Performances And Mechanisms For The Removal Of Carcinogenic Bromate And Hexavalent Chromium Lons From Water By Several Non-noble-metal-based Materials

Bromate and hexavalent chromium are commonly-found carcinogenic pollutants in drinking water,which are strictly regulated by the heath and environmental agencies.Catalytic reduction method to remove bromate is an efficient and clean process and adsorption method has the advantages of simple operation and high efficiency to remove hexavalent chromium.This study was focused on the catalytic reduction of bromate to non-toxic bromine ion and the removal of harmful heavy metal hexavalent chromium by high effective adsorbents.A series of non-noble-metal catalysts and MXene adsorbents were developed for effective removal of bromate and hexavalent chromium to below the national and international drinking water standards.1.A facile hydrothermal process was developed to synthesize CoS2 hollow spheres using cobalt(Ⅱ)acetate tetrahydrate as the Co source and carbon disulfide as the S2-source and.soft template,respectively.Compared to particle catalysts,hollow spheres catalysts were offered the advantages of the increased specific surface area and exposed more active sites.CoS2 hollow spheres as catalysts showed a superior catalytic activity in catalytic reduction of bromate in water under atmospheric pressure at room temperature.CoS2 hollow spheres showed a superior tolerance of co-existing ions and catalytic stability,which could serve as a substitution of noble metal catalysts for catalytic reduction of bromate in drinking.The catalytic reduction mechanism of bromate on CoS2 hollow spheres:H2 could adsorb onto CoS2 hollow spheres and create Co-active sites,H2 could then directly dissociate into hydrogen atoms on Co-active sites and reduce bromate adsorbed on CoS2 hollow spheres to bromide.2.Ti3C2Tx-O MXene with high-proportioned oxygen terminations,was prepared by a facile two-step method of HF etching followed with ethanol heating from Ti3AlC2 and tested in catalytic reduction of bromate as another non-noble metal catalyst.Compared to Ti3C2Tx,oxygen terminations in Ti3C2Tx-O MXene increased the catalytic reduction rate by a factor of 2,which was due to a proper interaction between Ti3C2Tx-O and H atom.Catalytic reduction tests conducted at differen temperatures showed that the catalytic bromate reduction reaction was endothermic with an Arrhenius activation energy of 46.4 kJ/mol.This work demonstrated that the application of two-dimensional MXenes could be expanded to various hydrogenation reactions as a replacement of noble metal-based catalysts.3.Nitrogen-containing MXene,Ti3CNTx was prepared by delaminating Ti3AlCN phase using the LiF/HCl-etching method and tested for reduction of heavy metal ions.Using Cr(Ⅵ)as an indicator,the tests demonstrated that Ti3CNTx could adsorb and reduce Cr(Ⅵ)to Cr(Ⅲ)more effectively than Ti3C2Tx MXene largely because the-NR2 surface groups on Ti3CNTx sheets and a large number of weaker Ti-N bonds replaced the stronger Ti-C bonds.In addition,both the-NR2 groups and Ti-N bonds on the Ti3CNTx surface enabled efficient adsorption of the reduced Cr(Ⅲ),resulting in complete removal of chromum ions in water.Within 5 min treatment,an excellent removal performance of 260 mg/g was achieved by Ti3CNTx MXene.4.A series of high quality Ti3AlCxN2-x MAX phases were made by a novel Sn assisted synthesis method at a relatively low temperature of 1350℃ for the first time.It was found from the XRD,SEM,and XPS analysis that N atoms had replaced C atoms in the Ti3AlCnN2-n crystal phases to form the solid solutions.In the product,the weight percentage of Ti3AlCN MAX phase could reach as high as 95%.By the HF etching process,Ti3AlC2-xNx phases could be successfully converted to accordion-like Ti3C2-xNxTx phase.The Sn assisted synthesis method will exploit high-yield double transition metal MAX and other solid solution MAX at a relatively low temperature and expand double transition metal MXene and MXene solid solution potential application.