Total Organic Carbon Removal Ratio Research Articles

Oxygen-doped and pyridine-grafted g-C3N4 for visible-light driven peroxymonosulfate activation: Insights of enhanced tetracycline degradation mechanism

A new metal-free O-doped and pyridine-grafted g-C3N4 (OCN/Py) was prepared and used to degrade tetracycline (TC) with peroxymonosulfate (PMS) as oxidant under visible light irradiation. Doping of O and graft of Py into CN framework could enhance the separation of photogenerated charge carriers, the adsorption of PMS and TC, as well as PMS activation. Experiments and theoretical calculations confirmed that the OCN/Py-PMS* complex was formed and mediated electron transfer for degrading TC. In addition, the ⋅O2−, •OH, surface-bound SO4•−, 1O2, especially holes contributed to TC removal. 97.0 % TC was degraded after 60 min visible light irradiation in the presence of 0.16 g L–1 OCN/Py and 0.5 mM PMS. The corresponding total organic carbon removal ratio and apparent rate constant were 97.37 % and 0.0568 min−1, respectively. This work provides an effective way to change CN structure for boosting TC degradation, and gives deep insights for the relationship among photogenerated charge separation, PMS activation and TC degradation.

Process optimization of dielectric barrier discharge reactor for chloroform degradation using central composite design

The increased detection of chloroform in wastewater and surface water bodies has become an environmental concern. However, conventional treatment methods such as biodegradation, incineration, and photocatalysis are challenging to scale up, inducing high operation costs and generating toxic byproducts. Hence, this study employs the Response surface methodology (RSM) to optimize chloroform degradation using a spray dielectric barrier discharge (DBD) reactor for the first time. The independent and interactive influence of the DBD parameters (power: 100–200 W, treatment time: 5–20 minutes, recirculating flow rate: 50–200 mL/M) and responses (chloroform degradation ratio (R), total organic carbon removal ratio (TOC)) were optimized using the Rotatable central composite design (RCCD), and a second-order polynomial equation was proposed to predict process efficiency. Results showed significantly predicted values (p < 0.05) and coefficient of determination of 0.96, and 0.93 for TOC and R, respectively, with treatment time and power having significant independent and interactive effects on the responses (TOC: p < 0.0001, R: p < 0.0001) concerning the variance analysis. Accordingly, the model yielded optimum recirculating flow rate, discharge power, and treatment time of 50 mL/min, 200 W, and 20 mins, respectively, which amounted to complete dechlorination of 300 mg/L chloroform, a 43% TOC removal ratio and an energy yield of 4.6 g/kW·h. In addition, the TOC removal ratio was enhanced with the increase in pH and conductivity of the solution.

Facile synthesis of carbon-doped CoMn2O4/Mn3O4 composite catalyst to activate peroxymonosulfate for ciprofloxacin degradation

In this work, carbon-doped CoMn2O4/Mn3O4 (C-CoMnO) composite catalyst were prepared via calcining solvothermal product of Mn2+, Co2+, and 2-methylimidazole in air. The catalytic performance of activating peroxymonosulfate (PMS) for ciprofloxacin (CIP) degradation was discussed. The results showed that the 0.1 g L−1C-2CoMnO could effectively activate PMS, resulting in 96.18% CIP was degraded within 60 min. The corresponding total organic carbon removal ratio and apparent rate constant were 81.33% and 0.043 min−1, respectively. The influencing factors, such as Co/Mn molar ratios, calcination temperature, PMS concentration, solution pH, and coexisting ions were explored. The optimal catalytic system exhibited good recycling performance, wide pH adaptation and high resistance to salt interference. SO4⋅−, ⋅OH, and ⋅O2−-based radicals, as well as 1O2 and surface complexes-based nonradicals were participated in CIP degradation. The CIP degradation pathways were proposed. Density functional theory calculations (DFT) indicated that PMS was more easily activated via Co/Mn synergism. The doped carbon was also contributed to PMS activation via facilitating electron transfer. This study may shed light toward the synergistic effect of metal oxides in PMS activation.

