Performance And Mechanism Of Bromate Removal Using Zn-Al Calcined Layered Double Hydroxides

With the rapid development of industry, water resources are widely polluted by organic contaminants. However, the conventional water treatment technology could not effectively remove organic compounds, as a result of failing to meet the Standard for Drinking Water Quality. Therefore, ozone-biological activated carbon process which combines physical adsorption, chemical oxidation, and biodegradation, has gradually become the main advanced drinking water treatment technology due to its highly efficient removal of organic pollutants. However, bromate as a disinfection product is usually formed by ozonation of water containing bromide. The International Agency for Research on Cancer (IARC) has classified bromate as a Group2B potential carcinogen, and China has set bromate the maximum contaminant level (MCL) for bromate in drinking water as10μg/L(GB5749-2006). As the present technologies for bromate control or removal have some deficiencies, it is urgent to develop new bromate removal technologies. Therefore bromate removal by Zn-Al calcined layered double hydroxides was investigated in this paper. Due to its unique structural memory effect, Zn-Al calcined layered double hydroxides can effectively remove bromate from water.Firstly, a series of carbonate intercalated Zn-Al layered double hydroxides precursor (Zn-Al LDHs) with Zn/Al atomic ratio of2,3and4were synthesized by co-precipitation hydrothermal method and then calcined at various temperature of300,400,500and600℃to obtain Zn-Al calcined layered double hydroxides (Zn-Al CLDHs). X-ray Diffraction (XRD), Zeta potential, BET surface area, Scanning Electron Microscopy (SEM), Fourier Transform Infrared Spectroscopy (FTIR) and Inductively Coupled Plasma-mass Spectrometry (ICP-MS) were conducted to investigate crystallinity, isoelectric point (IEP), BET surface area, particle size, morphology and chemical constitution of samples. The batch experiments of bromate removal using Zn-Al CLDHs were carried out, the results show that the optimum calcination temperature is500℃and under the same calcination temperature, Zn-Al CLDHs with Zn/Al atomic ratio of2have a better bromate adsorption capacity.Moreover, the effect of contact time, adsorbent dosage, pH, reaction temperature, coexisting anions on bromate removal using Zn-Al2CLDHs were investigated. It also shows that Zn-Al2CLDH(5) has the highest adsorbent activity for bromate, under the condition of pH=6.9, T=20℃, Zn-Al2CLDH(5)=0.8g/L,[BrO3-]0=100μg/L. Above90%bromate was removed within1h and the highest removal rate of97%was achieved after2.5h. Moreover, it shows Zn-Al2CLDH(5) adsorption capacity is nearly the same at pH range of4.5to9.5, but dramatically drops at pH over9.5. Adsorption of bromate on Zn-Al2CLDH(5) is exothermic reaction. As a result, with the increase of reaction temperature, the removal rate decreases. Both the type and amount of coexisting anions have great effect on bromate removal, and the negative effect of common anions decreased in the order of·PO43-> SO42-> F> Cl-> ClO3> NO3-. Meanwhile, Zn-Al2CLDH(5) shows good a stability and regenerability, indicating its excellent prospects for bromate removal.Moreover, it shows that chemical reaction is the rate-controlling step and the whole adsorption process fits well with the pseudo-second-order reaction kinetics model. Thermodynamic parameters of Gibbs free energy (△G0), standard enthalpy change (△H0), and standard entropy change (△S0) are negative, indicating bromate adsorption on Zn-Al2CLDH(5) was a spontaneous, exothermic and entroy-decreasing reaction.