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各种含Cd(II)工业废水的超标排放，会给环境和人类健康造成长期的威胁。文献报道的大部分含Cd(II)工业废水浓度为0-80 mg/L，常用的重金属废水处理方法，如化学沉淀法、溶剂萃取法等难以处理低浓度重金属废水。相比之下，吸附法对此类低浓度体系具有高效率、操作简单、产生污泥少等优点，在低浓度重金属废水处理领域具有广阔的应用前景。超顺磁性吸附剂不仅结构可设计、粒径可控，还可在外加磁场作用下移动富集、凝并及流动输送迁移，易于进行磁场操控，可避免传统固定床操作床层阻力大、能耗高、易沟流及不易连续操作等问题。然而已报道的用于Cd(II)处理的磁性载体分离技术存在吸附剂的吸附容量低、Cd(II)选择性分离困难，以及磁性吸附剂难以连续化规模分离等问题。针对以上问题，本论文围绕材料组成、界面性质对分离性能影响规律开展研究，发现界面官能团种类、密度对Cd(II)吸附容量、选择性的影响规律，研发获得了高吸附容量、高选择性的Cd(II)磁性吸附剂，并开发了适用于Cd(II)磁性吸附剂连续化规模分离的气助磁分离工艺及装置。 首先针对Cd(II)吸附容量低的问题，研究了官能团修饰密度对Cd(II)饱和吸附容量的影响。制备了一系列具有不同固定化氨基密度的乙二胺(EDA)功能化磁性聚甲基丙烯酸缩水甘油酯(PGMA)微球(m-PGMA-EDA)用于Cd(II)的吸附。实验结果表明，Cd(II)饱和吸附容量随着固定化氨基密度的增大而增大，但是两者之间并不完全呈线性正相关。N/Cd摩尔比随着氨基密度的增大而减小，当固定化氨基密度达到约1.25 mmol/m2后，N/Cd摩尔比趋近于4。说明提高吸附剂有效官能团的密度可提高Cd(II)吸附容量，且官能团修饰密度会影响吸附过程中官能团与Cd(II)之间的配比。m-PGMA-EDA微球对Cd(II)的最大吸附容量可达189.89 mg/g，与文献报道的Cd(II)磁性吸附剂相比，m-PGMA-EDA微球的Cd(II)吸附容量和吸附速率有了明显的提高。但是，Cd(II)的吸附容量受Zn(II)的影响较大，Cd/Zn分离因子约为0.5，说明m-PGMA-EDA对Zn(II)的选择性高于Cd(II)。通过分析FT-IR和XPS等结果表明吸附机理为氨基配位为主，羟基参与协同配位。 其次，针对Cd(II)选择性分离困难的问题，从软硬酸碱理论出发，将含软碱官能团-SH和-C=S的二硫代氨基甲酸盐(DTC)引入到聚乙烯亚胺(PEI)修饰的磁性PGMA微球上，得到mPGMA-PEI-S微球。研究结果表明，在pH为4-7范围内，mPGMA-PEI-S微球的Cd/Zn分离因子为3.36-4.00。吸附平衡时间为10 min，远短于文献中将PEI基二硫代氨基甲酸盐包埋于海藻酸盐微囊中所得吸附剂的Cd吸附平衡时间(24 h)。通过对X射线能量散射谱(EDX)、FT-IR以及XPS等结果的分析，表明mPGMA-PEI-S微球对Cd(II)的吸附机理为配位和阳离子交换作用机理。 为了进一步提高吸附剂的Cd/Zn选择性及循环使用性能，创新性地通过化学、物理修饰获得了双层负载萃取剂二(2-乙基己基)二硫代磷酸(D2EHDTPA)的多孔PGMA-D2EHDTPA吸附剂。实验结果表明，在pH为3-7范围内，Cd/Zn分离因子均大于80，最高可达330。采用PGMA-D2EHDTPA微球进行Cd(II)、Zn(II)分离，与文献中采用D2EHDTPA-甲苯体系进行Cd(II)、Zn(II)萃取分离(油水体积比为1:2，水相含1-4 mol/L的H2SO4)相比，有机溶剂用量大大减少了，操作条件更加温和。吸附剂用6 mol/L的HCl溶液再生后，可实现循环使用，6次循环之后Cd(II)吸附容量可维持在90%左右。通过FT-IR和核磁共振波谱(NMR)揭示了吸附剂与Cd(II)之间的相互作用机理为配位和阳离子交换协同作用机理。 最后，针对传统磁分离放大困难的问题，开发了适用于Cd(II)磁性吸附剂连续化规模分离的气助磁分离工艺及装置。间歇气助磁分离实验结果表明，Cd(II)负载的磁性PGMA微球能够在5 min以内达到90%左右的回收率。连续气助磁分离实验结果验证了Cd(II)负载的磁性PGMA微球规模化、连续化分离的可行性。;The emission of various Cd(II)-containing industrial wastewaters exceeding the emission standard will bring long-term threat to the environment and human health. The Cd(II) concentrations of most of Cd(II)-containing industrial wastewaters reported are 0-80 mg/L. Methods commonly used to remove heavy metal ions from wastewaters such as chemical precipitation and solvent extraction are difficult to treat low-concentration wastewaters. By contrast, adsorption is efficient, easy to operate, with less sludge disposal problems for the low-concentration systems, thus to be considered promising in the treatment of low-concentration wastewaters. Superparamagnetic adsorbents have tunable structure and size, and they can move, enrich, coagulate and transport under external magnetic field. Superparamagnetic adsorbents are easy to maneuvre under external magnetic field, and can avoid the problems of traditional fixed bed adsorption process including high pressure, high energy consumption, channeling and not easy for continuous operation. Magnetic carrier based separation technology has been reported to remove Cd(II). However, the reported magnetic adsorbents for Cd(II) removal showed low capacity. It is difficult to separate Cd(II) selectively. And the large-scale and continuous separation of magnetic adsorbents is not easy. Aim to deal with the issues mentioned above, this dissertation carries out a research on the effects of functional group type and density at the interface on the capacity and selectivity of adsorbents. Magnetic adsorbents with high capacity and enhanced selectivities towards Cd(II) were developed. Besides, gas-assisted magnetic separation process and devices suitable for the large-scale and continuous separation of magnetic adsorbents for Cd(II) were developed.Firstly, to address the problem of low capacity, the effects of immobilized functional groups density on Cd(II) adsorption capacity