Mineralization of pentachlorophenol by ferrioxalate-assisted solar photo-Fenton process at mild pH (2023)

This study successfully synthesized the Fenton catalyst of composite material (ACS) from activated carbon (SAC) derived from soybean residue and sludge (S) from the supply water treatment for the decomposition of paracetamol (PRC). ACS synthesized by SAC/S with the ratio of 1/1 and pyrolysis temperature of 600°C was found with the best performance. The characteristics of ACS were evaluated via SEM-EDS, S_BET, and XRD. Optimal conditions of the Fenton process were [H₂O₂]=70mM, pH 2, catalyst dose=0.5g/L, contacting time=30min, and 25°C for the decomposition of 99% PRC (C₀=500mg/L). The reaction followed the first-order kinetic model, with a reaction rate constant of k=0.1298 min⁻¹. The thermodynamic study indicated that the oxidation of PRC by the heterogeneous Fenton process catalyzed by ACS was a spontaneous (ΔG<0) and endothermic (ΔH>0) reaction. The research results indicated that carbon-based heterogeneous material can be used effectively for the Fenton catalysis of PRC in water. Soybean residue and sludge, among other solid wastes, can be utilized as feeding materials for manufacturing catalysts using the straightforward method as a promising approach to circular economy.

In recent years, visible light assisted advanced oxidation processes (AOPs) are appealing in the elimination of pollutants. Herein, an innovative and eco-friendly visible light enhanced Fe³⁺-nitrilotriacetic acid system for the activation of peroxydisulfate (Vis/Fe³⁺-NTA/PDS) was proposed for the removal of bisphenol A (BPA). Fe³⁺-NTA could be dissociated through ligand-to-metal charge transfer (LMCT) to realize the generation of Fe²⁺ for the continuous activation of PDS to remove BPA. The use of 0.10mM Fe³⁺, 0.10mM NTA and 1.00mM PDS led to 97.5% decay of 0.05mM BPA and 66.3% of TOC removal in 30min with the illumination of visible light at initial pH 3.0. The sulfate and hydroxyl radicals were proved to be the dominant species leading to BPA removal by means of radical scavenging experiments, radical probes and electron paramagnetic resonance (EPR) technique. The effects of various operating parameters, natural water constituents as well as different water matrices on BPA abatement were explored. The intermediate products of BPA degradation were identified and a possible transformation pathway was proposed. Briefly, this research provides an attractive strategy for the remediation of refractory wastewater using NTA assisted with visible light in the homogeneous Fe³⁺/PDS system.

Electro-Fenton (EF) process is considered as an efficient technique for removing recalcitrant organic pollutants from wastewater. To overcome the major drawback of low pH requirements in the classical Fenton process, then, chelating agents are introduced into the system which are capable of forming complexes with Fe³⁺/Fe²⁺, thereby enhancing the generation of reactive oxygen species (ROS) at circumneutral pH conditions. In this review, the basic mechanisms of EF process with chelating agents and their recent applications for removing various organic pollutants are discussed in detail. Also, the applications of chelating agents in the Fenton processes are reviewed. Further, the recent improvements in the electrochemical systems are also discussed.

This work demonstrated a novel N,N-bis(carboxymethyl)glutamic acid (GLDA) modified visible (vis) light assisted Fenton–like system which could effectively removal ciprofloxacin (CIP) at neutral pH. The removal rate of CIP in the GLDA/Fe(III)/H₂O₂/vis system was 97.9% within 120min and was approximately twice as high as that in the Fe(III)/H₂O₂/vis system. GLDA could significantly accelerate the Fe(III)/Fe(II) cycle under visible light irradiation. Radical scavenging experiments demonstrated that 74.2% of the Fe(II) in the GLDA/Fe(III)/H₂O₂/vis system originated from the ligand–to–metal charge transfer reaction between Fe(III) and GLDA. The hydroxyl radical was the dominant species for CIP degradation. H₂O₂ utilization kinetic modeling exhibited that 90.1% of H₂O₂ was used for CIP mineralization. The effects of experimental parameters and coexisting substances on the removal of CIP in the Fe(III)/GLDA/H₂O₂/vis system were investigated in detail. The intermediate products of CIP were explored via the high–performance liquid chromatography-mass spectrometry.

Carbon-bridge-modified malonamide (MLD)/g-C₃N₄ (CN) was prepared by copolymerization of MLD with urea and melamine and loaded with Fe₃O₄ for the high-efficiency removal of tetracycline (TC) in water under photo-Fenton. The prepared catalysts were characterized by SEM, TEM, N₂ adsorption–desorption analysis, XPS, XRD, and FTIR, which proved that the modification method successfully introduced the C bridge into the carbon nitride molecular system and increased the structural defects of the catalyst. The Carbon-bridge-modified MLD/CN/Fe₃O₄ also had good visible-light response and charge-separation and transport abilities in the photoelectrochemical test. Degradation results showed that the photo-Fenton degradation of TC reached 95.8%, and the mineralization rate was 55.7% within 80min at 80mM H₂O₂ dosage, 0.5g/L catalyst dosage, and near-neutral pH by 0.8MLD/CN/Fe₃O₄. Moreover, the oxidation products and mineralization pathways of TC were explored by LC-MS. Toxicity analysis indicated low environmental threat of the intermediates in TC mineralization. EPR analysis and H₂O₂ decomposition efficiency analyses showed an improvement in the H₂O₂ decomposition performance of 0.8MLD/CN/Fe₃O₄. This work could provide a valuable insight for the application of heterogeneous photo-Fenton technology in wastewater treatment.

