1 Introduction

Sulfonamide (sulfa) drugs are essential class of antibiotics for medication of diseases in humans as well as in animals [1, 2]. Despite, antibiotics are really required for treatment of humans, they are considered as one of the threatening pollutants for human health and ecosystem if they reach to the environment due to their involvement of complicated organic compounds [3,4,5]. The expired and unexploited sulfa drugs have several antagonistic impacts concerning humans and environment [6,7,8]. Thus, there is a great interest to ascertain operative and green treatment methodologies to remove or degrade such pollutants to safeguard human health. Traditional wastewater treatment plants were set to be ineffective for sulfa drugs degradation [9, 10]. However, antibiotics are known to impressively dispose to oxidation that can be a helpful method for antibiotics degradation [11,12,13,14,15,16,17,18]. Sulfafurazole or sulfisoxazole (SFZ) is one of sulfonamides that has antibiotic activity against Gram-negative and Gram-positive organisms. Little investigations were performed on the kinetics of oxidative degradation of sulfafurazole drug using different oxidants in various media [19,20,21].

There are many oxidants employed in oxidation reactions like permanganate, Mn(VII), [22,23,24,25,26,27,28,29,30,31,32,33,34,35], colloidal MnIVO2 [36,37,38,39,40,41,42,43,44], cerium(IV) [13, 14, 45,46,47,48,49,50,51,52], hexachloroplatinate(IV) [53,54,55,56,57,58], hexacyanoferrate(III) [59,60,61,62,63], etc. In addition, there are several investigations on the oxidation by chromic acid (H2CrVIO4) in acidic media [64,65,66,67,68,69,70,71,72]. Chromium (VI) oxide (CrVIO3) is considered as a substantial multi-electron oxidant commonly used in organic synthesis [17, 18, 73, 74]. It has a high toxicity to biological systems and carcinogenic but its reduced form, Cr(III), is approximately non-toxic [75, 76]. Thus, reductants can transform toxic Cr(VI) to Cr(III) compounds. Literature review revealed a paucity studies that interested on the oxidation kinetics by CrO3 [17, 18, 73, 74]. Therefore, our present research focuses on the description of the mechanism of CrO3 oxidation of sulfafurazole drug in both H2SO4 and HClO4 supported by a complete understanding of reactions’ kinetics. In this study, we investigated the influence of variation of the acidic medium used on the reactions’ kinetics and to understand the behavior of the reactants in these acids. Our research announce a hopeful appropriately and safe strategy with a twofold value for human and ecosystem: degradation of sulfafurazole drug and transformation of the toxic CrO3 to a relatively harmless Cr(III) compounds.

2 Results and discussion

2.1 Stoichiometry of the oxidative degradations of SFZ

The stoichiometry of oxidation of sulfafurazole (SFZ) using CrO3 in both acidic media was explored using spectrophotometry. Numerous sets of the reaction mixtures with various compositions of the main reactants (SFZ & CrO3 with a stock solution concentration of 0.1 M for each), at fixed [H+] (2.0 M) and ionic strength of 2.5 M, were reserved until realization of the reactions. Evaluation of unconsumed CrO3 showed a 1: 1.33 ± 0.07 stoichiometry (SFZ: CrO3); i.e. 3 mol of SFZ were reacted with 4 mol of CrO3. So, the reactions can be generally illustrated by Scheme (1).

scheme 1

The formation of Cr(III) was confirmed by the dark green precipitate, Cr(OH)3, which formed upon addition of (NH4)2S(aq) or NaOH solutions to the reaction medium.

2.2 Spectral changes

The spectral variations during sulfafurazole oxidation by CrO3 in 2.0 M of both H2SO4 and HClO4 solutions are introduced in Fig. 1(a) and (b), correspondingly. The documented spectra exhibited systematic disappearing of CrO3 band at λ = 349 nm with time. This behavior is a well-prove for oxidation of SFZ by CrO3 and reduction of the latter, Cr(VI), to Cr(III). Under similar circumstances, the decay occurred during the oxidation of SFZ in H2SO4 was discovered to be greater than that occurred in HClO4.

