Photo-Fenton degradation of phenol, 2,4-dichlorophenoxyacetic acid and 2,4-dichlorophenol mixture in saline solution using a falling-film solar reactor

Applying bottom ash as an alternative Fenton catalyst for effective removal of phenol from aqueous environment

In this study, coal bottom ash from a thermoelectric plant was tested as an alternative Fenton catalyst for phenol degradation in water. The effect of operating parameters such as initial pH, catalyst dosage and H2O2 concentration were evaluated. The characterization results indicated that the material has a mesoporous structure, with active species (Fe) well distributed on its surface. Under the optimal reaction conditions (6 mM H2O2, 1 g L-1 of catalyst and pH = 3), 98.7% phenol degradation efficiency was achieved in 60 min, as well as 71.6% TOC removal after 150 min. Hydroxyl radical was identified as the main oxidizing agent involved on the cleavage of the phenol molecule. After four consecutive reuse cycles, phenol degradation efficiency was around 80%, indicating good reusability and stability of the catalyst. Therefore, the obtained results demonstrated that the bottom ash presents remarkable activity for application in the Fenton reaction towards phenol degradation.

Stoichiometric excesses of H2O2 as dosimetry strategy: proof of concept for UVC-H2O2, dark-Fenton, and UVC-Fenton

Hydrochlorothiazide (HCT) is a pharmaceutical micropollutant highly toxic to the environment, being absolutely necessary to oxidize it completely to CO₂. Here, the variables stoichiometric H2O2 excess for (a) degradation and (b) mineralization are defined and used as metric to quantify the dosimetry of the H₂O₂. So that, dose of H₂O₂ qualifies being under- and over-dose respectively for values below and above such standards. In this work, these concepts have been elucidated across AOPs regarding the H₂O₂ degradation excess, whereas only UVC-Fenton was used regarding the H₂O₂ mineralization excess. At a H₂O₂ mineralization excess of 0.68 (equivalent to degradation excess of 36.74), oxidation via UVC-H₂O₂ enables absolute (100%) HCT degradation within 60 min; however, the mineralization of HCT demonstrated limited optimization for all AOPs employed in the beaker-like reactor of this work, being the underlying reasons investigated hereby. At best, 26.70% HCT mineralization was observed within 60 min of UVC photo-Fenton using an initial 2.00 H₂O₂ mineralization excess. Such mineralization of 26.7% is unexpectedly low considering that, in addition, the residual H₂O₂ concentration almost fully depletes within 30 min of UVC-Fenton oxidation. Taken all that together, the loss of H₂O₂ due its decomposition induced by the risen temperature from 28 to 70ºC very likely were the underlying reason preventing better mineralization performance. We successfully demonstrated 18% of mean efficiency of radical •OH consumption signals that the overheating is indeed a designer problem with the photo-reactor since a well-refrigerated photo-reactor shows a mean efficiency of 38% for the same H₂O₂ excess.

Evaluation and comparison of advanced oxidation processes for the degradation of 2,4-dichlorophenoxyacetic acid (2,4-D): a review

Organochlorine pesticides have generated public concern worldwide because of their toxicity to human health and the environment, even at low concentrations, and their persistence, being mostly nonbiodegradable. The use of 2,4-dichlorophenoxyacetic acid (2,4-D) has increased in recent decades, causing severe water contamination. Several treatments have been developed to degrade 2,4-D. This manuscript presents an overview of the physicochemical characteristics, uses, regulations, environmental and human health impacts of 2,4-D, and different advanced oxidation processes (AOPs) to degrade this organic compound, evaluating and comparing operation conditions, efficiencies, and intermediaries. Based on this review, 2,4-D degradation is highly efficient in ozonation (system O₃/plasma, 99.8% in 30 min). Photocatalytic, photo-Fenton, and electrochemical processes have the optimal efficiencies of degradation and mineralization: 97%/79.67% (blue TiO₂ nanotube arrays//UV), 100%/98% (Fe²⁺/H₂O₂/UV), and 100%/84.3% (MI-meso SnO₂), respectively. The ozonation and electrochemical processes show high degradation efficiencies, but energy costs are also high, and photocatalysis is more expensive with a separation treatment used to recover the catalyst in the solution. The Fenton process is a viable economic-environmental option, but degradation efficiencies are often low (50-70%); however, they are increased when solar UV radiation is used (90-100%). AOPs are promising technologies for the degradation of organic pollutants in real wastewater, so evaluating their strengths and weaknesses is expected to help select viable operational conditions and obtain optimal efficiencies.

