Recent development of electrochemical advanced oxidation of herbicides. A review on its application to wastewater treatment and soil remediation

Herbicides have been largely utilized during the last decades to maintain the quality and quantity of agricultural crop, ensuring the need of an increasing world food production. However, these synthetic organics are highly biorecalcitrant and stable at mild conditions and cannot be effectively destroyed in conventional wastewater treatment facilities. Among the advanced oxidation processes used to remove herbicides, electrochemical technologies have been recently developed at bench scale as potential powerful treatments. This review presents a critical, exhaustive and detailed analysis on the application of single and combined electrochemical advanced oxidation processes to remediate wastewaters and soils contaminated with common herbicides, covering the period 2010-2019. Nine kinds of treatments, including single methods like anodic oxidation, anodic oxidation with electrogenerated H2O2, homogeneous and heterogeneous electro-Fenton, photoelectro-Fenton, solar photoelectro-Fenton and photoelectrocatalysis, as well as combined ones involving hybrid and sequential processes, have been examined. The fundamentals of each technology are briefly described, and the main results obtained for the removal of the most used herbicide families from synthetic solutions and soil-washing effluents are carefully exposed and discussed. The role of generated oxidizing agents and/or photolytic reactions in photo-assisted processes is explained to justify the mechanisms proposed for herbicide mineralization. The comparative oxidation ability of the different methods is discussed. Finally, future challenges remarking the need of treating real agricultural wastewaters and contaminated soils, the construction of electrochemical systems with stable electrodes at industrial scale and the realization of techno-economic studies are envisaged

Kinetic Study of the Electrochemical Mineralization of m-Cresol on a Boron-Doped Diamond Anode.

The kinetics of the electrooxidation of m-cresol in aqueous solution was investigated in a one-compartment flow electrochemical cell with a boron-doped diamond electrode (BDD). Cyclic voltammograms recorded on BDD revealed that cresol oxidation takes place at a potential very close to the discharge of water. Under potentiostatic conditions, at a working potential lower than water discharge, a passive layer was rapidly formed on the electrode surface due to cresol polymerization. The anode fouling was not observed during electrolysis performed with the flow electrochemical cell operating under galvanostatic conditions. In this case, the decay of mcresol concentration followed a pseudo-first-order kinetics. The abatement of chemical oxygen demand (COD) showed that the kinetics of m-cresol oxidation was limited by mass transfer and that a full mineralization was achieved. A good agreement between predicted and experimental COD and instantaneous current efficiency values was obtained, although some deviations were observed at high current since the experimental data decreased faster than those predicted ones. These deviations can be explained by the occurrence of oxygen evolution which increases the mass transfer coefficient

Sequential use of a continuous-flow electrocoagulation reactor and a (photo)electro-Fenton recirculation system for the treatment of Acid Brown 14 diazo dye

The decolorization and TOC removal of solutions of Acid Brown 14 (AB14) diazo dye containing 50 mg L-1 of total organic carbon (TOC) have been first studied in a continuous-flow electrocoagulation (EC) reactor of 3 L capacity with Fe electrodes of ~110 cm2 area each. Total loss of color with poor TOC removal was found in chloride, sulfate, and/or hydrogen carbonate matrices after 18 min of this treatment. The best performance was found using 5 anodes and 4 cathodes of Fe at 13.70 A and low liquid flow rate of 10 L h-1, in aerated 39.6 mM NaCl medium within a pH range of 4.0–10.0. The effluent obtained from EC was further treated by electro-Fenton (EF) using a 2.5 L pre-pilot flow plant, which was equipped with a filter-press cell comprising a Pt anode and an air-diffusion cathode for H2O2 electrogeneration. Operating with 0.10–1.0 mM Fe2+ as catalyst at pH 3.0 and 50 mA cm-2, a similar TOC removal of 68 % was found as maximal in chloride and sulfate media using the sequential EC-EF process. The EC-treated solutions were also treated by photoelectro-Fenton (PEF) employing a photoreactor with a 125 W UVA lamp. The sequential EC-PEF process yielded a much higher TOC reduction, close to 90 % and 97 % in chloride and sulfate media, respectively, due to the rapid photolysis of the final Fe(III)-carboxylate complexes. The formation of recalcitrant chloroderivatives from generated active chlorine limited the mineralization in the chloride matrix. For practical applications of this two-step technology, the high energy consumption of the UVA lamp in PEF could be reduced by using free sunlight

Solar photoelectro-Fenton treatment of a mixture of parabens spiked into secondary treated wastewater effluent at low input current

