Electrochemical Reduction of Carbon Dioxide in an MFC–MEC System with a Layer-by-Layer Self-Assembly Carbon Nanotube/Cobalt Phthalocyanine Modified Electrode

Electrochemical reduction of carbon dioxide (CO₂) to useful chemical materials is of great significance to the virtuous cycle of CO₂. However, some problems such as high overpotential, high applied voltage, and high energy consumption exist in the course of the conventional electrochemical reduction process. This study presents a new CO₂ reduction technique for targeted production of formic acid in a microbial electrolysis cell (MEC) driven by a microbial fuel cell (MFC). The multiwalled carbon nanotubes (MWCNT) and cobalt tetra-amino phthalocyanine (CoTAPc) composite modified electrode was fabricated by the layer-by-layer (LBL) self-assembly technique. The new electrodes significantly decreased the overpotential of CO₂ reduction, and as cathode successfully reduced CO₂ to formic acid (production rate of up to 21.0 ± 0.2 mg·L^–1·h^–1) in an MEC driven by a single MFC. Compared with the electrode modified by CoTAPc alone, the MWCNT/CoTAPc composite modified electrode could increase the current and formic acid production rate by approximately 20% and 100%, respectively. The Faraday efficiency for formic acid production depended on the cathode potential. The MWCNT/CoTAPc composite electrode reached the maximum Faraday efficiency at the cathode potential of ca<i>.</i> −0.5 V vs Ag/AgCl. Increasing the number of electrode modification layers favored the current and formic acid production rate. The production of formic acid was stable in the MFC–MEC system after multiple batches of CO₂ electrolysis, and no significant change was observed on the performances of the modified electrode. The coupling of the catalytic electrode and the bioelectrochemical system realized the targeted reduction of CO₂ in the absence of external energy input, providing a new way for CO₂ capture and conversion

Novel Ion-Exchange Coagulants Remove More Low Molecular Weight Organics than Traditional Coagulants

Low molecular weight (MW) charged organic matter is poorly removed by conventional coagulants but contributes to disinfection byproduct formation during chlorination of drinking waters. We hypothesized that CIEX, a new Al-based hybrid coagulant with ion-exchange functional groups, would be new mechanistic approach to remove low MW organic matter during coagulation and would perform better than polyaluminum chloride (PACl) or metal–salt based coagulants. We measured coagulation performance using dissolved organic carbon (DOC) in a high hardness surface water. CIEX achieved excellent turbidity removal and removed 20% to 46% more DOC than FeCl₃, Al₂(SO₄)₃, or PACl, depending on dose. The improved DOC removal was attributable to better removal of low MW organic matter (<2 kDa). We further studied removal mechanisms in a model water containing a low MW organic acid (salicylic acid (SA)). CIEX achieved high removal of organic acids (>90% of SA) independent of pH, whereas removal by metal salts was lower (<15%) and was strongly pH dependent. CIEX ion-exchange capability is facilitated by its covalently bound quaternary ammonium group, which conventional coagulants lack. Plus, unlike other cationic polymers that react with chloramines to form <i>N</i>-nitrosodimethylamine (NDMA), CIEX has a low molar yield (9.3 × 10^–7 mol NDMA per mol CIEX-N)

Molecular Insights into the Transformation of Dissolved Organic Matter in Landfill Leachate Concentrate during Biodegradation and Coagulation Processes Using ESI FT-ICR MS

Landfill leachate concentrate is a type of refractory organic wastewater with high environmental risk. Identification of refractory components and insights into the molecular transformations of the organics are essential for the development of efficient treatment process. In this report, molecular compositions of dissolved organic matter (DOM) in leachate concentrate, as well as changes after anaerobic/aerobic biodegradation and coagulation with salts, were characterized using electrospray ionization (ESI) coupled with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). DOM in leachate concentrate were more saturated and less oxidized with more nitrogen and sulfur-containing substances (accounting for 50.0%), comparing with natural organic matter in Suwannee River. Selectivity for different classes of organics during biodegradation and coagulation processes was observed. Substances with low oxidation degree (O/C < 0.3) were more reactive during biodegradation process, leading to the formation of highly oxidized molecules (O/C > 0.5). Unsaturated (H/C < 1.0) and oxidized (O/C > 0.4) substances containing carboxyl groups were preferentially removed after coagulation with Al or Fe sulfate. The complementary functions of biodegradation and coagulation in the treatment of DOM in leachate concentrate were verified at the molecular level. Lignin-derived compounds and sulfur-containing substances in leachate concentrate were resistant to biodegradation and coagulation treatments. To treat leachate concentrate more effectively, processes aimed at removal of such DOM should be developed