Investigation on microwave absorbing properties of 3D C@ZnCo2O4 as a highly active heterogenous catalyst and the degradation of ciprofloxacin by activated persulfate process

This study investigated the degradation of ciprofloxacin (CIP) by activated persulfate with three-dimensional C@ZnCo2O4 (3D C@ZCO) under microwave irradiation. The 3D C@ZCO material was successfully prepared by a hydrothermal method. The morphology, microstructure, and microwave absorption properties were analysed by several characterization techniques. The effects of some influencing factors, such as the CIP concentration, persulfate dose, initial pH, 3D C@ZCO dose, and presence of organic matters and co-existing anions were researched in depth. The radical scavenger study document that SO4−, OH, and O2− radicals are prominent in the 3D C@ZnCo2O4-activated persulfate under microwave irradiation process. Under the following conditions, [CIP] = 10 mg/L, [3D C@ZCO] = 1.5 g/L, [persulfate] = 0.25 mM, [pH]0 = 6.5, microwave = 600 W, and T = 40 °C, the removal of CIP reached 90%, and the total organic carbon removal ratio reached 73.14% after 40 min. Toxicity tests indicated the significance of exploring the toxicity of intermediate products. Several stable intermediate products were identified, and three degradation pathways were speculated for CIP degradation.

Carbonate-enhanced catalytic activity and stability of Co3O4 nanowires for 1O2-driven bisphenol A degradation via peroxymonosulfate activation: Critical roles of electron and proton acceptors

Transition-metal catalysts (TMCs) for peroxymonosulfate (PMS) activation suffer from low stability (i.e. severe metal leakage and poor reusability) when maintaining high activity in water decontamination. An innovative carbonate (CO32−)-mediated method to synchronously enhance the catalytic activity and stability of TMCs was developed herein. In a model PMS/Co3O4 nanowire system for bisphenol A (BPA) degradation, the first-order kinetic constant and total organic carbon removal ratio were increased by 202.27% and 71.32% upon adding CO32−, respectively. Meanwhile, the cobalt release amount was significantly reduced from 4.90 to 0.03 mg/L, and the number of reuse with high efficiency (>90% of BPA removal within 10 min) was augmented from 1 to 3 times. The CO32− buffered pH decline to repress metal leakage, and promoted Co(III) reduction into Co(II) to avoid the over-oxidation of catalyst. Under the driving of CO32−, the dominated reactive species were switched from •OH/SO4•- to 1O2 accompanying the migration of catalytic center from Co(II) to Co(III). The Co(III) and CO32−/OH- acted as electron and proton acceptors, respectively, to accelerate PMS decomposition into SO5•- and subsequent generation of vast 1O2. This work proposes a green way to construct novel 1O2-based catalytic systems with excellent activity and stability for pollution remediation.

Methylene blue degradation by the VUV/UV/persulfate process: Effect of pH on the roles of photolysis and oxidation

This study investigated methylene blue (MB) degradation by the vacuum-ultraviolet/ultraviolet/persulfate (VUV/UV/PS) process using a mini-fluidic VUV/UV photoreaction system. Results show that MB degradation by the VUV/UV/PS process was significantly higher than that of the conventional UV/PS process, as the VUV photolysis of H2O and PS generated more reactive oxygen species (ROSs). HO• and SO4•−, identified as the main ROSs, were mostly consumed by dissolved organic carbon and Cl‒ in real waters, respectively. Additionally, the impacts of solution pH and the concentrations of PS, humic acid, and inorganic ions (HCO3‒, Cl‒, NO3‒, SO42‒, Fe(II), and Fe(III)) were systematically evaluated. The solution pH significantly affected the photon absorption distributions, as well as the contributions of photolysis and oxidation to MB degradation, resulting in different variations in the degradation rate constant and total organic carbon removal ratio with increasing solution pH. At all tested pH levels (3.0–11.0), particularly under acidic conditions, HO and SO4− were two predominant contributors to MB degradation, while VUV and UV photolysis contributed more when the solution pH increased. This study provides a highly efficient process for organic pollutant removal, which could be applied in water treatment.

