Kinetics and Mechanism Studies of Oxidation of Dibromothymolsulfonphthalein Toxic Dye by Potassium Permanganate in Neutral Media with the Synthesis of 2-Bromo-6-isopropyl-3-methyl-cyclohexa-2,5-dienone.

The oxidation of 3',3″-dibromothymolsulfonphthalein (DBTS) in neutral medium by potassium permanganate multi-equivalent oxidant has been studied spectrophotometrically. Pseudo-first-order plots showed inverted S-shape throughout the entire course of the reaction. The initial rates were found to be relatively fast in the early stages, followed by a decrease in the oxidation rates over longer time periods in the slow stage. Under pseudo-first-order conditions where [DBTS] ≫ 10 [MnO4 -], the experimental results showed a first-order dependence in [MnO4 -] and fractional-first-order kinetics in the [DBTS] concentration. The formation of 1:1 coordination intermediate complex prior to the rate-determining step was revealed kinetically. In addition, the intermediate species involving complexes of Mn(V) coordination has been detected. The oxidation product of DBTS was identified by Fourier transform infrared spectroscopy, ultraviolet-visible spectrophotometry, and gas chromatography-mass analysis. The obtained results indicated the formation of 2-bromo-6-isopropyl-3-methyl-cyclohexa-2,5-dienone as a derivative oxidation of DBTS.

Introduction

Bromothymol blue (BTB) is a chemical indicator for weak acids and bases with a molecular weight of 625 g/mol and a chemical formula of C27H28Br2O5S. The other name of this dye is dibromothymolsulfonephthalein (DBTS). DBTS has yellowish color in acidic medium, whereas the color gradually changes from green to blue when the pH increases. The main uses of DBTS are for testing pH, as a dye for painting plant tissue, in fish breathing tanks to determine the amount of carbonic acid, in biomedical applications,[1] and in the detection of lipids and phospholipids in thin-layer chromatography. In spite of the advantages of this dye, it can cause serious problems to human being such as the stimulation of the respiratory, digestive, skin, and eye diseases.[2] The removal of this dye from wastewater has been accomplished using a variety of techniques, including adsorption, sedimentation, chemical analysis, biological method coagulation, advanced oxidation, photodegradation, and membrane separation.[3−5] However, the techniques discussed above have several drawbacks. Biological approaches, for example, take a long time to decompose complex dyes, some commercial dyes are hazardous to specific microorganisms, and the process is not reusable.[6] The creation of colloids in wastewater during the coagulation process pollutes the environment.[7] Chlorine oxidation is a slow process that necessitates the use of reactive materials that are hazardous to carry and store.[8] As a result, there is a demand for more efficient and cost-effective methods of treating textile effluents that use the least amount of chemicals and energy.

The use of various oxidants such as potassium permanganate and hydrogen peroxide for the oxidation of toxic compounds is widespread. Organic compounds with carbon–carbon double bonds, aldehyde groups, or hydroxyl groups can be oxidized by potassium permanganate. A permanganate ion, being an electrophile, is strongly attracted to the electrons in carbon–carbon double bonds found in chlorinated alkenes, borrowing electron density from these bonds to generate hypomanganate diester, a bridging, unstable oxygen compound.[9] This intermediate product undergoes additional reactions such as hydroxylation, hydrolysis, and cleavage. Potassium permanganate has various advantages, including ease of handling and the fact that it is a readily soluble solid that is particularly successful in the treatment of water and wastewater.[10] Moreover, the oxidation by permanganate ions has several different pathways and is known as a multi-equivalent oxidant.[11,12] Once again, permanganate ions were also used as an oxidizing agent to purify water from toxic organic molecules[13,14] and to test the pharmaceutical formulations’ material. The kinetics of reducing permanganate ions by alcoholic polysaccharides in acidic[15,16] and alkaline solutions[17,18] have received much attention though.

The kinetics and oxidation processes of pectates,[18] methyl cellulose,[19] alginates,[20] carboxymethyl cellulose,[21] chondroitin-4-sulfate[22] carbohydrates, and BTB[23] by potassium permanganate have been studied in basic solutions. However, the oxidation of methyl cellulose,[15] pectates,[24] carboxymethyl cellulose,[25] BTB,[26]N-(2-acetamido)imino diacetic acid,[27] poly(ethylene glycol),[28] 2-butanol,[29,30] and mannitol[31] were investigated in acidic solutions. In these reactions, the pseudo-first-order plots were discovered to be reverse S-shape, and the reactions were found to proceed through free-radical intervention. However, to the best of our knowledge, the oxidation of DBTS dye using potassium permanganate in neutral medium has not been reported in the literature.

