The Study On The Removal Of Hexavalent Chromium From Groundwater Using Activated Carbon Fibre Supported Nanoscale Zerovalent Iron

The groundwater is one of valuable natural resources and is of importance to the survival of mankind.With the population increasing and economic progress,the heavy metals pollution of underground water is more serious,among which the contamination caused by chromium(Cr)has drawn public attention widely on account of its toxicity and some detrimental characteristics such as teratogenesis,carcinogenicity and mutagenicity.It primarily exists in groundwater in trivalent(Cr(III))and hexavalent form(Cr(VI)).Cr(VI)is highly toxic,soluble and mobile in aqueous solution.In contrast,trivalent chromium(Cr(III))is relatively less toxic and not easily mobile,and has comparatively chemical stability.Taking steps to transform Cr(VI)into Cr(III)can reduce its toxicity and form precipitation,thereby,we can bring the pollution under control with effect.In recent years,nanoscale zerovalent iron(nZVI)is one of the most promising materials used for environment modification because of its advantages including high reducing capability,large specific surface area,low cost and environmental benignity etc.However,oxidation and agglomeration are common problems of nZVI,which decrease specific surface area and reaction activity.Taking porous supporting materials as carriers can enhance the dispersity of nZVI and decrease agglomeration.The study intend to take activated carbon fibre(ACF)as supporter to prepare nZVI composite(nZVI-ACF composite)on account of its traits of cheap and available,large specific surface area and well-developed microporous structure,used to remove the Cr(VI)existed in underground water.It centres on the preparation and characterization of nZVI-ACF composite,the influencing factors and mechanism of Cr(VI)removal using nZVI-ACF composite.Specific research results are as follows:1.The nZVI and supported nZVI particles were prepared by frequently-used liquid phase reduction method.The effects of the kind of iron salts,the concentration of iron ion and sodium borohydride,the volume ratio of absolute ethyl alcohol and deionized water of solvent on the reaction activity of nZVI were investigated.Also,the morphology and structure of nZVI and nZVI-ACF composite were characterized by field emission scanning electron microscope(FESEM),energy dispersive X-ray detector(EDX),X-ray powder diffractometer(XRD),and specific surface area and particle size analyser.The results showed that n ZVI prepared by FeCl3?6H2O had higher reaction activity;The nZVI possessed the highest activity when the concentrations of iron ion and sodium borohydride were 0.045 mol L-1 and 0.5 mol L-1,respectively,and the volume ratio of absolute ethyl alcohol and deionized water of solvent was 3:7;the synthesized nZVI particles formed a prominent chain-like structure and the ACF was beneficial to elevate the dispersibility of bare nZVI particles;nZVI is amorphous,and the ACF had no impact on the structure of nZVI.The specific surface areas of nZVI and nZVI-ACF composite were 12.96 and 668.54 m2 g-1,respectively.2.The influences of mass ratio of nZVI and ACF of nZVI-ACF composite,the dosage of nZVI-ACF composite,the concentration of Cr(VI),pH and the co-existing ions on the removal efficiency of Cr(VI)were studied.The results indicated that at pH 3.0,98.34% of Cr(VI)removal efficiency was achieved from aqueous solution containing 8.99 mg L-1 of Cr(VI)by 0.40 g nZVI-ACF composite within 3 min.The removal of Cr(VI)enhanced with the increase of n ZVI:ACF mass ratio or dosage of nZVI-ACF composite.However,the Cr(VI)removal efficiency decreased with the increase of pH and the concentrations of Cr(VI).Some anions inhibited the removal of Cr(VI),especially in high concentration,such as NO3-and HCO3-.The negative impact of Mg2+,Ca2+ and Cl-was insignificant.3.The structure and surface composition of composite before and after reaction were analyzed by XRD and X-ray photoelectron spectrometer(XPS),designed to probe into the mechanism of Cr(VI)removal by n ZVI-ACF composite.The results manifested that Cr(VI)transformed into Cr(III)by n ZVI and ferrous ions,and formed some products,such as ?-Fe2O3 or FeCr2O4,Fe3O4,and Cr2O3 or Cr(OH)3.