Fig. 1
figure 1

Spectral variations through CrO3 oxidation of sulfafurazole (SFZ) in H2SO4 and HClO4 at 298 K. [SFZ] = 0.1, [CrO3] = 8.0 × 10–4, [H +] = 2.0, I = 2.5 M

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2.3 Effect of [CrO3]

The impact of CrO3 concentration on the oxidation rates was examined by changing its concentration in reactions’ mixtures at fixed [SFZ], [H+], I and T. The results indicated non-substantial variation in the values of the rate constants (kobs, calculated as the slopes of the first order plots) at several [CrO3] as listed in Table 1. This situation illuminated that the reactions disclosed first order reliance in [CrO3] which also confirmed by the good linearity of (ln Abs. vs. time) plots in both acidic media as appeared in Fig. 2.

Table 1 Dependence of kobs on [CrO3], [SFZ], [H+] and I at 298 K
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Fig. 2
figure 2

Effect of [CrO3] on the oxidation rates at 298 K. [SFZ] = 0.1, [H +] = 2.0, I = 2.5 M

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2.4 Effect of [SFZ]

The rate constant kobs was determined at various [SFZ] whilst other constituents were preserved constant. Rising [SFZ] was set to enhance the rates of reactions as presented in Table 1. Linear plots of kobs vs. [SFZ] with positively kobs axes’ intercepts were obtained, Fig. 3(a). Besides, log [SFZ] vs. log kobs plots were straight with gradients of 0.76 and 0.81 in H2SO4 and HClO4, correspondingly, as shown in Fig. 3(b). So, these reactions were fractional-first orders in [SFZ] in both acids. Values of R2 and standard deviation (S.D.) are added in Table S1 in the supporting information.

Fig. 3
figure 3

Plots of: a kobs vs. [SFZ], b log kobs vs. log [SFZ] at 298 K. [CrO3] = 8.0 × 10–4, [H +] = 2.0, I = 2.5 M

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2.5 Effect of [H.+]

The impact of [H+] on the rates of reactions was explored by performing the experiments at several [H+] (1.0–3.0 M) in both H2SO4 and HClO4 preserving all other variables stable. Augmenting [H+] was set to enhance the rates (Table 1). Linear plots of kobs vs. [H+]2 with positively kobs axes’ intercepts were acquired as demonstrated in Fig. 4(a) signifying fractional-second order credence in [H+]. Also, log [H+] vs. log kobs plots were straight with gradients of 1.51 and 1.64 in H2SO4 and HClO4, respectively, as presented in Fig. 4(b), confirming the fractional-second order reliance in [H+].

Fig. 4
figure 4

Plots of: a kobs vs. [H +]2, b log kobs vs. log [H +] at 298 K. [CrO3] = 8.0 × 10–4, [SFZ] = 0.1, I = 2.5 M

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2.6 Effect of ionic strength (I) and dielectric constant (D)

To clarify certain information regarding to the oxidation mechanism, the influence of I on the rates of reactions was examined. So, the kinetic runs were performed at various values of I (2.5–4.0 M). In both acidic media, the gained outcomes signified that varying the values of I were set to have insignificant effects on the rates as listed in Table 1. Moreover, the impact of D was inspected by varying the water– t-butanol compositions (vol%) in the studied media. Results revealed that kobs values were appreciably unaffected with the increase in t-butanol content, reduce in D, as presented in Table S2.

2.7 Effect of [Mn(II)]

To explore the existence of Cr(IV) as one of the predicted intermediates during oxidation reactions by Cr(VI), Mn(II) was added to the reaction mixtures with its various concentrations at constant other reaction constituents. The acquired outcomes illuminated reduction in the oxidation rates with increasing [Mn(II)] confirming the existence of Cr(IV).

2.8 Effect of [Cr(III)]

The dependence of the oxidation rates on the reduction product of Cr(VI), i.e. Cr(III), was studied. For this, Cr(III) was initially added to the reactions’ mixtures with several concentrations, (4.0–12.0) × 10–4 M, at fixed other constituents. The acquired outcomes illuminated no noteworthy impact of addition of Cr(III) on the oxidation rates.

