Industrial wastes from oil refinery and petrochemical industries, characterized by the presence of inorganic sulfur-containing pollutants such as sulfides, thiosulfates, sulfates and elemental sulfur, are held responsible for their high toxicity. The most fatal element of wastewater is sulfide sulfur, a toxin which causes severe damage to aquatic life . Therefore, the removal of sulfide sulfur from aqueous media has been investigated using different methods such as adsorption, precipitation, electrochemical or biological methods . Noticeably, chemical oxidation methods are most widely used to get rid of sulfide sulfur in wastewater since sulfide reacts with strong oxidizing agents (for instance, KMnO4, K2Cr2O7, O3, Ca(Cl)2O2, H2O2 and Cl2) to form sulfate. However, the cost of the chemicals and the pollution possibly caused by the oxidation agents are several disadvantages to consider [3-7].
As shown by previous studies [4-17], the most effective method of sulfide sulfur treatment in the wastewater involves its oxidation in an atmospheric oxygen in the presence of a catalyst. Several heterogeneous and homogeneous catalysts have been employed to catalyze the reaction between toxic sulfide and oxygen, forming less harmful sulfur compounds such as thiosulfate and sulfate. Transition metals and their compounds are also considered as the potential catalysts for selective oxidation of sulfide in a variety of industries. For example, metal oxides such as Fe2O3, CuO, Cr2O3, MnO2 supported on polymer matrix are extensively used for sulfide oxidation in wastewater refinery . Similarly, polymeric metal phthalocyanines and cobalt phthalocyanine sulfonates coordinately bound to polymers are also applied to oxidizing sulfides . Spinel-structured iron oxides such as ferrit MFe2O4 (M= Cu2+ and Co2+), magnetite (Fe3O4) and maghemite (Fe2O3) are also employed to catalyze sulfide oxidation . Lastly, vanadium antimonate VSbO4 [8,9] and metal chalogenides (CdS)  are used as catalysts for oxidizing sulfides. However, it is seen that in these methods, toxic substances of the catalysts, which are heavy metals, will partly dissolve in wastewater. In terms of financial issues, the removal of these compounds from wastewater during sulfide treatments can be very expensive. For these reasons, the development of an optimal and non-toxic catalyst for treating the sulfide-containing wastewater is critically important.
On another note, non-toxic and economical catalysts based on quinone-compounds such as 1,4-naphthoquinone, 1,2-naphthoquinone-4-sulfonic acid sodium salt, polynaphthoquinone, benzoquinone, duroquinone have been recently applied to sulfide sulfur purification, particularly in Petrochemical Industry of both Japan and the UK. However, the disadvantages of these catalysts include low catalytic activity, high catalyst losses and the complexity of the catalyst separation after oxidative treatments [11-16]. With the view to overcoming these drawbacks, we propose a new quinone-catalyst based on 3,3′,5,5′-tetra-tert-butyl-4,4′-stilbenequinone (hereinafter referred to as stilbenequinone or С30H42O2) that performs high catalytic activity, selectivity and stability in alkaline media. A straight-run kerosene fraction is used as the catalyst carrier owing to its low solubility in water  and its high boiling point . Furthermore, the good solubility of the catalyst in this kerosene fraction is taken into account. Consequently, the losses of the catalyst and its carrier during purification can be minimized.
The purpose of the present work is to establish a mechanism and kinetics of liquid-phase catalytic oxidation of sulfide sulfur in the presence of stilbenequinone.
The catalyst component — 3,3′,5,5′-tetra-tert-butyl-4,4′-stilbenequinone (С30H42O2) was synthesized as the procedure described in .
All the chemicals used in the experiments were reagent grade, including sodium sulfide (GOST 2053-77), technical gaseous oxygen in cylinders (GOST 5583-78) and kerosene fraction (GOST 10227-2013). The initial sodium sulfide concentration was varied from the range of 0.097 to 0.714 mol L-1 by dissolving an adequate amount of Na2S.9H2O (Russia, GOST 2063-77) in deoxygenated distilled water.
