Mesoporous molecular sieve MCM-41 for wastewater treatment

1 Synthesis and modification of mesoporous molecular sieve MCM-41

1.1 The basic principle of synthesis of MCM-41
Mesoporous molecular sieve MCM-41 generally uses surfactants with longer carbon chains as templates for synthesis. Surfactant consists of two parts, a hydrophilic layer and a hydrophobic layer. As it rises to a certain concentration in the aqueous solution, it will form a rod-like aggregate form. The concentration of the surfactant at this time is called the critical micelle concentration. The research on the synthesis mechanism of MCM-41 mesoporous molecular sieve mainly revolves around the state of surfactant in the solution system. The typical synthesis mechanisms are: liquid crystal template mechanism, synergy mechanism, charge matching mechanism, and phase structure shift mechanism.

1.2 The basic method of synthesizing mesoporous molecular sieve MCM-41
With the continuous deepening of research on MCM-41, many improvements have been made in the synthesis process, and many mesoporous molecular sieves MCM-41 with better structures and properties have been successfully synthesized. Congyan Chen et al. used an improved hydrothermal crystallization method to successfully prepare MCM-41 mesoporous molecular sieves with a very low silicon-to-aluminum ratio. Using NMR detection, no aluminum oxide octahedron was found in the framework. It was characterized by NH3-TPD. -41 acidity is similar to amorphous aluminum silicate. ACVoegtlin et al. investigated the effect of pH (pH 8.5~12) on the synthesis of pure silicon MCM-41 molecular sieve at room temperature, and synthesized a pure MCM-41 molecular sieve with a pore size of 2.6 nm at low alkalinity (pH=8.5). phase. Q. Huo et al. investigated the effects of cationic and anionic surfactants on synthesis under acidic conditions, and synthesized MCM-41 mesoporous molecular sieve when the surfactant concentration was lower than the critical micelle concentration. The method has a lower synthesis temperature and a smaller amount of template agent, which makes it easier to achieve scale-up production. The template required for the synthesis of MCM-41 is relatively expensive, and the process is cumbersome and the synthesis time is too long (2~7d). In order to overcome the above shortcomings, Sun Yan used microwave method for pretreatment, and the synthesis time was shortened to 10~15min. The demoulding still adopts the traditional method, calcining at 550℃ for 7h. Zhang Maisheng et al. carried out crystallization and demolding under microwave conditions. The crystallization time was 10-15 minutes, and demolding only required 1h.

1.3 Modification of mesoporous molecular sieve MCM-41
MCM-41 mesoporous molecular sieve has many advantages, but from a chemical point of view, its framework is mainly composed of amorphous SiO2. The pure silicon framework has weak acid strength and poor ion exchange capacity. These shortcomings will cause MCM-41 media The pore molecular sieve has a single chemical function. In order to expand the application fields of MCM-41, it needs to be modified. At present, the main modification methods include: metal heteroatom doping, active component loading and organic functionalization.

1.3.1 Metal doping
The main method of metal doping is to add the target metal ions to be doped into the hydrothermal reaction system before the hydrothermal synthesis. This method has successfully incorporated B, Bi, Zr and other atoms into the framework of MCM-41. The ion exchange method is to perform ion exchange between the template ions in the pores of the original molecular sieve powder without demolding and some metal ions to modify the internal framework. YeWang et al. synthesized V-MCM-41 by using an ion exchange method to exchange template ions with vanadium ions. T.A.Konovalova et al. used this method to synthesize Ni-MCM-41. The post-grafting method reacts with metal oxides, metal alkoxides, etc. through the silanol bonds on the surface of the channel silicon oxide, and the metal is fixed on the framework of MCM-41 through covalent bonds.

1.3.2 Load method
The loading method uses MCM-41 as the carrier, and loads the active components that need to be added in the pores through impregnation, co-precipitation, etc. to achieve the purpose of modification. There are many kinds of supported active components, such as metal elements, metal oxides, inorganic acids, heteropoly acids and organic bases.

