Mechanism On The Enhanced Performance Of Zero-valent Iron By Nickel For The Removal Of Chlorophenol And Chromium From Groundwater

To date,the groundwater was polluted seriously with the development of industry and agriculture,which was resulted from the discharge of large amounts of industrial wastewater and excessive use of pesticides and fertilizers.Chlorinated organic and heavy metal pollution is particularly serious.Zero-valent iron was investigated widely as an environmental in-situ remediation medium,which could be used to remove kinds of pollutants in water.The removal efficiency could be much enhanced by forming bimetals with zero-valent iron.But the mechanism was still unclear.Based on zero-valent iron technology,nickl was selected toprepare nanoscale Fe/Ni bimetal material?n-Fe/Ni?,while 2,4-dichlorophenol?2,4-DCP?and Cr???was selected as the target pollutant.The effects of H₂ and Fe²⁺on the removal of target pollutant was investaged,and the varitation of dissolved oxygen and redox potential was measured to investigate the mechanism for the reduction of 2,4-DCP and Cr???by n-Fe/Ni.Furthermore,the principal mechanism of Ni action in the reduction of2,4-DCP and Cr???was also investigated.The mechanism for the 2,4-DCP dechlorination by n-Fe/Ni was concluded as follows based on the investigation:the active hydrogen atom that transformed from H₂ under the catalysis of Ni was responsible for more than 90%of 2,4-DCP dechlorination by n-Fe/Ni and less than 10%of the dechlorination was attributed to the direct electron transfer from Fe⁰.Another corrosion product of Fe⁰,Fe²⁺,was not able to dechlorinate 2,4-DCP.The mechanism of Ni actioncould also be summarized accordingly:?i?Ni mainly acted as a catalyst to transfer H₂ into active hydrogen atom to dechlorinate 2,4-DCP;?ii?theacceleration of electron transfer from Fe⁰ by Ni had a positive effect on the 2,4-DCP dechlorination;and?iii?the Fe⁰ oxidization by oxygen could be reduced by Ni,which was also beneficial to 2,4-DCP dechlorination.The mechanism of Cr???reduction by n-Fe/Ni was concluded as follows:the direct electron transfer from Fe0 and the reducing capacity of Fe²⁺,one of the corrosion product of Fe⁰,are both responsible for the reduction of Cr???.H₂,another corrosion product of Fe⁰,does not contribute to Cr???reduction.The mechanism of Ni action could also be summarized accordingly:?i?Ni mainly accelerated the electron transfer from Fe⁰ to Cr???and promoted the corrosion of Fe⁰ to produce Fe²⁺,which accelerated the reduction of Cr???;?ii?the presence of Ni inhibited the oxidation of zero-valent iron,thus increased the removal capacity of Cr???.The Cr???was reduced to Cr?III?after reaction with n-Fe/Ni,and then retained on n-Fe/Ni in the form of FeCr₂O₄ precipitates,resulting in the sequestration of Cr???from the aqueous phase.The kinetic model and adsorption isothermal model for the removal of 2,4-DCP and Cr???from water by n-Fe/Ni were established,respectively,and the influence of various factors on the removal of 2,4-DCP and Cr???by n-Fe/Ni was also investigated.The results showed that the process for the removal of 2,4-DCP by n-Fe/Ni combined adsorption and reduction,and matched well with the first-order kinetic model and the Langmuir adsorption isothermal model.The removal rate was ranged from 0.3 min^-1to 0.7 min^-1,and the theoretical maximum adsorption-reduction capacity was 801.1 mg/g.The dechlorination rate of n-Fe/Ni on 2,4-DCP is relatively stable when the pH of the reaction system ranges from 3.0 to 9.0,and the dechlorination rate was higher than 95%.The Na⁺,Ca²⁺,SO₄^2-and Cl^-with the concentration in the range of 0 to 10 mmol/L had little effect on the dechlorination of2,4-DCP by n-Fe/Ni.However,the dechlorination of 2,4-DCP by n-Fe/Ni would be inhibited by the co-existing NO₃^-and HCO₃^-.The reaction between n-Fe/Ni material and Cr???matched well with the first-order kinetic model and the Langmuir adsorption isothermal model.The maximum adsorption-reduction capacity is 138.7 mg/g.The removal of Cr???by n-Fe/Ni was stable when the pH ranged from 4.0 to 9.0 and the removal rate was higher than 74%.Increasing the concentration of dissolved oxygen in water will inhibit the reduction and removal of Cr???.The co-existing Na⁺with the concentration range from 0 to 10 mmol/L had little effect on the reaction,while the co-existing Ca²⁺and HCO₃^-would promoted the removal of Cr???.However,the co-existing SO₄^2-,NO₃^-and PO₄^3-would inhibited the removal of Cr???by n-Fe/Ni,and the higher the concentration,the more obvious the inhibition.