Effects of water constituents on the stability of gas diffusion electrode during electrochemical hydrogen peroxide production for water and wastewater treatment.

Electrochemically producing hydrogen peroxide (H2O2) from oxygen reduction reaction (ORR) with natural air diffusion electrode (NADE) is an attractive way to supply H2O2 for decentralized water treatment. In this study, the stability of NADE during H2O2 electroproduction in varying water matrices were evaluated, including synthetic electrolyte solutions (0.05 M Na2SO4) with or without calcium ions (Ca2+, 200 mg/L) and/or humic acid (HA, 40 mg/L), as well as a selected municipal wastewater (92.7 mg/L Ca2+, 3.6 mg/L Mg2+, and 23.9 mg/L total organic carbon). The results show that NADEs maintained a good stability during H2O2 electroproduction in Na2SO4 solutions regardless of the presence of HA. However, Ca2+ (and Mg2+) could form significant amounts of mineral precipitates on the surface and in the internal pores of NADEs during H2O2 electroproduction. These mineral precipitates can negatively influence H2O2 production by impeding the oxygen, electron, and proton transfer processes involved in ORR to H2O2. Moreover, the mineral precipitates shifted the NADEs from hydrophobic to hydrophilic, which may promote H2O2 reduction to H2O at the NADEs. Consequently, the apparent current efficiencies of H2O2 production decreased substantially from initially ∼90% to 50%-70% as the NADEs were continuously used for 60 h in the Ca-containing solutions and selected wastewater. These results indicate that water constituents that are commonly present in real water matrices, especially Ca2+, can cause serious deterioration of NADE stability during H2O2 electroproduction. Therefore, proper strategies are needed to mitigate electrode fouling during H2O2 electroproduction with NADEs in practical water and wastewater treatment.

Comparison of emerging contaminant abatement by conventional ozonation, catalytic ozonation, O3/H2O2 and electro-peroxone processes.

The abatement of several emerging contaminants (ECs) in groundwater by conventional ozonation and three ozone-based advanced oxidation processes (AOPs) - catalytic ozonation with manganese dioxide (MnO2), conventional peroxone (O3/H2O2), and electro-peroxone (EP) - was compared in this study. The addition of MnO2, H2O2, or electro-generation of H2O2 during ozonation enhanced ozone transformation to hydroxyl radicals to different extent. These changes did not considerably influence the abatement of ECs with moderate to high ozone reactivities ( [Formula: see text] ), whose abatements were similar with >90 % during all four processes. In comparison, the abatements of ozone-refractory ECs (kO3< 15 M-1s-1) were lower during conventional ozonation (∼40-85 % abatement), but could be enhanced by ∼10-40 % during the three ozone-based AOPs. Besides enhancing ozone-refractory EC abatement, the three AOPs, especially the O3/H2O2 and EP processes, reduced considerably bromate formation compared to conventional ozonation. These results demonstrate that the EP process performs similarly as catalytic ozonation and O3/H2O2 processes in terms of EC abatement and bromate control. Considering its more convenient, flexible, and safer way of operation, the EP process may provide an attractive alternative to the two more traditional AOPs for water treatment.

Mechanism of ozonation enhanced formation of haloacetaldehydes during subsequent chlorination.

Haloacetaldehydes (HAs) are the third prevalent group of disinfection by-products of great health concern. A bench-scale study was performed to investigate the formation and speciation of HAs in raw and treated waters after chlorination and ozonation-chlorination. Pre-ozonation resulted in enhanced HA formation during subsequent chlorination, and the HA yields from ozonation-chlorination were 1.66 and 1.63 times higher than that from chlorination of raw and treated waters. The mechanism about the increase of HA formation during ozonation-chlorination was systematically investigated in this study. The results showed that acetaldehyde formed after ozonation was the dominant precursor for the enhanced HA formation during subsequent chlorination. Increase in pH and chlorine dose increased HA formation during acetaldehyde chlorination. Based on the kinetic studies on the HA formation during acetaldehyde chlorination and the HA stabilities with and without free chlorine, it was found that chlorine was incorporated into the α-hydrogen in acetaldehyde to form a sequence of mono-, di- and tri-chloroacetaldehyde. During this process, these three chlorinated acetaldehydes would also undergo base-catalyzed hydrolysis through decarburization and dehalogenation pathways. This study elucidated that acetaldehyde formed after ozonation resulted in the increase of HA formation during subsequent chlorination. This study also revealed the formation pathway of HA during chlorination of acetaldehyde, which would help to minimize HA formation at drinking water plants.