Hydrolyzed polyacrylamide biotransformation in an up-flow anaerobic sludge blanket reactor system: key enzymes, functional microorganisms, and biodegradation mechanisms.

Hydrolyzed polyacrylamide (HPAM) biotransformation in an up-flow anaerobic sludge blanket reactor including biodegradation performances, biodegradation mechanisms, key enzymes, and functional microorganisms was explored. Response surface methodology was applied to further improve HPAM degradation. The predicted degradation ratios of HPAM and CODCr were 46.2% and 83.4% under the optimal conditions. HPAM biodegradation ratio and total organic carbon removal ratio reached 40.5% and 38.9%. Total nitrogen concentration was dramatically decreased with the increasing fermentation time during the fermentation, while low ammonia nitrogen (NH4+-N) and nitrite nitrogen (NO2--N) were generated. NH4+-N and NO2--N increased slightly on the whole. Enzyme activity change was correlated with HPAM biodegradation. Dehydrogenase activity had a decline of 21.3-41.0%, and the minimum value occurred at 300mg/L of HPAM. Urease activity was varied from 28.7 to 78.7% and the maximal inhibition ratio occurred at 200mg/L of HPAM. Mechanisms for the biodegradation of HPAM were also explored by FT-IR, HPLC, and SEM. The results indicated that long-chain HPAM was broken into micromolecule compounds and the amide groups of HPAM were transformed into carboxyl groups. Based on the sequencing results on an Illumina MiSeq platform, Proteobacterias, Bacteroidetes, and Chloroflexi were turned out to be the critical microorganisms involved in HPAM degradation. This work lays a basis for HPAM-containing wastewater treatment and offers a support for water saving and emission reduction. It is of great significance to the sustainable development of oilfield.

Constructing a Z-scheme Heterojunction of Egg-Like Core@shell CdS@TiO₂ Photocatalyst via a Facile Reflux Method for Enhanced Photocatalytic Performance.

A well designed and accurate method of control of different shell thickness and electronic transmission in a Z-scheme core@shell system is conducive to obtaining an optimum photocatalytic performance. Herein, the Z-scheme heterojunction of egg-like core@shell CdS@TiO2photocatalysts with controlled shell thickness (13 nm, 15 nm, 17 nm, 22 nm) were synthesized by a facile reflux method, and the CdS@TiO2 structure was proved by a series of characterizations. The photodegradation ratio on methylene blue and tetracycline hydrochloride over the 0.10CdS@TiO2 composites with TiO2 shell thickness of 17 nm reached 90% in 250 min and 91% in 5 min, respectively, which was almost 9.8 times and 2.6 times than that of TiO2 and CdS on rhodamine B respectively under visible light. Besides, the higher total organic carbon removal ratio indicated that most of the pollutants were degraded to CO2 and H2O. The Z-scheme electronic transfer pathway was studied through radical species trapping experiments and electron spin resonance spectroscopy. Moreover, the relationship between shell thickness and photocatalytic activity demonstrated that different shell thickness affects the separation of the electron and holes, and therefore affected the photocatalytic performance. In addition, the effects of pollutants concentration, pH, and inorganic anions on photocatalytic performance were also investigated. This work can provide a novel idea for a well designed Z-scheme heterojunction of core@shell photocatalysts, and the study of photocatalytic performance under different factors has guiding significance for the treatment of actual wastewater.