The catalytic oxidation of inorganic sulfide was performed inside a 150 mL of three-neck-round-bottom flask. The mixture of sodium sulfide solution with its concentration of 0.143 mol L-1 – 0.714 mol L-1 and kerosene fraction, whose volumes were 40 mL and 20 mL respectively, was loaded into the reactor in the presence of a certain amount of the catalyst component. The reaction mixture was magnetically stirred at a constant speed of 500-1400 rpm. The oxygen from the cylinder was injected into the reaction mixture at 0 – 13 L h-1. The reaction temperature was maintained in the range of 50°C to 90°C by a thermally controlled magnetic stirrer.
The oxidation of inorganic sulfides by stilbenequinone was implemented in an inert atmosphere in a steel autoclave (150 mL) by using 40 mL of sodium sulfide solution with insignificant concentraton (0.097 mol L-1) and 20 mL of kerosene in the presence of an excessive amount of stilbenequinone (0.373 mol L-1). The reaction temperature varied from 50°C to 110 °C and the constant stirring was set at 1400 rpm in all experiments. Using a low concentration of inorganic sulfide and an excess of the stilbenequinone, this reaction is guaranteed to occur completely.
The quantitative content of sodium sulfide was identified by potentiometric titration method which was in accordance with UOP-209-00 (USA). Meanwhile, the sodium sulfate concentration was established by using spectrophotometry (Eros PE5300B, regime A, wavelength l=450 nm, path length L=50.0 mm) . The concentrations of hydrogen peroxide concentration, as well as of thiosulfate and sulfite were measured by iodometry . Infrared spectra (IR) of substances were recorded on the Perkin Elmer Spectrum Two FT-IR spectrometer. Melting point of substances were determined in open glass capillary tubes on a Buchi M-560 instrument. The concentration of 3,3′,5,5′-tetra-tert-butyl-4,4′-stilbenequinone was ascertained by spectrophotometry (Eros PE5300B, regime A, wavelength λ =500 nm, path length L=10.0mm).
Results and Discussion
The oxidation of sulfide sulfur in the presence of a stilbenequinone-based catalyst proceeds in a three-phase system «oxygen — kerosene fraction — aqueous alkaline solution of sulfide sulfur». Therefore, a sequence of experiments was conducted for the sake of process control. Specifically, the initial volumetric concentration of oxygen was varied by diluting with argon and variations in the rotational speed of the stirrer were also made from 500 to 1400 rpm whereas the initial reagent and catalyst concentration remained unchanged.
Fig. 1 shows the kinetic curves of Na2S oxidation at different stirring speeds in the presence of a catalyst based on stilbenequinone. According to the effect of stirring speed, the catalytic oxidation rate of sodium sulfide is technically influenced when the rotational speed of the stirrer varies between 500 and 1200 rpm. This means that the catalytic reaction occurs under diffusion control. On the other hand, the catalytic reaction rate is constant when the stirrer reaches a speed of 1200 rpm or over, indicating that the reaction is kinetically controlled. Therefore, a rotation rate of 1400 rpm was chosen for all subsequent experiments.
Fig.2 illustrates how the catalytic oxidation rate of sodium sulfide is influenced by both the feed rate of oxidant gas (a mixture of oxygen and argon) and its oxygen concentration. It is shown that at the stirrer rotation of 1400 rpm, the sulfide oxidation in the presence of stilbenequinone occurs in the kinetically controlled region which is bounded by the curve y = 0.0376x -1.66 and the line y = 0.0133 (Fig. 2).
The chosen ratio of kerosene fraction to sulfide solution is 1:2, which is optimal for catalytic sulfide oxidation in the presence of stilbenequinone. This stems from the maximum interface area between aqueous phase and liquid hydrocarbon phase (Table 1).
It is significant to study the kinetics of the catalytic oxidation of sulfide sulfur in the presence of stilbenequinone. To that end, a differential method with varying initial concentrations of reagents was adopted so that the order of the reaction with respect to reactants could be identified.
