Investigation Of Arsenic Removal In Groundwater By Fe-modified Biochar As A Permeable Reactive Barrier Material

Arsenic（As）is a worldwide contaminant,and the related environmental and public health issues have been studied for centuries.Developing the techniques of aqueous As removal in groundwater is of great practical significance.The permeable reactive barriers（PRBs）are passive,cost-effective and sustainable techniques that provide an in-situ approach to groundwater remediation.PRB,which is one of the most promising techniques in groundwater remediation,has experienced rapid growth in the last three decades.It is an emplacement of reactive materials in the subsurface designed to intercept a contaminant plume,providing a flow path through the reactive media,and transforming the contaminant into environmentally acceptable forms to attain remediation concentration goals down the gradient of the barrier.So far,zero valent iron（ZVI）has been the most widely investigated reactive material in As groundwater remediation by PRBs.However,passivation of ZVI shortens the longevity of PRB and coexisted ions like phosphate limited the efficiency of ZVI in high-As groundwater remediation.Developing the reactive material of aqueous As removal in groundwater and investigating the mechanisms of As stabilization by PRBs are of great practical significance to groundwater regulation and drinking water safety.Biochar has been widely reported in aqueous As removal as a cost-effective,sustainable,and renewable porous material with various functional groups and sufficient surface areas,while poor performance of pristine biochar is observed because of the weak affinity of negatively charged biochar to As species in natural aquatic systems.The novel Fe-modified biochar drives much attention by changing the surface charges of biochar.Fe-modified biochar is a iron-based material supported by the porous structure and properties-enhanced biochar,which is a win-win combination of these two types of reactive adsorbents.On one hand,the loaded ZVI or Fe oxides provide more reactive sites,improving the properties of biochar as well as the As removal capacity.On the other hand,the porous biochar support and encapsulate ZVI or Fe oxides,preventing the nano-materials from agglomerating.However,the composition of Fe-modified biochar is complicated,and the poor identification of species on Fe-modified biochar limited the mechanisms investigation of As removal in groundwater,resulting in insufficient research related to its role in PRBs for high-As groundwater remediation.Therefore,the present study prepares the novel biochar from poplar,invents different types of Fe-modified poplar biochars,evaluates the abilities of Fe-modified biochars in As removal,investigates the mechanisms of As removal,and reveals the role of Fe-modified poplar biochars in high-As groundwater remediation as a PRB filling material.The primary research progress is listed as follows:I)A novel Fe-modified poplar biochar was produced,the spatial distribution and speciation of Fe in Fe-modified biochars were characterized,and the interactions between Fe and biomass during pyrolysis were revealed.Characterization of the spatial distribution and speciation of Fe in Fe-modified biochars is critical for understanding the mechanisms of contaminant removal.Here,synchrotron-based techniques were applied to characterize the spatial distribution and speciation of Fe in biochars modified by Fe Cl₃ or Fe SO₄ and pyrolyzed at 300,600,and900°C,respectively.Chapter 2 is trying to provide a perspective and sustainable material,uncover the fate of Fe during pyrolysis,and offer information relevant to tailoring Fe-modified biochars to specific environmental applications.Confocal micro-X-ray fluorescence imaging（CMXRFI）results indicated Fe,sulfur（S）,and chlorine（Cl）diffused into the basic porous structure of the biochars and aggregated to the surface as pyrolysis temperature increased.Fe K-edge X-ray absorption near-edge structure（XANES）and extended X-ray absorption fine structure（EXAFS）spectra revealed maghemite（γ-Fe₂O₃）as the primary Fe species in the modified biochars and ZVI was observed when pyrolyzed at 600 or 900°C.II)The optimum type of biochar in As removal is screened out（Fe BC）,synchrotron-based techniques were combined with aqueous chemistry to uncover the mechanisms of As removal by Fe BC,and the role of oxygen in As removal by Fe BC was revealed.The Fe-modified biochar has been reported as promising in