Subcritical Water Oxidation Research Articles

Mineralization of clofibric acid by persulfate-promoted catalytic subcritical water oxidation process using CoFe2O4@SiO2 catalyst

The design and use of innovative treatment processes are very important in preventing the possible toxic effects of organic pollutants in aquatic environments. One of these methods is the subcritical water oxidation method, which has been used recently. In the current study, the mineralization of clofibric acid (CFA) was carried out under more effective and mild conditions using persulfate (PS) as an oxidant and CoFe2O4@SiO2 catalyst by the subcritical water oxidation (sub-CWO) process. Characterization of the synthesized catalyst was performed through XRD, FTIR, TEM and SEM–EDS analyses. In the CFA oxidation with persulfate-promoted catalytic Sub-CWO process, optimum working conditions was determined as 15 mM PS, 40 min, 383 K, and 0.3 g L−1 catalyst dosage using the response surface method and Box–Behnken design. The catalyst's efficiency remained relatively stable after three cycles under optimal conditions, resulting in a 97% total organic carbon (TOC) removal. Decomposition products were determined and a degradation mechanism was proposed.

Mechanism of catalytic subcritical water oxidation of m-nitroaniline and nitrogen conversion by CuCo2O4 catalyst

Catalytic subcritical water oxidation (CSWO) of M−nitroaniline (M−NA) was investigated in a fused quartz tube reactor (FQTR) to determine the effects of reaction parameters on M−NA mineralization rate, total nitrogen (TN) removal rate and nitrogen-containing products distribution. The results indicated that the TN removal rate was only 26.36 % at 200 °C, 200 % excess oxidant and 15 min residence time, while the TN removal rate reached 67.83 % at 360 °C in SWO of M−NA. Compared with SWO of M−NA, the mineralization rate and TN removal rate of CSWO of M−NA increased by about 10 % and 17.83 %, respectively. The main reason is mainly attributed to CuCo2O4 catalyzing the preferential reaction of ·NO2 with ·NH4, hastening N2 generation. Alternatively, the CuCo2O4 catalyst may induce a direct reaction between the nitro and amino groups on M−NA within the benzene ring to form N2. Moreover, degradation pathways and nitrogen migration mechanism of M−NA under SWO/CSWO conditions were proposed. These crucial findings are expected to guide the development of environmentally sustainable catalytic technologies towards addressing significant water pollution challenges.

Removal of imipramine using advanced oxidation processes: Degradation products and toxicity evolution

Pharmaceuticals are frequently detected in natural and wastewater bodies, and are very important in environmental toxicology because of their stable nature. Advanced oxidation methods used to remove contaminants are of great benefit, especially removing pharmaceuticals unsuitable for biodegradation. In this study, imipramine was degraded by anodic oxidation and subcritical water oxidation, which are advanced oxidation methods. The determination of degradation products was performed by Q-TOF LC/MS analysis. The genotoxicity and cytotoxicity of the degradation samples were determined by the in vivo Allium Cepa method. Among the anodic oxidation samples, the lowest cytotoxicity was obtained after using 400 mA current, and 420 min of degradation time. No cytotoxic effect was observed in any subcritical water oxidation sample. However, when 10 mM hydrogen peroxide as an oxidant was used at 150 °C and the reaction time was 90 min, the subcritical water oxidation sample showed a genotoxic effect. The results of the study showed that it is crucial to evaluate the toxicity levels of the degradation products and which advanced oxidation methods are preferred for removing imipramine. The optimum conditions determined for both oxidation methods can be used as a preliminary step for biological oxidation methods in the degradation of imipramine.

Treatment of Sugar Industry Wastewater by Using Subcritical Water as a Reaction Media

AbstractThe sugar industry is one of the most wastewater‐producing industries and it contains high content of organic and inorganic substances. Treating and reusing wastewater has significant importance because sugar industry needs to use a high volume of water. In this study, sugar industry wastewater was treated under subcritical conditions and the impacts of reaction temperature and duration over TOC removal percentage were investigated. Additionally, the impact of NaOH concentration over TOC removal percentage was examined. The highest TOC removal was obtained almost 95 % in the presence of 0.1 M of NaOH at 240 °C for 90 min of reaction duration. Treatment of sugar industry wastewater by subcritical water oxidation followed the second‐order reaction kinetic model and the activation energy was found as 11.41 kJ/mol. Furthermore, the intermediate products were identified via GC‐MS.