Figure 3 demonstrates the kinetic curves of sodium sulfide oxidation at its various initial concentrations and the plot of the logarithmic initial rate of this reaction (lgv0) against the logarithm of initial sulfide concentration (lg[Na2S]). The initial reaction rate is derived from the slope of a plot of the sulfide concentration against time. It is evident that the plot of the logarithmic initial oxidation rate of Na2S versus the logarithmic initial concentration of Na2S is straight line with a slope of 0.9509, i.e. the reaction has the first-order with respect to Na2S. This conclusion can be firmly reached thanks to the employment of an integral method. The linear dependence of the logarithm of the Na2S concentration upon the time lgCt = f(t) (Fig. 4) indicates that the liquid-phase catalytic oxidation of sulfide sulfur in the presence of stilbenequinone follows the rules of the first-order reaction in sodium sulfide.
A series of experiments was performed to analyze the effect of catalyst concentration on the sulfide oxidation, in which the initial amount of stilbenequinone in the kerosene fraction varied from 0.0011 mol L-1 to 0.072 mol L-1 (Fig. 5). It is seen that the initial oxidation rate of sodium sulfide is directly proportional to the stilbenequinone concentration in the kerosene fraction. However, this does not apply when the amount of stilbenequinone in the kerosene fraction is above 0.040 mol L-1. In the latter case, the initial oxidation rate of sodium sulfide remains constant. Based on the linear dependence of logarithmic initial rate of the sulfide oxidation (lgν0) on logarithm of initial catalyst concentration lg[stilbenequinone], the first-order of the reaction with respect to stilbenequinone is established.
The investigation into the impact of oxygen concentration on the catalytic sulfide oxidation (Fig. 6) shows that under the kinetic control, the plot of the logarithm of initial rate of Na2S oxidation versus the logarithm of initial oxygen concentration is a straight line with a slope of 1.074. In other words, the reaction is first-order with respect to oxygen.
Thus, it can be stated that the sulfide oxidation is first-order with respect to each reactant in the presence of stilbenquinone. As the sulfide oxidation proceeds with the excess of both oxygen and catalyst, the order of this reaction can be attributed to the pseudo-first with respect to sulfide. The first-order rate constant of this reaction is in agreement with the negative slope of plot of the logarithmic sulfide concentration ln[Na2S] versus time (Fig. 7a). A straight line is obtained by plotting the logarithmic rate constant of catalytic sulfide oxidation against inverse temperature as shown in Figure 7b. The slope of this line is -1,1132, which corresponds to an activation energy (Ea) of 9.25 kJ mol-1.
In comparison with other catalysts used in the sulfide sulfur oxidation in alkaline media, the stilbenequinone has the lowest activation energy (Table 2), which correlates with its high catalytic activity.
In our previous research , it was suggested that the oxidation of sulfide sulfur in the presence stilbenequinone is processed via a mechanism similar to the reaction of catalytic sulfide oxidation using benzoquinone. In this process, hydrogen sulfide is first absorbed into an aqueous solution containing the benzoquinone in the oxidized state. The absorbed hydrogen sulfide is oxidized by the benzoquinone, which is reduced to hydroquinone in the reaction. The formed hydroquinone is subsequently reoxidized to benzoquinone by contact with air in a separate step to complete the cycle. Therefore, the catalytic oxidation of sulfide sulfur in the test reaction may involve two steps. In the first stage, sulfide sulfur was oxidized by stilbenequinone with its reduction to 3,5,3′,5′-tetra-tert-butyl-4,4′-dihydroxy-1,2-diphenylethylene (hereinafter referred to as diphenylethylene or С30H44O2). The second step included the regeneration of the catalyst by oxidizing diphenylethylene to stilbenequinone in an alkaline medium (Scheme 1). Compared with sulfide oxidation reaction catalyzed by benzoquinone, both these steps of catalytic sulfide oxidation in the presence of stilbenequinone occur simultaneously.
