Gaseous trichloroethylene removal using an electrochemically generated homogeneous low-valent ligand-free Co(I) electrocatalyst by electro-scrubbing

Preparation of novel and recyclable chitosan-alumina nanocomposite as superabsorbent to remove diazinon and tetracycline contaminants from aqueous solution

This work presents a novel, strong and efficient adsorbent (CS@TDI@EDTA@γ-AlO(OH)) prepared through the green process using three components, chitosan, BNPs and EDTA using amide and ester bridges. An eco-friendly and easy approach was used for the preparation of this novel adsorbent, the low cost, easy access to the used materials, and the simplicity of the preparation method are some of the interesting advantages of this work. Also, this prepared adsorbent was used as an adsorbent to remove diazinon organophosphate poison and tetracycline antibiotic from aqueous solutions. In order to confirm the prepared adsorbent structure, the CS@TDI@EDTA@γ-AlO(OH) composite was investigated by various analyses including FT-IR, EDX, XRD, FESEM and TGA. The adsorption behavior of the adsorbent prepared for the removal of tetracycline and diazinon was investigated under different conditions by varying the concentration, temperature, the adsorbent dose, pH and contact time. Based on various tests, the highest diazinon adsorption capacity was obtained for 0.12 g/L adsorbent at pH 7 and 60 °C with 40 mg/L initial concentration. Also, the maximum adsorption capacity of the tetracycline was obtained for 0.12 g/L adsorbent at pH 9 and 60 °C with 30 mg/L initial concentration. The equilibrium results for diazinon and for tetracycline were in good accordance with the Langmuir and Freundlich isotherm models, respectively. Also, the highest adsorption capacities for diazinon at pH 7 and tetracycline at pH 9 were 1428.5 and 555.5 mg/g, respectively. Also the kinetic investigations revealed that the correlation factor (R2) of pseudo-second-order model obtained for the adsorption of diazinon and tetracycline was 0.9986 and 0.9988, while the coefficient k (g/mg.min) was 0.000084 and 0.0033, respectively. These results indicate that the adsorption of diazinon and tetracycline is pseudo-second-order kinetics model. Formation of hydrogen bonds between adsorbate and adsorbent as well as the high specific surface area and porosity of the adsorbent are the main mechanisms that contribute to the adsorption process. In addition, thermodynamic studies indicated that the adsorption of diazinon and tetracycline is a spontaneous endothermic process. The adsorbent prepared in this work was expected to have wide range of applications in wastewater treatment thanks to its good reusability in water and strong removal of diazinon and tetracycline compared to other adsorbents.

Combining Soil Vapor Extraction and Electrokinetics for the Removal of Hexachlorocyclohexanes from Soil

This paper focuses on the evaluation of the mobility of four hexachlorocyclohexane (HCH) isomers by soil vapor extraction (SVE) coupled with direct electrokinetic (EK) treatment without adding flushing fluids. SVE was found to be very efficient and remove nearly 70 % of the four HCH in the 15-days of the tests. The application of electrokinetics produced the transport of HCH to the cathode by different electrochemical processes, which were satisfactorily modelled with a 1-D transport equation. The increase in the electric field led to an increase in the transport of pollutants, although 15 days was found to be a very short time for an efficient transportation of the pollutants to the nearness of the cathode. Loss of water content in the vicinity of the cathode warns about the necessity of using electrokinetic flushing technologies instead of simple direct electrokinetics. Thus, results point out that direct electrokinetic treatment without adding flushing fluids produced low current intensities and ohmic heating that contributes negatively to the performance of the SVE process. No relevant differences were found among the removal of the four isomers, neither in SVE nor in EK processes.

Enhancing the mediated electrochemical reduction process combined with developed liquid-gas electrochemical flow sensors for sustainable N2O removal at room temperature

New electrocatalysts with high reduction efficiency are needed to upgrade the mediated electrochemical reduction for real applications. In addition, automation is required to quantify active electrocatalysts in alkaline media and air pollution. In this study, N₂O was removed sustainably by electrogenerated low valent nickel(I) phthalocyanine tetrasulfonate [Ni(I)TSPc] in 1 M KOH using an electroscrubbing system. Ni(I)TSPc electro generation and N₂O removal were automated by two (liquid/gas) electrochemical flow sensors, respectively. The Ni(I)TSPc was generated electrochemically up to 95% in 1 M KOH, and high removal efficiency (100%) was observed for 5 ppm N₂O and 90% for 10 ppm N₂O. A limiting potential change in the in-situ LSV of the chemically synthesized Ni(I)TSPc was taken and derived from the calibration plot and validated by an ex-situ potentiometric titration using an oxygen reduction potential electrode. Using the obtained calibration plot, electrogenerated Ni(I)TSPc allowed a direct determination in a liquid flow cell. The gas flow sensor developed using a KOH/Ni(II)CN₄ (TCN (II))-fabricated silver solid amalgam electrode showed an excellent response to N₂O concentrations up to 32 ppm. A calibration plot with known concentration was derived and validated by gas chromatography. The response time and sensitivity obtained were approximately 500s and -0.012 mA ppm^-1 cm^-2, respectively. The sensor stability test confirmed its good stability. Finally, the developed in-situ electrochemical flow sensors were applied to the sustainable automation of N₂O pollutant removal.

