Electrogenerated oxychlorides induced overlooked negative effects on electro-oxidation wastewater treatment in terms of over-evaluated COD removal efficiency and biotoxicity.

The high-efficiency and environmentally-friendly electro-oxidation (EO) would lose its competitive edge because of the production of oxychloride by-products (ClOx-), which has not yet drawn significant attention in academic and engineering communities. In this study, the negative effects of the electrogenerated ClOx- were compared among four commonly used anode materials (BDD, Ti4O7, PbO2 and Ru-IrO2) in terms of ClOx- interference on the evaluation of electrochemical COD removal performance and biotoxicity. Apparently, the COD removal performance of various EO systems were highly enhanced with increasing current density in the presence of Cl-, e.g., the amounts of COD removed by various EO systems from the phenol solution with an initial COD content of 280 mg L-1 at 40 mA cm-2 within 120 min decreased in the order: Ti4O7 of 265 mg L-1 > BDD of 257 mg L-1 > PbO2 of 202 mg L-1 > Ru-IrO2 of 118 mg L-1, which was different from the case with the absence of Cl- (BDD of 200 mg L-1 > Ti4O7 of 112 mg L-1 > PbO2 of 108 mg L-1 > Ru-IrO2 of 80 mg L-1) and the results after removing ClOx- by anoxic sulfite-based method (BDD of 205 mg L-1 > Ti4O7 of 160 mg L-1 > PbO2 of 153 mg L-1 > Ru-IrO2 of 99 mg L-1). These results can be ascribed to the ClOx- interference on COD evaluation, the extent of which decreased in the order: ClO3- > ClO- (where ClO4- cannot impact COD test). The highest overrated electrochemical COD removal performance of Ti4O7 may be associated with its relatively high production of ClO3- and the low mineralization extent. The chlorella inhibition ratio of ClOx- decreased in the order: ClO- > ClO3- >> ClO4-, which accounted for the biotoxicity increasement of the treated water (PbO2 68%, Ti4O7 56%, BDD 53%, Ru-IrO2 25%). Generally, the inevitable problems of overrated electrochemical COD removal performance and biotoxicity increasement induced by ClOx- should deserve significant attention and effective countermeasures should be also developed when employing EO process for wastewater treatment.

Differentiating the reaction mechanism of three-dimensionally electrocatalytic system packed with different particle electrodes: Electro-oxidation versus electro-fenton.

Recently, there are still some controversial mechanisms of the 3D electrocatalytic oxidation system, which would probably confound its industrial application. From the conventional viewpoint, the Ti4O7 material may be the desired particle electrodes in the 3D system since its high oxygen evolution potential favors the production of •OH via H2O splitting reaction at the anode side of Ti4O7 particle electrodes. In fact, the incorporation of Ti4O7 particles showed phenol degradation of 88% and COD removal of 51% within 120 min, under the optimum conditions at energy consumption of 0.668 kWh g-1 COD, the performance of which was much lower than those in many previous literatures. In contrast, the prepared carbon black-polytetrafluoroethylene composite (CB-PTFE) particles with abundant oxygen-containing functional groups could yield considerable amounts of H2O2 (200 mg L-1) in the 3D reactor and achieved a complete degradation of phenol and COD removal of 80% in the presence of Fe2+, accompanying a low energy consumption of only 0.080 kWh g-1 COD. It was estimated that only 20% of Ti4O7 particles near the anode attained the potential over 2.73 V/SCE at 30 mA cm-2 based on the potential test and simulation, responsible for the low yield of •OH via the H2O splitting on Ti4O7 (1.74 × 10-14 M), and the main role of Ti4O7 particle electrodes in phenol degradation was through direct oxidation. For the CB-PTFE-based 3D system, current density of 10 mA cm-2 was sufficient for all the CB-PTFE particles to attain cathodic potential of -0.67 V/SCE, conducive to the high yield of H2O2 and •OH (9.11 × 10-14 M) in the presence of Fe2+, and the •OH-mediated indirect oxidation was mainly responsible for the phenol degradation. Generally, this study can provide a deep insight into the 3D electrocatalytic oxidation technology and help to develop the high-efficiency and cost-efficient 3D technologies for industrial application.

Chlorate induced false reduction in chemical oxygen demand (COD) based on standard dichromate method: Countermeasure and mechanism.

Deliberate addition of mildly oxidative chlorate (ClO3-), so-called "chemical oxygen demand (COD) remover", into wastewater in China or electrochemical production of ClO3- from Cl- induces the false COD reduction, which would bring about false appearance of effluents meeting the COD discharge standards. In this study, an easy sulfite-based reduction method was developed for the first time to remove ClO3- from the water samples before COD determination to eliminate this interference of ClO3-. In this reaction system, keeping the reaction temperature of sulfite reducing ClO3- at 60 °C was crucial for fast ClO3- removal rate, fixed molar [sulfite]ini/[chlorate]ini ratio value and the synchronous exhaustion of sulfite and ClO3-, which were of great significance for the real application of this improved COD determination method. The ClO3- interference on COD determination could be successfully eliminated after 20 min reduction of ClO3- by sulfite at pHini 4.0∼6.0 with the molar [sulfite]ini/[chlorate]ini ratio value in the range of 5∼6 when concentration of ClO3- was below 5 mM. Despite of the involvement of SO4·- in the sulfite reducing ClO3- system, the degradation of organic matters by SO4·- could be greatly impeded due to the lessened dissolved oxygen for SO4·- production at high reaction temperature and the scavenging of SO4·- by sulfite. In this reaction system, ClO2, ClO2- and ClO- were also generated and could be further reduced by sulfite stoichiometrically via oxygen transfer process with Cl- as the final product. In general, this study pioneered an effective, fast and convenient method for COD determination of the ClO3--laden wastewaters and evaluating the real electrochemical wastewater treatment performance in terms of COD removal.