Formation of chlorinated biphenyls, diphenyl ethers and benzofurans as a result of Fenton-driven oxidation of 2-chlorophenol

Naturally Occurring Organohalogen Compounds-A Comprehensive Review

The present volume is the third in a trilogy that documents naturally occurring organohalogen compounds, bringing the total number-from fewer than 25 in 1968-to approximately 8000 compounds to date. Nearly all of these natural products contain chlorine or bromine, with a few containing iodine and, fewer still, fluorine. Produced by ubiquitous marine (algae, sponges, corals, bryozoa, nudibranchs, fungi, bacteria) and terrestrial organisms (plants, fungi, bacteria, insects, higher animals) and universal abiotic processes (volcanos, forest fires, geothermal events), organohalogens pervade the global ecosystem. Newly identified extraterrestrial sources are also documented. In addition to chemical structures, biological activity, biohalogenation, biodegradation, natural function, and future outlook are presented.

Dioxins and furans toxicity during the photocatalytic remediation of emerging pollutants. Triclosan as case study

The benefits of wastewater remediation technologies are offset in those cases where, as a result of operating conditions, harmful compounds are formed in the degradation routes of the original organic pollutants. This may be the case for the application of some advanced oxidation processes to wastewater containing precursors of dioxins and furans, as previously reported in the application of electrochemical and Fenton oxidation to degrade Triclosan and 2-chlorophenol. This work reports for the first time a detailed kinetic analysis of the formation of dioxins and furans during the photocatalytic treatment of aqueous samples containing 5-Chloro-2-[2,4-dichlorophenoxy] phenol, commercially known as Triclosan. After analysis of the PCDD/Fs concentration, the toxicity of the samples has been determined in terms of toxic equivalents (TEQ). TEQ values have been calculated, first with the group of 17 congeners with higher toxicity. Finally, a multivariable analysis and linear regression have been applied to reduce the significant number of congeners and optimize the analytical effort.

Comparison of treatability of four different chlorophenol-containing wastewater by pyrite-Fenton process combined with aerobic biodegradation: Role of sludge acclimation

Aerobic biodegradation combined with pyrite-Fenton process was used for the treatment of wastewater containing different chlorophenols (4-CP, 2,3-DCP, 2,4-DCP, 2,4,6-TCP). Fenton degradation using pyrite as the low cost iron catalyst was used as a pre-treatment step to lower the toxicity of CPs prior to aerobic biodegradation. Synthetic wastewater spiked directly with either 100 mg/L CPs or pyrite-Fenton pre-treated CPs was fed to the batch bioreactors inoculated with unacclimated or acclimated activated sludge using glucose as the C-source. The results show that the CP biodegradation under aerobic conditions was highly dependent on the type of CP treated. Except for 2,4-DCP, all other CPs investigated caused severe sludge toxicity, and thus significantly hindered glucose degradation by unacclimated sludge. The CP toxicity decreased in the order of: 2,4,6-TCP > 2,3-DCP > 4-CP > 2,4-DCP. The toxic effect was explained through an interaction of CPs with the lipid fraction of cell membrane. While the pyrite-Fenton pre-treatment improved the COD removal efficiency using unacclimated sludge, the sCOD removal efficiency was still less than the control reactor operated with no CP addition. With sludge acclimation, however, the sCOD removal efficiencies increased, and approached 74% for 2,4-DCP, 61% for 4-CP, 56% for 2,4,6-TCP and 46% for 2,3-DCP, suggesting an enhanced biomass tolerance to CP toxicity. On the other hand, the sludge acclimation combined with pyrite Fenton pre-treatment provided the best bioreactor performance for all CPs with the sCOD removal efficiencies reaching 81% for 2,4,6-TCP, 78% for 2,4-DCP, 73% for 4-CP and 62% for 2,3-DCP. This suggests that the dechlorination of CPs with Fenton process, in conjunction with sludge acclimation, not only reduced the sludge toxicity, but also enhanced the bioavailability of CP-containing wastewater for microorganisms, especially for highly chlorinated toxic CPs such as 2,4,6-TCP. Overall, the findings highlight the need for sludge acclimation for effective treatment of chlorophenol-containing wastewater by a combined pyrite-Fenton and aerobic biodegradation system.

Efficiency and mechanisms of rhodamine B degradation in Fenton-like systems based on zero-valent iron

Based on the Fe0/H2O2 heterogeneous Fenton system, the degradation of rhodamine B (RhB, an organic dye pollutant) was researched in this paper. The effects of initial pH value, concentration of H2O2, dosage of zero-valent iron (ZVI), and initial RhB concentration on RhB degradation by Fe0/H2O2 were studied. The results showed that when the initial pH = 4, dosage of ZVI was 9 mM, and concentrations of H2O2 and RhB were 8 mM and 0.1 mM, respectively, the color of RhB could be completely faded within 30 min, and the total organic carbon (TOC) removal percentage was about 63% after 120 min. The dissolved oxygen (DO) content and oxidation-reduction potential (ORP) were monitored during the reaction. Quenching experiments with methanol confirmed that the degradation of the dye was mainly due to oxidation by the ˙OH radical. Besides, the results from UV-Vis spectroscopy showed that the degradation of RhB was mainly due to the destruction of the conjugated oxygen hetero-anthracene in the RhB molecule. The solid-phase characterization of the ZVI samples after reaction confirmed that the original regular and slippery ZVI samples finally were corroded into rough and irregular lepidocrocite and magnetite. Two possible competitive reaction pathways for the degradation of RhB by Fe0/H2O2 were proposed by GC-MS analysis, which were attributed to the dissociation of ethyl radicals and the degradation of chromophore radicals.

