Removal Of Chlorite And Chlorate By Mg-Al Layered Double Hydroxides

The drinking water safety and our lives are closely interrelated. As the fourth-generation drinking water disinfectant, chlorine dioxide?ClO₂? has excellent characteristics which include high efficiency, low toxicity, broad spectrum and high-performance. However, the unavoidable formation of inorganic disinfection by-products in disinfection process, such as chlorite?ClO₂^-? and chlorate?ClO₃^-?, would have potential hazard toward public health. Chlorite and chlorate were classified as carcinogen and moderately toxic compounds by the International Agency for Research on Cancer?IARC?, respectively. According to Standards for Drinking Water Quality?GB5749-2006?, both the maximum contaminant level?MCL? of chlorite and chlorate in drinking water are 0.7 mg/L. With the limited ClO₂^-and ClO₃^-removal technologies and their shortcomings, the removal of ClO₂^-and ClO₃^-by Mg-Al layered double hydroxides?LDHs? and their calcined products were studied in this paper.A series of Mg-Al, Zn-Al, Mg-Fe LDHs, which have different metal composition and metal atomic ratios, were synthesized by co-precipitation hydrothermal method, and then calcined products were obtained under different calcination temperatures. The interlayer structure, thermal stability,surface properties, morphology and particle size of the samples were characterized by means of X-ray diffraction?XRD?, BET surface area,Fourier Transform Infrared Spectroscopy?FTIR? and Scanning Electron Microscopy?SEM? etc. The removal of ClO₂^-and ClO₃^-under different calcination temperatures by the synthetic LDHs with different metal composition and metal atomic ratios were studied. Results showed that the LDHs with calcination temperatures of 500? and metal atomic ratio of 2had the best adsorption effect on ClO₂^-and ClO₃^-. The adsorption capacity of Mg-Al and Mg-Fe LDHs were equivalent, however, both were better than the Zn-Al.On this basis, the effect of contact times, adsorbent dosages,co-existing anions, initial pH and concentrations on the ClO₂^-and ClO₃^-removal by 2Mg-AlC-500 were investigated. Meanwhile, the stability and recyclability of the above adsorbent were also studied. Result showed that the removal rate of ClO₂^-and ClO₃^-could reach 99% and 90% respectively for below conditions: the temperature of 25?, the pH value of 7.0, the2Mg-AlC-500 dosage of 0.8g/L, and the initial ClO₂^-and ClO₃^-concentration of 10 mg/L.Moreover, the kinetics research suggested that the adsorption of ClO₂^-and ClO₃^-by 2Mg-AlC-500 was consistent to the pseudo second-order reaction kinetics model. According to the results of thermodynamic research, the Langmuir adsorption isotherm was more suitable to characterize the adsorption process of ClO₂^-and ClO₃^-?2Mg-AlC-500 as the adsorbent?. Besides, the thermodynamic parameters indicated that the adsorption behaviours of 2Mg-AlC-500 on ClO₂^-and ClO₃^-were spontaneous exothermic processes.