Kinetics and Mechanism of Oxidation of Diethanolamine and Triethanolamine by Potassium Ferrate

The kinetics of oxidation of diethanolamine and triethanolamine by potassium ferrate(VI) in alkaline liquids at a constant ionic strength has been studied spectrophotometrically in the temperature range of 278.2 K-293.2 K. The reaction shows first order dependence on potassium ferrate(VI), first order dependence on each reductant, The observed rate constant (kobs) decreases with the increase in [OH], the reaction is negative fraction order with respect to [OH]. A plausible mechanism is proposed and the rate equations derived from the mechanism can explain all the experimental results. The rate constants of the ratedetermining step and the thermodynamic activation parameters are calculated.


Introduction
Potassium ferrate is a powerful oxidizing agent in the whole pH range, it is widely used as a water treatment agent [1][2][3] in 1970s.It can remove the phenolic, sulfide and other organic pollutants [4][5][6] which are residual in wastewater and also can oxidize the cyanide into NO 2 -, NO 3  -and HCO 3 -which are harmless to environment [7][8][9][10] .Ferrate as a very effective, selective oxidant which can remove effectively H 2 S, CH 3 SH 2 and NH 3 etc 11 odor substances in biological sludge.The treated sludge can be used as chemical fertilizer and soil conditioner, it is propitious to waste resource utilization.
Applied prospects of ferrate oxidation are becoming hotspot of research 12 .Oxidability of ferrate is stronger than potassium permanganate, ozone and chlorine.As a new water treatment agent, it has a trend to replace chlorine-atom.In recent years, James Carr etc. used potassium ferrate as water treatment agent, while they studied self-decomposition of potassium ferrate in a wide pH range (pH=2.53-9.31) 13and the reaction of oxidizing a variety of organic matters 14 .They had proposed rate equation which is applicable to the majority systems.The rate equation includes self-decomposition of potassium ferrate and the reaction of potassium ferrate with the substrate.They also established new methods to deal with kinetic data of such reaction systems 15 .However, all studies were not put forward the reaction mechanism to explain the experimental facts.
Diethanolamine and triethanolamine have a very wide range of uses, they can be directly used as surfactants for detergents and cleaning agent formulations; they also can be used for gas purification agent to remove carbon dioxide or hydrogen sulfide gas.In addition,diethanolamine is raw materials to synthesize drugs and it is also a cross linking agent for production of high resilience polyurethane foam.Triethanolamine is also used as preservatives,water repellent, analytical reagent and PH value regulators.In this paper, the kinetics and mechanism of oxidation of diethanolamine and triethanolamine by potassium ferrate were studied in detail.

Experimental
All the reagents used were of A.R. grade.All solutions were prepared with doubly distilled water.Potassium ferrate (K 2 FeO 4 ) was prepared by the method of Thompson et al 15 .The concentration of K 2 FeO 4 was derived from its absorption at 507 nm (ε = 1.15×10 3 L•mol -1 •cm -1 ).The solution of K 2 FeO 4 was always freshly prepared before use.KNO 3 and the Na 2 HPO 4 buffer solution were used to maintain ionic strength and acidity of the reaction, respectively.Measurements of the kinetics were performed using a TU-1900 spectrophotometer (Beijing, China) fitted with a DC-2010 thermostat (± 0.1 K, Baoding, China).

Kinetics measurements
All kinetics measurements were carried out under pseudo-first order conditions.The oxidant and reductant were dissolved in buffer solution which contained required concentration of KNO 3 and Na 2 HPO 4 .The reaction was initiated by mixing the Fe(VI) to reductant solution .The reaction process was monitored automatically by recording the concentration decrease of all the Fe(VI) species with time (t) at 507 nm with a TU-1900 spectrophotometer.All other species did not absorb significantly at this wavelength.

Product analysis
After completion of the reaction, adding K 3 Fe(CN) 6 to the solution have non-experimental phenomena, while adding K 4 Fe(CN) 6 prussian blue precipitate was generated; by adding 2,2-bipyridyl methanol solution have non-experimental phenomena also.It proves that the final reduction product of Fe(VI) is Fe(III) 16 .After completion of the reaction, the oxidation product was identified as aldehyde which was precipitated as 2, 4-dinitrophenylhy drazone derivative.

