Denitrification enhancement by electro-sorption/reduction using a layered metal oxide electrode loaded with Pd-Cu nanoparticles

Role of molybdenum compounds in enhancing denitrification: Structure-activity relationship and the regulatory mechanisms

The effect and regulatory mechanisms of molybdenum compounds (MoO2, MoS2, MoSe2 and MoSi2) on denitrification were investigated by structure-activity relationships, electrochemical characteristics, microbial metabolism analysis and bacterial community distribution. All the assessed molybdenum compounds exhibited the enhancement effect on denitrification, in the order of MoS2 > MoSi2 > MoSe2 > MoO2, with MoS2 increasing 7.08-fold in 12 h. Analysis of structure-activity relationships suggested that the molybdenum compounds with lower negative redox potential and higher redox reversibility were favorable for promoting denitrification. According to the morphology observation, the interactions between Mo compounds and denitrifying bacteria may be beneficial to extracellular electron transfer. Molybdenum compounds with electron transfer capability facilitated an increase in electron capacitance from 835.1 to 1011.3 μF, promoting the electron exchange rate during denitrification. In the denitrification electron transport chain, the molybdenum compounds upregulated nicotinamide adenine dinucleotide and denitrifying enzyme activity, as well as facilitated the abundance of quinone pools, ATP translocation, and cytochrome c related proteins. Moreover, Mo compounds enriched functional bacteria such as electroactive bacteria and denitrifying functional bacteria. Notably, Mo ions in molybdenum compounds may provide active sites for nitrate reductase, optimizing the electron distribution of the denitrification process and thus improved the partial denitrification efficiency. This work aimed to further understand the regulatory mechanisms of molybdenum on denitrification electron transfer in the compound state and to anticipate the catalytic role of Mo compounds for sustainable water treatment.

Pd Clusters Loaded with Multivalent Cu Foam for Superior Electrochemical Nitrate Reduction and Selective N≡N Bond Formation

The electrochemical denitrification of nitrate (NO3 -) in actual wastewater to nitrogen (N2) is an effective approach to reversing the current imbalance of the nitrogen cycle and the eutrophication of water. However, electrostatic repulsion between NO3 - and the cathode results in the low efficiency of NO3 - reduction reaction (NO3RR). Here, density functional theory (DFT) calculations are used as a theoretical guide to design a Pd cluster-loaded multivalent Cu foam (Pd/Cu2O-CF) electrocatalyst, which achieves a splendid 97.8% NO3 - removal rate, 97.9% N2 selectivity, 695.5 mg N g-1 Pd h-1 reduction efficiency, and 60.0% Faradaic efficiency at -1.3 V versus SCE. The projected density of states (pDOS) indicates that NO3 - and Pd/Cu2O-CF are bonded via strong complexation between the O 2p (in NO3 -) and Cu 3d (in Cu2O) with the input of voltage, which reduces the electrostatic repulsion and enhances the enrichment of NO3 - on the cathode. In-situ characterizations demonstrate that Pd[H] can reduce Cu2O to Cu, and subsequently Cu reduces NO3 - to nitrite (NO2 -) accompanied by in situ reconfiguration of multivalent Cu foam. NO2 - is then transferred to the surface of Pd clusters by the cascade catalysis and accelerates the breaking of N─O bonds to form Pd─N, and eventually achieves the N≡N bond formation.

Fabrication of a novel sensor based on Cu quantum dot and SH-SiO2 nanoparticles supported on copper-based metal organic framework (Cu QD-SH-SiO2@Cu-MOF) and its application for the simultaneous determination of norepinephrine, piroxicam and epinephrine

In this research, a novel electrochemical sensor with excellent sensitivity was fabricated based on Cu quantum dot (Cu QD) and SH-SiO2 nanoparticles immobilized on copper-metal-organic frameworks (Cu-MOFs) for determining piroxicam and simultaneous determination of norepinephrine, piroxicam and epinephrine. The nanoparticles were synthesized and characterized using FT-IR, EDX, FESEM, TEM and BET, and were subsequently used to modify carbon paste electrode. Cu QD-SH-SiO2@Cu-MOF for electrode modification possesses a distinctive structure and a high conductivity that raises the electron transfer rate and enhances the performance of electrochemical sensors. Square wave voltammetry was applied to investigate the redox properties of Cu QD-SH-SiO2@Cu-MOF/CPE, voltammograms showed three distinct anodic peaks at 0.41, 0.62 and 1.06 V in the presence of norepinephrine, piroxicam, and epinephrine. Various experimental parameters including the type and pH of electrolyte and scan rate were investigated. The calibration graph was obtained over the range 0.2-34285.0 μM including three linear segments. Also, the limit of detection was calculated as 0.05 μM of piroxicam. The introduced sensor was satisfactorily utilized for electrochemical determination of norepinephrine, piroxicam, and epinephrine in real samples. The obtained results using the introduced sensor were validated by high-performance liquid chromatography and the statistical tests confirmed the good agreement of them.

