The transport and retention of CQDs-doped TiO2 in packed columns and 3D printed micromodels
Graphical abstract
Introduction
Titanium dioxide nanoparticles (nano-TiO2) are widely used in commercial products, including sunscreens, paints, surface coatings, fuel cells and photo catalysts (Simonin et al., 2015; Xu et al., 2020). According to the statistics of the National Chemical Industry Productivity Promotion Center in China in 2019, the Chinese comprehensive output of titanium dioxide has reached 3.18 million tons, and increases in 7.69% year-on-year. As one of the most widely used catalysts, nano-TiO2 attracts much attention due to its low cost and favorable chemical stability (Pelaez et al., 2012). However, its large band-gap energy (3.2 eV for anatase and 3.0 eV for rutile) and the fast recombination of photoexcited electrons and holes result in the low response to ultraviolet light, which limits the applications of pristine nano-TiO2 (Yu et al., 2014; Tian et al., 2015). Such problem can be solved by doping with metal or nonmetal elements, coupling with metal-free carbonaceous materials (Camacho et al., 2018; Brunet et al., 2009; Park et al., 2009; Martins et al., 2016). Among many carbonaceous materials, carbon quantum dots (CQDs) is a good choice. CQDs is a novel class of nanocarbons with outstanding electronic properties and photostability, which also show high aqueous solubility and biocompatibility but low toxicity (Fernando et al., 2015; Tian et al., 2015). Moreover, the conjugated π-domains of CQDs can improve the photocatalytic activities of the doped compound, and the strong chemical interaction between the CQDs and TiO2 make electron transfer more efficient and faster (Sui et al., 2019). CQDs-doped TiO2 (C-TiO2) shows higher catalytic efficiency of hydrogen production and photodegradation ability of pollutants compared with pure TiO2 (Yu et al., 2014; Chen et al., 2016; Choi et al., 2018; Sui et al., 2019). Thus, C-TiO2 possibly becomes the catalysts with superior performance and wider applications in the future.
As a new type of catalysts, C-TiO2 has high catalytic efficiency and degradability to pollutants. For instance, C-TiO2 has a good degrading ability to Cr (VI) (Choi et al., 2018), and better degrading ability to rhodamine B than pure TiO2 under visible light (Pan et al., 2014). C-TiO2 nanoparticles (NPs) have great application prospects in wastewater treatment, hence may be released into the environment either artificially or unintentionally. The environmental hazard of NPs is closely related to its stability and mobility in aqueous system. Therefore, it is necessary to study the retention and transport of C-TiO2 in the environment media (surface, underground waters and soils) to predict its environmental risks. Present studies mainly focus on the catalytic performance of C-TiO2, however its processes in environmental media have not been investigated. Current researches have studied the transport and retention of TiO2 and CQDs in porous media. The influence of physical and chemical factors including solution chemistry, sand grain size, flow velocity, etc. on the transport and retention of NPs has been investigated via packed column experiments (Bradford et al., 2002; Wang et al., 2016). The transport and retention of TiO2 and CQDs in both saturated and unsaturated porous media are related to ionic strength (IS) and sand grain size. The retention increases with the increased IS because of the reduction of electrostatic repulsion; the mobility decreases with the decreased sand grain size because of the straining (Chen et al., 2012; Nebbioso and Piccolo, 2013; Lv et al., 2016). Dissolved organic matter (DOM) is ubiquitous in soil and water environments, which affects the transport of NPs in natural system (Nebbioso and Piccolo, 2013). The stability and mobility of TiO2 have been compared in soils with different DOM content (Khan et al., 2016). TiO2 shows stronger mobility in the presence of high DOM content, which increases the possibility of entering deeper soil layers or even groundwater. Humic substance, polysaccharides and proteins are typical DOM in nature, and humic acid (HA), alginate (Alg) and bovine serum albumin (BSA) are usually used to represent the three groups of DOM. HA can improve the transport of TiO2 in porous media because HA masks attachment sites on the sand surfaces favorable for the retention of TiO2 (Chen et al., 2018). In addition, DOM may cause different effects on the transport of TiO2. BSA shows the stronger enhancement on the stability and mobility of TiO2 than Alg in aquatic system (Ren et al., 2017). The high adsorption of BSA on TiO2 surface introduces vast negative charges, leading to static repulsion and removing positive charges of electrolytes in surrounding as well. Although the transport and retention of TiO2 in porous media have investigated, CQDs doping changes the physical and chemical properties of TiO2 and may result in different transport processes. Therefore, it is necessary to investigate the transport and retention of C-TiO2 under different solution chemistry to predict the mobility and fate of C-TiO2 in nature.