Co-culture of Bacillus coagulans and Candida utilis efficiently treats Lactobacillus fermentation wastewater

Co-culture of Bacillus coagulans and Candida utilis was firstly investigated in the efficient treatment of Lactobacillus fermentation wastewater (LFW) containing total organic carbon (TOC) of 22.0 g/L and total nitrogen (TN) of 2.4 g/L. The utilization of lactic acid by C. utilis was responsible for the relief of feedback inhibition to promote the growth of B. coagulans. The removal ratio of TOC by B. coagulans and C. utilis was only 9.1% and 22.7%, respectively, which was improved to 49.0% by co-culture. The removal ratio of TN by B. coagulans and C. utilis was merely 6.3% and 12.5%, respectively, which was also promoted to 44.6% by co-culture. Both the high growth of B. coagulans and the efficient removal of TOC and TN from LFW was achieved with the co-culture, which is not previously reported and very important in the production of probiotics with the resource utilization of LFW.

Optimized Fabrication of TiO2 Nanotubes Array/SnO2-Sb/Fe-Doped PbO2 Electrode and Application in Electrochemical Treatment of Dye Wastewater

A TiO2 nanotubes array/SnO2-Sb/Fe-doped PbO2 electrode with high oxygen evolution potential and enhanced electrochemical oxidation performance was successfully manufactured. The surface morphology and electrochemical performance of the electrode were characterized by a field emission scanning electron microscope, linear sweep voltammetry, and electrochemical impedance spectroscopy experiments. The effect of Fe doping concentration on the electrode performance was also investigated. This study shows the doping of Fe on a PbO2 electrode improved the micro-morphology and the conductivity of the electrode. The oxygen evolution potential was also increased to 1.95 V [versus a saturated calomel electrode (SCE)] via Fe doping. The optimal condition of Fe doping concentration was 0.02 M. The optimized electrode showed that the decoloration rate and total organic carbon removal ratio of methylene blue approached 98% and 96% after 30 min of electrochemical treatment, respectively. The accelerated lifetime of the optimized Fe-doped electrode was approximately 4.3 times larger than that of the undoped PbO2 electrode. A Ti/TiO2 nanotubes array/SnO2-Sb/Fe-doped PbO2 anode shows potential application in the electrochemical treatment of organic dye wastewater.

Advanced treatment of biologically treated coking wastewater by persulfate oxidation with magnetic activated carbon composite as a catalyst.

Advanced treatment of biologically treated coking wastewater (BTCW) using persulfate (PS) oxidation with magnetic activated carbon composite (CuFe2O4:AC w/w ratio of 1:1.5, denoted as 1.5-MACC) as a green catalyst was evaluated at ambient temperature (30 °C). Effects of PS (K2S2O8) and 1.5-MACC doses on PS decomposition and total organic carbon (TOC) removal in BTCW were also studied during 360 min. The results showed that the 1.5-MACC/PS system has a much better performance on TOC removal in BTCW than only 1.5-MACC or PS system. PS decomposition and TOC removal follow first-order kinetics in the 1.5-MACC/PS system. The optimum condition of the 1.5-MACC/PS system to treat BTCW is with a K2S2O8 dose of 4 g L-1 and 1.5-MACC dose of 5 g L-1. Under this condition, TOC in the PS oxidation effluent is 20.4 mg L-1 with a removal efficiency of 85.4%. TOC removal is a synergistic effect of adsorption and oxidation. TOC oxidation is due to the generation of ·SO4- via the activation of PS by CuFe2O4 impregnated AC. The gas chromatography-mass spectrometry (GC-MS) analysis revealed that phenol compounds and esters were removed significantly by the 1.5-MACC/PS system. When 1.5-MACC was used for the fourth time in the 1.5-MACC/PS system, the removal ratio of TOC was still over 62.2% in 360 min reaction. Thus, the 1.5-MACC/PS system has a potential practical application in treatment of BTCW.

Anchoring Tailored Low-Index Faceted BiOBr Nanoplates onto TiO2 Nanorods to Enhance the Stability and Visible-Light-Driven Catalytic Activity.

