Oil refinery wastewater is potential contributor to groundwater and surface water contamination. The wastewater in the refineries arising from various processes (such as pyrolysis oil, amine gas treating, alkalization of the crude oil) containing large quantities of sulfide sulfur is called sulfur-alkaline wastes (SAW) that is classified as hazardous material. The strength of this wastewater in terms of chemical oxygen demand is significantly high. The majority of chemical oxygen demand for SAW is from inorganic sulfides, with 2000 mg L-1 of sulfide concentrations [1]. The treatment of SAW in now an urgent environmental problem that demands immediate attention. Many methods have been applied for treating wastewater containing inorganic sulfides. At low sulfide concentrations, its detoxification process can be carried out by bacteriological oxidation. Nevertheless, at concentrations exceeding 70 mg L-1 the bacterial metabolism of sulfide is suppressed [2]. In this case, the chemical oxidation methods are the most widely used in industry to treat this wastewater, whereby the inorganic sulfide will be oxidized to form sulfate by air or pure oxygen in the presence of catalysts. Numerous catalyst types have been developed for sulfide oxidation, in which the transition metals and their compounds are the most commonly known. Metal oxide such as TiO2, MnO2, Cr2O3, Fe2O3, V2O5, CuO supported on the polymeric matrix are considered as the potential catalysts for sulfide oxidation, which exhibit high efficiency over a wide range of sulfide concentrations and pH levels [3-6]. Similarly, the transition metalphthalocyanines and chalcogenides (Co, Cd, V, Mo, Ni, etc.) are among the most popular and usually studied catalysts for sulfide-removal in industrial practice [7-9]. Among the different metal catalysts, the spinel structured iron oxides such as MFe2O4 (where M = Cu2+, Co2+, Ni2+, Mn2+), magnetite, maghemite as well as the chelated iron compounds revealed promising catalytic activity towards sulfide treatment [10-12]. The main disadvantage of using these catalysts are the release of heavy metals into the environment.
The modified carbon based on materials such as activated carbon, graphite and graphene have attracted much interest in recent years as promising catalysts in sulfide treatment [13-15]. The inert metal (Pt and Pd) modified activated carbon [16] and pyrolysed carbon supported cobalt porphyrin [17] as well as microorganisms supported on activated carbon [18] have also been proposed for sulfide oxidation. However, the high cost of these materials and the complex synthesis process as well as the blockage of active sites of carbon catalyst by sulfide oxidation products in the long run hinder their industrial application.
The quinones and their derivatives represent another class of powerful catalysts for sulfide oxidation, which have been applied commercially to a wide variety for sulfide removal applications in the UK and Japan Petrochemical Industry as well as in the United States. Highly efficient application of quinones as catalyst for sulfide sulfur oxidation would avoid the problems of existing processes mentioned above. Many authors [20-24] have investigated the catalytic oxidation of sulfide sulfur in the presence of quinones. It has been reported by Robert A.D [20] that the naphthoquinone complexes effectively accelerate the sulfide oxidation in both the Hiperion process and the Takahax process (used widely in Japan for treating refinery gas streams and digester biogas). Ueno H. [21] has been also argued that 1,4-naphthoquinone, 1,2-naphthoquinone-4-sulfonic acid sodium salt as well as hydroquinone have high activity in the catalytic sulfide oxidation of industrial wastewater. The shortcomings of these catalysts are high catalyst losses, complexity of catalyst separation and recovery after wastewater treatment. Surveys such as that conducted by Iwasawa [22] have shown that the polynaphthoquinone possessed a high catalytic efficiency for sulfide oxidation. However, during the course of sulfide oxidation, the formed sulfur was deposited on the surface of the polynaphthoquinone catalyst, and this led to a decrease in its activity.
To overcome the above-mentioned disadvantages, as quinone-catalyst we present a new 3,3′,5,5′-tetra-tert-butyl-4,4′- stilbenequinone that exhibits the high catalytic activity and stability towards the sulfide oxidation in concentrated alkaline media. The kerosene fraction has been used as catalyst carrier due to its low aqueous solubility and low volatility as well as satisfactory solubility of stilbenequinone. The main purpose of this study is to develop an understanding of catalytic efficiency of the stilbenequinone in the liquid-phase sulfide oxidation and attempt to apply it to industrial wastewater.

