Efficient electrocatalytic nitrate reduction in neutral medium by Cu/CoP/NF composite cathode coupled with Ir-Ru/Ti anode

Boosting seawater denitrification in an electrochemical flow cell

Nitrogen compounds in current seawater treatment processes typically are converted to nitrate, threatening seawater quality and marine ecology. Electrochemical denitrification is a promising technique, but its efficiency is severely limited by the presence of excess chloride ions. In this work, a flow-through cell went through an on-demand chlorine-mediated electrochemical-chemical tandem reaction process was designed for efficient seawater denitrification. Equipped with ultrathin cobalt-based nanosheets as the cathode catalyst and commercial IrO2-RuO2/Ti as the anode, the newly designed flow-through cell achieved nitrate removal efficiency that was about 50 times greater than the batch cell and nearly 100 % N2 selectivity. Moreover, nitrite and ammonia can also be removed with over 93 % efficiency in total nitrogen (TN) removal. Furthermore, the concentration of active chlorine in the effluent could be adjusted within two orders of magnitude, enabling on-demand release of active chlorine. Finally, this flow-through cell reduced the TN of actual mariculture tailwater (40.1 mg N L-1 nitrate) to only 5.7 mg N L-1, meeting the discharge standard for aquaculture tailwater of Fujian, China. This work demonstrates the paradigm of deep denitrification from ultra-concentrated chlorine ion wastewater using an on-demand active chlorine-mediated electrochemical-chemical tandem reaction process.

Incorporation of Graphene Oxide Quantum Dots in Gradient Layers of Polyethersulphone Nanofiltration Membranes for Nitrate Rejection from Aqueous Solution

Graphene oxide quantum dots (GOQDs) have been widely used to prepare nanofiltration membranes due to the merits of excellent dispersity, ultrasmall size, and unique properties related to graphene. In this study, we first prepared the polyethersulphone-based nanofiltration (PES-NF) membrane via an interfacial polymerization process using a piperazine and m-phenylenediamine mixed solution as the aqueous phase. Then GOQDs were incorporated into the top-down gradient structured layers (i.e., ultrathin layer, interlayer, and substrate membrane layer) of the nanofiltration membrane, and subsequently the effect of GOQD addition on the nitrate rejection was evaluated. Compared with the pristine PES-NF membrane without the incorporation of GOQDs, the fabricated NF membrane (GOQD/PES-NF-2) incorporating GOQDs at both the ultrathin layer and interlayer exhibits more remarkable performances (an acceptable permeation flux of 52.2 L m-1 h-1 and excellent nitrate rejection of 96.3% at 0.6 MPa), the permeation flux of this membrane increases by nearly 2.4 times, and its nitrate rejection also shows a slight enhancement (∼7.6%) compared with those of PES-NF. Remarkably, at the operating pressure much lower than that required by reverse osmosis membranes, the GOQD/PES-NF-2 membrane possesses an equivalent monovalent ion rejection to reverse osmosis membranes but a higher permeation flux. Furthermore, the result of a 7 day continuous stability test validates the excellent durability of the GOQD/PES-NF-2 membrane, and its antifouling and chlorine resistance performances also outperform those of the PES-NF membrane.

Electrocatalytic reduction of nitrate to ammonia by Pd/In modified Nickel foam electrode in aqueous solution

Nitrate pollution in surface water and ground water has drawn wide attention, which has brought challenges to human health and natural ecology. Electroreduction of nitrate to NH3 in waste water was a way to turn waste into wealth, which has attracted interest of many researchers. Using Nickel foam as substrate, we prepared Pd/In bimetallic electrode (NF-Pd/In) according to a two-step electrodeposition method. There are many irregularly shaped particles in the size range of 10 nm-100 nm accumulated on the surface of prepared NF-Pd/In electrode, which could supply high specific area and more active sites for nitrate electroreduction. FESEM-EDS, XRD and XPS analysis confirmed the uniform distribution of Pd and In on the surface of prepared NF-Pd/In electrode, with a mass ratio of 4.5/1. Above 96% of 100 mg/L NO3--N was removed and 95% of NH3 selectivity was reached after 5 h of reaction under -1.6 V vs. Ag/AgCl sat. KCl when using 0.05 mol/L of Na2SO4 as electrolyte. High concentration of NaCl (0.05 mol/L) in the test solution dramatically decreased the NH3 selectivity because the produced NH3 could be further oxidized to N2 by the formed HClO from Cl-. EIS tests indicated that the prepared NF-Pd/In electrode showed much lower electrode resistance than NF due to the adsorptive property and electrocatalytic ability for nitrate removal. Density functional theory (DFT) calculations indicated that the presence of In could promote the conversion of NO3- to *NO3 during the process of nitrate electroreduction to NH3. Circulating tests demonstrated the stability of prepared NF-Pd/In electrode.

Dual-Site W-O-CoP Catalysts for Active and Selective Nitrate Conversion to Ammonia in a Broad Concentration Window

Environmentally friendly electrochemical reduction of contaminated nitrate to ammonia (NO3 - RR) is a promising solution for large quantity ammonia (NH3 ) production, which, however, is a complex multi-reaction process involving coordination between different reaction intermediates of nitrate reduction and water decomposition-provided active hydrogen (Hads ) species. Here, a dual-site catalyst of [W-O] group-doped CoP nanosheets (0.6W-O-CoP@NF) has been designed to synergistically catalyze the NO3 - RR and water decomposition, especially the reactions between the intermediates of NO3 - RR and water decomposition-provided Hads species. This catalytic NO3 - RR exhibits an extremely high NH3 yield of 80.92 mg h-1 cm-2 and a Faradaic efficiency (FE) of 95.2% in 1 m KOH containing 0.1 m NO3 - . Significantly, 0.6W-O-CoP@NF presents greatly enhanced NH3 yield and FE in a wide NO3 - concentration ranges of 0.001-0.1 m compared to the reported. The excellent NO3 - RR performance is attributed to a synergistic catalytic effect between [W-O] and CoP active sites, in which the doped [W-O] group promotes the water decomposition to supply abundant Hads , and meanwhile modulates the electronic structure of Co for strengthened adsorption of Hads and the hydrogen (H2 ) release prevention, resultantly facilitating the NO3 - RR. Finally, a Zn-NO3 - battery has been assembled to simultaneously achieve three functions: electricity output, ammonia production, and nitrate treatment in wastewater.