To confirm this mechanism, the current study looks at the possibility of the occurrence of the sulfide oxidation with stilbenequinone (1st stage) at different temperatures in an inert medium (Table 3). Subsequently, the IR spectrum of the light-yellow powder precipitated out of this reaction at room temperature (supposedly diphenylethylene) is compared with that of stilbenequinone (Fig. 8).
The collected data has revealed that the reaction rate of sodium sulfide oxidation with stillbenequinone goes up as the temperature increases. As a rule, quinones under certain conditions are strong oxidants . In the stilbenequinone molecule, two cyclohexadiene fragments are separated by two methylene groups. This leads to the expansion of the conjugated electron system, resulting in an increase in the electron affinity [27,28]. It is also found out that in the IR spectrum of the proposed diphenylethylene, the absorption bands of the conjugated diene (Ar = C-C = Ar) 1640-1605 cm-1 and the carbonyl group (C = O) 1605 cm-1 characterized for stilbenequione disappear completely. In fact, there is an appearance of the absorption bands of the hydroxyl group (OH) 3627 — 3607, 1420, 1231 — 1133 cm-1 and the double bond (C = C) 960 cm-1 characterized for diphenylethylene. In comparing the melting point of diphenylethylene (240°C) with that of the authentic sample , its formation can be reaffirmed.
All stated results help strengthen the proposed mechanism of catalytic sulfide oxidation in the presence of stilbenequinone (Scheme 1). In this case, the stilbenequinone is a redox catalyst.
To gain further insights into the mechanism of catalytic sulfide oxidation, a detailed analysis of the final products of sodium sulfide oxidation with stilbenequinone in an inert atmosphere (stage 1) as well as the products of its catalytic oxidation in the presence of oxygen and stilbenequinone was carried out. The results were also put into comparison, as clearly shown in Figure 9.
It is known that the major products of inorganic sulfides oxidation with oxygen are thiosulfate, sulfite and sulfate in the presence of catalysts . Figure 9 shows that sodium thiosulfate and sodium sulfate are the main products of the sodium sulfide oxidation in the presence of stilbenequinone-based catalyst. It is also found out that the sodium sulfide is oxidized by stilbenequinone to form sodium thiosulfate in an oxygen-free environment.
To confirm the selective formation of thiosulfate in the catalytic oxidation of sulfide sulfur, the sodium sulfate concentration formed during the oxidation of sodium thiosulfate with and without the presence of a stilbenequinone-based catalyst was compared. The results in Figure 10 shows that the catalyst has no influence on the oxidation reaction of sodium thiosulfate. That is to say, additional factors promoting the oxidization of sodium sulfide to sulfate in the process of its catalytic oxidation in the presence of oxygen and stilbenequinone are expected.
Based on background of catalytic reaction, oxygen, as an oxidant, may simultaneously participate in two processes namely the sulfide oxidation and the catalyst regeneration. However, it is explored that a significant amount of hydrogen peroxide is formed in an aqueous alkaline medium during the latter process (Fig. 11).
The accumulation of hydrogen peroxide in the aqueous alkaline medium has been previously examined in the literature [31, 32]. In these works, the mechanism of hindered hydroquinone oxidation in the presence of alkaline as the catalyst was also investigated. The oxidative regeneration of stilbenequinone with the formation of hydrogen peroxide includes the main stages in Scheme 2. It is clearly that the active forms of oxygen (namely O2*-, HO2 and H2O2) are produced during the process of catalyst regeneration. These forms intensify the oxidization of sulfide sulfur to the maximum degree (SO42-).
