The Mechanism Of Reductive Removal Of Cr(?) By Oxalic Acid Or Nanoscale Zero Valent Iron In Aqueous Solution

Chromium widely exists in aquatic,terrestrial,and soil systems,caused by electroplating,tannery operations,chemical manufacturing,chromium mining,and ore processing.Chromium mainly exists in two valence states in the environment,namely hexavalent chromium(Cr(?))and trivalent chromium(Cr(?)).Cr(VI)with high solubility and mobility is extremely toxic even at very low concentrations.On contrast,Cr(?)is much less toxicity and low solubility than Cr(?),and Cr(?)is essential in trace quantities for sugar and lipid metabolism in humans.Thus,the one of most common approach to remediate Cr(?)pollution is to reduce Cr(?)into relatively less toxic Cr(?).Oxalic acid,mainly from the decomposition of organic matter and plant root exudates,is one of most widely natural existed organic acids.Oxalic acid shows relatively high reductive ability,we believe that soluble oxalic acid might play a vital role on the transfer and transportation of Cr(VI)species in the natural aquatic environment.Therefore,our work mainly aimed to study the performance and the mechanism of the Cr(?)reduction by oxalic acid under different reaction condtions.The detailed works were shown as the following:Part ?:Mn2+ is abundant in natural environment.We demonstrate that Mn2+ can greatly promote the Cr(?)reduction by oxalic acid at pH<5 via an induction period and a subsequent auto-acceleration process.The Cr(?)reduction rate constant during the late auto-acceleration process was about 10 times that of the initial induction period.The Mn3+ could be generated via oxidation of Mn2+ by Cr(?)in the presence of oxalic acid during induction period.The in-situ generated Mn3+might complex with oxalate and Cr(?)to produce a ternary complex,and Mn3+ can function as the central electron mediator to facilitate the electron transfer from electron transfer from C2HO4-to Cr(?)accelerating the Cr(?)reduction at auto acceleration period.This finding shed insight into the possible roles of widely existed Mn ions(Mn2+ and Mn3+)and oxalic acid in the transformation of Cr(?)in natural aquatic environment,and also provided an efficient way to remediate Cr(?)-containing acid wastewater.We subsequently test the applicability of Mn2-/oxalic acid method to remediate wastewater containing Cr(?)of concentration as high as 500 mg�L-1.The impacting factors including solution pH,the concentration of Mn2+ and oxalic acid on the reduction of Cr(?)were investigated.The results showed that lower pH,higher concentration of oxalic acid and Mn2+ are conductive to the reduction of Cr(?).It was interesting to found that 500 mg�L-1 of Cr(?)could be completely reduced with 200 ?mol�L-1 of Mn2+ and 35 mmol�L-1 of oxalic acid within 5 h.We also consider how to remove In situ generated Cr(?)complex(mainly existed as Cr(?)-oxalate complex)left in the wastewater through precipitation,which can be realized by simply adding CaO and CaCl2,as Ca2+ and OH-could precipitate oxalate and Cr3+ ions,respectively.After 24 hours precipitation reaction,the concentration of Cr(?)species left in the supernate was less than 0.2 mg/L,which suggsted that the supernate could meet the national discharged standards.Part ?:We investigated the effect of MnO2 on Cr(?)reduction by oxalic acid at differnt pH values.The results suggested that no promotion of Cr(?)reduction by oxalic acid was observed after addition of Mn02 at pH>5.The reaction of Mn02 with oxalic acid to generate Mn2+ and CO2 was the principal reaction at pH>5.Interestingly,the addtion of MnO2 could facilitate Cr(?)reduction by oxalic acid at pH<5.The characterzation experiments suggsted that the Mn(?)dimer complex and in situ generated Mn(?)play vital roles in Cr(?)redcution.Subsquently,the Cr(?)would coordinate with oxalic acid to for highly moblile Cr(?)-oxalate complex which could not absorb on MnO2 surface.This study shed light on the mechanism of MnO2 promoted Cr(?)reduction by oxalic acid at acid condtion(pH<5).Part ?:We found the formation of Cr(?)