Properties Of Three Types Of High Fluxmembrane In Removing Cadmium Ion From Aqueous Solution

As a typical heavy metal element, cadmium(Cd), which is pervasively applied as the raw materials for the manufacture of vacuum tubes, chemical stabilizers, bactericides, nickel-cadmium cells, solar cells, is one of the water environment contaminants. Membrane separation is a promising technology for cadmium ion(Cd2+) removal because of its simplicity of operation, stabilization of performance and convenience of maintenance. Many efforts have been made to develop membrane materials with high water flux, high rejection and simplicity of synthesis and energy saving membrane separation processes to remove Cd2+ from water realizing the combination of economic and environmental benefits.In the first part of this dissertation, a template-directing hydrothermal process with glucose as substrate, tellurium(Te) nanowires as template was applied to fabricate carbonaceous nanowire(CN). The subsequent solution-volatilization self-assemble process forged the CN into carbonaceous nanowire membrane(CNM) for removal of Cd2+ in aqueous solution. Elemental analyzer, energy dispersive spectrometer, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscope, surface area analyzer were applied to characterize CNM. Static adsorption and continuous water flow experiments were carried out to evaluate the performance of CNM. The effects of influence factors on Cd2+ rejection were also studied and optimized. The possible rejection mechanism was also discussed.The adsorption properties of CNM has been studied with static experiments, in which the effects of p H, adsorption time, initial Cd2+ concentration, temperature on adsorption performance were evaluated. The experimental data showed that the adsorption capacity is enhanced with temperature increased and the maximum adsorption capacity of CNM at 25 ℃ is 312.5 mg·g-1, the optimal p H is 5.0, adsorption equilibrium was reached in 20 min. The adsorption data was in accordance with pseudo-second-order kinetics model and the adsorption sites were the oxygen-containing groups(hydroxyl and carboxyl) on CNM surface were. The adsorption thermodynamics analysis proved the adsorption process is endothermic and chemically predominant, the adsorption activation energy is 61.4 KJ·mol-1. Except for adsorption process, the electric double layer formed by Cd2+ in bulk solution and CNM surface played an important role in Cd2+ rejection as well. In the continuous water flow experiments, a 352.3 L·m-2·h-1 water flux was obtained at 2 k Pa applied pressure, the Cd2+ rejection was 90.6% at 10 mg·L-1 initial concentration.The applied pressure driven membrane separation process is energy consuming. The second part of this dissertation focused on an osmotic pressure driven forward osmosis(FO) process to explore a more economic membrane process to remove Cd2+ from aqueous solution. In FO process, water molecules are driven by osmotic pressure difference and transport from low concentration feed solution(FS) to high concentration driving solution(DS), while the solute(Cd2+) in FS is rejected by a semi-permeable FO membrane. The FO membrane materials applied in lab-scale research are mainly composed of organic polymer, which contain a thin active separation layer and a porous support layer for enhancing mechanical strength. It is this two-layer structure that lead to the inner concentration polarization(ICP) reducing the water flux dramatically. In this part, stainless steel mesh was used as back-bone and Si O2 was prepared as active layer to synthesis quasi-symmetric thin film inorganic(QSTFI) membrane with micro-interface sol-gel process. The structural properties, chemical composition and surface charge properties of QSTFI membrane were characterized to discuss the Cd2+ rejection behavior.Without obvious phase-interface of active layer and support layer, QSTFI membrane could eliminate the ICP effectively. Besides, the membrane surface is negatively charged and water contact angle is 67.71°. With the special structural property and hydrophilic surface, QSTFI offered a water flux of 68.0 L·m-2·h-1(DS 2.0 mol·L-1 Na Cl, FS 10 mg L-1 Cd2+), which is higher than most organic polymer FO membrane. Electric double layer formed by Cd2+ and membrane surface could offered a rejection of 99.46%.In order to promote the performance of QSTIF membrane in Cd2+ removal, QSTFI membrane was modified with polyvingl alcohol(PVA) in the third prat of this dissertation. The comparison between modified QSTFI(PVA-QSTFI) membrane and pristine QSTFI membrane was conducted from the aspects of structural properties, surface charge properties and surface hydrophilicity. And the performance of PVA-QSTFI membrane was also discussed.PVA-QSTIF membrane surface is negatively charged and hydrophilic, and the quasi-symmetric structure of QSTFI membrane is maintained. High concentration PVA modification could block some part of the membrane pores lowering water flux. The optimal PVA concentration is 0.5 wt% obtained by gradient experiments from 0.1 to 1.5 wt%. When DS is 2.0 mol·L-1 Na Cl, FS is 10 mg·L-1 Cd2+, PVA0.5-QSTFI membrane offered a water flux of 76.48 L·m-2·h-1(12.5% higher than QSTFI) and a rejection of 99.58%.In conclusion, CNM, QSTFI and PVA-QSTFI membrane with high water flux were synthesis to remove Cd2+ from aqueous solution. And the water transport properties and Cd2+ rejection behavior of as-obtained membrane materials under low applied pressure or osmotic pressure was discussed. This dissertation may provide theoretical basis to novel membrane materials synthesis for heavy metals water pollution remediation and possesses scientific guidance significance.