Catalytical efficiency, mechanism and characterization of hydrolysed waste eggshell in the subcritical water oxidation of pistachio processing wastewater

Sustainable, effective and environmentally friendly methods are needed in wastewater treatment as quality water is necessary for a healthy life. Valorisation of solid food waste is also of great importance. This study examines the effectiveness of hydrolysed waste eggshells (HES), a green catalyst, in pistachio processing wastewater (PPWW) treatment using subcritical water oxidation (SWO). HES was prepared in the subcritical water medium (513 K, 100 bar of N2, 2 h) with a 92.3% yield. 88.8% of COD, 100% of both TPC and color removals of PPWW were achieved. The effects of independent variables such as temperature (363–403 K), treatment time (20–100 min), the concentration of H2O2 (0.25–1.25 M) and amount of catalyst (0–100 mg/100 mL) on the responses were investigated using response surface methodology (RSM). SEM, EDX and XRD were used to investigate the characterization of the waste eggshells. The percentage of Ca in HES increased with the hydrolysis, thus CaO increased the catalysis of hydrogen peroxide to form hydroxyl radicals.

Treatment of methyldiethanolamine wastewater using subcritical and supercritical water oxidation: parameters study, process optimization and degradation mechanism.

In this examination, sub/supercritical water oxidation (SCWO) in a batch reactor was employed to degrade methyldiethanolamine (MDEA). To do so, the impact of different operating parameters including temperature (300-500°C), time (0-100s), initial MDEA concentration (1000-4000ppm), oxidant coefficient (0.7-2), and pH (7.3-9.5) on MDEA degradation was separately and together investigated. Subsequently, the response surface method (RSM) was applied to optimize the operating condition of MDEA degradation. Based on the obtained results, a maximum amount of 97.4% MDEA degradation was achieved at the initial MDEA concentration of 1095ppm in optimal condition (i.e., oxidant coefficient: 1.913, temperature: 472°C and residence time: about 17s). Furthermore, according to the HPLC analysis, there was a negligible amounts of formic acid (CH2O2) and nitrous acid (HNO2) in the solution at the end of MDEA removal experiment. Eventually, the mechanism of MDEA degradation was acquired using molecular dynamics simulation (MDS), which had an acceptable coordination with the experimental results. In this way, the MDS results revealed that the presence of CH2O2 and HNO2 compounds in the products was related to the degradation of MDEA and their production as by-products during the SCWO experiments.

Water recovery from pistachio processing plant wastewater using subcritical water oxidation method

This study aims to apply an efficient and environmentally friendly subcritical water oxidation (SWO) method for the treatment of pistachio processing plant wastewater (PPPW). The effects of temperature (336.2–399.8 K), the concentration of H2O2 (0.08–0.92 M) and treatment time (6.4–73.6 min) on the oxidation of PPPW were investigated using the SWO method and experimental parameters were optimized using the response surface method. While the highest chemical oxygen demand (COD) removal was obtained as 53.8% in 73.6 min at 373 K using 0.5 M of H2O2, the highest obtained total phenol (TP) and color removals were 92.80%, and 99.6%, respectively, in 20 min at 373 K using 0.75 M of H2O2. R 2 values of COD, TP and color removal models were 0.9694, 0.9868 and 0.9649, respectively, and F values of the models were 35.24, 82.86 and 30.54, in the same order. In the vast majority of the experimental samples, the concentrations of nitrate, total phosphate, total nitrogen and suspended solids were found to be decreased while the nitrite concentration was increased after the SWO process. The effectiveness of SWO in water recovery from PPPW was demonstrated by the high, COD, TP and color removal values obtained.

Degradation of emerging contaminant coumarin based on anodic oxidation, electro-Fenton and subcritical water oxidation processes

The degradation of emerging contaminant coumarin was separately investigated in anodic, electro-Fenton and subcritical water oxidation processes. With respect to the anodic and electro-Fenton oxidation, the influence of constant current, treatment time and initial concentration of coumarin was studied. Regarding subcritical water oxidation, the effect of the oxidant concentration, temperature, treatment time and initial coumarin concentration was investigated. In anodic and electro-Fenton oxidation processes, coumarin degradation proceeded in a similar manner, achieving 99% degradation, after 180 min at a constant current of 200 mA. In both set-ups, further increasing the applied current lowered the degradation efficiency due to the formation of by-products and the increasing occurrence of side-reactions. The highest degradation of 88% was achieved in subcritical conditions, specifically at 200 °C, using 150 mM H2O2 and after 37.5 min of treatment. Under subcritical conditions, temperature was the most prominent parameter, followed by the H2O2 concentration. Under all methodologies, increasing treatment time had a small positive effect on coumarin degradation, indicating that time is not the most influential parameter. A comparison of the three methodologies in terms of performance as well as energy consumption and simplicity of operation highlighted the advantages of subcritical water oxidation.

