| As a cost-effective and eco-friendly material,zero-valent iron(ZVI)has been considered promising for water remediation.Nevertheless,the passivation and the resultant low efficiency of ZVI severely limited its pratical application.Recently,the development of enhanced ZVI technologies for water decontamination has become a research hotspot.It has attracted attention in recent studies that the mediation of oxidants could enhance the performance of ZVI in decontamination with merits including easy operation and low cost.Efficient removal of heavy metals and arsenic has also been achieved in the presence of oxidants(e.g.H2O2)in ZVI-packed bed column as a commonly employed type of reactor.However,the temporospatial evolution of ZVI as well as removal mechanisms of pollutants in the column system remains unclear.Moreover,the previously reported synchronous ZVI/oxidants system has shown insufficient removal of the contaminants due to competition for electron from ZVI with oxidants.To solve the above problems,this dissertation investigated the temporospatial evolution of ZVI during decontamination of As(V)and Se(VI)along a self-designed ZVI/H2O2 column in down-flow mode with a series of midway sampling ports.Furthermore,to address the electron competition effect,a ZVI based suspension with powerful reductive capability was prepared via pretreating ZVI/H2O system(prtZVI)with H2O2,resulting in a distinct reductive agent from the synchronous ZVI/oxidants system.The preparation parameters of prtZVI were optimized based on the response surface method(RSM),and the chemical compositions of the resultant prtZVI were characterized qualitatively and quantatively.Nitrate and nitrobenzene(NB)were selected as model pollutants to evaluate the reduction performance of prtZVI,where the effects of solution chemistry,the degradation products as well as the underlying mechanism were explored.Besides,the feasibility of prtZVI for decontamination from authentic wastewater containing nitroaromatic compounds was evaluated.In addition,to address the difficulty of prtZVI suspension in storage,transportation and dispensing,an alternative hybridized Fe0/Fe3O4/FeCl2 composite(BMI)in dry powder form was prepared for efficient water decontamination by a simple ball-milling process of the ternary mixture of ZVI,Fe3O4 and FeCl2·4H2O powders.Besides the improved applicability,BMI exhibited even higher reductive capability than prtZVI in decontamation from NB.In the ZVI/H2O2 column system,the addition of H2O2 significantly facilitated the corrosion of ZVI,thus enhanced the removal of As and Se.The corrosion degree of ZVI as well as the retention of As/Se decreased with the depth of the column,indicating the uptake of As and Se was highly dependent on the ZVI corrosion.In brief,the distributive removal percentages of As/Se in the column from Ll to L6 layer followed the order of 53.5%,24.5%,5.84%,2.51%,1.37%,1.03%for As and 42.6%,27.5%,6.69%,2.16%,1.03%,0.83%for Se,respectively,which suggests the dominant reactive zone was located in the L1 and L2 layers.With the continuous corrosion of ZVI,the breakthrough of H2O2 would activate the ZVI at lower positions,resulting in the reactive zone continuously shifting downward along the column.The temporospatial reductive evolution of aqueous As/Se along the ZVI/H2O2 column was revealed.The reductive transformation of aqueous As(?)and Se(?)occurred mainly in L2 to L3 layers.The aqueous As(?)concentration in L2 depicted a volcano shape with time.Whereas in L3,the As(?)increased from 0 mg/L to a plateau of 0.12-0.15 mg/L lasting from?150 to?750 BV,followed by a further rise up to?0.4 mg/L.The profiles of As(?)concentration at L4 to L6 were similar to that of L3 in shape,but followed a decreasing order in level.The profiles of Se(IV)in L1 and L2 were similar to that of As(?)in the same layers.In contrast,it almost overlapped with each other from L3 to L6 and kept a rising tendency.The reduction of aqueous As(V)and Se(VI)to As(III)and Se(IV)was favorably enhanced in the presence of abundant Fe2+ and limited H2O2,whereas remarkably inhibited at the areas with high concentration of H2O2.The retention of As(III)in the lower part of the column was observed while that of Se(IV)was negligible,owing to the different effects of pH on the adsorption of As(III)and Se(IV).Furthermore,the evolution of different oxidation states of As and Se retained in the column were identified by XPS,where the valence states were +5,+3,0 for As and +6,+4,0,-2 for Se,respectively.In addition,the As(?)/As ratio in the solid phase(>40%)was significantly higher than that in solution(<6%)among all the 6 layers,suggesting further reduction of adsorbed As(V)could occur in the solid phase.Likewise,the adsorbed Se(IV)/Se(?)could also be further reduced to its lower valences and the Se(0)was predominant in solid phase.The above results demonstrated the comprehensive mechanisms of As(V)/Se(VI)removal involving reduction and adsorption in the ZVI/H2O2 column.This dissertation proposed a novel and simple method to enhance ZVI reactivity via pretreating ZVI with H2O2/HC1 to obtain a suspension agent(prtZVI)with powerful reducing capability.The preparation parameters including pH,chemical dosage,and reaction duration were optimized on basis of RSM.Nitrate and NB were selected as probes to investigate the reductive performance of the prtZVI system.Highly efficient reduction of nitrate and NB by prtZVI over other widely used ZVI system was achieved at economcal dosage(2 g/L)and over wider applicable range(up to 9).In addition,the long-term storage of prtZVI system up to 30 days exerted negligible effect on its performance.Stoichiometric relation was observed between the removal of nitrate and consumption of dissolved Fe? by a linear regression,which indicated that Fe2+ was consumed with a ?NO3-/?Fe2+ stoichiometry of 1.36:1(?NO3-/?Fe2+).15N isotopic analysis revealed that NH4+ was the major aqueous product(? 95%)and N2 was the only gaseous one.Nitrosobenzene along with phenylhydroxylamine were identified as the intermediates during NB reduction while aniline was the sole final product.Compared with the virgin ZVI and nanosized ZVI,the prtZVI system exhibited much higher electron efficiency(50%-330%)for NB reduction as well as higher utilization ratio of Fe0,demonstrating better electron selectivity towards NB.The prtZVI system was consisted of various sub-systems including the sediment phase,colloidal phase,and liqiuid phase,all of which contributed to the reduction of nitrate and NB.The Fe3O4 coated ZVI core-shell material served as the reactive component.The Fe3O4-rich shell of prtZVI could not only enhance the adsorptive enrichment of nitrate/NB on the surface but also facilitate the electron transfer from the inner Fe0 core due to the improved conductivity of the shell.Moreover,the reaction of abundant Fe2+ with lepidocrocite into magnetite contributed to the depassivation of ZVI.In addition,Fe2+ and Cl-could favor the decontamination process via processing the shell of prtZVI and sustaining the corrosion of ZVI.With 2 g/L of prtZVI,efficient decontamination process was achived from nitroaromatic coumpound-rich real wastewater at circumneutral pH.This dissertation also proposed a method to prepare an iron-based agent(BMI)with high reductive reactivity via ball milling Fe0,Fe3O4 and FeCl2·4H2O powder.The BMI composite exhibited core-shell structure with Fe0 as the core and Fe3O4/FeCl2·4H2O(termed as Fe3O4/Fe?)mixture as the shell.ToF-SIMS showed that average thickness of the shell was-3.3 ?m.The Fe3O4/Fe? shell and the released Fe2+,Cl-as well as the structural Fe?(s)played a significant role in enhancing NB reduction by BMI.Compared with prtZVI,it only takes 30 min for BMI to remove equivalent amount of NB.BMI showed higher reductive reactivity and improved applicability than prtZVI. |
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