Removal of heavy metals in wastewater by using zeolite nano-particles impregnated polysulfone membranes
Introduction
The preservation of quality and adequate water resources to sustain the needs of modern society has become a major problem today, due to excessive increases in volumetric production of industrial effluents, as well as inadequate conventional wastewater treatments that do not meet the discharge limits proposed by the environmental and health organizations. The conventional processes of removing heavy metals generally include chemical precipitation, ion-exchange and electrochemical deposition [1] which have many disadvantageous, such as high energy requirements especially when the contaminant concentrations range from 10 to 100 mg/L, excessive toxic sludge production which is required for further treatment and a lack of quality of the treated water within acceptable limits [2]. On the other hand, adsorption is considered to be a simple and effective method used in wastewater treatment especially when with low cost sorbents. Zeolites have also been extensively used in the application of separation and purification processes because of their well-defined porous structures and offering mobility of alkali and alkaline earth metals, in order to compensate net negative charge between Si4+ and Al3+ in the framework [3], [4], [5], [6], [7]. Meanwhile, nanoparticles known as high efficient adsorbents are replaced with micrometer-sized counterparts due to their high specific surface area and interfacial activity. However, instability, difficulty in regeneration and agglomeration of the nano-adsorbents during adsorption process is still a challenging issue. In addition, separation of the nano-adsorbents from treated water is limited its practical application.
Membrane water treatments are successfully used in wastewater treatment and up to 98% rejection of cadmium ions through asymmetric PSf membrane has been reported [8]. However, rejected metals cannot be recovered through these treatments: in concentrated solution they need further treatment. As a result, more sophisticated membrane architectures are required to achieve desired objectives. A new class of membrane has recently been designed by combining polymeric materials with nano-particles for the specific water treatments [9]. The so-called hybrid membranes could provide desired outputs by simply tuning their hydrophilicity, their pore size, porosity, charge density and mechanical stability. Moreover, they introduce unique functionalities such as photocatalytic, antibacterial or adsorptive capabilities [10], [11], [12], [13], [14]. For example, water flux and rejection values of the PSf membrane were simultaneously improved by simply adding a zeolite 4A to the membrane’s selective layer [15].
In the following study, the adsorption and the filtration processes were coupled using a zeolite nanoparticles impregnated PSf membrane, which was used to remove the Ni2+ and the Pb2+ cations from synthetically prepared solution. In literature, homogeneous nano-composite membranes have been extensively studied in gas phase applications by utilizing the adsorption capabilities of the nanomaterials which alters the selectivity of the gas mixtures [16]. However, limited studies concerning about the filtration performance and the removal capacity of the hybrid membrane against heavy metals under dynamic conditions have been reported in literature. The objective of this study is to test the feasibility of the prepared membrane architecture for the removal of the metal ions. It is thought that, the combined approach can be used to improve water hydraulic permeability as well as metal sorption capacity of the membrane, while providing a continuous separation with low energy consumption and a complete treatment in a single step. For this purpose, zeolite nanoparticles were fabricated via conventional and microwave heating methods and then incorporated into a polysulfone. The physical and chemical structures of the prepared membranes were characterized by XRD, SEM-EDX, FTIR-ATR, TGA and water contact angle analyses. The effect of membrane fabricating conditions, including casting compositions, evaporating periods and the nanoparticles loading on the hydraulic permeability and on the metal sorption capacity of the membrane were examined.
Section snippets
Materials
Polysulfone beads (Mn = 22,000 g/mol) and N-methyl-2-pyrrolidone (NMP) for the preparation of ultrafiltration membranes were supplied by Sigma–Aldrich. During production of zeolite nano-particles, fumed silica having 0.007 μm particle size and sodium aluminate from Sigma–Aldrich were used. Sodium hydroxide pellets was purchased from Merck and nickel (II) chloride (NiCl2) and lead (II) nitrate (Pb(NO3)2) solutions as atomic spectroscopic standards were supplied from Fluka. Necessary dilutions were
Zeolite characterization
The powder XRD patterns of the zeolites which were prepared by hydrothermal and microwave heating techniques at different crystallization temperatures, are presented in Fig. 1. The starred peaks reflect the crystals of pure zeolite A [18]. From Fig. 1, it was observed that increasing the temperature enhanced the intensity and sharpness of the diffraction peaks in the case of the microwave method. Moreover, a higher temperature caused a decrease in the ratio of zeolite X and allowed furthermore
Conclusions
In the present study, NaX nanoparticles incorporated PSf composite membranes have been fabricated for the removal of Pb2+ and Ni2+ ions from aqueous solutions. The characterization of the nanoparticles revealed that the conventional hydrothermal method produced pure NaX with uniform and ultrafine particles. Asymmetric membranes in desired structures were produced by simply tuning the membrane’s fabricating conditions. Elimination of the evaporating stage and the addition of NaX simultaneously
Acknowledgements
The author would like to thank the Scientific and Technical Research Council of Turkey for the financial support through Grant 113Y159. Celal Bayar University, Food Engineering Department is gratefully acknowledged for providing AAS analysis and I would like to thank Olivier Hage for English proofreading of this text.
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