Degradation of isoniazid by anodic oxidation and subcritical water oxidation methods: Application of Box–Behnken design

Pharmaceutical compounds released into the aquatic environment are known to cause toxic effects on the environment. Isoniazid is widely used in the treatment of tuberculosis and is, therefore, frequently encountered in environmental waters. In this study, the degradation of isoniazid was investigated by anodic oxidation and subcritical water oxidation method which are members of Advanced Oxidation Processes. The Box–Behnken Design was used to determine the effects of current, initial concentration, and electrolysis time on mineralization in the anodic oxidation process, which carried out a cell with a Pt cathode and boron-doped diamond anode. The highest mineralization value of 78.14% was achieved at optimal conditions of 300 mA, 3 h, and 100 mg/L initial concentration. The degradation of Isoniazid was also investigated under subcritical water conditions using an ecological oxidizing agent, H2O2. The maximum mineralization rate of 72.23% was obtained when 100 mM H2O2 was used for a 90 min treatment at 125 °C for 100 mg/L Isoniazid solution in the subcritical water oxidation process. The LC-MS results showed that the degradation products obtained by AO and SWO methods were different from each other. Finally, possible degradation mechanisms are proposed according to the degradation products obtained for both processes.

The effect of different types of AOPs supported by hydrogen peroxide on the decolorization of methylene blue and viscose fibers dyeing wastewater.

Wastewater from the textile industry containing a high concentration of organic and inorganic chemicals has strong color and residual chemical oxygen demand (COD). Therefore, advanced oxidation processes (AOPs) are very good candidates to treat textile industry wastewater. In this study, we investigated the effect of different types of AOPs supported with hydrogen peroxide (H2O2) for the treatment of viscose fibers dyeing wastewater. Fenton, photo-Fenton, and Fenton-supported subcritical water oxidation (FSWO) processes were chosen as AOPs to compare the treatment efficiency of viscose fibers dyeing wastewater. The effects of solution pH, Fe2+ concentration, and H2O2 concentration on the treatment of viscose fibers dyeing wastewater were tested. The maximum color and COD removal efficiency was obtained corresponding to pH 2.5 for all oxidation methods when methylene blue (MB) dye solution was used. However, the maximum efficiencies were obtained at pH 3.0 for real textile wastewater decolorization. The MB dye removal efficiency was increased to 97.22, 100, and 100% for Fenton, photo-Fenton, and FSWO processes, respectively, when the addition of H2O2 concentration was adjusted to 125 mg/L. However, the maximum color removal efficiencies of viscose fibers dyeing wastewater were obtained 56.94, 61.26, 64.11% for Fenton, photo-Fenton, FSWO processes, respectively. As a result, the FSWO showed maximum color removal efficiencies.

Comparative degradation of 5-fluorouracil in aqueous solution by using H2O2-modified subcritical water, photocatalytic oxidation and electro-Fenton processes

This study investigated the degradation of the antineoplastic agent 5-fluorouracil (5-FU) widely applied to treat different cancers using different advanced oxidation processes such as electro-Fenton (EF), photocatalysis with TiO2, and H2O2-modified subcritical water oxidation. The treatment with the EF process was the most efficient compared to others. Interestingly, in the EF process, the oxidative degradation of 5-FU behaved differently depending on the anode used. At low currents (20 and 40 mA), Pt and DSA anodes performed better than BDD and Ti4O7 anodes. In contrast, at the higher current of 120 mA, the production of heterogeneous hydroxyl radicals (M(•OH)) became important and contributed significantly to the oxidation of 5-FU in addition to homogeneous •OH generated in the bulk solution. These latter have high O2-evolution overpotential leading to the high amount of physisorbed M(•OH) compared to Pt and DSA. The oxidative degradation of 5-FU was then performed by titanium dioxide-based photocatalytic oxidation and subcritical water oxidation processes, both of which showed a lower degradation efficiency and failed to achieve complete mineralization. Finally, a comparison was performed in laboratory-scale, taking into account the following performance indicators: the degradation efficiency, the mineralization power, the cost of equipment and reagents, and the energy required for the treatment of 5-FU.

