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

Application of intensified Fenton oxidation to the treatment of sawmill wastewater

The application of the Fenton process for the treatment of sawmill wastewater has been investigated. The sawmill wastewater was characterized by a moderate COD load (≈3gL(-1)), high ecotoxicity (≈ 40 toxicity units) and almost negligible BOD/COD ratio (5×10(-3)) due to the presence of different fungicides such as propiconazole and 3-iodo-2-propynyl butyl carbamate, being the wastewater classified as non-biodegradable. The effect of the key Fenton variables (temperature (50-120°C), catalyst concentration (25-100 mg L(-1) Fe(3+)), H2O2 dose (1 and 2 times the stoichiometric dose) and the mode of H2O2 addition) on COD reduction and mineralization was investigated in order to fulfill the allowable local limits for industrial wastewater discharge and achieve an efficient consumption of H2O2 in short reaction times (1h). Increasing the temperature clearly improved the oxidation rate and mineralization degree, achieving 60% COD reduction and 50% mineralization at 120°C after 1h with the stoichiometric H2O2 dose and 25 mg L(-1) Fe(3+). The distribution of H2O2 in multiple additions throughout the reaction time was clearly beneficial avoiding competitive scavenging reactions and thus, achieving higher efficiencies of H2O2 consumption (XCOD ≈ 80%). The main by-products were non-toxic short-chain organic acids (acetic, oxalic and formic). Thus, the application of the Fenton process allowed reaching the local limits for industrial wastewater discharge into local sewer system at a relatively low cost.

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?

Coupling Fenton and biological oxidation for the removal of nitrochlorinated herbicides from water

The combination of Fenton and biological oxidation for the removal of the nitrochlorinated herbicides alachlor, atrazine and diuron in aqueous solution has been studied. The H2O2 dose was varied from 20 to 100% of the stoichiometric amount related to the initial chemical oxygen demand (COD). The effluents from Fenton oxidation were analyzed for ecotoxicity, biodegradability, total organic carbon (TOC), COD and intermediate byproducts. The chemical step resulted in a significant improvement of the biodegradability in spite of its negligible or even slightly negative effect on the ecotoxicity. Working at 60% of the stoichiometric H2O2 dose allowed obtaining highly biodegradable effluents in the cases of alachlor and atrazine. That dose was even lower (40% of the stoichiometric) for diuron. The subsequent biological treatment was carried out in a sequencing batch reactor (SBR) and the combined Fenton-biological treatment allowed up to around 80% of COD reduction.

Degradation of chlorophenoxy herbicides by coupled Fenton and biological oxidation

A combined treatment for the degradation of the chlorophenoxy herbicides 2,4-D and MCPA in water by means of Fenton and biological oxidation has been studied. The chemical oxidation step was necessary to achieve an efficient removal of these pollutants due to their toxicity and low biodegradability. Aqueous herbicide solutions (180mgL(-1)) were subjected to Fenton oxidation upon different H2O2 doses (from the theoretical stoichiometric amount referred to initial COD to 20% of this value). The toxicity and biodegradability tests of the Fenton effluents suggested that the ones resulting upon treatment with 80% and 60% of stoichiometric H2O2 were the optimal for subsequent biological treatment dealing with 2,4-D and MCPA, respectively. These effluents were treated in a sequencing batch reactor achieving nearly 90% conversion of organic matter measured as COD.

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.

Case study of the application of Fenton process to highly polluted wastewater from power plant

This work investigates the application of Fenton process to the treatment of a highly polluted industrial wastewater resulting from the pipeline cleaning in a power plant. This effluent is characterized by a high chemical oxygen demand (COD>40 g/L), low biodegradability and quite a high iron concentration (around 3g/L) this coming from pipeline corrosion. The effect of the initial reaction temperature (between 50 and 90 °C) and the way of feeding H2O2 on the mineralization percentage and the efficiency of H2O2 consumption has been analyzed. With the stoichiometric amount of H2O2 relative to initial COD, fed in continuous mode, more than 90% COD reduction was achieved at 90 °C. That was accompanied by a dramatic improvement of the biodegradability. Thus, a combined treatment based on semicontinuous high-temperature Fenton oxidation (SHTF) and conventional aerobic biological treatment would allow fulfilling the COD and ecotoxicity regional limits for industrial wastewaters into de municipal sewer system. For the sake of comparison, catalytic wet air oxidation was also tested with poor results (less than 30% COD removal at 140 °C and 8 atm oxygen pressure).