Formation of chloronitrophenols upon sulfate radical-based oxidation of 2-chlorophenol in the presence of nitrite

Sulfate radical (SO₄^-)-based advanced oxidation processes (SR-AOPs) are promising in-situ chemical oxidation technologies widely applied for soil/groundwater remediation. The presence of non-target water constituents may interfere the abatement of contaminants by SR-AOPs as well as result in the formation of unintended byproducts. Herein, we reported the formation of toxic chloronitrophenols during thermally activated persulfate oxidation of 2-chlorophenol (2CP) in the presence of nitrite (NO₂^-). 2-Chloro-4-nitrophenol (2C4NP) and 2-chloro-6-nitrophenol (2C6NP) were identified as nitrated byproducts of 2CP with total yield up to 90%. The formation of nitrated byproducts is a result of coupling reaction between 2CP phenoxyl radical (ClPhO) and nitrogen dioxide radical (NO₂). As a critical step, the formation of ClPhO was supported by density functional theory (DFT) computation. Both 2C4NP and 2C6NP could convert to 2-chloro-4,6-dinitrophenol (2C46DNP) upon further treatment via a denitration-renitration process. The formation rate of 2C4NP and 2C6NP was closely dependent on the concentration of NO₂^-, solution pH, and natural water constituents. ECOSAR calculation suggests that chloronitrophenols are generally more hydrophobic and ecotoxic than 2CP. Our result therefore reveals the potential risks in the abatement of chlorophenols by SR-AOP, particularly when high level of NO₂^- is present in water matrix.

Photochemical reaction of tricresyl phosphate (TCP) in aqueous solution: Influencing factors and photolysis products

Organophosphate triesters (OPEs) have caused great concern as a class of emerging environmental contaminants due to their widespread use and their toxicity to organisms. However, the phototransformation behavior of OPE is still not fully understood, which is important for understanding their environmental fate. In the present study, the photodegradation of tricresyl phosphate (TCP), one of the most widely detected OPEs in aqueous environments, was investigated including the direct photolysis and in the presence of several natural water factors, NO₂^-, Fe³⁺ and humic acid. The degradation process followed the pseudo-first-order kinetics, with rate constant increasing slightly with increasing initial TCP concentration. The presence of NO₂^- and Fe³⁺ was observed to promote the photochemical loss of TCP, while humic acid played a negative role on TCP transformation. Electron spin resonance (EPR) analysis showed that carbon-centered radical was produced in the photolysis process of TCP, and hydroxyl radical contributed to the promotion of rate constant for Fe³⁺ and NO₂^-. Four photolysis products were tentatively identified by HPLC-LTQ-Orbitrap MS analysis, and the possible degradation pathways of TCP were proposed. These findings provide a meaningful reference for the fate and transformation of OPEs in natural water.

Role of complexing agents on oxidative degradation of chlorophenolic compounds by pyrite-Fenton process: Batch and column experiments

This study involves batch reactor and fixed-bed continuous flow experiments to determine the effects of complexing agents (e.g., tartrate and citrate) on the treatment of chlorophenolic (CP) compounds using heterogeneous Fenton system with pyrite mineral as the iron source. While the addition of organic ligands to the batch systems adversely affected CP removal, organic ligands had a beneficial effect on CP removal in column systems. Although the ligands extended the life span of pyrite-packed columns by removing surface oxidation products through the formation of soluble Fe-ligand complexes, the ligands competed against CPs for hydroxyl radicals (HO^*). The competitive effect was much higher in batch systems since pyrite loading was very low in order to generate sufficient hydroxyl radicals. On the other hand, at much higher pyrite loading of column experiments, the HO^* radicals generated during Fenton process were sufficient to overcome the competitive effect exerted by organic ligands. In spite of much higher Fe solubility in the presence of citrate, citrate was less effective in enhancing CP removal in column systems compared to tartrate since the competitive effect caused by citrate for HO^* radicals was more than that exerted by tartrate.