Experimental part

Catalyst Synthesis: The catalytic component — 3,3′,5,5′-tetra-tert-butyl-4,4′-stilbenequinone (C30H42O2) was prepared by the oxidation of 2,6-di-tert-butyl-4-methylphenol with hydrogen peroxide in the presence of a potassium iodide-based catalyst (Scheme 1) as decried in [ 25, 26].
Reagents. In our experiments, all the reagents were analytical grade and used without further purification. Kerosene fraction (Russia, GOST 10227-2013), AgNO3 (Russia, GOST 1277-75), FeSO4×7H2O (Russia, GOST 6981-94), technical gaseous oxygen in cylinders (Russia, GOST 5583-78). Aqueous sulfide solutions were prepared by dissolving appropriate amounts of reagent grade Na2S×9H2O (Russia, GOST 2063-77) in distilled water. A solution of sodium hydrosulfide was prepared according to the procedure used by James [27].
Catalytic oxidation of sulfide sulfur. The catalytic oxidation of inorganic sulfides was implemented in a 150 cm3 three-necked cylindrical glass reactor. A glass reactor was used to avoid any contact of the stock sulfide solution with metals, which would accelerate the sulfide oxidation. The reaction mixture containing 40 cm3 of an aqueous inorganic sulfide solution and 20 cm3 of a kerosene fraction was loaded into the reactor in the presence of certain amount of the catalyst component. The oxygen from the cylinder was continuously injected into the reaction solution at 0-13 L h -1. The reaction solution was stirred at speed of 500-1400 rpm and its temperature was maintained in the range of 50-90 °C by a thermally controlled magnetic stirrer at atmospheric pressure.
Experimental-analytical methods. The quantitative content of sulfide was measured by potentiometric titration with AgNO3 in accordance with UOP-209-00 (USA). The concentrations of thiosulfate and sulfite as well as of hydrogen peroxide were determined by the idometry described in [28,29]. Meanwhile, the concentration of sodium sulfate was determined by the spectrophotometric method [28].

Results and Discussion

The reaction of liquid-phase sulfide oxidation catalyzed by a stilbenequinone takes place in a three-phase system “oxygen — kerosene fraction – aqueous sulfide solution” and consequently the catalytic reaction efficiency depends not only on the chemical reaction rate, but also on the reactants diffusion rate. A series of experiments in which the oxygen supply rate and the rotational speed of the stirrer varied at the same initial reaction conditions was carried out to determine the process controlling the catalytic sulfide oxidation in the presence of stilbenequinone. The results of these experiments, as shown in Figure 1a-b, indicate that the catalytic sulfide oxidation rate was constants when the rotation frequencies of the stirrer exceeds 1200 rpm as well as the oxygen supply rate is above 1.2 L h-1, in other words the process occurs in the kinetically controlled region. In order to eliminate the effect of diffusion of reagents on the rate of catalytic sulfide oxidation, all subsequent experiments were performed at a stirring speed of 1400 rpm with the oxygen supply rate of 13 L h-1. Furthermore, a 1:2 ratio of kerosene fraction to sulfide solution is optimal for catalytic sulfide oxidation in the presence of stilbenequinone (see also in Fig. 1c), which may provide an explanation for the formation of maximum interphase area between aqueous phase and liquid hydrocarbon phase.
The stilbenequinone, which represents an aromatic unsaturated cyclic diketone characterized by complete conjugation, is a strong oxidant that plays a role of redox-catalyst in the liquid-phase sulfide oxidation [26]. Mechanism of the catalytic sulfide oxidation in the presence of stilbenequinone may take place as follows: in an aqueous solution, Na2S is hydrolyzed to NaHS that is then oxidized to form Na2S2O3 by stilbenequinone which is reduced to 3,5,3′,5′-tetra-tert-butyl-4,4′-dihydroxy-1,2-diphenylethylene (herein after referred to as diphenylethylene). The diphenylethylene is reoxidized to stilbenequinone by contact with oxygen to complete the catalytic cycle.