Molecularly imprinted photocatalyst with a structural analogue of template and its application

To realize selective mineralization of low-level chlorophenols (CPs) in the presence of high-level ordinary pollutants, molecularly imprinted polymers (MIPs) coated photocatalyst was prepared using substrate analog as template. The pseudo-template imprinted photocatalysts showed rapid decomposition ability toward a group of CPs. Based on the complete dechlorination and spectrophotometry, a new method was proposed to detect the total organochlorine on CPs in water samples. The method showed good linearity when the concentrations of the total organochlorine on CPs is in the range of 12.0-200.0μmolL(-1). The detection limit is 1μmolL(-1) for this method. When this method was applied to measure the total organochlorine of the CPs in both tap water and river water samples, an average recovery ranged from 96.3% to 105.1% was obtained with RSD values less than 5%. In this green and simple method, the common inorganic ions in water showed no interference for the detection. The determination of the total organochlorine on the CPs might be used for estimation of the toxicity and the persistence of the water samples.

Complete dechlorination of 2,4-dichlorophenol in aqueous solution on palladium/polymeric pyrrole-cetyl trimethyl ammonium bromide/foam-nickel composite electrode

The electrochemically reductive dechlorination of 2,4-dichlorophenol (2,4-DCP) in aqueous solution on palladium/polymeric pyrrole-cetyl trimethyl ammonium bromide/foam-nickel electrode (Pd/PPy-CTAB/foam-Ni electrode) was investigated in this paper. Pd/PPy-CTAB/foam-Ni electrode was prepared and characterized by cyclic voltammetry (CV), scanning electron microscope (SEM), X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) adsorption and inductively coupled plasma-atomic emission spectroscopy (ICP-AES). The influences of some experimental factors such as the dechlorination current, dechlorination time and the initial pH on the removal efficiency and the current efficiency of 2,4-DCP dechlorination on Pd/PPy-CTAB/foam-Ni electrode were studied. Complete removal of 2,4-DCP was achieved and the current efficiency of 47.4% could be obtained under the conditions of the initial pH of 2.2, the dechlorination current of 5 mA and the dechlorination time of 50 min when the initial 2,4-DCP concentration was 100 mg L(-1). The analysis of high performance liquid chromatography (HPLC) identified that the intermediate products were 2-chlorophenol (2-CP) and 4-chlorophenol (4-CP). The final products were mainly phenol. Its further reduction product cyclohexanone was also detected. The electrocatalytic dechlorination pathways of 2,4-DCP on Pd/PPy-CTAB/foam-Ni electrode were discussed. The stability of the electrode was favorable that it could keep dechlorination efficiency at 100% after having been reused 10 times. Results revealed that the stable prepared Pd/PPy-CTAB/foam-Ni electrode presented a good application prospect in dechlorination process with high effectiveness and low cost.

Comparative study of the destruction of polychlorinated dibenzo-p-dioxins and dibenzofurans during Fenton and electrochemical oxidation of landfill leachates

Two advanced oxidation processes (AOPs) that provide a broad-spectrum contaminant destruction option were applied to the treatment of the leachates, namely electrochemical and Fenton oxidation. Despite the similar efficiency in the oxidation of major organic contaminants, approximately 90% reduction of chemical oxygen demand after 180 min, our results showed a different behaviour of both AOPs in the oxidation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs). A concentration reduction of 73% for 1,2,3,4,6,7,8-HpCDD and of 71% for OCDD was reached after 180 min of electrochemical oxidation and the total toxic equivalent of the sample was reduced in 58%. However, Fenton oxidation followed a different trend and for similar operation times, it was detected an increase in the concentration of several PCDD/Fs congeners, specially stressed for the major congeners 1,2,3,4,6,7,8-HpCDD, OCDD and OCDF, and in the total toxicity of the sample between 12.5% and 128%. Comparison of the results and the rationale behind their difference are finally discussed.