Condensation By-Products in Wet Peroxide Oxidation: Fouling or Catalytic Promotion? Part I. Evidences of an Autocatalytic Process

The present work is aimed at the understanding of the condensation by-products role in wet peroxide oxidation processes. This study has been carried out in absence of catalyst to isolate the (positive or negative) effect of the condensation by-products on the kinetics of the process, and in presence of oxygen, to enhance the oxidation performance. This process was denoted as oxygen-assisted wet peroxide oxidation (WPO-O2) and was applied to the treatment of phenol. First, the influence of the reaction operating conditions (i.e., temperature, pH0, initial phenol concentration, H2O2 dose and O2 pressure) was evaluated. The initial phenol concentration and, overall, the H2O2 dose, were identified as the most critical variables for the formation of condensation by-products and thus, for the oxidation performance. Afterwards, a flow reactor packed with inert quartz beads was used to facilitate the deposition of such species and thus, to evaluate their impact on the kinetics of the process. It was found that as the quartz beads were covered by condensation by-products along reaction, the disappearance rates of phenol, total organic carbon (TOC) and H2O2 were increased. Consequently, an autocatalytic kinetic model, accounting for the catalytic role of the condensation by products, provides a well description of wet peroxide oxidation performance.

Condensation By-Products in Wet Peroxide Oxidation: Fouling or Catalytic Promotion? Part II: Activity, Nature and Stability

The deposition of condensation by-products onto the catalyst surface upon wet peroxide and wet air oxidation processes has usually been associated with catalyst deactivation. However, in Part I of this paper, it was demonstrated that these carbonaceous deposits actually act as catalytic promoters in the oxygen-assisted wet peroxide oxidation (WPO-O2) of phenol. Herein, the intrinsic activity, nature and stability of these species have been investigated. To achieve this goal, an up-flow fixed bed reactor packed with porous Al2O3 spheres was used to facilitate the deposition of the condensation by-products formed in the liquid phase. It was demonstrated that the condensation by-products catalyzed the decomposition of H2O2 and a higher amount of these species leads to a higher degree of oxidation degree The reaction rates, conversion values and intermediates’ distribution were analyzed. The characterization of the carbonaceous deposits on the Al2O3 spheres showed a significant amount of condensation by-products (~6 wt.%) after 650 h of time on stream. They are of aromatic nature and present oxygen functional groups consisting of quinones, phenols, aldehydes, carboxylics and ketones. The initial phenol concentration and H2O2 dose were found to be crucial variables for the generation and consumption of such species, respectively.

Oxidative degradation of chlorophenolic compounds with pyrite-Fenton process

Batch experiments, in conjunction with chromatographic and spectroscopic measurements, were performed to comparatively investigate the degradation of various chlorophenolic (CP) compounds (e.g., 2-CP, 4-CP, 2,3-DCP, 2,4-DCP, 2,4,6-TCP, 2,3,4,6-TeCP) by a modified Fenton process using pyrite as the catalyst. The batch results show that the CP removal by pyrite-Fenton process was highly dependent on chemical conditions (e.g., pH, CP and pyrite concentration), CP type, number and location of chlorine atoms on the aromatic ring. With the exception of 2,3,4,6-TeCP and 2,3-DCP, the CP removal decreased with increasing the number of chlorine constituents. While the main mechanism responsible for monochlorophenol removal (e.g., 2-CP and 4-CP) was the hydroxyl radical attack on aromatic rings, the CP removal for multichlorophenolic compounds (e.g., 2,3,4,6-TeCP) was driven by both: (1) hydroxyl radical attack on aromatic rings by both solution and surface-bound hydroxyl radicals and (2) adsorption onto pyrite surface sites. The adsorption affinity increased with increasing the number of Cl atoms on the aromatic ring due to enhanced hydrophobic effect. The TOC removal was not 100% complete for all CPs investigated due to formation of chemically less degradable chlorinated intermediate organic compounds as well as low molecular weight organic acids such as formic and acetic acid. Spectroscopic measurements with SEM-EDS, zeta potential and XPS provided evidence for the partial oxidation of pyrite surface Fe(II) and disulfide groups under acidic conditions.

Rethinking sulfate radical-based oxidation of nitrophenols: Formation of toxic polynitrophenols, nitrated biphenyls and diphenyl ethers

Sulfate radical (SO₄^-)-based oxidation of nitrophenols (NPs) have been widely studied; however, formation of potentially more toxic polynitroaromatic intermediates has been overlooked. In this contribution, we systematically investigated the degradation of four NPs by a SO₄^--based oxidation process. Degradation efficiency of NPs followed the order: 2-nitrophenol (2-NP) > 4-nitrophenol (4-NP) > 2,4-dinitrophenol (2,4-DNP) > 2,6-dinitrophenol (2,6-DNP). HPLC and LC-MS/MS analysis confirmed the formation of 2,4-DNP, 2,6-DNP and 2,4,6-trinitrophenol (2,4,6-TNP) during NPs transformation by SO₄^-, suggesting that both denitration and renitration processes occurred. Nitrogen dioxide radicals (NO₂) and phenoxy radicals are responsible for the formation of polynitrophenols. Coupling products including nitrated biphenyls and diphenyl ethers were also detected, which were proposed to be formed by combinations of resonance-stabilized radicals. Electron spin density and charge density calculation showed that ortho C-ortho C and ortho C-phenolic O were the most likely combination ways responsible for coupling products formation. ECOSAR program predicted that polynitrated diphenyl ethers and biphenyls had higher ecotoxicological effects on aquatic species such as fish and daphnia. Therefore, the formation of toxic polynitroaromatic intermediates in SO₄^--based advanced oxidation processes should be scrutinized before this technology can be safely utilized for water and wastewater treatment.

