Heterogeneous Catalytic and Non-Catalytic Supercritical Water Oxidation of Organic Pollutants in Industrial Wastewaters Effect of Operational Parameters

Abstract

This work reports supercritical water oxidation (SCWO) of organic pollutants in industrial wastewater in the absence and presence of catalysts. To increase the efficiency of the oxidation process, the SCWO of organic compounds in industrial wastewater was performed in the presence of various iron- and manganese-containing heterogeneous catalysts (Fe-Ac, Fe-OH, and Mn-Al). The catalytic and non-catalytic SCWO of organic compounds in wastewater from PJSC “Nizhnekamskneftekhim”, generated from the epoxidation of propylene with ethylbenzene hydroperoxide in the process of producing propylene oxide and styrene (PO/SM), was performed. The effect of operational parameters (temperature, pressure, residence time, type of catalysts, oxygen excess ratio, etc.) on the efficiency of the process of oxidation of organic compounds in the wastewater was studied. SCWO was studied in a flow reactor with induction heating under different temperatures (between 673.15 and 873.15 K) and at a pressure of 22.5 MPa. The reaction time ranged from 1.8 to 4.83 min. Compressed air was used as an oxidizing agent (oxidant) with an oxidant ratio of two to four. A pseudo-first-order model expressed the kinetics of the SCWO processes, and the rate constants were evaluated. In the present work, in order to optimize the operation parameters of the SCWO process, we used the thermodynamic properties of near- and supercritical water by taking into account the asymmetric behavior of the liquid–vapor coexistence curve.

1. Introduction

Water possesses unique thermodynamic and transport properties under critical and supercritical conditions. The importance of supercritical water oxidation (SCWO) is related to the unusual properties of water under critical and supercritical conditions. Due to its large compressibility under supercritical conditions, small variations in pressure lead to dramatic changes in density, causing sharp changes in other properties such as diffusivity, viscosity, dielectric, and solvation. Industrial wastewater treatment is one of the important applications of SCWO. For example, SCWO allows rapid and efficient destruction of organic compounds, with a conversion rate of 100% near the critical point. As a result, supercritical water (SCW) is widely used for the destruction of hazardous industrial wastes [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17] and as a medium to carry out chemical reactions. SCWO is one of the effective destruction methods for organic wastewater [6,7,8,9,10,11,12,13,14,15,16,17] and is used for the treatment of municipal sludge, heavy metals, toxic chemicals, and other harmful contaminants. Some authors [18,19] successfully used heterogeneous catalysts for phenol oxidation. The catalysts also increase the selectivity of phenol conversion and reduce the yield of byproducts (see below). Unlimited miscibility of organic compounds and oxygen with SCW ensures no interfacial mass transfer restrictions for SCWO, i.e., providing organic compounds and oxygen in a single phase at the reaction conditions, thus increasing the oxidation reaction rate.

It is apparent that the efficiency of using SCWO technology for industrial wastewater treatment strongly depends on the properties of water near the critical point. Liquid–vapor phase equilibrium properties of water near the critical point is extremely important for understanding the properties near- and supercritical conditions and successfully applying them to optimize the SCWO process. It is well-known that liquid–vapor symmetry near the critical point plays a crucial role in understanding the fundamental principle of critical and supercritical phenomena in fluids and their properties. For example, the law of rectilinear diameter is based on the liquid–vapor symmetry principle. According to the law of rectilinear diameter (or symmetry principle), the arithmetic mean of the densities’ (liquid and vapor) coexistence phases is a strictly linear function of temperature in the immediate vicinity of the critical point (in the asymptotic region). The symmetry assumes that the arithmetic mean of the coexisting liquid and vapor densities near the critical point is a constant; i.e., the diameter of the liquid–gas coexistence curve coincides with the critical isochore. However, our previous publications and other reported experimental liquid—vapor density data show that the diameter does have a slight inclination, i.e., deviation from the symmetry. This means that the liquid–vapor coexistence curve is asymmetric, especially in the non-asymptotic range around the critical point where the SCWO process is taking place. In the present work we are considering the influence of the deviation of the liquid–vapor coexistence curve of water from the symmetry in the non-asymptotic region (far from the critical point) on the SCWO process and its efficiency. According to the scaling theory of the critical phenomena the liquid–vapor coexistence curve can be presented as:

$$\Delta \rho =\pm B_0{t}^{\beta }\pm B_1{t}^{\beta +\Delta }+B_2{t}^{1-\alpha }-B_{3}t+B_4{t}^{2\beta },$$

where $B_i$(i = 0,4) are the adjustable system-dependent critical amplitudes of water, $\pm B_0{t}^{\beta }$ is the symmetric (asymptotic) term, $\pm B_1{t}^{\beta +\Delta }$ is the symmetric (non-asymptotic Wegner’s correction) term, $B_2{t}^{1-\alpha }$ is the “singular diameter” (the first non-analytical contribution to the liquid–gas asymmetry predicted by “incomplete” scaling), $B_4{t}^{2\beta }$ is the new non-analytical contribution of the liquid–gas asymmetry (“complete” scaling term), and $B_{3}t$ is the rectilinear diameter. As one can see, the coexistence curve diameter is singular:

$${\rho }_{\mathrm{d}}=1+\left(B_2{t}^{1-\alpha }-B_{3}t+B_4{t}^{2\beta }\right),$$

where $B_2$ and $B_4$ are the “incomplete” and ”complete” scaling asymmetry parameters. As one can see, the symmetric terms ($\pm B_0{t}^{\beta }$ and $\pm B_1{t}^{\beta +\Delta }$) of the liquid–vapor coexistence curve (see above) are not affected by the behavior of the coexistence curve diameter. Taking into account the deviation of the behavior of the liquid–vapor coexistence curve from the symmetry principle (asymmetric behavior) leads to the need to take into account the thermodynamic behavior of near- and supercritical water which, in turn, affects the control of the SCWO process. In the present work, in order to optimize the operation parameters of the SCWO process, we used the thermodynamic properties of near- and supercritical water by taking into account the asymmetric behavior of the liquid–vapor coexistence curve.