Determining the kinetic constants leading to mineralization of dilute carbamazepine and estradiol-containing solutions under continuous supercritical water oxidation conditions

Continuous subcritical and supercritical water oxidation experiments were conducted on dilute carbamazepine- and estradiol-containing synthetic solutions used to simulate the removal of model emerging pollutants from secondary municipal effluents. The operating conditions comprised 340–500 °C, retention time of 24–453 s and a stoichiometric oxidant ratio (O.C.) between 4 and 64. The transformation of the various species was determined at the outlet and by modeling a segmented non-isothermal reaction system. Four empirical power law kinetic models were established to represent both the pollutants' degradation and TOC removal efficiencies, using nonlinear multiple regression coupled with bootstrapping and K-fold cross-validation. The mineralization and degradation models for both pollutants yielded a R2 of 67–80.5% vs. the experimental results. Discussion on the various model assumptions revealed that attributing full model deviations to the constant oxygen concentration or to the laminar reactor flow, yielded a deviation of 6% and 15% in the removal efficiencies, respectively. However, the expected deviation of the models was lower than 0.32% at conditions leading to (almost) full mineralization (45–60 s, 480–500 °C and O.C.s of 5–10). The methodologies developed in the study are useful for interpreting future results obtained from SCWO of actual secondary effluent solutions.

Experiment of subcritical water (Fenton) oxidation treatment of methyl vanillin wastewater

In this paper the oxidative degradation process of methyl vanillin wastewater was studied by the subcritical water oxidation (HCWO) technology. A subcritical Fenton oxidation (HCFO) system formed while Fe2+ was added as a catalyst. The oxidation degradation kinetics of methyl vanillin wastewater was also studied. The results showed that the suitable process conditions for degradation of methyl vanillin wastewater by HCWO were as follows: temperature of 340 ​°C, pressure of 24 ​MPa, oxidant multiple of 1.5, residence time of 217.3 ​s (flow rate of 2.0 ​mL ​min-1). For methyl vanillin wastewater, the HCFO system has no obvious advantages compared to the HCWO system. The activation energy (Ea) of HCWO oxidized methyl vanillin wastewater reaction was 32.6 ​kJ ​mol-1, and the pre-exponential factor A was 5.64 s-1.

Fate of PAHs under subcritical and supercritical conditions in landfill leachate: Removal or formation?

In this study, fate of eight polycyclic aromatic hydrocarbons (PAHs) described as priority pollutants by the European Union during the subcritical and supercritical water oxidation (SCWO) of landfill leachate was investigated. The experiments were conducted under the temperature of 250–500 °C, pressure of 10–35 MPa, residence time of 2–18 min and dimensionless oxidant dose (DOD) of 0.2–2.3. The results revealed that the SCWO process had a high potential for degradation of PAHs except for four and more rings from leachate having high organic and inorganic pollutant loads. The removal efficiencies of chemical oxygen demand (COD), naphthalene (NAP), anthracene (ANT) and fluoranthene (FL) increased up to 98%, 97%, 95%, and 87%, respectively. However, the high removal efficiencies of COD did not directly correlate with the removal of PAHs. 2- to 4-ring PAHs increased in subcritical and supercritical conditions at 0.4 DOD. Additionally, PAH compounds such as pyrene and chrysene not found in raw leachate were detected in some treated samples at low DOD and high temperature. It was concluded that aromatic substances detected in the leachate could transform to naphthalene, anthracene and fluoranthene with merger/separation reactions. On the other hand, aldehyde, ketone and phenol-based compounds were detected as the transformation products when the oxidation mechanism was dominant. Therefore, optimum experimental conditions that aimed to both minimize PAHs formation and maximize COD removal were determined as 7 min, 2 DOD, 375 °C and 22.5 MPa.

Water recovery from textile bath wastewater using combined subcritical water oxidation and nanofiltration

Abstract In this study, textile dye bath wastewater was treated by subcritical water oxidation (SCWO) and pressure-driven membrane process. 54.5% of chemical oxygen demand (COD) reduction and 91.7% of color removal were obtained using eco-friendly SCWO method in the presence of H2O2. Box-Behnken design was performed to optimize the SCWO process and precision of the models were evaluated by ANOVA. Two different nanofiltration (NF270 and NF90) membranes were tested before and after SCWO pretreatment. The effect of membrane type (NF270 and NF90) and transmembrane pressure (10, 20, 30, 40 bar) were studied to improve COD reduction and colour removal efficiency. The results showed that steady-state permeate flux increased from 6.6 to 23.5 L/m2.h for NF270 at 30 bar and from 10.0 to 12.4 L/m2.h for NF90 membrane at 40 bar when SCWO was applied as the pretreatment method. The results depicted that SCWO pre-treatment increased the permeate flux almost 2–3 fold. COD was also tested for all permeates and the highest COD reduction efficiency was obtained for SCWO + NF90 combined process. The highest COD reduction and color removal efficiencies were obtained as 96.1% and 99.3%, respectively, for combined SCWO + NF90 process at 40 bar. The proposed combined SCWO + NF90 process was successfully used to recover wastewater from textile dye bath wastewater.