Precisely, sulfide sulfur is simultaneously oxidized by both stilbenequinone and the active oxygen forms. In addition, the possibility of partial non-catalytic oxidation of sulfide sulfur to form sulfate in a strong alkaline medium  cannot be ruled out according to the following reactions:
2HS- + 2O2 → S2O32- + H2O (1)
HS- + 2O2 + OH- →SO42- + H2O (2)
2S2- + 2O2+ H2O → S2O32- + 2OH- (3)
S2- + 2О2 → SO42- (4)
HS- + O2 + 2OH- → SO32- + H2O (5)
2S2- + 3О2 → 2SO32- (6)
2SO32-+ О2 → 2SO42- (7)
S2O32- + 2O2 + 2OH- → 2SO42- + H2O (8)
It is known that reaction (7) proceeds much faster than reactions (5) and (6) . Therefore, thiosulfate and sulfate ions are the main products of sulfide sulfur oxidation by oxygen. Obviously, the thiosulfate concentration decreases when the sulfide sulfur in the reaction solution is completely exhausted and the sulfate concentration (the final product of non-catalytic oxidation) increases (Fig. 12).
It should be noted that the hydrocarbon phase (i.e. kerosene fraction) contributes to the accumulation of hydrogen peroxide (Fig.11) in the reaction medium. This is due to the higher solubility of oxygen in the kerosene fraction compared to in aqueous medium , which increases the rate of non-catalytic oxidation of sulfide sulfur by oxygen (Fig.13). In other words, kerosene fraction carries the catalyst and concurrently, enriches oxygen in the reaction solution.
While studying the change of the stilbenequinone concentration in the saturated solution of oxygen (Fig.14), it is found that during the course of the catalytic reaction, the stilbenequinone concentration decreases to the lowest level and it increases only if the sodium sulfide has reached the point of complete exhaustion. This implies that the rate of sulfide sulfur oxidation by stilbenequinone is higher than that of catalyst regeneration and that the catalyst regeneration is the rate-limiting step in the catalytic oxidation of sodium sulfide in the presence of stilbenequinone.
In general, the role of stilbenequinone in the catalytic oxidation of inorganic sulfide is to create a new and efficient way of transferring electrons from the reducing agent (i.e. sulfide sulfur) to the oxidants (oxygen and its active forms) (Scheme 3).
Based on the presented data, it is likely to summarize the mechanism of the catalytic oxidation of sulfide sulfur to thiosulfate by the following reactions.
Na2S + H2O = NaHS + NaOH (9)
4С30H42O2 + 2NaHS + 3H2O → 4С30H44O2 + Na2S2O3 (10)
С30H44O2 + O2 □(→┴(OH¯) ) С30H42O2 + H2O2 (11)
In aqueous media, sodium sulfide is hydrolyzed to sodium hydrosulfide as shown in reaction (9). In reaction (10), sodium hydrosulfide in aqueous medium is oxidized by stilbenequinone (С30H42O2) to sodium thiosulfate and the reduction of stilbenequinone to diphenylethylene (С30H44O2). According to reaction (11), the stilbenequinone regeneration occurs by the oxidation of diphenylethylene with oxygen in an alkaline medium.
The experiments on the sulfur-alkaline waste of the petrochemical company SIBUR — KSTOVO Ltd. shown in Figure 15 have proved the likeliness of using stilbenequinone as a catalyst in industrial wastewater treatment. The effluent was completely degraded after thirty-five minutes of treatment in the presence of stilbenequinone-based catalyst. In contrast, the residue of sulfide in the effluent was 240 mg L-1 after 75 minutes of non-catalytic oxidation whereas the acceptable level of sulfide in wastewater is lower than 25 mg L-1.
In summary, the kinetics of aqueous sulfide oxidation in the presence of stilbenequinone-based catalyst has been presented as a first-order reaction with respect to sulfide sulfur, oxygen and catalyst. It is concluded that stilbenequinone plays a crucial role in transferring electrons from the reducing agent (i.e. sulfide) to the oxidants (such as oxygen and hydrogen peroxide). It has been proved that the process of catalyst regeneration is the rate-limiting step in the catalytic oxidation of sodium sulfide. The major products of this reaction include thiosulfate and sulfate in which the former is the product of sulfide oxidation by stilbenequinone while the latter is the final product of the oxidation of sulfide and thiosulfate being oxidized by hydrogen peroxide formed during catalyst regeneration.
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