Therefore, the aim of the present work is the elimination of toxic DBTS coloring dye from wastewater by potassium permanganate in neutral medium using the kinetic method. The oxidation in neutral medium was found to proceed through two distinct stages. The first stage was relatively quick that was observed via a spectrophotometric detection of intermediate species involving complexes of Mn(V) coordination. The oxidation product of the cited redox reaction was identified by Fourier transform infrared spectroscopy (FTIR), ultraviolet–visible (UV–vis) spectra, and gas chromatography (GC)–mass and was found to be 2-bromo-6-isopropyl-3-methyl-cyclohexa-2,5-dienone (BIMCDO).

Experimental Section

Materials

All the chemicals that are used in this study are of analytical grade. Potassium permanganate (KMnO4) was obtained from BDH, U.K., and DBTS was purchased from Aldrich Chemical Co. Ltd. The chemicals were dissolved in bi-distilled water to prepare the corresponding solutions. The other reagents were prepared and standardized in the same way as that mentioned previously in these papers.[32−35]

Kinetic Measurements

All kinetic measurements were performed under conditions of pseudo-first order where DBTS concentration exceeded oxidant concentration ([DBTS] ≫ 10 [MnO4–]0). The measurements of the absorbance change were conducted on the PerkinElmer (Lambda 750) spectrophotometer using a cell path length of 1 cm. The estimation method is the same as previously used.[32,36]Figure shows the UV/vis spectrum of DBTS (1.0 × 10–3 mol dm–3) oxidation by KMnO4 (4.0 × 10–4 mol dm–3). After the two solutions are mixed, an observed decrease in the absorption peaks of permanganate ions at 525 nm confirmed that the oxidation reaction proceeds. The decrease in the peak of permanganate ions with time is taken to monitor the progress of the oxidation reaction because it did not overlap with the other absorption peaks of other reagents in the reaction mixture. After 33 min, the MnO4– peak disappeared completely, which indicated the formation of some intermediates. A new peak at 382 nm, as observed in Figure a, revealed the formation of the intermediate complexes. With increasing the reaction time, the absorption at 595 and 710 nm increases, which indicated the formation of Mn(VI) and Mn(V) intermediate species, respectively (Figure b).

Figure 1

Spectral changes (250–800 nm) in the oxidation of DBTS by permanganate ions in neutral medium. [MnO4–] = 4.0 × 10–4 mol dm–3 and [DBTS] = 1.0 × 10–3 mol dm–3 at 25 °C.

Figure 2

Spectral variations in the oxidation of DBTS by permanganate ions in neutral media. (a,b) [MnO4–] = 4.0 × 10–4 mol dm–3 and [DBTS] = 1.0 × 10–3 mol dm–3 at 25 °C (reference cell: [MnO4–] = 4.0 × 10–4 mol dm–3).

The absorbance–time plots revealed that the oxidation reaction was found to proceed through two separate different stages. The first stage is relatively fast, which corresponded to the development of intermediate coordination complexes [transient species of blue hypomanganate(V) and green manganate(VI)] (Figure b). In the second slow stage, the intermediate was slowly decomposed to produce soluble colloidal manganese(IV) and BIMCDO as an oxidation product.

Synthesis of BIMCDO

BIMCDO was synthesized by dissolving 6.24 g of the DBTS powder in 250 cm3 of bi-distilled water. To avoid the development of aggregates, the powder was added to the solution gradually while the solution was constantly rapidly stirred. After the DBTS powder dissolved completely, the solution pH was adjusted to 7. A 250 cm3 solution containing 1.58 g of potassium permanganate and 2 g of sodium fluoride was then added to the DBTS solution in two steps over 2 h. The reaction mixture was stirred at room temperature for 48 h, the produced MnF4 was filtered out, and the solution was concentrated using a rotary evaporator to one-fifth of its original volume. After drying under vacuum, the obtained powder was subjected to FTIR, UV–vis spectrophotometry, and GC–mass analysis.[37−41]

Results and Discussion

Stoichiometry

At room temperature, the oxidation reaction was proceeded with differing initial concentrations of DBTS and MnO4–. The unreacted permanganate ion was measured on a regular basis until a consistent value was achieved. A stoichiometric mean of 1.0 mol has been found ([MnO4–]unreacted/[DBTS]0), which agrees with the following stoichiometric equationwhere C27H28Br2O5S and C10H13BrO denote the DBTS and BIMCDO, respectively.