Interactions of Gaseous 2-Chlorophenol with Fe3+-Saturated Montmorillonite and Their Toxicity to Human Lung Cells

The interactions of gaseous 2-chlorophenol with Fe³⁺-saturated montmorillonite particles in a gas-solid system were investigated to simulate the reactions of mineral dusts with volatile organic pollutants in the atmosphere. Results suggested that Fe³⁺-saturated montmorillonite mediated the dimerization of gaseous 2-chlorophenol to form hydroxylated polychlorinated biphenyl, hydroxylated polychlorinated diphenyl ether, and hydroxylated polychlorinated dibenzofuran. The toxicity of Fe³⁺-montmorillonite particles to A549 human lung epithelial cells before and after interaction with 2-chlorophenol was examined to explore their adverse impact on human health. Based on cell morphological analysis, cytotoxicity tests, and Fourier-transform infrared imaging spectra, surface-catalyzed reactions of Fe³⁺-montmorillonite with 2-chlorophenol increased the toxicity of montmorillonite particle on A549 cells. This was supported by increased cellular membrane permeability, the release of extracellular lactate dehydrogenase, and cell damages on cellular DNA, proteins, and lipids. Since mineral dusts are important components of particulate matter, our results help to understand the interactions of volatile organic pollutants with particulate matter in the atmosphere and their adverse impacts on human health.

Application of CWPO to the treatment of pharmaceutical emerging pollutants in different water matrices with a ferromagnetic catalyst

CWPO has proved to be effective for the treatment of representative pharmaceuticals (sulfamethoxazole, atenolol, metronidazole, diltiazem, trimethoprim and ranitidine) in different water matrices (ultrapure water, surface water, WWTP effluent and hospital wastewater). Complete removal of the pollutants and the aromatic intermediates was achieved using the stoichiometric dose of H₂O₂, a catalyst (Fe₃O₄/γ-Al₂O₃) load of 2gL^-1, pH 3 and temperature of 50-75°C. Accordingly, the ecotoxicity was reduced to negligible values. The degradation was faster when the pharmaceuticals were together, being the reaction time for the elimination of the most refractory species (metronidazole) shortened from 4h to 1h. The mineralization of the drugs was fairly different, being the most reactive species those containing several aromatic rings (X_TOC∼80%) and the most refractory that bearing an imidazolium ring (X_TOC∼35%). The water matrix affected the kinetics of the process but in all cases complete conversion of the drugs was reached within 1h. The presence of dissolved organic matter (surface water) seemed to promote drugs degradation while the occurrence of inorganic ions (real WTTP and hospital effluents) partially inhibited it due to scavenging effects. Remarkably, the process was successfully operated at the typical concentrations of main micropollutant sources (μgL^-1).

Environmental application of millimetre-scale sponge iron (s-Fe0) particles (IV): New insights into visible light photo-Fenton-like process with optimum dosage of H2O2 and RhB photosensitizers

In this study, we firstly develop the photo-Fenton-like system with millimetric sponge iron (s-Fe⁰), H₂O₂, visible light (vis, λ≥420nm) and rhodamine B (RhB), and present a comprehensive study concerning the mechanism. Thus, we investigate (1) the adsorption of RhB onto s-Fe⁰, (2) the photo-Fenton-like removal of RhB over iron oxides generated from the corrosion of s-Fe⁰, (3) the homogeneous photo-Fenton removal of RhB over Fe²⁺ or Fe³⁺, (4) the Fe³⁺-RhB complexes, and (5) the photo-Fenton-like removal of tetrabromobisphenol A (TBBPA). The results show that neither the adsorption process over s-Fe⁰ nor the photo-Fenton-like process over FeOOH, Fe₃O₄ and Fe₂O₃, achieved efficient removal of RhB. For comparison, in homogeneous photo-Fenton process, the presence of Fe³⁺ ions, rather than Fe²⁺ ions, effectively eliminated RhB. Furthermore, the UV-vis spectra showing new absorbance at∼285nm indicate the complexes of RhB and Fe³⁺ ions, adopting vis photons to form excited state and further eject one electron via ligand-to-metal charge-transfer to activate H₂O₂. Additionally, efficient TBBPA removal was obtained only in the presence of RhB. Accordingly, the s-Fe⁰- based photo-Fenton-like process assisted with dyestuff wastewater is promising for removing a series of persistent organic pollutants.

Phototransformation of 2,4,6-tribromophenol in aqueous solution: Kinetics and photolysis products

In this research, we studied the phototransformation of 2,4,6-tribromophenol (TBP) by UV irradiation in aqueous solution. Effects of typical environmental factors (i.e., pH and the initial concentration of TBP, Fe³⁺ and NO₂^-) were also investigated. Results showed that the transformation process followed pseudo-first-order kinetics. The transformation rate constant of TBP decreased with increasing initial concentration and increased with increasing Fe³⁺ and NO₂^- concentrations as well as increasing pH. Ten photoproducts were tentatively identified after irradiation of TBP, according to HPLC-LTQ-Orbitrap MS and GC/MS analysis. Among them, two dihydroxylated dibromobenzene compounds (di-OH-DBB) were found to be predominant over the reaction time, indicating that hydrodebromination of TBP could be the main phototransformation mechanism. In addition, the identification of eight OH-PBDEs and di-OH-PBBs suggested that photodimerization might also be a reaction pathway for TBP photochemical reactions.

