Removal Of Chromium(?) From Wastewater By Ca-aminoclay Coated Nanoscale Zero Valent Iron

Heavy metal pollution has become a hot and difficult problem in the field of environment.As a common heavy metal contamination,hexavalent chromium,Cr(?),is a pollutant in groundwater and hazardous to our living environment and human health because of its high mobility and toxicity.Thus,the control and treatment of Cr(?)from wastewater became the problems urgently needed solved now.In recent years,nano zero-valent iron(nZVI)is considering as one of the most efficient chemical reduction deposition process and has great potential for the remediation of wastewater.The nZVI has drawn an increasing attention due to its small particle size,large special surface area,high surface reactivity and easy recovery and recycling.However,the nZVI is prone to agglomerate,leading to some electrons of nZVI hard to used and easily oxidized by O2 and H2O exposing to air or water resulting in the waste of electrons.These problems lead to limit its application in the treatment of wastewater.In the present work,Ca-aminoclay(CaAC)was prepared and then coated on the surface of nZVI by the liquid-phase reduction method to from a novel CaAC-nZVI nanomaterials,which was proposed for Cr(VI)removal.Results are follows:(1)The preparation conditions of CaAC-nZVI nanomaterials were optimized.The optimum mass ratio of CaAC-nZVI was 0.15:1.When the dosage of materials was 0.2g L-1,temperature was 25?,pH was 5 and the initial concentration of Cr(?)was 20 mg L-1,the removal efficiency of Cr(VI)was up to 100%after 45 min reaction,which was much higher than that of bare nZVI(77.0%).The results proved that the modification of CaAC helped to increase the Cr(VI)removal efficiency of nZVI.Based on the characterization results of XRD,FTIR,TEM UV-VIS and Zeta potential,it found that CaAC had been success coated on the surface of nZVI,and the dispersivity of the nanomaterials was greatly enhanced compared with that of bare nZVI.(2)The effects of the process parameters,e.g.,initial concentration of Cr(?)?the dosage of CaAC-nZVI,initial pH,and temperature,on removal efficiency of Cr(VI)were measured.The optimum reaction condition was that the initial concentration of Cr(VI)was 20 mg L-1,the volume of simulated wastewater was 100 mL,the CaAC-nZVI dosage was 0.2 g L-1,temperature was 25?,pH was 5,and the maximum efficiency of removal of Cr(?)reached 100%after 45 min reaction.The influence of coexisting ions,e.g.,Cu2+?Ni2+?SO42-and PO43-on removal efficiency of Cr(?)were also carried out.It was found that Cu2+?Ni2+could enhance Cr(?)removal because that the galvanic battery action with Fe(0)could accelerate the electron transfer rate in this system.The effect of SO42-was almost negligible.Due to competitive adsorption between PO43' and Cr(VI),it caused the inhibition of Cr(?)removal.(3)The Cr(?)removal by CaAC-nZVI and nZVI were both in accordance with the pseudo-second-order adsorption kinetic model.The values of adsorption rate constant(ks)and removal ability(qe)of CaAC?nZVI system were both significantly higher than that of bare nZVI,which indicated that the reaction reactivity of nZVI was greatly enhanced by CaAC.(4)Based on the characterization analysis and experimental results,reaction mechanism of Cr(?)removal by CaAC-nZVI was clarified.Firstly,the negative Cr(?)species was accelerated adsorbed by the protonated amino groups(NH3+)on the CaAC on the surface of CaAC-nZVI nanomaterials.Secondly,the chemical reduction carried our,in which Cr(?)was reduced to Cr(?)while Fe was oxidized to form Fe(OH)2 and Fe(OH)3.Finally,some of Cr(?)reacted with Fe(?)to form the Cr(?)-Fe(?)co-precipitates,which then adsorbed on the surface of CaAC-nZVI.The other Cr(?)was removed from the partials and form CrxFe1-x(OH)3 coprecipitate reacted with Fe(OH)2 and Fe(OH)3 in water.