Enhanced sustainable electro-generation of a Ni (I) homogeneous electro-catalyst at a silver solid amalgam electrode for the continuous degradation of N2O, NO, DCM, and CB pollutants

This paper reports the sustainable and enhanced generation of a Ni(I) active electro-catalyst using AgSAE as a cathode material for the sustainable degradation of N₂O, NO, dichloromethane (DCM), and chlorobenzene (CB) by electroscrubbing in a series operation. The AgSAE electrode showed 1.66 times higher Ni(I) formation than the Ag metal electrode. The AgSAE achieved 20% ± 2% Ni(I) generation in a highly concentrated alkaline medium, whereas Ag metal only achieved 12% ± 2% Ni(I) generation at the same current density. Electrochemical impedance spectroscopy and voltammetric studies determined that the kinetics of the charge-transfer reaction was also preferential at the AgSAE, with the cathodic peak at -1.26 V vs. Ag/AgCl confirming Ni(I) formation. Initially, the change in the oxygen reduction potential and reduction efficiency of Ni(I) confirmed the removal of N₂O, NO, DCM and CB. In addition, the gas Fourier transform infrared (FTIR) spectrum revealed 99.8% removal efficiency of toxic pollutants. Therefore, the regeneration of Ni(I) confirmed the sustainable removal of toxic pollutants. Furthermore, the FTIR spectra revealed the formation of NH₃ during the reduction of N₂O and NO. On the other hand, DCM and CB were reduced to benzene derivatives in the solution phase. In addition, a plausible reduction mechanism was derived. As a result, the AgSAE cathode exhibited two-fold higher removal efficiency of N₂O, NO, DCM, and CB than the previously reported electrodes.

Assessing the viability of electro-absorption and photoelectro-absorption for the treatment of gaseous perchloroethylene

This work focuses on the development of electro-absorption and photoelectro-absorption technologies to treat gases produced by a synthetic waste containing the highly volatile perchloroethylene (PCE). To do this, a packed absorption column coupled with a UV lamp and an undivided electrooxidation cell was used. Firstly, it was confirmed that the absorption in a packed column is a viable method to achieve retention of PCE into an absorbent-electrolyte liquid. It was observed that PCE does not only absorb but it was also transformed into phosgene and other by-products. Later, it was confirmed that the electro-absorption process influenced the PCE degradation, favoring the transformation of phosgene into final products. Opposite to what is expected, carbon dioxide is not the main product obtained, but carbon tetrachloride and trichloroacetic acid. Both species are also hazardous but their higher solubility in water opens possibilities for a successful and more environmental-friendly removal. The coupling with UV-irradiation has a negative impact on the degradation of phosgene. Finally, a reaction mechanism was proposed for the degradation of PCE based on the experimental observations. Results were not as expected during the planning of the experimental work but it is important to take in mind that PCE decomposition occurs in wet conditions, regardless of the applied technology, and this work is a first approach to try to solve the treatment problems associated to PCE gaseous waste flows in a realistic way.

Sustainable removal of N2O by mediated electrocatalytic reduction at ambient temperature electro-scrubbing using electrogenerated Ni(I) electron mediator

Direct catalysis is generally proposed for nitrous oxide (N₂O) abatement but catalysis is expensive, requires high temperatures, and suffers from media fouling, which limits its lifetime. In the present study, an ambient temperature electroscrubbing method was developed, coupling wet-scrubbing with an electrogenerated Ni(I) ([Ni(I)(CN)₄]^3-) mediator, to enable N₂O reduction in a single process stage. The initial studies of 10 ppm N₂O absorption into 9 M KOH and an electrolyzed 9 M KOH solution showed no removal. However, 95% N₂O removal was identified through the addition of Ni(I) to an electrolyzed 9 M KOH. A change in the oxidation/reduction potential from -850 mV to -650 mV occurred following a decrease in Ni(I) concentration from 4.6 mM to 4.0 mM, which confirmed that N₂O removal was mediated by an electrocatalytic reduction (MER) pathway. Online analysis identified the reaction product to be ammonia (NH₃). Increasing the feed N₂O concentration increased NH₃ formation, which suggests that a decrease in electrolyzed solution reactivity induced by the increased N₂O load constrained the side reaction with the carrier gas. Importantly, this study outlines a new regenerable method for N₂O removal to commodity product NH₃ at ambient temperature that fosters process intensification, overcomes the limitations generally observed with catalysis, and permits product transformation to NH₃.

