Intensification of catalytic wet peroxide oxidation with microwave radiation: Activity and stability of carbon materials

A review of microwave-assisted advanced oxidation processes for wastewater treatment

Microwave (MW) technology has gained increasing interest in wastewater treatment due to its unique properties, such as fast and uniform heating, hot spots effect, and non-thermal effect. MW enhances the production of active radicals (e.g., OH, SO₄^-), which exerts a stronger integrated treatment effect in combination with advanced oxidation processes. Over the years, microwave-assisted advanced oxidation processes (MW-AOPs) have developed rapidly to degrade pollutants as innovative treatment approaches. This paper provides a detailed classification and a comprehensive review of MW-AOPs. The latest applications of MW in different advanced oxidation systems (oxidation systems, catalytic oxidation systems, and photochemical, electrochemical and sonochemical systems) are reviewed. The reaction parameters and performance of MW-AOPs in wastewater treatment are discussed, and the enhancement of pollutant degradation by MW is highlighted. In addition, the operating costs of MW-AOPs are evaluated. Some recommendations on MW-AOPs are made for future research. This review provides meaningful information on the potential development and evolution of MW-AOPs.

Developing Fe/zeolite catalysts for efficient catalytic wet peroxidation of three isomeric cresols

Five Fe/zeolite (i.e., Fe/ZSM-5-1, Fe/ZSM-5-2, Fe/ZSM-5-3, Fe/MCM-22, and Fe/MOR) were prepared and tested as catalysts in the catalytic wet peroxide oxidation process (CWPO). Their adsorption and catalytic effects on the removal of three isomeric cresols were systematically explored. Sufficient characterizations were carried out to illuminate the iron species dispersed on the catalysts' surface porous channels. Other properties of the catalysts such as the Si/Al ratio, crystalline structures, and morphologies were systematically studied. After loaded with iron, the catalysts maintained zeolite's framework, which possessed specific porous structures and surface areas. Interestingly, the Si/Al ratio seemed to be an important issue influencing the adsorption and catalytic degradation of cresols due to n-π interaction and the acceleration of HO• generation, respectively. The amount of framework-Fe and Fe³⁺Al-Si in Fe/ZSM-5-3 was the most, which was crucial for its better catalytic ability than the other Fe/ZSM-5 catalysts (71.19% for m-cresol conversion). In conclusion, the catalytic activities of all the Fe/zeolites followed the sequence: Fe/ZSM-5-3> Fe/ZSM-5-2> Fe/ZSM-5-1> Fe/MCM-22>Fe/MOR. For three cresols, m-cresol was more susceptible to the attack of HO• than p- and o-cresol because more positions of m-cresol could be easy to be approached by the oxidizing agent. Considering the mild reaction condition in this study, such as 30 °C, pH=4.0, and catalyst dosage=1.0 g/L, the Fe/ZSM-5-3 was a promising zeolite catalyst for the degradation of refractory contaminants in practical wastewater.

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.

On the Role of the Cathode for the Electro-Oxidation of Perfluorooctanoic Acid

Perfluorooctanoic acid (PFOA), C7F15COOH, has been widely employed over the past fifty years, causing an environmental problem because of its dispersion and low biodegradability. Furthermore, the high stability of this molecule, conferred by the high strength of the C-F bond makes it very difficult to remove. In this work, electrochemical techniques are applied for PFOA degradation in order to study the influence of the cathode on defluorination. For this purpose, boron-doped diamond (BDD), Pt, Zr, and stainless steel have been tested as cathodes working with BDD anode at low electrolyte concentration (3.5 mM) to degrade PFOA at 100 mg/L. Among these cathodic materials, Pt improves the defluorination reaction. The electro-degradation of a PFOA molecule starts by a direct exchange of one electron at the anode and then follows a complex mechanism involving reaction with hydroxyl radicals and adsorbed hydrogen on the cathode. It is assumed that Pt acts as an electrocatalyst, enhancing PFOA defluorination by the reduction reaction of perfluorinated carbonyl intermediates on the cathode. The defluorinated intermediates are then more easily oxidized by HO• radicals. Hence, high mineralization (xTOC: 76.1%) and defluorination degrees (xF−: 58.6%) were reached with Pt working at current density j = 7.9 mA/cm2. This BDD-Pt system reaches a higher efficiency in terms of defluorination for a given electrical charge than previous works reported in literature. Influence of the electrolyte composition and initial pH are also explored.