The FTIR spectra of DBTS and its corresponding oxidation product (BIMCDO) are shown in Figure For DBTS, the main characteristic bands can be ascribed as follows: 3481 and 3443 cm–1 (O–H stretching vibration), 2962 and 870 cm–1 (C–H stretching vibrations), 1605–1556 cm–1 (C=C stretching modes), 1473 and 1451 cm–1 (C–H bending vibrations), 1404 and 1381 cm–1 (C–O–H bending mode), 1347, 1158, and 1040 cm–1 (−SO3 group), 878 and 796 cm–1 (asymmetric and symmetric S–O–C stretching vibration), and 652 cm–1 (C–Br stretching vibrations). For BIMCDO, the characteristic bands can be assigned as follows: 3448 cm–1 (O–H stretching vibration), 2962 cm–1 (C–H stretching vibration), 1637 cm–1 (C=O stretching mode),[42] 1412 cm–1 (C–O–H bending mode), 1196 and 1021 cm–1 (−SO3 group very weak band), 1140 cm–1 (C=O bending mode), 1089 cm–1 (C–O stretching mode), and 618 cm–1 (C–Br stretching mode). The disappearance of the band located at 3481 cm–1 and appearance of the band at 1637 cm–1 in the FTIR spectrum of BIMCDO indicated the oxidation of the secondary alcoholic hydroxyl group (−CHOH) into the keto group (C=O).

Figure 3

FTIR spectra of DBTS and its oxidation product (BIMCDO).

Curves of Dependence of Time

The relations between ln absorbance versus time is presented in Figure . The obtained results are very unexpected, offering inverted S-shape curves showing that over the entire duration of the reaction, the oxidation kinetics are complex. The initial stage was relatively fast followed by sluggish stages with increasing the reaction time. This observation indicated that the oxidation reaction occurred over two distinct steps, involving the autoacceleration and induction processes, respectively. This action may be consistent with the following term of the rate law,[25] for the species that react rapidly and slowlywhere A is the absorbance at time t, A∞ is the absorbance at infinity, and constants P0 and B0 are absorption shifts. The rate constants of the autoacceleration period can be obtained by drawing a straight line throughout the fast stage and extrapolating the line back to zero-time P0. The ks of the slow stage was found from plots of the relation ln(A – A∞)(A∞ – A′) versus time (Table ). A typical plot is shown in Figure .

Figure 4

ln(absorbance)–time reciprocal plot in the oxidation of DBTS by permanganate ions in neutral medium. [MnO4–] = 4.0 × 10–4 mol dm–3 and [DBTS] = 1.0 × 10–3 mol dm–3 at 25 °C.

Table 1

Influence of Pseudo-First-Order Velocity Constants (kobs) on [MnO4–] and [DBTS] Materials in Neutral Medium Permanganate Ion Reduction by DBTS

	103kobs, s–1			103kobs, s–1
104[MnO4–]a, mol dm–3	kf	ks	103[DBTS]b, mol dm–3	kf	ks
2.0	1.33	0.077	1.0	1.27	0.075
4.0	1.27	0.075	2.0	1.82	0.160

[DBTS] = 1 × 10–3 mol dm–3.

[MnO4–] = 4 × 10–4 mol dm–3. Experimental error ± 4%.

Effects of the Reaction Rate on [MnO4–] and [DBTS]

ln(absorbance) against time plots showed that the redox reaction in [MnO4–] is of first order in sequence, with good straight lines for more than two half-lives of the end of the reaction. Not only pseudo-plotting but also independent of the oxidation rates at different initial permanganate concentrations ranging from 1 × 10–4 to 5 × 10–4 mol dm–3 have confirmed this effect. A fractional-first-order in [DBTS] was obtained from the relationship (ln kobs = n ln[DBTS]), as shown in Figure , where kobs is pseudo-first-order rate constants for fast and slow stages (kf and ks), respectively. The straight lines were obtained by plots of 1/kobs against 1/[DBTS], giving a positive intercept. The present redox system shows the creation of a 1:1 intermediate complex, as shown by Michaelis–Menten kinetics (Figure ). The values of kf and ks for both fast and slow stages have been determined using the least squares method and are summarized in Table .

Figure 5

Plots of ln kobs against ln[DBTS] in neutral medium to oxidize DBTS through potassium permanganate. [MnO4–] = 4.0 × 10–4 mol dm–3 at 25 °C.

Figure 6

Plots of 1/kobs vs 1/[DBTS] in neutral medium to oxidize DBTS through potassium permanganate. [MnO4–] = 4.0 × 10–4 mol dm–3 at 25 °C.

