A heterogeneous electro-Fenton (EF) process is applied for degrading high-molecular-weight polyacrylamides (PAMs). Degradation efficiencies of the PAM compound with molecular weight of 15 000 kDa at different cathodic potentials are compared in terms of solution viscosity, NH4+–N concentration, chemical oxygen demand (COD), and total organic carbon (TOC). The degradation products of PAM are identified, and anaerobic biodegradability of the products is evaluated. Partial mineralization of PAM is observed in the heterogeneous EF process, producing polymeric products with newly generated ether and ketone groups. As the cathodic potential changes from −0.3 to −1.1 V (vs SCE), the mineralization efficiency of PAM increases from 20.8% to 32.9%. The PAM degradation in the heterogeneous EF process is proposed to follow a novel pathway starting with the cleavage of polymer backbone at the head-to-head linkage and ending with a termination reaction between the carbon-centered radical and oxygen-centered radical. The polymeric products from PAM degradation demonstrate favorable anaerobic biodegradability. Thus, the heterogeneous EF process has great potential as a pretreatment unit before anaerobic digestion for the treatment of nonbiodegradable high-molecular-weight PAMs.Keywords: Catalysis; Chain recombination; Heterogeneous electro-Fenton process; Polyacrylamide; Radical termination;

•Oxidation of RhB at the graphite anode of undivided electro-Fenton cell.•O2 demonstrated enhancement on RhB degradation at the graphite anode.•N, N-de-ethylation and cleavage of COC bonds happened on the RhB molecular.•Unsaturated functional groups in graphite anode catalyzed oxidation of RhB by O2.•Anodic current was necessary to activate catalytic centers in graphite.Undivided electro-Fenton cells are widely applied in wastewater treatment due to their high efficiencies in degrading hazardous pollutants. Anodic oxidation may play a role in the pollution removal in undivided electro-Fenton cells, which is usually ignored by researchers. In this work, the anodic oxidation possibly involved in undivided electro-Fenton cells was investigated with a graphite paper as the anode electrode and Rhodamine B (RhB) as the probe pollutant. Partial degradation of RhB was observed when an external voltage of + 2.0 V was applied to the graphite anode with continuous O2 supply. The degradation efficiency of RhB was enhanced by decreasing the pH of electrolyte. Total decolorization of RhB was achieved after 2 and 12 hrs of electrolysis at pH 3.0 and 6.8, respectively. Neither the direct oxidation nor the oxidation by reactive oxygen species mechanism was responsible for the degradation of RhB at the graphite anode. Alternatively, the anodic oxidation process was manipulated by the electro-oxidation of unsaturated functional groups in graphite. An anode-catalyzed O2-oxidation pathway was proposed in which RhB was virtually oxidized by the O2, whereas the unsaturated functional groups in graphite played a catalyzing role. The anodic current was necessary to initiate such a reaction by activating the catalytic centers in these unsaturated functional groups.

•Performance degradation of iron-fed fuel cell is attributed to iron contamination.•Iron contaminant is presented in the form of α-FeO (OH).•The α-FeO (OH) forms fouling layers on surfaces of electrode and membrane.•The α-FeO (OH) migrates into the membrane to destroy the membrane structure.•Electro-oxidation kinetics of Fe (II) is delayed by iron contamination.The iron-fed fuel cell is an effective technology to recover iron and electricity from acid mine drainage (AMD). However, this technology suffers from the problem of performance degradation which significantly reduces its power output during long-term operation. In this work, the performance degradation of iron-fed fuel cell is comprehensively evaluated with the objective to elucidate the mechanisms involved in such a phenomenon. The iron contamination is identified as the main cause responsible for the performance degradation of fuel cell. The iron contaminant is present in the form of α-FeO(OH), which is the main product recovered by the iron-fed fuel cell. Both the electrode and membrane are deteriorated by iron contamination, whereas the membrane deterioration is more significant. Fed-batch experiments demonstrate the performance loss of fuel cell due to contamination of membrane is more than 50% greater than the performance loss due to contamination of electrode. The α-FeO(OH) contaminant not only forms fouling layers on the surfaces of carbon electrode and membrane, but also migrates into the membrane to damage the membrane structure. As a result, both the charge transfer and mass transfer resistances of fuel cell are dramatically increased, which leads to delayed electro-oxidation kinetics of Fe(II).

