Remediation Of Hexavalent Chromium Contaminated Groundwater With Integrated Fe~0-PRB Fillers

Chromium and its compounds are important basic raw material in the industry and the lives of people.However,chromium-containing waste water and residues in the chromium salt producing process,especially the piling up of chromium slag,have brought heavy contamination to soil and groundwater around China.Permeable reactive barrier(PRB)technology,with the advantages of in situ remediation,stable operation and low cost,has sprung up promptly in recent years,and has broad prospects.Permeable reactive barrier based on zero-valent iron(Fe~0-PRB)has been widely applied in the remediation of Cr(?)-contaminated groundwater due to its strong reductive capacity,elevating reaction rate,easy access and inexpensiveness.However,traditional Fe~0-PRB fillers have the drawbacks of agglomeration,easy passivation and low utilization efficiency of Fe~0,impeding spread and application of this technology.Aiming to solve these issues of traditional Fe~0-PRB fillers in the remediation of Cr(?)-contaminated groundwater,this manuscript focused on study of preparation and optimization of PRB fillers.Two kinds of integrated FeO-PRB fillers from organic and inorganic aspect respectively were developed and their performance on the removal of hexavalent chromium was conducted on laboratory scale.With sodium alginate-polyvinyl alcohol(SA-PVA)as immobilization supports,cast iron powder embedded,the organic coated FeO-PRB fillers were prepared.Through the orthogonal experiment,the optimum preparation conditions were attained:200-300 mesh cast iron powder,1%SA and 1%PVA.The organic coated fillers prepared under these conditions had 90.34%removal of Cr(?)(initial Cr(?)/Fe=0.02)at the reaction time of 120 h.The filler had large specific surface area,and its surface was rough and porous,with great reticular pores inside.This porous structure made the Cr(?)-contaminated groundwater have great chance to react with Fe~0 inside sufficiently,increased active sites,prevented agglomeration of Fe~0,thus the filler can maintain relatively high reaction rate with great mass transfer performance.With bentonite as basic framework,sludge as pore former and cast iron powder added,the inorganic calcining Fe~0-PRB fillers were prepared after high temperature calcination.Through the orthogonal experiment,the optimum preparation conditions were attained:200-300 mesh cast iron powder,40%sludge,calcination temperature of 800? and calcination time of 60 min.The inorganic calcining fillers prepared under these conditions had 87.25%removal of Cr(?)(initial Cr(VI)/Fe?0.02)at the reaction time of 120 h.The filler had great specific surface area,and its surface was rough and porous,with mesoporous structure inside,which prevented agglomeration of Fe~0 and enhanced the reduction efficiency of Fe~0.Besides,the filler had enough compressive strength and abrasive resistance.Solution pH has great impact on the removal of chromium using the integrated FeO-PRB fillers.Both two integrated fillers are greatly recyclable.For two fillers,the Cr(VI)removal process follows the pseudo first-order kinetics.Both fillers can enlarge the specific surface area of Fe~0,prevent agglomeration of Fe~0,thus improving the utilization rate of Fe~0.The observed rate constant of organic coated fillers was 2.09 times that of inorganic calcining fillers therefore the former have an advantage over the latter in terms of reaction rate,wheres the latter have far greater compressive strength,and could better meet the requirements of engineering application.