Abatement of 2,4-D by H2O2 solar photolysis and solar photo-Fenton-like process with minute Fe(III) concentrations

The Photo-Fenton-like (PF-like) process with minute Fe(III) concentrations and the Hydrogen Peroxide Photolysis (HPP), using Xe-lamp or solar light as sources of irradiation, were efficiently applied to eliminate the herbicide 2,4-D from water. PF-like experiments concerning ferric and H₂O₂ concentrations of 0.6 mg L^-1 and 20 mg L^-1 respectively, using Xenon lamps (Xe-lamps) as a source of irradiation and 2,4-D concentrations of 10 mg L^-1 at pH 3.6, exhibited complete 2,4-D degradation and 77% dissolved organic carbon (DOC) removal after 30 min and 6 h of irradiation respectively whereas HPP (in absence of ferric ions) experiments showed a 2,4-D reduction and DOC removal of 90% and 7% respectively after 6 h of irradiation. At pH 7.0, HPP process achieved a 2,4-D abatement of approximately 75% and a DOC removal of 4% after 6 h. PF-like exhibited slightly improved 2,4-D and DOC removals (80% and 12% respectively) after the same irradiation time probably due to the low pH reduction (from 7.0 to 5.6). Several chlorinated-aromatic intermediates were identified by HPLC-MS. These by-products were efficiently removed by PF at pH 3.6, whereas at neutral PF-like and acid or neutral HPP, they were not efficiently degraded. With natural solar light irradiation, 10 and 1 mg L^-1 of 2,4-D were abated using minor H₂O₂ concentrations (3, 6, 10 and 20 mg L^-1) and iron at 0.6 mg L^-1 in Milli-Q water. Similar results to Xe-lamp experiments were obtained, where solar UV-B + A light H₂O₂ photolysis (HPSP) and solar photo-Fenton-like (SPF-like) played an important role and even at low H₂O₂ and ferric concentrations of 3 and 0.6 mg L^-1 respectively, 2,4-D was efficiently removed at pH 3.6. Simulated surface water at pH 3.6 containing 1 mg L^-1 2,4-D, 20 mg L^-1 H₂O₂ and 0.6 mg L^-1 Fe(III) under natural sunlight irradiation efficiently removed the herbicide and its main metabolite 2,4-DCP after 30 min of treatment while at neutral pH, 40% of herbicide degradation was achieved. In the case of very low iron concentrations (0.05 mg L^-1) at acid pH, 150 min of solar treatment was required to remove 2,4-D.

Comparative study on 2,4-dichlorophenoxyacetic acid and 2,4-dichlorophenol removal from aqueous solutions via ozonation, photocatalysis and non-thermal plasma using a planar falling film reactor

Ozonation and advanced oxidation processes based on photocatalysis (P.C.) and non-thermal plasma generated in a dielectric barrier discharge (DBD) in different gas atmospheres were compared for the degradation and mineralization of 2,4-dichlorophenoxy acetic acid (2,4-D) and 2,4-dichlorophenol (2,4-DCP) in aqueous solutions, using a planar falling film reactor with comparable design. The energetic yields (G₅₀) as measure of the efficiencies of the different methods are for 2,4-D in the order DBD/Ar-Fenton>ozonation>DBD/Ar>P.C.ozonation>DBD/Ar:O₂≫DBD/Air>P.C.oxidation. For 2,4-DCP the order is ozonation≫DBD/Ar-Fenton>P.C.ozonation>DBD/Ar>DBD/Ar:O₂≫P.C.oxidation>DBD/Air. The degradation by using ozone is very effective, but it should be noted that the mineralization measured by the total organic carbon (TOC) removal is low. The reason is the formation of stable towards ozone intermediates, especially low chain carboxylic acids. The fate of these intermediates during the degradation with the different methods has been followed and discussed.

Reduced graphene oxide-silver nanoparticle composite as visible light photocatalyst for degradation of colorless endocrine disruptors

Sunlight-induced degradation of organic pollutants is an ideal approach for environmental pollution control and wastewater treatment. Although a variety of photocatalysts have been designed toward this goal, efficient degradation of colorless organic pollutants by visible light is a challenging issue. Here, we show that a reduced graphene oxide (rGO)-based composite with silver nanoparticle (rGO-Ag) can act as an efficient visible-light photocatalyst for the degradation of colorless organic pollutants. We have developed a simple, large-scale synthesis method for rGO-Ag and used it for the degradation of three well-known endocrine disruptors (phenol, bisphenol A, and atrazine) under UV and visible light. It is found that photocatalytic efficiency by rGO-Ag under visible light is significantly higher compared to that of rGO or silver nanoparticles. It is proposed that Ag nanoparticles offer visible-light-induced excitation of silver plasmons, and conductive rGO offers efficient charge separation and thus induces oxidative degradation of the organic pollutant. This approach can be extended for sunlight-induced degradation of different organic pollutants.