Role of the Activated Carbon Surface on Catalytic Wet Peroxide Oxidation

Assessment of Pretreatments for Highly Concentrated Leachate Waters to Enhance the Performance of Catalytic Wet Peroxide Oxidation with Sustainable Low-Cost Catalysts

Matured compost, derived from a mechanical and biological treatment (MBT) plant, was used as a precursor to produce catalysts through hydrothermal and thermal carbonization, HC and PC, respectively. HC and PC displayed suitable properties to act as catalysts in the catalytic wet peroxide oxidation (CWPO) treatment of the highly polluted leachate waters generated in the same MBT plant (TOC0 = 27 g L−1; COD0 = 60 g L−1; BOD5,0 = 23 g L−1). The influence of catalyst loading and pH were studied, considering multiple additions of H2O2. The best experimental conditions found were T = 80 °C, pH0 = 3.0, 7.2 g L−1 of HC catalyst, 85.7 g L−1 of H2O2, added in five batches in one-hour intervals between each addition. Under these experimental conditions, removals of 43%, 52%, 93%, 82%, 35%, 95% and 93% for the COD, TOC, BOD5, aromaticity, chlorides, turbidity and color number (CN) were, respectively, observed. Ion exchange resins and coagulation–flocculation were studied as pretreatment options to reduce the complexity of the leachate waters and enhance the CWPO results. Both strategies resulted in higher mineralization and enhanced the consumption efficiency of H2O2 (ηH2O2). The sequential treatment using coagulation–flocculation and CWPO with PC catalyst showed the best results, achieving abatement of 94%, 70%, 98%, 93%, 31%, 96% and 95% for COD, TOC, BOD5, aromaticity, chlorides, turbidity and CN, respectively.

Post-treatment of real municipal wastewater effluents by means of granular activated carbon (GAC) based catalytic processes: A focus on abatement of pharmaceutically active compounds

Pharmaceutically active compounds (PhACs) widely present in urban wastewater effluents pose a threat to ecosystems in the receiving aquatic environment. In this work, efficiency of granular activated carbon (GAC) - based catalytic processes, namely catalytic wet peroxide oxidation (CWPO), peroxymonosulfate oxidation (PMS/GAC) and peroxydisulfate oxidation (PDS/GAC) at ambient temperature and pressure were studied for removal of 22 PhACs (ng L^-1 level) that were present in secondary effluents of real urban wastewater. Concentrations of PhACs were measured using Ultra Performance Liquid Chromatography - Triple Quadrupole Mass Spectrometry (UPLC-QqQ-MS/MS). Catalytic experiments were conducted in discontinuous mode using up-flow fixed bed reactors with granular activated carbon (GAC) as a catalyst. The catalyst was characterized by means of N₂ adsorption-desorption isotherm, mercury intrusion porosimetry (MIP), elemental analysis, X-ray fluorescence spectroscopy (WDXRF), X-ray diffraction (XRD), thermal gravimetry and differential temperature analyses coupled mass spectrometry (TGA-DTA-MS). Results indicate that the highest efficiency in terms of TOC removal was achieved during CWPO performed at optimal operational conditions (stoichiometric dose of H₂O₂; TOC removal ~ 82%) followed by PMS/GAC (initial PMS concentration 100 mg L^-1; TOC removal ~73.7%) and PDS/GAC (initial PDS concentration 100 mg L^-1; TOC removal ~ 67.9%) after 5 min of contact time. Full consumption of oxidants was observed in all cases for CWPO and PDS/GAC at contact times of 2.5 min, while for PMS/GAC it was 1.5 min. In general, for 18 out of 22 target PhACs, very high removal efficiencies (> 92%) were achieved in all tested processes (including adsorption) performed at optimal operational conditions during 5 min of contact time. However, moderate (40 - 70%) and poor (< 40%) removal efficiencies were achieved for salicylic acid, ofloxacin, norfloxacin and ciprofloxacin, which can be possibly attributed to insufficient contact time. Despite high efficiency of all studied processes for PhACs elimination from urban wastewater effluent, CWPO seems to be more promising for continuous operation.

Performance of Iron-Functionalized Activated Carbon Catalysts (Fe/AC-f) on CWPO Wastewater Treatment

