Application Of Porous Flow-through Ti₄O₇ Electrode Based Electrolysis Combined With Microfiltration For The Removal Of Silica Or Chromium From The Water

Silica is widely present in natural water bodies and it will be introduced into industrial production with the utilization of nature water,which can lead to reverse osmosis membrane contamination during the zero release process.Heavy metal chromium has a certain risk of leakage due to its widespread use as an industrial chemical.which can pose a threat to the safety of drinking water sources（e.g.,groundwater,surface water,etc.）due to its hypertoxicity and stong mobility.In light of this,an electrochemical-microfiltration coupling system was developed.In this system,a porous Ti₄O₇ membrane was utilized as both the flow-through cathode and microfiltration membrane,and a porous polytetrafluoroethylene microfiltration membrane（PTFE）was used to separate the electrolytic cell into a cathode chamber and an anode chamber.The alkaline environment generated by the cathode can induce the precipitation of Mg²⁺in the form of Mg（OH）₂,at the same time silica can be remove by co-precipitation,and the generated precipitate can be retained by the membrane.Results show that with the increase of initial Mg²⁺concentration,the removal efficiency of silica increased at first and then kept constant,and the optimal molar ratio of Mg²⁺to silica was10:1.When the current density increased from 0.5 m A cm^-2 to 1 m A cm^-2,the removal efficiency of Mg²⁺increased from 20%to 51%and the removal efficiency of silica increased from 41%to 78%.As the current density increased further,although the removal efficiency of magnesium continued to increase accordingly,the removal efficiency of silica remained essentially the same.The presence of HCO₃^-in the water can consume the OH^-produced by the cathode,which lead to a decrease in the removal efficiency of Mg²⁺and silica.The removal efficiency of Mg²⁺and silica can be maintained by appropriately increasing the current density.In addition,the formation of Ca CO₃ had almost no effect on the removal of silica.As deposits accumulated on the membrane surface,the transmembrane increased.The removal of scale from the membrane surface can be achieved by performing polarity reversal at a current density of 10 m A cm^-2 for 10minutes.The specific energy consumption was found to be between 0.34 and 0.79 k Wh m^-3,which meets the energy requirements of the electrochemical silica removal process(（27）1.0 k Wh m^-3).In this study,the electrochemical-microfiltration coupling system was also used for simultaneous Cr（Ⅵ）reduction and Cr（Ⅲ）immobilization.The porous Ti₄O₇ membrane enables simultaneous reduction of Cr（Ⅵ）,precipitation of Cr（OH）₃,and filtration separation of the Cr（OH）₃ particles.As compared with the flow-by mode,the flow-through mode exhibited a 3-fold increase in the mass transfer rate constant and therefore100%of Cr（Ⅵ）was reduced when passing through the Ti₄O₇ cathode.Since the presence of the PTFE membrane in the electrochemical cell can effectively retard the diffusion of H⁺generated in the anode chamber to the cathode chamber,the p H of the cathode chamber can reach about 11.0 regardless of the initial p H changing from 5.0 to 10.0,which favored the formation of Cr（OH）₃ particles,thereby facilitating the removal of total Cr（Cr（total））from water.Thus,variation of influent p H exhibited negligible effects on the Cr（Ⅵ）and Cr（total）removal efficiency.Under the optimum condition(initial Cr（Ⅵ）concentration500 ppb,current density 1 m A cm^-2,flux 287 L m^-2 h^-1),the Cr（total）removal efficiency could achieve at as high as 95%with residual Cr（total）concentration of 27μg L^-1.As the reaction progressed,the insoluble Cr（OH）₃ enriched on the Ti₄O₇ membrane gradually passivated the membrane,resulting in a decrease of the Cr（Ⅵ）reduction efficiency to 84%after 30 h of operation.In this regard,a periodic polarity reversal process was used,in which the Ti₄O₇ membrane was used as the anode,which can generate H⁺and the strong oxidizing^·OH to achieve the dissolution of the deposits on the membrane surface and their oxidative conversion to soluble Cr（Ⅵ）.Results show that the performance of the passivated Ti₄O₇ electrode was fully restored after 30 min of electrolysis when a current density of 5 m A cm^-2 was applied in the backwash process.Generally,this process can be considered as a promising and suitable treatment method for Cr（Ⅵ）detoxification from distributed water with a very low specific energy consumption of 0.109 k Wh m^-3.In summary,the electrochemical-microfiltration coupling system used in this study can achieve the removal of silica and hexavalent chromium from water bodies,while the polarity reversal strategy can be used to achieve the removal of membrane pollution.This study provids a new idea for the removal of inorganic ions from water bodies.The system requires no acid or alkaline reagents for the entire process,which is a green and efficient water treatment technology.