For organic synthesis companies, the most challenging aspect of minimizing their impact on the environment is the disposal of organic waste. In most cases, the industrial liquid wastes are multicomponent, chemically inhomogeneous mixtures of compounds with different physical, chemical, and biological properties. Industrial wastewater covers a variety of materials. Various organic wastes, such as petroleum hydrocarbons, heavy metals, and alkaline salts are the three types of contaminants in aqueous organic waste. Oil and gas wells have been and are increasingly becoming greater sources of wastewater containing organics. Deep processing of chemical waste is an important scientific and practical task. In the present work, we studied the effect of various factors (temperature, process duration, heterogeneous catalysis, oxygen excess coefficient, and, as a result, chemical oxygen demand) on the efficiency of oxidation of the components of aqueous organic waste (aqueous alkaline effluent) from industrial propylene epoxidation with the ethylbenzene hydroperoxide process [20,21], under supercritical aqueous conditions. Previously, this industrial waste stream was treated with a sodium hydroxide solution in order to neutralize acids and decompose the used catalytic complex based on expensive molybdenum [22]. The presence of organic sodium salts and residual alkali, which are emulsifiers, prevent the separation of the wastewater formed after treatment into aqueous and organic phases by settling, distillation, or rectification. As a result, a waste stream containing a significant amount of oxygen-containing aromatic derivatives, sodium hydroxide, organic sodium salts, molybdenum derivatives in the form of salts, and organic complexes of various compositions is subject to thermal neutralization.

Components contained in the wastewater, such as styrene, phenol, methylphenylcarbinol, acetophenone, ethylbenzene, propylene glycol, molybdenum, etc., are of great value for chemical production as feedstock. For example, the consumption of molybdenum (metal powder) for the preparation of catalytic complex is 48 tons/year, from which, in the process of separation and purification of reaction products, at least 90%, passing into the industrial waste stream, is irretrievably lost (see our previous publications [23]). Based on the results of previous studies [23], it was concluded that in order to confront the problem of utilizing molybdenum-containing wastewater, a two-stage process using working media in the supercritical media is advisable. To remove the main mass of the aforementioned components from the wastewater of PO/SM processes (production of propylene oxide and styrene at PJSC “Nizhnekamskneftekhim”), except for molybdenum (the first stage), a supercritical fluid extraction process was recommended (see References [23,24]).The problems for the second stage were the oxidation of the residues of the components of the wastewater under supercritical conditions (catalytic, CSCWO, and non-catalytic, SCWO process) and the simultaneous precipitation of the inorganic component of the wastewater, including the molybdenum to be separated. In our previous series of publications [21,22,23,24,25,26], we have successfully studied chemical reactions in supercritical media for reaction mixtures. However, even under supercritical conditions, without the use of catalysts, to achieve deep oxidation of organic compounds, high temperatures (673.15 to 973.15 K) and pressures (22 to 35 MPa) are required [16,27]. In addition, free radical reactions occurring in the non-catalytic SCWO process can lead to the formation of undesirable byproducts of incomplete (partial) oxidation. Therefore, the use of catalysts in the SCWO process is of interest, both from the point of view of an increase in selectivity towards deep oxidation of organic derivatives and a simultaneous decrease in the operating temperature of the oxidation process. It has been experimentally proven that the use of a catalyst increases the rate constant of the target reaction. Kosari et al. [28] also reported comparative results on the oxidation of tributilphosphate (TBF) in the temperature range from 643 to 753 K using the oxides Ag (I), Cu (II), Fe (III), Mg (II), and Zn (II) as catalysts. The heterogeneous catalysts based on transition metal oxides (Mn, Ti, Cu) were studied in SCWO processes by several authors [28,29,30,31,32,33,34]. In these works, the reaction mechanism and the chemical and morphological changes of catalysts in the SCWO process of phenol oxidation were discussed. Based on the results of comparative experiments and critically analyzed previous studies, we have concluded that the most effective catalyst for SCWO treatment of organic pollutants is bulk Mn (IV) oxide.

The main goal of the present work is to study the effect of various factors on the SCWO process characterization, including operation parameters such as temperature, pressure, heterogeneous catalysis, oxygen excess ratio (OER), organic concentrations, and reaction time, on the efficiency of catalytic and non-catalytic oxidation, under supercritical water conditions, of the components of aqueous alkaline wastewater formed from propylene epoxidation processes. Aqueous organic waste from the propylene epoxidation process significantly affects the local ecosystem if left untreated.

2. Experimental

2.1. Materials

The industrial wastewaters were sampled from PJSC Nizhnekamskneftekhim (aqueous organic waste from PJSC). Waste from the process of propylene oxide and styrene production can find its way into the environment through the wastewater discharged by industrial facilities of PJSC Nizhnekamskneftekhim. Wastewater is generated from the epoxidation of propylene with ethylbenzene hydroperoxide in the process of producing propylene oxide and styrene (PO/SM). An industrial organic wastewater sample examined for this study contains 40% water. The qualitative and quantitative composition of industrial wastewater samples was determined using the methods of UV and infrared spectroscopy (IS), as well as liquid chromatography (LC). Spectrophotometric analysis was performed on a double-beam scanning spectrophotometer with a Lambda 1050 (PerkinElmer) double monochromator. The wavelength range was 175–3300 nm, and the resolution was ≤0.05 nm for the ultraviolet region and ≤0.20 nm for the infrared region. We found three peaks in each of the spectra, corresponding to phenol, methyl phenyl ketone, and benzene ring. The data obtained are fitted spectra for average spectra. Chromatographic analysis was performed using a Flexar series LC (PerkinElmer) to quantify organic pollutants. Shooting mode: eluent (carrier gas) A: water—75% vol, acetonitrile—24% vol, acetic acid—1% vol; eluent (carrier gas) B: water—25% vol, acetonitrile—75% vol; pump mode—gradient, 0 min—100% A, 5 min—100% A, 15 min—100% B, 25 min—100% A; flow rate—1 mL/min; detector—diode-matrix, λ = 254 nm; column—C18 Bio, 150 mm × 4.6 mm × 5 μm.