1.3.3 Organic functionalization
The main purpose of organic group modification is to further expand its application range by changing its surface properties while maintaining the original excellent properties of the mesoporous molecular sieve. Methods for organic functionalization of mesoporous molecular sieves mainly include grafting, co-condensation, and ordered mesoporous silicone. In 1996, D.J. Macquarrie used polycondensation to introduce 3-aminopropyltrimethyl(eth)oxysilane, a common organic basic group functionalization reagent, into mesoporous silica. H.P.Lin et al. modified MCM-41 with a chloropropyl group, and then connected organic bases such as pyrrolidine and pyrimidine to the surface of the channel. Berenguer? Juan Murcia et al. used grafting to link sulfhydryl (-SH) to MCM-41, which has a higher adsorption capacity for Hg2+ than the unmodified MCM-41. S. Carloni et al. used post-oxidation method to successfully introduce a strong acidic group—SO3H on the surface of MCM-41. J. Frasch et al. used propyltriethylsilane as a functionalization reagent to synthesize n-propyl functionalized MCM-41.

2 Application of Mesoporous Molecular Sieve MCM-41 in Wastewater Treatment
2.1 Adsorption of metals in wastewater
In the water body, heavy metals cannot be biodegraded, and can only be transformed, dispersed and enriched between various forms, and exist in the form of compounds or ions. Positively charged heavy metal ions are easily adsorbed by negatively charged colloidal particles in water. After functional modification, MCM-41 can effectively adsorb heavy metals in water by grafting sulfhydryl or amino groups on the surface, especially for removing trace heavy metals in water. D.Pérez? Juan Quintanilla et al. [23] used 3-chloropropyltriethoxysilane as a functionalization reagent, introduced chlorine functional groups on the inner surface of MCM-41, and modified the sample with 2-mercaptothiazoline to obtain The surface of MCM-41 containing sulfhydryl groups has been studied for the adsorption of mercury ions in water. The maximum adsorption capacity is 0.7mmol/g. The research group used 5-mercapto-1-methyl-1-hydro-tetrazole to modify MCM-41, prepared MTTZ-MCM-41, and carried out Zn2+ adsorption research. Under the optimal adsorption conditions (pH=8, stirring for 2h), the adsorption capacity reached 1.59mmol/g, while the adsorption capacity of unmodified MCM-41 was only 0.01mmol/g. Under the condition of pH=8, the nitrogen and sulfur atoms in the functional group are coordinated with Zn2+, but under the condition of low pH, only the sulfur atom is coordinated. The presence of ethanol and other metal ions (Cu2+, Mn2+, Ca2+ and Mg2+) in the solution did not have a great influence on the adsorption effect of MTTZ-MCM-41. After three regeneration cycles, MTTZ-MCM-41 still maintains an excellent adsorption effect. Zhang Cui et al. modified MCM-41 with mercaptopropyltrimethoxysilane to remove Pb2+ in the water. At the same time, they investigated the influence of pH and adsorption time on the removal effect, and found that higher pH conditions are more conducive to the removal of Pb2+. At low pH The H+ in the solution has a competitive effect on the adsorption site, the best adsorption time is 9h, and the maximum adsorption capacity is 36.39mg/g. K.F.Lam et al. [26] grafted aminopropyl and carboxyl groups on MCM-41 to selectively separate Cr2O72- and Cu2+ in water. In the single-component and two-component adsorption tests, when the pH is less than 3.5, NH3+-MCM-41 can adsorb 100% of Cr2O72- in water without adsorbing other cations. At pH 1.5-5.5, COO-Na+-MCM-41 only adsorbs Cu2+.