Mineralization of 2-chlorophenol by sequential electrochemical reductive dechlorination and biological processes

In this work, a novel approach was applied to obtain the mineralization of 2-chlorophenol (2-CP) in an electrochemical-biological combined system where an electrocatalytic dehydrogenation process (reductive dechlorination) was coupled to a biological denitrification process. Reductive dechlorination of 2-CP was conducted in an ECCOCEL-type reactor on a Pd-Ni/Ti electrode at a potential of -0.40V vs Ag/AgCl(s)/KCl(sat), achieving 100 percent transformation of 2-CP into phenol. The electrochemically pretreated effluent was fed to a rotating cylinder denitrifying bioreactor where the totality of phenol was mineralized by denitrification, obtaining CO2 and N2 as the end products. The total time required for 2-CP mineralization in the combined electrochemical-biological process was 7.5h. This value is close to those previously reported for electrochemical and advanced oxidation processes but in this case, an efficient process was obtained without accumulation of by-products or generation of excessive energy costs due to the selective electrochemical pretreatment. This study showed that the use of electrochemical reductive pretreatment combined with biological processes could be a promising technology for the removal of recalcitrant molecules, such as chlorophenols, from wastewaters by more efficient, rapid, and environmentally friendly processes.

Efficient photodecomposition of rhodamine B by an Fe-based metallic glass in an oxalic acid solution

Fe–Si–B amorphous ribbons has been fabricated and effectively employed as a heterogeneous catalyst in the Fenton system with oxalic acid.

Assessment of PCDD/Fs formation in the Fenton oxidation of 2-chlorophenol: Influence of the iron dose applied

Toxic polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) may be formed during remediation of chlorinated phenols via Fenton oxidation. To highlight the need for monitoring the production of toxic byproducts in these reactions, this work assessed the influence of iron dose (0.09-0.36 mM) on the Fenton oxidation of 2-chlorophenol (2-CP, 15.56 mM), a potential precursor of PCDD/Fs, by quantifying 2-CP removal and mineralization rates as well as byproducts yields, including PCDD/Fs. Although the increase in the iron dose showed positive contribution to 2-CP oxidation, under the operating conditions of the current study (H2O2 at 20% of the stoichiometric dose and 20 °C), there was no effect on the mineralization rate, and TOC and chlorine balances were far to be closed, depicting the presence of chlorinated organic byproducts in the reaction medium. After 4 h of treatment, the total PCDD/Fs concentrations increased by 14.5-39 times related to the untreated sample when the iron doses tested decreased from 0.36 to 0.09 mM, with preferential formation of PCDFs over PCDDs and dominance of lower chlorinated congeners such as tetra and penta-PCDD/Fs. The treatment with the highest iron dose (0.36 mM) exhibited the lowest PCDD/Fs yields and was thus most successful at mitigating toxic byproducts of the Fenton oxidation, leading to lower sample toxic equivalence (TEQ) value.

Identification of oxidative coupling products of xylenols arising from laboratory-scale phytoremediation

Oxidative coupling reactions take place during the passage of xylenols through a laboratory-scale helophyte-based constructed wetland system. Typical coupling product groups including tetramethyl-[1,1'-biphenyl] diols and tetramethyl diphenylether monools as stable organic intermediates could be identified by a combination of pre-chromatographic derivatization and GC/MS analysis. Structural assignment of individual analytes was performed by an increment system developed by Zenkevich to pre-calculate retention sequences. The most abundant analyte turned out to be 3,3',5,5'-tetramethyl-[1,1'-biphenyl]-4,4'-diol, which can be formed by a combination of radicals based on 2,6-xylenol or by an attack of a 2,6-xylenol-based radical on 2,6-xylenol. Organic intermediates originating from oxidative coupling could also be identified in anaerobic constructed wetland systems. This finding suggested the presence of (at least partly) oxic conditions in the rhizosphere.

Hydrothermal carbonization of poly(vinyl chloride)

Poly(vinyl chloride) (PVC) was subjected to hydrothermal carbonization in subcritical water at 180-260 °C. Dehydrochlorination increased with increasing reaction temperature. The release of chlorine was almost quantitative above ∼235 °C. The fraction of organic carbon (OC) recovered in the hydrochar decreased with increasing operating temperature from 93% at 180 °C to 75% at 250 °C. A wide array of polycyclic aromatic hydrocarbons (PAHs) could be detected in the aqueous phase, but their combined concentration amounted to only ∼140 μg g(-1) PVC-substrate at 240 °C. A pathway for the formation of cyclic hydrocarbons and O-functionalized organics was proposed. Chlorinated hydrocarbons including chlorophenols could only be identified at trace levels (low ppb). Polychlorinated dibenzodioxins (PCDDs) and dibenzofurans (PCDFs) could not be detected. The sorption potential of the hydrochar turned out to be very low, in particular for polar organic pollutants. Our results provide strong evidence that hydrothermal carbonization of household organic wastes which can be tied to co-discarded PVC-plastic residues is environmentally sound regarding the formation of toxic organic products. Following these findings, hydrothermal treatment of PVC-waste beyond operating temperatures of ∼235 °C to allow complete release of organic chlorine should be further pursued.