In order to verify the accuracy of the wastewater sample’s contents, two methods, UV and infrared spectroscopy, were used. UV and infrared spectrograms of an initial wastewater (aqueous organic waste) sample are shown in Figure 1 and Figure 2.

As a result of the study in the UV region, peak characteristic of the benzene ring (λ ~ 206 nm), methyl phenyl ketone (λ~ 249 nm, 287 nm, 319 nm (implicit)), and phenol (λ ~ 268 nm) were found (see Figure 1). For the infrared region, peaks of the same compounds were found: phenol—λ ~ 3300 nm; methyl phenyl ketone—λ ~ 1640 nm; peak characteristics of the benzene ring—λ ~ 670 nm.

Figure 3 shows the chromatographic analysis of the initial wastewater by the content of aromatic hydrocarbons and glycols.

The quantitative composition of the wastewater sample is provided in

Table 1. Composition of the industrial wastewater sample.

Samples	Phenol/ mg/L	1-Phenylethanol/mg/L	Methyl Phenyl ketone/mg/L	Toluene/ mg/L	Styrene, mg/L	Ethylbenzene, mg/L	Propylene Glycol, mg/L
Initial wastewater	1256.64	1449.32	2598.225	0.03	92.588	0.046	1370

. The main characteristics of the real industrial wastewater sample are provided in

Table 2. Main characteristics of the real wastewater sample.

pH	13.25
Ash content/%	15.01
Density/kg/m3	1175.0
COD/mgO2/l	335.000

.

Before the experiment, the initial wastewater sample was mixed with distilled water in a volume ratio of 1:20.

2.2. Synthesis of Catalysts

Catalytic supercritical water oxidation (CSCWO) is one of the most environmentally friendly advanced oxidation processes [35,36]. It constitutes a promising technology for the treatment of organic pollutants in industrial wastewaters (see above). Various heterogeneous catalysts, including metal oxides, have been extensively studied to enhance the efficiency of CSCWO (see review [35]). For the present study, clay with a mineral composition (in mass%) of quartz (34%), calcite (24%), mica (20%), kaolinite (17%), feldspar 3 (%), and anatase (1%) was used as a catalyst carrier. The choice of the catalyst carrier is determined, firstly, by the stability of the physical–mechanical properties of ceramic materials in supercritical fluids and low activity of selected mineral phases in catalytic reactions of oxidation. The sequence for the preparation of a catalytic system based on a ceramic carrier is: (1) mixing an air-dried powder of a mineral mixture (screened through a sieve with a cell diameter of 0.2 mm) with a dense aqueous suspension of a catalyst in a ratio of 70:30 wt %; (2) molding from a wet mass by rolling a granule with a diameter of ~5 mm; (3) air drying of granules; and (4) thermal treatment of granules in air at a temperature of 873 K for 3 h. Montmorillonite of high concentration was obtained by enrichment of bentonite clay of the “Monamet 1N1” (Metacley) brand by the sedimentation method. The content of montmorillonite in the samples after enrichment, according to X-ray diffraction data, is 95% mass. A pilling solution containing oligomeric iron hydroxycations was prepared by slowly adding a 0.4 M NaOH solution to a 0.2 M Fe(NO₃)₃ solution with constant vigorous stirring until the OH/Fe molar ratio of 2 was reached. The resulting mixture was aged at room temperature with constant stirring for 6 h. The pilling solution at room temperature was added dropwise to a 1% smectite suspension with constant stirring until the Fe/smectite ratio reached 20 mmol/g of aluminosilicate. Then, the suspension was stirred overnight at room temperature. After exposure, the target product was separated from excess salts by repeated centrifugation with distilled water and converted into a dense suspension. The pilling solution was synthesized according to the following procedure: (1) preparation of an aqueous solution of iron (III) chloride (27 g FeCl₃∙6H₂O); (2) introduction of sodium acetate into the solution (27.2 g CH₃COONa∙3H₂O); (3) concentration of the resulting solution in a vacuum desiccator until crystals of [Fe₃(CH₃COO)₇OH·2H₂O]Cl precipitated, which, after washing and drying, were used to obtain a solution with a concentration of 0.05 M. Ion exchange for Na+ cations was carried out at room temperature by adding, dropwise, a pilling solution to a 1% suspension of aluminosilicate in the iron/aluminosilicate ratio of 1 mmol/g, with constant stirring for 1 h. To completely replace interlayer cations, the procedure was sequentially repeated 5 times. After exposure, the target product was separated from the excess of salts by repeated centrifugation with distilled water and converted into a dense suspension. The elemental composition of the obtained catalysts is provided in

Table 3. Elemental composition of Fe-pillared aluminosilicate derived from cation complex of triaquahexaacetate trioxo iron.

Sample	Elemental Composition (Mass %)
	SiO2	Al2O3	Fe2O3	MgO	CaO	Na2O
Fe-pillared aluminosilicate (hydrocomlex oligocations	33.7	11.4	53.5	1.4	-	-
Fe-pillared aluminosilicate (triaquahexacetate trioxo iron cations)	39.1 h	12.5	46.9	1.5	-	-

.

The manganese-containing catalyst was prepared from Mn-Al layered double hydroxides using co-precipitation method [37]. A mixed solution of 1 M Mn(NO₃)₂ and 1 M Al(NO₃)₃ in a 2:1 molar ratio was simultaneously mixed with a 1 M NaOH solution under constant stirring and pH control at 9 under an argon atmosphere. The resulting products were mixed for 3 h and then filtered from excess salts by repeated centrifugation with distilled water and transferred into a dense suspension. Samples of the prepared catalysts were named Fe-Ac, Fe-OH, and Mn-Al, respectively.

