Reduction Of Bromate By Chemical Method And Electrochemical-Coupled Anaerobic Microbial Community

Bromate is an oxyhalid disinfection byproduct（DBP） during chlorination or ozonation of bromide-containing water, which has been classified as a group II carcinogen（as a possible human carcinogen） by the International Agency for Research on Cancer（IARC）. Nowada ys, the pollution of bromate become more serious due to chlorination and ozonation widely used in water purification, and a large number of food additives（potassium bromate） discharged into surface water. In this paper, the chemical, electrochemical and the bio-electrochemical technology are investigated for remediation of bromate, more details are as follows:（1） Firstly, the Fe（II）-Al（III） layered double hydroxides（Fe-Al LDHs） were prepared by the ultrasound-assisted co-precipitation method, which have adsorption-reduction ability for reduction of bromate. The Fe-Al LDHs particles were characterized by X-ray Diffractometer（XRD）, Scanning Electron Microscopy（SEM）, Thermogravimetry Differential Scanning Calorimetry（TD-DSC） and Fourier Transform Infrared Spectroscopy（FT-IR） spectrometer. Batch experiments were performed to study the reaction mechanism. It was found that The assistance of ultrasound irradiation promoted the formation of the hydrotalcite-like phase and then improved the removal efficiency of bromate. When the initial bromate concentration was 7.81 μmol/L, the Fe-Al LDHs with irradiation time of 30 min exhibited the optimum removal efficiency, which can remove bromate completely within 120 min. The effects of solid-to-solution, initial bromate concentration, initial p H and coexisting anions on the bromate removal were investigated. The results showed that the influence of coexisting ions was slight. The possible mechanism for bromate removal by Fe-Al LDHs involved two-step reaction: firstly, bromate in solution was absorbed quickly onto the Fe-Al LDHs by ion exchange between sulfate and bromate. Then, the adsorbed bromate was reduced by Fe2+ and the innocuous reduced products, bromide, entered into the solution. In addition, the fixed-bed column experiments were carried out in continuous model to study the reaction kinetics. Two kinetic models, namely BDST and Thomas model were applied to predict and determine the characteristic parameters. The results indicated that the whole breakthrough curve at various experimental conditions was effective fitted by the Thomas model. but BDST model was effective to fit the initial part（1-10%） only. The maximum removal capacity（N0） calculated by Thomas model reached 71.01 μmol/g at flow rate 3 m L/min. Based on the value of N0, lower flow rate and higher bed depth was benefit to remove bromate by Fe-Al LDH in fixed-bed column（in chapter 2）.（2） This chapter explored a new approach to reduce bromate base on electrochemically induced pitting corrosion of Ti anode. Difference from conventional electrochemical reduction of bromate occuring at the cathode, this study investigated the reduction process of bromate via electrochemically induced pitting corrosion of titanium（Ti） anode. The Ti electrode was oxidized to produce reactive titanium ions, e.g., Ti²⁺ or Ti³⁺, led by electrochemically induced pitting corrosion. These multivalent Ti species effectively reduced BrO₃^- to Br^-. The pitting potential（EPit） of Ti electrode was 1501 m V at p H 7.0. Related e xperiments were also conducted to evaluate the effects of terminal potential, electric current, initial p H and initial bromate concentration on bromate reduction. Experimental results showed that it was beneficial to reduce bromate at lower p H and lower in itial bromate concentration. However, the product analysis showed that the amount of reduced bromate was not in accordance with that of generated Br-, and a 19.8% loss of bromine mass should contribute to the formation of solid by-products（Ti O₂）. Even if the coexisting anions(Cl-, NO₃^-, and SO₄^2-), bromate at initial concentration of 100 μg/L also could be reduced to below the maximum contaminant level（MCL） of 10 μg/L, as well as the Cl- and NO₃^- were reduced simultaneously. The decrease in chloride concentration should be attributed to the oxidation of Cl- to Cl₂. Nearly 80% of NO₃^- was removed during electrochemical reduction process, which should be attributed to the conversion of NO₃^- to NH₃ or N₂. Although bromate reduction by electrochemically induced corrosion of Ti is feasible, its practical application is limited by the high energy costs and unwanted Ti dissolution（ in chapter 3）.（3） An auto-hydrogenotrophic rotating biofilm-electrode reactor（RBER） was designed for bromate removal. The RBER combined electrochemical method with biological method, and the community of denitrifying bacteria immobilized on the cathode surface could completely utilize hydrogen（H₂） as the electron donor, which was internally produced by the electrolysis of water. The cathode of RBER was designed to automatically rotate so as to enhance the immobilization of biofilm on the cathode and achieve the complete mixing condition. The running tests confirmed that the RBER system could reduce 150-800 μg/L bromate to below 10 μg/L under autotrophic conditions. The reduced bromate was considered to be roughly equivalent to the amount of bromide in effluent, indicating that bromate was completely reduced to bromide without accumulation of by-products. The results of different current and hydraulic retention time tests suggest that when H₂ delivery was insufficient to completely reduce both electron acceptors, nitrate reduction out-competed bromate reduction for electrons from H₂. The maximum bromate reduction rate estimated by the Monod equation was 109.12 μg/L·h when the electric current was 10 m A and HRT was 12 h than the rates that compared with fixed-film bioreactor, ion exchange membrane bioreactor and fixed bed column reactor, the bromate reduction rate in RBER was greater. It was proposed that the electron transfer process in RBER produced H₂ on the surface of the ACF cathode, and the microbial cultures a ttached closely on the cathode which could completely utilize H₂ as electron donors for reduction of bromate, while bromate and nitrate could reduce completely when H₂ was not limiting in RBER（in chapter 4）.（4） The high-throughput sequencing was employed to investigate the microbial communities of six biofilm samples from different stage of RBER. According to 16 S r RNA gene sequencing, the bromate-reducing bacteria are phylogenetically diverse, representing the Firmicutes, Proteobacteria, Bacteroidetes and Actinobacteria. Relative abundances of these bacterial communities represented 99.1% of all phylum in the biofilms at 90 th days. The dominant genus-level bacteria were Bacillus, Pseudomonas and Lactococcus. These communities represented 64.3% of all genus in the biofilms when bromate is served as the sole electron acceptor. Moreover, three genus communities including Exiguobacterium（7.37%）、Arthrobacter（1.81%） and Chlorobium（0.52%） were appeared in this stage, its might be the specific bromate-reducing bacteria（in chapter 5）.