| In recent years,due to the continuous mining and smelting of arsenic(As)and antimony(Sb)minerals and the extensive and unreasonable application of its compounds,the events of water contamination with As and Sb have occurred frequently all around the world.Therefore,it is urgent to find an effective technology to treat water contamination with As and Sb.Fortunately,electrocoagulation(EC)has shown the greatest potential for application in water treatment with easy operation,strong controllability,no addition of chemicals and high removal efficiency.In this study,As and Sb removal from wastewaters by EC were investigated with perspectives of single factor analysis and response surface optimization design,characterization of EC products,adsorption kinetics and thermodynamic studies,and numerical simulation of EC system.As a result,effects of operating parameters on As and Sb removal efficiencies were systematically discussed,and the optimization of EC process was obtained.Besides,removal mechanism and adsorption behavior involved in As and Sb removal were deeply explored.Furthermore,the electric field,flow field as well as mass transfer models in EC system were build,which could establish and improve the theoretical basis of As and Sb removal from wastewaters in EC process.Results of single factor analysis indicated that more than 99%As and Sb were removed by EC with hybrid Fe-Al electrodes.Lower overpotential and higher reaction rate were obtained when electrodes were connected in monopolar series mode.There was a positive correlation between current density and removal efficiencies of As and Sb.The optimum range of pH was observed at 5.0-7.0.The increase of initial concentration led to the decrease of removal efficiency.However,when the treatment time was prolonged,As and Sb removal improved.As(III)removal was preferred with the increase of dissolved oxygen,but Sb(III)removal declined.NaCl as electrolyte reduced the power consumption and displayed better EC performance.The presence of NO3-and SO4-had little influence on As and Sb removal.However,PO4-competed with As and Sb for adsorption sites,resulting in the decrease of removal efficiency,and more remarkable inhibition was found with higher PO43-concentration.Results of response surface design indicated that aeration intensity and treatment time had significant influence on As removal,and Sb removal were affected greatly by current density,pH,as well as treatment time.The optimal reaction conditions by model prediction were as follows:current density 13.19 A/m,pH 7.0,aeration rate 0.32 L/min,and treatment time 20 min for As removal;current density 28.67 A/m2,pH 5.24,initial concentration 790.13p,g/L,and treatment time 89.17 min for Sb removal,which were also verified by experiments.Based on theses studies,more than 99%As and Sb were removed from actual wastewater near Xikuangshan area by EC with hybrid Fe-Al electrode.Results of removal mechanism and adsorption behavior studies involved in As and Sb removal indicated that As(?)oxidation mechanism was related to the active intermediate of Fe(IV)in the dissolved oxygen environment.In a two-stage reaction process(graphite anode reaction and iron anode reaction),As(?)was converted into As(V)which was easier to remove,and the treatment time decreased.Sb(?)was effectively reduced,and the total Sb removal efficiency was improved in EC process with iron anode and copper cathode.ESEM/EDS,FTIR,XRD and XPS analysis showed that EC products possessed micron level of microcrystal structure.Al(?)addition caused the expansion of Fe(?)oxide/hydroxide crystal lattice,leading to the increase of specific surface area and its activity.EC products were mainly composed of aluminum and iron hydroxides/oxyhydroxides such as Al(OH)3,AIOOH,Fe3O4/Fe2O35,?-[FeO(OH)],and no As-Fe/Al or Sb-Fe/Al precipitates were expected to form,confirming that adsorption onto iron and aluminum hydroxides/oxyhy-droxides was the main mechanism involved in As and Sb removal.According to adsorption kinetics,isothermal adsorption and thermodynamic studies,the adsorption process was mainly chemical adsorption with monomolecular layer adsorption,and the temperature was beneficial to the adsorption process.Results of numerical simulation of EC system indicated that the primary current distribution was effectively improved by changing the electrode distance or adding 1 cm insulator at both edges of the anode in the sequencing batch EC reactor.Electrode configuration 2 and 3 cm electrode distance indicated that more uniform primary current distribution and higher electrode current efficiency were obtained.In the continuous flow EC reactor,with the increase of flow rate,the tail of the residence time distribution curve decreased,and the peak was shifted to the right,leading to the reduction of stagnation zone and the increase of mass transfer in EC reactor.However,the formation of flocs and its effective combination with pollutants were inhibited when the flow rate was too high.According to the simulation of mass transfer process,when the treatment time was 5 min,the rate of electrolysis/hydrolysis was greater than that of mass transfer.Therefore,Fe2+ was focus on the anodic area,OH-mainly existed in the cathodic area,and Fe(OH)2 distributed between anode and cathode.When the treatment time was 15 min,the contact area between Fe2+ and OH-as well as distribution of Fe(OH)2 increased.It was proved that the concentration of Fe2+ and OH-were the lowest at the outlet of the EC reactor.The concentration and distribution of Fe(OH)+ were not affected by the treatment time,and once produced,it was immediately consumed in the following hydrolysis reaction.In summary,this study provided a theoretical basis for the selection of operating parameters and optimization of the EC reactor for As and Sb removal.Besides,the mechanism involved in EC process was further established and consummated,which was significant to the treatment of increasingly serious water contamination with As and Sb. |
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