2.3. Catalyst Characterization

The characteristics of the catalysts and samples of the resulting inorganic precipitate were determined by an X-ray diffraction using a powder diffractometer (D2 Phaser, Bruker) with a standard Bragg-Brentano (2θ) reflection geometry and a goniometer radius of 141.4 mm. Radiation source was CuK_α and NiK_β, the sample rotation in its own plane was 30 rpm, with a scanning rate of 0.02° min⁻¹. For analysis, the precipitate obtained after 26 experiments was preliminarily divided on sieves into fractions (>1 mm; 0.5–1 mm; 0.25–0.5 mm; 0.16–0.25 mm; and <0.16 mm). In addition, the fractions were divided into magnetic and non-magnetic components. The additional fractionation into the magnetic component was primarily carried out to facilitate phase diagnostics. The obtained diffraction spectra were processed using the DIFFRAC.SUITE software package. The DIFFRAC.EVA software module (V-3.1) and the PDF-2 Release 2013 diffraction database were used to estimate the phase composition. The DIFFRAC.TOPAS software module (V-4.2) was used to quantitatively calculate the phases and refine the structural parameters. To determine the elemental composition, X-ray fluorescence analysis was used (spectrometer SUR-02 RENOM-FV, Moscow, Russia).

2.4. Catalyst’s Characteristics

The texture characteristics study of the catalysts was performed on the basis of the analysis of nitrogen adsorption–desorption isotherms at 77.4 K, obtained on an automatic analyzer of specific surface area and pore size NOVA 2200 E (Quantachrom, FL, USA). Processing of the results was performed using the NovaWin 11.04 (build 02) program. The specific surface calculation of the samples was carried out by the Brunauer–Emmett–Teller method (BET analysis). Gurvich’s method was used to calculate the total pore volume. The calculations of the specific surface area and volume of microspores were carried out using the t-plot method. The surface, volume, and size distribution of mesopores were determined by the Barrett–Joyner–Halenda (BDH) method from the desorption branch of the isotherm.

2.5. SCWO Reactor

SCWO of organic industrial aqueous waste from propylene epoxidation process was carried out using a batch reactor. The reactor was working in continuous regimes with a flow-through reactor containing a catalytic section with a fixed bed of a heterogeneous catalyst located internally and externally (Figure 4). The reactor, with a volume of 1.07 L and induction heating, consisted of a casing with thermal insulation. A thick-walled tube was located vertically inside the casing, the lower end of which is equipped with a collector for removing dry residues. A tube of shorter length was coaxially installed inside the pipe, connected to the reaction mixture inlet fitting. Around the outer tube, at some distance, was a copper coil-inductor, the upper and lower ends of which were equipped with terminals for supplying high-frequency current. The residues obtained during the work can be used for further processing to extract valuable and expensive components. The catalytic section was a tube-shaped reactor containing a fixed bed located therein for loading a heterogeneous catalyst. The volume of the section itself, without heterogeneous catalysts located inside it, was 2.15 L. The volume of the mixture pumped through the catalytic section was dependent on the volume of the loaded catalyst, taking into account its porosity. The experimental apparatus was additionally equipped with an air compressor (Nardi Pacific C 30, Vicenza, Italy), high-pressure liquid pump (LLC Dosing Equipment Plant Areopag, Moscow, Russia), high-frequency induction heater (China), and refrigerating machine (HolodMash, S. Petersburg, Russia). The use of a high-frequency induction heater provided high heating uniformity and high speed.

2.6. Chemical Oxygen Demand

Chemical oxygen demand (COD) test was used for measuring the initial and remaining (residual) COD in sludge, and the COD removal efficiency after SCWO treatment. The degree of efficiency of the process of organic compounds’ oxidation of wastewater was estimated using COD characterization of the process, which is a quantitative indicator of the degree of pollution of wastewater and industrial waste. COD analysis was performed using a photometric COD analyzer, “Expert-003-COD”, with a thermo-reactor for 26 samples. The central concept of the method for COD measuring is the treatment of a wastewater sample with sulfuric acid and potassium dichromate at a given temperature in the presence of silver sulfate (oxidation catalyst) and mercury (II) sulfate, used to reduce the effect of chlorides. The COD value in a given concentration range is carried out by measuring the optical density of the solution under study at a given wavelength (of 430 or 605 nm, depending on the measurement range) using the calibration dependence of the optical density of the solution on the COD value. Based on the results of the COD assessment, the degree of conversion (X) was calculated as:

$$X=1-\frac{{\mathrm{COD}}_f}{{\mathrm{COD}}_i},$$

(1)

where COD_i is the chemical oxygen demand of initial wastewater, mgO₂/l and COD_f is the chemical oxygen demand after oxidation reaction process of the wastewater organic compounds, mgO₂/l. The excess of oxygen used (α) (oxidant coefficient, OC) is calculated as:

$$\alpha =\frac{|O_2{|}_a}{|O_2{|}_s},$$

(2)

where $|O_2{|}_a$ is the actual concentration of oxygen (oxygen in air) supplied to the reactor in mg/l and $|O_2{|}_S$ is the theoretical oxygen concentration based on wastewater COD in mgO₂/L, i.e., the OC defined as the ratio of the supplied amount of oxidant in each experiment to the amount theoretically required for complete oxidation of organic matters according to the reaction stoichiometry.