The stilbenequinone as the other quinones is a two-electron oxidizer that in the process of catalytic sulfide oxidation accepts an electron pair from sulfide and is reduced to diphenylethylene with the simultaneous production of elemental sulfur. The formation of elemental sulfur during the course of sulfide treatments in the presence of various quinones has been previously shown in the literatures [20, 22, 24]. In these works, the mechanism of liquid phase hydrogen sulfide oxidation catalyzed by the naphthoquinone complexes, duroquinone as well as by benzoquinone derivatives was also investigated. It has been proved that after absorbed into an aqueous solution containing a quinone, the hydrogen sulfide is oxidized to elemental sulfur. It is well known [30-33] that the elemental sulfur (even with ordinary rhombic sulfur) in the alkaline solution disproportionates rapidly to form thiosulfate and sulfide at elevated temperature (above approximately 50°C) (Equation 1).
6OH- +4S S2O32- +2S2- + 3H2O (1)
In a study which set out to identify the mechanism of thiosulfate formation in liquid-phase oxidation of sulfide by organic compound, Jesse L.B и George S. F [30] found that at the initial moment of the reaction, sodium sulfide is oxidized to elemental sulfur, which further disproportionates to form thiosulfate as described in Equation 1.
In view of all that has been mentioned above, the thiosulfate formation from the reaction of sulfide with stilbenequinone may occur as follows (Scheme 2): the stilbenequinone oxidizing HS- to S0 is converted to phenoxy anion which further reacts with H2O to yield the diphenylethylene and hydroxide anion OH-. The elemental sulfur S0 produced straightway reacts with hydroxide anions to form thiosulfate with the simultaneous production of sulfide. The following expressions are offered as a possible explanation of this process:
S2- + H2O → HS- + OH- (2)
С30H42O2 + HS- + H2O → С30H44O2 + S + OH- (3)
4S + 6OH- → S2O32- + 2S2- + 3H2O (4)
With the view to confirming the elemental sulfur formation during catalytic sulfide oxidation in the presence of stilbenequinone, the order of this reaction with respect to sulfide and stilbenequinone was determined using a differential method with varying initial concentrations of reagents. Figure 2 illustrates the plots of the logarithmic initial rate of this reaction (lgv0) against the logarithmic concentrations of sulfide lg[Na2S] and stilbenequinone lg[cat]. It is evident that these plots are straight lines with slopes of approximately 1. In other words, the reaction has the first-order with respect to both sulfide and stilbenequinone. This conclusion helps strengthen the proposed mechanism of sulfide oxidation with stilbenequinone that one molecule of stilbenequinone reacts with exactly one molecule of sulfide to produce elemental sulfur as described in Eq. 3.
The catalyst regeneration in process of catalytic sulfide oxidation play a crucial role that not only restores the catalytic activity of stilbenequinone but also simultaneously produces a significant amount of hydrogen peroxide (H2O2). In Figure 3, the H2O2 accumulation is plotted as a function of reaction temperature in the process of catalyst regeneration based on stilbenequinone. The H2O2 formation in the alkaline solution has been previously confirmed in the literature [34,35] which describe the mechanism of hindered hydroquinone oxidation in the presence of alkaline based-catalyst. The oxidative regeneration of stilbenequinone with the formation of hydrogen peroxide involves the main stages as described in Scheme 3. It is clearly that besides hydrogen peroxide, other active forms of oxygen namely O2*-, HO2 are produced during the process of catalyst regeneration
In our previous research [26], it was indicated that the catalyst regeneration was a limiting step in the catalytic sulfide oxidation in the presence of stilbenequinone. Therefore, the rate of catalytic sulfide oxidation largely depends upon the regeneration rate of stilbenequinone. The results of studying the impact of nature of sulfide on its catalytic oxidation in the presence of stilbenequinone (Fig. 4) show that although the oxidation rate of NaHS solution (with pH = 9.1~9.5) by stilbenequinone exceeds that of Na2S solution (pH = 13.5~14), the rate of its catalytic oxidation is lower than that of latter solution. This rather contradictory result may be due to difference between these solutions in concentration of OH- anion, which catalyzes the process of stilbenequinone regeneration with the formation of strong oxidants (namely active oxygen forms) as mentioned above.
