In situ growing directional spindle TiO2 nanocrystals on cellulose fibers for enhanced Pb2+ adsorption from water
Introduction
Heavy metal pollution has always been a concern worldwide because these metals are toxic and nonbiodegradable [1], [2]. Their presence in water poses significant threat to environment and public health. Lead (Pb2+) is regarded as one of the most toxic heavy metals. The current allowable maximum contamination level of Pb2+ in drinking water set by US EPA is 15 ppb. Chemical precipitation, ion exchange, membrane separation, electrochemical method and adsorption have been used to remove heavy metals from aqueous solutions. Among them, adsorption is a promising method because of its low cost and high efficiency, especially for low-level heavy metal treatment in wastewater. The key to the success of adsorption process lies in the selection of appropriate adsorbents [3], [4].
Nanoparticles (NPs) are attractive and promising adsorption materials for application in water treatment due to their high specific surface area, reactivity and mobility [5], [6], [7]. Interestingly, the use of TiO2 for removing heavy metals and other pollutants from water has been extensively studied due to the TiO2’s low-cost, non-toxicity, stability and easy availability [8], [9], [10], [11].
However, nanoscale particles, when synthesized, in storage, during transportation or in use, tend to agglomerate and lose a fair amount of surface area which reduces their intended efficiency for target applications. When applied in a fixed bed for adsorption, these particles can cause some practical issues including inevitable particle loss and its induced secondary pollution, as well as high flow resistance [12]. As reported, the mobility of pollutants can often be enhanced or slowed by orders of magnitude because of the presence of colloidal particles in aquatic systems [13], [14]. Although TiO2 is thought to be environmentally benign, its accidental release to aquatic systems could still cause significant environmental risks. An effective approach to overcome the problems is to fabricate hybrid nanocomposite by immobilizing ultrafine particles onto materials orders of magnitude bigger [15].
Cellulose is the most abundant natural polymer on Earth [16], [17]. Cellulose and its derivatives have demonstrated adsorption properties for heavy metals, especially when properly modified [18], [19], [20]. Cellulose fibers (CF) are soft and easy to process, they can be readily made into a form of filter bed/paper using traditional wet laid technology for practical applications. In addition, the abundant hydroxyl groups on CF surface can act as efficient hydrophilic sites to accelerate nucleation and growth of inorganic particles, thereby controlling the morphology, crystallinity and particle size. As reported elsewhere, such readily available cellulose matrices offered an ideal platform, either as templates or precursors [21], [22], [23], [24], [25], [26], [27], for the design and fabrication of hybrid nanocomposite with hierarchical structure. As such, various synthetic strategies have been attempted to anchor nanoTiO2 onto cellulose matrix, such as dip coating [28], [29], self-assembly [30], [31], in situ hydrolysis [32] and hydrothermal treatment [25], [33]. However, most of these TiO2/cellulose composites are characterized as dense nanoTiO2 coatings covering the whole cellulose surface, which are not intended for heavy metal removal by design. In this work, we used CF to immobilize TiO2 NPs for water treatment by taking advantages of not only the adsorption efficiency of the composite material, but also the material’ s greenability. We attempted not to compromise the needed surface area of the NPs by purposely controlling particle growth, so a scattered pattern of particle distribution on cellulose was obtained for effective adsorption. Phrased it in other words, we aimed to avoid forming a nanoparticle film on cellulose to sacrifice the surface area of the particles and the flexibility of the composite material in order to further process the material into a form of filter bed, for more convenient applications.
Considering the essential binding strength of nanoTiO2 on cellulose, which is critical for applying the material under strong hydrodynamic conditions, here we chose a simple method, in situ hydrolysis, to synthesize TiO2/CF composite. Meanwhile, we used a titanium oxysulfate-sulfuric acid complex hydrate (TiOSO4·H2SO4·H2O) as the precursor. This material is a low-cost and easy-to-handle chemical as opposed to other titanium salts. The as-synthesized TiO2/CF nanocomposite showed unique surface morphology, especially for the nanoTiO2 crystals because of their restrained growth on the cellulose template. More interestingly, the composite exhibited good Pb2+ adsorption performance in aqueous solutions in both static batch adsorption experiments and dynamic filtration experiments using a filter bed made of the composite fibers. The composite also showed good selectivity and regenerability for repeated use. And finally, the adsorption mechanism was discussed based on XPS and ATR-IR experiments, as well as surface area measurement of the composite.
Section snippets
Materials
TiO2 (rutile, 35 nm) was obtained from Beijing Dk Nano technology Co., Ltd. Titanium oxysulfate solution (TiOSO4·H2SO4·H2O, ∼15 wt% in dilute sulfuric acid) was purchased from Aldrich. Cellulose fibers of 10–30 μm diameter were supplied by Beijing Ronel Engineering Materials Co., Ltd., and washed with distilled water before use. The sulphuric acid (H2SO4) with a concentration of 95–98% was supplied by Beijing Chemical Works. All other chemicals were analytical grade and used as received.
Synthesis of TiO2/cellulose fiber composite
In a
Synthesis and characterization of TiO2/CF composite fibers
As described previously, in situ hydrolysis was used to fabricate hierarchical TiO2/CF composites with nanoTiO2 crystals immobilizing on micron-sized cellulose fibers. The morphology and content of TiO2 in the TiO2/CF composites were dependent upon the synthesis conditions (temperature, time, acid concentration and reactant concentration) (Table S1) [32]. It was found that dense and well-dispersed TiO2 particles could be readily obtained within 4.5 h of reaction, and the shape of the particles
Conclusions
NanoTiO2 was successfully immobilized on the surface of cellulose fibers by in situ hydrolysis. Cellulose played an important role in directing the nucleation and crystal growth of TiO2, resulting in the formation of spindle rutile TiO2 nanocrystals. The capacity of the composite is 4 times higher than pure CF. However, the capacity calculated based on sole spindle TiO2 adsorption in the TiO2/CF composite is 371.0 mg/g, far exceeding the 20.0 mg/g of commercial TiO2 used alone. The nanocomposite
Acknowledgements
This work is supported by the Chinese Academy of Sciences under the Talented Program and the National Nature Science Foundation of China (Grant no. 51302267).
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