Characterization of the Interfacial Joule Heating Effect in the Electrochemical Advanced Oxidation Process

Abstract

The electrochemical advanced oxidation process (EAOP) has gained popularity in the field of water purification. During the EAOP, it is in the boundary layer of the anode–solution interface that organic pollutants are oxidized by hydroxyl radicals (•OH) produced from water oxidation. Applying current to an anode dissipates heat to the surroundings according to Joule’s law, leading to an interfacial temperature that is much higher than that of the bulk solution, which is known as the “interfacial Joule heating” (IJH) effect. The modeling and experimental results show that the IJH effect had an inevitable consequence for the activity of •OH, rate constants, and mass transport within the boundary layer. The interfacial temperature could be increased from 25 to 70.2 °C, a value mostly doubling that of the bulk solution (33.6 °C) at the end of a 120 min electrolysis (10 mA cm–2). Correspondingly, the •OH concentration available for oxidation of organic pollutants was much lower than that calculated at a constant temperature of 25 °C probably due to H2O2 formation via •OH dimerization. The enhanced •OH diffusion resulting from strengthened molecular thermodynamic movement and decreased kinematic viscosity of the solution also drove •OH to move far from the anode surface and thus extended the maximum thickness of the boundary layer. The oxidation rate was positively correlated to the interfacial temperature, the activation energy, and the number of activated molecules, indicated by a 1.57–2.28-fold increase depending on the target organic compounds. The finding of the IJH effect prompts a re-examination of the literature based on a realistic rather than a constant temperature (e.g., 20–30 °C), the case reflected in a number of prior studies that does not exist virtually, and reconsideration of behaviors that can be attributed to the change in temperature during EAOP.

Low-Temperature Removal of Refractory Organic Pollutants by Electrochemical Oxidation: Role of Interfacial Joule Heating Effect

“…The positive correlation between the increase in the oxidation rate and the activation energy was in good agreement with the results reported in our previous study. 33 This means that • OH-mediated oxidation of organic compounds occurred at an interfacial temperature that was virtually much higher than the bulk solution, as though the low temperature of the bulk solution did not act on interfacial reactions at all. This is due to the fact that the half-life of • OH is extremely short compared with the rate at which they are produced on the anode surface, making them exist only in a thin layer (i.e., Nernst diffusion layer with a thickness smaller than 1.0 μm 48 ) near the anode surface, and the heating of this layer precedes that of the bulk solution.…”

“…The ohmic heating phenomenon has been universally observed in fuel cells, sterilization, and electrochemical organic synthesis. − In our previous studies, we proposed the interfacial Joule heating (IJH) effect to demonstrate the formation of the temperature gradient between the anode/solution interface and the bulk solution in the electrochemical advanced oxidation process. As a result of the IJH effect, the reacted interfacial temperature could reach 70.2 °C, a value mostly doubling that of the bulk solution (33.6 °C) . In this case, organic pollutants are oxidized within the boundary layer of the anode/solution interface (i.e., Nernst diffusion layer with a thickness of several hundred nanometers) at a temperature much higher than that of the bulk electrolyte.…”

“…As a result of the IJH effect, the reacted interfacial temperature could reach 70.2 °C, a value mostly doubling that of the bulk solution (33.6 °C). 33 In this case, organic pollutants are oxidized within the boundary layer of the anode/solution interface (i.e., Nernst diffusion layer with a thickness of several hundred nanometers) at a temperature much higher than that of the bulk electrolyte. This may not only provide a most likely explanation to the reason why efficient abatement of organic pollutants can be achieved at low temperatures but may also suggest a new approach that allows alleviating the restriction of low temperature to water decontamination in cold regions.…”

A tubular electrode assembly reactor for enhanced electrochemical wastewater treatment with a Magnéli-phase titanium suboxide (M-TiSO) anode and in situ utilization

“…Moreover, it is necessary to transfer heat from the reactor to the outside to keep the temperature at the desired set point. Pei 42 indicated that at 70.2 °C, the concentration of the ˙OH radical used for removing organic pollutants was much lower than the concentration at 25 °C. Panizza 43 pointed out that the current efficiency of the reactor was significantly improved when the temperature in the reactor was reduced from 60 °C to 20 °C.…”

“…Moreover, it is necessary to transfer heat from the reactor to the outside to keep the temperature at the desired set point. Pei 42 indicated that at 70.2 C, the concentration of the cOH radical used for removing organic pollutants was much lower than the concentration at 25 C. Panizza 43 pointed out that the current efficiency of the reactor was signicantly improved when the temperature in the reactor was reduced from 60 C to 20 C. The temperature elds of both reactors were greatly consistent with their velocity elds, in which the temperature eld distribution in the TEAR was more uneven (Fig. 7B), indicating that the spiral mixer can efficiently accelerate heat transfer.…”