aqueous heavy metals removal,while a detailed screening experiment should be conducted to evaluate the optimal pyrolysis condition.The decisive role of O₂ has been recognized in the fate of As,while the redox conditions of previous As removal are usually ambiguous.In Chapter 3,6 types of un-modified and Fe Cl₃-modified biochars will be used to remove As（III）and As（V）from simulated groundwater.Then,the kinetic and isothermal experiments will be conducted,and dynamic speciation will be monitored to uncover the mechanisms of As removal by certain types of Fe-modified biochar under different redox conditions.Fe modification resulted in better As（III）and As（V）removal abilities of poplar biochars,and biochars pyrolyzed at higher temperatures showed higher efficiencies.Fe Cl₃-modified biochars pyrolyzed at 900°C showed excellent abilities in As（III）and As（V）removal,and it was used to remove aqueous As（III/V）in further experiments.As removal by Fe BC was well fitted by pseudo-second-order kinetic model and Langmuir model.Speciation of As in Fe BC,along with the kinetic experiments,was analyzed by synchrotron-based XANES spectra.In As（III）-spiked systems,As（III）was observed in all Fe BC samples;LCF-calculated percentages of As（III）in Fe BC declined from 82 to48%for oxic conditions and from 100 to 78%for anoxic conditions;As（V）was observed in Fe BC along the 24 h,increasing from 18 to 52%,under oxic conditions;As（V）was not observed in Fe BC under anoxic conditions until 3 h,with the percentage increasing to 22%after 24 h.In As（V）-spiked systems,As（V）accounted for 100%through the kinetic experiments under both oxic and anoxic conditions.Combination of aqueous and solid analyses of As speciation helps uncover the mechanisms of As removal in Fe BC-added systems.Fe BC removed As（III）from As（III）-spiked systems by surface-oxidation following adsorption.As（V）was first reduced,re-oxidized in solutions,and then adsorbed to Fe BC in As（V）-spiked systems.Both As（III）and As（V）were bidentate corner-sharing complexed to Fe oxides/hydroxides on Fe BC,with As coordinated to Fe at～3.4（?）according to As EXAFS modeling.The removal efficiencies for As（III）and As（V）increased from 86.4%and 99.2%under anoxic conditions,respectively,to>99.9%when O₂ was available.The supply of O₂ improved As（III）oxidation but inhibited As（V）reduction,resulting in more As（V）but less As（III）in oxic systems compared to anoxic ones.Transformation of As species in solutions and different charges of As（III）and As（V）are likely responsible for the O₂-promoted As stabilization.Combination of aqueous and solid analyses of As speciation helps uncover the role of O₂ in Fe BC-added systems.Fe BC removed As（III）from As（III）-spiked systems by surface-oxidation following adsorption,where oxidation of As（III）was promoted by O₂.As（V）was first reduced,re-oxidized in solutions,and then adsorbed to Fe BC in As（V）-spiked systems,where reduction of As（V）was inhibited in the presence of O₂.III)The effect of co-exited ions was evaluated in As removal,Fe XANES LCF and As XANES LCF were coupled to reveal the role of Fe-modified poplar biochars in high-As groundwater remediation as a PRB filling material,and the fate of Fe-modified poplar biochars in high-As groundwater remediation as a PRB filling material at the co-existence of phosphate was uncovered.The Fe BC shows excellent abilities in As removal under both oxic and anoxic conditions,while the column experiment has not been conducted to explore the performance of Fe BC in continuous flow columns.Limited work has been done related to continuous groundwater by PRB,nor have the interferences of competing ions been evaluated.To bridge the gap between Fe BC and PRBs for As groundwater treatment is of great significance.In Chapter 4,interferences were involved to evaluate possible effects.Further,the role of Fe BC in high-As groundwater remediation as a PRB filling material,as well as the co-existence of phosphate,was investigated.In general,the Fe BC with interferences still comes up with better ability than BC for both As（III）and As（V）stabilization,indicating the overall potential of Fe-modified biochars in As-contaminated groundwater remediation.The co-existence of 0.5 m M Na Cl,Na SO₄,Na H₂PO₄,and Na HCO₃ influenced the As（III）and As（V）removal in different degrees.Limited influence of HCO₃^-was found in the As-spiked systems because the working solutions were prepared with Ca（HCO₃）₂-saturated water.All other ions inhibited the As（V）removal by Fe BC.The