-oxalate complex proceeded via esterification process under acid conditions and the possible elemental compositions and structure of Cr(?)-oxalate complex was identified by high resolution mass spectrometry(HRMS).And then,the photoactivity of Cr(?)-oxalate complex and the its effects on photoreduction of Cr(?)were systematically explored based on the results of experimental and theoretical results.The results suggested Cr(?)could be reduced via photoinduced electron transfer from oxalate to chromate through a coherent HOMO-LUMO electronic transition within CrC2O72-complex under visible light illumination.LC-HRMS was also used to determined the intermediates of Cr(?)photoreduction by oxalic acid and proposed a possible Cr(?)photoreduction pathway.We systematically investigated the mechanism of Cr(?)reduction by oxalic acid in Mn2+/oxalic acid sytem,in MnO2/oxalic acid system or under visible light illumiantion,respectively.These experiment results suggested that oxalic acid played an important role in determining the oxidation states and existence speciation of chromium in the natural aquatic environment.The Cr(?)reduction to Cr(?)with oxalic acid could reduce the toxicity of chromium,which was considered as a environmentally benign process.While,Cr(?)species were still existed in the aquatic environment after Cr(?)was reduced,and these Cr(?)species are potentially harmful to human health.The removal of Cr(?)from aquatic environment with adsorbents could elinimated the threat of Cr(?)pollution thoroughly.Nanoscale zero-valent iron(nZVI)is a special kind of ZVI with particle size less than 100 nm in diameter.Because of nZVI's high surface area and high reactivity,it was widely used in adsorbing heavy metals.nZVI is also kind of reactive transition metal with reductive property.It can directly transfer electrons to the Cr(?)and transform flexibile Cr(?)species into inflexibile Cr(?)species during the adsorption processes.which could facilitate the removal of Cr(?)from aquatic environment.nZVI were found to possess a core-shell structure which was composed of a metal iron core encapsulated by an iron(oxyhydr)oxide shell of 6-10 nm.It was generally accepted that the oxide shell have dual effects on Cr(?)removal.On the one hand,the oxide shell would inhibit electron transfer from the metal core to the outside shell,which disfavored the reductive removal of Cr(?).On the other hand,the oxide shell would provide the reactive sites for Cr(?)adsorption on the particle surface.However,the specific influence of oxide shell on Cr(?)adsroption with nZVI was still unknown,and the related mechanism was need to explored.Part IV:In this study,we investigated the influence of iron oxide shell thickness and the amount of surface bound ferrous ions on the anoxic Cr(?)removal performance of core-shell Fe@Fe2O3 nanowires(a special kind of nanoscale zero valent iron)in order to clarify their Cr(?)removal mechanism in detail.It was found the surface area normalized Cr(?)removal rate constants of Fe@Fe2O3 nanowiresfirst increased with increasing the iron oxide shell thickness and then decreased,suggesting that Fe@Fe2O3 nanowires possessed an interesting core-shell structure dependent Cr(?)removal property.Meanwhile,the Cr(?)removal efficiency was positively correlated to the amount of surface bound Fe(?).This result revealed that the core-shell structure dependent Cr(?)removal property of Fe@Fe2O3 nanowires was mainly attributed to the reduction of Cr(?)by the surface bound Fe(?)besides the reduction of Cr(?)adsorbed on the iron oxide shell via the electrons transferred from the iron core.The indispensable role of surface bound Fe(?)was confirmed by Tafel polarization and highresolution X-ray photoelectron spectroscopic depth profiles analyses.X-ray diffraction patterns and scanning electron microscope images of the fresh and used Fe@Fe2O3 nanowires revealed the formation of Fe(?)/Cr(?)/Cr(?)composite oxides during the anoxic Cr(?)removal process.This study sheds a deep insight into the anoxic Cr(?)removal mechanism of core-shell Fe@Fe2O3 nanowires and also provides an efficient Cr(?)removal method.