Two commercial activated carbon were functionalized with nitric acid, sulfuric acid, and ethylenediamine to induce the modification of their surface functional groups and facilitate the stability of corresponding AC-supported iron catalysts (Fe/AC-f). Synthetized Fe/AC-f catalysts were characterized to determine bulk and surface composition (elemental analysis, emission spectroscopy, XPS), textural (N2 isotherms), and structural characteristics (XRD). All the Fe/AC-f catalysts were evaluated in the degradation of phenol in ultrapure water matrix by catalytic wet peroxide oxidation (CWPO). Complete pollutant removal at short reaction times (30–60 min) and high TOC reduction (XTOC = 80 % at ≤ 120 min) were always achieved at the conditions tested (500 mg·L−1 catalyst loading, 100 mg·L−1 phenol concentration, stoichiometric H2O2 dose, pH 3, 50 °C and 200 rpm), improving the results found with bare activated carbon supports. The lability of the interactions of iron with functionalized carbon support jeopardizes the stability of some catalysts. This fact could be associated to modifications of the induced surface chemistry after functionalization as a consequence of the iron immobilization procedure. The reusability was demonstrated by four consecutive CWPO cycles where the activity decreased from 1st to 3rd, to become recovered in the 4th run. Fe/AC-f catalysts were applied to treat two real water matrices: the effluent of a wastewater treatment plant with a membrane biological reactor (WWTP-MBR) and a landfill leachate, opening the opportunity to extend the use of these Fe/AC-f catalysts for complex wastewater matrices remediation. The degradation of phenol spiked WWTP-MBR effluent by CWPO using Fe/AC-f catalysts revealed pH of the reaction medium as a critical parameter to obtain complete elimination of the pollutant, only reached at pH 3. On the contrary, significant TOC removal, naturally found in complex landfill leachate, was obtained at natural pH 9 and half stoichiometric H2O2 dose. This highlights the importance of the water matrix in the optimization of the CWPO operating conditions.

Simulation and Optimization of the CWPO Process by Combination of Aspen Plus and 6-Factor Doehlert Matrix: Towards Autothermal Operation

This work aims to present an industrial perspective on Catalytic Wet Peroxide Oxidation (CWPO) technology. Herein, process simulation and experimental design have been coupled to study the optimal process conditions to ensure high-performance oxidation, minimum H2O2 consumption and maximum energetic efficiency in an industrial scale CWPO unit. The CWPO of phenol in the presence of carbon black catalysts was studied as a model process in the Aspen Plus® v11 simulator. The kinetic model implemented, based on 30 kinetic equations with 11 organic compounds and H2O2 involvement, was valid to describe the complex reaction network and to reproduce the experimental results. The computer experiments were designed on a six-factor Doehlert Matrix in order to describe the influence of the operating conditions (i.e., the different process temperatures, inlet chemical oxygen demands, doses of H2O2 and space time) on each selected output response (conversion, efficiency of H2O2 consumption and energetic efficiency) by a quadratic model. The optimization of the WPO performance by a multi-criteria function highlighted the inlet chemical oxygen demand as the most influential operating condition. It needed to have values between 9.5 and 24 g L−1 for autothermal operation to be sustained under mild operating conditions (reaction temperature: 93–130 °C and pressure: 1–4 atm) and with a stoichiometric dose of H2O2.

Fitting Biochars and Activated Carbons from Residues of the Olive Oil Industry as Supports of Fe- Catalysts for the Heterogeneous Fenton-Like Treatment of Simulated Olive Mill Wastewater

A series of biochars and activated carbons (ACs) was prepared combining carbonization and physical or chemical activation of cheap and abundant residues of the olive oil industry. These materials were used as Fe-support to develop low-cost catalysts for the heterogeneous Fenton-like oxidation of simulated olive mill wastewater (OMW), the highly pollutant effluent generated by this agroindustry. Commercial ACs were also used as reference. All catalysts prepared were extensively characterized and results related with their performances in the catalytic wet peroxide oxidation (CWPO). Results showed a linear relationship of the textural properties of the catalysts with the adsorptive and catalytic performance, as well as the preferential adsorption and degradation of some phenolic compounds (caffeic and gallic acids) by specific interactions with the catalysts’ surface. Despite the best performance of catalysts developed using commercial supports, those prepared from agro-industrial residues present some advantages, including a smaller catalyst deactivation by iron leaching. CWPO results show that catalysts from physically activated olive stones are the most promising materials, reaching total organic carbon and toxicity reductions of 35% and 60%, respectively, as well an efficient use of H₂O₂, comparable with those obtained using commercial supports. This approach showed that the optimized treatment of this type of residues will allow their integration in the circular economic process of the olive oil production.

Review on Activated Carbons by Chemical Activation with FeCl3

This study reviews the most relevant results on the synthesis, characterization, and applications of activated carbons obtained by novel chemical activation with FeCl3. The text includes a description of the activation mechanism, which compromises three different stages: (1) intense de-polymerization of the carbon precursor (up to 300 °C), (2) devolatilization and formation of the inner porosity (between 300 and 700 °C), and (3) dehydrogenation of the fixed carbon structure (>700 °C). Among the different synthesis conditions, the activation temperature, and, to a lesser extent, the impregnation ratio (i.e., mass ratio of FeCl3 to carbon precursor), are the most relevant parameters controlling the final properties of the resulting activated carbons. The characteristics of the carbons in terms of porosity, surface chemistry, and magnetic properties are analyzed in detail. These carbons showed a well-developed porous texture mainly in the micropore size range, an acidic surface with an abundance of oxygen surface groups, and a superparamagnetic character due to the presence of well-distributed iron species. These properties convert these carbons into promising candidates for different applications. They are widely analyzed as adsorbents in aqueous phase applications due to their porosity, surface acidity, and ease of separation. The presence of stable and well-distributed iron species on the carbons’ surface makes them promising catalysts for different applications. Finally, the presence of iron compounds has been shown to improve the graphitization degree and conductivity of the carbons; these are consequently being analyzed in energy storage applications.

