Reductive Removal Of Bromate In Drinking Water Using NZVI Modified By Nitrogen Doped Carbon

The shortage and unsafety of drinking water are serious problems that need to be resolved urgently in China,especially in the central and westrern regions.Disinfection is essential to ensure the quality of drinking water.Ozone is wildly used as a disinfectant for drinking water.However,when using ozone as the disfectant,bromate,a kind of disinfection byproduct,will be produced.Bromate is classified as a Group 2B carcinogen,by the International Agency for Research on Cancer?IARC?,and the"Standards for drinking water quality"revised by China in 2006 stipulates that the maximum contaminant level of bromate in drinking water is 10?g L^-1.It is crucial to develop an efficient treatment for bromate removal in drinking water.Nanoscale zero-valent iron?nZVI?has gain extensive attention during the past decades as a most promising material for the remediation of contaminated groundwater and wastewater.It has been proven efficient in the reductive removal of various pollutants including bromate.The advantages of nZVI include the large surface area,high reactivity,and low cost.Nevertheless,pristine nZVI often suffers from leaching,fast aggregation originating from the high surface energy and intrinsic magnetic interactions and easy surface oxidation in air/water,which significantly reduces the reactivity and stability of nZVI and limits its practical applications.To address this issue,the nZVI composite materials modified with nitrogen doped carbon,was designed and prepared by two different strategies and used for bromate reductive removal.X-ray diffraction?XRD?,transmission electron microscope?TEM?,X-ray photoelectron spectroscopy?XPS?and other characterization methods were used to analyze the structural properties of the material.Potentiometric mass titration method and water contact angle measurement were used to characterize the surface properties of these materials.The effects of carbonation temperature,composite dosage,initial concentration and p H were also studied.The main research results are as follows:?1?A novel nZVI material motified by nitrogen doped carbon?i.e.,nZVI@MOF-CN?was prepared in situ through a high-temperature carbonthermal process,by using a metal organic framework MIL-100?Fe?,as the self-sacrificing template and and an ionic liquid,1-butyl-3-methylimidazoloum chloride as the nitrogen source.The characterization results show that iron nanoparticles with a diameter of about 30 nm were evenly dispersed in the nitrogen doped carbon,and most of the nanoscale iron was in zero charge.The content of nZVI in the composite increased with the increase of carbonization temperature,but nitrogen content decreased simultaneously.The reduction activity results showed that bromate reduction by nZVI@MOF-CN followed the pseudo-first-order kinetic model.nZVI@MOF-CN-600,which was obtained by carbonization at 600,had the highest efficiency for bromate reduction.The iron content normalized pseudo-first-order rate constant(k_m,L min^-1 g^-1)of nZVI@MOF-CN-600was determined to be 7.06×10^-1 L min^-1 g^-1,which was approximately 45 and 149times higher than that of tested commercial nZVI(1.56×10^-2 L min^-1 g^-1)and carbon modified nZVI(i.e.,nZVI@MOF-C-600,4.75×10^-3 L min^-1 g^-1).This observation suggested the significant enhancement effect of N-doped carbon modification on nZVI activity.Moreover,nZVI@MOF-CN-600 exhibited a good removal efficiency for bromate in a wide p H range?4.0-9.0?.The high reduction activity and ease of synthesis highlight the potential application of nZVI@MOF-CN for bromate removal.?2?Another different nZVI material with nitrogen-doped carbon shell,nZVI@DCN,was prepared by a double coating strategy.After nZVI particles were obtained from hydrogen reduction of Fe₃O₄,which was synthesized by hydrothermal method,dopamine and ethylenediamine/carbon tetrachloride were used as the first and second carbon and nitrogen precursors,respectively.The characterization analysis results showed that iron particles with a diameter of 20-40 nm were coated by nitrogen doped carbon shell with a thickness of about 4 nm.Initial activity(r₀,m M g^-1 Cat h^-1)was used to characterize the reaction activity of the materials for bromate reduction.By comparing the initial activity of these series of materials,it was found that the adsorption is the rate-limiting step in the nZVI reduction reaction for bromate.The PZC and the surface hydrophilicity of the material strongly affect the adsorption of bromate and the reduction efficiency.The reduction activity results show that the r₀ for bromate reduction by nZVI@DCN-800 was determined to be 14.54 m M g^-1 Cat h^-1,which was approximately 2.26 and 16.34 times higher than that of tested nZVI and nZVI@C-600.Beside the high reduction activity,nZVI@DCN-800 has a very low iron leaching level(24.18?g L^-1)compared to nZVI(1015.65?g L^-1)and single layer coated materials(nZVI@CN-600,184.13?g L^-1).After 5 cycles of reduction or using environment water as the background solution,removal activity of nZVI@DCN-800 was barely changed,suggesting that it had good stability.Thus,nZVI@DCN is a promising nZVI materials for practicl applications.