In this work, a fantastic one-dimensional (1D) BiOBr/TiO2 nanorod (NR) heterojunction composite was rationally proposed and designed from the perspective of molecular and interface engineering. The fabricated intimately connected interfacial heterojunction between two-dimensional BiOBr nanoplates and 1D TiO2 NRs acts as an interfacial nanochannel to promote efficient interfacial charge migration and separation of photogenerated electron-hole pairs. As a result, 1D BiOBr/TiO2 NR heterojunctions exhibited outstanding visible-light photocatalytic activities and sustained cycling performance. Under visible-light irradiation for 120 min, the reduction efficiency of Cr(VI) over the TB-2 sample (molar ratio: n(Ti)/n(Bi) = 2:1) is as high as 95.4% without adding any scavengers. Furthermore, the sample also shows excellent photodegradation activity of RhB with a much higher apparent rate constant of 0.49 min-1 and 88.5% total organic carbon removal ratio. Furthermore, the corresponding mechanism of enhanced photocatalytic activity is proposed according to comprehensively investigated results from photoluminescence spectroscopy, photoelectrochemical measurement analysis, and radical trapping experiments. This study provides an attractive avenue to design and fabricate highly efficient 1D NR heterojunction photocatalysts, which possessed a high application value in the field of environmental remediation, especially for wastewater purification.

Electrochemical degradation of acidified reed pulp black liquor with three-dimensional electrode reactor

The electrochemical degradation of reed pulp black liquor containing lignin pretreated by acidification method was investigated using a three-dimensional electrode reactor. Using activated carbon as particle electrode, the effects of pH value, reaction temperature, electrolysis time and current on residual concentration of total organic carbon (TOC) were discussed in detail. The optimal conditions were obtained: pH 2.5, influent flow rate of 200 mL/min, 25 °C, 300 mA and 2 h of electrolysis time, and the removal efficiency of TOC maintains at 35.57 %. The results of the electrochemical method indicate that •OH radicals are produced in activated carbon anode in the electrolysis process and then adsorbed on the activated carbon surface. Microcell consists of •OH radicals and the absorbed lignin. With the microcell reaction, the lignin is degraded, while the anodic polarized curve illustrates that the lignin is obviously oxidized in the anode. The contributions of direct and indirect electrolyses to the TOC removal ratio are about 50%, respectively.

High thermostable ordered mesoporous SiO2–TiO2 coated circulating-bed biofilm reactor for unpredictable photocatalytic and biocatalytic performance

Utilizing high thermostable ordered mesoporous SiO2–TiO2 as a precursor, macroporous polyurethane foam (PUF) as a floating biofilm carrier, the photocatalytic circulating-bed biofilm reactor (PCBBR) is fabricated via ultrasonic vibration and deposition approach. The prepared SiO2–TiO2/PUF carrier is characterized in detail by X-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, N2 adsorption, scanning electron microscopy and energy dispersive spectroscopy. The results indicate that the ordered mesoporous SiO2–TiO2 network can be maintained and the presence of SiO2 can inhibit the anatase-to-rutile phase transformation during 800°C calcinations. Furthermore, the prepared SiO2–TiO2/PUF carrier presents a hierarchical macro/mesoporous structure, filling the bacterium to the channels. The PCBBR exhibits good synergic effect for the refractory phenolic wastewater, and the total organic carbon removal ratio of high toxic 2,4,5-trichlorophenol is up to 97.5% after hydraulic retention time for 3h, which is ascribed to the hierarchical macro/mesoporous structure in favor of pollutants adsorption, efficient photon utilization and microorganism loading. This novel ordered mesoporous SiO2–TiO2 coated circulating-bed biofilm reactor is promising in the environmental field.