were studied. A series of ethanediamine-modified magnetic poly(glycidyl methacrylate) (m-PGMA-EDA) microspheres with different amine density were synthesized as the adsorbents for Cd(II). The results showed that Cd(II) saturation adsorption capacity increased with the immobilized amine density. But they didn’t show absolute linear relation in the whole range of amine density. The molar ratio of N to adsorbed Cd decreased with the increase of amine density, and reached a minimum value about 4 when the amine density reached 1.25 mmol/m2. These studies demonstrated that Cd(II) adsorption capacity could be significantly improved by increasing the immobilized functional groups density, and the density of immobilized functional groups could influence the complex ratio of functional group to Cd. The maximum Cd(II) adsorption capacity of m-PGMA-EDA was 189.89 mg/g. The Cd(II) adsorption capacity and rate of m-PGMA-EDA were obviously improved, compared with the reported magnetic adsorbents. However, Zn(II) had significant impact on Cd(II) adsorption capacity and the Cd/Zn selectivity factor was about 0.5. This indicated that m-PGMA-EDA had stronger affinity for Zn(II) than Cd(II). The coordination between amine groups and Cd(II) was the primary adsorption mechanism, and hydroxyl groups had synergistic coordination effect, concluded by analyzing the results of FT-IR and XPS.Next, to solve the problem that selective separation of Cd(II) is difficult, dithiocarbamate (DTC) containing -SH and -C=S groups was introduced onto polyethyleneimine (PEI) modified magnetic PGMA microspheres based on the HSAB (Hard and soft acids and bases) theory. The obtained adsorbent was designated as mPGMA-PEI-S. The Cd/Zn selectivity factor was 3.36-4.00 at pH 4-7 for mPGMA-PEI-S. The equilibrium time was 10 min, much shorter than that of the adsorbent prepared by encapsulating PEI-based dithiocarbamate in alginate matrix, which was 24 h. It could be concluded from the analysis of EDX, FT-IR and XPS results that cation exchange and coordination interactions existed in the adsorption process of Cd(II) onto mPGMA-PEI-S.To further enhance the Cd/Zn selectivity and recyclability of the adsorbent, two layers of di(2-ethylhexyl) dithiophosphoric acid (D2EHDTPA) were innovatively impregnated into PGMA microspheres through chemical and physical interaction respectively, and the obtained adsorbent was designated as PGMA-D2EHDTPA. The results showed that the Cd/Zn selectivity factor for PGMA-D2EHDTPA was greater than 80, even up to 330 at pH 3-7. Cd/Zn separation with PGMA-D2EHDTPA used less organic solvent and the operation condition was milder, compared with Cd/Zn separation using D2EHDTPA-toluene extraction system with 1-4 mol/L H2SO4 in the aqueous phase and the volume ratio of oil to water was 1:2 in the reference. The adsorbents could be reused after desorption by 6 mol/L HCl solution, and 90% of the adsorption capacity maintained after 6 adsorption-desorption cycles. Adsorption mechanisms included cation-exchange and coordination interactions, elucidated by FT-IR and nuclear magnetic resonance (NMR). In the last part of the study, gas-assisted magnetic separation process and devices suitable for the large-scale and continuous separation of magnetic adsorbents for Cd(II) were developed, aiming at solving the difficult scaling up of traditional magnetic separation. In the interval experiments, the recovery of Cd(II)-loaded magnetic sorbents could reach 90% within 5 min. The continuous gas-assisted magnetic separation experiments proved the feasibility of scalable and continuous separation of Cd(II)-loaded magnetic PGMA microspheres.
|董婷婷. 磁性聚合物吸附剂的制备及对Cd(II)的吸附性能研究[D]. 中国科学院研究生院,2018.|
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