Contact time τ (or residence time of liquid phase of the raw material in the flow reactor) was calculated as:

$$\tau =\frac{V}{Q_1+Q_2}\frac{v_0}{v_r}·60,$$

(3)

where V = 1.07 l is the reactor volume (l); ν₀ = 0.84 m³∙kg⁻¹ and ν_r are the specific volumes of initial effluent (industrial wastewater) at room temperature, atmospheric pressure, and reaction condition (m³∙kg⁻¹), respectively; Q₁ = 62–166 mL/min is the aqueous dilute effluent supply (feed) rate; and Q₂ = 41–99 l/min is the air supply rate. The values of ν_r were determined by calculation.

3. Results and Discussions

3.1. Textural Characteristics of Catalysts

The main quantitative textural characteristics of the synthesized catalysts’ manganese-containing sample are summarized in

Table 4. Textural characteristics of manganese-containing catalyst.

SBET/m2∙g−1	VΣ (P/P0 = 0.99)/cm3∙g−1	Micropores (t-Plot Method)		Dave/nm	Porous Structure (in the Mesopores)
		Smp/m2	Vmp/cm3∙g−1
23.8	0.0410	4.8	0.0021	7.0	Polymodal

and

Table 5. Volume distribution and surface mesopores by pore diameter (method BET, desorption branch of the isotherm).

Pore Diameter Range/nm	Mesopore Volume Fraction a/%	Specific Surface Fraction b/%
2–5	20.3	50.3
5–10	20.7	27.8
10–20	27.0	16.0
above 20	32.0	5.90

^a Total volume of the pores by BET 0.0347 cm³/g; ^b total surface by BET 14.4 m²/g.

, as an example. The distribution of mesopores by pore size is qualitatively illustrated in Figure 5.

As can be seen, the catalyst is a structure containing both meso- and micropores, with a predominant contribution to the pore volume by the mesoporous component. The size distribution of mesopores is polymodal, with about 30% of the volume occupied by pores with a diameter of over 20 nm. The main contribution to the specific surface of the sample (~ 80%) is made by pores with a diameter between 2 and 10 nm. In general, the textural characteristics of the synthesized catalysts are comparable with the catalyst for the liquid-phase hydrogenation of ketones (including acetophenone) Ni-70 (NiO-MgO) [38], characterized by a specific surface area of 23.2 m²/g, a pore volume of 0.065 cm³/g, and a polymodal distribution of mesopores with a maximum pore size of 6 nm.

3.2. Phase Composition of Catalysts

The phase composition diffractograms of the initial catalyst carrier and Fe-pillar aluminosilicates are depicted in Figure 6 and Figure 7.

Diffractograms show diffraction peaks characteristic of quartz, mica, calcite, kaolinite, and feldspars. After thermal treatment at 873.15 K, there are no peaks of kaolinite, which transforms into meta-kaolinite, in the diffractogram.

XRD analyses were used to identify crystalline phases present in the material. The diffraction patterns show the maxima of the basal and total diffraction of Fe-pillar aluminosilicates, iron oxides (maghemite, hematite), and silicon dioxide impurities (quartz, cristobalite). The interplanar spacing of 1.78 nm for acetate complexes, as compared to the same at 1.5 nm for hydroxide complexes, indicates a more efficient incorporation of oxides into the interlayer space of aluminosilicate. Iron oxides have a very low crystallinity, with an average crystallite size of 2 to 3 nm (according to Scherrer), which indirectly indicates a close relationship with the aluminosilicate matrix and potentially high catalytic activity. XRD results for the manganese-containing catalyst are depicted in Figure 8.

The main phase in the Mn-Al mixing catalyst is manganese oxide (hausmannite). In addition, gamma aluminum oxide and mixed oxides of aluminum and manganese were present in the sample. The manganese oxide crystal sizes were within 17 nm.

3.3. The Results of Oxidation of the Industrial Wastewater

The oxidation process was performed for 5 v/v % diluted wastewater in non-catalytic (SCWO) and catalytic (CSCWO) processes within the temperature range from 673.15 to 873.15 K and at pressure of P = 22.5 MPa with an oxygen excess ratio (OER) of 2.0 to 4.0 at the reaction time (from 1.8 to 4.83 min).

3.4. Chromatographic Analysis

The results of the chromatographic analysis of 5 v/v % wastewater and reaction products derived in non-catalytic (SCWO) and catalytic (CSCWO) processes are presented in

Table 6. Aromatic hydrocarbons and their oxygen-containing derivatives’ contents in the initial wastewater and in the non-catalytic (SCWO) and catalytic (CSCWO) reaction products ^a.

Sample/mg∙l−1	Initial Wastewater	Diluted Wastewater	Sample-1	Sample-2	Sample-3	Sample-4	Sample-5
Phenol	1256.6	62.8	10.139	1.462	<1.000	17.8	10.94
1-Phenyl-ethanol	1449.3	72.466	7.204	2.8004	<1	7.64	4.9
Methylphenylketone	2598.2	155.8	20.391	3.019	1.94	62.8	22.24
Toluene	0.03	0.002	<0.001	<0.001	<0.001	n/a	n/a
Styrene	92.6	4.61	1.506	0.539	0.55	1.607	1.24
Ethylbenzene	0.046	0.0019	<0.001	<0.001	<0.001	n/a	n/a
Propylene glycol	1370	69.5	n/a	n/a	n/a	n/a	n/a

^a Sample-1: T = 773.15 K, P = 22.5 MPa, τ = 2.91–3.11 min, OER = 2; Sample-2: T = 823.15 K, P = 22.5 MPa, τ = 2.91–3.11 min, OER =2; Sample-3: T = 823.15 K, P = 22.5 MPa, τ = 2.91–3.11 min, OER =3; Sample-4: With using of catalyst carrier, T = 673.15 K, P = 22.5 MPa, τ = 2.91–3.11 min, OER = 2; Sample-5: With using Fe-Ac catalyst, T = 673.15 K, P = 22.5 MPa, τ = 2.91–3.11 min, OER = 2 (Figure 5).

and depicted in Figure 9.