Treatment of agrochemical wastewater by subcritical water oxidation method: chemical composition and ion analysis of treated and untreated samples

This work aimed at offering an effective and environmentally friendly approach to the real wastewater of an industrial agrochemicals production plant containing high amounts of many hazardous compounds that threaten human health. The removal of total organic carbon (TOC) (59.45%) and colour (97.92%) of the wastewater was achieved using subcritical water oxidation method. The fate of the detected compounds was followed by GC–MS analysis. Several pollutants such as phenol, fumaric acid, chlorpyrifos, penconazole, brassilexin, buprofezin, etoxazole, pyriproxyfen and 2-naphthalene-sulphonic acid which are bio-refractory and harmful to human health, were effectively degraded. Inorganic ions exist in the wastewater or formed through the process and their possible effects on the applied method were analysed. The central composite design was used to optimise the method and fully evaluate the single or combined effects of the method parameters on the removal rates. The precision of the applied design models was evaluated employing ANOVA, Regression coefficients and validation analysis. F and P and R2 values were obtained as 107.43, <0.0001, and 0.9898, respectively in the TOC removal model and 39.45, <0.0001, and 0.9726 in the colour removal model.

Application of ultrasound-assisted and subcritical water oxidation methods in the mineralisation of Procion Crimson H-EXL using response surface methodology and artificial neural network

Eco-friendly methods, the ultrasound-assisted oxidation (UAO) and the subcritical water oxidation (SWO) methods, were applied to mineralise the widely used commercial reactive azo dye, Procion Crimson H-EXL in the presence of H2O2. 72.20% and 72.86% of total organic carbon removal were achieved in the UAO and SWO methods, respectively. The Box-Behnken design (BBD) was applied to design the experimental processes and optimise both methods. ANOVA and validation tests were performed to assess the employed models. F and P values were obtained as 36.72 and <0.0001 in the UAO method, respectively, and 605.97 and <0.0001 in the SWO method, respectively. The artificial neural network (ANN) was applied in both the UAO and the SWO methods. The predictive performance of the BBD and ANN models were evaluated and compared to each other over R2, root mean square error and absolute average deviation values.

Multi-response central composite design of the mineralization and removal of aniline by subcritical water oxidation method

Aniline is used by many industrial organizations and is released into the environment in high amounts and pollutes the water. Due to the damages to human health, there is a need for effective and environmentally friendly methods to remove this compound from the water. The aim of this work is to mineralize and remove aniline by investigating total organic carbon removal and aniline removal, respectively, via using the eco-friendly method, subcritical water oxidation and green oxidizing agent, H2O2. The effect of process variables such as temperature, treatment time and H2O2 concentration were evaluated. Total organic carbon removal of aniline was obtained as 92.73% in 80 min of treatment time and in the presence of 60 mM of H2O2 at 403 K. 99.21% of aniline removal was achieved based on the UV spectrophotometric analysis at these conditions. The response surface method was used to optimize the performed model and the effect of the above-mentioned parameters. The reliability of the method was provided by ANOVA.

Application of the eco-friendly subcritical water oxidation method in the degradation of epichlorohydrin

Degradation of epichlorohydrin was investigated using subcritical water oxidation method in the presence of hydrogen peroxide. Degradation rate was monitored by means of total organic carbon (TOC) analysis. The central composite design was used to determine optimal TOC removal conditions and modelling experimental process. The effects of all experimental variables (temperature, oxidant concentration of hydrogen peroxide and treatment time) on the TOC removal rates were evaluated and the theoretical prediction model was proposed. Reliability of the employed model was evaluated using ANOVA. F value and the p-value of the model were found to be 84.60 and &lt;0.0001, respectively. 93.78 % of TOC removal was achieved in the degradation of epichlorohydrin at 373 K of temperature and 75 min of treatment time using 90 mM of H2O2.

An approach to apply eco-friendly subcritical water oxidation method in the mineralization of the antibiotic ampicillin

Mineralization of ampicillin, a semi-synthetic β-lactam penicillin, was investigated in subcritical water medium in the presence of hydrogen peroxide. Optimisation of the employed method was performed by the central composite design of the response surface methodology. Effects of each independent variable such as temperature, treatment time, and concentration of the oxidizing agent and their co-effects on TOC removal percentages were evaluated. 81.59% of TOC removal was obtained at 403 K of temperature, 80 mM of hydrogen peroxide and 147.3 min of treatment time. The reliability of the method was assessed by ANOVA and the theoretical equation was proposed for TOC removal of ampicillin. F value was obtained as 113.88 and p-value was obtained to be lower than 0.0001. The applicability of the theoretical model was proved by a series of validation experiments.