Aqueous mixtures of methyl, ethyl and propyl paraben (MeP, EtP and PrP) prepared in real urban wastewater with low conductivity were treated by solar photoelectro-Fenton (SPEF) process at low input current (j = 10 mA cm-2) using a pre-pilot plant with an electrochemical reactor equipped with an air-diffusion cathode to electrogenerate H2O2 and a boron-doped diamond (BDD) or RuO2-based anode. Comparative trials in simulated water matrices with or without Cl− in the absence of natural organic matter (NOM) always led to a slower decay of parabens concentration and total organic carbon (TOC). This was mainly due to the superior regeneration of Fe2+ from photoreduction of Fe(III) complexes formed with NOM in real wastewater compared to that from Fe(OH)2+. In all matrices, a catalyst concentration as low as 0.20 mM Fe2+ was enough to ensure the production of ¿OH in the bulk from Fenton's reaction. SPEF with BDD yielded a complete removal of parabens in 180 min and 66% mineralization at 240 min. This gave rise to the greatest mineralization current efficiencies reported so far, up to 1000%, with a low energy consumption of 84 kWh (kg TOC)-1. The synergy between homogeneous and heterogeneous catalysis, which allowed the efficient dosage of ¿OH and M(¿OH) at low j, with simultaneous action of high UV power from sunlight justified such a good performance. Analogous apparent rate constants were determined for MeP, EtP and PrP. Slower decays were found with RuO2-based anode due to its lower oxidation power. As a result, the MCE was 425% as maximum, but a lower energy consumption of 52 kWh (kg TOC)-1 was needed. Since the role of active chlorine was of minor importance, the formation of toxic, refractory chloroderivatives was minimized. All by-products were transformed into malic, formic and oxalic acids prior to total mineralization

Editorial of the special issue on advanced electrochemical technologies for environmental applications

This special issue of Separation and Purification Technology gathers 27 articles, which are related to keynotes and oral or poster presentations at the 2nd European Workshop of Electrochemical Engineering entitled ‘New Bridges for a New Knowledge on Electrochemical Engineering’. The workshop was held from 1st to 5th October 2017 in Barcelona (Spain), as a Joint Event of the 10th World Congress of Chemical Engineering (WCCE10). This congress was promoted by the World Chemical Engineering Council (WCEC), the European Federation of Chemical Engineering (EFCE) and the European Society of Biochemical Engineering Sciences (ESBES) to approach researchers and specialists in all areas of chemical engineering and to improve their strategy for the development of innovative processes that will be vital for the society of tomorrow. The joint event was promoted by the Working Party on Electrochemical Engineering (WPEE) of the EFCE and co-organized with the Spanish Excellence Network on Environmental and Energy Applications of the Electrochemical Technology (thus being the 2nd Workshop of E3TECH Network). It took place at Fira de Barcelona, one of the most important trade fair institutions in Europe

Treatment of cheese whey wastewater by combined electrochemical processes

This study shows the good performance of a sequential electrochemical methodology, consisting in electrocoagulation (EC) followed by an electrochemical advanced oxidation process (EAOP), to treat raw cheese whey wastewater at laboratory and pre-pilot scales. In EC, different electrode materials like Fe, Al and stainless steel (AISI 304 and ASI 316L) were tested. Among EAOPs, photoelectro-Fenton (PEF) and electrochemical oxidation (EO) with active anodes like Pt or DSA and non-active ones like boron-doped diamond (BDD) were studied. At both scales, the optimum anode/cathode combination in EC was Fe/AISI 304, which yielded the highest total organic carbon (TOC) removal of 22.0%-27.0%. This is due to various effects on organic compounds: (i) coagulation promoted by Fe(OH)3 flocs, (ii) cathodic reduction, and (iii) oxidation with generated active chlorine. At small scale, the resulting wastewater was further treated by PEF at pH 3.0. The highest TOC removal was achieved using the BDD, owing to the great oxidation power of hydroxyl radicals. In contrast, total nitrogen was abated much more rapidly with active anodes because of the attack of active chlorine on N-compounds. At pre-pilot scale, the post-treatment of conditioned wastewater made by EO with a BDD/Pt flow cell combined with UVA irradiation yielded the highest TOC removal, i.e., 49.1%. The high energy consumed by the UVA lamp would be a drawback at industrial scale, which could be overcome by using sunlight

Blue LED light-driven photoelectrocatalytic removal of naproxen fromwater: Kinetics and primary by-products