Enhanced electro-reduction of NO to NH3 on Pt cathode at electro-scrubber

Besides cheaper electrodes used in NH₃ product formation during NO degradation by mediated electrochemical reduction (MER), a specific electrode that can perform direct electrochemical reduction (DER) and MER of NO is an added advantage. In the present study, a Pt electrode was used to examine NO degradation through NH₃ formation during the electro-scrubbing process. Initially, the DER of NO was tested on a Pt electrode to determine if the DER of NO is possible. The NO degradation by only absorption, DER on Pt, and MER using electrogenerated [Ni(I)(CN)₄]^3- showed that a combination of DER and MER increased the NO degradation efficiency. In addition, the online FTIR spectra obtained under different conditions showed that the product formed was NH₃, either from the DER or MER during electro-scrubbing. The feed gas flow rate and feed concentration results of NH₃ formation revealed an additional chemical reaction that was influenced by the Pt electrode in addition to the DER and MER processes. Furthermore, the degradation efficiency of NO when using the Pt electrode increased to 90% compared to that of the Cu electrode (65%), which showed that Pt follows a combination of DER and MER processes. Based on the gas-phase FTIR results of NH₃ formation during NO degradation, higher NH₃ production (0.32 mg/h) was obtained when using a Pt electrode than that using a Cu electrode (0.21 mg/h), highlighting the specificity of the Pt electrode in NH₃ formation during the degradation of NO gas.

Studies on Effective Generation of Mediators Simultaneously at Both Half-Cells for VOC Degradation by Mediated Electroreduction and Mediated Electrooxidation

Of the several electrochemical methods for pollutant degradation, the mediated electrooxidation (MEO) process is widely used. However, the MEO process utilizes only one (anodic) compartment toward pollutant degradation. To effectively utilize the full electrochemical cell, an improved electrolytic cell producing both oxidant and reductant mediators at their respective half-cells, which can be employed for treating two pollutants simultaneously, was investigated. The cathodic half-cell was studied first toward maximum [Co^I(CN)₅]^4- (Co⁺) generation (21%) from a [Co^II(CN)₆]^3- precursor by optimizing several experimental factors such as the electrolyte, cathode material, and orientation of the Nafion324 membrane. The anodic half-cell was optimized similarly for higher Co₃(SO₄)₂ (Co³⁺) yields (41%) from a Co^IISO₄ precursor. The practical utility of the newly developed full cell setup, combining the optimized cathodic half-cell and optimized anodic half-cell, was demonstrated by electroscrubbing experiments with simultaneous dichloromethane removal by Co⁺ via the mediated electroreduction process and phenol removal by Co³⁺ via the MEO process, showing not only utilization of the full electrochemical cell, but also degradation of two different pollutants by the same applied current that was used in the conventional cell to remove only one pollutant.

Innovative reductive remediation of carbon tetrafluoride at room temperature by using electrogenerated Co1

Among the non-CO₂ greenhouse gases, carbon tetrafluoride (CF₄) is the most recalcitrant and should be eliminated from the atmosphere. In the present study, a non-combustion electroscrubbing method was used in an attempt to degrade CF₄ with an electrogenerated Co¹⁺ mediator in a highly alkaline medium. The initial absorption experiments revealed 165mgL^-1 CF₄ gas dissolved in 10M NaOH. Different mediator precursors, [Co(II)(CN)₅]^3-, [Ni(II)(CN)₄]^2-, [Cu(II)(OH)₄]^2-, and [Co(II)(OH)₄]^2-, were used and the electroscrubbing results showed that the electrogenerated Co¹⁺ or [Co(II)(OH)₄]^2- precursor effectively degraded up to 99.25% of the CF₄ gas. The variations in [Co(II)(OH)₄]^2- reduction efficiency and cyclic voltammetry revealed CF₄ degradation followed by electrogenerated Co¹⁺ mediated reduction. The increased zeta potential (+6mV) of the electrogenerated Co¹⁺ showed that the degradation reaction occurs preferably at the solution interface. Electroscrubbing for CF₄ removal and the resulting products were controlled by the carrier gas. Air and H₂ carrier gases lead to the formation of CHF₃ and COF₂. N₂ as the carrier gas caused 99.25% degradation with ethanol as a product. An 80% CF₄ degradation efficiency with CHF₃ as the product was observed when a mixture of N₂ and air was used as the carrier gas.