Although there has been a lot of research on the kinetics of permanganate ion oxidation of organic, inorganic, and alcoholic macromolecules in acidic solutions as multi-equivalent oxidants. Several problems about oxidation mechanisms in terms of electron transfer and intermediate states in rate-determining processes remain unresolved. Therefore, it is very important to know, whether the transition was carried out through a series of electron transitions (two electron transfer or more) or a sequential one-electron transfer procedure: Mn7+ to Mn6+ to Mn5+ in a sequence or Mn7+ to Mn5+ to Mn3+ in a single step. Therefore, it is important to know whether the pathways for the electron transfer process are outer-sphere or inner-sphere type.

According to the obtained kinetic results, the speculated oxidation mechanism of DBTS by permanganate ions involved attack of permanganate oxidants on the center of the substrate DBTS, giving intermediate complex (C1). This step was followed by the formation of second intermediate complexes (C2), which was decomposed slowly giving the final oxidation product and MnO2, as shown in eqs –5.where product is the BIMCDO and red is MnO2.

The rate constants were calculated as a function of the concentrations of substrate change using the following equationwhere [DBTS]T represents the total analytical concentration of the DBTS reducing agent. The rate-law expression is written as

Comparing eqs and 7 and rearrangement, the following equation was obtainedwhere *K′ = [MnO4–]/kK1[DBTS].

From eq , the plots of (1/kobs) versus (1/[DBTS]) were given straight lines with positive intercepts on (1/kobs) axes and small intercept observed in the Michaelis–Menten plot (Figure ), and this intercept could be negligible.

The disturbance of the alteration in spectra (Figure ) can suggest that the original quick portion of the reaction to oxidation is not the true phase of electron transfer. Therefore, the initial rapid part of oxidation may be attributed to a fast formation of an intermediate between the reactants. Moreover, several experiments have been performed to detect the hypomanganate(V) intermediate as a transient species. As seen in Figure b, we were able to detect intermediate Mn(V) formation, while increasing the absorption at wavelength 710 nm, which suggests the formation of intermediate Mn(V) complex. The suggested oxidation mechanism of DBTS by permanganate ions was discussed as the following: a fast attack of the permanganate oxidant on the center of the substrate, giving the intermediate complexes (C1) [DBTS–MnVO43–] prior to the rate-determining step. Such complexation is followed by the transfer of electrons from the substrate to the oxidant in the rate-determining step to give the second intermediate complex (C2) [DBTS–MnIVO44–] with the subtraction of H2O through the initial fast stage of oxidation. Then, the formed MnIV reacts with the substrate to form intermediate complexes, followed by the slowly decomposed intermediate complex, giving the final oxidation product and MnO2 (Scheme ).

Scheme 1

Mechanism of Oxidation of 3′,3″-Dibromothymolsulfonphthalein by Permanganate Ions in Neutral Solutions

UV–Vis Spectrophotometer Characterization

The UV–vis spectra of DBTS and BIMCDO are shown in Figure . The DBTS showed two peaks at λmax = 276 and 430 nm, whereas the BIMCDO exhibits three absorption peaks at λmax = 238, 270, and 420 nm. These results confirmed the formation of new compounds during the oxidation reaction.

Figure 7

UV–vis spectra of DBTS and BIMCDO samples.

Mass Spectra Characterization

GC–mass spectrum of the BIMCDO sample was recorded to confirm the oxidation product of DBTS by permanganate in neutral medium, as shown in Figure . The m/z = 230 suggested the formation of BIMCDO.

Figure 8

Mass profile of the oxidation product of DBTS (BIMCDO).

Conclusions

The oxidation of toxic 3′,3″-dibromothymolsulfonphthalein dye by MnO4– at pH ∼ 7 was studied using the UV–vis spectrophotometric technique. By plotting ln (absorbance) against time, the inverted S-shape curve shows that the redox reaction occurs through two distinct stages. The first stage is relatively fast followed by decrease in the oxidation rates, called the induction period, which was shown to be linear over a longer period. Kinetic evidence for the formation of 1:1 intermediate complex has been observed. The increase in the absorption peak at λmax = 710 nm is observed, suggesting the development of intermediate hypomanganate(V) as a transient compound. The oxidation product of this dye was confirmed by a variety of techniques such as such as FTIR, UV–vis, and GC–mass. Finally, the results obtained from this paper showed that it can easily remove toxic dyes form wastewater by a simple oxidation process using potassium permanganate.