城市与区域生态国家重点实验室知识库

      甲基硅氧烷类有机物，作为工业添加剂大量应用于纺织行业、涂料行业等，也作为助剂加入到个人洗护用品中，最终汇入污水处理厂和垃圾填埋场中。在后续的厌氧化处理过程中，会进一步分解成为小分子的挥发性甲基硅氧烷（Volatile methylsiloxanes，简称VMS），存在于并严重妨碍了厌氧后沼气的资源化利用。此外，许多硅基聚合物、弹性体和树脂在生产中，也使用到VMS中间体，在此过程中有大量VMS逸散进入到大气中，参与了大气反应生成二次气溶胶。因此，从资源化利用和工业污染控制角度来说，VMS的污染控制均具有重要意义。本研究研究考察了多孔炭和分子筛类纳米吸附剂对特征VMS污染物六甲基二硅氧烷（Hexamethyldisiloxane，简称L2）的吸附过程和吸附机理，阐明影响吸附剂吸附性能的主要影响因素。研究的主要内容及取得的结论和成果如下： 
      （1）多孔炭类吸附剂对L2的吸附性能
      采用颗粒活性炭、活性炭纤维和介孔炭作为吸附剂，通过静态蒸汽吸附的方法研究了吸附剂对L2的吸附等温线，并计算了等量吸附热和吸附速率常数。
研究结果表明，Dubinin-Astakhov (D-A)方程可较好地对三种吸附剂吸附L2的吸附等温线进行拟合；三种吸附剂对L2的等量吸附热和吸附动力学有较大的差别，这种差别推测主要是由于吸附剂孔径尺寸不同引起的。
      （2）分子筛对L2的吸附性能
      考察不同结构的分子筛（Hbeta、SBA-15、MOR和MCM-41）对L2的吸附性能，研究结果表明分子筛对L2的初始吸附作用力大小为Hbeta > SBA-15 > MOR > MCM-41，这与各分子筛的孔径尺寸有关；此外，三维孔道更有利于L2的扩散，当Hbeta分子筛尺寸为1纳米时，对L2的扩散速率即接近于3纳米的二维孔道的MCM-41，进一步采用大孔道尺寸的SBA-15，扩散速率并没有显著增加，而小孔道的MOR对L2的扩散速率较低。
      （3）影响分子筛对L2的吸附性能的主要因素
      当分子筛的孔径小于L2的分子动力学直径时，吸附质难以进入吸附剂内部发生吸附作用；MOR、ZSM-5和ZSM-35三种分子筛对L2无吸附作用；微孔孔容和微孔面积是决定分子筛吸附量的重要因素，而介孔孔容不利于L2的有效吸附。
 

      Volatile methylsiloxanes (VMS) are widely used as industrial additives in textile industry, paint industry, etc. They are also added to personal cleaning products as additives, and eventually discharged into sewage treatment plants and landfills. In the subsequent anaerobic treatment process, VMS will be further decomposed into small molecules, which exist and seriously hinder the utilization of anaerobic biogas. In addition, many VMS intermediates are also used in the production of silicon-based polymers, elastomers and resins, during which a large number of VMS escape into the atmosphere and participate in the formation of atmospheric secondary aerosols. Therefore, from the perspective of resource utilization and industrial pollution control, removal of VMS is of great significance. In this study, nano-adsorbents, including porous carbon and zeolites were selected as adsorbents to investigate their adsorption properties towards characteristic VMS pollutant, i.e., hexamethyldisiloxane (L2). The main factors affecting the adsorption performance of adsorbents were elucidated. The main contents, achievements and conclusions of the study are as follows:
      (1) Adsorption properties of porous carbon adsorbents towards L2
Static vapor adsorption method was used to investigate the adsorption of granular activated carbon, activated carbon fibers and mesoporous carbon towards L2. Their isosteric heats of adsorption and adsorption rate constants were calculated.
       The results show that Dubinin-Astakhov (D-A) equation can well fit the adsorption isotherms of three adsorbents. There are great differences in their adsorption heats and kinetics of L2 among the three adsorbents, which might be mainly due to the different pore sizes of adsorbents.
       (2) Adsorption properties of zeolites towards L2
      The adsorption properties of different zeolites (Hbeta, SBA-15, MOR and MCM-41) on L2 were investigated. The results show that the initial adsorption interaction of zeolites on L2 follows an order of Hbeta > SBA-15 > MOR > MCM-41, which might be related to the pore size of each zeolite. In addition, three-dimensional pore channels are more conducive for the diffusion of L2. Adsorption rate constant of Hbeta with a pore size about 1 nanometer is close to that of MCM-41 with a pore size of 3 nanometers. The diffusion rate constant of SBA-15 does not increase significantly even through its channel size is larger, while the diffusion rate of L2 on small channel MOR is even lower.
       (3) Main factors affecting the adsorption performance of zeolites towards L2
      When the pore size of zeolite is smaller than the molecular dynamics diameter of L2, it is difficult for adsorbates to enter into the adsorbent to adsorb. Thus, no L2 was adsorbed on MOR, ZSM-5 and ZSM-35. Micropore volume and micropore area are important factors determining the adsorptive capacity of molecular sieves, while mesoporous volume is not conducive to the effective adsorption of L2.