Quantitate analysis of the initial and diluted wastewaters and the reaction products (see Table 5) shows the considerable reduction of the main components of the wastewater, in particular, of phenol (from 62.8 to 1.0 mg/L), 1-phenylethanol (from 72.5 to 1.0 mg/L), and methylphenylketone (from 155.8 to 1.94 mg/L).

3.5. Chemical Oxygen Demand and Conversion Coefficient

The results of COD determination and conversion rate (X), calculated for all experimental data derived in non-catalytic (SCWO) and catalytic (CSCWO) processes in the presence of Fe-Ac, Fe-OH, and Mn-Al catalysts, are presented in

Table 7. COD of non-catalytic SCWO reaction products as a function of temperature, reaction time, and oxygen excess ratio (OER).

	Oxygen Excess (OER) 2				Oxygen Excess (OER) 2.5
	Reaction Time τ Minute				Reaction Time τ Minute
T, K	1.8	2.91	4.08	4.83	1.8	2.91	4.08	4.83
673.15	18598	16875	14763	12914	18296	16136	14320	12708
723.15	16040	13783	13056	12367	15793	13110	12855	12604
773.15	12358	8942	8125	7682	11895	8471	7993	7541
823.15	2983	2420	2403	2386	2910	2319	2305	2291
873.15	2185	2017	1802	1988	2125	1933	1800	1709
	Oxygen excess (OER) 3.0				Oxygen excess (OER) 4.0
	Reaction time τ minute				Reaction time τ minute
T, K	1.80	2.91	4.08	4.83	1.80	2.91	4.08	4.83
673.15	17369	15396	13222	11355	16443	14186	12125	10362
723.15	14747	12438	11829	11249	13701	11429	10803	10210
773.15	10450	8000	7804	7437	9006	7194	6882	6583
823.15	2830	2218	2105	1998	2750	2017	1905	1800
873.15	1958	1848	1754	1665	1891	1580	1488	1400

,

Table 8. COD of CSCWO reaction products (Fe-OH, Mn-Al, and with a catalyst carrier) as a function of temperature and the oxygen excess ratio at reaction time of τ = 2.91 min.

	Oxygen Excess (OER) 2			Oxygen Excess (OER) 3			Oxygen Excess (OER) 4
T/K	CSCWO (Fe-OH)	CSCWO (Mn-Al)	catalyst carrier	CSCWO (Fe-OH)	CSCWO (Mn-Al)	catalyst carrier	CSCWO (Fe-OH)	CSCWO (Mn-Al)	catalyst carrier
673.15	11250	11045	15325	10950	10080	14965	9250	8755	14535
723.15	9001	8955	13900	8255	8010	12358	7450	7020	12495
773.15	5820	5753	8733	5340	5205	8096	4735	4485	7145
823.15	1733	1635	2502	1505	1405	2015	1393	1025	1945
873.15	636	610	1915	615	595	1795	576	580	1398

,

Table 9. COD of CSCWO reaction products with a catalyst based on Fe-Ac as a function of temperature, reaction time, and oxygen excess.

	Oxygen Excess (OER) = 2			Oxygen Excess (OER) = 3	Oxygen Excess (OER) = 4
	Reaction Time τ = min			Reaction Time τ = 2.91 min	Reaction Time τ = min
T, K	1.80	2.91	4.08		1.80	2.91	4.08
723.15	12495	12124	10613	10785	10681	9731	8529
773.15	8335	8088	7080	6725	6680	6430	5640
798.15	4697	4558	3990	4060	3714	3665	3216
823.15	2130	2067	1809	1850	1729	1677	1533

and

Table 10. Conversion rate for the SCWO and CSCWO reaction products as a function of temperature, OER, and type of catalyst at reaction time of τ = 2.91 min.

OER	Reactor’s Temperature, K	SCWO	CSCWO (Fe-Ac)	CSCWO (Fe-OH)	CSCWO (Mn-Al)	Catalyst Carrier
		X	X	X	X	X
2	673.15	0.749	0.809	0.832	0.835	0.772
	723.15	0.795	0.820	0.866	0.866	0.793
	773.15	0.867	0.879	0.913	0.914	0.867
	823.15	0.964	0.969	0.974	0.975	0.963
	873.15	0.970	0.988	0.990	0.990	0.971
3	673.15	0.741	0.819	0.837	0.850	0.777
	723.15	0.781	0.839	0.877	0.880	0.816
	773.15	0.844	0.899	0.920	0.922	0.879
	823.15	0.957	0.974	0.977	0.979	0.970
	873.15	0.971	0.989	0.990	0.991	0.973
4	673.15	0.789	0.832	0.862	0.869	0.783
	723.15	0.830	0.855	0.889	0.895	0.814
	773.15	0.893	0.904	0.929	0.933	0.893
	823.15	0.970	0.975	0.979	0.984	0.971
	873.15	0.977	0.990	0.991	0.991	0.979

.

The analysis of the obtained data (see Table 7) shows that the greatest (maximal) impact (degree of the oxidation) on the results of the oxidation reaction was exerted by the excess of oxygen and the residence time of the wastewater in the reactor. With increasing temperature up to 773.15 K, COD of the reaction product monotonically decreases. However, at temperature of 823.15 K, rapid decreases of COD were observed. Upon increasing the residence time of the wastewater in the oxidation reactor from 1.8 to 4.83 min, the value of COD was reduced by 1.5 times (see Table 7). This is typical for all values of oxygen excess ratio (OER). The minimal value of COD ~1400 mgO₂/l (see Table 7) was reached for an entire range of changes in excess oxygen at the temperature of 873.15 K and at the residence time of 4.83 min.

