Selective Adsorption Of Pollutants By Functionalized Mesoporous Molecular Sieves MCM-41 From Aqueous Solution

Water pollution has become a serious problem with the development of socioeconomics. The conventional process of micropolluted water treatment was found to be obviously insufficient with the improvement of water quality demands. Thus, it is necessary to develop a high efficient, economic and convenient water treatment technology. Considerable attentions have been paid to adsorption technologies as efficient and versatile methods for water treatment. However, most of normal adsorbents have a low adsorption capacity and poor selectivity. Therefore, based on the hot issue of micropolluted water treatment, functionalized mesoporous silica materials were synthesized in this thesis as a selective adsorbent for water treatment. The adsorption behavior and mechanism were investigated in detail in order to provide effective technical support of water quality safety. In addition, functionalized mesoporous silica materials also offered an effective solution and method for water treatment of special pollutants.Concerning with the characteristics of pollutants, investigation were conducted to study the adsorption mechanism and application prospect in water treatment by using mesoporous molecular sieves (MCM-41). The content of this thesis consists of four parts: (1) investigation on the nitrobenzene adsorption by MCM-41 from aqueous solution; (2) study on the nitroaromatics adsorption by methylation MCM-41 (CH₃-MCM-41) and discussion on the adsorption mechanism; (3) research on the anionic dyes adsorption by ammonium-functionalized MCM-41 (NH₃⁺-MCM-41); (4) investigation on the Hg (II) adsorption by thiol-functionalized MCM-41 (SH-MCM-41).Adsorption of nitrobenzene onto MCM-41 from aqueous solution was investigated systematically using batch experiments. Results indicated that nitrobenzene adsorption capacity was 30 times and 10 times higher than phenol and p-nitrophenol, respectively. The nitrobenzene adsorption was initially rapid and the adsorption process reaches a steady state after 1 min. The equilibrium time was independent of initial nitrobenzene concentration. The amount of nitrobenzene adsorbed decreased with an increase of temperature, pH and ionic strength. However, the amount of nitrobenzene adsorbed onto MCM-41 did not show notable difference in the presence of cations (K⁺, Na⁺, Ca²⁺) and humic acid (0⁵0mg/L). The desorption process showed a reversibility of nitrobenzene adsorption onto MCM-41. After regeneration, the nitrobenzene adsorption decreased. Based on the results, it suggests that the adsorption is primarily brought about by hydrophobic interaction between nitrobenzene and MCM-41 surface.As the structure of MCM-41 could collapse by mechanical compression through the hydrolysis of siloxane bonds in the presence of adsorbed water, Trimethylchlorosilane was used to modification of MCM-41. Characterization results indicated that pore diameter and specific surface decreased, and the stability was improved after methylation; methyl group loading (calculated by C) was 5.74%. Researches were conducted in a batch reactor system to study the adsorption behavior of nitroaromatics by CH₃-MCM-41. Results showed that nitroaromatics can be effectively removed by CH₃-MCM-41 than MCM-41. And the adsorption process was exothermic. One obvious feature was that substituents with strong electron withdrawing properties strongly enhanced adsorption and substituents with electron donating group decreased adsorption. Both pH and ionic strength had a little effect on non-ionized chemicals adsorption. However, for ionized chemicals, the amount of adsorption decreased with the increase of pH and increased with the increase of ionic strength. Competition adsorption can be neglected for nitroaromatics adsorption by CH₃-MCM-41. From the evidence given above, the formation of an electron donor-acceptor complex between electron donor functions at the siloxane surface and the aromatic ring system of the nitroaromatics may be postulated as a plausible mechanism of interaction. Through breakthrough experiments, dynamic capacity of the fixed-bed column decreased with the increase of flow rate and the adsorption rate increased with the increase of flow rate. 3-aminopropyltrimethoxysilane was used to modification of MCM-41.Characterization results indicated that pore diameter and specific surface decreased and the structure characteristics keep unchanged after modification; the N loading was 1.8%. NH₃⁺-MCM-41 was used to study the adsorption behavior of four anionic dyes (Methyl orange (MO), Orange IV (OIV), Reactive brilliant red X-3B (X-3B), and Acid fuchsine (AF)) from aqueous medium. Results indicated that NH₃⁺-MCM-41 had a high affinity to four types of anionic dyes, MO, OIV, X-3B and AF. The amount of dye uptake was independent of initial dye concentration. Langmuir model was successfully applied to show that the adsorption was localized to a monolayer. The adsorption capacity followed the order of MO>OIV>AF>X-3B. The adsorption process was endothermic. The adsorption kinetic studies showed that the adsorption process followed intraparticle diffusion model and more than one process affected the adsorption. The low values of the intraparticle diffusion rate constant indicated the significant influence of intraparticle diffusion on the kinetic control. At pH range from 4.0 to 8.0, the adsorption capacity of dye was slightly changed by pH. However, at pH above 8.0, the adsorption capacity of dye decreased significantly. The zeta potential of NH₃⁺-MCM-41 decreased after dye was adsorbed, showing that the electrostatic interaction was considered as the main mechanism for the dye adsorption. In the presence of competitive anions, dye uptake was notably inhibited by the anion of very soft acid.Finally, 3-mercaptopropyltrimethoxysilane was used to modification of MCM-41. Characterization results indicated that pore diameter and specific surface decreased and the structure characteristics keep unchanged after modification; the S loading was 4.04%. SH-MCM-41 was used to remove Hg (II) from aqueous solution. Results indicated that the pseudo-second-order kinetic model was successfully applied to fit the experimental data and adsorption isotherms can be fitted well using Langmuir model. The adsorption capacity increased with the increase of temperature, showing that the adsorption was endothermic. Hg (II) can form complex with–SH in 1:1. The solution pH had no significantly effect on Hg (II) adsorption by SH-MCM-41. However, the adsorption capacity decreased with the increase of NaCl concentration. Pb (II) and Cd (II) can not be adsorbed by SH-MCM-41. Finally, the desorption experiments showed that SH-MCM-41 had a high affinity to Hg (II).