2.9 Effect of temperature

To evaluate the activation quantities, the oxidation rates were examined at different temperatures (288–328 K) at fixed other variables. The results illuminated that augmenting temperature was set to increase the oxidation rates as presented in Table S3. The activation quantities of k2 (k2 = kobs / [SFZ]), were computed (see Table 2) via Eyring and Arrhenius graphs as illustrated in Fig. 5(a) and (b), respectively.

Table 2 Activation quantities of k2. [CrO3] = 8.0 × 10–4, [SFZ] = 0.1, [H+] = 2.0, I = 2.5 M
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Fig. 5
figure 5

a Eyring, b Arrhenius plots of k2. [CrO3] = 8.0 × 10–4, [SFZ] = 0.1, [H +] = 2.0, I = 2.5 M

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2.10 Free radical intervention test

The feasible attending of free radicals in the present reactions was examined by supplement of acrylonitrile to the reactions’ mixtures, preserved for 4 h in an inert medium. Then, dilute the mixtures with methanol. These tests were positive where dense white precipitates were developed designating intervention of free radicals through the reactions signifying that the reactions were proceeded via generation of free radicals.

2.11 Reactions mechanism

It was stated [77] that CrO3 is hydrolyzed in water to form chromic acid as represented by Eq. (1)

$${\text{CrO}}_{3} + {\text{H}}_{2} {\text{O}}^{{}} \mathop \rightleftharpoons \limits^{{}} {\text{H}}_{2} {\text{CrO}}_{4}$$
(2)

The acquired positive free radical tests favored the involvement of Cr(V) species in the reactions. Also, the reduction in the oxidation rates with increasing [Mn(II)] verified the participation of Cr(IV) in the present reactions [67, 68].

On the other hand, in sulfa drug structures, two groups were identified as the protonation sites in acidic media [78, 79]. In sulfafurazole (SFZ), protonation occurs at the aromatic primary amino group and the anilinic NH2 group [80, 81]. The obtained strongly pH-dependent with fractional-second order dependence is considered as a significant prove for protonation of sulfafurazole (symbolized by S) which can be illustrated by Eq. (2),

$${\text{S}} + 2{\text{H}}^{ + } \mathop \rightleftharpoons \limits^{{K_{1} }} {\text{SH}}_{2}^{{2 + }}$$
(3)

where, SH22+ represents the protonated species of SFZ which is considered as the reactive species in the existing reactions’ kinetics. Also, the lower than unity order in [SFZ] refers to a complexation of the active species of both SFZ and the oxidant according to the following equation,

$${\text{SH}}_{2}^{{2 + }} + {\text{H}}_{2} {\text{CrO}}_{4} \mathop \rightleftharpoons \limits^{{K_{1} }} \left[ {{\text{SH}}_{2} - {\text{ H}}_{2} {\text{CrO}}_{4} } \right]^{{2 + }}$$
(4)

This was kinetically confirmed by the acquired positive intercepts in 1/kobs vs. 1/[S] plots [82] as presented in Fig. 6 (b). The obtained trivial effects of I and D on the oxidation rates accorded with the reactions happening amongst an ion and a neutral molecule [83, 84], i.e. between SH22+ and H2CrO4. The formed complex, [SH2 – H2CrVIO4]2+ (C), was gently decomposed in the slow stage producing free radical SFZ and Cr(V) reactive intermediates,

$$\left[ {{\text{SH}}_{2} - {\text{ H}}_{2} {\text{Cr}}^{{{\text{VI}}}} {\text{O}}_{4} } \right]^{{2 + }} \mathop {\mathop \to \limits_{{{\text{slow}}}} }\limits^{{k_{2} }} {\text{S}}^{.} ({\text{free radical}}){\mkern 1mu} + {\text{ 3H}}^{ + } + {\text{ H}}_{2} {\text{Cr}}^{{\text{V}}} {\text{O}}_{4}^{ - }$$
(5)
Fig. 6
figure 6

Confirmation of the rate-laws: a Eq. (5, b Eq. (6)

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This stage is followed by successive rapid stages to produce the final oxidative degradation products.

