Electrochemical removal of gaseous benzene using a flow-through reactor with efficient and ultra-stable titanium suboxide/titanium-foam anode at ambient temperature

Catalytic oxidation technology is currently considered as a feasible approach to degrade and mineralize volatile organic compounds (VOCs). However, it is still challenging to realize efficient removal of VOCs through catalytic oxidation at room temperature. In our study, a novel flow-through electrocatalytic reactor was designed, composed of porous solid-electrolyte, gas-permeable titanium sub-oxides/titanium-foam (TiSO/Ti-foam) as anode and platinum coated titanium foam (Pt/Ti-foam) as cathode. This device could oxidize nearly 100% of benzene (10 ppm) to carbon dioxide at a current density of 1.2 mA/cm² under room temperature. More importantly, the device maintained excellent stability over 1000 h. Mechanism of benzene mineralization was discussed. Hydroxyl radicals generated on the TiSO/Ti-foam anode played a crucial role in the oxidation of benzene. This study provides a promising prototype of the electrochemical air purifier, and may find its application in domestic and industrial air pollution control.

High-efficiency electrochemical removal of Cd(II) from wastewater using birnessite-biochar composites: Performance and mechanism

Birnessite has been widely used for electrochemical removal of heavy metals due to its high pseudocapacitance. Incorporation of carbon-based materials into birnessite can enhance its conductivity and stability, and synergistically improve the electrochemical adsorption capacity due to the double-layer capacitor reaction derived from carbon-based materials. In this study, biochar was successfully incorporated with birnessite at various ratios to synthesize composites (BC-Mn) for effective electrochemical removal of cadmium (Cd(II)) from water. The effects of cell voltage, initial pH, and recycling performance of BC-Mn were evaluated. As a result, the electrosorption capacity of BC-Mn for Cd(II) exhibited gradual increases with increasing birnessite content and reached equilibrium at a Mn content of 20% (BC-Mn20). The Cd(II) adsorption capacity of BC-Mn20 rose at higher cell voltage, and reached the maximum at 1.2 V. At pH 3.0-6.0, the electrosorption capacity initially rose until pH 5.0 and then approached equilibrium with a further increase in pH value. The Cd(II) electrochemical adsorption capacity of BC-Mn20 in the solution could reach 104.5 mg g^-1 at pH 5.0 for 8 h at 1.2 V. Moreover, BC-Mn20 exhibited excellent reusability with a stability of 95.4% (99.7 mg g^-1) after five cycles of reuse. Due to its superior heavy metal adsorption capacity and reusability, BC-Mn20 may have a promising prospect in the remediation of heavy metal polluted water.

Electrochemical nitrate removal by magnetically immobilized nZVI anode on ammonia-oxidizing plate of RuO2-IrO2/Ti

Ammonium as the major reduction intermediate has always been the limitation of nitrate reduction by cathodic reduction or nano zero-valent iron (nZVI). In this work, we report the electrochemical nitrate removal by magnetically immobilized nZVI anode on RuO₂-IrO₂/Ti plate with ammonia-oxidizing function. This system shows maximum nitrate removal efficiency of 94.6% and nitrogen selectivity up to 72.8% at pH of 3.0, and it has also high nitrate removal efficiency (90.2%) and nitrogen selectivity (70.6%) near neutral medium (pH = 6). As the increase of the applied anodic potentials, both nitrate removal efficiency (from 27.2% to 94.6%) and nitrogen selectivity (70.4%-72.8%) increase. The incorpration of RuO₂-IrO₂/Ti plate with ammonia-oxidizing function on the nZVI anode enhances the nitrate reduction. The dosage of nZVI on RuO₂-IrO₂/Ti plate (from 0.2 g to 0.6 g) has a slight effect (the variance is no more than 10.0%) on the removal performance. Cyclic voltammetry, Tafel analysis and electrochemical impedance spectroscopy (EIS) were further used to investigate the reaction mechanisms occurring on the nZVI surfaces in terms of CV curve area, corrosion voltage, corrosion current density and charge-transfer resistance. In conclusion, high nitrate removal performance of magnetically immobilized nZVI anode coupled with RuO₂-IrO₂/Ti plate may guide the design of improved electrochemical reduction by nZVI-based anode for practical nitrate remediation.

Remediation of heavy metal contaminated soils by organic acid extraction and electrochemical adsorption

Remediation of heavy metal contaminated soils remains a global challenge. Here, low-molecular-weight organic acids were used to extract Cu and Zn from polluted soils, and the extracted heavy metals were subsequently adsorbed by activated carbon electrodes. The electrochemical adsorption mechanism as well as the influence of pH, organic acid type and voltage were investigated, and the soil remediation effect was further evaluated by the cultivation of rape. After extraction by citrate at initial pH 8.3 and electrochemical adsorption at 0.9 V for 7 d, the concentrations of total and bioavailable Cu in soils decreased from 1090 to 281 to 391 and 52 mg kg^-1, and those of Zn decreased from 262 to 39 to 208 and 30 mg kg^-1, respectively. Cu and Zn ions were mainly electrochemically adsorbed on the carbon cathode and anode, respectively, resulting in decreases of their concentrations to below 1 mg L^-1 in the leachate. The presence of organic acids improved the remediation performance in the order of citrate > oxalate > acetate. The decrease in the initial pH of citrate solution enhanced the removal rate of Zn, while seemed to have no effect on that of Cu. The removal capacity for heavy metals decreased with decreasing cell voltage from 0.9 to 0.3 V. In the rape cultivation experiment, the Cu and Zn contents in shoot and root were decreased by more than 50%, validating the soil remediation effect. The present work proposes a facile method for heavy metal removal from contaminated soils.