The presence of heterogeneous catalysts and their effect on the industrial wastewater treatment process characteristics are provided in Table 8 and Table 9, and depicted in Figure 10. The dependence of COD on oxidation process parameters is similar to those previously established for the non-catalyst oxidation process; i.e., with an increase of the temperature, residence time, and oxygen excess, the COD decreases. The values of COD for reaction products presented in Table 7 and Table 8 show that the presence of the catalyst carrier exerts a slight effect on the oxidation process. The effect of synthesized heterogeneous catalysts in the process of oxidation indicates the appropriateness of their use. Thus, the values of COD in the oxidation processes with the use of catalysts Fe-OH and Mn-Al, at the same process conditions, are lower by 1.5 to 3.0 times in comparison with non-catalyst SCWO reaction. In particular, the most effective catalyst for lowering the COD of industrial wastewater, among those studied, was the manganese-containing catalyst. This catalyst can be recommended for use in SCWO processes of wastewater treatment. The derived results indicate that characteristics of the treated wastewater samples met the ecological requirements for the composition of industrial water (COD < 1000 mgO₂/l).

The impact of residence time on the efficiency of the wastewater treatment using the CSCWO process have been studied using the example of an Fe-Ac catalyst (see Table 9). In this case, in comparison with non-catalyst SCWO reaction at the same conditions, COD of the wastewater samples decreases by 20 to 40%.

The degree of the oxidation usually depends on the feed composition, residence time, and process conditions. Table 10 shows the degree of conversion of the initial wastewater calculated from Equation (1). The greatest degree of conversion is shown by the process using a catalyst based on manganese, at which the maximum value of 0.991 is achieved under the conditions: T = 873.15 K and OER (oxygen excess) = 3.0.

3.6. Phase Contents (Composition) of the Inorganic Residues

The results of the phase contents of the inorganic residues generated from the SCWO treatment are provided in

Table 11. Phase composition (contents) of the inorganic residue generated from the SCWO processes.

Fraction	Mass %	Phase Composition, Mass %
		Fe2O3	Fe3O4	MoO3	ZnO	NiO	Zn5(CO3)2(OH)6
Magnetic	63.4	8.9	8.0	60.6	7.6	10.5	4.5
Not magnetic	36.7	0.6	-	48.1	19.8	4.3	27.1
Total	100	5.9	5.1	56.0	12.1	8.2	12.7

and depicted in Figure 11.

The bulk sample of the inorganic residue consists mainly of molybdenum oxide, with oxides of zinc, nickel, and iron in smaller amounts. The magnetic fraction contains iron oxides.

3.7. Kinetic of the Process of Deep Oxidation of Organic Compounds under SCF Conditions

The industrial wastewater under study includes many different organic compounds, some of which have not been identified; therefore, it is convenient to use the COD indicator as a parameter that indicates the total concentration of the oxidizable components of the system (initial compounds and intermediate products). In this case, the chemical reaction rate is defined as the change in COD value with reaction time:

$$rate=\left|\frac{d\left[COD\right]}{d\tau }\right|=k_{eff}{\left[COD\right]}^x{\left[C_{O_2}\right]}^y{m}_{cat}^z,$$

(4)

where x, y, z is the reaction order of COD (organic components), O₂ (oxidizer) and m_cat catalyst (in the case of a non-catalytic experiment, z = 0); and k_eff is the reaction rate constant which can be presented via the Arrhenius equation (see below Equation (7)). The Equation (4) has been simplified due to following assumptions:

(1)

The determination of the oxygen concentration in the condensed phase under experimental conditions is extremely difficult; therefore, as an assumption, we accept that this value does not depend on both the temperature and the composition of the system, despite the fact that both of these parameters affect the gas solubility in the case when the condensed phase is a liquid;

(2)

In the all-catalytic experiments, the mass of the loaded catalyst was the same.

Therefore, (4) can be rewritten as:

$$\left|\frac{d\left[COD\right]}{d\tau }\right|=k_{eff}^{’}{\left[COD\right]}^x.$$

(5)

Figure 12 shows experimental data in the $\ln \frac{{\left(\mathrm{COD}\right)}_{\tau }}{{\left(\mathrm{COD}\right)}_0}$ versus τ projection for each fixed temperature. Since, for the systems under study, a linear dependence of $\ln \frac{{\left(\mathrm{COD}\right)}_{\tau }}{{\left(\mathrm{COD}\right)}_0}$ on residence time is observed (see Figure 12), it can be assumed that the reaction order in terms of oxidized components (COD) is equal to one (x = 1). Integration of the kinetic Equation (5) on residence time τ leads to the following equation (pseudo-first-order expressions):

$$ln\frac{{\left(\mathrm{COD}\right)}_{\tau }}{{\left(\mathrm{COD}\right)}_0}=-k_{eff}^{’}\tau ,$$

(6)

where [COD]_τ and [COD]₀ represent the chemical oxygen demand of the organic component of wastewater at the current τ and initial times τ=0, respectively. As one can see from Figure 12, the present experimental results confirm the correctness of the present model. Thus, the values of reaction rate constant $k_{eff}^{’}$ were estimated as an experimental slope of the plot of $ln\frac{{\left(\mathrm{COD}\right)}_{\tau }}{{\left(\mathrm{COD}\right)}_0}$ versus that of residence time τ at the fixed temperature. The activation parameters $E_a$ of the non-catalytic and catalytic oxidation reactions were determined using the Arrhenius plot $ln{k}_{eff}^{’}$ vs. $T^{-1}$ based on the data provided in Table 7 to 9. The Arrhenius equation for reaction rate constant $k_{eff}^{’}$ in logarithmic form can be expressed as:

$$ln{k}_{eff}^{’}=lnA-\frac{E_a}{RT},$$

(7)

where A is the pre-exponential factor or high temperature limit of reaction rate constant at T → ∞; $E_a$ is the activation energy, J·mol⁻¹; R is the gas constant (8.314 J·mol⁻¹·K⁻¹); and T is the temperature, K. According to Arrhenius Equation (7), experimental $ln{k}_{eff}^{’}$ is a linear function of $T^{-1}$, the slope of which is equal to $\frac{E_a}{R}$, while the intercept for $T^{-1}=0$ is related to $\ln A$. The experimental reaction rate constants $k_{eff}^{’}$ at different temperatures are plotted to obtain A and $E_a$. The derived values of the Arrhenius parameters ($E_a$ and ln A) for the oxidation reaction of 5 v/v % wastewater in the catalytic (CSCWO with a catalyst of Fe-Ac) and non-catalytic (SCWO) processes are presented in

Table 12. Parameters of the Arrhenius equation for the oxidation reaction of 5 v/v % wastewater in the catalytic (CSCWO with a catalyst of Fe-Ac) and non-catalytic (SCWO) systems as a function of OER.