Recently, the micromodel experiments are developed (Baumann and Werth, 2004, Wan and Wilson, 1994), which allow the real time observation to the transport of NPs. Such observation cannot be achieved by the packed column experiments. At present, the micromodels are mainly made by photolithography, including photoetched silicon, glass models and Polydimethylsiloxane models (Liu et al., 2012; Meng et al., 2016; Jung et al., 2018). The complex production processes, high cost and harsh operating environment of the photoetched micromodels limit their wider application (Zhang et al., 2020). As an emerging technology, 3D printing has attracted wide attention and developed rapidly. It has multiple applications in biological microfluidics, construction, and medical treatment (Niu et al., 2013; Zhang et al., 2014; Gosselin et al., 2016). In recent years, 3D printing has been used in the production of two-dimensional micromodels, which are used to observe foam movement, wetting and imbibition of liquid, and to calculate the permeability of fluid in the medium (Gambaryan-Roisman, 2014; Osei-Bonsu et al., 2017; Ahkami et al., 2019). Due to the convenient preparation, simple procedure and low cost, 3D printed micromodels may provide a new tool to study the transport of NPs in porous media.
This research aims to explore the transport and retention of emerging NPs (C-TiO2) in porous media. The effects of ionic strength, typical DOMs (humic substance, polysaccharides and proteins) and sand grain size on C-TiO2 transport are investigated via packed columns. The deposition mechanism of C-TiO2 on a pore space, and the spatial/temporal changes in pore network were studied using 3D printed micromodel.
Section snippets
Synthesis of CQDs and C-TiO2
The CQDs and C-TiO2 were synthesized by a hydrothermal method (Khodadadei et al., 2017). Citric acid (4.0 g) and urea (4.0 g) were dissolved in 60 mL deionized (DI) water (18.25 MΩ) and stirred vigorously for 30 min to obtain a homogeneous suspension. Then, the insoluble matters were removed through 0.45 μm and 0.22 μm filtration membranes, respectively. The solution was transferred into a 100 mL Teflon-lined stainless-steel autoclave at 180°C for 12 hr, then it was naturally cooled to room
Characterization of particles
The XRD spectrum of CQDs has two superimposed broad reflections: these two broad peaks are attributed to amorphous carbon composed of aromatic carbon sheets oriented in a random way (Appendix A Fig. S5) (Choi et al., 2018), meaning that the CQDs is composed of disordered carbon and amorphous carbon (Zhu et al., 2013). The XRD spectrum of pure TiO2 shows well-resolved diffraction peaks at 27.4°, 36.0°, 41.1°, 54.2°, 56.5° and 68.8° (Fig. 1a), which can be assigned to (110), (101), (111), (211),
Conclusion
This study systematically studies the transport and retention of emerging NPs (C-TiO2) in packed sand columns and 3D printed two-dimensional micromodels. The transport of C-TiO2 is inhibited by the increased IS and decreased sand grain size, but promoted by the increased DOM concentration. C-TiO2 cannot pass through the fine-sand media at all, hence fine sand may be a good filter for such nanomaterials in nature. The promotion efficiencies of three types of DOM follow the order of HA > Alg >
Acknowledgments
This work was supported by the National Natural Science Foundation of China (No. 41773110), the National Natural Science Foundation of China-Shandong Joint Fund (No. U2006214), and the Shenzhen Science and Technology Research and Development Funds, China (No. JCYJ20180301171357901).
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