The air–cathode fuel cell approach is promising for ferrous (FeII) ion removal from acid mine drainage because iron and electricity are simultaneously recovered in the treating process. Here we show that electricity generation from FeII can be enhanced by amending chelating anions to facilitate FeII oxidation at the anode of the fuel cell. A series of FeII-fed fuel cells were operated with various chelating anions, including carboxylate, phosphate, and borate ligands. The average power densities of these fuel cells varied over a wide range from 0.08 ± 0.5 to 107.85 ± 1.50 mW m–2. Citrate-amended fuel cells operated at pH 8–9 and carbonate-amended fuel cells operated at pH 6–8 exhibited greater charge-recovery efficiencies than others, which ranged from 93.5% to 96.1%. The redox potential of an anodic solution and redox activity of FeII were two important factors affecting the electrooxidation of FeII in fuel cells.

Due to the high redox activity of Fe(II) and its abundance in natural waters, the electro-oxidation of Fe(II) can be found in many air-cathode fuel cell systems, such as acid mine drainage fuel cells and sediment microbial fuel cells. To deeply understand these iron-related systems, it is essential to elucidate the kinetics and mechanisms involved in the electro-oxidation of Fe(II). This work aims to develop a kinetic model that adequately describes the electro-oxidation process of Fe(II) in air-cathode fuel cells. The speciation of Fe(II) is incorporated into the model, and contributions of individual Fe(II) species to the overall Fe(II) oxidation rate are quantitatively evaluated. The results show that the kinetic model can accurately predict the electro-oxidation rate of Fe(II) in air-cathode fuel cells. FeCO3, Fe(OH)2, and Fe(CO3)22- are the most important species determining the electro-oxidation kinetics of Fe(II). The Fe(II) oxidation rate is primarily controlled by the oxidation of FeCO3 species at low pH, whereas at high pH Fe(OH)2 and Fe(CO3)22- are the dominant species. Solution pH, carbonate concentration, and solution salinity are able to influence the electro-oxidation kinetics of Fe(II) through changing both distribution and kinetic activity of Fe(II) species.

•Fabricate Fe3O4/GF composite as electro-Fenton catalyst from acid mine drainage.•Manipulate air–cathode fuel cell to precipitate iron oxides on GF.•Activity of Fe3O4/GF depends upon iron loading and Fe3O4 morphology in composite.•Solution pH and composition in fuel cell affect electro-oxidation of Fe(II).•Distribution of Fe(II) species in fuel cell controls structure of Fe3O4/GF.The recovery of iron oxides from acid mine drainage (AMD) has attracted extensive research attention due to the double advantage of waste minimization and resource recovery. Recently a novel air–cathode fuel cell approach was proposed to in-situ utilize ferrous iron (Fe(II)) in the AMD for the fabrication of Fe3O4/graphite felt (GF) composite as the cathode of electro-Fenton process. In the present work, the influence of fuel cell operating parameters, including solution pH, carbonate concentration and Fe(II) concentration, on the catalytic activity of prepared Fe3O4/GF composite is adequately elucidated. The highest activity is observed on the composite obtained from the fuel cell operated with 30 mM Fe(II) and 50 mM carbonate at pH 7.5. The activity of Fe3O4/GF is strongly dependent upon iron loading and Fe3O4 morphology in the composite. Higher iron loading generally induces higher catalytic activity, and the Fe3O4 aggregate is catalytically less reactive relative to the well-dispersed one. The precipitation of Fe(III) oxides on the GF through electrochemical oxidation of Fe(II) plays a key role in determining the structure of Fe3O4/GF composite. Solution pH and composition in the fuel cell affect such a process by manipulating the distribution of Fe(II) species in aqueous solution and on the GF.