Pyrite is widely used in Fenton reaction for degradation of pollutants and exhibits great potential for environmental remediation, however, its efficiency is greatly compromised by extra H₂O₂ and pH adjustment. Herein, a pyrite based green solar photo-Fenton system for carbamazepine (CBZ) treatment is constructed, involving the use of simulated sunlight and natural organic acids with in situ-generated H₂O₂ and without extra pH adjustment. The addition of organic acids including tartaric acid (TA), citric acid (CA), and ascorbic acid (AA) can form complex with iron in pyrite, which promotes the Fe(II) dissolution. Upon irradiation, pyrite could be excited to produce photoelectrons, which would reduce oxygen to produce H₂O₂ through a two-step route assisted by organic acids. The simulated sunlight and organic acids promoted the in-situ production of H₂O₂ and Fe(II) species, sustaining an efficient Fenton reaction. This produced massive hydroxyl radical (OH), as demonstrated by the active species capture experiment. Compared with no degradation of CBZ under pure pyrite, the degradation efficiency of CBZ reached to 70%, 60%, and 53% in pyrite/TA, pyrite/CA, pyrite/AA within 30 min under simulated solar light irradiation, respectively. This work reports the first use of natural pyrite, a typical Fe-mineral semiconductor, to produce OH for CBZ degradation through natural additive assisted Fenton reaction excluding the adding extra H₂O₂ and pH adjustment.

Chemosphere, Volume 229, 2019, pp. 200-205

In the present study, ascorbic acid (AA) induced persulfate activation was investigated for the further exploration of organic pollutants oxidation by persulfate. We interestingly found that AA showed a significant catalytic activity to persulfate. Under neutral pH and room temperature condition, about 71.3% of pentachlorophenol (PCP, 10 mg L⁻¹) was decomposed in 180 min with 40 mmol L⁻¹ persulfate and 1.0 mmol L⁻¹ AA, while only 15.4% and 3.2% of PCP was removed by alone persulfate and AA respectively. The result of EPR spectra identified sulfate radical (SO₄^•-) and hydroxyl radical (OH) were generated during the reaction between persulfate and AA. Quenching experiments confirmed that both SO₄^- and OH contributed to the decomposition of PCP. With the addition of AA augmented from 0 to 1 mmol L⁻¹, the PCP degradation ratio continuously increased. However, excess AA could consume the generated reactive oxygen species (ROSs) that led to the inhibition of PCP degradation. Meanwhile, the PCP degradation by persulfate-AA was strongly pH dependent. The PCP degradation rate was declined as the initial pH increased from 3.5 to 10.5. At pH above 12.5, the base activation began to predominate over AA activation of persulfate. Furthermore, it was observed that the AA inducing persulfate activation was related to the extent of AA ionization, while C₆H₈O₆ promoted the highest persulfate activation for the PCP degradation, and C₆H₆O₆²⁻ induced the lowest persulfate activation. This study indicates the high potential of AA induced persulfate activation for treatment of organochlorine contaminated water.

Water Research, Volume 163, 2019, Article 114861

Hydrogen peroxide (H₂O₂) is used widely as Fenton's reagent for organic wastewater treatment. However, the application range of the optimum Fenton reaction is narrow, needing to adjust pH before and after treatment. Besides, the disproportionation of H₂O₂ and generated iron precipitation also confine the normal operation of Fenton method. To overcome the drawbacks of the traditional Fenton process, a Fe(II) catalyzed calcium peroxide (CaO₂) Fenton-like process assisted by oxalic acid (OA) for aqueous organic pollutants degradation was proposed (Fe²⁺/OA/CaO₂). The methyl orange (MO) as a typical organic pollutant, its removal performances by this Fe²⁺/OA/CaO₂ system were evaluated. In the optimized conditions, 99% of MO was degraded within 15 min, and 38% mineralized after 180 min when the molar ratio of Fe²⁺: OA: CaO₂ was 1: 2: 2 (Fe²⁺ = 1.5 mM). Radicals detection indicated that hydroxyl radical (HO^•) was the main reactive species for the MO elimination. Furthermore, density functional theory calculation was in good agreement with the experimental results, which proved that the Fe²⁺/OA/CaO₂ could improve the circulation between Fe²⁺ and Fe³⁺, promoting the oxygen reactive species generation and pollutant removal. The main intermediates were identified and the degradation pathways were proposed based on the results of the mass spectrum analysis, and the attack of HO^• was suggested as the main function for the MO decomposition. The matrix effects of water constituents on the performance of Fe²⁺/OA/CaO₂ were investigated, and the results showed that a certain amount of Cl⁻, NO₃⁻, HCO₃⁻, and HA affected the elimination than SO₄²⁻. Finally, the attempt of actual wastewater disposal indicated the synergistic system possessed good potential for future practical application.