The formation of intermediate and final reaction products is also investigated. Findings suggest that the products of catalytic sulfides oxidation in the presence of stilbenequinone are thiosulfate and sulfate in which the former is the product of sulfide oxidation by stilbenequinone while the latter, presumably, is the product of sulfide and thiosulfate being oxidized by active oxygen forms produced during the catalyst regeneration. In addition, the results of our previous behavioral research [25] have shown that the stibenequione does not influence the thiosulfate oxidation. Besides, the possibility of non-catalytic oxidation of sulfide sulfur by oxygen to form sulfate in a strong alkaline medium [21] cannot be excluded. Precisely, as shown in Figure 5, the concentration of thiosulfate decreases after the sulfide sulfur in the reaction solution is completely exhausted. Meanwhile, the concentration of sulfate being the final product of non-catalytic oxidation increases.
Based on the data presented above, the catalytic cycle of sulfide sulfur oxidation in the presence of stilbenequinone can be described as shown in scheme 4. The role of stilbenequinone in this catalytic cycle is to create an efficient way of transferring electrons from the sulfide sulfur to the oxidants (i.e. oxygen and its active forms). It should be note that in this process the kerosene fraction plays not only the role of the catalyst carrier but also that of depositing oxygen in the reaction solution (Fig.3).
The rate of catalytic reaction depends upon various factors such as temperature of the reaction, the nature and action mechanism of catalyst, the nature of reactants, etc. The results obtained from studying the impact of catalyst amount on the sulfide oxidation rate are presented in Figure 6. It is evident that at a low stilbenequinone concentration, the rate of catalytic sulfide oxidation is even lower than that of non-catalytic process. In our opinion, this phenomenon might be related to the formation of diphenylethylene which is well known as an antioxidant.
As shown by previous studies [36-39], the reaction of non-catalytic sulfide oxidation proceeds through a free-radical chain mechanism, in which after reaching a critical concentration of active peroxide radicals, the oxidation reaction begins to occur:
HS- + O2 → HS· + O2¬- (5)
HS· + O2 → HO2 + S (6)
HS· + O2- → S + HO2- (7)
S + O2 +OH- → S2O32- + H2O (8)
Diphenylethylene, is a phenolic antioxidant, reacts with the active radicals formed during the oxidation of sulfide sulfur by oxygen and therefore terminates the reaction chain. This leads to a decrease in the rate of non-catalytic sulfide oxidation. It should be note that with increasing stilbenequinone concentration in reaction solution, the catalytic oxidation rate of sulfide sulfur also goes up. That is explained by the influence of stilbenequinone in the catalytic process.
The stilbenequinone has a high catalytic activity in the reaction of catalytic sulfide oxidation. The ratio of the rate of catalytic sulfide oxidation to that of its non-catalytic oxidation at 90 °C is 17. Stability is one of the most important parameters of a catalyst. Practical experiments (Table 1) have shown that activity of stilbenequinone remains unchanged after 9 consecutive catalytic runs with a total duration of up to 22.5 h. In this case, the number of revolutions of the catalytic cycle (i.e turnover number) identified by the ratio of mole of oxidized sulfide to stilbenequinone mole is 121.15.
A wastewater containing sulfides such as the pyrolysis wastewater of «SIBUR- KSTOVO» ltd has been tested to evaluate the applicability of stilbenequione to sulfide treatment in industry. The concentration profile of sulfide with respect to reaction time for catalytic oxidation of the pyrolysis wastewater is presented in Fig.7. It is clear that during the early stages of non-catalytic oxidation reaction the sulfide concentration remains nearly constant. This induction period has been reported previously by many researches [38-40]. As mentioned above, the typical of non-catalytic sulfide oxidation is governed by a radical mechanism. Once a critical concentration of free radicals is reached, the induction period will be completed, in other words sulfide oxidation stage begins. The experimental result shows that in the presence of stilbenequinone the complete treatment of sulfide is achieved only in 30 min, as against over 90 min in non-catalytic process.
