Design of chemically treated activated carbon fibers for mercury removal and advanced membranes for water purification

mercury removal;activated carbon fibers;chemical modifications;crosslinking;solvent resistance;multilayer polyelectrolyte;antifouling

Yao, Yaxuan

关键词: mercury removal;    activated carbon fibers;    chemical modifications;    crosslinking;    solvent resistance;    multilayer polyelectrolyte;    antifouling;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/45632/Yaxuan_Yao.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

There are mainly two projects in this thesis, one is to develop chemically treated activated carbon fibers for mercury removal from power plant effluents, and the other one is to design advanced nanofiltration membranes for water purification with specific features, i.e. solvent resistance and antifouling properties. The current technologies for mercury removal involve the use of chemically treated activated carbon powder has had limited success. These systems present practical problems in dealing with the large amount of absorbents required to insure quantitative removal of the Hg. The system we developed depends on using a chemically treated high surface area activated carbon fibers (50~600 m2/g), which provides very effective contact efficiency with the power plant effluent. For chemical modifications of activated carbon fibers, sulfur and bromine containing groups were introduced into the carbon matrix. Generally, sulfur impregnations decrease surface area and pore volume but increase the Hg uptake capacities when compared to untreated activated carbon fibers. For our sulfur-treated samples, sulfur atoms were incorporated into the carbon matrix in the form of sulfide and sulfate. The sulfide groups appeared to be more effective for mercury removal than sulfate, which was probably because the lone pairs of sulfide groups could act as the interaction site for Hg adsorption, or at least the initial point of attachment. Three approaches were explored for bromination; namely, 1) bromination using Br2 vapor, 2) bromine deposition by an electrochemical reaction and 3) impregnation of bromine using KBr solution. Both static and dynamic tests were carried out to measure the mercury adsorption performances of these brominated samples. For the brominated samples treated by Br2 vapor and electrochemical method, they showed stable mercury adsorption performance (30% to 33% removal) up to 3 months, which showed great potential promising for commercialization. A possible mechanism for mercury adsorption, which was likely to involve the formation of oxidized mercury complexes (e.g. [HgBr]+, [HgBr2] and [HgBr4]2-), was also discussed. Besides the chemical structures, pore properties also play an important role on mercury adsorption performance at room temperature. Usually, micropores are mainly responsible for mercury adsorption while mesopores may serve as transportation channels. However, physical adsorption capability decreases due to desorption at high temperatures. Nanofiltration membranes can be used to separate salts and small molecules from the solution by applying a pressure. By far the most processes is dealing with aqueous solutions, however, with the emerging of membranes usage in food applications, petrochemical applications and pharmaceutical areas, the membranes suited for applications in organic media are required. To address this problem, the crosslinking of polyimide membranes is a commonly known method to prepare membranes suitable for solvent resistant nanofiltration. In this work, preparation of crosslinked membranes of P84 copolyimide asymmetric membranes using branched polyethylenimine (PEI) at different reaction temperatures was studied. The rejection sequence of CaCl2 > NaCl > Na2SO4 indicated a positively charged membrane surface. Additionally, the resultant membranes were very stable in dimethyl formamide (DMF), a harsh aprotic solvent. Even after soaking in DMF for 1 month, there were no significant changes in membrane performance or membrane structure. Fouling caused by organic impurities such as proteins, humic substances and polysaccharides is another concern for membrane processes. Polyelectrolyte multilayer (PEM) films consisting of sulfonated poly (ether ether ketone) sPEEK alternating with selected anionic layers were developed for fouling resistant properties. Two novel variables were introduced in our approach, a) the use of pressure and b) organic solvents, during the alternating physisorption of oppositely charged polyelectrolytes on porous supports through the electrostatic self-assembly. It was shown that the use of pressure and/or organic solvent systems could increase the salt rejection of the PEMs by several times while still remaining a high water flux. The PEMs also had a better antifouling property in comparison with NTR 7450, a commercial NF membrane with a sulfonated surface.