System	Temperature Range, T/K	Activation Energy ${\mathit{E}}_{\mathit{a}}$/kJ·mol−1	ln A	${\mathit{k}}_{\mathbf{eff}}^{’}$·102/s−1 (at 773 K)
SCWO (non-catalytic)	673–873	23.7 ± 4.5	−0.694 ± 0.7	1.250
CSCWO (catalytic Fe-Ac)	723–873	22.8 ± 5.1	−0.524 ± 0.8	1.705

.

As can be seen, the statistical uncertainty of the Arrhenius parameters for non-catalytic and catalytic reaction systems do not differ significantly; however, the addition of a catalyst to the system increases the oxidation rate by about 1.4 times (based on the calculated values of the rate constants for an average temperature of 773 K).

The present results indicate that the molybdenum, iron, and nickel compounds contained in the effluent are catalysts for the oxidation of the organic components of the effluent by molecular oxygen, and the addition of an extra amount of a heterogeneous catalyst does not sufficiently affect the oxidation rate. The values of the apparent activation energy ($E_a$) characterizing the overall oxidation processes in the system under study are comparable with those obtained earlier in the study of deep electrocatalytic oxidation of acetaldehyde (29–33 kJ/mol [39]) and catalytic oxidation of toluene (33 kJ/mol [40]). However, these significantly exceeded the value of $E_a$ for deep catalytic oxidation of isopropanol (3–5 kJ/mol [41,42]).

4. Conclusions

Oxidation of organic components of molybdenum-containing industrial wastewater was performed under supercritical conditions (at a pressure of 22.5 MPa and in a temperature range from 673.15 K to 873.15 K), with an oxygen excess ratio of 2 to 4), and in the presence of various types of heterogeneous catalysts. The results indicated that nearly 100% of naphthalenes and more than 97% of alkanes could be destroyed at temperatures above 748 K, for t > 2 min, and OC > 2.5. The results of X-ray diffractometer (XRD) and scanning electron microscope (SEM) analyses suggested that SCWO can efficiently remove organic pollutants from aqueous organic waste epoxidation of propylene with ethylbenzene hydroperoxide. A solid-phase residue (solid product) containing oxides of molybdenum, nickel, zinc, and iron was generated by the SCWO treatment of the wastewater. A decrease in aromatic oxygen-containing compounds of the wastewater was observed for both the non-catalytic and catalytic SCWO reactions. The effect of synthesized heterogeneous catalysts in the process of oxidation indicates the appropriateness of their use. A considerable increase, by 20%, in the efficiency of the SCWO utilization process of wastewater in the presence of a heterogeneous catalyst was found. The present results demonstrated that among all of the catalysts (Fe-Ac, Fe-OH, Mn-Al), a manganese-containing catalyst (Mn-Al) provides the best result in the oxidation process; with a manganese-containing catalys, the organic compounds’ removal efficiency was increased by 7.5 to 20% (depending on catalysts type) with respect to non-catalytic oxidation. The values of COD in the oxidation processes using catalysts Fe-OH and Mn-Al under the same process conditions are lower by 1.5 to 3.0 times in comparison with non-catalyst SCWO reaction. A manganese-containing catalyst can be recommended for use in SCWO processes of wastewater treatment to improve the oxidation process. The derived results indicate that characteristics of the treated wastewater samples met the ecological requirements for the composition of industrial water (COD < 1000 mgO₂/l). In addition, temperature and pressure showed an influential effect on the organic compounds’ decomposition efficiency. The greatest degree of conversion (the best operating conditions for organic compounds in industrial wastewater treatment) was found with the process using a catalyst based on manganese under the conditions: T = 823.15 K, P = 25 MPa, OER within n = 3, and τ = 2.91 min. SCWO was very effective for industrial wastewater treatment, and over 99.1% COD removal could be achieved under these conditions. A considerable reduction in the main components of the wastewater, in particular, of phenol (by 59%), 1-phenylethanol (by 38%), and methylphenylketone (by 25%), was observed after SCWO and CSCWO treatment of the industrial wastewater samples.

The greatest (maximal) impact (degree of the oxidation) on the results of the oxidation reaction was exerted by the excess of oxygen and the residence time of the wastewater in the reactor. We experimentally observed rapid decreases of COD at a temperature of 823.15 K. By increasing of the residence time of the wastewater in the oxidation reactor from 1.8 to 4.83 min, the value of COD was reduced by 1.5 times. This is typical for all values of OER. The minimal value of COD ~1400 mg O₂/l was reached for the entire range of changes in OER at the temperature of 873.15 K and at residence time of 4.83 min. COD decreased with an increase in temperature, residence time, and OER. The impact of residence time on the efficiency of the wastewater treatment using CSCWO process (catalyst Fe-Ac), in comparison with non-catalyst SCWO reaction at the same conditions, showed that COD of the wastewater samples decreased by 20 to 40%. In addition, based on the results of the kinetics of the process of organic compound oxidation under SCF conditions, it was concluded that molybdenum, iron, and nickel compounds contained in the effluent are catalysts for the oxidation of the organic components of the effluent with molecular oxygen. The addition of an extra amount of a heterogeneous catalyst does not significantly affect the oxidation rate.