Cross-linked poly(vinyl alcohol)/modified α-manganese dioxide composite as an innovative adsorbent for lead(II) ions
Graphical abstract
Introduction
Heavy metal ions are among the major pollutants in the environment (Fu and Wang, 2011). For instance, the lead ion is a type of poisonous heavy metal ions, which the solubility of them decreases with increasing the pH of the solution. Lead is widely used in industrial processes (e.g., battery, plating, petrochemical, printing pigments, photographic materials), and eventually, is released into water bodies (Ren et al., 2012; Wang et al., 2007). Heavy metal ions are classified as “cancer risk” materials for humans. In addition, it has adverse effects on the nervous, reproduction, and immunity systems. This ion enters the bloodstream and can be accumulated in the bones (Xu et al., 2017; Yang et al., 2011). Therefore, novel technologies are required to diminish the level of heavy metal ions to a lower environmentally acceptable level. Hence, various techniques like ion-exchange, coagulation, adsorption, reverse osmosis, electrodialysis, and membrane filtration were developed for the heavy metal ion elimination from the aqueous media (Guo et al., 2014; Stafiej and Pyrzynska, 2007). One of the common methods is adsorption that has a low economic cost and high efficiency for the water and wastewater heavy metal ion treatment. This simple and cost-effective method is highly efficient in a wide range of concentrations and flexible in design and operation (Zhang et al., 2017; Ong et al., 2018).
Polymer composites are a novel class of light, cheap, and flexible materials made at ambient temperatures. They have a high potential for adsorbing contaminants, for example, heavy metal ions (Kango et al., 2013; Mallakpour and Motirasoul, 2017a). Poly (vinyl alcohol) (PVA) has a hydrophilic nature, which can readily form a film and act as a suitable matrix for the preparation of valuable polymeric composites (Rajaeian et al., 2015). Since PVA contains numerous hydroxyl groups, not only make it water soluble but also give it the capability for forming useful interactions with heavy metal ions. Cross-linked PVA could be beneficial for water treatment, because of its non-toxic nature and insolubility in the aqueous media (Hui et al., 2015).
α-Manganese dioxide (α-MnO2), an inorganic material, commonly acts as an additive for the reinforcement of polymeric matrixes. α-MnO2 nanorods may enhance the adsorptive ability of the composites, due to the existence of hydroxyl groups on their surfaces, having a relatively small size, and especially its morphology (Mallakpour and Motirasoul, 2017b). Subsequently, incorporation of them into a cross-linked PVA matrix may improve the adsorption property of the obtained composite. On the other hand, the surface of these nanorods needs to be modified in order to increase their dispersion in the cross-linked PVA matrix and create great interactions and compatibility among them. Hence, silane coupling agents are widely used for the surface modification of the nanostructures (Arun et al., 2014).
In this study, we prepared a new cross-linked composite based on PVA and modified α-MnO2 nanorods with 3-aminopropyltriethoxysilane (APES) for Pb2+ removal from the water solution. PVA was crosslinked with glutaraldehyde (GA) in order to prevent its dissolution in the aqueous media. Also, this will result in easy separation of the obtained composite film from water solution after the sorption process. Moreover, we investigated the adsorption efficiency of composite for Pb2+ removal using factors such as adsorbent amount, solution pH, temperature, the initial concentration of Pb2+, and time duration. For examination of the impact of the α-MnO2 nanorods surface modification on the Pb2+ adsorption by the adsorbent, composite of cross-linked PVA containing unmodified α-MnO2 was also fabricated and used as a control substance.
Section snippets
Chemicals
Manganese (II) sulfate monohydrate (MnSO4·H2O), APES, hydrochloric acid (HCl), 98% sulfuric acid (H2SO4), sodium hydroxide (NaOH), PVA with a molecular weight of 145000 g mol−1 and 99% hydrolysis were chemicals of Merck Company (Germany). Also, 25% aqueous solution of GA and potassium permanganate (KMnO4) were purchased from Sigma-Aldrich Company (Germany). Lead (II) nitrate [Pb(NO3)2] was supplied by Cica-Reagent (Japan).
Characterizations
A Topsonics ultrasonic device with output power of 100 W and 20 kHz
Results and discussion
The APES has two different functional groups amino and ethoxy groups along with alkyl chain moiety. The amine group alongside with the alkyl chain could have good compatibility with the PVA chains. Furthermore, it can separate nanorods of α-MnO2 from each other to some extent and reduce attractions among them. On the other hand, ethoxy groups are readily hydrolyzed to OH groups and subsequently react with hydroxyl groups of nanorods of α-MnO2 via so-gel process, which will cause surface
Conclusions
In this study, the composite film based on PVA and α-MnO2-APES was developed as an effective adsorbent for Pb2+ removal. Factors affecting the Pb2+ adsorption amount such as adsorbent quantity, solution pH, primary concentration of Pb2+, time, and temperature were studied. It is believed that the PVA-GA/α-MnO2-APES composite may find a promising practical application as a proper adsorbent for Pb2+ removal in terms of easy preparation, simple separation, and reasonable adsorption capacity. Here,
Acknowledgments
Research Affairs Division Isfahan University of Technology, Isfahan, I. R. Iran for financial support is acknowledged. The authors also thank Dr. H. Farrokhpour, Dr. F. Azimi, Dr. V. Behranvand, Mr. F. Tabesh, and Mrs. S. Rashidimoghadam for their helpful discussion.
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