Overview of the PCDD/Fs degradation potential and formation risk in the application of advanced oxidation processes (AOPs) to wastewater treatment

Polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) are a family of unintentionally produced persistent organic pollutants (POPs) that have received considerable public and scientific attention due to the toxicity of some of their congeners, more specifically those with chlorine substitution in the 2,3,7,8 positions. The environmental management and control of PCDD/Fs is addressed at a global level through the Stockholm Convention that establishes that POPs should be destroyed or irreversibly transformed in order to reduce or eliminate their release to the environment. Several technologies, including advanced oxidation processes (AOPs) such as photolysis, photocatalysis and Fenton oxidation, have been considered as effective methods for destroying PCDD/Fs in polluted waters. Nevertheless, during the remediation of wastewaters it is critical that the treatment technologies applied do not lead to the formation of by-products that are themselves POPs, especially if PCDD/Fs precursors or chlorine are present in the reaction medium. Despite the high effectiveness of AOPs in the oxidation of major contaminants, scarce references deal with the monitoring of PCDD/Fs in the course of the oxidation process, revealing that a detailed assessment of non-combustion technologies with respect to PCDD/Fs formation is still lacking. This study reports a review of the state of the art related to the potential remediation and/or formation of PCDD/Fs as a result of the application of AOPs for the treatment of polluted waters, warning on the correct selection of the operating conditions.

Efficient degradation of rhodamine B using Fe-based metallic glass catalyst by Fenton-like process

An efficient heterogeneous catalyst, Fe-based metallic glass (Fe–Si–B amorphous ribbon), was successfully prepared for Fenton-like degradation of rhodamine B (RhB) by a melt-spinning method. The catalyst was characterized using XRD and SEM. The effects of various reaction parameters such as H2O2 dosage, temperature, initial pH value, Fe–Si–B dosage and initial RhB concentration on the degradation of RhB were studied. Almost complete degradation of RhB (20 mg L−1) was achieved within only 10 min by 0.5 g L−1 Fe–Si–B catalyst and 1.6 mM H2O2 at pH 3.0 at 295 K. Kinetic analyses showed that the degradation process could be described by a pseudo-first-order kinetic model. The catalytic stability was also investigated and it was found that the Fe–Si–B catalyst exhibited good structural stability and no loss of performance even after three cycles. It was concluded that the Fe–Si–B amorphous ribbon was a potential heterogeneous Fenton-like catalyst for industrial wastewater treatment.

The critical role of the operating conditions on the Fenton oxidation of 2-chlorophenol: assessment of PCDD/Fs formation

This work assesses the influence of the operating conditions H2O2 dose (20 or 100% of the stoichiometric amount), temperature (20 or 70°C), and the presence of chloride in the oxidation medium in the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) during Fenton treatment of aqueous samples of 2-chlorophenol, 2-CP, one of the strongest precursor of PCDD/Fs. After 4h of oxidation in the experiments carried out with 20% H2O2 chlorinated phenoxyphenols and biphenyls, which are intermediates in PCDD/Fs formation, as well as PCDD/Fs were observed, resulting in concentrations 11 times higher than in the untreated sample. Additionally, when NaCl was also present in the reaction medium, PCDD/Fs were formed at higher extent, with a total concentration 74.4 times higher than in the untreated 2-CP solution. Results depicted a preferential formation of PCDFs over PCDDs, with dominance of lower chlorinated PCDD/Fs (tetra and penta-PCDD/Fs). Besides, the formation of the most toxic PCDD/Fs congeners (2,3,7,8-PCDD/Fs) was not favored under the operating conditions used in this work.

Degradation of chlorophenols and alkylphenol ethoxylates, two representative textile chemicals, in water by advanced oxidation processes: the state of the art on transformation products and toxicity

Advanced oxidation processes based on the generation of reactive species including hydroxyl radicals are viable options in eliminating a wide array of refractory organic contaminants in industrial effluents. The assessment of transformation products and toxicity should be, however, the critical point that would allow the overall efficiency of advanced oxidation processes to be better understood and evaluated since some transformation products could have an inhibitory effect on certain organisms. This article reviews the most recent studies on transformation products and toxicity for evaluating advanced oxidation processes in eliminating classes of compounds described as "textile chemicals" from aqueous matrices and poses questions in need of further investigation. The scope of this paper is limited to the scientific studies with two classes of textile chemicals, namely chlorophenols and alkylphenol ethoxylates, whose use in textile industry is a matter of debate due to health risks to humans and harm to the environment. The article also raises the critical question: What is the state of the art knowledge on relationships between transformation products and toxicity?