Cl^-and SO₄^2-inhibited the As（III）removal by Fe BC.What noticeable is that the H₂PO₄^-/HPO₄^2-promoted the As（III）removal and enhanced the efficiency to 96.7%.For Na H₂PO₄-added systems,the obvious decrease in p H,total Fe,and Ca²⁺concentrations could result from Fe-Ca-PO₄co-precipitation,resulting in better abilities of As removal.Simultaneous removal of As（III）and phosphate was investigated.For the first 3 h,As（III）and phosphate were stabilized by Fe BC at a relatively slow rate by adsorbing to Fe OOH or functional groups on the surface of Fe BC.After 3 h,the As and total phosphorus concentrations decreased faster,which could be the period that co-precipitation happened.A column experiment was conducted to evaluate the role of Fe-modified poplar biochars as PRB filling material in groundwater As（III）remediation.Fe BC PRB played an excellent role in remediation of synthetic As（III）-groundwater.It takes 37.5 PVs to total breakthrough at the average flow rate of～2.0 m L h^-1.The effluent As concentration increased gradually,while the effluent total Fe concentration decreased.Higher Fe concentrations were observed in side ports closer to effluent,indicating that Fe was released from the packed reactive materials and carried by the working solutions.Samples were collected at the end of the column experiments,and LCF analyses of Fe XANES and As XANES were used to identify the speciation.ZVI corrosion and As（III）reduction were observed in Fe BC.The ferrihydrite was formed when～45 PVs As（III）-containing solution was pumped through it,which is more obvious in Fe BC closer to the effluent port.The As（III）in the column stayed at a stable proportion（～60%）,while the trend of transforming As（0）to As（V）was obvious,which indicated As（III）can be reduced by Fe BC but re-oxidized gradually on ferrihydrite.Another column experiment was conducted to evaluate the role of Fe-modified poplar biochars as PRB filling material in groundwater As（III）remediation at the co-existence of phosphate.It takes 31.8 PVs to total breakthrough at the average flow rate of～3.0 m L h^-1.The As profile front migrated in a similar trend through columns in the presence of phosphate,while a better capacity in retaining As was observed.The maximum t Fe concentration was lower when phosphate was involved,like the results of batch experiments,indicating the detention of Fe by phosphate.There were stable stages of profile Fe in both columns.Higher step differences between side ports were found at the co-existence of phosphate during the Fe stable stage,which indicated the mobilization and immobilization of Fe were more frequently happening.The Fe and As speciation on Fe BC were identified by LCF analysis of Fe XANES and As XANES at the end of the experiment.When the phosphate-As（III）-spiked solution was pumped into the column,the transform between ZVI and ferrihydrite was not obvious,indicating that the phosphate decelerated the corrosion of ZVI.Compared with the phosphate-free column,the general proportion of As（0）in this column is higher,while the average percentage of As（III）is lower,which indicated more reduced conditions when phosphate was pumped together with As（III）into the column.Besides,the gradient of As（0）and As（V）percentages from bottom to top is higher than that of the phosphate-free column,which could be explained by the higher flow rate.The As（III）in the phosphate-free column stayed at a stable proportion（～60%）,while the percentage of As（III）decreased along the flow when phosphate was added,indicating that the co-precipitated As（III）at the presence of phosphate was readily to be oxidized to As（V）.The co-precipitation of phosphate with Ca or Fe,such as hydroxyapatite,could be the major reason for promoted As detention by adsorption or co-precipitation.The detained As（III）by Ca/Fe-phosphate could be further transformed to As（V）on Fe BC.In general,novel Fe-modified poplar biochars are produced in the present study,speciation and distribution of Fe in biochars were comprehensively characterized,the abilities and mechanisms of As stabilization by the Fe-modified biochar were investigated,and the role of Fe-modified biochar as a PRB material for As（III）removal from synthetic groundwater was revealed.The new finding of the present study is as follows:the present study revealed the immigration and redox transformation of As in synthetic groundwater in the presence of Fe-modified biochar,and the excellent performance of Fe-modified biochar in a simulated PRB was investigated.