Electrochemical oxidation of 2,4,5-trichlorophenoxyacetic acid by metal-oxide-coated Ti electrodes

Electrochemical oxidation of 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) over metal-oxide-coated Ti anodes, i.e., Ti/SnO2–Sb/Ce–PbO2, Ti/SnO2–Sb and Ti/RuO2, was examined. The degradation efficiency of over 90% was attained at 20min at different initial concentrations (0.5–20mgL−1) and initial pH values (3.1–11.2). The degradation efficiencies of 2,4,5-T on Ti/SnO2–Sb/Ce–PbO2, Ti/SnO2–Sb and Ti/RuO2 anodes were higher than 99.9%, 97.2% and 91.5% at 30min, respectively, and the respective total organic carbon removal ratios were 65.7%, 54.6% and 37.2%. The electrochemical degradation of 2,4,5-T in aqueous solution followed pseudo-first-order kinetics. The compounds, i.e., 2,5-dichlorohydroquinone and 2,5-dihydroxy-p-benzoquinone, have been identified as the main aromatic intermediates by liquid chromatography-mass spectrometry. The results showed that the energy efficiencies of 2,4,5-T (20mgL−1) degradation with Ti/SnO2–Sb/Ce–PbO2 anode at the optimal current densities from 2 to 16mAcm−2 ranged from 8.21 to 18.73kWhm−3.

Electrochemical Degradation of Triclosan at a Ti/SnO2‐Sb/Ce‐PbO2 Anode

Electrochemical degradation of an emerging contaminant, triclosan (5‐chloro‐2‐(2,4‐dichlorophenoxy) phenol) by a Ti/SnO2‐Sb/Ce‐PbO2 anode was investigated. The degradation efficiency attained &gt;99.9% during 5 min of electrolysis at all influential factors, i.e., applied current density (2–10 mA/cm2), pH 3–11, inter‐electrode distance (1–4 cm), initial concentration (0.5–5 mg/L triclosan) and supporting electrolyte (10 mmol/L NaCl). Total organic carbon removal ratio achieved 79.7% at the optimal conditions after 2 min of electrolysis. The electrochemical degradation of triclosan followed pseudo‐first‐order kinetics. The intermediate products namely 2,4‐dichlorophenol, 5‐chloro‐3‐(chlorohydroquinone) phenol and 2‐chloro‐5‐(2,4‐dichlorophenoxy) benzene‐1,4‐diol were prominently detected using liquid chromatography–tandem mass spectrometry. A degradation mechanism of triclosan at the Ti/SnO2‐Sb/Ce‐PbO2 anode was proposed based on the intermediates. The energy consumption of triclosan (4 mg/L) degradation at different electrode distances (1–4 cm) was 0.466–2.225 kWh m−3. The Ti/SnO2‐Sb/Ce‐PbO2 anodes can be employed preliminary for rapid degradation of triclosan in wastewater.

4-Nitroaniline Degradation by TiO2Catalyst Doping with Manganese

Stainless steel anode covered with layer film of TiO2doped with manganese was utilized to decompose 4-nitroaniline in rectangular borosilicate glass reactor, while stainless steel mesh was chosen as cathode; the anode and cathode were connected to the direct-current power; meantime two 60 W (λmax= 365 nm) UV lamps were used as light source. The microstructures on TiO2before and after being doped with manganese were analyzed by energy disperse X-ray (EDX) and X-ray diffraction (XRD). The performance of degradation of 4-nitroaniline was evaluated by analyzing cracking ratio of 4-nitroaniline ring, the chemical oxygen demand (COD), and total organic carbon (TOC) in remaining solution. Monitored parameters during all the photocatalytic reaction including dissolved oxygen, direct voltage, and radiation dosage of ultraviolet rays were investigated. When dissolved oxygen concentration, direct voltage, and radiation dosage of ultraviolet rays were, respectively, equivalent to 9 mg/L, 24 V, and 1200 μW/cm2, the degradation ratio of 4-nitroaniline reached maximum. The experimental results indicated that cracking ratio of 4-nitroaniline ring and the removal ratio of COD and TOC were, respectively, more than 99%, 85%, and 80% when reaction was run for 10 hours. The values of COD and TOC were, respectively, less than 16 mg/L and 8 mg/L while the experiment was finished.