Quantitative assessment of the formation of polychlorinated derivatives, PCDD/Fs, in the electrochemical oxidation of 2-chlorophenol as function of the electrolyte type

The electrochemical degradation of 2-chlorophenol (2-CP) on boron-doped diamond (BDD) anodes was carried out using two electrolytes, NaCl and Na2SO4. Both electrolytes supported complete mineralization of 2-CP, but faster rates of degradation were observed in NaCl. After 4 h of oxidation, the total organic carbon (TOC) balance neared 100% with Na2SO4 for identifiable compounds, whereas 4 mM of TOC remained unidentified with NaCl. Since chlorophenols are known to be precursors of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), a rigorous assessment of intermediate products was carried out. When near complete mineralization was achieved, the use of NaCl resulted in the concentration of total PCDD/Fs increasing 2.68 × 10(4) times compared to the untreated sample, and to toxicity values several times higher than the maximum level established by U.S. Environmental Protection Agency for water ingestion. When Na2SO4 was used, the increase in total PCDD/Fs concentration was 134 times lower than with NaCl and there was no significant 2,3,7,8-PCDD/Fs formation. Thus, we emphasize the importance of electrolyte selection in electro-oxidation processes, especially when PCDD/Fs precursors are initially present or may be formed in the treated water samples.

Chlorophenols breakdown by a sequential hydrodechlorination-oxidation treatment with a magnetic Pd-Fe/γ-Al2O3 catalyst

Degradation of chlorophenols by a sequential combination of hydrodechlorination (HDC) and catalytic wet peroxide oxidation (CWPO) using a new magnetic Pd-Fe/γ-Al2O3 catalyst has been studied. This catalyst is active in both hydrodechlorination of chlorophenols and decomposition of H2O2 for the oxidation of organic compounds. The sequential combination of HDC and CWPO allows overcoming some of the drawbacks of both treatments applied independently. The HDC step achieves the complete dechlorination of chlorophenols, so that the subsequent CWPO does not lead to the formation of highly toxic chlorinated by-products and reduces significantly the organic load of the effluent. The results showed that the presence of iron in the Pd catalyst improved significantly its hydrodechlorination rate, achieving the complete dechlorination of chlorophenols in a short reaction time (≈ 15 min), giving rise to phenol and cyclohexanone. The CWPO of synthetic mixtures of phenol and cyclohexanone showed that a high phenol concentration promotes the oxidation of all the organic species, but the presence of cyclohexanone seems to hinder the formation of aromatic radicals limiting the effectiveness of the CWPO step. Therefore, the effective combination of HDC and CWPO requires that the HDC step achieves the complete dechlorination of chlorophenols but no further hydrogenation is needed. The Pd-Fe/γ-Al2O3 catalyst showed a high activity in both HDC and subsequent CWPO of chlorophenols being easily separated and recovered from the reaction medium due to its ferromagnetic properties. In spite of a moderate loss of activity, the complete dechlorination of chlorophenol and a negligible ecotoxicity of the final effluents were maintained upon successive applications of HDC + CWPO in a four-cycles test.

Hydrothermal carbonization of olive mill wastewater

Hydrothermal carbonization (HTC) is an emerging technology to treat wet biomasses aimed at producing a biochar material. Herein, olive mill wastewater (OMW) was subjected to HTC. Mass balance considerations provide evidence that the yield of biochar is low (~30%, w/w), which is associated with a low fraction of carbohydrates in OMW. The combination of different preparation schemes, pre-chromatographic derivatization reactions and GC/MS analysis for the analysis of organic compounds in aqueous HTC-solutions allowed to identify and quantify a wide array of analytes which belong either to intrinsic constituents of OMW or to characteristic HTC-breakdown products. Biophenols, such as hydroxyl-tyrosol (OH-Tyr), tyrosol (Tyr) account for the most abundant members of the first group. Most abundant breakdown products include phenol and benzenediols as well as short-chain organic acids. Secoiridoids, such as decarbomethoxy ligostride aglycon and decarbomethoxy oleuropein aglycon, all of them being typical components of OMW, are less abundant in HTC-solutions.

Formation of 2'-hydroxy-2,3',4,5'-tetrabromodipheyl ether (2'-HO-BDE68) from 2,4-dibromophenol in aqueous solution under simulated sunlight irradiation

Hydroxylated polybrominated diphenyl ethers (HO-PBDEs) have received significant attention due to their toxicities and universal presence in the environmental matrices. However, their origins are not fully understood. We explored the feasibility of the generation of HO-PBDEs through photochemical processes from bromophenol, a commonly detected pollutant with anthropogenic source in freshwater and natural source in the marine environment. The results showed that when 2,4-dibromophenol (2,4-diBP) was irradiated in aquatic solutions under simulated sunlight, significant amounts of 2'-hydroxy-2,3',4,5'-tetrabromodipheyl ether (2'-HO-BDE68) were rapidly formed as the dimeric product of 2,4-diBP. The formation of 2'-HO-BDE68 intensified with the increase of light intensity and with the initial concentration increase of 2,4-diBP, whereas it weakened with an increase in pH. Moreover, Fe(III) and fulvic acid played important roles in the formation of 2'-HO-BDE68. This study provides important insight into a possible source of HO-PBDEs from bromophenols in natural aquatic systems through photochemical approaches.