High-efficiency As(III) oxidation and electrocoagulation removal using hematite with a charge-discharge technique

Arsenite (As(III)) is generally removed by adsorption or coprecipitation after being oxidized to arsenate (As(V)). Electrocoagulation is regarded as an effective and environment-friendly method for arsenic (As) removal from wastewater. However, some disadvantages including the passivation of electrode and high energy consumption limit its wide application. Herein, a multi-cycle galvanostatic charge-discharge technique was employed to remove aqueous As(III) using hematite prepared through a microwave-assisted hydrothermal reaction. When charge-discharge experiments were conducted at the potential window of -0.8-0 V (vs. SCE) in As(III) solution with NaCl as the background electrolyte, ClO^- intermediates and the counter electrode at high potential contributed much to As(III) oxidation. As(V) was adsorbed on ferrihydrite generated from the re-oxidation of released Fe²⁺, forming FeAsO₄ precipitate. A higher removal ratio of As(T) was achieved at initial pH 7.0 compared with that at initial pH 5.0 and 9.0. When the hematite mass was 4, 10 and 15 mg, the removal ratio of As(T) reached 55.2%, 79.6% and 98.6% after 600 cycles of charge-discharge. The periodic redox reactions of hematite electrodes occurred in each charge-discharge process, effectively avoiding the passivation of electrode. Additionally, the electrochemical system can be used as a supercapacitor for power output. The present work provides a novel strategy for high-efficiency As(III) immobilization and removal from aqueous solution.

Improved removal capacity of magnetite for Cr(VI) by electrochemical reduction

Aqueous hexavalent chromium (Cr(VI)) poses serious threats to ecological environments. Magnetite is a potential adsorbent for Cr(VI). However, its adsorption capacity is limited due to the formation of Fe(III) oxide coating on magnetite surface. Herein, constant potential reduction was conducted to improve the Cr(VI) removal capacity of magnetite, and the influence of pH, potential, and supporting electrolytes including KNO₃, KCl, and K₂SO₄ on the adsorption capacity was also investigated. The results showed that the highest Cr(VI) reduction percentage reached 93.7% with a total Cr removal capacity of 514.7 mg g^-1 at optimized pH 2 and -0.2 V (vs. SCE) in supporting electrolyte of KNO₃. Cr(VI) was reduced to Cr(III) on the surface of magnetite due to the direct electrochemical reduction at low potentials and reduction by Fe²⁺_aq electrochemically generated from magnetite. The Cr(III) was subsequently removed and easily separated due to the formation of Cr(OH)₃ precipitate on magnetite surface when KNO₃ and KCl were used as supporting electrolyte; however, when K₂SO₄ was used instead, Cr(OH)₃ precipitate was not observed. The decrease in pH and electrical potential was found to facilitate the reduction and removal of Cr(VI). This work proposes a facile method to enhance Cr(VI) removal by iron oxides.

Fabrication and characterization of a Cu-Pd-TNPs polymetallic nanoelectrode for electrochemically removing nitrate from groundwater

A novel Cu-Pd-TNPs (Copper-Palladium-TiO₂ Nanopores) polymetallic nanoelectrode was fabricated, and then used to catalytically reduce dissolved nitrate in groundwater. The aim was to develop a high efficient nanoelectrode for removing nitrate from groundwater. The Cu-Pd-TNPs polymetallic nanoelectrode was fabricated by plating Pd onto a TiO₂ nanoporous matrix and then plating Cu onto the layer which is previous coating. TiO₂ nanopores on the Cu-Pd-TNPs electrode surface gave the electrode a large specific surface area, and the Pd and Cu nanoparticles gave the electrode a high nitrogen to hydrogen ratio and a high nitrate reduction activity. Scanning electron microscopy images indicated that the Cu-Pd-TNPs polymetallic nanoelectrode was porous with lamellar deposits. The elements on the Cu-Pd-TNPs electrode surface, identified by energy-dispersive X-ray spectroscopy, were Ti, Pd, Cu, and O. The Cu-Pd-TNPs electrode gave a high nitrate reduction rate, removing 287.3% nitrate more than that was removed by a Ti nanoelectrode under the same conditions. The optimal NaCl concentration, at which the electrode effectively removed nitrate and produced as few byproducts as possible, was determined. Nitrate was completely removed using the Cu-Pd-TNPs electrode with a Pt anode at a NaCl concentration of 0.5 g L^-1, little ammonia and almost no nitrite were detected in the treated solution. Using a constant current density, temperature strongly affected nitrate removal, but the initial nitrate concentration affected the removal rate little. Maximum nitrate was removed at pH 3 when the other conditions were constant.