Here, we demonstrate the viability of a ZnO/TiO2/Ag2Se thin-film composite synthesized on FTO to degrade the drug naproxen in aqueous solutions by visible-light photoelectrocatalysis (PEC). The experiments were made with 100 mL of solutions containing 5 mg L−1 drug and 50 mM Na2SO4 at natural pH, using a cell equipped with a Pt wire as cathode and the composite as photoanode exposed to a 36Wblue LED lamp. Total degradation was achieved after 210 min of electrolysis at anodic potential of +1.0 V/Ag|AgCl. This resulted from the oxidative action of hydroxyl radicals formed via direct anodic water discharge and through mediated water oxidation by photogenerated holes. The degradation rate decreased at higher naproxen concentration, but the treatment efficiency became higher due the deceleration of the parasitic reactions involving hydroxyl radicals. In chloride medium, the photoanode showed a large ability to produce active chlorine, which contributed to the oxidation of the target molecule. LC-QToF-MS analysis of treated solutions revealed the generation of four primary naphthalenic by-products, from which the initial degradation route of naproxen is proposed

Els Qüestionaris com a recurs docent en l'aprenentatge autònom. Resultats d'una prova pilot en un curs de Química bàsica.

En aquest article es presenten uns nous qüestionaris on line dissenyats per millorar la comprensió dels continguts de l"assignatura Química Bàsica II i, alhora, propiciar el treball autònom dels estudiants. En el grup pilot que ha disposat dels qüestionaris, els resultats acadèmics dels estudiants que els han realitzat diverses vegades han estat millors que els dels alumnes que no els han utilitzat amb regularitat. Els resultats acadèmics del grup pilot també ha estat millor que els de cursos anteriors

On the performance of electrocatalytic anodes for photoelectro-Fenton treatment of synthetic solutions and real water spiked with the herbicide chloramben

The destruction of the herbicide chloramben in 0.050 M Na2SO4 solutions at natural pH has been studied by photoelectro-Fenton with UVA light (PEF). The trials were carried out in a cell equipped with an air-diffusion cathode for H2O2 generation and different electrocatalytic anodes, namely active IrO2-based and RuO2-based electrodes and non-active boron-doped diamond (BDD) and PbO2 ones. Similar removal rates were found regardless of the anode nature because the herbicide was mainly oxidized by ●OH formed from Fenton's reaction, which was enhanced by UVA-induced photo-Fenton reaction. The use of an IrO2-based anode led to almost total mineralization at high current density, as also occurred with the powerful BDD anode, since photoactive intermediates originated from ●OH-mediated oxidation were degraded under irradiation with UVA light. The good performance of the IrO2-based anode in PEF was confirmed at different current densities and herbicide concentrations. The presence of Cl- in the medium caused a slight deceleration of herbicide removal as well as mineralization inhibition, owing to the production of active chlorine with consequent formation of persistent chloroderivatives. Seven aromatic products along with oxalic and oxamic acids were identified in sulfate medium. Five aromatic derivatives were detected in Cl--containing matrix, corroborating the generation of organochlorine compounds. In secondary effluent, larger mineralization was achieved by PEF with a BDD anode due to its high oxidation ability to destroy the chloroderivatives, although an acceptable performance was also obtained using an IrO2-based anode

Diffusion-charge transfer characterization of a rotating cylinder electrode reactor used for the complete electrocatalytic removal of nitrate from water

Groundwater nitrate contamination is an emerging threat in stressed regions under intensive farming although, lately, efforts to valorize such residues are highly encouraged. Here, electrochemical nitrate removal has been investigated as a versatile strategy for this purpose, using a reactor equipped with a cheap central Fe-based rotating cylinder electrode (RCE) as cathode and six concentric Ti|IrO2 plates as anodes. The study of the effect of Ecath and rotational speed (ω) on NO3- electroreduction from a synthetic aqueous solution with high conductivity revealed the feasibility of complete nitrate removal, which only required 100-120 min at Ecath = -1.80 V vs Hg|Hg2SO4|sat. K2SO4 within the ω-range of 100-500 rpm. The concentration decays agreed perfectly with a first-order kinetics. NH3 was accumulated as main product, being partly volatilized due to the quick solution alkalization, whereas NO2- was not found. Linear sweep voltammetries demonstrated the high electrocatalytic activity of carbon steel RCE as compared to inactive stainless steel. Koutecky-Levich analysis showed that the reduction process with carbon steel at Ecath from -1.80 V involved 8 electrons. The participation of H radical in the reduction mechanism was ascertained by electron paramagnetic resonance. The mass transport and charge transfer of the RCE reactor were characterized under turbulent flow by means of the dimensionless Damköhler (Da) number, as well as from the Sherwood-Reynolds-Schmidt (Sh-Re-Sc) analysis. A mixed regime with a prevalence of mass transport control was determined at Ecath from -1.8 V. The Sh = 0.70Re0.46Sc0.356 correlation obtained for this reactor may serve to guide the scale-up of electrochemical NO3- removal as more electrocatalytic cathode materials are developed. Successful NO3- elimination from solutions with low conductivity that mimicked groundwater is finally reported