Removal and degradation pathway study of sulfasalazine with Fenton-like reaction

The Fenton-like degradation of sulfasalazine solution is studied in this work. The effects of reaction parameters such as Fe(3+) concentration, initial H(2)O(2) dosage and the reaction temperature are evaluated. For sulfasalazine of 100mg/L, the removal of sulfasalazine, chemical oxygen demand (COD) and total organic carbon (TOC) reached 99.5%, 84.2% and 41% in 60 min with 0.20mM Fe(3+) and 16 mM H(2)O(2) at 35°C, respectively. The complexed Fe(3+) presents a reaction constant of 0.062 min(-1)mM(-1) while that of free Fe(3+) is 2.526 min(-1)mM(-1) for sulfasalazine degradation. LC-MS technology was used to analysis the possible degradation intermediates. The degradation of sulfasalazine principally begins with the attack of hydroxyl radical on the azo-group as well as the sulfanilamido group. Both intramolecular rearrangement and bimolecular reaction occur simultaneously after the hydroxyl radical attack. Further attack of the active oxidative species results in the cleavage of the aromatic rings and the production of CO(2). The degradation of industrial sulfasalazine wastewater with a COD of 3425 mg/L has also been achieved by Fenton reaction with different dosage of H(2)O(2). Relatively better removal efficiency is observed at moderate Fe/H(2)O(2) molar ratio from 1/5 to 2/5 for industrial sulfasalazine wastewater treatment.

Assessment of the generation of chlorinated byproducts upon Fenton-like oxidation of chlorophenols at different conditions

Homogeneous Fenton-like (H(2)O(2)/Fe(3+)) oxidation proved to be highly efficient in the degradation of monochlorophenols but some important issues need to be considered depending on the operating conditions. When using the stoichiometric amount of H(2)O(2) and a dose of Fe(3+) in the range of 10-20mg/L, complete breakdown of 4-CP up to CO(2) and short-chain acids was achieved. Nevertheless, when substoichiometric amounts of H(2)O(2) or lower concentrations of iron were used, significant differences between the TOC measured and the calculated from the identified species were found. These differences were attributed to condensation byproducts, including chlorinated species, formed by oxidative coupling reactions. PCBs, dioxins and dichlorodiphenyl ethers were identified. A solid residue was also formed consisting mainly in carbon, oxygen and chlorine including also Fe. The occurrence of these highly toxic species must be carefully considered in the application of Fenton oxidation to wastewaters containing chlorophenols. The possibility of reducing costs by lowering the H(2)O(2) dose below the stoichiometric one needs to take this into account.

Degradation of rhodamine B by Fe(0)-based Fenton process with H2O2

Degradation of rhodamine B by Fe(0)-based Fenton process with H(2)O(2) was investigated. The effects of H(2)O(2) dose, Fe(0) dose, initial concentration of rhodamine B and initial pH value on the degradation of rhodamine B were examined. The results showed that the degradation and mineralization of rhodamine B occurred with low dose of H(2)O(2) and Fe(0). The intermediates of rhodamine B were analyzed with UV-Vis spectrophotometry and ion chromatography and the mechanism of oxidative degradation of rhodamine B was also discussed. The reactive oxygen species (·OH) produced in Fe(0)-based Fenton process with H(2)O(2) is the key to the degradation of rhodamine B by ways of N-de-ethylation, chromophore cleavage, ring-opening and mineralization.

Aromatic intermediate formation during oxidative degradation of Bisphenol A by homogeneous sub-stoichiometric Fenton reaction

The elimination of Bisphenol A (BPA) from contaminated waters is an urgent challenge. This contribution focuses on BPA degradation by homogeneous Fenton reagent based on reactive ()OH radicals. Pronounced sub-stoichiometric amounts of H(2)O(2) oxidant were used to simulate economically viable processes and operation under not fully controlled conditions, as for example in in situ groundwater remediation. Aside from the most abundant benzenediols and the monohydroxylated BPA intermediate (which were detected as stable intermediates in earlier contributions), a wide array of aromatic products in the molecular weight range between 94 Da (phenol) and approximately 500 Da could be detected, the overwhelming majority of which have not been reported thus far. The identification was carried out by GC/MS analysis of trimethylsilyl ethers. The structural assignments were confirmed through the use of fully deuterated [(2)H(16)] BPA as the substrate, as well as using retention indices calculated on the basis of the increment system. The occurrence of aromatic intermediates larger than BPA, which typically share either a biphenyl- or a diphenylether structure, can be explained by oxidative coupling reactions of stabilized free radicals or by the addition of organoradicals (organocations) onto BPA molecules or benzenediols. The hydroxycyclohexadienyl radical of BPA was recognized to play central role in the degradation pathways. Ring opening products, including lactic, acetic and dicarboxylic acids, could be detected in addition to aromatic intermediates. Since some of those intermediates and products are recalcitrant to further oxidation under the conditions of sub-stoichiometric Fenton reaction, they should be carefully considered when designing and optimizing Fenton-driven remediation systems.