Denitrification enhancement by electro-adsorption/reduction in capacitive deionization (CDI) and membrane capacitive deionization (MCDI) with copper electrode

The green and efficient removal of nitrate (NO₃^-) in groundwater is a primary concern nowadays, and membrane capacitive deionization (MCDI) is an emerging technology for the removal of nitrate (NO₃^-) from water. In this study, a novel electrochemical system for nitrate denitrification removal was established, wherein the economic non-noble metal copper was used as the electrode material to achieve harmless removal of nitrate in a single electrochemical cell. The effects of applied voltage, initial NO₃^- concentration, and co-existing matters on NO₃^- denitrification removal during electro-adsorption/reduction system were deeply investigated. The results showed that the NO₃^- denitrification removal increased with raised voltage and in proportion to the initial NO₃^- concentration within certain limits, wherein the removal rate reached a maximum of 53.3% in the single-solute solution of 200 mg L^-1 NaNO₃ at 1.8 V. Nevertheless, overhigh voltage or initial NO₃^- concentration would have a negative effect on nitrate removal, which was caused by multiple factors, including side reactions in the solution, fouling of activated carbon fiber and anion exchange membrane, and corrosion of copper electrode. The presence of NaCl also had a negative effect on the removal of nitrate, which was mainly caused by fouling of ACF/IEM and redox reaction on account of the chloride ions. This study provides a potential economical alternative for the NO₃^- denitrification removal to achieve a more environmentally friendly outcome.

An overview of in-situ remediation for nitrate in groundwater

Faced with the increasing nitrate pollution in groundwater, in-situ remediation has been widely studied and applied on field-scale as an efficient, economical and less disturbing remediation technology. In this review, we discussed various in-situ remediation for nitrate in groundwater and elaborate on biostimulation, phytoremediation, electrokinetic remediation, permeable reactive barrier and combined remediation. This review described principles of each in-situ remediation, application, the latest progress, problems and challenges on field-scale. Factors affecting the efficiency of in-situ remediation for nitrate in groundwater are also summarized. Finally, this review presented the prospect of in-situ remediation for nitrate pollution in groundwater. The objective of this review is to examine the state of knowledge on in-situ remediation for nitrate in groundwater and critically evaluate factors which affect the up-scaling of laboratory and bench-scale research to field-scale application. This helps to better understand the control mechanisms of various in-situ remediation for nitrate pollution in groundwater and the design options available for application to the field-scale.

Selective electrosorption of Ca2+ by MXene cathodes coupled with NiAl-LMO anodes through ion intercalation

Capacitive deionization (CDI), or electrosorption, is a desalination technology that exhibits significant potential; however, its major technical requirement of selective ion separation poses a challenge for its further practical application. Herein, a titanium carbide (MXene)-layered electrosorption electrode with high selectivity for Ca²⁺ was fabricated. The prepared MXene electrode had many surface hydroxyl functional groups that serve as adsorption sites for Ca²⁺. Ca²⁺ was successfully inserted into the interlayers of the MXene cathode and formed a strong interaction with [Ti-O] bonds during the capacitive deionization process. When a Ni-Al layered metal oxide anion intercalation electrode was employed as the counter electrode, Ca²⁺ adsorption by the MXene electrode was significantly enhanced due to the valence compensation balance effect. The maximum Ca²⁺ electrosorption capacity of the MXene electrode reached 1011.82 mg per gram effective MXene material, which is 6.3 times higher than that of Na⁺ based on the Langmuir adsorption isotherm model. The MXene electrode exhibited prominent selectivity for Ca²⁺ ions in the presence of Na⁺ and Mg²⁺. The Ca²⁺/Mg²⁺ selectivity factor for electrosorption reached 2.63, and Ca²⁺/Na⁺ selectivity factor could achieve 9.84, respectively. After five electrosorption/desorption cycles, the Ca²⁺ removal rate only decreased from 46.96% to 45.34%, suggesting that the MXene electrode has excellent stability. Our study demonstrated a novel CDI electrode and technical approach for softening water.