2.2 Adsorption of anions in wastewater
Under neutral conditions, because the surface of MCM-41 contains -SiOH and -Si-O-Si- functional groups, its surface is negatively charged, so to achieve the adsorption of inorganic anions in the solution, it needs to be modified to make MCM-41 The surface is positively charged. T. Yokoi et al. used Fe3+ coordinated amino-modified MCM-41 (Fe/NN-MCM-41) to remove toxic anions in water, namely arsenate, chromate, selenate and molybdate. It was found that the modified MCM-41 structure was damaged to a certain extent and the specific surface area and pore size were greatly reduced. The maximum adsorption capacity of each toxic anion was 1.56, 0.99, 0.81, and 1.29 mmol/g, respectively. A. Benhamou et al. first reamed MCM-41 through post-treatment, and used dodecyl dimethyl tertiary amine (DMDDA), lauryl amine (DDA), hexadecyl amine (HDA) for amino functionalization, and the chromate The maximum adsorption capacity is 134.6, 159.2, 184.6mg/g; the maximum adsorption capacity for arsenate is 95.2, 119.6, 86.5mg/g, which is 5~10 times higher than the adsorption capacity of untreated MCM-41 .

2.3 Adsorption of organic matter in wastewater
CKLee et al. [29] studied the effects of MCM-41 on the three basic dyes rhodamine B, crystal violet (CV), methylene green (MG) and two acid dyes, acid red 1 (AR1) and food coloring brilliant blue ( EG) Adsorption in aqueous solution, the results show that MCM-41 with stable pore structure can be used as a very effective adsorbent. QingdongQin et al. used 8-aminopyrene-1,3,6-trisulfonic acid trisodium salt (APTS) as a functional reagent to modify MCM-41 to obtain NH3+-MCM-41, and studied the effect of methyl orange in solution. The maximum adsorption capacity of, Orange IV, Reactive Brilliant Red X-3B and Acid Fuchsin were 1.12, 1.09, 0.34, 0.43mmol/g, respectively. The effects of pH and anions on the adsorption were investigated. Experiments show that the amino functionalized MCM-41 has a high affinity for four anionic dyes. In the range of pH 4.0~8.0, the adsorption capacity did not change significantly; when the pH was greater than 8.0, the adsorption capacity decreased due to the increase of hydroxide groups and the structural instability of NH3+-MCM-41. The presence of weak acid ions (CO32-, HPO42-) will significantly inhibit dye adsorption. After the dye was adsorbed, the electromotive force on the surface of NH3+-MCM-41 decreased significantly. The electrostatic reaction on the surface was the main mechanism of the adsorption reaction, which also explained the influence of pH and competing anions on the adsorption. The experiment of QingdongQin research group proved that MCM-41 can adsorb nitrobenzene in aqueous solution in a short time, and the adsorption law can better conform to the Langmuir equation; and as the pH increases from 1.0 to 11.0, the adsorption rate decreases from 54.3% 18.1%; ionic strength increased from 0.001mol/L to 0.1mol/L; adsorption capacity decreased from 2.12mol/L to 1.81mol/L. This is because methanol and acetone exist in the solution, and the co-solvent effect affects the adsorption of nitrobenzene, while the presence of humic acid has no effect on the adsorption.

3 Conclusion
Mesoporous molecular sieve MCM-41 has an ordered pore structure, high specific surface area and large pore volume. After modification, it has many advantages such as fast adsorption rate, high adsorption rate and large adsorption capacity. However, further studies are needed in some aspects: (1) Maintain the adsorption efficiency and stability of the adsorbent after regeneration. The functionalized adsorbent showed excellent performance in the first adsorption, but after desorption, the attachments on some functional groups did not detach, causing the functional groups to lose their adsorption activity. Increasing the reuse rate of the adsorbent and enhancing its service life in a wastewater environment can reduce process costs and make it easier to realize its industrialization. (2) Most of the current researches focus on the adsorption treatment of a single pollutant, and the water conditions of industrial wastewater are more complex, so research should be conducted on complex water quality. With the continuous research and exploration of the synthesis and modification mechanism of MCM-41, and the gradual maturity of the synthetic preparation process, mesoporous molecular sieve MCM-41 will surely become one of the important solid adsorbents for industrial wastewater treatment.