Efficiency And Mechanism Of Molybdate Removal From Polluted Water By Electrocoagulation

Molybdenum（Mo）is a refractory rare metal that is required by animals and plants.It is widely used in metallurgy,manufacturing,agriculture,aerospace,medicine,and other fields.A large amount of waste water,waste residue,and tailings are produced during the process of mining for molybdenum.If they are not properly treated,they will seriously pollute the groundwater,surface water,and soil surrounding the molybdenum mine.Molybdenum exists primarily in water as molybdate(Mo O₄^2-).Chemical precipitation,adsorption,ion exchange,constructed wetland,and coagulation sedimentation are currently the most common methods for removing molybdate from water.Electrocoagulation（EC）is an electrochemical water treatment technology that is widely used to remove various contaminants from water,including organic matter,oil,heavy and metal-like metals,bacteria,viruses,and so on.However,there are few studies on the removal of molybdate from water using EC.Therefore,this study used EC to remove molybdate from water,so as to provide theoretical guidance and data support for subsequent practical application,optimized the process parameters of EC such as electrode material,number of electrodes,electrode connection method,and current density,and investigated the effects of initial p H,background electrolyte,and sodium chloride concentration on the removal of molybdate by EC and explored the mechanism of molybdate removal by EC,and the main findings are as follows:The best operating parameters for molybdate removal by EC:four aluminum electrodes,monopole parallel connection,current density of 2.5 A/m~2 operating parameters,under this condition,the EC operation for 60 min can reduce the initial concentration of 0.3mg/L,p H 6.7 molybdate simulated aqueous solution of Mo（VI）concentration to the requirement of molybdenum concentration in the sanitary standards for domestic drinking water the Mo（VI）removal efficiency was as high as 95%,and the energy consumption generated by EC was 0.15 k Wh/m~3,with a total cost of 13.48 RMB/m~3.The anions in the background electrolyte competed with Mo（VI）,and the removal effect of Mo（VI）was:no electrolyte（98.9%）>sodium chloride（96.0%）>sodium nitrate（73.5%）>sodium sulfate（22.8%）,and the low conductivity under electroless condition resulted in high energy consumption,while sodium chloride provided a certain conductivity and reduced energy consumption,and dissolved the anode oxide film to improve the current efficiency.Sodium nitrate and sodium sulfate passivate the anode resulting in high energy consumption and increase the p H of the solution and cathode losses due to side reactions,XRD analysis shows that the flocs produced under different background electrolyte conditions are aluminum hydroxides.Kinetic analyses indicated that the removal of molybdate from water by EC at different current densities and different initial Mo（VI）concentrations was consistent with a pseudo first-order kinetic model.XRD analysis of the flocs in this paper found that the crystalline structures of the precipitates and the scums at 2.5 A/m~2 and Mo（VI）concentration of 0.3mg/L were the same,mainly Bayerite and Bohmite,while the precipitates and the scum at50 A/m~2 and Mo（VI）concentration of 150 mg/L had different crystalline shapes,with the precipitates being Bayerite,Bohmite and Gibbsite.The precipitates were Bayerite,Bohmite and Gibbsite,while the scums was mainly Bayerite and Bohmite,and no other substances were produced under these two conditions.SEM/EDX revealed the presence of Mo in both precipitate and scum at 50 A/m~2 and Mo（VI）concentration of 150 mg/L.XPS analysis showed a new peak at 232.9 e V in the full-sweep spectrum,the presence of molybdenum in flocs was demonstrated,while the spectrum of Mo 3d showed peaks at 235.7 e V and 232.6e V binding energy for Mo（VI）.FTIR analysis showed that the flocs were hydroxides of aluminum,and the reaction would be more intense at high current density.The comparison of molybdate removal using EC flocs and direct molybdate removal by EC showed that there were adsorption and charge neutralization in the process of EC.The comparative experiments on the removal of molybdate by chemical coagulation and EC showed that a continuous accumulation process of EC was more conducive to the removal of molybdate.The mechanism of removing molybdate from water by EC was mainly:Al³⁺dissolved by aluminum anode under the action of DC electric field combined with OH^-produced by cathode to form Al（OH）_3（s）,while Al³⁺would also be hydrolyzed into Al OH²⁺,Al（OH）₂⁺,Al（OH）_3（aq）,Al（OH）₄^-,polymer and aluminum hydroxide,these species would electrically neutralize and electrostatically adsorb the molybdate.As the reaction proceeded,large amounts of aluminum hydroxide were produced,which accumulate and gradually grow in large volume,netting and sweeping the molybdate in water,accelerating its precipitation and eventually forming precipitates,while some flocs with low density would adhere to the hydrogen bubbles generated by the cathode and remove molybdate by air flotation.