Facilitated OH¯ diffusion via bubble motion and water flow in a novel electrochemical reactor for enhancing homogeneous nucleation of CaCO3.

Electrochemistry is a potential method for water softening. An essential disadvantage is OH¯ ions from water electrolysis accumulate on cathode surface, inducing the generation of the insulating CaCO3 layer and then interrupting the electrochemical reaction. In order to propel OH¯ diffusion into the bulk solution instead of aggregation at cathode, we designed an electrochemical reactor, whose electrodes were placed horizontally in the middle of the reactor, and the bubbles created by water electrolysis move upward, while the water flows downward. The visual evidence displayed that the unique reactor structure allowed OH¯ to spread to almost all the solution rapidly. Average pH value of bulk solution reached 10.6 in only 3 min. Therefore, homogeneous nucleation of CaCO3 in bulk solution would take primary responsibility for water softening, and the softening efficiency is up to 212.9 g CaCO3/h/m2, higher than reported results. The reactor is easy to scale up, providing a new idea for the softening of circulating cooling water.

Spatial and temporal regulation of homogeneous nucleation and crystal growth for high-flux electrochemical water softening.

Electrochemical softening is an effective technology for the treatment of circulating cooling water, but its hardness removal efficiency is limited because that nucleation and growth of scale crystals depended on cathode surface. In this study, a novel method was proposed to break through this limit via spatiotemporal management of nucleation and growth processes. A cube reactor was divided into cathodic chamber and anodic chamber via installing a sandwich structure module composed of mesh cathode, nylon nets, and mesh anode. Using this continuous-flowing electrochemical reactor, OH ̄ generated by water electrolysis was rapidly pushed away from cathode surface by water flow and hydrogen bubbles movement. As a result, a wide range of strongly alkaline regions was rapidly constructed in cathodic chamber to play a nucleation region, and homogeneous nucleation in liquid phase replaced heterogeneous nucleation on cathodic surface. Furthermore, the growth process of scale crystals in alkaline regions was monitored in situ. It took only 150 s of residence time to grow to 500 nm, which may be easily separated from water by a microfiltration membrane. With this new method, the precipitation rate was 290.8 g/(hˑm2) and corresponding energy consumption was 2.1 kW·h/kg CaCO3, both were superior to those reported values. Therefore, this study developed an efficient electrochemical softening method by spatial and temporal regulation of homogeneous nucleation and crystal growth processes.

Photocatalytic ozonation of organic pollutants in wastewater using a flowing through reactor.

Photocatalytic ozonation (PCO/O3) process is a promising technology for mineralizing refractory organics in wastewater. In this study, we described an efficient approach to improve the mass-transfer performance of PCO/O3 by using a helical photocatalytic module (HPM) in an annular UVC reactor. Under hydraulic retention time (HRT) of 19 min and influent phenol concentration of 33 mg/L (TOC 26 mg/L), TOC removal of 91.5% was obtained during a PCO/O3 process with HPM, while TOC removal was only 58.1% without HPM (UVC/O3). This flowing through reactor displayed good stability in a continuous test lasting 20 h. The electric energy required to reduce TOC by one order of magnitude per cubic meter of solution was calculated to be 10.23 kWh/(m3 order), which supported that the PCO/O3 process in this flowing through reactor was energy-efficient compared with other processes (24.30-68.75 kWh/(m3 order)). The steel-rolling wastewater after biological treatment was taken as a target. Under the HRT of 57 min and initial COD of 124 mg/L, COD in effluent dropped to 45.8 mg/L and met the discharge standard of pollutants for municipal wastewater treatment plant of China.

Electrochemical activation of peroxymonosulfate in cathodic micro-channels for effective degradation of organic pollutants in wastewater.

Persulfate may be electrochemically activated into sulfate radicals (SO4•-) or hydroxyl radicals (•OH) by accepting electrons on cathode. Although electro-activated persulfate has displayed good performance in oxidation of organic pollutants in wastewater, both yield and availability of radicals are still limited because the electrostatic repulsion resists the contact between persulfate anions and cathode. In this study, a flow-through cathode (FTC) with well-ordered micro-channels was fabricated via carbonization of wood. The solution containing persulfate ions flowed through these micro-channels and then activation of persulfate was confined and performed in the micro-channels, which enhanced remarkably the contact between persulfate ions and cathode. Under the residence time of 10 min and other optimized conditions, the decomposition rate of persulfate in FTC displayed 3.78 folds of enhancement compared with that on a flow-by cathode (FBC). EPR signal of •OH produced in FTC was also higher distinctly than that on FBC. The average removal rates of phenol and TOC in FTC were 97.9 % and 39.6 %, respectively, which were 2.61 times and 2.57 times as much as that on FBC. Cycling experiments indicated that this FTC had good stability. Therefore, activating persulfate in FTC is an efficient strategy to enhance the yield and availability of radicals.

Construction of a Microchannel Electrochemical Reactor with a Monolithic Porous-Carbon Cathode for Adsorption and Degradation of Organic Pollutants in Several Minutes of Retention Time.

A monolithic porous-carbon (MPC) electrode was fabricated to simultaneously intensify mass transfer and enhance reaction activity. The MPC involved channel arrays (about 50 µm of diameter for each channel) with mesopores and micropores in channel walls. The abundant surface pores may improve the reaction efficiency of the reduction of O2 to produce H2O2 and •OH. The function of channel arrays was to shorten the mass-transfer distance not only from O2 to the electrode surface but also from pollutants to the electrode surface and •OH. A microchannel electrochemical reactor was assembled to evaluate the performance of the MPC cathode. For 20 mg/L of phenol, sulfamethoxazole or atrazine, effluent concentration and total organic carbon (TOC) decreased down to 1.5 and 3 mg/L, respectively, in a retention time of only 100-300 s. Phenol removal was dominated by the MPC cathode, and the contribution of cathodic adsorption, cathodic degradation, and anodic reaction was 46, 33, and 8%, respectively. The proper working potential for the MPC cathode was +0.26 to +0.6 V versus reversible hydrogen electrode; in this potential range, no scaling was observed. For the real surface water (the initial TOC was 41.5 mg/L), TOC in effluent (the retention time was 335 s) was stable at 31.0 mg/L.