Reaction intermediate
1,10-Phenanthroline was added to the reductant solution, then it was mixed with K 2 FeO 4 solution, purple disappears and at the same time orange appears, which indicates that Fe(phen) 3 2-has generated in the process of the reaction 16 .It proves that Fe(II) has once appeared in the reduction process of Fe(VI) to Fe(III).

Evaluation of pseudo-first order rate constants
Under the conditions of [reductant] 0 >>[Fe(VI)] 0 , the plots of ln(A t -A ∞ ) versus time t gives straight lines, details of the evaluation were described in our previous work 17 .

Rate dependence on [reductant]
At fixed [Fe(VI)], [OH -], ionic strength I, the values of k obs were determined at different temperatures.The k obs were found to be increased with the increase of reactant concentration.The plots of k obs versus [reductant] were linear.For the plots passed through the grid origin (Figure 1 and Figure 2), the reaction was first order with reductant.

Rate dependence on [OH -]
Under fixed [Fe(VI)], [reductant], ionic strength I and temperature, k obs values were decreased with an increase of [OH -], The order with respect to OH -was found to be negative fractional.The plots of 1/k obs versus [OH-] were liners (Figure 3 & 4).

Reaction mechanism
James Carr 14 has given the rate equation as follows where [S] represents substrate concentration.According to James Carr 14 , the first two terms are contribution of K 2 FeO 4 self-decomposition rate to the reaction system when there is no substrate.In this article, under the experimental conditions, the self-decomposition rate of K 2 FeO 4 is far less than oxidation rate of reductant reaction, so we get the rate equation: rate=k[FeO 4 2-][R].In essence, the results were consistent with James This experiment was performed at pH = 9.83 and 10.14, then there is HFeO both have a small percentage in the system.The concentration of -4 HFeO is small, but it is very easy to form complex with reductant in the presence of hydrogen atom and the complex has higher activity.Under the attack of hydroxyl, the complex dissociates into Fe(IV) and product, then Fe(IV) with another molecule of reductant further react to generate Fe(II) and product.Therefore, reaction is mainly through -4 HFeO to realize.According to discussion, the following reaction mechanism is proposed: 4) is the rate-determining step, where R stands for reductant.As the rate of the disappearance of [

2-4
FeO ] was monitored, the rate of the reaction can be derived as Equation ( 9) can be obtained from (3): Substituting equation ( 9) into (8), we can get the following equation ( 10 Equation (10) suggests that the reaction should be first order with respect to Fe(VI); equation (11) suggests that the order with respect to R is unity.The plot of 1/ k obs versus [OH -] derived from equation ( 12) at constant [R] is linear with positive intercept.These are consistent with the experimental phenomena.
Meanwhile, the plots of 1/k obs versus [OH -] were liner at different temperatures.From their slopes and equation ( 12), the rate-determining step constants (k 2 ) were evaluated and the thermodynamic activation parameters date were obtained (Table 1) 18 .It is noteworthy that according to equation (12) and Figure 3 & 4, we can get the values of k' under corresponding temperature and then, substituting the k', k 2 and [OH -] into equation (11), we can calculate the rate constants in corresponding [R], we found that the calculated value is very close to the experimental value (Table 2 and Table 3).This also illustrates the equation ( 12) is correct and the reaction mechanism we supposed is reasonable.

Conclusion
Based on the above discussion and results, we can know that the reaction of potassium ferrate with diethanolamine and triethanolamine both are completed by double-electron transfer.At the same time, we also observe the rate of the rate-determining step of triethanolamine is quicker than that of diethanolamine and the rate constants of the ratedetermining step of triethanolamine at different four temperatures are larger than those of diethanolamine.The effect of [OH -] and the activation parameters are all in support of the mechanism and consistent with experimental phenomena.
In the reaction system, we also observe that the activation energy of experiment is very small, but the entropy of activation has a big negative value.So according to the literature 19 , it is reasonable that the reaction rate is not too fast.

Table 1 .
Rate constants (k 2 ) and Thermodynamic activation parameters of the rate-