The Enhanced Catalytic Performance and Stability of Ordered Mesoporous Carbon Supported Nano-Gold with High Structural Integrity for Glycerol Oxidation

Ordered mesoporous carbon (OMC) supported gold nanoparticles of size 3-4 nm having uniform dispersion were synthesized by sol-immobilization method. OMCs such as CMK-3 and NCCR-56 with high surface area and uniform pore size were obtained, respectively, using ordered mesoporous silicas such as SBA-15 and IITM-56 as hard templates, respectively. The resulting OMC supported monodispersed nano-gold, i. e., Au/CMK-3 and Au/NCCR-56, exhibited excellent performance as mild-oxidizing catalysts for oxidation of glycerol with high hydrothermal stability. Further, unlike activated carbon supported nano-gold catalysts (Au/AC), the OMC supported nano-gold catalysts, i. e., Au/CMK-3 and Au/NCCR-56, show no aggregation of active species even after recycling. Thus, in the case of Au/CMK-3 and Au/NCCR-56, both the fresh and regenerated catalysts showed excellent performane for the chosen reaction owing to an enhanced textural integrity of the catalysts and that with remarkable selectivity towards glyceric acid. The significance of the OMC supports in maintaining the dispersion of gold nanoparticles is explicit from this study, and that the activity of Au/AC catalyst is considerably decreased (∼50 %) upon recycling as a result of agglomeration of the active gold nanoparticles over the disordered amorphous carbon matrix.

Regeneration of siloxane-exhausted activated carbon by advanced oxidation processes

In the context of the biogas upgrading, siloxane exhausted activated carbons need to be regenerated in order to avoid them becoming a residue. In this work, two commercial activate carbons which were proved to be efficient in the removal of octamethylcyclotetrasiloxane (D4) from biogas, have been regenerated through advanced oxidation processes using both O3 and H2O2. After the treatment with O3, the activated carbon recovered up to 40% of the original adsorption capacity while by the oxidation with H2O2 the regeneration efficiency achieved was up to 45%. In order to enhance the H2O2 oxidation, activated carbon was amended with iron. In this case, the regeneration efficiency increased up to 92%.

Biogas upgrading: optimal activated carbon properties for siloxane removal

A total of 12 commercial activated carbons (ACs) have been tested for the removal of octamethylcyclotetrasiloxane (D4) in dynamic adsorption experiments using different carrier gases and D4 concentrations. Characterization of the ACs included several physical and chemical techniques. The D4 adsorption capacities were strongly related with the textural development of the ACs. Results showed that the optimum adsorbent for D4 is a wood-based chemically activated carbon, which rendered an adsorption capacity of 1732 ± 93 mg g(-1) using 1000 ppm (v/v) of D4 with dry N2 as the carrier gas. When the concentration of D4 was lowered to typical values found in biogas, the adsorption capacity was halved. The presence of major biogas compounds (i.e., CH4 and CO2) and humidity further reduced the D4 adsorption capacity. The polymerization of D4 over the surface of all ACs was found to be relevant after prolonged contact times. The extent of this phenomenon, which may negatively affect the thermal regeneration of the AC, correlated reasonably well with the presence of phenolic and carboxylic groups on the carbon surfaces.

Adsorption of volatile sulphur compounds onto modified activated carbons: effect of oxygen functional groups

The effect of physical and chemical properties of activated carbon (AC) on the adsorption of ethyl mercaptan, dimethyl sulphide and dimethyl disulphide was investigated by treating a commercial AC with nitric acid and ozone. The chemical properties of ACs were characterised by temperature programme desorption and X-ray photoelectron spectroscopy. AC treated with nitric acid presented a larger amount of oxygen functional groups than materials oxidised with ozone. This enrichment allowed a significant improvement on adsorption capacities for ethyl mercaptan and dimethyl sulphide but not for dimethyl disulphide. In order to gain a deeper knowledge on the effect of the surface chemistry of AC on the adsorption of volatile sulphur compounds, the quantum-chemical COSMO-RS method was used to simulate the interactions between AC surface groups and the studied volatile sulphur compounds. In agreement with experimental data, this model predicted a greater affinity of dimethyl disulphide towards AC, unaffected by the incorporation of oxygen functional groups in the surface. Moreover, the model pointed out to an increase of the adsorption capacity of AC by the incorporation of hydroxyl functional groups in the case of ethyl mercaptan and dimethyl sulphide due to the hydrogen bond interactions.