A floating macro/mesoporous crystalline anatase TiO2ceramic with enhanced photocatalytic performance for recalcitrant wastewater degradation

A macro/mesoporous anatase TiO2 ceramic floating photocatalyst has been successfully synthesized using highly thermally stable mesoporous TiO2 powder as a precursor, followed by a camphene-based freeze-casting process and high-temperature calcinations. The ceramics are characterized in detail by X-ray diffraction, Raman spectra, scanning electron microscopy, transmission electron microscopy and N2 adsorption-desorption isotherms. The results indicate that the TiO2 ceramics present hierarchical macro/mesoporous structures, which maintain high porosity and high compressive strength at the optimal sintering temperature of 800 °C. The ordered mesoporous TiO2 network still possesses high thermal stability and inhibits the anatase-to-rutile phase transformation during calcinations. The obtained ceramics exhibit good adsorptive and photocatalytic activity for the degradation of octane and rhodamine B, and the total organic carbon removal ratio is up to 98.8% and 98.6% after photodegradation for 3 h, respectively. The roles of active species in the photocatalytic process are compared using different types of active species scavengers, and the degradation mechanism is also proposed. Furthermore, the ceramics are recyclable, and no clear changes are observed after ten cycles. In addition, the ceramics are also active in the photodegradation of phenol, thiobencarb, and atrazine. Therefore, these novel floating photocatalysts will have wide applications, including the removal of floating organic pollutants from the wastewater surfaces or the removal of soluble organic pollutants from wastewater.

Influence of oxidation coefficient on product properties in sewage sludge treatment by supercritical water

Oxidation coefficient plays a significant role in sewage sludge treatment in supercritical water, such as supercritical water oxidation (SCWO) and supercritical water partial oxidation (SWPO). In this work, the influences of oxidation coefficient (n) on sewage sludge treatment in supercritical water are investigated systematically and the corresponding reaction mechanisms are discussed objectively. Moreover, corrosion properties of stainless steel 316 considered as reactor material are also explored. The results show that H2 yield first rises and then decreases with an increase in n. Its maximum value is approximately 190.4 ml/L when sewage sludge is disposed at 450 °C, 25 MPa, n = 0.6 and a residence time of 2.5 min. Under the reaction conditions of 540 °C, 25 MPa, n = 2.0 and 2.5 min, liquid product COD (Chemical Oxygen Demand) removal ratio, TOC (Total Organic Carbon) removal ratio and ammonia removal ratio of liquid product can reach up to 99.95%, 99.8% and 99.7%, respectively. However, if coupling SWPO and SCWO, we can obtain a certain amount of H2 and CH4, achieve the above removal ratios even at 450 °C and a lower total n (0.74), and meanwhile liquid products can meet corresponding discharge standards. However, special attention should be paid to reactor material corrosion in the above SWPO. We also confirm that stainless steel 316 undergoes more severe pitting corrosion in the absence of oxygen, compared with general corrosion in the excessive oxygen environment.

Enhanced decomposition of (4-chloro-2-methylphenoxy) acetic acid by combined ultrasonic and oxidative treatment

Combined ultrasonic (US) irradiation and sodium peroxodisulfate (Na2S2O8) treatment has been investigated for promotion of both decomposition of (4-chloro-2-methylphenoxy) acetic acid (MCPA) and mineralization of organic residues. This treatment is expected to accelerate both reactions, because the US cavitation effect promotes the production of radicals, such as SO4−· and OH·, by the decomposition of Na2S2O8 and water. In this study, decomposition experiments were performed on 100 ppm MCPA aqueous solutions in a sonoreactor at reaction temperatures of 298–333 K with US irradiation alone, Na2S2O8 treatment alone, and the combination of US and Na2S2O8 treatment. It was found that the combined treatment achieved a higher MCPA decomposition rate and total organic carbon (TOC) removal ratio than either treatment alone. The decomposition ratios of both MCPA and TOC increased with reaction temperature, and especially steep increases were observed at 333 K due to a significant promotion of thermal decomposition of Na2S2O8. The production of radical species was also promoted by the combined treatment. These results suggest that the higher MCPA decomposition rate and TOC removal ratio are due to the increased formation of sulfate and hydroxyl radicals via thermal and US decomposition of Na2S2O8.