The suggested mechanism guides to derive the following rate-law expressions (see Appendix S1 in the supporting information),

$$\frac{1}{{k_{{{\text{obs}}}} }}\, = \,\left( {\frac{{{1} + K_{1} {\text{[H}}^{ + } {]}^{{2}} }}{{k_{1} K_{1} K_{2} [{\text{H}}^{ + } ]^{2} }}} \right)\frac{1}{{[{\text{S}}]}} + \frac{1}{{k_{1} }}$$
(6)
$$\frac{1}{{k_{{{\text{obs}}}} }}\, = \,\left( {\frac{{1}}{{k_{1} K_{1} K_{2} [{\text{S}}]}}} \right)\frac{1}{{[{\text{H}}^{ + } ]^{2} }} + \frac{{1}}{{k_{1} K_{2} [{\text{S}}]}} + \frac{1}{{k_{1} }}$$
(7)

Equations (5) and (6) prerequisite that the graphs: 1/kobs vs. 1/[S] at fixed [H+] and 1/kobs vs. 1/[H+]2 at fixed [S] must be linear with positively 1/kobs axes’ intercepts, as were acquired in both acidic media, Fig. 6(a) and (b), respectively, proving the legality of the proposed mechanism. The rate constant k1 and the equilibrium constants K1 & K2 at 298 K were evaluated via Eqs. (5) and (6) and are inserted in Table 3.

Table 3 Values of k1, K1 and K2 at 298 K
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2.12 Activation parameters

The obtained activation quantities are presented in Table 2. The high negatively ΔS# recommends construction of a rigid intermediate through the reactions [85]. Also, the positive values of ΔH≠ and ΔG≠ manifests that the intermediate construction in the rate-determining step, as proposed in the reactions mechanism, was endothermic and non-spontaneous, correspondingly [86]. The acquired higher Ea≠ illuminated that the slow stage was the decomposition of constructed complexes to yield the degradation products [17].

3 Experimental

3.1 Materials

Most employed chemicals in this research work were of Sigma-Aldrich and Fluka. Bidistilled water was used as a solvent to make all solutions. Sulfafurazole (99.6%) was utilized as supplied. Fresh solution of chromium trioxide, CrO3 (Sigma-Aldrich) was made prior to each run by dissolving the sample in bidistilled water and it was standardized spectrophotometrically [17, 18]. Solutions of H2SO4 (Fluka, 97%) and HClO4 (Sigma-Aldrich, 70%) were made by dilution with bidistilled water. Solutions of Na2SO4 and NaClO4 were made to fix the ionic strength (I) in H2SO4 and HClO4 solutions, correspondingly. t-Butanol was utilized to examine the impact of dielectric constant (D) of the reactions media.

3.2 Kinetic measurements

Kinetic measurements were conveyed out underneath pseudo-first order circumstances where [SFZ] >  > [CrO3]. The values of I of the reactions’ media were attained fixed at 2.5 mol dm–3. The progresses of the reactions were monitored via spectrophotometric tool by detecting the deterioration of the absorbance of Cr (VI) with time at λ = 349 nm using a temperature– accurate double-beam Shimadzu UV-1800 spectrophotometer. The first order rate constant plots (ln Abs. vs. time) were nearly linear and the values of kobs were calculated as the slopes of these plots. The mean values of about three distinctive runs were taken and were reproducible within ± 4%.

4 Conclusions

  1. 1.

    The kinetics of oxidative degradation of sulfafurazole (SFZ) by CrO3 in both H2SO4 and HClO4 were examined.

  2. 2.

    The reactions in both acidic media showed a 1: 1.33 ± 0.07 stoichiometry (SFZ: CrO3).

  3. 3.

    The rate of oxidation of SFZ in H2SO4 was discovered to be higher than that observed in HClO4. 4. The activation quantities were calculated and discussed.

  4. 4.

    A proposed mechanism for the oxidative degradation was made.

  5. 5.

    The rate-law expressions were also derived.

  6. 6.

    This research announce a hopeful appropriately and safe strategy for degradation of SFZ antibiotic to safeguard the human health and ecosystem.