| Cr(VI),generating from rinsing of plated articles,dragout from chromium bath and spent chromate passivation solutions,often coexist with methyl orange(MO)dye,which has a strong biological toxicity and difficulty in treatment.Due to their significantly difference in properties,the previous technologies were used to remove Cr(VI)or MO alone,and few researches paid attention to the simultaneous treatment.Flow-through electrode,developing from the combination of filtration membrane and electrochemical technology,allows solution to pass through the electrode vertically,which makes the collision probability of pollutants with the electrode larger and the mass transfer rate higher.Although it has been used in Cr(VI)electro reduction,the used electrode materials have some drawbacks,including expensive,difficulty in preparation,easy corrosion and difficult to practical engineering application.Also,Cr(VI)was the only pollutant removed in the previous studies,but there is no report on the simultaneous removal of Cr(VI)and MO by using flow-through electrode system.In order to solve the above two problems,a commercial graphite felt was selected as the flow-through electrode to reduce Cr(VI)by electrochemical method.Moreover,influencing factors and mechanism of simultaneous removal of Cr(VI)and catalytic oxidation degradation of MO were explored through using the modified graphite felt to utilized the oxygen from anodic oxygen evolution reaction and in-situ generation of H2O2.The main results are as follows:(1)Feasibility of demulsification,de-complexed and Cr(VI)reduction by flow-through electrode cell were investigated.Results showed surface charge redistribution of oil droplets occurred under the action of electrochemistry or electric field,and resulted in the demulsification of oil droplets on the flow-through electrode surface,which makes the demulsification efficiency is 1.6～8.0 times than that of baffle-flow and straight-flow type;The slow mass transfer rate caused by electrostatic repulsion can be overcome by using commercial porous graphite felt as flow-through cathode.Cr(VI)removal in flow-through system conformed to the pseudo first-order kinetic model and the kinetic constant is-0.072 min-1,which was 2.3 times than that of plate system.Fe(Ⅲ)can combine with negatively charged Cr(VI)(Cr2O72-and HCrO4-)to form positively charged FeCrO4+,which can accelerate the interfacial mass transfer rate and improve Cr(VI)to reduce to Cr(III).Results indicated Cr(VI)removal rate was 99.9%after treating the influent with a pH 2.0 and 117.0 mg/L Cr(VI)at 300.0 mA and 10.0 mL/min.Energy consumption in per unit area of graphite felt is 0.5 × 10-3 kWh/m2 when treating 100.0 L this solution.(2)When graphite felt was used to simultaneously remove Cr(VI)and MO,anodic oxygen evolution reaction will lead to the accumulation of oxygen in the front of graphite felt,which increase the systemic internal resistance with a sharp rise in cell voltage and also result in the deterioration of Cr(VI)removal.Under acidic(pH 2.0)conditions,H2O2 reacts with Cr(VI)rapidly to form Cr(V)and finally to Cr(III),while the active substance was produced to remove MO.Cr(VI)and MO cannot be simultaneously removed because the in-situ formed H2O2 is not sufficient.Loading PTFE can increase the dissolved oxygen concentration at the electrode interface and accelerate the oxygen mass transfer rate at the electrode interface,which provide a theoretical basis for the efficient in-situ generation of H2O2.(3)PTFE/CNT/GF flow-through electrode was successfully prepared by impregnation-heating curing method.PTFE/CNT/GF had gas diffusion channel and permitted the excess gas generated by electrolysis to be discharged in time,hence the system has good stability.In the single pass mode,increasing current and decreasing flux can increase H2O2 production,which can produce 40.0 mg/L～45.0 mg/L H2O2 at pH 2.0～5.0.Increasing current,decreasing flux,pH and Cr(VI)concentration in influent can improve Cr(VI)removal efficiency.Under the optimal conditions,the Cr(VI)removal efficiency is up to 99.9%.PTFE/CNT/GF flow-through electrode can generate H2O2 in situ by using anodic oxygen evolution reaction as oxygen source.H2O2 reacted with Cr(VI)to form intermediate product of Cr(V)which can further combine with H2O2 to form·OH.which can oxidize MO into ester and transform itself into Cr(III),thus realizing simultaneous removal of Cr(VI)and MO.Effluent was affected by influent pH.Cr(VI)and MO concentration,flux and current.When treating the pH 2.0.100.0 mg/L Cr(VI)and 50.0 mg/L MO in influent under 250.0 mA and 1.5 mL/min,the MO and Cr(VI)removal rate was about 99%and 99.9%,respectively.(4)PTFE/CB/GF flow-through electrode was successfully prepared by using cheap carbon black instead of carbon nanotubes.H2O2 production.Cr(VI)and MO removal efficiency were also affected by influent pH,Cr(VI)and MO concentration,flux and current.In single-pass model,the highest H2O2 production in effluent reached 40.0 mg/L at pH 2.0.200.0 mA and 1.5 mL/min,while Cr(VI)was totally removed after treating the influent containing 100.0 mg/L Cr(VI)and pH 2.0 at 1.5 mL/min and 200.0 mA.When treating the influent containing 100.0 mg/L Cr(VI)and 50.0 mg/L MO,both contaminants can be completely removed by a single pass experiment. |
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