detail in mechanism 5.1. Reaction 5.16 is a simple outersphere electron transfer and should produce a simple first order
dependence on pH. The observed data are much more consistent with an inner sphere adsorption of the ligand.
The same experiments with MnOOH and EDTA have recently been reproduced by Tian and Stone at the Johns Hopkins University (35). They used capillary electrophoresis to identify the products of the reaction. They observed (in addition to unreacted free EDTA) an Mn(Il)-EDTA species, EDTriA (ethylenediaminetriacetate), an Mn(II)EDTriA complex species, and an unidentified product peak. They saw no evidence of Mn(III)EDTA product, in agreement with the results presented here.
There are few data on the oxidation state of the solids because the solids dissolved so fast that it was difficult to collect enough sample to analyze. There did seem to be a slight drop in the average oxidation state of the solids from 3.0 to about 2.8. This would agree with the mechanism in equations 5.17 to 5.20 where the average oxidation state could be lowered if reaction 5 .20 proceeded slowly.