In the last few decades, advanced oxidation process based on Fenton reaction chemistry (H2O2/Fe) have been extensively implemented to remove hazardous compounds during wastewater remediation [42-43]. The Fenton reaction was discovered in 1894 and to date, the importance of this reaction for industrial wastewater treatment has been recognized. The Fenton reaction describes the activation of hydrogen peroxide (H2O2) by iron ion Fe2+ to form strong oxidizing radical species (primarily HO•) via a complex reaction sequence [45-47]:
Fe2+ + H2O2  → Fe3+ + HO•+ OH- (9)
Fe3+ + H2O2  → Fe2+ + HO2• + H+ (10)
The hydroxyl radical (HO•) is the most reactive species of oxygen. The strong standard oxidation potential and high bimolecular reaction rate constants as well as no-selective reactivity of the hydroxyl radical are primarily responsible for high-efficiency of advanced oxidation processes in treating toxic compounds of industrial wastewater [48].
With the goal of taking advantage of the hydrogen peroxide formed during the catalyst regeneration to intensify the catalytic oxidation of sulfide sulfur in the presence of stilbenequinone, a new catalytic system based on stilbenequinone/Fe has been investigated, in which the formed hydrogen peroxide reacts with additional ferrous ion to form Fenton reaction that enhances the sulfide degradation.
The sulfide-oxidation efficiency with respect to reaction time in the presence and absence of various catalysts is presented in Fig.8. It is evident that the catalytic oxidation rate of the concentrated sulfide solution in the presence of stilbenequinone significantly increases when adding amount of iron (II) sulfate to the reaction mixture. In comparison with the sulfide oxidation catalyzed by stilbenenquinone, the initial rate of its oxdiation in the presence of a Fe/stilbenequinone has been increased almost ten times. It should be noted that the ferric salt of sulfate was chosen to form Fenton reaction because the sulfate does not affect the sulfide oxidation rate in the presence of stilbenequinone [25].
The important factors affecting the degradation of pollutants by Fenton process are operating pH, temperature and dosages of iron and H2O2. The effect of temperature and dosages of iron on the degradation of Na2S using stilbenequinone /Fe is shown in Fig.9. From this figure can be seen that conversion of Na2S gradually increases with the increasing temperature to 70 oC, and then distinctly decreases with the further increasing the temperature up to 90oC. A possible explanation for this result may be due to accelerated decomposition of H2O2 and decreasing the dissolved oxygen into solution at high temperatures.
In the Fenton reaction, the rate of degradation generally increases with increasing amount of Fe2+, reaches a maximum at definite dosages of Fe2+ and decreases with further adding [49,50]. The similar phenomenon has also been observed in sulfide oxidation catalyzed by stilbenequinone/Fe, which may be explained due to recombiation of hydroxyl radicals with Fe2+, contributing to the OH• scavenging capacity (Eqs.11) [51].
Fe2+ + OH• → Fe 3+ + OH- (11)
In addition, when the Fe2+ concentration significantly increased, a great amount of Fe3+ ions which was generated through Eqs.9 and Eqs.11 also react with OH- ions to form insoluble ferric hydroxide [52]. This leads to reducing the overall oxidation efficiency.


— Recent research has provided a more complete understanding of the catalytic process occurring during the sulfide oxidation in the presence a stilbenequinone based oxidation-reduction catalyst. In this case, the sulfide sulfur can be oxidized, not only by stilbenequinone and oxygen but also by the active oxygen forms which formed in process of catalyst regeneration.
— The laboratory test has been demonstrated that the stilbenequinone can be used for treating industrial wastewater containing sulfide sulfur.
— It has also been shown that the combination of stilbenequinone with iron (II) salt leads to a significant increase in catalytic oxidation rate of sulfide.


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