Journal of Hazardous Materials, Volume 319, 2016, pp. 51-60

The effects of electrochemical oxidation (EO), Fered-Fenton and solar Fered-Fenton processes using a recirculation flow system containing an electrochemical cell and a solar photo-reactor on biochemically treated landfill leachate were investigated. The most successful method was solar Fered-Fenton which achieved 66.5% COD removal after 120min treatment utilizing the optimum operating conditions of 47mM H₂O₂, 0.29mM Fe²⁺, pH₀ of 3.0 and a current density of 60mA/cm². The generation of hydroxyl radicals (OH) are mainly from Fered-Fenton process, which is enhanced by the introduction of renewable solar energy. Moreover, Fe²⁺/chlorine and UV/chlorine processes taking place in this system also result in additional production of OH due to the relatively high concentration of chloride ions contained in the leachate. The energy consumption was 74.5kWh/kg COD and the current efficiency was 36.4% for 2h treatment. In addition, the molecular weight (MW) distribution analysis and PARAFAC analysis of excitation emission matrix (EEM) fluorescence spectroscopy for different leachate samples indicated that the organics in the leachate were significantly degraded into either small molecular weight species or inorganics.

Journal of Photochemistry and Photobiology A: Chemistry, Volumes 303–304, 2015, pp. 1-7

This study evaluates the use of the Fe(III)-EDDS complexing agent as an alternative to conventional acidic pH photo-Fenton for the treatment of the pesticide imidacloprid in natural water. The main objective is to estimate whether Fe(III)-EDDS is a viable alternative when treating waters containing high concentrations of contaminants. To this end, the mode of action of Fe(III)-EDDS is examined in the presence of carbonates, at different aeration conditions and at different temperatures. The role of HO₂/O₂⁻ radical species and some aspects of the Fe(III)-EDDS complexation mechanism are discussed. Degradation kinetics and toxicities of generated intermediates are compared between Fe(III)-EDDS and conventional photo-Fenton treatment. An overview of the benefits and limitations of the process is presented.

Chemical Engineering Journal, Volume 286, 2016, pp. 508-516

Leachate, a highly polluted wastewater, is generated from landfill containing complex compositions which have great impact on the environment. The advanced treatment of biochemically treated landfill leachate using electrochemically assisted UV/chlorine process (UEC) in a flow reactor was investigated. The effects of pH and current density on COD and NH₃-N removal by UEC process were evaluated. At the pH value of 5.0, current density of 60mA/cm², the removal efficiencies for COD and NH₃-N were observed as 77% and 87%, respectively. The electrochemical process alone (EC) could obtain relatively high NH₃-N removal but very low COD removal, compared with which, the UEC process significantly promoted the removal efficiency of COD while that of NH₃-N increased to a very limited extent. Most of NH₃-N was converted into N₂ and little nitrogen compounds and chloramines were remained in the leachate after 8h treatment. Moreover, the variations of active chlorine and chloride ions were monitored in the EC and UEC systems, and a possible mechanism was proposed for the UEC process. The in situ electro-generated active chlorine and photo-generated radical species should be responsible for the efficient degradation of landfill leachate. The total energy consumption and current efficiency were 216.5 (kWh/kg COD) and 17.5%, respectively, for 8h treatment. In addition, the molecular weight (MW) percentage distribution analysis and PARAFAC analysis of excitation emission matrix (EEM) fluorescence spectroscopy indicated that the organic matters in the leachate were significantly degraded into the fractions with small molecular weight.

Journal of Cleaner Production, Volume 208, 2019, pp. 1393-1402

The removal of benzophenone-3 (BP-3), a ubiquitous pollutant in municipal wastewater treatment facilities, was optimal by means of a sequential electrocoagulation (EC)/UVA photoelectro-Fenton (PEF) treatment. Overall mineralization was attained upon combination of EC (Fe/Fe cell, 15 mA cm⁻², 20 min) with PEF (boron-doped diamond/air-diffusion cell, 33.3 mA cm⁻², 720 min), being superior to EC/electro-Fenton (EF) and requiring shorter time than single PEF. In EC, an Al/Al cell yielded the largest removal of BP-3 in a simulated matrix at pH 11.0 due to precipitation of its neutral form caused by a substantial pH drop, with optimum current density of 15 mA cm⁻². EC of BP-3-loaded urban wastewater at natural pH was quite effective also with a Fe/Fe cell, being preferred since it provided the required metal catalyst for subsequent treatment. Among the electrochemical advanced oxidation processes tested, PEF was superior to electrochemical oxidation with electrogenerated H₂O₂ (EO-H₂O₂) and EF, especially when using the boron-doped diamond instead of a RuO₂-based anode, due to the oxidation of generated active chlorine and hydroxyl radicals, along with the photolytic action of UVA irradiation. GC-MS revealed the formation of 14 cyclic products in PEF treatment, two of them being also formed during EC.